Abstract:
A pretensioner for a three point vehicle safety restraint seat belt, has a mounting for one end of the seat belt. The mounting is a first member fixed to a load bearing part of the vehicle. A second member is attached to the end of the seat belt. A means is provided for moving the second member relative to the first member in a pretensioning direction in response to activation of a crash sensor. The mounting may be a flexible cable that functions as a slider bar. A piston-cylinder force reservoir may be used either to pull the slider bar via a carriage mounted on a rail, or to pull the cable directly. Return motion is preferably inhibited. The arrangement provides a space saving pretensioner suitable for two-door vehicles while allowing access to the vehicle rear.

Description:
This is a Continuation-in-Part of application Ser. No. 10/874,911 filed Jun. 24, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a seat belt pretensioner that can be used in a vehicle. 
     BACKGROUND OF THE INVENTION 
     Traditionally a seat belt comprises a length of seat belt webbing connected at three points to load bearing parts of a vehicle. Typically one end is bolted to a door sill on one side of the seat, and is arranged to pass laterally across the hips of the vehicle occupant to a buckle mechanism fixed to the vehicle on the opposite side of the seat, and then diagonally across the torso of the vehicle occupant to a further fastening point on the B pillar of the door. The buckle mechanism engages a buckle tongue slideably attached to the webbing. 
     To increase the comfort of the vehicle occupant restrained by the seat belt a retractor is attached to the pillar end of the webbing. This allows the webbing to pay out under relatively low loads to enable limited movement of the vehicle occupant, for example to reach in-car entertainment controls or storage compartments. The retractor is biased to keep the webbing relatively taut about the vehicle occupant and a locking element is included to lock the retractor against webbing pay out in the event of a crash being detected. For example, an acceleration sensor activates if the vehicle undergoes rapid acceleration or deceleration indicative of a crash. 
     In recent years, pretensioners have been introduced to rapidly pull in a length of seat belt webbing to tighten the seat belt about the vehicle occupant in of a crash. This takes up any slack that may have developed in the seat belt and helps to more correctly position the vehicle occupant in the seat to maximize the effect of the seat belt protection and of any secondary safety restraint such as an airbag. 
     Pretensioners comprise a force reservoir such as a pyrotechnically operated gas generator to provide an impulse of sufficient magnitude to tighten the seat belt in a short space of time, ideally before the crash takes full effect. A typical known pretensioner may use rotational means to wind in a length of seat belt webbing, for example by rotating the retractor spool in a webbing rewind direction to take in the required length of webbing prior to the retractor locking against webbing pay out. 
     However, known pretensioners tend to be bulky, and are particularly difficult to use for the driver and front passenger seats of a two door vehicle because of the requirement to allow access to the rear of the vehicle past the front seats. Using a traditional retractor pretensioner mechanism in a front seat of a two door vehicle causes an unacceptable obstruction. 
     Seat travel is greater in a two door vehicle than in a four door vehicle to provide access to the rear seat and to accommodate this the door sill end of the webbing is usually attached to a so-called slider bar of well known design, instead of being bolted to the floor. This allows the sill end of the webbing to be moved longitudinally forward and rearward to facilitate rear seat access and front seat movement. The present invention provides an improved pretensioning mechanism that can be used in two-door, front seat applications. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a pretensioner for a two point vehicle safety restraint seat belt, comprising a mounting for one end of the seat belt, the mounting comprising a first member fixed to a load bearing part of the vehicle and a second member attached to the end of the seat belt, and means for moving the second member translationally relative to the first member in a pretensioning direction in response to activation of a crash sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation view of a pretensioner according to a first embodiment of the present invention. 
         FIG. 2  is an enlarged side elevation view of part of the pretensioner of  FIG. 1  before pretensioning. 
         FIG. 3  is an enlarged side elevation view of part of the pretensioner of  FIG. 1  after pretensioning. 
         FIG. 4  is a side elevation view of an alternative embodiment of the pretensioner of  FIG. 1 . 
         FIG. 5  is an exploded perspective view of the carriage and rail arrangement of  FIG. 1 . 
         FIG. 6  is a side elevation view of a pretensioner according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the first embodiment shown in  FIG. 1 , a slider bar  10  is positioned near to a front seat  12  of a vehicle. One end of a seat belt webbing  14  passes around the slider bar  10  and is free to move back and forth along the slider bar  10 . 
     The seat belt webbing  14  is of a conventional design and is attached at one end to a retractor mounted, near a to seat, to a load bearing part of the vehicle such as a vehicle side pillar (not shown). The webbing passes through a shoulder support also attached to the side pillar and has a buckle tongue, which is insertable into a buckle (not shown) located on the other side of the seat. 
     When in use, the seat belt webbing  14  is at one end of the slider bar  10  in the load bearing position shown in  FIG. 1 . When the seat belt is not in use the end of the webbing  14  may be moved in a rearward direction along the slider bar  10  so that it does not obstruct access to the rear seat of the vehicle. 
     One end of the slider bar  10  is attached to a carriage  16  that is mounted on a rail  18 . The use of a rail is particularly beneficial since the pretensioning action can be constrained to a linear motion in one direction in a simple manner and without the need for complicated or bulky rotating parts. A cable  20  extends between the carriage  16  and a pyrotechnic unit  22 . The pyrotechnic unit  22  is of a known type and contains a piston within a cylindrical housing and a gas generator. The gas generator is pyrotechnically activated to provide an impulse that forces the piston in a rearward direction and tensions the cable  20 . Such a pyrotechnic unit is particularly suited to this application since it provides an impulse of the required magnitude over a short time period. Positioning the piston and cylinder below the rail advantageously reduces the overall size of the pretensioner. The piston-cylinder arrangement may incorporate means to allow the second member to only move in a pretensioning direction, for example by ratchet teeth on the inside of the cylinder and at least one cooperating tooth on the piston. 
       FIG. 2  shows one arrangement of the carriage  16  and the rail  18  in greater detail. The rail  18  is attached at each end to support members  26  that are fixed to a load bearing chassis member  24  by bolts  28 . The load bearing chassis member  24  may be a chassis member extending in a longitudinal direction down each side of the vehicle, or a load bearing door sill and provides a suitable load bearing anchorage zone for the slider bar  10  adjacent to and slightly to the rear of each of the vehicle doors. 
     Prior to pretensioning, the carriage  16  is positioned for normal use of the seat belt webbing  14  at its forward most position on the rail  18  as shown in  FIG. 2 . Upon sensing an acceleration of the vehicle above a predetermined criterion, a crash sensor, in a known manner, generates a signal indicative of a crash that causes the pyrotechnic unit  22  to fire, creating a tension in the metal cable  20 . The tension in the cable  20  pulls the carriage  16  and the slider bar  10  in a rearward direction i.e. the direction indicated by the arrow A in  FIG. 2 . The carriage  16  can be arranged to move rearwardly a distance in the range of about 50 mm to about 150 mm depending on the vehicle size and performance requirements. 
     The sudden movement of the slider bar  10  in a rearward direction provides tension in the seat belt webbing  14 , which takes up any slack in the webbing  14  and pulls a vehicle occupant backwards into the seat  12  to correctly position the vehicle occupant in the seat  12  to maximize the benefit of the seat belt and correctly position the vehicle occupant for maximum effect of any secondary restraint such as an airbag. 
       FIG. 3  shows the position of the carriage  16  immediately after pretensioning. After the pyrotechnic unit has fired, the carriage  16  is prevented from returning to its original position by the forward momentum of the vehicle occupant during a crash, by a ratchet mechanism within the carriage. Locking the pretensioner against return movement in this way prevents a loss of tension in the seat belt after pretensioning has been carried out 
     In  FIG. 5  is an exploded view showing the ratchet mechanism. The rail  18  is attached at each of its ends to respective support members  26  by bolts  36  passing through cylindrical holes  38  at each end of the rail  18 . The bolts  36  are secured with appropriate washers or spacers  55  and nuts  56 . Locking ratchet teeth  30  are formed in one surface of the rail  18 . The carriage  16  has two end plates  40  and a base plate  42  attached to a side wall  44 . A locking lever  32  extends from the base plate  42 . Each of the end plates  40  and the locking lever  32  have a slot, allowing the rail  18  to pass through the center of the carriage  16 . 
     An inner surface of the locking lever  32  is in contact with the teeth  30  on the surface of the rail  18  and is orientated at an angle such that the locking lever  32  can pass over the teeth  30  in one direction. However the locking lever  32  is biased such that if it tries to move in the opposite direction, a locking edge of the inner surface of the locking lever  32  will be caught against a tooth  30  on the upper surface of the rail  18  and prevent the carriage  16  from sliding in a reverse direction. To facilitate this the teeth  30  have a saw-tooth form. 
     The slider bar  10 , the carriage  16  and ratchet mechanism, the rail  18 , the support members  26  and the bolts  28 ,  36  and  46  are all preferably made of metal. Ratchet mechanisms are known for different seat belt restraint applications and so the pretensioner of the present invention can advantageously be constructed using standard parts and manufacturing processes, and thus offers a relatively low cost locking mechanism. The ratchet mechanism and the carriage  16  may be formed from parts of a height adjuster traditionally used to alter the height of the belt shoulder support. 
     In  FIG. 4  the pyrotechnic unit is shown stored beneath the rail  18  and the bolts  36  and corresponding holes  38  in the rail may be used to fix the pyrotechnic unit  22  in relation to the rail  18 . The operation of the pretensioner in  FIG. 4  is substantially the same as that shown in  FIG. 1 , except that the cable  20  bends 180° to cater for the different orientation of the pyrotechnic unit  22 . 
       FIG. 6  shows an alternative embodiment in which the slider bar of the first embodiment is replaced by a flexible cable  50 . The flexible cable  50  thus preferably form a loop, in the general form of a slider bar, to provide the flexibility of movement for the seat belt webbing and access to the rear of a vehicle, required in two door vehicle applications. The flexible cable may be directly attached to, or form at least a part of, the piston of the piston-cylinder arrangement which is preferably operated pyrotechnically. Thus this embodiment can provide an even more space saving alternative for two-door applications of pretensioners. 
     The seat belt webbing (not shown) is looped round the cable  50  in the same manner as with the slider bar  10  of  FIG. 1  and the cable provides similar versatility of movement for the webbing mounting, and at least the same degree of access to rear seats. 
     One end  51  of the cable  50  is attached to a load bearing part of the vehicle such as the door sill, and the other end is attached to a piston  53  of a piston-cylinder pyrotechnic unit  52  which is fixed to a load bearing part of the vehicle such as the floor of the vehicle, particularly to retain the door profile and avoid inhibiting access through the door. 
     The cable  50  forms a loop profile as shown in the figure and can be encouraged to hold such a profile in normal use by retaining clips or by an elastomeric plastic coating. Alternatively a flexible tube may cause the cable  50  to increase its stiffness to the appropriate degree. 
     When a crash sensor indicates that a sudden deceleration or acceleration is taking place, the pyrotechnic unit  52  is pyrotechnically activated to release gas to push the piston along the cylinder in the direction indicated by the arrow A, causing the cable  50  to retract to the profile indicated by the broken line  50 ′. This causes the seat belt webbing mounted on the cable  50  to be pulled back in the direction indicated by the arrow A thus effecting the required pretensioning. 
     A ratchet or another form of non-return mechanism may be built into the pyrotechnic unit  52 . 
     Of course elements of the embodiments described may be combined. For example the cable  50  could be attached to an arrangement such as the carriage  16  sliding on the rail  18 , of  FIG. 1 . The pyrotechnic unit  52  could then be connected to the carriage  16  by another cable such as  20  in  FIG. 1  and be mounted either in line with, or below, the rail  18 . 
     Pretensioners can be constructed according to the invention having reduced or eliminated obstruction to rear seat access, smaller package sizes and which are attached to an appropriate door sill anchorage zone. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.