Driving device especially for return stiffening of a safety belt in an automatic safety belt wind-up device

An apparatus comprising a reel to tighten a safety belt in response to detonation of an explosive charge, an elongated tubular member to be connected in fluid communication with the explosive charge, a piston disposed in said elongated tubular member, said piston being movable along said elongated tubular member toward one end portion of said tubular member under the influence of high pressure gas conducted from the explosive charge upon detonation of the explosive charge, flexible connector means connected with said piston and safety belt reel for rotating the safety belt reel to wind up the safety belt and for stopping movement of said piston along said tubular member with said piston spaced from the one end portion of said tubular member in the absence of breaking of said flexible connector means under the influence of force applied against said flexible connector means by said piston, and stop means disposed in the one end portion of said tubular member for stopping said piston upon breaking of said flexible connector means under the influence of force applied against said flexible connector means by said piston, said stop means including a body of resiliently deformable material having surface means for defining a cavity with an axial extent which is greater than the axial extent of said piston to enable leading and trailing end portions of said piston to enter the cavity during movement of said piston after breaking of said flexible connector means, said cavity having a side surface which tapers inwardly from an entrance to an end portion having a cross sectional size which is substantially less than a maximum cross sectional size of said piston to enable said piston to radially compress the material of said stop means as said piston moves in the cavity after breaking of said flexible connector means.