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.
Ensuring an Optimal Gap Height
To keep the starting 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 8000sprag clutches.
|GMN Series||Starting Gap||Nominal Torque Gap||Maximum Torque Gap|
|FE 400||4.00 mm||4.06 mm||4.07 mm|
|FE 8000||8.33 mm||8.46 mm||8.60 mm|
If the gap height exceeds the maximum value due to over torquing or from tolerance stack up outside the GMN recommended range, the contact angle of the sprags to the shaft and housing will start increasing and can either cause the sprags to slip or roll over. So, it is important to limit the design parameters with the tolerances and application torque to prevent the sprags from exceeding the maximum gap height and operational zone on the engagement curve.
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