Abstract: A screwdriver comprises a planetary gear carrier rotatably carrying a plurality of circumferentially displaced planet gears meshing with a pinion driven by the motor of the screwdriver and with teeth provided on the inner surface of a coupling element in form of a hollow cylinder surrounding the carrier. The coupling element is also provided at one end with coupling teeth axially displaced from the first-mentioned teeth. The coupling element is movable by a fluid-operated piston from a first position to which it is biased by springs, in which the coupling teeth thereon are in mesh with corresponding coupling teeth on the carrier to rotate with the same speed as the latter, to a second position in which the coupling teeth on the coupling element are in mesh with stationary teeth on a housing surrounding the carrier and the coupling element to hold the latter against rotation.