Torque - Work done and Power transmitted

The work done and power transmitted by a constant torque

Work done

Work done is the force multiplied with the distance moved by the force - and can be expressed as

W = F s                                  (1)


W = work done (J, Nm)

F = force (N)

s = distance moved by force (s)

For an angular motion

Angular motion - force, torque, work and power

the work done can be expressed as

W = F θ r

   = T θ                                     (2)


W = work (Joules)

θ = angle (radians)

r = radius (m)

T = torque or moment (Nm)

Power transmitted

Power is the ratio between the work done and the time taken and can be expressed as

 P = W / dt

    = T θ / dt

    = T ω

    = 2 π n T 

    = 2 π (nrpm / 60) T

    = 0.105 nrpm T                                  (3)


P = power (Watts)

dt = time taken (s)

ω =  θ / dt = 2 π n = angular velocity (rad/s)

n = speed (rev/s)

nrpm = speed (rev/min, rpm)

Note! - a machine must rotate to produce power! A machine with no rotation can deliver torque - like an electric motor - but since no distance is moved by force - no power is produced. As soon as the machine starts to rotate power is produced.

Example - required Torque to produce Power

A machine rotates with speed 3000 rev/min (rpm) and consumes 5 kW. The torque at the shaft can be calculated by modifying (3) to

T = P / 2 π n

    = (5 kW) (1000 W/kW) / 2 π (3000 rev/min) / (60 sec/min)

    = 15.9 Nm 

Related Topics

  • Mechanics - Forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more

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