| 摘要 |
Rotatable input and output hubs each have opposed friction faces separated by a bearing means through which at least a substantial torque is transmitted. A helical spring is attached to the output hub and surrounds a portion of the input and output hubs, but is normally out of engagement with the input hub. A pressure ring also surrounds portions of the input and output hubs and in turn is surrounded by the helical spring. The pressure ring is composed of at least three segments, each having a pair of beveled faces directed toward one another and opposing oppositely beveled faces on the input and output hubs, respectively. The free end of helical clutch spring is preferably magnetically selectively drawn into contact with the driven input hub by an energizing magnetic field which in turn causes the spring to wrap down and exert radially inward pressure on the pressure ring. The input hub includes a portion of a magnetic circuit including an air gap into which the last turn of the helical spring is drawn as the magnetic field is energized.
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