Tensioning device for use particularly with vehicle safety belts

A tensioning device for use in tensioning an element, such as a vehicle safety belt, comprising a casing, a cylinder having front and rear ends defined within said casing, a first piston axially movable within said cylinder, a rod firmly fixed to said first piston and having means for attaching said element to be tensioned, an expansion chamber defined between said first piston and said front end of said cylinder, means for supplying gas at high pressure to said expansion chamber to move said first piston in a first direction along said cylinder and exert a tensioning force on the element attached to said rod, an auxiliary piston included in the first piston structure, a frangible element for holding said auxiliary piston in an initial position and adapted to fracture when the pressure in said expansion chamber reaches a predetermined level, permanently open passage means communicating the pressure in said expansion chamber to said auxiliary piston, and piston stopping means actuated in response to displacement of said auxiliary piston away from said initial position following fracture of said frangible element, to prevent movement of said first piston relative to said cylinder in the said first axial direction, but allow movement of said first piston relative to said cylinder in the opposite direction.