How In-Line Bearingless Torque Sensors Can Help Increase Turbo Machinery Efficiency

The first phase of any effort to boost the efficiency of a piece of machinery in order to lower its operating costs and reduce power consumption is gathering accurate data on the machine’s initial performance, particularly torque data. The ability to measure torque reliably is crucial for applications ranging from determining the level of force required to lower an automobile’s window to testing a tugboat’s high-horsepower engines. The nature of turbo machinery can make it especially challenging to measure torque accurately. Electric motors, pumps, compressors and turbine engines can all generate high torques at high RPMs. In addition, torsional vibration can sometimes lead to premature failures in the driveline. Historically, this has created limitations on how torque can be measured both accurately and safely. This article addresses some of the basics associated with minimizing torque measurement uncertainty to allow making more informed modifications to a machine’s design or operating parameters. As every former first-year engineering student should recall, “torque” is defined as the amount of force needed to rotate an object about an axis, fulcrum, or pivot. The mathematical equation for torque is “force multiplied by distance.” To illustrate, imagine a foot-long lever arm attached to the center of a wheel; if hanging a one-pound weight on the end of the lever arm makes the wheel turn, the force needed to turn the wheel can be described as 1 lb-ft of torque.

Most bearingless torque sensors use foil strain gauges as the means of measuring stress (Figure 2). The strain gauges are configured and wired in what’s known as a Wheatstone bridge. A voltage (preferably an AC voltage to improve noise immunity) is applied to the circuit. This is known as an AC carrier frequency amplifier. When the sensor experiences mechanical torque, the resistance of the Wheatstone bridge changes, which outputs an analog voltage that is proportional to the torque applied. Because the rotor and stator are not in physical contact with each other, the analog voltage on the rotor is converted to a digital signal, which is then sent to the stator via a digital telemetry system. There are two antennas, one on the rotor that spins and one on the stator that is stationary. The stator receives the digital signal and converts it into any one of a number of usable outputs. If done correctly, this method of measuring torque can be very accurate and highly repeatable. However, there are limitations as to how fast the electronic parts can spin. Some bearingless torque sensors, such as the HBM T40B or T12, can also measure RPMs or angle of rotation with a reference pulse.

Keeping most of the weight near the bearing of the power absorber reduces the “sag” in the driveline. If a torsional analysis shows a critical speed still exists in the testing RPM range, a support bearing may be needed. Given that a bearingless torque sensor is unsupported in the driveline, using a “dual flex” type coupling is generally recommended to remove any angular and parallel misalignments. It is very important to remove any parasitic loads that could damage the torque sensor during use. Parasitic loads, depending on the type and size, will also add errors to the torque reading. In summary, lowering operating costs, reducing emissions, and improving product quality are becoming more important every year, making the need for more accurate test data and more reliable test equipment critical. Fortunately, very accurate ways of measuring rotating torque in applications with higher operating speeds are increasingly available. Inserting a torque sensor directly into a driveline to capture and study dynamic data can help engineers improve the performance and efficiency of their turbo machinery. High response times make it possible to measure torsional vibrations that may lead to mechanical failures on the production floor. By eliminating parts that contribute to errors in the torque data, simplifying the driveline design can minimize test stand uncertainty. Bearingless torque sensors are one important new way to help to reduce data uncertainty while decreasing equipment maintenance requirements and, potentially, downtime.