Lever 1, turning about fixed axis A, has lug b which has screw 3 for compressing spring 2. In arm c of lever 1 is slanting hole d through which rod 4 slides. Rod 4 is subject to the action of spring 5. Due to the action of spring 2, lever 1 tends to bend rod 4 and locks it in any given position. When lever 1 is turned counterclockwise about axis A until it runs against stop a, the slanted hole becomes aligned with rod 4 which is thereby released and raised by spring 5. The force exerted by spring 2 can be adjusted by screw 3. This adjusts the bending moment acting on rod 4.