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Consider using worm gears because the worm gear can turn the gear but the gear cannot turn the worm gear, this allows from some serious "stayin put" power.
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Note: the "anti-back-drive" property of worms depends on a bunch of factors (number of threads on the worm, friction in the system, lead angle on the worm). It is not a guarantee if you just put a worm/worm gear in your drive system that you will be impervious to back drive.
Worms also have an added inefficiency due to sliding contact between the worm and worm gear. This is a function of the contact friction between the two. Use lubrication. And be prepared to lose a little of your "available work" to friction (which is not as big a problem with spur gears).
Worms and worm gears experience significant thrust loads! The thrust load in our drive system was sufficient to push the snap rings holding the gears into the proper position out of the groove. The worm would also grab the key in the shaft and shove the shaft around. We ended up putting in spacers to keep everything in place.
Make sure you properly support all shafts. You might want to use angular contact bearings on each end of both your worm shaft and your worm gear shaft in order to accommodate both radial loads and thrust loads.
We designed our first worm and gear system last year and are still crawling up the learning curve. I think it is probably worth the effort. But, there are a few more quirks than we expected.
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Excessive use of pneumatics is a NO NO!, if you are going to employ pneumatics into a drive train, be sure to keep the range of movement to a minimun (for instance short strokes). Example... a gearbox, though a very risky undertaking in itself for a rookie, or even some veteren teams... use short stroke cylenders to and little room for error.
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Maybe they'll give us air motors some year.
For those of you who have used pneumatics, how many things can you drive at once? Do you experience "sag" in your devices as the match proceeds?
Can you charge your air tanks to 200 psi before a match? Or just to 60 psi? This seems to be an extra source of potential energy and therefore a good thing.
Andrew, Team 356