Abstract: A jack comprising a screw and a nut member, the displacement of which along the screw produces a change in the distance between the active portions of the jack. The jack has a catch which prevents collapse if the nut member, due to worn threads, loses its grip on the screw. The catch is achieved by dividing the nut member into a threaded load-carrying portion and a threaded portion which is axially unloaded during normal thread engagement and which follows the load-carrying portion and serves as a stop if the latter should lose its grip on the screw.