Directly measuring power transmission loads

Strain gauges and load cells can simplify diagnostics and troubleshooting.

By Phillips D. Rockwell

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Mechanical power transmission components usually perform reliably. When a system fails or doesn’t perform properly, it’s usually for one of the following reasons:

  • Excessive loading beyond design capabilities.
  • Misalignment.
  • Inadequate mounting that allows components to move under load, resulting in misalignment.
  • Insufficient, improper or contaminated lubrication.
  • Unfavorable environmental conditions, such as extreme temperatures or chemical and physical contaminants.
  • Improper maintenance procedures, such as forcing belts onto pulleys without first reducing center distance or installing bearings on shafts by pressing on the outer race.
  • Defective components.

While the failure analyst must address every item in the above list, this article deals primarily with the first item—loads.

A drive train is a group of components that deliver mechanical power from a prime mover, typically an electric motor, to a load. In most cases, power is available from the prime mover in the form of a rotating shaft. The drive train normally delivers power to the load at lower speed and greater torque than is available directly from the prime mover. To operate successfully, the drive train components must sustain loads without damage or excessive wear.

Component manufacturers

Component manufacturers generally can’t help determine the magnitude of externally applied loads, but they don’t want unhappy users. Therefore, their catalogs often provide a wealth of relevant information, including formulae to calculate loads within the drive train once applied loads are known, and information for evaluating component suitability for an application. If there are problems with components, it would be well worthwhile to read the application information in the catalogs carefully.

Also, manufacturers are typically more than willing to help with specific application questions not addressed in the catalogs.

Load determination

In many applications, it’s difficult to determine loads. Some obvious clues can be obtained from:

  • Motor current readings.
  • Traditional engineering analysis.
  • Comparison with existing systems.
  • Assistance from component manufacturers.
  • Research in reference books, trade journals, conference proceedings and the Internet.

If these approaches are impractical or otherwise fail to give satisfactory results, it may be useful to measure the loads directly using strain gauges or load cells.

Strain gauges

These small devices detect strain (i.e. “stretch” or “compression”) in the surface on which they are mounted. They are essentially resistors whose resistance changes when stretched or compressed. With a constant excitation current passing through it, the strain gauge is “read” by detecting changes in voltage associated with the change in resistance.

Strain gauges are bonded securely to the surface of the part being investigated. Accurate positioning gives accurate data. Because strain gauges generate small signals—usually 1 to 20 millivolts at full scale—electrical noise can be a problem. It’s possible for the noise level to be greater than signal strength. There are several ways to compensate:

  • Install a signal conditioner, which amplifies the signal and filters out noise, as closely as possible to the strain gauge.
  • Keep strain gauge leadwires as short as possible because they act like antennas for picking up noise.
  • Use conventional shielding techniques, such as steel conduit and shielded, twisted cable.

When applying noise filters, don’t over-filter. If it’s not clear whether an oscilloscope trace is showing noise or signal or both, turn off the gauge excitation to eliminate the signal. Any remaining amplitude must be from electrical noise. When selecting strain gauge instruments, look for a switch that disables excitation current without turning off the amplifier. Not all instruments have this feature.

Temperature changes can induce errors. Temperature effects can be minimized by:

  • Using temperature-compensated strain gauges.
  • Controlling the temperature of the test area within a narrow range.
  • Calibrating and testing at the same temperature.
  • Using circuit designs with inherent temperature compensation, such as half and full bridges.

When properly installed and protected, strain gauges can be used in harsh environments, including temperatures from -400 to +500° F, and submerged in oil, water and other liquids. Strain gauges have even been installed in the root fillets of gear teeth (see Figure 1).

Figure 1

Load cells

For our purposes, the term load cell means strain gauge load cell. A strain gauge load cell is a load-sensing device with pre-installed strain gauges. The gauges are arranged to control sensitivity to various kinds of loads. For example, torque-sensing load cells are optimized to detect torque and have no sensitivity to moment (bending), tension or compression. Similarly, tension-compression load cells are optimized to detect tension and compression and have no sensitivity to torque or moment. Users never deal directly with strain gauges in load cells; they simply connect the leadwires to an amplifier.

Special load cells are available in the form of pins, bolts and washers.

Strain gauges versus load cells

The primary advantage of commercial load cells over strain gauges is lower cost. Load cells are typically less difficult and time-consuming to install. While load cells are accurate devices, it can be difficult to place them so they see a load that accurately reflects drive train loads.

The advantage of a strain gauge is a more direct and accurate readings because it can be put directly in the power path. This is particularly advantageous for investigating shocks, where a device not directly in the power path would likely see diminished shock loads.

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