A connecting part (8) forming a tension device comprises an elbow lever (16) with short lever arm (20) and long lever arm (21) in active connection therewith via a tension bearing (17). The long lever arm includes a tubular connection comprising a guide tube (24) holding the tension spring (9), an adjusting tube (34) with detent rod (36) and a connecting tube (22,22a) connecting these tubes together. The free ends of the lever arms of the elbow lever are attached to the movable tension jaws (10,11) by swivel bearings (18,19) and the tensioned lever (16) is secured by a detachable lock (33). The connecting tube has an inner thread (23) whereby the closed end of the guide tube is screwed into one end and the adjusting tube into the other.
