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Gearboxes are not 100% Efficient
The effeciency term in the equations for the stall torques are a catch all for a ton of losses that occur at the interfaces of the parts: The losses at the bearing, the losses as one gear tooth meshes with another, the losses due to squishing grease out of the way, etc.
For normal spur gears, with reasonably well designed bearings, the effeciencies should be in the mid to high 90's. As a practical matter, with one of a kind gears running in one of a kind gearboxes designed under insane time pressures, I think it is wise to use 90%. It should be conservative, but you will thank your stars for the extra torque when you need it.
Joe J.
P.S. I agree, that the motor curves you discussed are very important for FIRST folks to understand. DrJoesMotorCalc.xls in the whitepapers section of these very forums will calculate all four of them for you. All you need are 4 pieces of information: FreeSpeed, FreeCurrent, StallTorque, & StallCurrent for any one voltage and the spreadsheet will give you all 4 charts at whatever voltage your gearbox designin' heart desires!
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