Q: Hi. I am building an ant spinning drum. I want to use a 4 oz. drum. Would the Radio Shack 9VDC Micro Super High Speed Motor work? It runs at 24,000 rpm with 36 g:cm stall torque. I would probably gear it down to 4 or 5 to 1. Thanks, your web site has helped me learn a lot.

A: That Radio Shack motor is a very small, inexpensive, general purpose, low-power unit designed to run a small cooling fan or the like. It has a lot of speed, but not nearly enough torque for an effective antweight spinner weapon. It would take a very long time (6 or 8 seconds) to spin the weapon up to speed, and you don't have much spin-up time in a small antweight arena.

Staying with an inexpensive brushed motor, I'd suggest a Speed 300 geared around 2:1 for a 4 ounce drum about 2" in diameter. These motors were used to great effect in the VDD antweight kits to power their spinning weapons.

Q: Would the Speed 300 motor work the best or could I get something better? I dont mind paying for it.

A: There's always 'something better', but exactly what that might be depends on your design and your skill as a builder. It takes more than power to make an effective weapon, and too much power can be worse than too little.

The Axi 2208/20 brushless motor would be a 'top end' powerplant for an antweight spinner, but it would require a brushless motor controller, a more robust weapon drive system, and a higher level of precision all around. For a first spinner, I'm going to suggest sticking with the Speed 300 - it'll be plenty.

A: No secrets to the weapon in Code:Black. With a 12 HP internal combustion weapon spinning a low-mass undercutter blade in a lightweight robot you can spin the weapon pretty much as fast as you like. However, weapon speed is only one part of the spinner success formula. There is plenty of information about spinner design in this archive.

A: Mark J. here: horsepower is only part of the spinner equation. The idea is to store kinetic energy in the rotating mass of the weapon and release that stored kinetic energy on impact. My own rule: an effective spinner will have a minimum 20 joules of stored kinetic energy per pound of weight class and will be able to spin up to at least half that level before an opponent can attack them.

There are many trade-offs in spinner design: speed versus 'bite', kinetic energy versus maneuverability, horsepower versus rotational mass, optimum mass placement versus durability, etc. Read thru this archive for plenty of discussion on spinner design.

Opinion: Warhead's weapon was awesome to watch, but the gyroscopic effect of the spinning mass made maneuverability a real issue. Too much weapon, I think. With a different design that 22 horsepower might be more useable - or it might become a real disaster.

That other robot, who's name has been recently banned from the page, had no control issues and ample weapon power. I don't have enough info to calculate the weapon's energy storage, but it was certainly plenty. Given a choice between the two robots, I'd rather drive [name deleted] than Warhead.

A: Beta's hammer has a claimed 3000 joules of energy. The robot was an interesting experiment, but usless without a suitable arena in which to fight.

A: Mark J. here: the 7 HP I cited (I like to round up) was the version used at BB 4.0. The earlier version had about half that power. I don't know of any motor upgrade for BB season 5. Their best result came in BB 3.0 with the less-powerful engine.

A: Not even close. I know of at least one 150 gram UK antweight with a pneumatic flipper and there are plenty of 1 pound US antweight flippers.

A: Terrorhurtz uses a true rack and pinion weapon drive similar in concept to the drive on 'The Judge' but without the chain and sprocket bodge. The design on Killerhurtz was a much more complicated system involving lever arms, gears, and chains. It worked, but the rack and pinion is superior.

A: Building a combat robot is roughly equal parts engineering skill, past experience, and 'that looks about right'. Sometimes an 'improvement' in one aspect of a robot can lead to unexpected consequences in other areas. I suspect that the style and position of Warhead's impact teeth were chosen to avoid stalling the weapon. A more aggressive tooth design might have caused trouble -- Team Razer is known to do extensive testing.

A: Whether the impact surfaces of a spinning weapon are sharp or blunt is a function of the intent of the weapon. If the weapon is designed to penetrate, the impact area should be sharp - but it risks becoming stuck in the gash or possibly in the arena itself. If the intent is to deliver a smashing blow, a blunt impact zone is easier to maintain, stronger, and less prone to damage.

A: Mark J. here: My comment was based on engine power - 'Mechavore' claimed 7 horsepower and 'Warhead' claimed 22. I don't have enough information on the weapon from either robot to calculate kinetic energy storage. You are correct that the non-linearity of the torque curve for an internal combustion engine makes weapon calculations difficult - but not impossible.

A: Most tournament rulesets require a maximum 'spin-down' time for rotary weapons. The tournament director does not want to hold up the tournament while a weapon takes five minutes to stop spinning. If your weapon won't come to a full stop in the alloted time (currently 60 seconds for BattleBots and RFL) you'll be disqualified - so yes, very important!

Q: If you had a spinner with no brake system for stopping your weapon at the end of a match, could you just hit it against the arena walls to stop it? It dosen't specify in the RFL rules.

A: Nope. The RFL ruleset does specify. Section 10.2 says:

Use of the arena wall does not qualify as 'self-contained'.

A: Tool steel is at least 50% harder than even the best 7075-T6 aluminum. When it comes to turning energy into damage, there is no subtitute for harness. An aluminum weapon blade could do more damage to itself than to the opponent. Plus, aluminum doesn't make those pretty (judge influencing?) sparks when it hits.

Q: You say "An aluminum weapon blade could do more damage to itself than to the opponent", but why does 'Mechavore' keep using aluminum blades instead of hardened tool steel blades? Weight issues, or just Robert L.'s mistake?

A: I think we have a difference in terminology. 'MechaVore' did not have an aluminum blade, it had an aluminum disk with large hardened steel impact teeth. This is entirely standard for rotating disk weapons. The aluminum alloy provides a high strengh to weight ratio for the body of the weapon and and the steel teeth provide hardness at the impact site to do the damage.

A: Diesector had several upgrades for BB 5.0: new jaws, new batteries, new tires, and new hammers. I suspect that the hammer change was largely cosmetic -- the new hammers looked much more aggressive than the previous design and better matched the look of the whole robot. Diesector's hammers never did a great deal of damage, but they helped in the 'aggression' scoring.

Q: And what is Warhead's spinning dome's height-adjustability used for?

A: The adjustable weapon height allowed the team to 'aim' the weapon at critical parts of the opponent. Low target, low spinner. High target, high spinner.

A: Middleweight 'SubZero' did indeed have a powerful flipper and a very good 26 win, 10 loss record with three tournament wins. I suspect that little is said about the 'bot because the flipper design is entirely standard. However, there have been comments about the legality of the components used in the flipper. The pneumatic cylinder was rated 250 psi by the manufacturer, but was being operated at 850 psi with only minimal modification. I'm puzzled about how they got this grenade thru tech inspection.

A: Flexible how? There are slip clutches available that will prevent a large torque load from being transmitted along the shaft. There are universal joints that will allow axial flex, but I don't think you want your weapon flopping around at the end of the shaft like that. The best option to isolate the weapon from the motor is to use a v-belt drive that will also allow a suitable speed reduction/torque increase. You really don't want to direct drive a weapon from an internal combustion engine.

A: Mark J. here: for a given weight and diameter the three shapes will store different amounts of kinetic energy at a given speed. Examples, all weighing 10 kilos and spinning at 1200 RPM:

• a bar 1 meter long and by 0.1 meter wide stores 6882 Joules of kinetic energy;
• a disk 1 meter in diameter stores 9927 Joules of kinetic energy;
• a cylinder 1 meter in diameter by 0.3 meter tall stores 19,728 Joules of kinetic energy.

Other trade-offs go into designing a spinning weapon - I could fill a lengthy chapter in a robot design book - but the essential consideration is a balance between efficiency, durability, and simplicity.

A: Again, I can't get excited about ranking combat robots by weapon/weight/tournament.

• 'Hammertime' used high-pressure nitrogen regulated to about 200 PSI to power it's weapon.

• 'No Apologies' had a spring powered axe/spike that was reset by an electric winch. It also had a pair of lifting arms.

• 'The Judge' was CO₂ powered.

A: In arenas where it's possible to win by throwing an opponent out of the arena, a monster flipper is a viable design choice. In a fully enclosed arena, they don't carry much benefit - they are more of a novelty.

Q: But what caused 'Toro' and 'T-Minus' got Giant Nuts? They're really powerful and I think they are belonged to 'Monster Flippers',too. Aren't they? [Chinese Forum]

A: Mark J. here: Inertia Labs was one of the premiere teams in Combat robotics. They were experienced, skilled, well funded, and loved innovation. Importantly, they knew how to learn from their combat experience and improve a good robot over time to make it a great robot (they also knew when to give up on a poor idea). I'm sure they could succeed in building a winning robot with any type of weapon they might choose.

A: See the description of 'Ringmaster' further down in this archive.

A: I guess you're giving a lot of points for destruction. In my book, top spinners win championships.

• How can you not include 'Ziggo' (13-3, 2 championships)?

• How about 'Hazard' (17-1, 3 championships)?

• 'Minion' qualifies as a spinner (12-4, 2 championships).

• and the often overlooked 'Backlash' (13-4, 1 championship) probably belongs in the mix.

I think my 5th robot would be 'Shovelhead'. His record at BattleBots was only 3-3, but he has since gone on to win 5 tournaments and amass an impressive 39-15 record.

Q: I asked you a question about top 5 spinner but I made a mistake: not in all Battlebots, just in the heavyweight of Battlebots.

A: That becomes a very narrow category. I'm not a big enough fan of spinners to try to sort out which amongst a lackluster group of robots are the 'top heavyweight spinners at BattleBots'. Outside of 'SOW' and 'Warhead' the heavyweight spinners just weren't all that impressive - IMHO.

A: There is a great deal more to a robot than the power of its weapon. 'Hexadecimator' (21-9) was a well constructed robot that did everything well: quick, controllable, powerful, and well driven. When you have that combination going you can use a pointy stick for a weapon and still win matches.

A: I've never seen inside. I could only speculate.

A: Some builders share a lot of information and some don't. The only details I have on Voltarc/Voltronic are that the lifter is electric and the main drive motors are made by Leeson.

A: Reliability is an issue with internal combustion engine (ICE) spinners. A few teams have that licked.

Electric weapons are simpler and more reliable, while ICE gives the potential for greater power at lower weight (and higher noise level).

A: Some designs work better in a specific size range. That's why there are no eagle-sized mosquitoes or mouse-sized elephants.

A: It's been tried, with very poor results. It's way too slow and does way too little damage. Think about the pressure and time needed to drill a hole in armor material. If your opponent is sitting still long enough for you to do that, he's already lost the match.

A: I was quite surprised when 'Warhead' was beaten by 'Overkill' at BattleBots 5.0. I thought it was going all the way.

There are many approaches to building a spinner weapon. Some builders like the simplicity of a single-piece bar, while others attempt to gain an advantage through complex design with several different materials. Warhead's aluminum dome covers their powerful internal combustion engine and acts as armor as well as adding to the rotational mass of the weapon. It was easiest to machine the dome from a single chunk of aluminum and add on the impact teeth.

A: Team Razor wasn't very keen on sharing construction details about their 'bots. I do know that the weapon dome on Warhead was aluminum, and it would certainly make sense to use a tool-steel alloy for the blades.

A: Mark J. here: you're entitled to your opinion on 'awesome', but 'Warhead' is an 'Honorable Mention' in the Hall of Fame and 'MechaVore' is still waiting outside.

Warhead had more than three times the power of MechaVore's weapon, and the display of gyroscopic forces put on when Warhead fully spun-up has never been matched. I'm frightened just thinking about it! Suggest that you go watch a few videos of Warhead and see if your opinion changes.

A: No, it's too weak. It was only intended as a brace in the self-righting sequence.

A: Simplicity. No moving parts, low weapon weight requirement, and an attack that develops quickly and works well against many types of robot.

A: Razer has an adjustable 'torsion bar' suspension that allows very precise positioning of the front wedge 'snout'. The information I have is unclear about whether the suspension is adjustable remotely.

A: Complete Control was the original 'clampbot'. Starting with a conventional pneumatic lifter, Derek added a pneumatic arm on the lifter platform itself that clamped downward to hold the opponent in place. Once clamped, the entire platform lifted upward with the opponent in a firm grip. Awesome!

I built my first 12 pound wedge bot, yet to compete, and I am wondering about adding a weapon. With the 1-1/2 pound steel wedge my robot only weighs 10 pounds 13.5 oz. I was thinking of making another lighter wedge and putting some type of lifting mechanism using a servo or linear actuator. I know both of those are slow but it does not need to be that fast. Any advice? Thanks, Daniel.

A: Glad to hear that you found the advice here helpful, Daniel.

My advice is to enter a competition with the robot as it is. You may discover a critical use for that extra 18 ounces of weight allowance. If you're pleased with the performance of your chassis and drive train you can think seriously about modifications to give you some additional offense.

Electric lifters are effective in the sub-light classes, and judges seem to like them. You are right to say that they do not need to be fast -- they do not need to lift very high either. Even a simple servo-powered hinged wedge can be very useful and can add to the versatility of the robot, but give the robot a shake-down in combat before you start modifications.

A: What... the weapon RPM? I'd start by looking at their websites. See the for info on finding archived websites for teams that aren't on the web anymore. Many teams consider this type of data 'top secret' and do not make it available.

There is much more to a rotary weapon than just RPM. Serch this archive for other design criteria.

A: Builders have been working on flywheel powered flippers for quite some time. There is a discussion thread at the RoboWars Australia Forum. The problem with such a design is building a clutch mechanism that can withstand the brutal force instantaneously transferred from the flywheel to the flipper without tearing itself apart.

I have not seen Warrior SKF up close, but unless you can match the budget and resources available at Team Whyachi I would suggest you try a more conventional approach.

A: It takes more than a powerful weapon to make a champion robot. I say this all the time: a successful combat robot has all of it's components and systems working together in balance.

A super-weapon on a chassis that doesn't work well to use that weapon is not going to perform well. A well designed chassis with a poorly set-up radio system will have a very rough time. A robot that's hard to repair and maintain won't get far in a tough tournament. The weapon is probably the least important system on a combat robot.

By the way, I think a 4 win 2 loss record is pretty successful. They were stopped at BattleBots 5.0 only by the barbaric 'Warhead'.

Q: Why did you say The Matador had a "next generation" flipper? What was so different about it?

A: Inertia Labs refered to it as a new generation. Here's what they had to say on their website:

A: Mark J. here: a well designed combat robot is not just a set of components bolted together. The design of each system must be made with consideration to the rest of the machine. Recommending a weapon drive without knowing anything about the design for the rest of the robot would be folly.

If I'm given a weapon design I can comment on potential flaws, but I have to assume the designer has an overall plan that will incorporate the proposed weapon into a well balanced robot.

Read down thru this archive a little to find an example of integrating a weapon into a beetleweight design.

See also the about 20 questions down.

A: Why would you want to? On impact your opponent and your 'bot will be thrown clear of each other and your blade can start to spin back up.

I can't think of any simple circuitry to accomplish an electrically detected contact shut-down.

A: A dead shaft is fixed in place and does not rotate with the weapon. The weapon and the sprocket/pulley are fixed together and have bearings that allow them to rotate on the dead shaft as a unit. This eliminates the problems associated with trying to secure the weapon to a small diameter shaft. Search this archive for more tips on dead shafts.

A: A steel striker bar will make very pretty sparks when hitting titanium armor. I suppose you could add a titanium or magnesium insert to your steel blade to make sparks when attacking steel armor. Aluminum and plastic armor are too soft to make sparks no matter what you hit them with, and nobody would believe sparks off plastic anyhow.

Experienced judges will not be impressed by sparks, but they could help get the audience behind you.

Do you envision any problems with implementing or using this design? Do you know if anyone has used a similar idea? Do you have suggestions on materials to build the disc and teeth out of? I don't plan to go too crazy with the weapon RPM as I know my robot's weapon mount won't be indestructable. Thanks again for all the help, guys!

A: Mark J. here: I don't like the idea of putting the entire stress of impact on a hinged tooth. Even an antweight spinner will transmit a big slug of energy thru the impact tooth and I think it's best to have that solidly anchored.

Suppose only one of the teeth gets rotated - you have a serious imbalance problem that would require shutting down the weapon to reset. Worse, one of the hinges gets tweeked and the tooth is stuck either in or out.

A 4" diameter disk rotating at 3000 RPM has more than 500 g's of centrifugal acceleration acting at the edge (calculator). The 'long arm' of your tooth would need to be held in place very firmly to keep from 'rotating out' prematurely. I can't think of a simple design to hold it in place, let it swing out when triggered by an impact on the short arm, and return it to position at lower RPM.

I have seen spinners with 'free swinging' impact bars that are held outward only by centrifugal action. The idea was that the bar could deliver a good blow, then rotate out of the way to prevent the weapon from stopping. This produced a series of rapid impacts that was impressive to the judges, if not as damaging as a single big impact.

Insect class rotating weapons are typically single piece steel to take great abuse. Larger disks are often aluminum with tool-steel teeth, but I've seen large disks made of steel, titanium, end even UHMW Polyethylene.

I have to believe someone would have thought of this before, and the fact that I don't see this kind of thing in common use makes me think it's somehow a bad idea. What do you think?

A: Mark J. here: first, I like your plan to put off building a spinner until you have more experience. Many builders aim to dominate the sport with their first project. That usually ends in frustration. Compliments on your patience.

Some builders reading this answer will scoff, but your idea has theoretical merit. To understand why requires a short lesson in the characteristics of Permanent Magnet Direct Current (PMDC) motors.

Take a look at the chart. A PMDC motor generates maximum torque at stall, and that torque falls off linearly to zero at maximum RPM. Power is the product of RPM and torque, and that product is greatest at 50% of the maximum RPM.

In a conventional spinner weapon, the motor strains against the large rotational mass of the weapon to slowly reach the 'fat' part of the power curve, where it really pumps energy into the weapon. Quickly, the RPMs move beyond the peak power zone and the motor again struggles to add more energy with decreasing torque and power.

Your 'sliding weight' design achieves an effect similar to a continuously variable transmission. The weapon initially has a smaller Moment of Inertia (MOI), allowing it to accelerate more quickly into the peak power zone of the motor output curve. If the sliding weights are designed to start moving outward as the motor approached its output peak and completed their transit shortly after the motor power peak, the motor will spend a greater percentage of the spin-up time near peak power output and would average greater power output which would give a shorter spin-up time.

So, on paper the idea works. How much benefit there is to be gained depends on the amount of mass you are able to shift and how accurately your spring loading system controls the weight progression. The drawback: increased complexity on a component that will take a lot of abuse. It might be simpler to just pop for a more powerful weapon motor; simplicity has merit.

A: Mark J. here: aerodynamic and mechanical drag are the limiting factors on absolute top speed of a rotating weapon for a given amount of power; add more power and you can get more speed. However, if you spin your weapon too fast it won't be able to catch and 'dig in' to your opponent. It will just 'skitter' over the surface of their armor. Also extremely important in an insect class robot is spin-up time. The best weapon takes all these factors into consideration. Play around with the parameters in the Team Run Amok Spinning Weapon Excel Spreadsheet to put everything in balance.

Q: How many joules can a beetleweight bar spinner realistically dish out?

A: See the above question. Joules alone are not the answer. You see builders claiming astronomical numbers of joules, but a well-balanced design is going to win more matches.

Q: How much weight is usually alloted to a robot's weapon system - something like 'Totally Offensive'?

A: 'Totally Offensive' is an extreme example -- blade and motor alone make up more than 1/3 of the entire robot weight. Add in the weapon supports, battery, geabox, and controller and I'd guess at around 60% of the robot weight devoted to the weapon. A more typtical weight allotment might be 35%.

Q: I played around with the spreadsheet a bit and came up with two beetleweight configurations:

1. A 0.4x0.04x0.004m (The obscene length represents weight concentrated towards the ends of a 25cm blade) steel bar, running on a Axi 2820/10 at 4:1. Giving a 0.41 second spin-up. With max RPM and joules of 3600 and 500 respectively weighing in around 24 ounces.

2. A 0.2x0.04x0.004m steel blade on an Axi 2808/20 at 3:1, giving 0.17 second spin-up, and max stats of 4900 RPM and 85 joules.

A: I get very different numbers for your weapon configurations, so I started over.

First, don't extend the length of a spinning bar to reflect extra mass out at the ends. That will overestimate the moment of inertia. The spreadsheet gets kinda clunky when trying to get this right, but here's how you do it:

Q: What are you using for stats on the Axi 2808/20, because when I run it a 4:1 it gives a spin-up RPM of 2426, not 2816

A: Per the Axi website the Axi 2808/20 puts out 1490 RPM per volt and has a terminal resistance of 105 mohm. At 12 volts that's 17,880 RPM. Divided by 4 gives 4470 RPM. Take 63% for the spin-up calculation and that gives 2816 RPM.

Q: Do you need a motor controller to power an Axi, or can you just use a relay?

A: Axi motors are brushless. Brushless motors require a motor controller to operate - they won't run without one.

A: Hydraulics are rare but they are used, usually in heavyweight robots because industrial products are too large and heavy for lighter robot classes. Pumps and actuators sized for smaller robots are available, but they tend to be low-power and expensive -- see recent posts in this archive for a source.

A: Suggest you take a look at the BioHazard Mechanical Design page. There are several photos and explanatory text covering their use of linear actuators to power a 4-bar lifter. Briefly, the actuators are attached to the front lifter bars via a bellcrank. The bellcrank allows the actuators to generate maximum force when the mechanism has the least mechanical advantage in the lift. Great design!

A: Electric linear actuators work fine for a lifter, but are way too slow for a flipper. You need pneumatics for true flipper speed. There are several posts about pneumatic system parts and links to design help in this archive.

You have quite a bit of design work to do before I can recommend a specific components. Will the liting mechanism be a 4-bar mechanism like BioHazard, a single-pivot unit like Silverback, or some alternative design? How high do you want to lift?

If you do decide to go with a linear actuator I'd suggest a supplier who knows the needs of robot builders. Have a look at the actuators offered by Team Delta and Trossen Robotics.

A: Mark J. here: very hard to calculate because of so many variables: differing armor mounting systems, alloy type, heat treatments, pick cross-sectional area, attack angle, etc. I've never seen even 1/4" titanium pierced by such weapons. I'm told that a 30.06 armor-piercing round will penetrate 7/16" 'hardened titanium alloy'. Such a round carries close to 4000 Joules of energy.

Newtonian physics (action/reaction) effectively prohibits rams or overhead axes from developing enough power to do the job. Team Hurtz worked on this problem a lot.

A: Linear internal combustion actuators are not allowed under section 7.2 of the current RFL Standard Extensible Rule Set. Rotary internal combustion engines (ICE) are allowed for weapon power at the discretion of the event organizer.

A: That depends on what you consider to be a 'crusher' and what you call 'successful'. There were plenty of self-defined 'crushers' at Robot Wars -- off the top of my head:

• 'Big Nipper'
• 'Cerberus'
• 'Chompalot'
• 'Crusher'
• 'Crushtacean'
• 'Kan Opener'
• 'Mantis'
• 'Ming 3'
• 'Pincer'
• 'R.O.C.S.'
• 'Snake Bite'
• 'Tiberius'
• 'Tough as Nails'
• and even one version of 'Suicidal Tendencies'.

A: Take a read thru this archive and you'll find that I'm not a big fan of spinners. The truth is that wedges are more sucessful than spinners -- see my dad's analysis of tournament results. My advice is to refine your wedge and get it competitive before you move on to an active weapon.

A: Mark J. here: holes for the screw teeth? Use that drill press. Set-up a cradle for the drum that will position the perpendicular axis of the drum directly below the axis of the drill bit and still allows it to rotate and slide left/right to line up the position of the hole. Mark off the hole positions and drill away.

A: I was only 9 at 'Robotica' and I remember how much I loved Robot Combat back then. Good to have you here!

The arguement over which weapon is 'best' has been going on since the first robot fight. There is a discussion at the Combat Robot Wiki and the topic has been addressed in books and in the on-line forums.

This really is too big a topic to discuss fully here, but you'd probably be interested in the results of my dad's study of results from combat robot tournaments. The study shows that, on average, robots with active weapons (spinning blades, drums, lifters) actually do less well than robots with passive weapons (wedges, spikes, rammers). Particulary for your first robot, I'd recommend keeping it simple and avoiding any type of active weapon.

If you are determined to use an active weapon, the data from the weapon study indicates that lifters are the most successful active weapon style -- at least in small robots. A servo-powered lifter is very easy and inexpensive to implement in an antweight robot.

A: With a weapon rotating at operational speed there is no advantage to the eggbeater structure on impact. The weapon is rotating so fast that penetration of the opponent into the weapon before impact is a small fraction of an inch. A shallow impact bar on a drum is every bit as effective as the eggbeater bar.

There are several posts about eggbeaters and drums in this archive that deal with construction, rotational speed, and impact bars. The question you reference came in after the other posts and asked if the two designs didn't basically do the same thing, and my answer generated a good deal of off-line discussion.

To clarify my original statement, a drum weapon the same weight and diameter as an eggbeater will have greater rotational inertia. Someone pointed out that an eggbeater can be built to a larger diameter and have the same mass as a smaller drum and therefore have greater rotational inertia. This is true, but it just points out the trade-offs and complexity of weapon design. Generally speaking, an eggbeater is simpler to build but a drum is more effective.

A: Suggest you find a copy of Combat Robot Weapons by Chris Hannold for a good discussion on weapon pros and cons. Very briefly:

• Overhead axe weapons can attack the usually weaker top armor of an opponent but must accelerate the weapon to high energy levels in very little time.

• Spinners can more slowly build up kinetic energy in the rotational mass of their weapon for a devastating impact, but must deal with gyroscopic forces and the large 'kickback' from their own weapon.

A: That mini hydraulic system is expensive! A pump, cylinder, valve and assorted lines and fittings looks like about $700, and they don't give you the weight of the system or the volume output of the pump.

Crushing weapons require a great deal of force and very strong chassis, The crushing pincer on Robot Wars heavyweight champion 'Razer' had 18,000 pounds of force -- 100 times it's own weight! So, 40 to 60 pounds of hydraulic force is not going to make much of a crushing weapon and neither is a geared-down drill motor.

Q: Would it be an effective crushing system on an antweight? Would I be able to multiply that 60 pounds of force with a lever like 'Razer' did when there cylinder force was 3 tons and now the tip of the claw has 9 tons? Sorry for bombarding you with questions -- I just really like this sport. I can't help it!!!

A: If I didn't like questions, I wouldn't have this site, Anthony!

There are a couple problems I see with using this system in an antweight:

• Although the component weights are not given, the dimensions are. The pump alone is 6" long and 1.5" in diameter. That looks like a RS-385 motor powering it. I'd guess the pump alone might weigh half a pound. You wouldn't have enough weight allowance left to build a robot strong enough to withstand the force of it's own weapon.

• Gardentrucking supplies components for builders of scale trucks and construction equipment. These builders are after realistic lifelike performance from the hydraulic system, and in this case lifelike means 'slow'. Adding a force-multiplying lever system to would slow this down even more: three times the force means one-third the speed.

A: Either will have to be geared down to perform well with most spinner designs, but it's generally easier to work with a high torque motor. This is why the high-torque 'outrunner' style brushless motors are popular for spinner power. High RPM means high gear reduction and potential problems keeping drive belts in place on small drive sprockets.

A: See the , about 20 questions down. A good place to start with a weapon around that size would be the brushless Axi 4130/16 or the DeWalt 18 volt drillmotor.

Q: I've built a spinner weapon similar to 'Son of Whyachi'. It weighs 7 lbs. The total bot weight is 22 lbs. What is the ideal motor rating for my weapon? The maximum voltage I can use is 24 volts.

A: More power is better for a spinner motor, so 'ideal' is as much power as you can reasonably get. The Axi motor I referenced above would be a good choice. Geared down about 3:1 and running at 24 volts it would give excellent performance. Use the Team Run Amok Spinning Weapon Excel Spreadsheet to check performance of your specific weapon with various motors.

At Motorama I'm going to run it as an undercutter. What other similar components could I use for mounting the weapon to the shaft? I could use the Shaftloc again since an undercutter won't be taking the same kind of vertical hits, but I'd rather use something more proven and reliable. Thanks.

A: Mark J. here: you don't have a lot of options for securing a hub to a live shaft that small. Hobbyweight mid-cutter Fiasco uses a custom hub that clamps the blade in place and is held to the shaft with four set screws. I'm not a fan of set screws! Given the options, your current solution may be the best choice; it sounds like that hit you took would have broken something else if the Shaftloc hadn't failed.

The most reliable method of securing a hub or other item to a round live shaft is to broach a keyway into the shaft and the hub and inserting a key to prevent rotation. This isn't practical for a 1/4" shaft, and neither is cross-drilling a hole thru the shaft and hub and inserting a hardened pin. I recommend using a dead shaft in insect class spinners for just this reason.

A: The thickness needed for a wedge depends on the unsupported length of the material, the angle of the wedge, the type of the support it receives, and the punishment to which it will be exposed. There's no way I can even make a guess about the minimum thickness you'll need for your specific design. Follow my rule for armor: make it as thick as you can and still make weight.

A: Cool is building what you want to build. If other builders don't like what you build, beat the bolts out of their robots and smile while you're doing it.

A: Selection of a motor and drivetrain for a spinning weapon needs more input than just the weight of the blade. I'd suggest reading thru the many posts about spinning weapons in this archive and the archive for a start. If you have access to Excel, you can download our Team Run Amok Spinning Weapon Excel Spreadsheet to help you calculate the performance of your weapon design with various motor options. Something like an Axi 2808 brushless motor with a belt drive reduction around 4:1 would be a good starting point.

Q: I want the blade to be a 15cm by 3cm by 1cm 10 ounce steel bar for my beetle weight robot. Is this going to be too powerful and what sort of motor/gearhead should I use? I looked at the Axi 2808 but I can't find gears or pulley small enough to fit on the shaft, can you suggest a place?

A: A steel blade that size weighs closer to 12 ounces than 10, so I'll assume a thickness of 8mm to cut the weight down to 10 ounces. With a 4:1 reduction and an Axi 2808/20 running on a 3-cell LiPo battery, the weapon will spin up very quickly to about 4000 RPM and pack nearly 50 Joules of energy -- plenty for a beetle, but certainly not 'too powerful'.

4mm shaft diameter is just a little over 5/32" You can drill out 1/8" bore timing belt pulleys to 4mm. Robot Marketplace sells 1/8" bore MXL timing pulleys that would work well for this purpose. Belt drive is prefered over gears for a spinning weapon.

Q: Thanks Aaron. One more thing, I keep trying to calculate stall torque of a brushless motor using your spread sheet and I keep coming up with numbers like 1.41N-m which I know is obscene. What am I doing wrong?

A: Maybe nothing. Brushless motors can pump out short bursts of really huge power. Just make sure you get the correct numbers for RPM/volt and internal resistance. Plugging numbers for the Axi 2808/20 into the Run Amok Spinner Spreadsheet (1490 RPM/volt, an internal resistance of 105 mOhm, and 11.1 volts) I get an estimated 0.68 N-m stall torque.

A: An effective weapon is integrated into the structure of the robot. The time to think about a weapon is at the start of the design process, not after the 'bot is built and you discover that have a little weight allowance left.

I'd spend the extra weight on armor, but if you're determined to tack on a weapon you'll need to take a look at your design and component layout and think about what type of weapon would make sense. Consider if there is room for a servo-powered lifter and if that would make sense in your design.

Q: Hi again -- I had the 7oz beetle weapon question. Is it possable to build an effective beetle spinner with 7 oz? Thank you!

A: I'm gonna say no. You'd want at least 5 ounces of rotating mass for an effective beetle spinning weapon. That doesn't leave much for motor, drivetrain, and support structure.

You really need to decide what your robot is going to be in the design phase and build the robot to meet that goal. If you just keep adding things on 'til you top out on weight you're going to have a robot that tries to do too many things and ends up being good at nothing. Use that extra seven ounces of weight allowance to make the robot better at what it already does well.

A: Glad you like the site!

'Messin with Sasquatch' used a hole saw spun by (I'm guessing) a drill motor. This type of weapon has been tried quite a few times in various weight classes and has never proven effective.

Think about how you use a hole saw, how much pressure you have to apply, and how long it takes to cut thru a stationary object. Then think about trying to cut into another moving robot with the pressure a beetleweight could apply. The best you could do is to scratch up a little Lexan.

Pick a robot with a better record than 0-1 to emulate.

Q: So if a powered hole saw like on 'Messin with Sasquatch' isn't effective, then what's the coolest most effective weapon I can make on a beetle? I don't have enough weight to make an effective spinner.

A: 'Cool' and 'effective' are often two very different things in robot combat. Lifters win a higher percentage of matches than anything else in the beetle weightclass, but most people don't consider them to be 'cool'. Spinners do very poorly overall, but builders think they are 'cool' so they keep building lots of them. Read thru this archive and take a look at our analysis of weapon success.

I know you recommend against direct drive weapon motors, but I will be using a Robot Marketplace blade hub, and the brushless motor will be directly mounted to .9 aluminum. There will be another piece of aluminum on the other side of the motor to support the shaft to prevent bending.

A: Mark J. here: did you wonder why other builders don't do this? Several problems:

• I think you've miscalculated the stall torque of the GWS brushless motor. I can't find a quoted figure for internal resistance, but from size and performance parameters I estimate 170 mΩ. This gives a stall-torque estimate of 0.29 N-m, not the 1.30 N-m you were apparently using. This stretches the calculated spin-up time to 1.02 seconds.

• The spinner spreadsheet was designed for weapons spinning much slower than what you propose. It does not take into account aerodynamic drag, and a 110 mm diameter saw blade with big teeth spinning that fast is gonna have a whole lot of drag. That will stretch spin-up time and reduce top RPM.

• The ESC you pick to use with the motor may limit amperage to the motor via a 'soft start' feature. This will reduce the maximum torque and even further stretch spin-up time.

• Driving the 'bot is gonna ba a real problem. A blade spinning that fast will generate sufficient gyroscopic force to make turning the robot very difficult.

• At speed, you'll have less than 0.00025 second between passing blade tips. That's not going to 'dig in' to your opponent, it'll just skitter over the surface.

Q: I asked about connecting a GWS motor directly to a VDD1 blade, and you suggested gearing it down. I was wondering if the timing pullies from Robot Marketplace are a good choice? I am having trouble understanding what all of the things like pitch and flange diameter mean. I also don't know how to connect the the blade to the outer peices of aluminum. Thanks for your help!

A: The timing belt and pulley solution will work well, but the simplest and possibly least expensive solution would be to to use the VDD Polycarbonate Gearbox Kit.

Your GWS motor is small enough to fit right in place, the weapon shaft is attached with widely spaced bearings to support the load, and a set of gears are included for the reduction drive. The pinion gear will have to be drilled out to fit onto the 3 mm shaft on your GWS motor. It will save you from building a weapon shaft support and from figuring out the correct belt size and sprocket diameters.

Q: Hi Aaron, I have another question about my gws brushless motor, and VDD1 blade setup. Would an alternative solution to using a gearbox be leaving it direct drive, but not passing 1/3 throttle? I left all of the numbers for my motor the same, except I changed the rpm to 1/3 of what I had originally. I got these numbers, and they seemed okay:

What do you think?

A: You're still using the wrong numbers to calculate torque and spin-up time. On the 'Instructions' page of the spreadsheet enter '170' as the Ri (internal resistance) value, and '2160' as the Kv (rpm/volt) value. A drop in throttle will effect both RPM and torque -- if you're going to only use 1/3 throttle, reduce the Voltage to '3.7'. The calculated stall torque value now drops to 0.10 N-m.

With these changes, I get a 63% spin-up time of 1.03 seconds, with the weapon hitting 35 Joules in about 2.5 seconds.

I know that setting up a gear reduction drive for a spinning weapon is somewhat difficult, but if you look around at your competitors you'll notice that they all do it even though it adds weight and complexity. There is more to a good spinning weapon than just Joules of stored energy. The best approach has proven to be a gear reduction. This reduces load on the motor, helps to isolate it from impact shock, increases available torque to carry the weapon past initial impact, and reduces peak amperage draw from the battery/ESC.

Trust the other builders that have done the experimental work on this. Use a gear or belt reduction.

Extra thought: for a beetle, you might consider mounting two blades on the hub. A few drops of epoxy between the blades would hold them in alignment, and the doubled mass would double the weapon energy. Spin-up time would still be good if you use a gear reduction.

A: Mark J. here: different builders use different techniques. Hall of fame member 'Ziggo' used a pre-made 20" diameter wok from a kitchen supply store for the spinner body. You can read a write-up of Ziggo's construction at the Team Ziggo website.

Other techniques include use of a sheet metal roller to create a cone that can be clipped to the proper height, and machining the entire shell from a single block of metal.

A: That's the standard use of vertical spinners like 'Nightmare' -- a couple of impact 'knockers' are bolted opposite each other on the edge of the spinning disk to strike the opponent. When a circular saw blade is used for the disk, a blade with large teeth is used to catch and throw the other 'bot. This design is particularly popular in antweight 'bots that fight in arenas where an instant win can be scored by tossing your opponent off the arena platform.

Browse this archive for tips on spinning weapons.

A: 'Ziggo' was the product of a lot of hard work and experimentation. Builder Jonathan Ridder started building combat robots in 1995. His first attempts were not terribly successful, but he kept at it and learned from his experiences.

There isn't any real secret to the power of Ziggy's spinner. Quoting from the Team Ziggy website:

Insect class full-body spinners have a serious problem. As I have pointed out often, insect arenas are small and there is very little time to spin the weapon up to effective speed before your opponent is on top of you. Build for the situation in which you will be fighting.

A: Mark J. here: I don't have enough information about 'Mangi' to make even a rough calculation on weapon energy. I've written to Al Kindle for additional info. As a pure guess I'd say 'Helios Sport' hits a little harder, but 'Mangi' is quicker. They did have one head-to-head fight won by 'Helios Sport', but they are both beautifully made top-flight robots.

UPDATE: Al got right back to me with details on Mangi's weapon. According to my calculations, 'Mangi' with 78 Joules is harder hitting than 'Helios Sport' at 50 Joules.

A: Mark J. here: the spreadsheet I have is just a few basic kinematic formulas strung together with a couple 'best guess' fudge factors. I put it together to get some order of magnitude comparisons between electric and pneumatic hammers. It has no documentation, is useable only over a small range of values without recalibration, and it relies on some questionable physics shortcuts. I don't think it would be of any use at all as a design aid.

A full-blown hammer simulator would be kind of a big deal. The motor torque continually changes with increasing speed, there is no good way to calculate the inertia of the armature from available data, and the effect of gravity on the hammer changes non-linearly all along the arc.

My best advice, as has often been repeated on the Ask Aaron webpage, is to use the construction examples provided by other builders as a starting point. In your case I'd carefully examine Helios Sport from Team Cosmos. Best luck!

A: Yes, 'The Butcher' from Inertia Labs had a massive pneumatic powered spinning weapon which was not very successful. Pound for pound, electric batteries store much more energy than compressed gas cylinders. Electric motors are also very efficient (80%) compared to compressed gas engines (15%). Pneumatics are great for quick bursts of power, but not efficient for continuous motion.

Q: Would the hammer get more damage if I increased the arm to 18" and lightened the hammer head to 3oz?

A: Mark J. here: assuming a 180 degree hammer arc and taking a guess at torque losses to armature acceleration:

• the Speed 400 motor geared 25:1 will provide about 5 G's acceleration to a 4 ounce weight on a 12" arm;

• the hammer head will complete a 180 degree arc in about 0.2 second;

• the velocity of the hammer head at the end will be about 31 feet per second;

• the kinetic energy of the hammer head at impact will be around 5 Joules.

Weapon energy is a product of the acceleration provided by the motor and the length of time that acceleration is applied. Changes to the gear ratio, length of the arm, or mass of the hammer will change the weapon acceleration, but will be offset by the change in time available before impact. As long as these design elements remain anywhere near reasonable you'll still get about the same weapon energy output. To get more energy output, you'll need more motor power input.

Q: Is 5 joules strong for a beetle hammer?

A: A strong hammer runs close to 10 Joules per pound of weight class -- about 30 joules for a beetle. Electric hammers just can't put out enough power.

Q: How many joules did 'Helios Sport's hammer have?

A: Mark J. here: 'Helios Sport' is a 30 pound 'sportsman class' robot with a 3 pound hammer on the end of an 18" arm powered by a DeWalt 14.4 volt motor at 18 volts. That works out to about 50 Joules -- comparable to a 5 Joule hammer on a beetle. You can see from the video of 'Helios Sport' vs. 'Bounty Hunter' that the hammer isn't effective.

A: Mark J. here: not quite. The 'weight' input is the constant force pressing down on the end of your lifter. The motor torque needed to offset that force is shown as the green line in the output chart. Note that the torque needed varies with the position of the lifter, but the maximum torque available from your gearmotor is constant. This means that the maximum lift force will vary with lifter position.

Also, the motor torque shown in the graph is the amount needed to just offset the weight on the lifter. Permanent magnet electric motors produce maximum torque at stall (zero speed), so a gearmotor with the torque shown in the graph could hold position against that much weight at that position but could not lift it further. Aim for a gearmotor with about twice the maximum torque shown for the weight you want to lift.

A: 'The Matador' from Inertia Labs claims the most powerful flipper title with 20,000 pounds of lifting force. Team Run Amok's The Gap is a heavyweight that last weighed in at 208 pounds with an empty CO₂ tank.

Q: Is Matador's flipper more powerful than Toro's?

A: What did I just say? Matador's is the more powerful. Toro's superheavyweight CO₂ flipper had 7000 pounds of lift. The Matador's 'next generation' flipper had 20,000 pounds of lift in a heavyweight.

Q: Aaron, do you know how many pounds of force Ziggy's lifting arm has? Do you know what psi Ziggy's pneumatic system runs on? Could you give me a link to Ziggy's website if it has one?

A: CM Robotics claims 14,000 pounds of lifter force for 'Ziggy', but the 4-bar flipper mechanism makes comparison with direct-acting flippers like 'Toro' difficult.

Ziggy's flipper runs on high-pressure air, someplace in the 3000 to 5000 psi range.

Some robot teams, such as Ziggy's CM Robotics, don't have websites. It isn't that they aren't capable, it's just that they have a problem with sharing. : ^ )

A: Mark J. here: that number comes from CM Robotics -- I don't know how they are calculating. In a conventional single-pivot lifter it's fairly easy to apply a little geometry to figure the lifter force based on the force available at the pneumatic cylinder (surface area of piston times available pressure), the angle at which the force is applied, and the leverage between the force application point and the point of lift. Ziggy has a four-bar flipper mechanism, which makes the calculation more difficult. They may simply be giving the raw force available at the cylinder.

Actual flipper performance depends not only on the maximum force available, but also on the gas flow capacity of the pneumatic system. Valves and hoses must be large enough to flood the cylinder with an instantaneous burst of high-pressure gas or the 'flip' becomes a gentle 'lift'.

A: The Matador, like all of Inertia Labs' robots, was beautifully made and well thought out. The amount of force in it's weapon was certainly overkill. A little less weight devoted to the weapon and a little more to a more controllable chassis might have worked out better.

It's record was 4 wins and 2 losses -- the last loss to the fearsome 'Warhead' at BattleBots 5.0. I don't remember the match, but there may not have been enough of 'The Matador' left to put back together.

A: There are certainly some gearmotors that will work for your purpose, but it is also a fairly simple task to hack a servo for continuous rotation.

The power you'll need from a gearmotor (or hacked servo) will depend on the length of the lifter arm attached to it. For maximum speed and reliability, select a gearmotor that would just stall with 1.67 times the weightclass on the end of the lifter. Here's the formula to calculate the desired torque:

Example - a gearmotor for a 4" lifter arm on a beetleweight should have:

The B231 231:1 gearmotor running at 12 volts would be a good choice. You could also use the BaneBots 64:1 24mm gearmotor running at 7.2 volts, but it's higher speed could be difficult to control on a lifter.

Some builders believe you can use a gearmotor with much less torque than we recommend. There is an extended discussion on gearmotors for lifting arms in this archive: search the page for 'RS-540'.

A: That's a fairly complete list -- you might want to remove 'lifter'. Active weapons add several layers of additional complexity onto an already complex undertaking. I recommend that first-time builders keep their robots very simple. You'll have enough new things to worry about with battery maintenance, R/C system set-up, traction issues, ESC mixing, driving, radio interference, wireing, tournament procedures, and repair problems.

A: The front wedge on the current #1 ranked heavyweight is mounted on a heavy rod that pivots in very sturdy mounts attached to the chassis. This allows the front of the wedge to pivot down and scrape along the surface of the arena, or drop and lie flat if the 'bot is inverted. The pivot point is low -- near the centerline of the robot -- which helps prevent the wedge from folding back under on heavy impact.

Zero-clearance wedges have a drawback: they can get hung up on joints in the arena floor. In a smooth arena a low-pivot hinged wedge can be a plus.

A: Mark J. here: it was in interesting experiment: a pneumatic powered rotary weapon with a claimed 150 horsepower. Record: 1 win, 1 loss. The problem was that it isn't possible to effectively store enough energy in the form of compressed gas to power such a weapon for more than a very few seconds. Pound for pound, electric batteries store much more energy than compressed gas cylinders.

A: That's way better than decent, assuming that you have a reasonable spin-up time.

Q: What size screws would be best for use as teeth on a 2.5" OD Beetleweight drum spinning at 18,000 RPM?

A: Mark J. here: unless that is a very narrow drum, it isn't reasonable to try to spin it at 18K RPM. Anything less than perfect dynamic balance will shake your 'bot around like jello in an earthquake. Even if it's perfectly balanced to start with, it won't be after the first hit. Worse, with two rows of teeth a tooth will pass by every 2 milliseconds -- you can't get any 'bite' into your opponent with teeth passing by that fast.

Slow down the drum by a factor of at least two. I'd suggest even slower. Speed and Joules are less important on a small drum than are bite and torque. For teeth try 3/16" hex cap screws. Anything smaller might shear off.

A: 'Crack Torch' never won a fight. Although they are crowd favorites, flame weapons are ineffective in any weight class. Your opponent would have to sit still while you positioned your flame weapon and held it long enough to do some damage. If your opponent is sitting that still, he's disabled and you've already won the match.

You can research the fight history of any robot at Botrank.com.

Q: What about Texas Heat? It is an effective lightweight flame thrower robot that is currently ranked 2nd. Wouldn't a similar design work for an antweight?

A: 'Texas Heat' is an effective wedge robot designed by the very experienced and well financed CM Robotics. It would be equally effective without the flame weapon. Flames are for show.

A: Small drums may have the rotating can of a brushless outrunner motor press-fit into the inner diameter of a custom machined drum. The motor mount becomes the support for one end of the drum. This design is simple and rugged, but gives you no gear reduction control for maximum drum speed and spin-up time.

I've also seen larger drum weapons with an internal motor mounted on a hollow non-rotating 'dead shaft' with the drum supported on bearings. Power wires can be run to the motor thru the center of the dead shaft. Power was transferred to the inner surface of the drum by a friction drive. In general, an external motor with a belt drive is a simpler and better choice.

A: 'Blendo' was a great robot and a real benchmark in robot design. Their Briggs & Stratton internal combustion engine was rated at 5 HP -- not much by today's standards for heavyweight spinner power.

A: There are several tips on 4-bar mechanism design in this archive. The T.i. Combat Robotics 4-Bar Simulator is a valuable design tool and a good place to start.

A: Combat robots are generally expensive. Larger combat robots are more expensive. You'd better have an overall budget of at least a couple grand to cover a minimal lightweight flipper.

Standard pneumatic components are off-the-shelf products available thru industrial suppliers. If you can put your system together from these standard components you can keep the price reasonable. Pricing for a budget CO₂ low-pressure (150 psi actuation pressure) lightweight pneumatic system might run something like this:

• an actuator cylinder: less than $100
• a useable 5-port valve: about $150
• a small paintball regulator: about $100
• a small CO2 paintball tank: less than $30
• assorted connectors, hoses, valves, guages, electronics, and mounts: another $100.

How much will it weigh? That's like asking how much a rock weighs -- it depends on the rock. A basic lightweight system might weigh as little as 6 or 8 pounds, but adding on performance will quickly add to your weight as well as the bulk of the system. Pneumatics take up a lot of room, so your 'bot may need to be larger which makes for more weight in the armor and chassis as well.

There is also the matter of assembling a safe and reliable system. Pneumatics are not for beginners. They can generate a lot of force and move very quickly. That is a recipe for severe injury if you don't know what you're doing. A first-time builder has plenty to worry with just getting the radio, electrical, and mechanical systems of a simple combat robot working correctly. Don't add on the complexity of an active weapon 'til you get the basics down.

A: Mark J. here: Although it is an interesting theoretical design, a flywheel powered flipper has a number of engineering challenges that render it inferior to pneumatic flippers. The idea is to store energy from an electric motor in a rotating disk, just like a conventional spinning weapon, but then to engage a clutch mechanism to transfer that stored energy via a linkage to a flipping arm or platform. The clutch must be able to engage quickly, transmit enormous force, and disengage quickly to prevent excess energy from ripping the weapon apart.

I've seen a few clever designs on paper, but no one has actually built a successful flywheel flipper. The complexity, weight, and expense would be much greater than a pneumatic system of similar capacity. Stick with pneumatics.

A: Mark J. here: HPA is High Pressure Air (78% nitrogen, 20% oxygen) right out of the atmosphere. HPA systems in combat robots typically have their tanks pressurized at 2500 psi or above -- likely much greater than the capability of your compressor. The RFL rules require HPA pressure tanks to be rated for al least 120% of the maximum pressure they will be used at, and to have a current hydro test certifying that capacity. High pressure paintball tanks are commonly used in small 'bots, and SCUBA tanks in larger 'bots.

Before you go any farther, read the Team Da Vinci Understanding Pneumatics page. That will answer a lot of questions you didn't even know you should be asking.

A: Mark J. here: there have been several attempts to build a successful invertible 'full body' spinner, with wheels sticking out both top and bottom. Ringmaster fought as a heavyweight at BattleBots 5.0 and had a good run, winning 4 fights before losing to Omega-13.

A rotating ring weapon is typically supported by a number of small wheels attached around the upper and lower perimeter of the chassis. The wheels run in grooves in the inner surface of the ring, supporting the ring and allowing it to rotate.

A: Mark J. here: it sounds like you're planning a small diameter weapon with the intent to grind away at the bottom armor rather than a traditional impact spinner weapon. What you're building is more of a mobile power tool than a sledge hammer. Joules of stored kinetic energy don't apply -- you're not going to rely on stored energy to do damage, but on continuous application of power.

You'll need enough power to keep the weapon spinning while in contact with the other 'bot and their full weight bearing down on the spinning blade/bar/grinder. I don't have any simple formulas to calculate the power you'll need for that. I'd suggest keeping the RPMs relatively low and gearing for torque.

A: Mark J. here: each of the power options you list have strengths and weaknesses:

• Pneumatics components are widely available and can provide high thrust, speed, and design flexibility. They do require additional support equipment and add complexity to the confined space inside your robot.

• Linear actuators are self-contained, compact, and have the same design flexibility found in pneumatic systems without the need for additional support equipment. They are, however, much slower and potentially less powerful than pneumatic systems.

• An electric gearbox capable of supplying the torque needed for a 12 kilo robot would be bulky and heavy. The T.i. Combat Robotics 4-Bar Simulator can give you torque requirements for a gearbox powered 4-bar lifter. You might be surprized by the torque required.

A similar amount of lift should work well with a pneumatic system. You may be able to get much more, but you did say 'lifter' and not 'flipper'.

A: A true 'press fit' involves machining the inside of the drum to a diameter a couple of thousandths of an inch smaller than the outside diameter of the motor and using a hydraulic press to shove the motor into place. If your machining isn't accurate, the drum won't run true.

A: For weapon power, two E-150s would weigh nearly twice as much as an S28-150 and produce 1/3 the power. Weight is the greatest challenge in building a combat robot. You need to make the best use of every ounce. The weight you save in the motor can go into the spinner disk for more stored energy and damage potential, while the added power will improve your spin-up time and maximum RPM. Well worth the added expense.

A: If you're building a large weapon I would recommend putting all available weight into the battery, motor, and weapon mass rather than on structures that do not increase the impact and energy of the weapon. The Robot Wars housebot 'Dead Metal' used a circular saw weapon that swung forward and down, but a cutting saw does not produce anywhere near the kick-back force of a rotary impact weapon.

Every action produces an equal and opposite reaction, so your weapon arm would need to absorb the same amount of force that the weapon inflicts on your opponent. That would take a massively reinforced arm assembly! Keep it simple.

A: The energy stored in a spinning weapon is dependent on the speed of the disk, the mass of the disk, and how the mass is distributed on the disk. There is also the question of the time needed to spin the weapon up to an effective energy level. Search this archive for information on calculating weapon energy, appropriate energy levels, and spin-up time.

Q: How were the chains attached to the weapon sprockets on 'The Judge'?

A: Careful examiniation of the photo at right shows that the last chain link is bent inward toward the shaft and riveted thru the sprocket. An animation of this weapon is in this archive -- scroll down about 40 questions.

A: Yep.

A: There are a lot of variables in calculating weapon battery requirements, but we gave a 'rule of thumb' and approximate formulas in an earlier post. Search the archive for 'battery capacity'.

Q: I took a look at your formulas to figure out weapon battery requirements and added them into your Spinning Weapon Excel Spreadsheet. It's formatted exactly like your work, and the formulas are all correct. Would you like to add it to your spreadsheet?

A: Nice idea! Michael Maples sent me his addition to the spreadsheet and it's way cool. I've replaced the downloadable file with the new version. Thanks, Michael.

A: Mark J. here: ideally you should be able to meet the full stall current demand of your weapon motor. Current equals torque, and if you can't supply the current the spin-up time of your weapon will suffer. From a practical standpoint, if you can supply 80% of your weapon motor stall current you'll be fine.

Note that LiPoly batteries can be damaged by current drain above their rated capacity. If your LiPoly pack is rated below stall current, you might want to use a servo slower module to 'feather in' the throttle to your weapon ESC and avoid the big start-up amp rush.

A: How about the classic DeWalt 18 volt drill motor? At 24 volts it kicks out 1.5 horsepower and it's less than a third the weight of the S28-150 magmotor. The problem with it and other small brushed motors is high RPM. You'll have to figure out a weapon drive that will stay together at 20K RPM.

Check out the performance of your weapon/motor combination with the Team Run Amok Spinning Weapon Excel Spreadsheet.

A: Mark J. here: 'Green Wave' was a middleweight spinner with the weapon motor mounted above the robot body on a short, stationary (dead) shaft. The motor support shaft also functioned as the axle upon which the bar weapon spun. The design allowed the bar weapon to be mounted very low on the chassis.

'Green Wave' had serious problems with weapon bearing disintegration caused by heavy off-axis loads on impact. To prevent this problem with your 'bot, the weapon bar assembly should be supported by two bearings separated on the shaft to spread the load and prevent twisting. I'd suggest one bearing in the belt drive pulley and a second bearing in a flange mount on the bottom of the weapon bar.

A: Eggbeaters take enormous punishment. Most are cut from a single plate of metal for greatest strength.

A: Mark J. here: come on, guys -- you've got to give me more info than the general type of weapon and the weight class! It's like asking, "What should I feed my yellow pet?" Dog? Canary? Goldfish? See comment in the next question below on 'how thick'.

A: Mark J. here: my standard answer to a 'how thick' question is, "as thick as you can make it and still make weight." In this case, a thicker drum wall will mean more energy storage and more durability. Like most design factors in a combat robot, there is a trade-off in adding weight to one system while pulling weight out of another.

A typical drum makes up around 20% of the total weight of the 'bot. About 3/16" aluminum might be a good starting point for a lightweight drum, but the size, speed, and design of the drum can modify this.

A: Optimum drum tooth length depends on tooth spacing and the axial speed of the teeth, not the size of the drum. You want your target to get as far inside the arc of the rotating teeth as possible in order to get maximum 'bite'. Faster spinning drums require wider tooth spacing and run shorter teeth, while slower spinning drums can have closer and deeper teeth.

You need to calculate the amount of time between teeth when the weapon is up to speed, then calculate how far your 'bot is likely to move forward in that time period when attacking. That distance is the optimum tooth length.

Alternately, since your attack speed is so variable, you could just stick the teeth out about 3/8" and go for it!

A: Bolts are not ideal drum teeth -- but they are inexpensive, easy to obtain, and simple to replace. The simple blunt impact is fairly effective, plus a little time with a hand file or Dremel tool can put a nice, sharp edge on a bolt head.

A: Mark J. here: the Team Whyachi TWM3R gearbox has a nominal 1 inch titanium shaft with a 3/8-16 tapped hole in the end and a .25 inch keyway down the side. This makes it convenient to lock a keyed weapon hub onto the shaft. I'm sure that Team Whyachi would be willing to machine a suitable weapon hub for you.

Note that a keyed hub has no slip or give and will transmit all weapon shock loads back to the gearbox, potentially destroying it. I prefer a slipable weapon drive like a v-belt, but the Team Whyachi gearbox is built to take high loads. Your choice.

A: Mark J. here: If you are using hardenable metal it's a waste not to harden them, but I see a lot of non-hardened drum teeth. For longest life and greatest damage, harden the teeth.

A: There is no maximum pressure specified in the RFL rules, but all components in the system must be rated for the pressure used. Above 250 psi, components must be over-rated to 120% of the pressure used. Systems operating above 2500 psi require pre-qualification by the event officials.

Consult section 7 of the RFL rules for details.

A: Son of Smashy used an electric winch to reset the spring after each strike. The difficult part in building a set-up like this is the release mechanism that allows the weapon to fire then re-engages the winch. In theory it could be done in any weight class but pneumatic systems are more powerful, quicker to reset, and can be built from off-the-shelf components. Spring powered weapons are rare for a reason.

Search the archive for 'winch' for other posts on this topic, and scroll down about eight questions for the post on 'Red Square'.

A: Axes are neither popular nor effective weapons, but I've seen hardware store axes, picks, hammers, and chisels used. High energy weapons usually have custom made impact pieces.

A: The Etek motors have very large armature intertia which will cause the rotor to shift on the shaft if stopped abruptly. This effectively destroys the motor. To prevent this you should drive a rotary weapon with a slipable system like a v-belt. You may need several parallel belts to handle the power output of twin Eteks. Gears, chains, or timing belts will lead to trouble.

Interestingly, the battery capacity needed for a rotary weapon is more dependent on the weight, geometry, and speed of the weapon than on the motor powering it. An example, with formulas, of calculating spinning weapon battery capacity requirement is given in a previous answer in the Radio and Electrical archive.

A: You can use a slice of well-frozen pepperoni pizza if you like -- the result will be much the same. Neither the pizza nor the knife are designed to take large loads at the pointy end. Either will simply shatter on good impact. You need something that you could hammer into concrete. Repeatedly.

A: KillerHurtz is an unusual robot in nearly all design areas: all-plastic chassis, combined differential and pivot steering, computer controlled ESCs, and a pneumatic actuated overhead axe weapon with a chain drive.

The weapon is powered by a 100 mm bore pneumatic cylinder operating at 150 psi. The cylinder is attached to a lever directly connected to a large chain sprocket. A chain carries the force to a smaller sprocket attached to the axe. There is a good photo on an archive of their site: KillerHurtz Design page.

Q: Did KillerHurtz use a double-acting pneumatic cylinder?

A: Yes. The pneumatic plumbing appears to be incomplete in the photo -- see the KillerHurtz pneumatics page for details.

A: Frostbite's thresher is simply eight short vertical bars arrayed along a common central support shaft. Mount the shaft on bearings and spin it with a belt drive.

Mark J. here: the most efficient design for a spinner weapon is a drum -- it can store more energy for size and weight than other configurations. A disc weapon is less efficient, and a rotating bar even less efficient. Multiple short bars, like the thresher, are very poor at storing energy.

A: Mark J. here: it takes just a little extra work. Input a double-thick bar of the correct length and width to represent the two side bars on your eggbeater. Manually calculate the mass of your two crossbars and represent them with a cylinder the diameter of your weapon with a thickness that brings it's mass equal to the calculated mass of your crossbars. That will get you very close to correct.

A: There is plenty of information on drum weapons in this archive. You may also want to get a copy of Combat Robot Weapons by Chris Hannold -- it has specific information on many types of weapons in detail that I cannot afford to provide here.

A: A leaf spring is a curved metal plate that has been tempered to 'spring back' to it's curved shape after being flattened. One end of the leaf spring is securely fastened to a chassis and the other end is winched down to flatten the spring, This stores a large amount of energy in a thin and flat profile. When another 'bot is on top of the spring the free end is released and the spring tosses the other 'bot skyward.

The principle is simple, but designing the winch and release mechanism is tricky. Team Whyachi's middleweight 'Red Square' was a successful leaf spring flipper -- ranked 15th, with a record of 19 wins and 12 losses.

Q: Would it be possible to make a spring powered flipper in a featherweight?

A: Possible, sure -- but there are reasons that spring flippers are not a popular design. Team Whyachi was successful because they are a very experienced team with great resources.

Q: How would I make a spring powered flipper in a hobbyweight?

A: No answer that I can give in a couple of paragraphs is going to help you much. Very few spring flippers have been built due to the difficulties in constructing a releaseable winch system to reset the spring. The mechanism from an automotive power seat would be a good start, but custom machine work would be needed for the release spool. I'd suggest building something else.

A: Mark J. here: Newtonian physics states, "For every action there is an equal and opposite reaction." When your spinning weapon strikes your opponent, equal forces are applied to both robots.

If the front edge of your weapon is spinning upward: the force applied to your opponent will be upward and your robot will be subject to an equal downward force. Since a solid surface is holding your robot up, nothing much will happen to it. Since nothing except gravity is holding your opponent down, they will fly end over end into the air -- greatly impressing the judges and spectators in your favor.

If the front edge of your weapon is spinning downward: the force applied to your opponent will be downward and your robot will be subject to an equal upward force. Since a solid surface is holding your opponent up, nothing much will happen to them. Since nothing except gravity is holding your robot down, you will fly end over end into the air -- greatly impressing the judges and spectators, but not in your favor.

Upward spinning, please.

A: Mark J. here: The 'best way' depends on a balance of your budget, experience, skills, and expectations.

• Pneumatic lifters are very quick and powerful but require complex, bulky components and a fair bit of weight allowance. Pneumatics are applicable to either a simple single-pivot lifter or a more efficient 4-bar design.

• Electric actuator lifters are typically slow but are comparatively compact, safe, and simple to support and maintain. Again, they can be used with single pivot or 4-bar lifters.

• Driving one arm of a 4-bar lifter with a gearmotor requires a heavy gearbox capable of handling enormous torque, and careful design work -- but the result can be quick and effective.

A: I can't find a picture of the weapon used by middleweight 'El Diablo' at BattleBots 5.0, but I did find some construction notes in a build report:

The weapon was made from four 1" thick polyethylene discs, each with a central bearing riding on a stationary steel shaft. Bridging across the four discs on opposite sides were two 1/4" thick 6160 aluminum plates with bolts threaded thru from the back side for 'teeth'. The entire assembly weighed 24 pounds and was reduction belt-driven by a Bosch GPA 750 motor to about 1300 RPM.

'El Diablo' went 0 for 1 at BB 5.0, losing their opening match to 'Ankle Biter'. Heavyweight 'El Diablo Grande' did better with a similar weapon, winning 3 matches before a loss to 'MechaVore'.

You can find pictures of El Diablo and El Diablo Grande as they fought at BB 5.0 at robotcombat.com, under 'Event Reports'.

Thanks! I'd forgotten about their extensive collection of BattleBots photos.

It looks like the team changed the polyethylene and aluminum design before the competition. Those appear to be four thick aluminum discs with steel striker hooks bolted in. I don't think I could recommend a similar design -- a drum weapon of the same size, weight, and speed would store much greater energy than the 4-disc setup.

A: Drum weapons are not projects for the average builder without access to and experience with precision machine tools. An error of a couple hundredths of an inch will leave you with a useless paperweight. A competent machine shop could certainly make a custom drum weapon for you, but the cost would be prohibitive (hundreds of dollars). Consider switching to an eggbeater weapon -- much easier to construct and balance. See also the previous post on balancing rotary weapons.

A: Clan MacCanIKill's 'Ziggy' has a 4-bar flipper mechanism with a pneumatic actuator acting on the front bar. Click the photo for a larger image. The pneumatics operate on high-pressure nitrogen at up to 3000 psi. Most pneumatic flippers use carbon dioxide gas which gives more 'flips' per tank but which also restricts their maximum pressure to about 850 psi.

In spite of the high-pressure nitrogen pneumatics, I don't believe that Ziggy is more powerful than other top-rank flippers. The 'laydown' position of Ziggy's actuator cannot match the mechanical efficiency of the twin upright actuators in Toro.

A: 6AL-4V titanium, about 5/16" thick should do.

A: Team Entropy never had large or complete site, and what they had was taken down about a year ago. I can direct you to an archived version of their site that has a picture of 'Redrum'.

Q: I once heard an announcer say that the new Redrum's internal combustion engine is located INSIDE the drum! Is that true? Is that even possible?

A: Yes, and yes. It's a very big drum. If you think about a drum as being kinda like a full body spinner on it's side, you might get more comfortable with the idea.

A: My advice: stay away from Internal Combustion Engines (ICE) in combat robots. They seem like such a good idea, but every team I've ever seen use an ICE has suffered multiple problems at tournaments:

• the engine becomes inexplicably difficult to start;
• the engine stalls at idle waiting for the match to start;
• the engine delivers one hit and quits;

A: Mark J. here: 'SlamJob' kinda cheated. Scott Kincaid built an ingenious system that used two pneumatic cylinders that worked in sequence to accelerate the weapon arm. Power is not evenly applied over the swing, but it worked very well. See if you can figure it out from this early construction photo of SlamJob. Hint: neither of the cylinders are physically attached to the axe arm, except for a retract cable.

`The Judge' did not cheat. Jacha Little built a system that evenly powers the hammer throughout a 180 degree swing. You can find descriptions and pictures at the Team Mechanicus Website (archived).

Q: I looked at the pneumatic system for the hammer on 'The Judge' on their website. It's kind of confusing to me. It looks like a rack and pinion but it's backwards, and has chains. Do you think you can explain it better to me?

A: You've correct, it is a reversed rack and pinion. The pneumatic cylinder pushes and pulls the 'rack' in a straight line. The 'pinion' is the pivot point of the hammer: it swings the hammer in an arc as the rack is moved.

Their 'rack' is actually a pair of heavy power chains, and their 'pinion' is a pair of chain sprockets. One chain is wrapped clockwise around the sprocket and attached, the other chain is wrapped counter clockwise around the other sprocket and attached. One chain is always under tension and can transmit power to the hammer. There is less chance of 'jumping a tooth' under high loads with the chain system.

Q: In the illustration, which side is the front end of 'The Judge'?

A: The layout in 'The Judge' has the pneumatic cylinder at the rear of the robot, so 'front' would be to the right.

Q: So, The Judge's weapon can be compared to a spring loaded weapon, in the sense that natural physical forces fire the weapon (spring) and a special mechanism is required to reset the weapon into a ready position (winch), right?

A: No: the pneumatic actuator powers the weapon in both directions. I've modified the illustration to better show the action. Hammer motion and force is roughly equivalent in either direction.

Q: How does 'The Judge' get so much power from retracting the cylinder?

A: 'The Judge' uses a 4" bore double-acting cylinder that produces (almost) equal power when extending or retracting. Gas pressure to the actuator is under microprocessor control to quickly vent the cylinder for quick cycling. Take a look at the Team Da Vinci pneumatics page for an explanation of double acting cylinders and for a different animation of the hammer on 'The Judge'.

A: Our heavyweight flipper has a very large CO² tank that will power the weapon thru multiple matches. We also designed the tank mounts and connections to allow quick removal and installation of the tank without tools. With careful design tank changes aren't a problem, and if you enjoy tossing your opponent in the air there really isn't an alternative.

Note: High Pressure Air (HPA) tanks can be refilled with the tank in the robot, but CO² tanks need to be 'pre-chilled' and filled with a specific weight of liquefied gas -- not easy to do with the tank in the robot. Check with your event organizer for specific pressure tank refill regulations at the event.

A: Many: spin-up time, optimum speed... the same elements that apply to larger rotary weapons apply to fairys. Read thru this section of the archive and you'll find plenty of pointers.

A: Go take a look at the Antweights - VDD Kits page at the Robot Marketplace for parts and complete saw weapon kits.

The idea of a spinner weapon is to store energy in a rotating mass and release a big burst of energy into a your opponent, sending them flying. if you make the weapon too 'small and light', it will just run into your opponent and stop. Browse this section of the archive for design ideas.

A: Not a great idea. See the previous post in the Ask Aaron Archive.

A. Sewer Snake is from the stable of Matt and Wendy Maxham's Team Plumb Crazy. It does better than just compete -- it was the 2005 RFL heavyweight champion, it is the #1 historic ranked heavyweight robot at botrank.com, and is a member of the Combat Robot Hall of Fame.

I wrote to Matt and Wendy for some details on Sewer Snake's weapon. They sent the following description and photos:

Q: Isn't Sewer Snake currently a superheavyweight?

A. Sewer Snake fought very briefly (and unsucessfully) as a superheavyweight in 2006. It returned to the heavyweight rankings to fight at the Combots Cup in 2007.

A. Drum weapons are usually cut from a section of thick-walled tube, or machined from a solid block of metal. Getting a rolled sheet of thick metal perfectly round and well balanced would be very tough. There is a lot of material about drum weapons on this archive page.

Q: Where would I find the tube I would need to make my drum?

A. It would help if I knew whether you were building an antweight or a superheavyweight. The Robot Marketplace metals department has a good selection of aluminum and steel tube in assorted sizes.

A: Mark J. here: something tells me you're not building a combat robot. An H-bridge provides on/off/forward/reverse control for a motor -- too crude for proper combat robot control. Our robots run either full ESC control for drive motors or relay H-bridge control for lifter weapons. We don't have any experience with selection of H-bridge IC chips.

Q: I'm using a resistor in series with the motor to lower the voltage when the current is big. The H-Bridge would also take a voltage drop, right?

A: No. Resistors are not voltage control devices -- they are current limiters. Adding a resistor in series on the motor circuit will limit the maximum current the circuit can pull, but because of the variable back EMF the motor generates in operation the voltage the motor 'sees' will depend on the load the motor is under. At low load conditions the voltage across the motor brushes (and in the rest of the circuit) will be very close to full available voltage. At high loads, both the voltage and current at the motor will be reduced by the series resistor. A series resistor in the motor circuit beyond the H-bridge will not impact the voltage at the H-bridge.

Q: I'm not interested in speed control but in torque/current/dutycycle and direction control, with a high current output (for the same FF-180 BaneBots motor). Would an ESC do the work for this purpose? If so, which do you think is a better choice, the H-bridge, or the ESC?

A: Yes, an ESC can control torque/current (they are equivalent in a permanent magnet DC motor) and provide directional control. The H-bridge and a series power resistor can perform similar functions. If I knew what you were actually using the motor and controller for I might be able to give an opinion on which is 'better'.

Q: Remember the questions about a worm gear driven robotic gripper I posted before? I want to control the pinch force of the gripper and of course it must be able to use the full power of the BaneBots motor. Since I also need to control direction, I thought of an H-Bridge like the LMD18200T, but its operating voltage is from 12 to 55V.

A: Yes, welcome back! Go take a look at Pololu Robotics line of motor controllers. I'm sure you can find something there that will suit your application. In particular, the 'MC33887 Motor Driver Carrier' looks like a winner, but there are several other options. Add a series power resitor to the motor and you'll be in great shape. Or you could go with an ESC...

Q: If I were to use an ESC for this purpose, can you tell me how to connect the ESC to control torque/current instead of speed?

A: The term 'Electronic Speed Controller' is not really accurate: the device controls the power available to the motor rather than directly controlling speed. It effectively controls amperage, which directly controls torque and, only indirectly, speed.

Q: OK, so the signal I need to send to the ESC would be the same as the signal needed to control a servo, right? A 'high' (4 to 6 volt) pulse on a 20 millisecond period where:

• 1.0 to 1.5 millisecond pulse is proportional high to low power 'forward' rotation;
• 1.5 millisecond pulse is neutral - no rotation; and
• 1.5 to 2.0 millisecond pulse is proportional low to high power 'reverse' rotaton.

A: You've got it exactly correct -- the same signal you sent to the servo you're replacing with the gearmotor. You can control torque by limiting the pulse width to a range nearer the 1.5 millisecond 'neutral' range.

Note that 'forward' and 'reverse' are relative -- you can always reverse the motor leads if the direction is not correct for your application.

Q: What ESC would you recommend - perhaps the BaneBots 3 Amp continuous / 9 Amp peak ESC?

A: The Banebots ESC should work very well for you, It is small, thermally protected, reversible, and has a fail-safe startup routine.

A: Team Raptor's successful series of lightweight robots (Alpha / Beta / Gamma Raptor) all used 4-bar electric lifters. I believe the short rear arm of the lifter was powered by a high-reduction gearmotor -- possibly an Astro Flight cobalt.

Team Raptor's website is no longer on-line, but you can access an archive of their site from 2005.

A: Mark J. here: electric motors can power effective lifters, but a flipper must unleash enormous power within a very short time period -- no practical electric motor can deliver that much instantaneous power. You could use an electric motor to spin up a big flywheel and then release the stored energy into a flipper, but the weight and complexity of the system would be prohibitive. Pneumatic flippers are cheaper, lighter, simpler, and more effective.

Q: I didn't necessarily mean "flipper". Is it possible to create a platform that rises up and forward (like Firestorm from the UK Robot Wars) without using a pneumatic cylinder or linear actuator of any kind?

A: Sure! It wouldn't be as fast or powerful as a pneumatic system, but it can certainly work. Driving the platform from a shaft at the hinge point requires a lot of torque and a heavy duty gearbox, but other designs -- like a modified 4-bar linkage (animation at right) -- can reduce the required torque and simplify construction and control elements.

A: BioHazard's lifter uses a pair of highly modified 12 volt linear actuators running at 24 volts. Estimated power from each of the twin actuator motors is over 1 horsepower, with each actuator delivering 1400 pounds of thrust. The motors used are not standard robotics items. More information on the actuators can be found at the BioHazard website.

A: A price tag of nearly $1000 is certainly one reason. The Perm PMG 132 motor can crank out 34+ horsepower at 72 volts and would make a mighty weapon motor for a heavy or superheavy 'bot. However, that much power requires a big stack of battery capacity and a very serious motor controller. Overall, you might be better off with an internal combustion powerplant for a mega-weapon drive.

A: I don't think you've read closely enough. Most of Team Run Amok's active weapons, from the lifter in our beetleweight champion 'Zpatula' to the Bosch-powered spinner on our heavyweight Robot Wars competitor 'Run Away', are controlled by relays. We don't have anything against them -- in the right application.

The choice of a mechanical or electronic relay versus an ESC for weapon control depends on many factors. For very large weapons, a reliable relay or contactor may cost as much as an ESC and will certainly weigh more. The high amperage surge that comes along with using a relay is hard on the batteries, and for a very large weapon the capacitor you suggest would have to be huge to be any use at all against a multi-hundred ampere spike.

As far as combat robots being 'doomed to be ripped apart', we have never lost any electronics to opponent caused damage. Maybe we're lucky, or maybe we just build 'em right.

A: Yes. The BaneBots 125:1 36mm planetary RS-540 gearmotor could be used on a 12" lifting arm for a hobbyweight, if the shaft was well supported. It should provide over 20 pounds of stall force at the end of the arm at 12 volts. You might also consider the 256:1 42 mm planetary RS-550 gearmotor for it's larger diameter shaft and greater torque.

Q: You told the guy that asked about the RS540's on a lifter to consider the 256:1 RS-550 motor. I dont know about the 42mm gearbox, but the 38mm 256:1 gearbox's durability is very poor. You may want to say something to that guy before he gets the 256:1 and it busts on the first attempt. Maybe recommend the 64:1 since it has way more than enough torque, or preferably the 48:1, again way more than enough torque.

A: The BaneBots 256:1 38mm gearbox is a 4-stage unit and is not offered with the RS-540 or RS-550 motors. The 125:1 RS-540 38mm gearmotor is a stronger 3-stage design -- the same as the 64:1 and 48:1 boxes -- and should be fine in the hobbyweight application I recommended. The 256:1 42mm gearbox is much stronger than the 38mm gearbox and should also be fine for a small lifter.

I don't agree that the 64:1 and 48:1 gearboxes with an RS-540 motor would provide 'way more than enough torque'. For reliability and maximum lift speed, you want to load the system to no more than 60% of stall torque at maximum lift weight. With a 12" arm and allowing for gearing losses, the RS-540 motor with a 64:1 box stalls at about 10 pounds, and the 48:1 box stalls at less than 8 pounds.

Q: Unless you are fighting a very tall, thin bot all 12 lbs will never be on the lifting appendage at one time, unless some is higher than the point, like say a 6" wide bot is on the arm, with one edge at the tip and extending towards the shaft, and in that case it wouldn't be stall torque / 12", it would be stall torque / 6".

A: The needed torque is calculated by measuring from the pivot point to the location of the center of mass of the opponent -- not to the closest edge. In your example, the center of mass would be 9" from the pivot. If your arm tip is under the center of mass, you'll need full-weight lift capacity regardless of the width of the oponent.

Having the full weight of the opponent on the end of your lifter is not as unusual as you think. The end of the arm can and does lodge in 'bot openings, and large lift torque is needed when your opponent is pinned against the arena wall.

Q: No one designs their lifters with that much torque. It's a waste of weight and generally unrealistic unless you have a super huge lifting spatula or whatever at the end of the lifting arm. Biohazard is the greatest heavyweight lifter of all time, and it couldn't lift 220 pounds out at the end of it's arm. I'm not trying to criticize your answer, just trying to help another bot builder.

A: Mark J. here: we design our lifters to lift the full weight of the class they enter, and then some. Other builder do as well. For example, Team Cosmos' very successful hobbyweight 'IO' has a minimum 15 pound lift capacity. Carlo Bertocchini claims that Biohazard will lift 220 pounds, and the photo at right appears to show it doing just that.

Team Run Amok's lifters do have large surfaces that focus the lift toward the end of the arm, but the original questioner did not specify a design or even a weight class.

As for being a waste of weight, the BaneBots 125:1 36mm gearbox weighs the same as their 64:1 and 48:1 boxes. With the higher gear reduction you get greater lift capacity, less motor stress, lower amperage draw, and faster lift rate at heavy loadings. I'll back-up Aaron's recommendations.

32.32 in this case represents G, the gravitational constant. This isnt the kind of question you normally answer, but is my math correct? I know that the answer probably won't be realistic, but it would be fun to have a stat like that.

A: Mark J. here: You're not quite ready for the physics final exam. Joules don't directly convert to distance, but can convert to height. You're also mixing english and metric units -- keep the height in meters, mass in kilos, and the gravitational constant in metric units (9.8, not 32.3). The equation itself is correct, but can be further simplified:

So, a weapon with 6000 joules of energy has the potential to toss a 100 kilogram opponent: 6000 / (100 * 9.8) = 6.1 meters high. You're correct in saying the number will not be realistic, since it assumes a conversion of all of the weapon energy directly into vertical motion of your opponent.

Q: Wow! That formula yields 18.7 meters of potential height for a hobbyweight with only 1 Kj of energy. That really makes me appreciate how much power that is.

To calculate distance, I figure that an object may get thrown at about a 45 degree angle from a drum hit, which allows the trajectory to be modeled with height equal to distance -- a trajectory ratio of 1:1 with the height and distance both half of the original height: ( 1000 joules / ( 5.45 kilos * 9.8 ) ) / 2 = 9.4 meters high and 9.4 meters far. Is that about right?

A: You're more than a little off on your calculation of trajectory. With a 45 degree launch angle, the energy will be equally divided between vertical and horizontal vectors. The veritcal speed slows, stops, and then reverses under gravitational acceleration. However, the horizontal speed of an object thrown on a ballistic path stays constant (negating drag). As a result, an object thrown with identical initial vertical and horizontal speed (45 degree angle) will travel four times further than the maximum height it reaches. The formulas are:

Your maximum height calculation is correct, but the theoretical maximum distance thrown would be 37.4 meters. There is a very nice trajectory calculator about half-way down the page at the HyperPhysics Trajectory webpage.

Q: For every action there is an equal and opposite reaction, so won't half of the energy of the weapon hit go to throw my 'bot backward a distance equal to the distance my opponent is thrown forward?

A: The vertical component of the hit energy will act against the arena surface, resulting in no movement of your 'bot and full transfer of that energy into the height of your opponent's trajectory. The horizontal component of the hit energy will act equally on both 'bots, but your 'bot will be sliding across the arena surface with high friction resistance, while your opponent will be flying thru the air with minimal resitance. You'll get a little 'kick back' distance, but it will not be anywhere close to the distance your opponent will fly.

There are lots of factors that will reduce the actual distance the opponent will travel, but remember that this topic started out as just a 'brag number' -- a maximized ideal just for shock and awe.

A: Mark J. here: MOI is a measure of the resistance of an object to a change in rotation. It is dependent on the mass and shape of the object, and the axis of rotation. If you want to do it the hard way:

• Flat rectangular bar, axis thru midpoint: MOI = 1/12 Mass * ( Width² + Length²) -- yes, that is 1/12th, not 1/2.
• Disk, spinning on symmetry axis: MOI = 1/2 Mass * Radius²
• Tube, spinning on symmetry axis: MOI = 1/2 Mass * ( Inside Radius² + Outside Radius²)

The easy way is to use the Team Run Amok Spinning Weapon Excel Spreadsheet and get the MOI plus the spin-up time for your weapon as a bonus.

Q: OK, I got the MOI, but how do I calculate spin up time?

A: See the previous article on kinetic energy and weapon spin-up time for a link to a Paul Hills' site with all the math. The Team Run Amok Spinning Weapon Excel Spreadsheet will calculate the spin-up time for you.

Q: How many joules of energy is average for a spinning mass weapon on a hobbyweight?

A: See the previous article on kinetic energy and weapon spin-up time.

Q: My hobbyweight has a drum with an MOI of 21 lb-in². It's being driven by a 24V DeWalt motor at 24V. How much capacity should the battery for the DeWalt have?

A: Energy required for spin-up is dependent on the gear reduction you choose for the drum. I'm guessing you'll want about a 3:1 reduction to keep the drum RPM reasonable. That'll spin it up to 1000 joules in 1.8 seconds. That's gonna be some weapon!

There are too many variables to calculate a 'hard number', but I'd estimate that a high-amp 600 mAH pack would give you enough energy for a three-minute match. Common practice is to use a single battery for weapon and drive motors. See the article on battery selection.

A: Mark J. here: all of the blue input boxes (material density plus all dimensions) must be filled in for a bar, disk, or tube before mass and MOI can be calculated. If that isn't the problem, your spreadsheet may have been corrupted -- download a new copy.

The differing shapes, placement, and number of teeth on a drum spinner made it problematical to include a simple calculation box in the spreadsheet for them, but if you get a little creative you can approximate their presence. Calculate the total mass of the teeth, then increase both the outer radius and thickness of the tube to add in the extra mass. Alternately, for just a few teeth, increase the radius of the endcap(s) to bring the mass up.

A: How quickly you need to spin up depends on:

• The size of the arena;
• The ability of your 'bot to dodge the first attack; and
• How much energy you have in your weapon.

A: See the previous post on pneumatics and 4-bar lifters. The solenoid valve systems are electrically operated and connect to your radio system via a R/C Switch Interface.

Q: Where can I buy parts for a pneumatic hobbyweight flipper?

A: Hobbyweight and larger combat robots use standard industrial pneumatic components.

• For actuator cylinders, valves, tubing, and connectors, look in your phone book under 'Hydraulic Equipment and Supplies'.
• On-line try www.mcmastercarr.com. -- search for 'pneumatic cylinders' to get started.
• For small pressure tanks and regulators, check with your local or on-line paintball gear stores.

A: Mark J. here: The questions are getting tougher. A full answer to your question would make a good chapter in an advanced physics text.

Simplest case - where the flipper is positioned directly below the opponent's center of mass, and the flipper force perfectly vertical and uniform for the entire stroke length:

Complex cases - if you want to calculate the height achieved from off-center flipper hits, non-constant flipper force, and non-linear flipper vectors, break out your calculus text. You'll need to calculate - amongst other things - the moment of inertia for a specific opponent and axis of rotation. That ain't easy, and life's too short. For more information on Newtonian mechanics, visit the Hyperphysics website.

A: Search the Ask Aaron Archive for 'winch' to see previous answers. There's a reason you don't see many spring-powered weapons on combat robots: pneumatic systems are more powerful, faster, safer, and easier to build.

A: Take a look at Team Boomer's Fright Knight for a good example. Note that their weapon shaft is supported both above and below the plane of the disc. The supports form a triangular shape and are anchored directly and firmly to the chassis. The shaft anchor point is heavily gusseted with flat metal plate to spread the load to the supports. The support arms are also no longer than they need to be.

Q: Well, that type of design works well for 'Fright Knight', but I was talking about a 48 inch bar similar to 'Hazard' in design. I needed to know: how do I keep it from simply pulling out of the bearing at the base of the robot?

A: Sorry -- my mind reading is a little weak. You'll still want to support the shaft at two locations, above and below the drive pulley. If the pulley is pinned to the weapon shaft and tubular spacers are inserted as needed, the pulley will locate the shaft.

A: The hammer system is actually operated by chains and sprockets that emulate a rack and pinion. You can find descriptions and pictures at the Team Mechanicus Website (archived).

A: One... two... three... hey, it only has three bars! How come nobody ever counted before? The base counts as the fourth 'bar'.

A: Read the previous post about Breaker Box. The problem isn't power, it's finding a gearbox that can handle the huge amount of torque. A single RS-540 could provide enough power for a quick lift of 200 plus pounds at a gear reduction of around 1600:1, but the gearbox would have to handle 80,000 in-oz of torque!

A: You're going to transmit more torque than you probably think, but yes you can get away without bearings. Keep the axle size fairly large and lubricate with a little grease. Better still, use oilite bushings on the axles -- cheap and light.

A: Sure, but support the weapon shaft on both sides of your saw with bearings or you'll destroy the gearbox. Try the 5:1 ratio box for a small spinning weapon.

A: Electric lifters require great gobs of torque. See the earlier articles on electric lifters and 4-bar lifters. An electric hammer has pretty much the same trouble -- it's very difficult to get enough power to make one effective. Pneumatics work much better.

A: Mark J. here: yes, designing a 4-bar lifter system is 'sticky'. I recommend giving the T.i. Combat Robotics 4-Bar Simulator a try. It gives charts of torque and lift, as well as lift height and lateral displacement.

Our beetleweight lifter Zpatula uses a gearbox to power the front arm of the 4-bar lifter. The bar lengths between pivot points) are:

• Front: 3.375"
• Rear: 2.25"
• Top: 4.5"
• Bottom: 5.625"

Q: Ok, I got my measurements. How much weight should it be able to lift? I was thinking 30lbs would be enough for the featherweight sportsman class, since it would hardly ever experience the full weight of the opponent. About how high should it be able to lift?

A: I like your reasoning on the weight the lifter should be able to lift, but remember that the 4-bar simulator gives you the torque needed to HOLD a weight at a given position -- you'll need excess torque to be able to LIFT that weight. But you're right when you say the lifter will only rarely see full opponent weight.

How high is a tricky question that depends a bit on your attack strategy. The usual lifter attack involves breaking the opponent's traction and then shoving them into a wall, rather than trying to turn them over while everybody just stands still. I think 12 or 14 inches would be plenty.

The question you aren't asking is: how fast should it lift? Speed will depend on the weight on the lifter, and faster is certainly better. If you're set-up to 'hold' 30 pounds at stall, then a 15 pound load will take your lifter motor down to about half the no-load speed. I'd aim for no slower than 2 seconds to full lift at half load.

Q: One last question then I swear I'll leave you alone about the 4-bar lifter ; - ) According to the T.i. 4-bar calculator, my design needs a gearmotor with 1000 in-lb of torque to lift 30lbs of bot. That's a lot in a small enough package for a featherweight, so how do I calculate the thrust requirements for a linear actuator actuated 4-bar lifter?

A: You've discovered why there are so few electric powered lifters. Powering the shorter rear bar of the 4-bar assembly or shortening the overall length of the lifter can reduce the torque requirement, but it's still big.

Powering the assembly with a linear actuator (like BioHazard does) trades off torque for thrust, but the total power requirement remains the same. At the point of travel that calls for peak torque, you're still gonna need 1000 pounds of thrust at a right angle to a 1 inch lever arm, or 500 pounds of thrust on a 2 inch lever arm, or ( 1000 / X ) pounds of thrust on a 'X' inch lever arm. See the photo of BioHazard's twin linear actuator set-up to see how Carlo Bertocchini does it.

Don't forget that your chassis has to put up with the force of all that thrust as well. The actuator will push just as hard on the chassis mount as it does on the lifter assembly. Kinda makes you appreciate Carlo's design, eh?

A: Mark J. here: servos do not generally have a thermal shut-down. They are often used in applications where a shut-down could be more disastrous than risking damage to the servo! The HSR-5995TG pulls more than 5 amps at stall, which will quickly heat up the power controller and motor.

Failure mode will likely be abrupt rather than gradual -- a puff of smoke and zero response. You can reduce the heat build-up and stress by running the servo at lower voltage, but this will also reduce the available torque and speed of the servo.

Grippers in 'real world' robots generally use a worm drive gear train rather than the spur gears used in your servo. A worm drive does not usually require motor power input to hold against back-force on the output, so the drive motor can be powered down and the gripper will still hold.

Q: Do you know of some gearmotor with torque-size-power comparable to that of the HSR-5995TG servo with worm gear train included?

A: Nothing that powerful and that small. Tamiya makes several small motor/gearbox combinations that include a worm-drive stage, but their output is considerably less than your servo. You might investigate worm-drive mechanisms for automotive power antennas. They are larger, but have a cable output that would allow flexibility in positioning the gearmotor itself.

Q: Do you know someplace to buy a miniature worm gear train?

A: Try the Stock Drive Products/Sterling Instruments site. They have a large selection of small worms and wheels.

A: Mark J. here: the efficiency of gearboxes depends on their design, but forget about the gearing losses -- trying to put 476 foot-pounds of torque thru a BaneBots gearbox would instantly turn it into scrap. You'll need a gearbox rated for that type of torque -- search industrial suppliers like McMaster-Carr or Grainger, or look at truck/ATV winches.

Q: How much torque at the axle of that 20 inch long arm do you think I should have, since it will be a middle weight and the lifting attachment will weigh a lot?

A: Your torque calculations are about correct -- you'll want to aim for about twice as much lift as the load you expect to have on your lifter. Does your lifter arm really have to be that long? The problem is getting a gearbox heavy enough to handle that amount of torque. Why don't you write to Jim Smentowski and see if he'll tell you what gearboxes he uses on Breaker Box?

Q: OK. I talked to Jim and I think I should use an NPC motor because of their low rpm and high torque. McMaster-Carr also has worm gear reducers up to 60:1, but I still need to know how the gearbox efficiency will impact my torque output. How do I calculate the real output of my motor through a gearbox?

A: You will lose some power with any type of gear reduction. The general equation is: output power = input power * gearbox efficiency

Gearbox efficiency depends on many factors:

• the type of gears used (spur, bevel, crown, worm);
• the number of gear meshes under load;
• the number and type of internal bearings;
• the gear clearances in the design;
• the lubricant used; and
• the design and construction of the shaft seals.

This is a good time to remind builders that the best way to figure out if something works is to look at similar designs built by other builders. If you're building something entirely new, you're kinda on your own. Best luck!

A: Torque reaction overhead blades/spikes are legal and easy enough to build, but they would not be effective under the current damage/aggression scoring system.

For an explanation of the operating principle, see the Toecrusher website.

A: Mark J. here: `Toe-Crusher' uses a torque-reaction hammer weapon - a very simple weapon based on applying the torque of acceleration and braking to swing an overhead axe. Have you ever seen a motorcycle do a reverse-wheelie? The rider hits the front brake hard enough that the rear wheel comes up off the ground and the bike balances on the front wheel. If the rider braked even harder, the entire bike would flip over and crash into the ground in front of the braking wheel. If the motorcycle accelerates very hard, the opposite happens and the front wheel lifts off the ground.

A torque-reaction hammer works the same way. The robot is built with the weight balanced on the axle and a long arm with a hammer or pick on the end. When it accelerates, the hammer `pops a wheelie' and is thrown over to the rear of the `bot. When it decelerates, the hammer does a reverse-wheelie and is thrown over to the front of the `bot. Later versions of 'Toe-Crusher' had a wedge attachment stayed in contact with the floor by pivoting on bearings at the axles. Although showy, you don't get a lot of force out of this type of weapon and it is difficult to drive effectively.

A: Mark J. here: you'll need to know the stall torque and top speed of your electric motor, the shape and dimensions of the spinning weapon, the type of material you'll use to make the weapon, and the gear reduction between the motor and the weapon. The math gets a little sticky, but I wrote an Excel spreadsheet that will do the calculations for you:

As a rule of thumb, you'll want a minimum of 20 joules of energy per pound of weight class, and you'll want to spin-up to 10 joules per pound within 2 seconds. A modern high-energy spinner might have more than ten times that energy! You might have a little more spin-up time in a big arena, or if your 'bot is nimble enough to protect the weapon from premature impact. Smaller arenas used by sub-lightweight 'bots will require quicker spin-ups.

If you want to wade thru the math yourself, go to Paul Hills' page on Spinning Disk Weapons

A: Sorry, no. The hobbyweight class is dominated by passives (wedges, rammers, dustpans), spinning weapons (mostly drums/eggbeaters), and a few lifters. Spinning weapons store power delivered by their motors over several seconds of spin-up time as kinetic energy. A hammer that gains power over a fraction of a second from a similar motor just can't put together enough energy to do any damage.

A: The design of 4-bar systems is very technical and I've never found a good primer on the web. There is a 'Four Bar Front Bar' calculator that could be helpful if you were building an electric lifter.

You can get a very good education in general robot pneumatic systems at the Team Da Vinci Robotics pneumatics page. Remember: high-pressure pneumatic systems are potentially VERY dangerous. Don't get in over your head.

A: Yes, but it has several drawbacks and may not be considered legal under current RFL rules. Not everyone interprets the rules the same way -- check with your event organizer before you build!

You'd need to add a pressure rated 'T' fitting between the tank and the system shutoff valve before the pressure regulator. A second shutoff valve on the other branch of the 'T' connects to a male quick-disconnect refill fitting.

This adds weight, bulk, and complexity to the system. I think it's easier to design the CO2 tank mounts for easy release. You can then carry the tank (instead of the whole robot) over to the refilling station.

A: 'SJ' (it used to be called 'Slam Job') has a classic overhead pneumatic pickaxe -- two pneumatic cylinders acting together on a pivoting arm. Getting a pneumatic system to work over a long weapon arc is tricky. SJ uses one short cylinder to give the weapon an initial 'boost' at an optimum angle until the working angle for the second longer cylinder improves.

A: You'll need a narrow two-wheel high-speed drive and a long sturdy boom with a sharp pick or blunt heavy weapon on the end. The rest of the 'bot is much like any other 'bot. Search the archives for 'thwackbot' for more tips.

Thwackbots are not currently popular. The RFL Judging Guidelines demand constant aggression to score well and a simple thwackbot cannot spin and move toward an opponent at the same time.

A: Drum weapons are most effective when the teeth are spaced far enough around the drum circumference to allow the other 'bot to get close enough to the drum for the teeth to dig in and get some 'bite'. Don't put on too many teeth!

I don't know if big screws are better than custom teeth, but they're inexpensive and easy to replace! Make sure you have plenty of material depth in the drum to support the threaded shaft of the screw.

A: Mark J. here: It would help if I knew what size 'bot you're building. When in doubt, go bigger.

Screws and bolts are designed to take tension stress along their axis and are not ideal for enduring the shear stress that will be imposed when you use them on a spinning drum weapon. Their advantage is that they are inexpensive and easy to replace. Use a coarse thread -- it will be easier to remove when damaged.

A: Mark J. here: no -- weapon RPM is limited only by bearing friction, transmission loss, and aerodynamic drag. A very heavy weapon will eventually spin-up to the same RPM as a lighter weapon. However, given the information you list, you can calculate the approximate RPM of the weapon after a given number of seconds spin-up time:

If you'd like to learn more about the physics of kinetic energy weapons, read thru Paul Hills' page on Spinning Disk Weapons.

A: Mark J. here: the simple answer is 'all you can manage'. Successful FBS 'bots typically spin a shell representing around 20% of the total robot mass at speeds around 3000 RPM. I'll let you do the math to calculate the KE of such a weapon. The Team Run Amok Spinning Weapon Excel Spreadsheet will help.

A: Mark J. here: yes, milling a Full Body Spinner shell from one big chunk of metal does waste material, but I've seen it done. More commonly, somebody finds a short section of large diameter pipe at a scrap yard and welds a top onto it. I've also seen a tube formed from thick plate with a set of forming rollers, seam welded, and a top added. The key is precision to keep the finished product well balanced.

A: Flame weapons are allowed at the discretion of the event organizer. Check with the specific event you plan to enter for eligibility and special rules.

Note that flamethrowers are not effective robot weapons. They are for very experienced builders who just want to show off. If you have to ask how to build one, you aren't experienced enough to attempt it. I'm not going to encourage anyone to try dangerous construction beyond their skill level -- build something else.

A: Interesting question! I figure anything more than 45 degrees isn't a wedge, it's a sloped 'brick'. How much lower than 45 degrees you go is a function of how the wedge is going to be used. A very shallow angle is useful for defense and sneaking in under spinning weapons. A steeper angle is better for ramming attacks. A curved 'scoop' can be effective both for offense and as a 'spinner killer' -- I'd say that's ideal.

Q: What material would you use the create the "curved scoop"?

A: Titanium would be great, or steel if you have enough weight allowance.

A: Mark J. here: The more precisely a weapon is made, the better the balance is likely to be. If the weapon was hand drilled and cut, it isn't going to be anywhere close. Assuming the mounting bearings spin freely, you can 'static balance' the weapon:

• Remove the connection to the weapon motor (belt, chain, friction drive...) so that the weapon spins free.
• Position the robot so the weapon shaft is horizontal (parallel to the floor) and the weapon has room to spin.
• Give the weapon a spin with your hand and let it coast to a stop.
• Mark the part of the weapon that is at the bottom, closest to the floor.
• Repeat the spinning and marking about a dozen times. The part of the weapon that has the most marks on it is too heavy.
• Remove some material from the heavy part of the weapon, or add some weight to the opposite (light) side.
• Repeat the spinning, marking, and weight adjustment until there is not an obvious heavy spot.

A: Sure! Get hold of a copy of Robot Combat: Weapons by Chris Hannold, or Kickin' 'Bot by Grant Imahara. They both cover basic weapon construction materials, design, and technique.

A: Very ambitious plans! I would suggest starting with something simpler for your first 'bot. Exotic weapons won't do a lot of good until your basic mechanical systems are up to the task.

The current top-bots have heavy, high-speed rotating 'kinetic energy' weapons that hit like a speeding truck. The preferred armor in the heavyweight class is 1/2" thick titanium. Get to a tournament and see what the competition looks like before you decide on a design.

A: The first step is to carefully read the Robot Fighting League rules. The RFL rules are used by most robot competitions in the US. That will answer your questions about nail guns (not legal) and huge wheels (legal, but you're not gonna get thru 'bot armor that easy). Search the archive for suggestions on books for robot construction.

A: Nice choice! Your gear reduction will depend on the size of your undercutter blade. Take a look at Johnson Junior -- a S-280 powered undercutter with a big 8.5" blade and a gearbox. They're running a 5:1 reduction which seems about right for a blade that big. A smaller and lighter blade would use less reduction -- maybe 3:1 for a 5" blade. You can use the Team Run Amok Spinning Weapon Excel Spreadsheet to pick the best gear ratio.

Most builders like belt drives over gear drives for weapons because they require less precision, can absorb impact shock loads, and won't 'jam' as easily. A well designed gearbox can still be effective and rugged.

A: Brushless hobby motors are mostly sold to model aircraft builders. On a model airplane a 'firewall' is the flat panel at the front of the plane where the motor is attached.

Robot weapons put large side-loading forces on the motor shaft. Support the motor by mounting it as close to the weapon drive as possible -- 'behind the firewall' style.

Mark J. here: Engines in early airplanes were prone to catch fire. The 'firewall' was the solid bulkhead behind the engine that prevented flames from entering the cockpit. In that context it wouldn't make much sense to mount an engine behind the firewall, but the term survives.

A: For an antweight, that's probably too thick. For a heavyweight, that's probably too thin.

A: Team Inevitable Destruction's 'Cadaver' has simply hard-mounted the can of a brushless outrunner motor inside one end of a drum and put a support bearing in the other end. The force of impact is taken directly by the motor bearings, which is hard on the motor. A belt drive would isolate the motor from impact forces, give a faster spin-up, and would keep the weapon speed down enough to 'grab and toss' an opponent.

A: You think lifters are easy? I guess you can make an antweight lifter out of a servo without much trouble, but a bigger 4-bar linkage lifter is not easy.

A better question might be, "What is the simplest weapon that is effective at winning matches?" The answer might surprise you. My dad looked at the results from 20 recent robot tournaments to see what type of weapons did best. Take a look at the results: What Weapons Win?

A: A drum is just a large diameter metal tube with end plates and a few teeth welded to the outside. It's empty inside except for the shaft. The weapon shaft needs strong support, like any spinning weapon. Spin it with a belt drive from your weapon motor.

A: It's like the difference between a rotating bar and a disk. An eggbeater (like Team Sawzall's 'Switchblade') can't store as much kinetic energy as a drum that weighs the same -- the weapon has less rotational inertia. They're still effective, and more durable than a thin-walled drum.

A: Same as for any other rotating weapon: solid supports, correct RPM, protected drive, good balance, and quick spin-up. Drums are particularly hard to balance, so use extra care in precision construction.

A: The weapon/pulley shaft must be supported so that it remains aligned and does not wobble. This requires two bearing supports at separate locations along the shaft on either side of the pulley and/or the weapon. You'll need to adapt or fabricate bearing mounts strong enough to both support the correct pulley alignment and absorb the weapon loading.

Take a look at Team Basenji's antweight 'Bitsy Blade' at the Robot Riots 5 photos page for an example of a belt driven antweight spinner.

A: Technical question, Mark J. here: Briefly, gear drives require precision alignment and spacing to function properly. They can operate at very high RPM, but are not terribly efficient at transmitting power. Belt and pulley systems can tolerate some misalignment and are capable of absorbing the sudden shock loading a rotary weapon can produce.

Timing belts have small raised teeth along the inner surface of a flat belt that mesh with grooves in the pulley to reduce slippage and increase power capacity. They are very efficient at transmitting power and can operate at higher speed than regular belts.

I like belt drives for rotational weapon systems like bars, disks, and drums. The ratio of the number of grooves on the larger pulley to the number of grooves on the smaller pulley is your gear ratio. You'll probably want to try something around 4 to 1 for a large bar weapon.

Robotcombat.com has a selection of small timing belts and pulleys suitable for antweights in their in their Mechanical and Drive Components section. Their timing belt page has a calculator to help with correct belt length selection. Tower Hobbies has inexpensive, ready-made gearboxes designed for model airplanes that would be useful for an ant weapon. Search for `gearbox' at their site.

A: Wow - another technical question - Mark J. here: Power is calculated as the product of torque and RPM, so the real answer to your question is that both are equally important. A spinning weapon stores power in the rotating mass of the weapon and unloads it destructively onto your opponent. You need as much spinning mass as possible, enough torque to spin that mass up quickly enough for it to be effective before your opponent can get to you and stop it, and as much speed as possible to store more energy in the weapon. A spinning weapon is all about stored power!

A: Technical question - Mark J. here: Gyroscopic forces can cause odd effects for robots with a spinning mass weapon. Problems come when turning changes the orientation of a line running thru the weapon axle. With a horizontal spinning weapon, the axle points up and down, so turning the robot does not change the direction that line points -- no problem, as long as the weapon is well balanced.

With a vertical spinning weapon, turning the robot does change the orientation of the weapon axle. This results in forces that resist the turning motion and lift one side of the robot. The magnitude of the force is dependent on the speed of rotation, the mass of the weapon, and the radius of gyration. You can get full details on calculating gyroscopic forces at: www.freestudy.co.uk/dynamics/gyroscope.pdf

A: Different builders have different ideas about what's 'best'. It depends on your driving style and your skill as a builder. I see lots of different weapon types winning, but I notice robots with spinning drum weapons doing very well in the 12 to 60 pound classes.

A: Mark J. here: a 4-bar linkage is a simple arrangement of four mechanical links (like rods or beams) with pivot connectors on each end linking them together into a roughly rectangular shape. By careful selection of the relative lengths of the links used, you can create complex movement arcs and gain torque or speed without gear reduction. The Wikipedia has diagrams of 4-bar linkages in their mechanical linkages section.

Robots sometimes use 4-bar links to control and position lifter arms efficiently and allow them to scoop in a sweeping upward and outward arc rather than a simple single-pivot backward rotation. Aaron's beetleweight 'Zpatula' uses a 4-bar linkage in its electric lifter.

A: I like 'bots that have a lot of pushing power and good control. I think the best weapon is to be all over your opponent and don't let up -- hit 'em 'til they break!