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An actuator for a seat belt tightening device for a vehicle using a piston received in a cylinder so as to be actuated by an explosive expansion of a propellant in a chamber defined in the cylinder behind the piston. To ensure a required strength of the coupling between the piston and a cable for tr

An actuator for a seat belt tightening device for a vehicle using a piston received in a cylinder so as to be actuated by an explosive expansion of a propellant in a chamber defined in the cylinder behind the piston. To ensure a required strength of the coupling between the piston and a cable for transmitting the actuating force and to simplify the process of attaching the cable to the piston, the piston is cast by metal or alloy with an end of the cable embedded therein. Thus, the piston may be used as cast and no extra work is required to tie the cable to the piston. To improve the sealing capability of the piston, a skirt having a flared edge made of synthetic resin may be attached to the rear end of the piston.

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1. An actuator for connection to a seat belt pulling means for clamping a part of a seat belt and retracting said clamped seat belt for tightening the seat belt for a vehicle, comprising: a frame; a cast metal piston receiving a cylinder mounted on said frame, said cylinder having a substantiall

1. An actuator for connection to a seat belt pulling means for clamping a part of a seat belt and retracting said clamped seat belt for tightening the seat belt for a vehicle, comprising: a frame; a cast metal piston receiving a cylinder mounted on said frame, said cylinder having a substantially closed base end; a deceleration sensor mounted on said frame for detecting a deceleration level indicative of a vehicle crash; a propellant received in a case on said frame and communicating with a chamber on said frame defined between a rear end surface of said piston and said base end of said cylinder and provided with means for ignition thereof upon detection of said deceleration sensor; and cable means connected at one of its ends with said piston and at its other end with said seat belt pulling means for clamping and retracting; said one end of said cable means being cast in said piston. 2. An actuator for tightening a seat belt according to claim 1, wherein said cable means consists of a wire rope. 3. An actuator for tightening a seat belt according to claim 1, wherein an annular skirt member having a flared circular edge is coaxially attached to a rear end of said piston so as to seal off said chamber defined behind said piston by said edge cooperating with an inner circumferential surface of said cylinder. 4. An actuator for tightening a seat belt according to claim 3, wherein said annular skirt member is made of synthetic resin. 5. An actuator for tightening a seat belt according to claim 1, wherein said piston is substantially entirely encased in a cover made of synthetic resin. 6. An actuator for tightening a seat belt according to claim 5, wherein said cover is integrally provided with an annular skirt member having a flared circular edge extending coaxially from a rear end of said piston so as to seal off said chamber defined behind said piston by said edge cooperating with an inner circumferential surface of said cylinder. 7. An actuator for connection to a seat belt pulling means for clamping a part of a seat belt and retracting said clamped seat belt for tightening the seat belt for a vehicle, comprising: a frame; a cast metal piston received in a cylinder mounted on said frame, said cylinder having a substantially closed base end; a deceleration sensor mounted on said frame for detecting a deceleration level indicative of a vehicle crash; a propellant received in a case on said frame and communicating with a chamber on said frame defined between a rear end surface of said piston and said base end of said cylinder and provided with means for ignition thereof upon detection of said deceleration level by said deceleration sensor; cable means connected at one of its ends with said piston and at its other end with said seat belt pulling means for clamping retracting said clamped seat belt, said one end of said cable means being cast in said piston; and an annular synthetic resin skirt member having flared circular edges and coaxially attached to a rear end of said piston so as to seal off said chamber defined behind said piston by the edge cooperating with an inner circumferential surface of said cylinder. 8. An actuator for tightening a seat belt according to claim 7, wherein said skirt member is an extension of a cover substantially entirely encasing said piston. 9. An actuator for connection to a seat belt pulling means for clamping a part of a seat belt and retracting said clamped seat belt for tightening the seat belt for a vehicle, comprising: a frame; a cast metal piston received in a cylinder mounted on said frame, said cylinder having a substantially closed base end; a deceleration sensor mounted on said frame for detecting a deceleration level indicative of a vehicle crash; a propellant received in a case on said frame and communicating with a chamber on said frame defined between a rear end surface of said piston and said base end of said cylinder and provided with means for ignition thereof upon detection of said deceleration level by said deceleration sensor; and cable means connected at one of its ends with said piston and at its other end with said seat belt pulling means for clamping and retracting said clamped seat for tightening said seat belt; said one end of said cable means being flared and being cast in said piston.