This invention relates to vehicle passenger safety restraint pretensioning devices. In a vehicle safety restraint system it is usual to provide a seat occupant with a seat belt comprising a lap portion of webbing passing from an anchorage point across the lap of the occupant to a buckle tongue and thereafter from the buckle tongue diagonally across the torso of the occupant and via a shoulder height D-ring to an emergency locking retractor. A modern seat belt retractor is a low friction mechanism requiring only a light rewind spring.

This contributes to optimum comfort for the occupant but means that the seat belt webbing may rest only lightly against the body of the occupant. In order to minimise forward movement of an occupant following lock-up of the retractor, it has increasingly become the practice to provide vehicle sensitive means to cause pretensioning of the belt webbing at the onset of a crash condition. Such pretensioning can be effected for example by operating the retractor to rewind the belt or alternatively by effectively shortening the buckle anchorage.

One known type of pretensioner acts by rotating a retractor spool onto which the seat belt webbing is wound, the retractor spool being driven by a spring or other rotary drive. Since under normal conditions modern retractor spools must be free to move to pay out or take in seat belt webbing in the interests of passenger comfort, then known pretensioners of this type include a mechanical clutch which mechanically connects the drive means to the retractor spool only under crash conditions.

There are disadvantages in the use of mechanical clutches for this application since they tend to comprise several movable components, which increases the cost of manufacture, and decreases their reliability. In addition, when the clutch engages, a shock load is imposed on the pretensioner. Since take-up of seat belt webbing must be extremely rapid, these shock loads may be considerable.

In accordance with the present invention, there is provided a pretensioner for a vehicle safety restraint comprising two coaxial rotatable members having opposing surfaces spaced by a cavity filled with a coupling fluid, the opposing surfaces being shaped and relatively positioned such that motion of one member relative to the other member is transmitted through the coupling fluid to the other member and wherein one member is connected to a webbing winding spool of a restraint retractor, and the other member comprises drive means actuable in response to a crash condition. Preferably, the arrangement is such that the coupling fluid acts as a torque convertor.

Preferably, the fluid containing cavity is located in a radial direction between the two rotary members.

In a preferred embodiment of the invention, the first rotary member is formed as an outer rotor within which is a cavity containing the fluid and the second rotary member which is formed as a turbine.

Preferably, the outer rotor comprises inwardly extending vanes projecting 'into the fluid cavity, and the turbine comprises outwardly extending vanes projecting into the fluid filled cavity.

Preferably, the outer rotor and the turbine are contained in a sealed outer casing.

In a particularly pref-erred embodiment of the present invention, the first rotary member is adapted to be driven by reaction forces exerted by expanding gas.

Preferably, the expanding gas is generated by at least one gas generator and is' emitted in an approximately tangential direction from the first rotary member.

There is also provided, in accordance with the present invention, a seat belt retractor provided with a pretensioner as described above.

In accordance with still a further aspect of the present invention, there is provided a pretensioner for a vehicle safety restraint provided with drive means in the form of at least one gas generator adapted to emit gas in a direction generally tangential to a rotatably mounted member, thereby driving the member rotationally.

The use of a pretensioner with a fluid actuated coupling, in accordance with the invention, provides for smooth take up of belt retraction forces without the shock loads associated with mechanical systems.

Further, while the type of clutch used in known pretensioners cannot increase the torque on the retractor spool, the fluid actuated coupling can act as a torque convertor, providing torque which is large when the difference in rotational velocity between the drive means and the retractor spool is large, and which declines as this difference decreases. In a pretensioning sequence, the above mentioned difference of velocities is largest at the start of the sequence, when the spool is stationary. Thus the largest torque is exerted to initially accelerate the

retractor spool, which is clearly desirable.

By virtue of this increase in torque, it is possible to use drive means of low torque which operate at high rates of rotation, such as a gas efflux drive. A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying single figure, which is a cross section through a pretensioner according to the invention.

The figure shows a sealed, stand alone unit with working parts contained in .an outer casing 2, of circular cross section. In use, the outer casing is attached or fixed, in relation to a retractor housing, to a seat belt retractor mechanism. The spool axle of the retractor projects into the pretensioner to engage with a shaped aperture 4 at the centre of a rotatably mounted turbine 6, so that rotation of the turbine 6 causes rotation of the retractor spool. In the present embodiment, the shaped aperture 4 is square: it could of course be any shape which will grip the spool axle strongly enough to rotate it. The turbine 6 comprises a circular body portion from which a number of curved turbine vanes 8 project. The vanes extend in a radially outward direction.

The turbine 6 is coaxial with and mounted within an outer rotor 10. The outer rotor 10 comprises an inner surface with a circular cross-section facing the turbine 6 and a number of curved rotor vanes 12, extending from the inner surface towards turbine 6, i.e. in a radially inward direction, but curved in the opposite direction to the turbine vanes 8. A space 14 between the turbine 6 and the inner surface of the outer rotor 10 is filled with oil or other suitable

fluid .

In operation of the pretensioner, the outer rotor 10 is rotationally driven, and the rotor vanes 12 create powerful eddy currents within the oil in space 14. These eddy currents act on the turbine vanes 8 exerting a torque on the turbine 6 and causing it to rotate thus rotating the retractor spool.

Driving of the outer rotor 10 is achieved using gas generators mounted in peripheral portions 9 of the outer rotor 10. The outer surface of the outer rotor 10 is cut away adjacent the portions 9 in three places, to form shoulders 16 in each case through which gas is expelled in the direction of arrows B.. The reaction force from this process drives the outer rotor 10 rotationally. The three gas generators may be simultaneously or sequentially fired, depending on the torque characteristic desired. Electric or electronic timing apparatus may be used, of standard constructions.

Simultaneous firing will produce a very high initial torque, reducing rapidly, whereas phased firing will produce a sustained torque over a longer period.

The outer casing 2 and the outer rotor 10 may be injection moulded in a plastics material; in view of the large forces to which it is subjected, however, the turbine 6 is preferably die cast.