| 摘要 |
A selective drive mechanism comprises a driven drive shaft (2), a drive nut (4), locking means (6) and an operable control arm (10). The drive nut (4) is disposed about the drive shaft (2) and is adapted to be selectively driven by the drive shaft (2). The locking means (6) is associated with the drive nut (4) and drive shaft (2) and comprises a helical coil spring mounted about said drive shaft (2). The operable control arm (10) is arranged to selectively engage with and operate the locking means (6). The mechanism (3) is arranged such that with the control arm (10) in a first position the locking means (6) is arranged to lock the drive nut (4) to the drive shaft (2) so that the relative positions of the drive nut (4) and drive shaft (2) are fixed. With the control arm (10) however in a second position the locking means (6), drive nut (4) and drive shaft (2) are arranged so that the drive nut (4) can move relative to the drive shaft (2). Preferably a number of such drive mechanisms (3) are mounted on a common shaft (2) in order to provide a multiple selective drive mechanism (40).
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