Drag Torque in Sprag Clutch Applications

The primary function of a sprag clutch is to transfer torque in one direction and freewheel in the other direction.

Most of the design attention is rightfully focused on selecting the best clutch for the torque transmission to maximize number of engagements and prevent over-torque situations. However, it is also important to consider the sprag clutch interaction when in the freewheel state.

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.

Fortunately, GMN’s FE 400 and FE 8000 series clutch insert elements have virtually no drag torque.

They rely on a thin film of lubrication between the sprags and the races to keep a boundary layer, preventing metal-to-metal contact in the freewheeling state. This means that they are able slide with ease and have minimal wear throughout the life of the clutch.

In fact, the drag torque on these GMN clutches is so low that it is virtually impossible to even measure it. Lubrication choices will have a greater impact on the drag torque than the insert elements themselves. For design considerations, it is safe to assume a near 0 drag torque.

The drag torque is a little more notable in units that have built-in bearing support and some sort of seal to keep the lubrication inside the clutch.

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.

Interested in GMN Sprag Clutches?

Check out our Sprag Clutch Guide resources below for more information.