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Mechanical Design
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Mechanical Facts and FAQs:
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 5 to 15 miles per hour. A few robots can do 20 MPH or more.
1) Use a motor with a built-in gearbox. 2) Buy a speed reducer. 3) Use gears. 4) Use chain. 5) Use belts.
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.
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.
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.
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 Pro/E 3D CAD software.
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.
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.
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. BUILD
YOUR OWN COMBAT ROBOT. Everything you need to plan, design, and build your own
battling robot! 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.
In all things, the supreme excellence is simplicity. Everything should be made as simple as possible, but not simpler. A complex system that works is invariably found to have evolved from a simple system
that works. 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. 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. Making the simple complicated is commonplace; making the complicated simple... that's
creativity. Only those who have the patience to do simple things perfectly ever acquire the skill
to do difficult things easily. Genius is one per cent inspiration, ninety-nine per cent perspiration. Try not. Do or do not. There is no try.
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I used a four-bar mechanism to get 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 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.
The vendor rates the actuators for only
400 pounds of force. They are very 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.
