A chuck (10) for retaining the shank of a tool bit or accessory, having a fixed diameter, inserted in a central channel (42) at a distal end of said chuck (10), comprising at least one torsion spring (14) mounted axially within a housing (12) and having an inner dimension and shape comparable to said shank's outer dimension and shape, and mechanical means (32a, 32b, 34) for winding and unwinding said torsion spring to respectively grip or release said shank, said winding being in a rotational direction consistent with a normal operating rotational direction of said chuck such that said shank (10) tends to further wind said torsion spring (14) for further self-tightening in normal operation.