|GMN Recommendation: Only steel should be used as the mating surfaces for the sprags|
|At the very basic level, the sprags ability to engage and stay connected against the mating surfaces is based on the sliding friction of the surfaces. The angle of engagement between the sprag and the mating surfaces is designed so that the tangent of the maximum engagement force is less than the force to overcome static friction of steel. That means if steel is used and everything else is within the design requirements, the sprags will not slip. However, if other materials are used, even if they meet all the other requirements, the change in coefficient of static friction could cause the sprags to slip. For this reason, GMN recommends only steel mating surfaces.|
|GMN Recommendation: A hardness of HRC 60 to 64 with an effective hardness depth of at least 1.3mm|
|In addition to the type of material for the mating surfaces, the hardness and hardness depth are also critical in proper engagement of the sprags. As the sprags engage, they elastically deform the mating surfaces until an equilibrium is reached between the surface pressure applied by the torque and the resistance from the mating surfaces. If the surfaces are soft due to insufficient hardness or hardness depth, there will be less resistance to the sprags which will cause them to engage higher up on the engagement curve. That could result in a substantially decreased overall torque capacity and could cause premature failure.|
|GMN Recommendation: A surface finish, Rz, of less than 2.5μm|
|In overrunning operation, the surface finish of the mating components plays a major role in the life of the sprags. If the surface is too rough, the lubrication will not be enough to prevent degradation of the sprags as they slide over the surfaces.|
|Note: the system is unable to incorporate all the material requirements for the mating surfaces GMN offers sprag clutch products that incorporate hardened steel inner and outer rings.|
Wall Thickness Requirements
|GMN Recommendation: The sprags should engage against a solid shaft and the housing should have a ratio of the housing outer diameter to the housing inner diameter be 1.4 or greater|
|Similar issues to the insufficient hardness and hardness depth can occur if the wall thickness of the mating surfaces is too thin. As the sprags start to push into the surfaces, if the walls are too thin, they won’t be able to resist the elastic deformation as much, decreasing the torque capacity.|
In some applications these recommendations may not possible, and that doesn’t mean that the sprags will fail. However, a decreased torque capacity should be expected depending on how far the actual application conditions deviate from GMN’s recommendations.
For more information on wall thickness and material requirements for sprag clutches, contact our team of engineers at GMN Bearing USA.
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