Torque sensor helps prove gearbox reliability

A test rig built at Centa Transmissions is enabling the company to guarantee that the precision gearboxes it supplies to nuclear industry customers will not fail prematurely. Despite its critical role, the rig is essentially quite simple, in that a motor drives the test unit against a load created by one of Centa's own industrial disc brakes. The test runs initially for three hours at the full working load, and then is increased to 300 per cent load for another hour. At the heart of the rig is a non-contact sensor that constantly monitors the torque in the gearbox, generating a performance profile that can be compared with the ideal performance standard.

The sensor is a TorqSense unit, developed and manufactured by Sensor Technology. Because it is a non-contact, the TorqSense does not create a dynamic load that would otherwise have to be accounted for in the analysis of the output signal.

Michael Sykes, managing director of Centa Transmissions, says: "The output reading we get is a true refection of the performance of the gearbox, we do not even have to allow for a constant offset. This means our calculations, which we share with the client, inspectorate, etc, etc, are very simple and easily understood even by non-technical people."

Gearbox application

The duty the gearboxes are destined for takes place in an environment where reliability has to be 100 per cent. They are used in completely automated scoop mechanisms that collect small amounts of 'high-activity liquor' from the reactor cooling systems, which is sealed into thick-walled ceramic flasks for long-term storage until the radioactivity has decayed to safe levels.

Sykes adds: "This is at the very core of the nuclear plant, where a component or system breakdown would mean shutting down all operations for months, automated/unmanned removal of the faulty parts, sealing into a secure flask and automated installation of a replacement. The cost would be millions of pounds - at the very least. To avoid this, everything has to be lifetime guaranteed to demanding criteria."

Instead of making physical contact with the output shaft of the gearbox, the TorqSense unit uses a radio frequency (RF) link. This picks up signals from two piezo ceramic combs glued onto the shaft, which vary as the combs deflect under the effects of rotation. As the torque increases, the combs change their electrical resistance proportionally to the change in frequency of the Surface Acoustic Waves (SAWs) caused by the rotating shaft.

Sykes explains: "The combs function as strain gauges that measure changes in resonant frequency of the rotating shaft. The really clever bit is that the RF coupling is used to supply power to the strain gauges - being piezo-based, they need less than one milliwatt of power, which is transmittable as a radio wave."

Centa's rig therefore does not need any slip rings, the effectiveness of which would be difficult to maintain during extended and/or demanding test run duties. Other advantages claimed to be inherent with SAW technology, and therefore TorqSense sensors, include no load imparted to the drive mechanism under investigation, a broader signal bandwidth than other analogue-based technologies and elimination of electromagnetic interference.

Sykes concludes: "Simplicity is a great virtue for the nuclear installations. There is less that could go wrong, maintenance is easier, inspections and approvals are simpler. In this respect, both our test rig and Sensor Technology's TorqSense are ideal for the industry."

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