Testing Servo Motor Torque

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Mike RigsbyMike Rigsby 03/13/2018 at 18:590 Comments

The servo motors I am currently using claim to have a torque rating of 20 kg-cm (277.6 oz-in). Rather than just accepting an advertised claim, I thought it would be a good idea to know if the motor could really produce such torque.

My test rig involves a 1" x 2" pine board attached to a servo motor. At a distance of 21" from the servo shaft, I have taped 8.3 ounces of lead weight.  Twenty-one inches multiplied by 8.3 ounces yields 174.3 ounce-inches of torque. The 8 ounce wood arm (assuming even distribution and that the center is 11 inches from the arm) contributes 88 ounce inches to the torque required. Adding 174.3 to 88 yields 262.3 ounce inches of torque. I didn't play with weights to find the absolute max, but I have no reason to question the 277.6 oz-in advertised.

Under load, it looks like this:

As an interesting side note, the current required to hold the arm horizontally (at 7 volts) is about .4 amps. If the current limit on the power supply is less than 2.5 amps, the servo motor won't rotate the arm.