Tensioning device capable of damped return movement and retensioning

A tensioning device which also provides for damped return movement and subsequent retensioning, such as for use with a vehicle safety belt comprises a casing, a piston movable within the casing by high pressure gases issuing from a pyrotechnic charge, a rod attached to the piston and carrying an attachment buckle for the belt, and a piston jamming mechanism for preventing movement of the piston in one direction but allowing damped movement thereof in the opposite direction, the piston and/or any of the other additional pistons possibly included in the device, having a part slidable relative thereto with a compression spring located therebetween for urging the piston and associated part axially apart at the end of damping. The pistons and associated parts form concentric members, the springs being located between them and abutting against an inwardly directed shoulder on the piston and external shoulder on the parts.