Torque monitoring − the secret weapon

Vertical Roller Mill in a Cement Plant
Vertical Roller Mill in a Cement Plant
Typical components needed for measuring torque
Figure 1. Typical components needed for measuring torque
Broken teeth on the gearbox of a bucket elevator
Figure 2. Broken teeth on the gearbox of a bucket elevator

Torque monitoring is an ever-increasing trend, present in industries like cement, marine and power. It provides users with a deeper understanding of the operational demands of their machines when it is difficult to diagnose a problem through standard vibration analysis. This way, users can make the necessary corrections to ensure smoother operation and avoid unexpected failures.

Machines are tools that are specially designed to perform specific tasks. In order to do that, they convert energy into work. Let us look at a bucket elevator for example. The energy flows through each of the components of the machine train, from the motor through the gearbox to the bucket elevator. The motor turns electrical energy into mechanical energy, the shaft of the motor and the input shaft of the gearbox are linked together through a coupling, and the output shaft of the gearbox is connected to the bucket elevator’s driving shaft. Like any ordinary chain, a machine train is only as strong as its weakest link.
Depending on the operational demands of the bucket elevator, which we can see as the load, the amount of power delivered by the motor will vary. The actual force making every shaft rotate is called torque, and it is directly proportional to
the power. This means that when we want to maintain a certain operating speed on our process, and the load increases,
the motor must deliver more power, and that is achieved by increasing the torque.

Taking a closer look at the machine elements involved into this transmission chain, such as all the gear teeth of the gearbox, the coupling, the different shafts, we can find a great number of parts that can break, if they are not able to resist the torsion due to the applied torque. Breakage in any of these elements can be attributed to any number of reasons, ranging from design flaws, bad material quality, wrong selection of the machine components, incorrect operation of the machine, or extreme operating conditions that were not catered for in the machine’s original design, just to name a few.

What is condition monitoring?
Condition monitoring is one of the key aspects of predictive and proactive maintenance strategies. By analogy to a doctor and his patient, the maintenance specialist is in charge of monitoring the health of his machines. However, instead of measuring blood pressure, pulse, respiratory rate, etc., the maintenance specialist measures vibration, torque, oil quality and temperature, among others. By doing this, he is able to quickly detect changes in the machine’s condition and to take the necessary steps in order to get ahead of any potential failure, keeping the machine and the production running smoothly. The benefits of a properly implemented condition monitoring program are numerous: fewer unplanned shutdowns, lower maintenance costs, fewer spare parts needed in stock, increased productivity and uptime, and an extended machinery lifetime.

 

 

Strain gauge and mounting place on the shaft
Figure 3a. Strain gauge on the shaft
Static torque readings, stored every second
Figure 3b. Mounting place on the shaft
Dynamic torque measurement during machine startup
Figure 4. Static torque readings, stored every second
Dynamic torque during operation at full load
Figure 5. Dynamic torque measurement during machine startup
Frequency spectrum from the dynamic torque signal
Figure 6. Dynamic torque during operation at full load
Figure 7. Frequency spectrum from the dynamic torque signal

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.

 

 

Conclusion

When it comes to protecting your assets, combining torque measurements with condition and process monitoring offers a unique advantage. By closely analyzing torque, you can directly assess the operational requirements of a machine, detect incorrect operation and even design flaws.

Measuring torque can be performed as a service, carried out temporarily with a portable data collector, or installed permanently, whereby the torque signal will be continuously analyzed by an online condition monitoring system. No matter the age of the machine, torque measurement can be implemented at
any time. For production critical machines, it is highly recommended to
implement an online vibration and torque monitoring system. In most cases, the repair costs of an unexpected machine breakdown and the total value of the production loss can easily outweigh the initial investment of the implementation of the said system.

 

Published in: World Cement (03/2014)

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