The safety belt is wound around an inertial reel (10).One end of the reel is surrounded by a disc pulley (44) normally out of contact with the reel and wound around by a cable (22). The cable is secured to the pulley with one end while the other end is looped over a pulley (30) at one end of a piston drive (13) and secured onto the reel mounting. The piston is moved away from the reel by a servo drive on collision.The servo drive is provided by a pyrolytic charge (17). The disc pulley is crushed by the cable tension and pulls onto the reel, twisting the reel to tighten it
