Torque is among the most important of all the measured quantities in applications ranging from characterizing high-power gas turbines, to determining the level of force required to open a screw cap on a medication container. As anyone who studied physics in high school should recall, torque is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt. However, measuring torque accurately can be far from straightforward. This article offers an overview of one approach to reducing uncertainty in torque measurement using an example of turbine engine testing to illustrate the process.
This testing application involved a company that was repurposing large turbine engines, specifically jet engines, by converting them to run on either diesel fuel or natural gas. These converted engines would be used to generate electrical power in remote locations without access to power lines, such as off-shore oil-drilling platforms, undeveloped regions, etc.
As part of this repurposing project, the goal was to create a test stand that would give the engineers the torque data they needed to make design changes that would help them optimize fuel efficiency. Acquiring accurate torque data on the engines’ performance is a critical part of this research. Although the firm had been converting jet engines to electrical power generators for many years, they recently decided to switch to in-line rotating torque sensors to improve their testing accuracy by minimizing torque measurement uncertainty.
The testing requirements called for torque sensors with full-scale capacity ranges of 200 Nm, 1 KNm, 2 KNm, and 130 KNm. The three smaller-capacity sensors had to share the same dimensional package size and operate at speeds up to 22,000 RPM. The largest torque sensor had to operate at up to 4,000 RPM. Like the rest of the test stand, the torque sensors used had to be extremely durable and reliable, given that a testing sequence might run anywhere from a few hours to a few months. Each sensor had to have two separate torque outputs to allow for greater confidence in data accuracy and to serve as a backup if one output failed during testing.
Previously, the test stand had incorporated a non-rotating torque measurement technique that used a lever arm and a load cell attached to a floating or bearing-supported dynamometer (a large electric motor used as a drive or a load) to measure reaction torque (Figure 1). The dynamometer was supported on either side by trunnion bearings. In this method, the lever arm is attached to the dynamometer, and the distance between the center of the shaft and the center of the load cell multiplied by the force equals torque. Although it is a technique that has been around for more than half a century, there are some advantages to measuring reaction torque with a lever arm and load cell. For example, alignment is not critical because the sensor isn’t located on the rotating shaft. Overload protection can be relatively simple to implement. In addition, the system is easy to calibrate because there’s no need to open the rotating shaft.
Despite these advantages, however, there are also some disadvantages associated with measuring reaction torque with a level arm and load cell. The first of these is low dynamic response (typically up to 20 Hz) because the mass of the dynamometer acts as a low-pass filter, which increases the uncertainty in the torque measurement. In addition, the dynamometer must be supported on bearings, which can apply additional torque to the measurement because of friction; these bearings also demand regular maintenance.
Adding an In-Line Torque Sensor
The new torque measurement technique the client chose involved removing the lever arm and load cell, fixing the dynamometer in place so it no longer turned on the trunnion bearings, then adding an in-line torque sensor (Figure 2). This technique employs a flange-to-flange style torque sensor using a digital telemetry system, so it’s totally non-contact, with minimal backlash and support bearings.