The tension lever (2) is connected for articulated movement to the cam (1) to swivel about a bolt (3) parallel to the rotary axis of the cam. The swivel area is restricted by stops to an angle of about 180 degrees so that the tension lever is active when the cam is tensioned and after tensioning can be folded against the cam. - The unfolded tension lever is transversely movable across the cam and further stops can be provided which during displacement of the lever abut to ensure that the lever entrains the cam during release.
