Screwing heads

A screwing head, comprising shank means adapted to be rotatably driven, said shank means having a longitudinal axis and said shank means having an end portion of annular cross-section, said end portion of annular cross-section defining a seat, a screwdriver bit comprising a shank part, said shank part being located in said seat, locking members for releasably locking the shank part of the screwdriver bit within the seat so that the screwdriver bit can be rotated by the shank means, said locking members being mounted in the end portion of annular cross-section for movement transversely of the longitudinal axis of the shank means between radially inner positions in which the locking members lock the shank part of the screwdriver bit for rotation with the shank means, and radially outer positions in which the locking members release the shank part of the screwdriver bit to permit relative rotation between the shank means and the shank part of the screwdriver bit, an annular casing mounted on the shank means for longitudinal movement relative thereto, said casing surrounding the annular end portion of the shank means and having an internal surface, said internal surface comprising a portion of relatively small internal diameter and a portion of relatively larger internal diameter, said casing being movable longitudinally relative to the shank means from a locking position in which the small internal diameter portion holds the locking members in the radially inner positions and a release position in which the locking members can move radially outwardly into the larger internal diameter portion, and spring means biasing the casing into its locking position, the casing being movable from its locking to its release position by engagement with a workpiece upon a predetermined depth of screwing being attained.