Motor torque (momentum) is one of the motor ratings used to indicate the rotary motor force produced on its output shaft. The motor torque rating is usually captured at the point when the motor is stalled. The torque measurement unit is (Nm) in the metric system or (ft-lbs) in the US system. But the principle is the same torque is a multiplication of lever length (r) and force(F) applied to lever:
I=r x F
So there is a simple experiment on how to measure motor torque quite accurately without any fancy measuring tools.
All you need is a motor of which torque will be measured, a nominal power supply of motor, thread long enough, a mass of known weight, ruler, and a table or other lifted surface where the motor will be fixed.
Attach mass to one end of the string and another to the motor shaft to turn the windings. Fix the motor at the edge of the table so that string with mass would hang. Start the motor and led the rotating shaft to wind the thread while lifting the mass. In the beginning, the motor will turn very easily but later when the winding radius will increase, so the stopping force will also increase. And at last, the motor will stall. So this is the moment when you have to stop the motor and measure the radius of winding. Then take the formula and calculate the torque. For instance if mass m=0.1kg; radius of winding r=0.01m; acceleration due to gravity g=9.8m/s2.
I=0.01 · 0.1 · 9.8=0.0098 = 0.01(Nm)
If the motor will wind up all thread try to attach heavier mass. This technique will help compare different motors or test motors torque and compare it to documentation to see how reliable this technique is.
Robots Builder Guide 1992 – Fred G. Martin