A controlled-friction device comprising a first element having a second element mounted thereon to slide in a sliding direction, and friction retaining means arranged to exert between the two elements an opposing force against relative movement of the two elements. The friction retaining mechanism comprises a friction member mounted securely to the first element to rub against the second element via a friction surface extending parallel to the sliding direction, a presser member for exerting a thrust force on the friction member, urging the friction member against the second element in a direction normal to the relative movement of the elements, and a linkage linking the friction member to the first element causing the friction member to separate from the second element during movement in either direction of the second element relative to the first element.