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Unread 30-08-2004, 10:16
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Re: White Paper Discuss: Team 179 conceptual - single speed - 3 motor gearbox

Quote:
Originally Posted by Tristan Lall
Which 24 pitch gears are you using? Consider the hardened steel ones that SDP/SI sells--they worked nicely for us last year. By all accounts, they're fantastically strong. For a low-speed, high-torque gear, I'd be wary of regular steel gears, and would absolutely rule out aluminum, brass or plastic (again, experience...we chewed up some rather expensive aluminum gears last year, and they were only in the middle of the gearbox). Edit: To clarify, in terms of 24 pitch gears, I'd be careful about regular steel gears. Regular, un-hardened steel 20 pitch gears will be fine--that's what we use for the gears beside the shifter, though having them hardened wouldn't be a bad idea.
We used unhardened 24 pitch gears in our intermediate stage of our 2003 gb without difficulty. I'm pretty sure the face width was 0.2"

We used 20 pitch gears for all of our 2004 gearbox gears. The pinion diameter of the smaller gear became too small to use with the axle stock that we had picked for 24 pitch. We shrunk the face width down to about .1" for the intermediate gear and .4" for the final output gear. None of our gears last year experienced any wear.

I agree with your comments about aluminum gears. I would not use them in any drive system gear box (low strength, galling, expense).

Brass is not much weaker than steel and is stronger than aluminum. However, I seem to remember that brass gears are more expensive than steel gears for the same pitch, etc. Nonetheless, if I didn't have a choice, I'd use them in an intermediate stage of a drive system gear box without qualms. I'd do a basic strength calculation first, though.

Plastic gears should not be used anywhere except the zeroth stage of a drive system gearbox.

BTW, we buy gear stock from SDP-SI and cut our gears to desired width. It is less expensive than buying standard gears and machining them to our specifications. When trying to accommodate wear, you can always increase face width (if you have space/weight) rather than increasing tooth thickness (reducing pitch or increasing module). If your teeth are wearing or your key ways are wearing, increasing width gives you improved performance in both areas.

Also, we used hex axles and hex broaches in 2004 rather than key ways. The load on each facet of the hex is much lower than the load on a key way for the same sized shaft. You can reduce shaft sizes or even go to aluminum axles versus a similar keyed shaft. This reduces weight in the heaviest part of the robot without sacrificing performance. The machining time to broach hex holes and make bearing fits on the end of hex shafts is less than the time to make similar bearing fits on round axles, broach a key way, and machine a key way in the shaft.

The only down side that I have found is that hex broaches are expensive ($200 for a 1/2" broach) and keeping the broach aligned while broaching takes a little art or a fixture.

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