| 摘要 |
An expansible shaft including a hollow axially-extending body and operatively coupled core engaging and retracting sections within the body. The core engaging section includes a core engager movable relative to the body in a direction generally perpendicular to the axis of the body and biased towards a core engagement position in which it projects beyond the wall of the body; and the retracting section includes an actuator arranged to overcome the bias and move the core engager radially inwardly from its core engagement position.
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