Sprag clutches operate in either an engaged or freewheel state. In the engaged state, the sprags are locked against the shaft and housing and torque is being transmitted. Because the sprags are firmly pressing into the shaft and housing, there are no speed effects that would limit the sprag operation, which means there is no limiting speed from the clutch.
When the sprags are in the freewheeling state, speeds typically become an important factor to consider.
Most sprag clutches have a limiting speed. This value is not a ceiling indicating that if the sprags freewheel above that speed, they will fail. Rather, it’s the maximum freewheeling speed where the sprags can still engage. That’s because of the balance between the centrifugal forces acting on a sprag and the spring trying to counteract the centrifugal forces.
Calculating Maximum RPM
Most sprags have a center of mass out in front of the pivot point. So, the centrifugal force creates a torque that causes the sprag to want to lay down and lose contact with the shaft and housing.
This torque can be calculated as the distance from the pivot point to the center of mass (sZ) multiplied by the centrifugal force (FZ).
The spring will work against this torque by applying its own torque in the opposite direction calculated as the distance from the pivot point to where the spring pushes down on the sprag (s) multiplied by the spring force (F).
There is a point where the torque from the centrifugal force becomes larger than the torque from the spring force and the sprags lose contact. That’s where the maximum RPM value comes from.
*An important note: These limiting speeds are not the shaft or housing speeds but the sprag clutch speed. When sprag clutches are floating, they will be rotating at some speed between the shaft and housing speeds. It’s difficult to calculate exactly what speed that would be because it is highly dependent on sliding friction.
To overcome the lift-off effects, some clutch manufacturers put drag clips on the cages to encourage the sprags to stay connected to the component that doesn’t rotate or rotates at the slower rate of the two between the shaft and housing. However, drag clips encourage increased wear on the surface acting against the other component.
It is also possible to increase the spring force in high-speed applications, but there is a trade-off between increased maximum RPM ability and freewheeling wear.
As the spring force increases, the amount of force the sprag exerts against the shaft and housing surface during freewheeling increase, which will decrease the sprag life due to the sliding wear.
The GMN Solution: FE 8000 Series
GMN has designed the FE 8000 series sprags in a special way that eliminates the possibility of sprag lift-off at any speed.
For this series, the center of mass is moved behind the pivot point, which means that the centrifugal forces have the opposite effect on these sprags. The torque direction is reversed. Instead of creating a torque that works against the spring and makes the sprags want to lay over, the torque now encourages the sprags to want to stand up more. This eliminates the need for drag clips and removes any speed limit.
Note: If the sprag clutch has integrated seals, then the speed limit no longer represents the lift-off speed but rather the maximum speed the clutch can operate with the contact seals. Operating above that limit would produce excessive wear on the seals and cause them to fail prematurely.
Interested in GMN Sprag Clutches?
Check out our Sprag Clutch Guide resources below for more information.