The functioning parts fitted partly on the swivel part and partly on the fixed frame are connected by a slide member which has a recess in which at least one movable blocking element is located. One of the function parts is wedge shape at the end where an entrainment member is located so that this wedge can press the movable blocking element towards a wall of the slider guide whilst the function part is supported about the opposite wall of the slider guide. The blocking elements can be formed by rolling elements or brake elements.
