The crank has a tab (6) joined to a three part ring (12). A carrier (9) is connected to a drive which engages laterally in a recess (8) in a bearing part (4) of the ring. It also has a working engagement with a positioning part (3) of the ring. Wave-shaped humps and hollows (11) are formed on the face sides of these parts so that they engage each other when a load is applied to the tool.
