How do you measure rotary torque?

Rotary torque is the force required to cause something that is free to rotate to do so continuously (360º). Rotary torque can be measured by a rotary torque sensor. A rotary torque sensor or transducer measures rotary torque by converting torque into an electrical signal.

Selecting a torque sensor for your application depends upon a number of considerations, such as long-term reliability, physical constraints, portability and budget.

One way to measure rotary torque is to strain gauge the shaft. In this process, one or more strain gauges are bonded directly to the shaft that rotates. As the shaft deforms due to applied torque, so does the resistance in the bonded, foil strain gauge. A Wheatstone bridge converts the resistance change into a calibrated output signal.

Direct torque sensor measurement is generally preferred to remote or indirect methods of calculating torque. Another way to measure rotary torque is to mount a transducer in the machine train as an in-line, pre-gauged sensor.

Wireless torque cells consist of a rotary torque transformer connected in-line with standard industry flanges. The rotary torque transformer is coupled to the stationary portion of the assembly by wireless transmission. A stationary loop antenna induces power into an embedded antenna on the rotating torque cell.

The induced power supplies the excitation voltage to the strain gauges and powers the radio transmitter mounted in the torque sensor. The radio transmitter then modulates the strain signal for transmission to a stationary antenna.

Still, another way to measure rotary torque, is to use a clamp-on collar. A clamp-on torque cell is a pre-calibrated, bending beam mounted between two collars that clamp onto the shaft. Appropriately spaced knife-edges provide an accurate, reliable shaft torque measurement without marring or modifying the shaft. A clamp-on torque cell handles shaft diameters from three to 32 inches, and as much as 100,000 hp.