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when shopping on digi-key or other electronics distributor, I can see the Torque - Holding spec, given in oz-in and mNm.

I understand Nm in a torque context when the force is applied as a lever at a distance from the rotation point (for instance when applying 1 N force on a wrench, 20 cm from the rotational point, would give 0,2Nm).

But what does it mean when we are talking about motor torque? It seems to me that it is like opposite: the motor is applying force at the rotational point, so the distance is always 0.

Does it mean motor specs are always measuring the force the motor is applying 1 meter away from it? I am trying to have a good intuition of what Nm mean when looking at motors, because tutorials online only talk about wrenches or other cases when the force is applied away from the ration point.

Thanks a lot!

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  • $\begingroup$ How would you calculate the torque if there was a gear of diameter 5cm on the end of the shaft? $\endgroup$
    – Solar Mike
    Commented May 5, 2020 at 5:21
  • $\begingroup$ G=10 (close enough :-) ). Work in kg.m say 100 Nm = 10 kgm. Visualise a 10 kg mass on a 1 marm. You can se the effects of the torque by the bend in a suitable dimentioned arm and measure it with load clls on the arm - or by the ability to make a clamp at the hub slip or ... numberous ways. If you now twist that arm rotationally on a plane at right angles to gravity you will again induce a bend or measured strain or slippage. The same effect results from the same torque. A useful approximation is watts = kg.m x RPM. ie power = torque x rotational speed. ... . $\endgroup$
    – Russell McMahon
    Commented May 5, 2020 at 9:33
  • $\begingroup$ ... The above is derived from: "power = force x distance per second" and the units just happen to cancel nicely. |Summary - torque is "twisting power" It doesn't matter how you produce it. $\endgroup$
    – Russell McMahon
    Commented May 5, 2020 at 9:33

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Let's say you have a motor with torque rating of 10 Nm and it came in a box with just a shaft.

The rating just means if you attach a pulley with R =1 meters you get ten Nm torque. But if you attach a 0.2 meters pulley you get 5*10=50 Nm torque. Of course your belt runs 5 times slower.

Shaft torque/ demand torque = radius of your pulley in meters.

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  • $\begingroup$ Thank you! but what if I decide to install a load directly on the motor shaft? How can I tell the amount of work my motor can do? Do I use the rotor radius instead? for example: I found a stepper with 230mNm and has a radius of 2cm. does it mean I would get 230/50 so less than 5 Nm? $\endgroup$
    – JCSB
    Commented May 5, 2020 at 15:01
  • $\begingroup$ 230 x50= 11500mNm available torque. $\endgroup$
    – kamran
    Commented May 5, 2020 at 15:44

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