On Sun, 7 Sep 2008 02:45:31 -0400, "Tom Gardner"
wrote:


I would step down from the 20.

If you step down to below load speed and then step back up, the max
torque in the system is greater than load torque. Not that it matters
in this case but I thought I'd mention it.



It just seems natural to reduce to speed, I don't know why. Would the 20
box be more robust and run cooler, last longer, etc.?

If designed to the same specs for efficiency, lifetime and power
output the boxes should be quite similar. Gear volume generally
depends on power level (speed = diameter, torque = thickness, power
= torque * speed, power in = power out + losses).

I don't know what the case might be for the two particular examples
you have in mind.