How to Calculate Belt Drive Torque

Written by brad painting
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How to Calculate Belt Drive Torque
A bike chain is a human-powered drive belt. (pinions and folding tool image by Radu Razvan from

Drive belts are flat, rubber loops used in various machines to rotate parts that the motor cannot rotate directly. In cars, a drive belt controls the power steering and other accessories. In a dryer, the drive belt rotates the clothes chamber. Drive belts (or sometimes chains) are even used for large applications such as escalators and moving walkways. A drive belt is only as strong as the motor that propels it, and thus depends on the motor's power rating.

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  1. 1

    Calculate the motor's power. Electrically, power equals voltage multiplied by current. If you multiply these two specifications for a motor, you will get a quantity of volt-amperes, or watts. A motor rated at 500 volts and 100 amperes would consume 50,000 watts, or 50 kilowatts.

  2. 2

    Correct the base power rating for losses. Although a motor may be rated at 50 kilowatts, only a certain percentage will transform into the mechanical power of the shaft due to the motor's efficiency. Multiply the base amount of volt-amperes by the efficiency to obtain the motor's mechanical power. If a 50,000 volt-ampere motor has an efficiency of 80 per cent, its mechanical output will be 40 kilowatts.

  3. 3

    Calculate the torque output of the motor. The power of the motor equals its torque times its angular velocity (defined as the number of radians the shaft moves per second). Therefore, you can easily obtain the torque output by dividing the motor power by its angular velocity. A 40 kilowatt motor with an angular velocity of 10 radians per second would have a torque of 40,000W divided by 10 rad/s, which equals 4,000 Newton-meters.

Tips and warnings

  • Note that the calculated torque is the amount of torque that the drive belt can possibly supply. Each cylinder in the loop will only absorb a certain amount of it based on its rotational inertia (the resistance to rotation based on mass and geometry), the frictional resistance of its bearings, and its speed of rotation.

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