The Basics of Torque Measurement

Torque can be divided into two major categories, either static or dynamic. The methods used to measure torque can be further divided into two more categories, either reaction or in-line. Understanding the type of torque to be measured, as well as the different types of torque sensors that are available, will have a profound impact on the accuracy of the resulting data, as well as the cost of the measurement.

In a discussion of static vs. dynamic torque, it is often easiest start with an understanding of the difference between a static and dynamic force. To put it simply, a dynamic force involves acceleration, were a static force does not.

The relationship between dynamic force and acceleration is described by Newton’s second law; F=ma (force equals mass times acceleration). The force required to stop your car with its substantial mass would a dynamic force, as the car must be decelerated. The force exerted by the brake caliper in order to stop that car would be a static force because there is no acceleration of the brake pads involved.

Torque is just a rotational force, or a force through a distance. From the previous discussion, it is considered static if it has no angular acceleration. The torque exerted by a clock spring would be a static torque, since there is no rotation and hence no angular acceleration. The torque transmitted through a cars drive axle as it cruises down the highway (at a constant speed) would be an example of a rotating static torque, because even though there is rotation, at a constant speed there is no acceleration.

The torque produced by the cars engine will be both static and dynamic, depending on where it is measured. If the torque is measured in the crankshaft, there will be large dynamic torque fluctuations as each cylinder fires and its piston rotates the crankshaft.

If the torque is measured in the drive shaft it will be nearly static because the rotational inertia of the flywheel and transmission will dampen the dynamic torque produced by the engine. The torque required to crank up the windows in a car (remember those?) would be an example of a static torque, even though there is a rotational acceleration involved, because both the acceleration and rotational inertia of the crank are very small and the resulting dynamic torque (Torque = rotational inertia x rotational acceleration) will be negligible when compared to the frictional forces involved in the window movement.

This last example illustrates the fact that for most measurement applications, both static and dynamic torques will be involved to some degree. If dynamic torque is a major component of the overall torque or is the torque of interest, special considerations must be made when determining how best to measure it.