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Design and Theory

When an actuator is under load and the power to the motor is cut the actuator could back drive.  Or in other words the load could continue to move or not hold a load when the actuator not powered.  In most cases this is not desirable.  The following video discusses how actuators can be designed to prevent back driving.

How back driving is prevented

How a wrap spring brake works

Wrap spring brakes are commonly used as load holding brakes in industrial actuators.  Wrap spring brakes work well and are very reliable.  But are difficult to describe how they work in the actuator.  If interested in how a wrap spring brake actually does it's job read on.  Otherwise proceed to the next topic.

The following describes the brake's function under different conditions:

Extending Under Compression Load

When extending under a compression load the wrap spring slips on compression hub. Compression needle bearing rotates. All other parts stationary.

Holding Compression load

When holding a compression load the wrap spring couples the center hub to compression hub. The compression needle bearing does not rotate. The compression friction disc will not allow load to back drive screw.


Retracting Under Tension Load

When retracting under a tension load the wrap spring slips on tension hub. The tension needle bearing rotates. All other parts remain stationary.


Holding a Tension Load

When holding a tension load the wrap spring couples the center hub to tension hub.  The tension needle bearing does not rotate. The tension friction disc will not allow load to back drive screw.

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