A pretensioner is disclosed which, when a vehicle suddenly decelerates, a large quantity of gas is generated, and the gas flows into a cylinder from a gas-generating a device. As a result of pressure of the gas, a movable pulley moves instantaneously together with a piston, so that the wire trained around the movable pulley is pulled. An amount by which a wire is pulled is twice an amount by which a wire is pulled in cases where the wire is directly connected to the piston, as in conventional devices. Accordingly, the wire can be pulled by the same amount by using only half of a length of a conventional cylinder, thus making it possible to manufacture a pretensioner which has a short cylinder.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a pretensioner which is used in a webbing 
take-up device, and which causes a webbing to be applied closely to a 
vehicle occupant when a vehicle suddenly decelerates. 
2. Description of the Related Art 
A seat belt device must insure the safety of a vehicle occupant by tightly 
restraining the vehicle occupant by a webbing when the vehicle suddenly 
decelerates. 
The pretensioner is designed such that the webbing can be freely taken up 
or pulled out without any interference with the rotation of the take-up 
shaft during ordinary running of the vehicle. When the vehicle suddenly 
decelerates, the take-up rotary force of the pretensioner is transmitted 
to the take-up shaft of the webbing by a clutch mechanism provided between 
the take-up shaft and the pretensioner, so that the webbing is taken up in 
the direction which restrains the vehicle occupant. 
The take-up rotary force of such a pretensioner is obtained in the 
following manner. A gas is fed under pressure into a cylinder so that a 
piston is caused to move suddenly, thus causing instantaneous rotation of 
a rotating drum around which a wire, one end of which is connected to the 
piston, is wound. The rotary force of the rotating drum is transmitted to 
the take-up shaft of the webbing by the clutch mechanism. 
However, in conventional pretensioners equipped with a cylinder and piston, 
the cylinder must be lengthened in accordance with the amount off rotation 
of the take-up shaft. Accordingly, the cylinder portion protrudes from the 
main body of the webbing take-up device, so that a given attachment space 
must be provided inside the passenger compartment. 
SUMMARY OF THE INVENTION 
In view of the aforementioned, the object of the present invention is to 
provide a pretensioner which makes it possible to shorten the length of a 
cylinder. 
A pretensioner relating to the present invention which forcibly rotates a 
webbing take-up shaft when a vehicle suddenly decelerates and which tenses 
a webbing in a direction of restraining a vehicle occupant, comprises: a 
rotating drum connected to an axial end portion off said webbing take-up 
shaft via a clutch mechanism; a cylinder into which gas, which is 
generated when the vehicle suddenly decelerates, flows; a piston disposed 
within said cylinder and receiving gas pressure of the gas which flows in 
said cylinder; a movable pulley which is axially supported by said piston; 
and a wire, one end of said wire being wound around said rotating drum, 
another end of said wire being fixed to a bottom portion of said cylinder, 
and an intermediate portion of said wire being trained around said movable 
pulley. 
In the pretensioner of the present invention, when the vehicle suddenly 
decelerates, a large amount of gas is generated and the gas flows into the 
cylinder. Due to the pressure of the gas, the movable pulley moves 
instantaneously together with the piston, so that the wire trained around 
the movable pulley is pulled. The amount by which the wire is pulled is 
twice the amount by which the wire is pulled in cases where the wire is 
directly connected to the piston, as in conventional devices. Accordingly, 
the same amount of wire can be pulled with half the length of a 
conventional cylinder, thus making it possible to manufacture a 
pretensioner which has a short cylinder. 
Further, since the pressure of the gas is received by the bottom surface of 
the piston and does not act on the movable pulley, the movable pulley, 
which is axially supported in slots formed in side walls or the piston, 
can rotate smoothly without being affected by the gas pressure. 
When the wire is pulled, the rotating drum around which the wire is wound 
rotates, and the rotary force is transmitted to the take-up shaft via the 
clutch mechanism, so that the webbing is tensed in the direction which 
restrains the vehicle occupant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a webbing take-up device 12 equipped with a pretensioner 10 of 
the present embodiment. The webbing take-up device 12 has a frame which is 
formed from a rectangular plate 14 and a pair of leg plates 16 which are 
bent at right angles from the sides off the plate 14 so as to be parallel 
to each other. 
The leg plates 16 axially support both ends of a take-up up shaft 20 such 
that the take-up shaft 20 is rotatable. The tip end of a webbing 22 
applied to a vehicle occupant is engaged with the take-up shaft 20, and 
the webbing 22 is wound in a roll form on the take-up shaft 20. 
A rotating drum 18 which is positioned on the outside of one of the leg 
plates 16 is coaxially connected to one end of the take-up shaft 20 via a 
clutch mechanism 23. A wire 24 is wound around the rotating drum 18, and 
an end portion of the wire 24 is fixed to the rim portion 18A of the 
rotating drum 18. When the rotating drum 18 begins to rotate, the clutch 
mechanism 23 is actuated so that balls 21 are sandwiched between a central 
shaft 20A of the take-up shaft 20 and narrow-width portions 19A of a cam 
groove 19 formed at a shaft portion of the rotating drum 18, so that the 
take-up shaft 20 rotates together with the rotating drum 18. 
Further, a cylinder 26 is attached alongside the webbing take-up device 12. 
The cylinder 26 is a rectangular tube which is open at one end and which 
has a bottom portion 28 at the other end. A guide passage 34 which 
communicates with a gas-generating device 32 is connected to a circular 
hole 30 formed in the bottom portion 28. A gas-generating agent (enhancer 
or the like) and a detonator, which are not shown in the drawings, are 
accommodated in the gas-generating device 32. When an acceleration sensor 
detects the sudden deceleration of the vehicle, the detonator is 
detonated, and the gas-generating agent is caused to react by the 
resulting spark so that a large quantity of gas is generated. 
Further, a box-shaped piston 36, which has an opening 35 provided at the 
top and which has a bottom surface 36A, is disposed inside the cylinder 
26. Slots 38 are formed along side walls 36B from the opening 35. Shaft 
portions 42 which protrude from sides of a movable pulley 40 are axially 
supported in the slots 38 such that the shaft portions 42 are rotatable. 
An intermediate portion of the wire 24 which is wound around the rotating 
drum 18 is trained on the movable pulley 40. The direction of movement of 
the wire 24 is changed by means of a fixed pulley 46 which is axially 
supported by a shaft 44. The wire 24 is passed through a through-hole 48 
formed in the bottom portion 28 of the cylinder 26, and is trained over 
the movable pulley 40. The wire 24 is then passed through a through-hole 
58 formed in the bottom portion 28, and is fastened to the cylinder 26 by 
a stopper 60 (see FIG. 2). 
Next, operation of the pretensioner 10 of the present embodiment will be 
described. 
As is shown in FIG. 2, the piston 36 is positioned at the bottom portion 28 
of the cylinder 26 during ordinary running off the vehicle. In this case, 
the take-up shaft 20 is not connected to the rotating drum 18 by the 
clutch mechanism. Accordingly, the take-up shaft 20 rotates freely so that 
the webbing 22 can be taken up or pulled out. 
When the vehicle suddenly decelerates, the acceleration sensor of the 
gas-generating device 32 reacts so that the detonator is detonated, and 
the gas-generating agent is caused to react by the resulting spark so that 
a large quantity of gas is generated. The gas flows into the cylinder 26 
via the guide passage 34 and instantaneously pushes the movable pulley 40 
upward together with the piston 36. In this case, since the gas is 
received by the bottom surface 36A of the piston 36 and does not directly 
contact the movable pulley 40, the movable pulley 40 can rotate smoothly 
without being affected by the gas pressure. As a result, the wire 24 is 
pulled into the cylinder 26 so that the rotating drum 18 rotates in the 
direction indicated by arrow A in FIG. 3. When the rotating drum 18 
rotates, the clutch mechanism is actuated so that the rotary force of the 
rotating drum 18 is transmitted to the take-up shaft 20, thus causing the 
take-up shaft 20 to rotate integrally with the rotating drum 18. As a 
result, the webbing 22 can be forcibly tensed in the direction which 
restrains the vehicle occupant. 
Thus, as a result of the use of the movable pulley 40, the amount by which 
the wire 24 is pulled in is twice the amount by which the wire 24 is 
pulled in cases where the wire 24 is directly connected to the piston 36. 
Accordingly, even in cases where a short cylinder is used, the take-up 
shaft 20 can be rotated by the same amount as in the case of a 
conventional long cylinder.