TechArchive

This article was originally written in the period 1995-2000

Terry Allen, Managing Director of Industrial Measurements, reviews the various types of torque measurement products.

Historically, torque measurement was confined to test bed applications, usually as part of product development or final product test, and was only rarely considered as a measuring technique for end users. The standard configuration for test beds was to mount the loading machine, either water brake, dc or ac generator, on a bearing assembly which allowed the loading machine to pivot around the drive axis. The resultant torque reaction was measured by a spring balance.

There were, however, severe drawbacks to a spring balance arrangement, in that the measuring instrument was torsionally soft which produced large angular movements in the torque reaction arm. Whilst this arrangement was very satisfactory for steady state running, it had limitations, and was liable to errors, on internal combustion engine testing where the instantaneous torque fluctuations are high.

With the development of accurate and reliable strain gauge load cells, the performance of swinging arm dynamometers was greatly improved and a well engineered system is capable of making dynamic torque measurements. There are, however, some inherent limitations in the ultimate accuracy of swinging arm machines. Because the load machine has to be suspended on trunion bearings, which are usually ball or roller types, and the angular movements are very small, the load throughout the life of the machine is taken only on one roller or one ball. There is also a potential problem with cooling water pipes or electrical cables producing a torque load on the machine. With care, however, the effect can be reduced to acceptable levels.

There is also an important economic consideration to be taken into account, in that the trunion bearing assembly forms a very large proportion of the machine cost, and electrical machines need specially designed end casings to accept the bearing assembly. These economic considerations have led to an alternative technique for torque measurement, using torque transducers actually mounted in the machine drive line where the measurement system detects the torsional stress produced in the drive.

Torque transducers

The general concept of placing a measuring device in a machinery drive line to measure torque sounds, on the face of it, very simple but, unlike most transducers, they have to transmit what they are measuring and any mechanical failure in the transducer usually results in a total loss of drive between the drive and driven machinery. The transducer must also be insensitive to axial loads or bending loads which may occur due to misalignment in the drive line, and it must be unaffected by speed. Last, but not least, it must have a traceable calibration.

Phase Displacement Torquemeters

Torque transducers have been used with varying degrees of success from the early 1930s. One of the earliest systems for large shafts was installed on the liner ‘Queen Mary’ and it needed a measuring length of some 4 metres of the final drive shaft. It employed the phase displacement principle of measurement. There appear to be no existing reports of its performance, although if the shear modulus of the steel was known, it could well have achieved an accuracy of 5% or better, bearing in mind all the electronics at that time would be based upon early triodes or tetrode thermionic valves and long-term accuracy stability may have been a problem. The phase displacement torquemeter was for many years the only type available, and was developed by Rolls Royce in the 1950s. This well-established system measures the twist between a pair of toothed flanges integral with a shaft of known stiffness. These generate sinusoidal signals in magnetic pickups in the form of internally toothed rings and circumferential coils. The resultant phase change of the two signals varies linearly with torque and is nowadays accurately measured by digital electronics.

The principal advantages of the system are the absence of rotating electronics allowing very high speed and temperature capability and hence reliability. They are therefore particularly suited to condition monitoring of large turbomachines in the oil, gas and petrochemical industries. They are widely used for test stand applications including aero engine and accessory development which require both high accuracy (0-1%) and high speed capability (up to 140,000 rpm). Typically these are pedestal mounted can be user calibrated, and have the facility to readily interchange torsion shafts to cover a wide torque range at full capacity. Whilst they do not require any rotating electronics, they do require a long gauge length in order to achieve an angular displacement which can be measured reliably. The stator assembly surrounding the shaft requires very careful engineering as the system cannot differentiate between relative movements of the teeth on the shaft or the teeth on the stator coils. Such transducers have an important part to play in the overall spectrum of torque measuring transducers.

Strain gauge torque transducers

The next major development of torque transducers came in the late 1950s with the development of reliable strain gauges and associated bonding techniques. Initially, virtually all strain gauge work was used on a temporary basis, mainly in the aircraft industry on components and structures, and this led to a feeling amongst engineers that strain gauges were, in fact, only suitable for temporary measurement situations, and it is true that even today we still find a certain reluctance to believe that strain gauges are suitable for long term measurements. But if we consider just the load cell market there are literally tens of thousands of load cells, based on strain gauge principles, which are in use all day, every day, offering long term stability with very high accuracy.

The first strain gauge torque transducers employed a system of slip rings in order to make the connections from the casing to the shaft. Several companies manufacture this type of transducer which is particularly suited to the lower speed end of the market associated with automatic component assembly machines and automatic control of fastener systems.

Because the slip rings are carrying only millivolt signals from the strain gauges, the materials for both the slip rings and the brushes have to be very carefully selected, the normal procedure being to use coin silver for the slip rings and silver graphite for the brush gear. The limiting performance for any slip ring assembly is based on the maximum contact sliding speed for the combination of materials selected.

The biggest step forward in torquemeter design came about with the availability of high performance, high temperature electronic components developed for the aerospace and military markets. This gave instrumentation engineers a whole new technology which would realise the full potential of the strain gauge torquemeter and offer a reliable, cost-effective product capable of being engineered into most drive line configurations.

A most versatile type of transducer utilises strain gauges with a non-contacting ‘telemetry’ data transmission system. There are few limitations on shaft length or diameter, although measurement accuracy can be affected by shaft lengths less than half their diameters. The system is integrated into proprietary spacer couplings and is also available for retrofit applications on shaft diameters up to one metre and above. Thus it is suitable for many industries, including Aerospace, Automotive, Food, Marine, Power Generation and Steel. It allows torque measurement from around 0.1Nm to several MNm and can provide fast transient response at maximum speeds.

The data transmission system between rotating and stationary coils is not affected by dirt, oil, grease or water and can allow high degrees of misalignment. Some retrofit types, however, do require accurate alignment at installation.

Several companies have transducers aimed at a particular market sector. However, perhaps there is no such thing as a standard torque transducer because there are virtually infinite combinations of shaft speeds, torques and coupling types.

The Future

Existing products are being updated and institutions are looking at using laser technology and other methods of torque measurement. However, it will take an invention or something equally revolutionary to change the technology of torque measurement, whilst maintaining the versatility provided by existing products.

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