A power-operated tool, preferably a screw-tightening tool

A power-operated tool is fitted with a drive, whose driving shaft (17) can, by means of a toothed connecting coupling (36) whose disconnection depends on the reaction torque and a disconnecting, toothed coupling (20), be coupled with a driven shaft (45). When a predetermined reaction torque occurs, part (35) of coupling (36) is moved axially (Figures 3a and 4a) against bias of spring (37) to interrupt drive to the driven shaft (45), and to allow cam teeth (59) on part (25) of the coupling (20) to engage cam teeth on the portion of the driven shaft (45) which forms part of the coupling (36) so as to effect axial movement of the parts (25,19) of the coupling (20) against bias of springs (29,21). Further rotation of the driving shaft (17) causes the parts (25, 35) to be returned by springs (29, 37) to their initial positions, whilst part (19) is held in a disconnected position (Figure 6) by balls (54) until the power supply is terminated.