1. Field of the Invention
This invention relates to backstopping clutches and, in particular, to an improved frictional insert for use in such clutches.
2. Disclosure of Related Art
Backstopping clutches are used to prevent transfer of a back drive torque from a driven device to a driving, or torque generating, device. The clutch is typically disposed between the output shaft of the driving device and the input shaft of the driven device. The clutch allows torque to be transferred from the output shaft of the driving device to the input shaft of the driven device in order to rotate the input shaft in either rotational direction. The clutch, however, prevents a back driving torque from being transferred from the input shaft of the driven device to the output shaft of the driving device.
A conventional backstopping clutch includes a hollow outer race having a generally cylindrical inner surface. The clutch also includes an input member disposed within the outer race and coupled to the output shaft of the driving device. A pair of opposed brake shoes are coupled to the input member and rotate with the input member as torque is transferred from the driving device to the driven device. The brake shoes have generally arcuate surfaces that correspond to, and are normally spaced from, the inner surface of the race. The clutch further includes an output member that is connected to the input shaft of the driven device and one or more output pins. Each of the output pins has a first portion disposed between the two brake shoes and a second portion disposed within a corresponding opening in the output member. A lubricant is generally used within the clutch to reduce wear on the components of the clutch.
During normal operation, rotation of the output shaft of the driving device causes a corresponding rotation in the input member of the clutch, and consequently, the brake shoes, output pins and output member of the clutch, to thereby transfer torque to the input shaft of the driven device. In the event that the driven device attempts to generate a back drive torque through the clutch, rotation of the output member causes the output pins to become skewed. As a result, the brake shoes are forced apart from one another and into frictional engagement with the inner surface of the outer race of the clutch.
The above-described clutch often include hardened inserts disposed within slots formed in the arcuate surfaces of the brake shoes. The inserts frictionally engage the inner surface of the race when the brake shoes are forced apart as described above and are provided to reduce wear on the brake shoes. After a period of time, however, the hardened inserts in conventional clutches tend to "hydroplane" or slip relative to the inner surface of the race due to the presence of the lubricant within the clutch. As a result, the inserts do not frictionally engage the race of the clutch and the clutch may fail to prevent transfer of a back driving torque.
There is thus a need for an improved back-stopping clutch that will minimize or eliminate one or more of the above-mentioned deficiencies.