Sprag Clutch Gap Height

Sprag clutches transmit torque when the sprags engage against the shaft and housing.

This engagement occurs because the sprags are just slightly larger than the gap between the shaft and housing. At the top and bottom of the sprag are special surfaces that are designed to produce an optimal clamping angle against the shaft and housing. These surfaces are called the engagement curve.

If the sprag tries to engage outside this engagement curve, it could slip or roll over. It is critical that the sprags make contact on the engagement curve. But just being on the engagement curve isn’t enough because, as torque increases, the contact point on the engagement curve moves up. The higher the sprag starts on the engagement curve, the less capacity there is remaining for added torque.

This all demonstrates just how important the gap is between the shaft and the housing, because the starting gap value dictates where the sprags contact along that engagement curve or if they are even able to hit the engagement curve.

Our in-house engineers are here to answer questions you may have about sprag clutch gap height in your application. Contact us today!

Ensuring an Optimal Sprag Clutch Gap Height

To keep the starting clutch gap height controlled as much as possible but also allow room for manufacturing, GMN recommends the following shaft and housing tolerances and runouts:

These tolerances and runouts ensure that the sprags will engage in a correct spot on the curve with sufficient room left on the curve for torque transfer no matter where the resultant starting gap is due to the tolerance and runout stack up.

As the sprags engage and roll up the engagement curve, they press outwardly radially against the shaft and housing causing the gap to expand. GMN has set nominal and max torque values and gap heights based on resultant Hertzian pressure values and locations on the engagement curve. Below is a table of the gap heights for the GMN FE 400 and FE 8000 sprag clutches.