Torque in Servo Motors

October 24, 2009

Updated on June 10, 2019 

What is torque in servo motors?

When specifying servo motors for applications like a new machine or CNC retrofit, there are many specifications regarding servo motor performance and sizing. One of the most confusing is the differing torque specifications of the motor. Torque can be specified in Newton meters (Nm) or more commonly foot pounds (lb-ft) or inch pounds (lbf-in).

1Nm  =  0.74 lbf-ft  =  8.85 lbf-in
heidenhain torque motor

Torque – Also called moment of force, it can be defined as the mechanical work generated by the turning effect produced when force is applied to a rotational axis. Or the measure of how much a force acting on an object causes that object to rotate.

Stall Torque (MO) – The torque that is produced by a device when the output rotational speed is zero or the torque load that causes the output rotational speed of a device to become zero – i.e. to cause stalling.

Max. Torque (Mmax) – Also known as peak torque, the greatest amount of torque the motor can generate for a very short time period (typically specified in ms). Put another way, this is required only intermittently, not for sustained periods of time.

Root Mean Square (RMS) Torque – This measurement considers how torque varies and sustains during different stages of a motor’s operation, including accelerations, constant velocity, deceleration and dwell.

Rated Torque (MN) – The maximum continuous torque available at the rated speed that allows the motor to do the work without overheating (typically specified at a temperature). This is the working range of the motor.

When specifying servo motors for a machine tool application specifically, Rated Torque is the most commonly accepted specification. Since shop floor and plant environments are rarely consistent across the board, this kind of measurement, one that takes into account operating conditions, is critical. Ambient temperature has an especially significant impact on torque performance. Servo motors have recommended maximum temperature environments and when that temperature is exceeded, rated torque tends to lessen in relation to RPM.  The opposite is also true when ambient temperatures are dramatically below recommendations.

How to Measure Servo Motor Torque on a Machine

torque curve

Calculating, measuring and monitoring the torque of your servo motors is an important step to ensure your system performs as it should, when it should, regardless of application. Monitoring changes in torque usage or output can also inform maintenance and help prevent catastrophic failure. Here are a few ways you can measure servo motor torque:

Torque Testers or Sensors

These tools can be attached to the engine shaft and provide measurements as the servo motor runs clockwise or counterclockwise. A tester is more of a quality control device used to calibrate during downtime while a sensor can provide real-time readings and record torque measurements.

Velocity Sensors

To use velocity sensor readings to figure torque rating, monitor the acceleration and velocity at the moment of inertia when moving a large load. You’ll find the relationship between torque and speed provides insight into how the motor is performing.

Force Sensors

Use a force sensor at a known distance by multiplying the distance by force minus any known variables or inaccuracies.

Torque Curves

Torque curves are common and simple visual representations that explain a lot about a servo motor’s torque performance. They make it quick and easy to see how things like speed (e.g. RPMs), temperature, peak torque and continuous torque relate.

Top Trends in Direct Drive Motor Technology

Motor technology is advancing rapidly and that includes increases in force density, of which torque is a key element. In fact, getting more output from smaller systems tops our recent list of six trends in direct drive motor technology:

1. Increases in Force Density

2. More Size Options

3. Simplified Integrations

4. More Winding Options

5. New Ways of Integrating

6. Greater Efficiency

To read more about the latest trends in direct drive motor technology, click here. And to learn more about how motors are deployed specifically in CNC machines, from the characteristics that make them useful to how they differ from stepper motors, click here.

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