Torque is the turning effect of a force.
A force acting away from an axis can produce torque. Torque equals force multiplied by the perpendicular distance from the axis to the force’s line of action. It is measured in newton metres (N·m).
Pushing a door near its hinges produces less turning effect than the same perpendicular push near its handle.
Distance changes the turning effect.
At a lever arm of 0.2 m it produces 20 N·m. At 0.4 m the same force produces 40 N·m.
These distances are perpendicular lever arms. If the direction of the force changes, simply using the full length of the lever may give the wrong result.
Speed tells us how quickly the shaft turns.
Rotational speed is often given in revolutions per minute (rpm). Angular speed expresses the same motion in radians per second (rad/s). One revolution is 2π radians.
A shaft can turn quickly with little torque, or turn slowly while transmitting substantial torque. Neither speed nor torque alone describes its power.
Mechanical power combines torque and speed.
- TorqueTurning effect in N·m.
- Angular speedRotation in rad/s.
- Shaft powerTorque × angular speed, in watts.
60 rpm is one revolution per second, or approximately 6.28 rad/s. Shaft power is approximately 126 W.
Holding and moving are different duties.
A stationary brake can resist torque while transmitting no rotational mechanical power, because its speed is zero. A motor producing torque at zero speed may still draw electrical power and generate heat.
A lift machine must meet the required torque and speed over a journey. Electrical input also covers losses; it is not identical to useful mechanical output.
A closer look
During a changing-speed journey, some torque accelerates rotating parts and connected loads. Torque can also oppose motion, as in braking. The signs of torque and speed indicate the direction of energy transfer.
Check your understanding
Does twice the torque always mean twice the shaft power?
Reveal the explanation
Only if angular speed stays the same. Power depends on both torque and speed.