A compact web spool tightener, for achieving a high ratio pretensioning of webbing of a seat belt, having a contiuously meshed epicyclical gear-train system (9, 10, 11) that is engaged both under normal and suddenly decelerated states, to provide a non-collision rotary drive to the spindle (3) for tightening the web (14) of a seat belt and a reduced number of driving means (17) arranged in tandem in a shorter conduit that are in flow communication with a piston (6) to provide a rotary drive to the gear-train, on sudden deceleration of the vehicle.
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