Drag Torque in Sprag Clutch Applications

What is Drag Torque?

The sprag clutches use springs acting on the sprags to ensure that they maintain contact with the inner and outer race when freewheeling to be ready for the transition to torque transmission with minimal rotation.

The only time the sprags can lose this contact is if the clutches use the FE 400 series sprag and the speed gets high enough that the centrifugal forces cause the sprags to lift off the races. (This does not happen with the FE 8000 series sprags due to their center of gravity and pivot point.) With that exception, there is always going to be a pressure exerted by the sprags on the raceways

This pressure results in a drag torque, providing some resistance to the freewheel rotation.

Depending on the magnitude of the drag torque, it can have a sizeable impact on the system. Often in the design process, consideration must be taken to overcome the drag torque which could factor into the specification of motors or other components.

GMN has two sealed clutch options:

• The FK series ball bearing clutch uses grease lubrication and has contact seals. These seals result in a drag torque of 0.01 to 0.05 Nm.
• The FPD and FND complete units have oil lubrication. They require heavier contact seals to prevent the oil from egressing. This results in a slightly higher drag torque of 0.1 to 0.3 Nm.

These sprag clutch options create optimal drag torque across applications. To learn more, contact our team of in-house engineers.