Tightening device for the gripping jaws of a plate stretcher

A tightening device for the gripping jaws of a plate stretcher comprises one push rod for each pair of gripping jaws sliding in guides in the gripping head in the stretching plane, and having hinged thereto two link rods which are also hinged to the gripping jaws, the gripping jaw moving synchronously rearwards for the closing operation and forwards for the opening operation, in two guides disposed in V-formation to each other. The link rods are coupled to the gripping jaws in a disengageable manner to avoid damage to the link rods and tightening device if tearing of the plate takes place during the stretching operation, or on account of any difference in the gripping of the plate by the upper and lower gripping jaw causing generation of very large but different forces on the two link rods. The link rods are in the form of angle levers each having a long and short arm pivotally connected to the hydraulically mobile push rod. Each short arm is pivotally connected to a separate hydraulic cylinder disposed on the push rod, and the long arm is releasably coupled by way of mouth-shaped openings to entrainment cams on the gripping jaws so that they can be uncoupled therefrom by swivelling.