20 pitch, 14.5° pressure angle, 0.375 in face width, steel and iron gears are commonly and cheaply available from Boston and Martin; that's probably why most teams use them. In general they're strong enough for drivetrains, but might not be sufficient for an arm.
You don't need to use 20 pitch on the high-speed stages, though; I've sucessfully used 32 pitch and 0.7 module, 20° pressure angle, 0.1875 in face width aluminum gears to gear motors together (linking the old Bosch drill motors, Fisher-Price motors and CIM motors). With that 1.4 HP combination, the gears ran safely (marginal wear after 2 regionals, championships and an off-season, including some impressive collisions and violent driving) at the equivalent of a 1.5:1 reduction from the CIM motors. Of course, I also tried to use those same gears at a total of 5:1 with respect to the CIM (i.e. 1.5:1 × 3.3:1 ≈ 5:1), and those broke in testing.
I'm also a fan of the 32 and 24 pitch hardened steel gears from
Stock Drive Products. To replace the ones that broke, we used the 24 pitch, 20° pressure angle ones and never had an issue.
The final stages (parallel reductions, selected by dog shifter) of that gearbox used steel and iron 20 pitch gears, as described above. There was visible wear after the season, but it may have been due to the misalignment of the output gears due to somewhat inadequate support against the dog shifter. They weren't ideal for that gearbox, but they didn't fail.
Using 20 pitch gears can simplify things greatly, but just be aware that there are better solutions to keep weight and inertia down.
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Originally Posted by someone
Another question: would a 16 pitch worm be sufficently strong?
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For what? I'd seriously advise against worms for the drivetrain, because of their efficiency characteristics (often between 50% and 80%) and (depending on the type) point-loaded teeth. Unless you're investing in extremely expensive high-efficiency, high-strength worms and worm gears, avoid them like the plague for your main drive.