| 摘要 |
<p>A rotatable shaft element (311) has a disc part secured thereto. An electromagnet (314) is located relative to the part (312) so that an armature (315) of the electromagnet (314) which engages the circumference of the part (312) is also maintained at a minimum distance from the electromagnet (314) even when the latter is de-energised, but can enter a recess (313) to arrest rotation in one direction. Rotation in the opposite direction causes the armature (315) to return to the periphery of the part (312). Energisation of the electromagnet (314) allows the recess (313) to pass the armature (315) in either direction. The electromagnet (314) is thereby required to act on the armature (315) only when the latter is close to the electromagnet, with consequent reduction of input power required.</p> |
