Products › Torque Sensors › Rotary vs reaction
A torque sensor measures torque in one of two ways. A rotary sensor turns with the shaft and reads torque while it spins; a reaction sensor stays fixed and reads the torque that tries to twist its mount. Picking the wrong one is the most common torque-measurement mistake, so this guide explains how each works and how to choose.
What is a rotary torque sensor
A rotary (dynamic) torque sensor sits in line with a turning shaft and rotates with it. A strain-gauge bridge on the shaft deforms under torque, and a slip ring or contactless electronics carries the signal off the spinning rotor. It reads torque continuously at speed, and many models add speed so they can report power.
Use it when the torque is on something that turns: a motor, gearbox, pump or dynamometer. Our rotary range runs from the micro and general WTQ-1050/1060 shaft units to the bearingless N40B and the encoder-equipped WTQ2500.
What is a reaction torque sensor
A reaction (static) torque sensor does not rotate. It is fixed between the load and a rigid frame, and it measures the reaction torque that the load feeds back into the mount. With no rotating parts, there are no slip rings or bearings to wear, so it is simpler to install and lower cost.
Use it when the torque is held or turns only a little: torque-wrench calibration, fastener-tightening checks, motor and pump brackets, or limited-rotation jobs. Our reaction range covers the general flagship, the WTQ-9500 flange unit and the compact MRT micro sensor.
Rotary vs reaction at a glance
| Point | Rotary (dynamic) | Reaction (static) |
|---|---|---|
| Motion | Turns with the shaft | Fixed, does not rotate |
| Measures | Torque on a spinning shaft | Torque on a stationary mount |
| Speed | Up to 15,000 rpm (to 30,000 with N40B) | 0 rpm |
| Moving parts | Slip ring or contactless electronics | None to wear |
| Install | Coupled in line, needs alignment | Bolted flange-to-flange, simple |
| Relative cost | Higher | Lower |
| Best for | Motor, gearbox and dyno testing, drivetrains | Torque-wrench calibration, brackets, fastener torque |
How to choose
Three questions settle most cases:
- Does the shaft spin during the measurement? If yes, you need a rotary sensor. If the part is held or barely moves, a reaction sensor is simpler and cheaper.
- What is the torque range and speed? Match the rated capacity above your peak torque, and check the maximum rpm for rotary units.
- How will it mount and signal? In-line shaft coupling or flange-to-flange, and a mV/V bridge or an amplified 4-20 mA or voltage output for your controller.
When in doubt: closed-loop control and continuous running point to rotary; calibration, audits and tight budgets point to reaction.
When to use each
Choose rotary for motor and engine dynamometers, gearbox and pump test stands, and any drivetrain where torque must be read while the shaft turns. Start with the rotary torque sensors.
Choose reaction for torque-wrench and tool calibration, fastener-tightening verification, and bracket or limited-rotation torque. Start with the reaction torque sensors. Not sure which fits your rig? Tell us the application and we will point you to the right unit.
FAQ
What is the difference between a rotary and a reaction torque sensor?
A rotary sensor turns with the shaft and measures torque while it spins; a reaction sensor stays fixed and measures the torque fed back into its mount. Rotary is for spinning shafts, reaction is for static or limited-rotation torque.
Can a reaction torque sensor measure rotating torque?
Only briefly and not continuously. A reaction sensor has no slip rings, so it cannot follow a shaft that keeps turning. For continuous rotation use a rotary sensor.
Which is more accurate?
Both reach 0.1 to 0.3% of full scale. A reaction sensor avoids slip-ring noise and is often the steadier choice for calibration; a rotary sensor is the only option once the shaft must spin.
Which is more cost-effective?
A reaction sensor is usually lower cost because it has no slip rings, bearings or rotating electronics, and it installs more simply.
How do I choose a torque sensor for my application?
Decide whether the shaft spins (rotary) or is held (reaction), match the torque range and speed, then pick the mounting and output your system needs.
Request a quote
Tell us the application, torque range and shaft speed, and we will configure the right rotary or reaction unit.