Quote:
Originally Posted by StevenB
Time to back up and make sure I understand what's going on. My initial assumption was that the gearbox acts like a constant load on the motor, causing both reduced speed and reduced stall torque. This was incorrect.
If I understand correctly, there are two separate forces acting on the gearbox: One of them (windage) is loosely proportional to the speed of the motor, caused by churning of the lubricant and friction we can't get rid of. The other is loosely proportional to the forces on the gear teeth, and thus to the torque on the gearbox.
Since the power out of the motor is the product of speed and torque, each reduced by their respective loss factors, the product of the loss factors gives efficiency. Thus, when an single efficiency number is posted, it most likely represents both speed and torque loss.
However, the split is not necessarily even - 90% overall efficiency could be the result of 92% torque efficiency and 97.8% speed efficiency, or any other split. Experience is the easiest and most useful way to determine what the separate speed/torque loss factors are.
Is this right, or am I off in the weeds?
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The gearbox puts a parasitic torque load on the motor. This parasitic torque is a function of the speed and the output torque load on the gearbox:
Parasitic_Torque = f(output_speed, output_torque)
To the motor, the parasitic torque looks just like an additional external torque load. So it slows down the motor.
It's too bad that gearbox manufacturers don't provide this data.
It's simple to get one data point by measuring the motor's free current with and without the gearbox. Using the motor's known torque constant, this could be used to calculate free speed with the gearbox. Getting a second datapoint requires measuring torque or setting a known torque load, which requires equipment most teams don't have. If the motor can tolerate stall for several seconds without damage (perhaps even at a reduced voltage), a torque wrench with a special fitting could be used to measure the stall torque with and without the gearbox.