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Mechanical Design                     
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Photo Courtesy of Andrew Lindsey

 

Mechanical Facts and FAQs:

  • Can I bolt my motors directly to my wheels?

No. You will need to have some sort of speed reduction unless you have a very small wheel attached to a very slow motor. A typical wheel/motor combination for a heavyweight might be 8" wheels with a 5000 rpm motor. If there was no speed reduction this would produce a speed of 175 feet per second, or 119 miles per hour. Most of the robots in BattleBots run in the range of about 6 to 15 miles per hour. A few robots can do 20 MPH or more. Here are some good gearmotors.

 

  • How do I reduce the speed of the motor?

1) Use a motor with a built-in gearbox.
This is probably the best all-round solution for a novice robot builder. It requires the least amount of skill and effort to implement. Just buy it, and bolt it on. If you want high performance, you need to find a high performance motor with an efficient speed reducer. This could be hard to locate, and/or expensive. Many motor/gearbox combinations have worm gearing - cheap, but inefficient.

2) Buy a speed reducer.
Some very nice planetary-gear speed reducers are available, unfortunately they can be very expensive, typically costing more than the motor itself.

3) Use gears.
Spur gear reduction can be very efficient and robust, but using gears for speed reduction should only be attempted by people who are very skilled in the shop and behind the drawing board. You may need to hold tolerances of +/- .001. Don't forget the lubrication requirements, the faster its spinning, the more likely it is that you will need to run the gears submerged in oil.

4) Use chain.
Probably the easiest type of speed reduction to make. Efficiency can approach 97%, and tolerances are not too tight for the typical midnight engineer. Chains should not be used at extremely high RPM. They may pop off if the sprockets are not properly aligned, or if the tension is not correct. It is a good idea to use chain tensioners to take up the slack and compensate for stretch unless the sprockets are very close together. Of the ten chains in BioHazard, the four longest have chain tensioners.

5) Use belts.
This refers to timing belts, not V-belts. Timing belts have the same pros and cons as chains except for the following: Belts can be used at much higher RPM than chains; Efficiency can approach 98%; and they weigh slightly less. Belts are wider than chains, and a multistage reducer can take up a lot of space.

 

  • How did you do the speed reduction in BioHazard?

I used ten #25 chains and 24 sprockets in BioHazard. The two-stage chain reduction, and all the misc. drive components for powering each of the six wheels are squeezed into the 2.5" x 2.5" space inside the extruded aluminum square frame tubing. All moving parts use ball bearings.

 

  • I've got a 20,000 RPM motor. Can I use worm gear reduction?

Worm gear speed reducers are inefficient, and have way too much friction to be used at 20,000 RPM (at least for the first stage). That speed is also too high for chain reduction. Your only options are gears or belts. Timing belts are probably your best bet.

 

  • How does the BioHazard's arm work?

BioHazard ArmA four-bar mechanism gets the tip of the arm to move with a slight forward arc while it travels up. This seemed like a good motion for flipping other robots over. Four-bar mechanisms are very popular in machine design. The trick is to determine the optimum length for each of the four bars to get exactly the motion you want.

The arm itself is one bar. I made this as long as possible to get the most height. The length of the dual lifting bars determines how high the arm reaches, and the rear control bar determines the angle of the arm relative to the base. The base itself is considered a bar in four-bar linkages. The linear actuators also move through an arc so this might actually be considered a five-bar mechanism.

BioHazard FrontThe actuators rotate a bellcrank which is attached to a 3/4 diameter shaft. the bellcranks rotate the shaft which is held in place by four bearings. The lifting bars are also attached to the shaft and they rotate up and forward as the shaft turns.

A tremendous amount of torque is transmitted through the shaft. The hard part was to invent a method of attaching the bellcranks and lifting bars to the shaft so that they would survive such high torque loads. I wanted to stay away from welding, so I ground two flats on the shaft for each item that was attached to it. The bellcranks are shaped like big open-end wrenches which fit tightly over the flats on the shaft. I used Airmet100 steel for all these components, and I think they are all stressed to a high percentage of their yield points when lifting another robot. Other than the axles and a few other small components, this is the only place I used steel in BioHazard. A great concinnity of design is required to squeeze this mechanism into a space just 3" tall. One of the things that makes this possible is the use of 3D CAD software.

 

  • Should I support my wheels on both sides, or is one side enough? How are BioHazard's wheels supported?

BioHazard's wheels are supported on both sides. They have a hollow 5/8" axle, which does not turn. The frame rails are made from 3" X 3" extruded aluminum square tubing with 1/4" wall thickness. The axles are solidly mounted in the frame rails, and the 4" diameter wheels are mostly inside the extruded tubing. They stick out about 1" which gives BioHazard about 1" of ground clearance. The motors are bolted directly to the tubing.

Wheels which are supported on only one side, or "overhung", are much more likely to get damaged. Overhung wheels is a valid design option, but I would use at least 1" diameter axles for heavyweights.

 

  • How did you get so much power out of the electric linear actuators in BioHazard's arm?

The two linear actuators that power BioHazard's arm only have about 3.5 inches of travel, and they go that distance in a little less than a second. To lift another 220 pound robot, they each have to push with a force of over 1,400 pounds. The motors are the 12 volt motors that came with the actuators, but I am running them at 24 volts. They spin at 12,000 RPM, and I think that they put out over one horsepower each. Speed reduction is about nine to one. By the end of a match, the motors are too hot to touch.

Linear ActuatorThe vendor rates the actuators for only 400 pounds of force. They are heavily modified to get the 1400+ pounds of push out of them. I changed the position of the motors, and I used a 5/8" wide timing belt for power transmission. I replaced the dozen or so bronze bushings in each unit with ball and needle bearings. I also removed a device that prevents back driving, and I shaved some weight off of them with some careful machining. The trunions that came with them were much too small to handle the increased push, so I made new, larger diameter ones.

The main power screws use ball bearings instead of the more common Acme threads. This makes them very efficient. I think that the modifications I made increased the mechanical efficiency of the units to about 90%. This allows for easy back-driving of the arm, I can even push the arm back down with my hand.

 

Here is a list of books that have information about mechanical design. Most of them have been useful in the design of BioHazard. All are available from RobotBooks.com.


BOOKS

BUILD YOUR OWN COMBAT ROBOT. Everything you need to plan, design, and build your own battling robot!

MECHANISMS & MECHANICAL DEVICES SOURCEBOOK. By Nicholas P. Chironis. If you get just one item on this page, this should be the one. "Preinvented" linkages, cams, gears, clutches, brakes, and mechanisms. 1700 illustrations. Everything you need for a simple wedge, or a complicated walking robot. Available from RobotBooks.com.

507 MECHANICAL MOVEMENTS. By Henry T. Brown. This small book was originally published in 1893. These mechanisms are simple, and the 19th century technology is well suited for amateur garage workshops. Available from RobotBooks.com.

INGENIOUS MECHANISMS FOR DESIGNERS AND INVENTORS. By Jones, Newell, and Horton. This is a four volume set with a total of about 2100 pages of very detailed drawings and descriptions. This advanced text is not as easy as the other texts, but much more complete. Available from RobotBooks.com.

MACHINERY'S HANDBOOK. This is one of the standard reference books for mechanical engineers, and machinists. 2500 pages. Available from RobotBooks.com.

BUILDING ROBOT DRIVE TRAINS. The first volume in the new "Robot DNA" series by McGraw-Hill/TAB Electronics. It is just what robotics hobbyists need to build an effective drive train using inexpensive, off-the-shelf parts.

Visit RobotBooks.com for more book recommendations.

 

  • What do the experts say?

In all things, the supreme excellence is simplicity.
-- Henry Wadsworth Longfellow

Everything should be made as simple as possible, but not simpler.
-- Albert Einstein

A complex system that works is invariably found to have evolved from a simple system that works.
-- John Gall

Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction.
-- E. F. Schumacher

You know you've achieved perfection in design, not when you have nothing more to add, but when you have nothing more to take away.
-- Antoine de Saint-Exupery

Making the simple complicated is commonplace; making the complicated simple... that's creativity.
-- Charles Mingus

Try not. Do or do not. There is no try.
-- Yoda

 

BioHazard

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