The entire purpose of a sprag clutch is to be able to transfer torque when the sprags are engaged. This makes torque capacity one of the most important factors considered when selecting a clutch.
Torque Capacity in Action
When freewheeling, the sprags are in sliding contact with the shaft and housing. When the relative rotation changes, the sprags lock against the shaft and housing surfaces. As the torque starts to increase, the sprags elastically deform the mating surfaces which provides the resistance to the continued twisting of the sprags until an equilibrium is achieved between the torque transmitting and the resistance from the mating surfaces.
Because the sprags are creating that elastic deformation, the torque capacity of the clutches comes down to what happens between the sprags and the mating surfaces. The contact area of the sprags creates a pressure against the mating surfaces that is causing the elastic deformation.
That mating pressure is called Hertzian pressure, which is a term used for contact stresses or pressures between two curved components. If that pressure gets too high, the sprags and the mating surfaces will go from elastic to plastic deformation which will cause system failure. GMN uses a value of 4,200 MPa for the nominal pressure, and 4,600 MPa for the maximum pressure.
How Will I Know My Torque Capacity?
The torque capacity is then determined working backwards from the nominal and maximum Hertzian pressures factoring in the surface area and then the number of sprags.
For this reason, simple things like going from a Z-Spring to an M-Spring sprag can increase the torque capacity because the sprags with the M-Spring don’t feature the spring notch on the outside surface of the sprag which results in higher surface area.
Similarly, using a wider sprag will also increase the torque capacity because of the increased surface area per sprag. Also, going with a larger diameter clutch will allow for room for more sprags which divides up the torque, allowing for more per sprag for higher torque.
To learn more about GMN Sprag Clutches, torque, and Hertzian pressure, contact us today!
Interested in GMN Sprag Clutches?
Check out our Sprag Clutch Guide resources below for more information.