Quality Magazine

The Basics of Torque Testing

September 1, 2011

What is torque and when is torque testing necessary?

Torque is the angular force required to turn something. For example, the force required to turn the steering wheel as you round a corner, or the force provided by an airplane engine to turn a propeller are both simple examples of torques. Torque testing involves measuring the amount of torque being applied to an object.

Two of the more common applications for torque testing are in fastening and in products with rotating parts such as motors, engines or transmissions. By measuring and analyzing the torque characteristics in such applications it is possible to accurately determine not only the quality of the part or process, but also the root cause of a wide variety of defects.



Torque testing is essential on products with rotating parts such as engines.

How is torque measured?

Torque testing is accomplished by inserting a torque transducer between the tool applying the force and the item against which the force is being applied. There are two different approaches to torque measurement: reaction and in-line. In-line torque testing measures the torque required to turn the rotating part; reaction measures the amount required to prevent the part from rotating. Specialized sensors are available for each approach.

Common Applications

Assembling a large or complex product can often involve hundreds, thousands or even millions of fasteners, many of which are bolts or screws that are tightened by a torque driver of some sort. When a fastener fails, the integrity of the entire product can be affected, so it is important for the manufacturer to be able to ensure that each fastener has been properly tightened and has not been damaged or weakened during the fastening process.

The conventional approach to ensuring a well-tightened fastener is to turn until a pre-defined maximum torque value is reached. However, that may not be enough to ensure that the fastener was properly installed. For instance, if it was mis-threaded, the part won’t have turned through the proper number of rotations, typically hitting the maximum torque value much earlier than it should have. To eliminate such defects, a more comprehensive measurement is required.

One proven approach is to use process signature analysis to characterize whether the fastener was properly installed. Process signatures capture the complete waveform of critical manufacturing processes, providing unparalleled insight into variables that contribute to product quality. In this case, the entire torque-versus-angle signature would be collected and analyzed. The total rotation angle when the maximum torque was achieved would quickly reveal the mis-threading, identifying not only the presence of the compromised fastener, but also the root cause of the defect.



Distinctive variations in the process signature indicate the root cause of quality issues or part failure.

Torque-to-turn

These measurements are an essential validation and diagnostic tool for any assemblies where the primary function involves rotating parts, such as engines, motors, transmissions or axles. There are two basic categories of torque-to-turn tests: breakaway torque and running torque. Breakaway torque is defined as how much torque is required to start a part’s rotation from a stationary position. Running torque is the determination of how much torque is required to keep the part rotating at a constant angular velocity once it starts rotating.

To explain breakaway torque in more detail, consider a crankshaft within an engine. How much torque is required to get the crankshaft moving? This is the torque necessary to overcome the inertia of the crankshaft and the static friction between it and the engine block; the greater the friction, the greater the breakaway torque required. This can be caused by a number of problems: insufficient oil, out of tolerance parts, damaged or defective bearings, debris around the crankshaft, amongst others. As in the case of the fastener, the manufacturer could simply record the peak torque value and use that to determine if the engine assembly is good or bad. However, to determine the source of the problem requires more detailed analysis. With a testing modality that uses process signatures, the entire torque versus time signature is recorded. The breakaway torque value is extracted as the maximum applied torque before the crankshaft begins to move. If the part displays a high breakaway torque value, the manufacturer can identify the cause of the problem by examining the process signature. For example, if there is debris, this will present as “chattering” in the waveform, or if components are misaligned, this will appear as undulations in the signature.

Distinctive variations in the process signature indicate the root cause of quality issues or part failure.

To explain running torque, further consider the crankshaft example. How much torque is required to keep the crankshaft moving? Again, a measurement of the average torque will indicate if there’s a problem, but reveal nothing of the root cause. By measuring torque versus angle at high resolution, the details of the waveform curve provide insight into the process. When applied to an internal combustion engine, for example, the peaks and valleys as each of the pistons fire can actually be seen, making it possible to identify issues associated with the pistons and seals related to the amount of compression that is achieved. In an electric motor, the effects of each of the stator brushes on each rotation can be seen. Damaged components have telltale signs that appear in the waveform – bearings show up as “nicks” in the signature each time the bearing rotates, damaged gears show up as a cyclic discrepancy that reappears on each gear rotation, geometric imperfections in the gears (for example, gears that are not perfectly circular) can show up as a slower undulation superimposed upon the usual waveform characteristics.

Summary

Torque measurements are essential tools used by manufacturers to determine good assembly from bad. When using an approach that utilizes process signatures, torque measurements can also provide valuable information on the root cause of problems identified in the test. By performing high-resolution measurements and collecting the entire waveform rather than merely considering maximum or average values, manufacturers are better able to identify and address problems at the source.