Torque is best described as twisting effort. A simple example is using one’s wrist to turn a screwdriver to tighten a screw. The torque (twisting effort) to tighten the screw originates in the wrist. It is then transmitted through the screwdriver and ends up tightening the screw.
All electric motors and reciprocating engines (steam, petrol or diesel) which have a rotating shaft output, produce torque. From the motor or engine’s shaft, torque is usually transmitted to where it is needed by means of pulleys and belts, or by pairs of gears.
Torque can be multiplied by these pulley or gear drives. Meccano modellers would well know that a small pulley or gear on a motor, in mesh with a larger one, makes a drive appear to get more powerful. The downside is that the rotational speed of the driven pulley or gear is reduced in direct inverse to the torque multiplication.
This is because the power output of a motor is the multiple of torque and RPM. At a given motor power level, if torque is increased by gears or pulleys, speed (as RPM) is reduced proportionally, and vice versa. There is no such thing as getting something for nothing in engineering!
Torque is actually closely related to force. A force can create torque, and torque can create a force. Think of a rack and pinion. If the rack has a force applied in a line along its length, it will rotate the pinion and its shaft. The torque will then be transmitted along the shaft to where it is needed in a mechanism.
The opposite is also true: torque in the shaft will rotate the pinion and push the rack with a straight-line force to be used for some useful task. An electric motor used to open and close a sliding door is a good example.
It might then be thought that a crank can achieve the same motion with a force, but this is not the case. Returning to the rack and pinion example above, it will be noted that the rack is at a fixed distance from the centre of rotation of the pinion. However, a crank rotates in a circle on each revolution in relation to the applied line of force from the piston through the centre of the crank’s shaft. A connecting rod is used to cope with the changing angular alignment, but the reality is that use of a crank will result in a sinusoidal relationship between the piston force and the torque in the shaft.
This relationship is seen in a vertical reciprocating engine where the stationary piston starts descending from top dead centre where it was imparting no torque on the crankshaft (because the piston and crank are in a straight in line). When the piston reaches maximum speed halfway down the cylinder, the crank is at 90 degrees to the piston’s force and maximum torque is applied through the crank to the shaft. At bottom dead centre where the piston momentarily stops, torque on the shaft is zero again. The process repeats on the upward stroke of a double-acting steam engine. With a single acting petrol or diesel engine, the shaft torque actually becomes negative, because the rotating crank has to push the piston back up to top dead centre. Torque in a crankshaft is therefore anything but constant through each revolution.
A spanner is another example of conversion of force to torque. A novice user will quickly find that maximum tightening of a nut occurs when the spanner is held at right angles to their pulling arm.
In the metric system, the unit of torque is the Newton-metre (Nm), where Newton is the amount of force and metres is the offset distance of the force from the centre of rotation of the torque. Thus, a spanner of length 250mm (0.25m) being pulled by a force of 98 Newtons (10kg) would result in a torque of:
98 x 0.25 = 24.5 Nm.
From the above discussion we should realise that as humans, we are lucky to be able to produce rotational torque from our wrists, or a straight-line force from our arms, as required. Not many animals can do that!