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Torque Measurement on Rotating Shafts using Strain Gauge


Torque measurement is of great importance to the engineers, users, and manufacturers of test benches in various industries. It is a crucial parameter used to determine the condition and performance of most of the mechanical measuring systems such as pumps, rotational cutting equipment, gearbox shafts, vehicle axles, and electric motors. Furthermore, measuring torque also helps in reducing downtime, improving product quality, and maximising the energy efficiency of the test equipment.

For instance, in the automotive industry, the demand is to produce vehicles that offer lower fuel consumption, greater operating safety, longer-lasting reliability, and higher levels of comfort. The right test bench will ensure that the vehicles are properly calibrated and the measurement sit within the set regulations and are accurate. In this case, measuring torque of the rotating shaft of the test bench equipment helps in determining the mechanical power of the rotating system and the amount of torque it can withstand. Based on these torque readings, process engineers can speed up or slow down the process, shut down the equipment to avoid any failure or diagnose any potential breakdown in the equipment.

To measure torque by acquiring reliable data from moving objects, strain gauges are very popular due to their simple integration. In this article, we are going to discuss the method of measuring torque on a rotating shaft using strain gauges and the factors that impact the measurement.

How is Torque Measured?

A strain gauge converts torque into an electrical signal. The sensor is bonded to a rotating shaft that deforms when a torque is applied. Generally, four strain gauges are attached to the rotating shaft in the form of a Wheatstone bridge circuit. They are placed precisely at 45 degrees with the shaft axis with gauge 1 and 3 being diametrically opposite, as must gauge 2 and 4. When torque is applied to the shaft, the shaft gets twisted to the direction of rotation, thereby producing shear strain. This causes elongation in gauges 1 and 3 and compression in gauges 2 and 4. These changes in the strain gauges lead to an increase in the circuit resistance due to tensile strain generated by one pair of gauges and a decrease in the circuit resistance due to the compression strain generated by the other pair. This results in an unbalanced bridge, which produces an electrical output corresponding to the applied torque.

In the case of a rotating object, it is challenging to receive the signals from the attached strain gauges and to continuously power them, as compared to static objects. Hence, to transmit power or signal between the rotating strain gauge and the stationary signal receiver, slip rings are used. A slip ring consists of a series of conductive bands that are positioned around the rotating element of the assembly and a group of stationary brushes that contact the rings and transmit the sensor’s signals. This allows for a smooth transfer of electrical power and sensor signal from the strain gauges mounted on the shaft.

However, measuring torque on a rotating shaft using strain gauges is impactful only when the gauge is operated in a low-to-moderate rotating speed. Higher rotating speeds will severely degrade their performance. Also, in heavy-duty applications that deal with dust, dirt, extremes of temperatures, and vibration, the reliability and accuracy of torque output may get compromised. Excessive vibrations can cause wear due to the limited lives of brushes and the rings which eventually lead to premature failure. Similarly, high temperatures can have an impact on the electrical connections of slip rings and bushes and promote corrosion. 

Alternative Solutions

In order to overcome the shortcomings of slip rings of non-performance at high rotating speeds and susceptibility to wear, there are several non-contact measurement systems available. These systems are based on the principles of phase measurement, magnetism, and surface acoustic waves. They generally include pre-installed strain gauge, along with pre-installed slip ring to power the strain gauge bridge and receive signal from the rotating shaft. They are configured without drag parts or bearing, thereby eliminating the wear parts and increasing rotating speeds.

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