Abstract:
A pretensioner for a three point vehicle safety restraint seat belt has a mounting for one end of the seat belt. The mounting has a first member fixed to a load bearing part of the vehicle and a second member attached to the end of the seat belt. A force reservoir, such as a pyrotechnic unit, moves the second member relative to the first member in a pretensioning direction in response to activation by a crash sensor.

Description:
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 safety restraint comprises a length of 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, 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 attached to the belt webbing in a slidable manner. 
     To increase comfort for the vehicle occupant restrained by the belt, a retractor is fitted at the pillar end of the webbing. This allows the webbing to pay out under relatively low loads to enable limited movement of the restrained 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 dangerous situation 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 webbing to actively tighten the belt about the vehicle occupant in the event of a crash condition being sensed. This takes up any slack that may have 
     developed in the belt and helps to more correctly position the vehicle occupant in the seat to maximise the effect of the 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 belt in a short space of time, ideally before the crash pulse takes full effect. A typical known pretensioner uses 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, seat of a three 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 three door vehicle causes an unacceptable obstruction. 
     Seat travel is greater in a three door vehicle than in a four door vehicle in order to provide such access 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 fixedly 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 three-door, front seat applications. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a pretensioner for a three 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 
       For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a schematic side view of a pretensioner according to the present invention. 
         FIG. 2  is an enlarged view of part of the pretensioner of  FIG. 1  before pretensioning. 
         FIG. 3  is an enlarged view of part of the pretensioner of  FIG. 1  after pretensioning. 
         FIG. 4  is a side view of an alternative arrangement of the pretensioner of  FIG. 1 . 
         FIG. 5  is an enlarged perspective view of the carriage and rail arrangement of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a slider bar  10  positioned adjacent to a front seat  12  of a vehicle. One end of 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, adjacent a 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  to provide suitable compact anchorage with flexibility to allow access to rear seats in a three-door vehicle. 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  which is mounted on a rail  18 . The carriage may advantageously be arranged to slide in only the pretensioning direction along the rail upon activation of the force reservoir, being restrained from return motion in the opposite direction, for example by a ratchet mechanism comprising teeth incorporated into a surface of the rail. 
     Locking the pretensioner against return movement in this way prevents a loss of tension in the seat belt after pretensioning has been carried out. Ratchets 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. 
     A cable  20  extends between the carriage  16  and a pyrotechnic unit  22 . A means for moving the carriage  16  comprises a force reservoir such as a pyrotechnically operated unit  22 . Such a pyrotechnic unit is particularly suited to this application since it provides an impulse of the required magnitude over a short time period. 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 which forces the piston in a rearward direction and tensions the cable  20 . The activation of the force reservoir drives the piston along the cylinder and the piston pulls the cable and thus pulls the carriage along the rail. 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. Pretensioners can be constructed according to the invention with reduced or eliminated obstruction to rear seat access, smaller package sizes and which are attached to an appropriate sill anchorage zone. Positioning the piston and cylinder below the rail advantageously reduces the overall size of the pretensioner. 
       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 means of bolts  28 . Such a load bearing chassis member  24  may be a chassis member extending in a longitudinal direction down each side of the vehicle, such as 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, at the right hand side as shown in  FIG. 2  of the rail  18 . Upon sensing an acceleration of the vehicle above a predetermined criteria, a crash sensor, in known manner, generates a signal indicative of a crash condition which causes the pyrotechnic unit  22  to fire, creating a tension in the metal cable  20 . Advantageously the crash sensor is activated when the vehicle exceeds a predetermined acceleration or deceleration threshold. The tension in the cable  20  pulls the carriage  16  and the slider bar  10  in a rearward direction i.e. the direction of arrow A in  FIG. 2 . The carriage  16  can be arranged to move rearwardly a distance in the region of 50 to 150 mm depending on the vehicle size and 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  so as to correctly position the vehicle occupant within the seat  12  in order to maximise the benefit of the seat belt and correctly position him for maximum effect of any secondary restraint such as an airbag. 
       FIG. 3  shows the position of the carriage immediately after pretensioning. After the pyrotechnic unit has fired, the carriage  16  is prevented from returning to its original position under the forward momentum of the vehicle occupant during a crash, by a ratchet mechanism within the carriage  16  which is shown more clearly in  FIG. 5 . 
     The rail  18  is attached at each of its ends to respective support members  26  by means of bolts  36  passing through cylindrical holes  38  at each end of the rail  18 . 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  has a slot, allowing the rail  18  to pass through the centre of the carriage  16 . 
     An inner surface of the locking lever  32  is in contact with the teeth  30  in 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 thus prevents the carriage  16  from sliding in a reverse direction. To facilitate this the teeth  30  may be of 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. The ratchet mechanism and the carriage  16  may be formed from both 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° in order to cater for the different orientation of the pyrotechnic unit  22 . 
     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.