How to measure torque
Strain gauges connected as a Wheatstone bridge and protected against humidity and contamination, are applied to the twisting shaft (Figure 1). As the gauges are deformed due to the stresses generated on the shaft, its electrical resistance changes proportionally. A voltage signal is forced to pass through the strain gauges in order to measure this electrical resistance. The signal is amplified, transmitted via a rotating antenna, and then picked up by a stationary receiving antenna. Lastly, it flows to an evaluation unit where it is rectified and measured just as any other process parameter signal. Knowing how much the surface of the shaft has elastically deformed, the dimensions of the shaft, and its modulus of rigidity (a physical property of the shaft’s material), it is possible to determine the torque that the shaft is subject to.
Torque measurement on a bucket elevator
The Service and Diagnostic Centre of PRÜFTECHNIK Condition Monitoring performed torque measurements on a bucket elevator in a cement factory. This service was ordered by the customer because of damage and breaking of some gear teeth inside the gearbox (Figure 2). The customer had reported that the gearbox had been functioning for several years without problems. However, in the last two years, there were two separate incidents only seven months apart.
The strain gauges and the necessary equipment were installed on the output shaft of the gearbox (Figure 3). Alongside the torque measurement components, accelerometers were also mounted at key measurement points. In order to correlate the torque and the vibration readings, VIBGUARD, a multi‑channel condition monitoring system developed by PRÜFTECHNIK was used to allow the synchronous measurement of all signals.
The machine was monitored while on standard production overnight, on the next day in idle state (no load) and during start-up.
Initial static torque readings showed slight vibrations during normal operation (Figure 4), however they are not uncommon in such type of machines, and they do not surpass the nominal torque of the output shaft of the gearbox.
The next step was to measure the load during the startup of the machine. Since this process is short in time and highly variable, the dynamic torque must be measured, which is the AC component of the signal. In order to do this, a high-resolution signal was recorded. Figure 5 shows that although the highest peak is just above the 29.400 Nm nominal torque, a negative torque appears, which is undesirable in any gearbox, and reflects an unusual behavior of the fluid coupling.
Lastly, dynamic torque was also measured while on full load operation. Analyzing the time signal, oscillations could be clearly identified, but with additional superimposed higher amplitude shocks, which are separated roughly 45 seconds from each other. From the original signal, an FFT analysis was made in order to find the frequencies of the torsional vibrations (Figures 6 and 7).
In the frequency spectrum, a dominant peak was seen at 3,5 Hz, which is the passing frequency of the buckets.
It was recommended to the customer to reduce the amount of oil in the fluid coupling, or to reduce the tension in the bucket belt, in order to eliminate the negative torques. In the long term, buckets should be reduced in weight, or the gearbox should be replaced for a stronger one.