The present invention relates to a power supply mechanism, comprising a threaded plunger rod (40) a drive nut (42) threadedly connected to the threaded plunger rod a manually operable tensioning mechanism (126) a drive member (70) rotatably connected to said tensioning mechanism (126) spring force means (78) operably connected to said drive member (70) such that said spring force means is tensioned when said tensioning mechanism (126) and said drive member (70) are rotated in a first direction said drive member (70) being connected to said drive nut (42) by a first engagement mechanism (66, 68) for providing a rotational release of said drive member (70) relative said drive nut (42) when said drive member (70) is rotated in said first direction by said tensioning mechanism (126) and providing a rotational lock of said drive member (70) relative said drive nut (42) in a second direction opposite said first direction when said drive member (70) is rotated by said spring force means (78), said first engagement mechanism allowing a longitudinal movement of said drive nut (34) in relation to the drive member (70).
