From what it sounds like, you want it to drive like a car like our current year’s robot. You will need an Ackerman steering and a differential to allow the robot to move correctly. For more information and pictures, go here. Ackerman steering works much like a car steering, it points the front wheels in a direction and then the rear wheels drive it forward or backward, following the direction of the front wheels. The robot will generally move in an arc. A differential is required because the rear wheels are so far apart that the robot actually follows 2 arcs. Obviously, one of the arcs will be longer than the other, so one wheel has to make more rotations than the other. A differential allows this (mechanically), without having to skid wheels across the ground.
The positive side about the differential is that the motors are coupled together to make 1 output, since the rear wheels operate at one speed. For this reason, you’ll want to use the CIM motors for your drive system (the drills have timing, they spin faster in one direction than the other). The singe output means you’ll only need 1 gearbox, cutting the amount of parts you need in half, but also the parts of the gearbox/differential will need to be twice as durable, since they need to cope with the stress of 2 CIM motors instead of 1. I recommend no less than 3/8" shafts and keyways. Then, use 2 pneumatic cylinders to create a 4 speed on-the-fly shifting gearbox (2 reductions per cylinder). Each cylinder will shift between 2 gear choices. As each engages, you’ll have a different gear ratio, giving you 4 different speeds. If you space the reductions enough, you’ll have a very low speed, a medium-low speed, a medium-high speed, and a high speed. There are many whitepapers around here on how a 2 speed shifting gearbox works. Just copy it twice and do the math for the reductions. If you have a 2:1 reduction (gear up) and a 1:2 reduction (gear down), copied twice, the possibilities are 4:1, 1:1, and 1:4. Essentially you have a 3 speed using 2 shifting stages. Now the trick is to change those so they don’t match, such that you have 4 different reductions. Now you have a 4 speed gearbox.
You don’t need a clutch because you are using an electric motor and not an engine. A engine can’t afford to stall, because it is a combustion process and must be continually running to continue the process. If the process stops when you are engaging gears, you’ll either shear some gears from the massive torque, sieze up your engine, or bust a piston. In either case, it won’t be pretty. Electric motors don’t have that problem because you can turn them on and off in an instant, and nothing seriously major goes wrong if they stall.
The on-the-fly pneumatic shifting works by brute force. As the robot is moving, the pneumatic cylinder forces the gear off the current gear that it is locked with (they slide apart), and slides it along the shaft until it hits the 2nd gear. These cylinders have anywhere from 30-180lbs of force (30 is plenty). As the opposing gear rotates (becasue the robot is moving) eventually the gears will mesh and the cylinder will force the gear to slide into position. Usually the gear slides on a keyway, so that no matter where the gear is positioned, the entire shaft rotates. I suggest you hit the whitepapers section of the website if you want to read more on this and see some designs. I recommend the document on a 2 speed gearbox designed by 116 I believe. It’s the easiest to build and the easiest to understand. The design (above) by 226 would be a good (and easy) one to implement as well. I can already think of a good picture of how it would look and work.
If you don’t understand how gears/gearboxes work, then you have a lot of catching up to do.
This task is daunting for even a veteran team to pull off. If you are truly serious about it, do some research. Anyone can do it, it’s all about the amount of time you have. If you are truly serious about it, then I guess we’ll see you next year with the design. There’s no doubt about it you’d win a technology award if you can pull it off.
