example a hydraulic or a pneumatic cylinder by means of a three point articulated system or toggle lever mechanism comprising an intermediate connection link 19 hinged between the clamping arm 12 and the actuator 15 more precisely in the example of fig1 the cylinder 15 comprises a piston member 16 having a rod 17 provided with a fork 18 hingedly connected at 20 to one end of an elastically yieldable connection link 19 the other end of the link 19 is hinged at 22 to a crank lever 21 integral with or fastened to the clamping lever 12 by means of the same pivot shaft 13 the fork 18 of the piston rod 17 the connection link 19 and the crank lever 21 define a three point articulated system or toggle lever mechanism having a dead centre condition beyond which the system with the cylinder 15 in a non operative condition becomes irreversible to maintain the arm 12 in the clamping condition for the piece p as shown in fig1 reference number 23 indicated a guide roller for the fork 18 while reference number 24 indicates a pad at the forward end of the guide for roller 23 to stop the fork 18 and to reduce the noise when reaching the stop position for the clamping arm 12.