What is a Sprag Clutch?
A sprag clutch goes by many names: One-way bearing, freewheel clutch, cam clutch, overrunning clutch. Whatever you call it, sprag clutches are a critical mechanical device in many heavy industrial applications—from automatic transmissions to torque converters, industrial winches to helicopter rotors.
The Design & Functionality of a Sprag Clutch
Sprag clutches are designed to transfer torque in one relative direction, referred to as the “engaged direction,” and to disengage—or “freewheel”—in the other direction. That means in the engaged direction, there is no relative rotation allowed between the two components connected by the sprag clutch.
Both components rotate at the same rate and that rotation is dictated by the driving component. In the disengaged direction, the two components are free to rotate at different rates and directions as long as the driving component doesn’t attempt to over rotate the driven component in the engaged direction.
The core component to these clutches are the sprags. The sprags themselves are larger than the gap between surfaces that they operate against. The sprags have curved surfaces on the top and bottom called the engagement surfaces and use a spring to keep them in constant contact with the operating surfaces. When the clutch is in the freewheeling state the sprags slip against the operating surfaces of the two components because the relative rotation is in the direction that causes the sprags to want to lay over.
When that relative rotation changes, the driving component causes the sprags to stand up in the gap and engage on their engagement surfaces. As they engage, they press into the operating surfaces elastically deforming the surfaces.
As the torque increases, the level of engagement and elastic deformation increases until an equilibrium is met. At that point, the torque is transferred through the clutch and the driving component causes the driven component to rotate at the same speed as one connected unit. This is maintained until the relative rotation is changed again and the sprags disengage.
We hope this has answered your questions about how a sprag clutch works. Contact us and let us know what you think!
Interested in GMN Sprag Clutches?
View our Sprag Clutch Guide resources below for more information!