Abstract:
An adaptive load limiting seat belt retractor particularly adapted for motor vehicle applications. The retractor provides multiple levels of load limiting through the use of a pair of internal torsion bar elements. The torsion bar elements control the tension force on the seat belt webbing during an impact in a manner which provides desirable crash management characteristics. The retractor can be switched between high and low load limiting modes through activation of a pyrotechnic pretensioner. When the pyrotechnic pretensioner is not activated, the retractor provides a low load level characteristic. Pretensioner activation switches the retractor into a high load level operating condition. A high load limiting level is provided when the pretensioner is not activated if the spindle rotation exceeds a limited level in the low load limiting condition. A degressive load limiting feature may be provided to cause load limiting to begin at a higher level than the high load limiting level, and then step down the high load limiting level.

Description:
FIELD OF THE INVENTION 
     This invention relates to a seat belt retractor of a type used as part of a seat belt occupant restraint system, particularly adapted for motor vehicle applications. 
     BACKGROUND OF THE INVENTION 
     Seat belt retractors are in common use in motor vehicles around the world as part of an occupant restraint system for providing occupant protection. Seat belt retractors store belt webbing which is deployed across an occupant in the typical so-called “active” type belt system, in which the occupant manually fastens the belt. The seat belt retractor typically incorporates a torsion rewind spring which enables an internal spindle to store a spool of seat belt webbing. The retractor allows extension of the belt during fastening and retraction when it is unfastened. The basic functions of the retractor are to provide convenient storage of belt webbing, enable occupant movement when the belt is fastened, and control the extension of belt webbing upon the occurrence of a crash event. 
     Significant advances have been made in recent decades in the area of motor vehicle occupant restraints. In addition to passive restraints, such as inflatable air cushion restraint systems, the area of belt restraint systems has also undergone significant advancement. Two areas of advancements in retractor design are particularly noteworthy. Pretensioning devices are often provided which are typically pyrotechnically actuated and forcibly wind up the belt retractor to reduce slack in the webbing upon the detection of a crash event. By reducing the slack in the webbing by pretensioning, the belt is able to couple with the occupant early in the crash sequence to provide control of the occupant&#39;s displacement relative to the vehicle. Taking up webbing slack and tightening the belt at the initial portion of the crash sequence also enables belt loading to be managed better while restraining the occupant. Pretensioning also helps provide proper positioning of the belt webbing on the occupant&#39;s body during a crash event. 
     Another area of significant development in seat belt retractors is providing seat belt webbing load limiting. Early retractors simply had spindles which were rigidly locked, typically by an inertia sensitive device which locked the spool to the retractor frame. Upon retractor locking, loads exerted on the belt webbing would result in some extension of the webbing through stretching of webbing and deflection of the retractor and other belt system components. However, the extension of the webbing in retractors without load limiting features was not tailored in a precise manner. Accordingly, these retractors could result in high loads applied to the occupant which can lead to less than optimal restraint performance. To improve performance, designers developed load limiting systems for retractors. Load limiting systems typically employ the use of a torsion bar coupled between the webbing spool and the inertial locking device which provides controlled torsional deflection in response to belt webbing loads. The presence of the torsion bar will “soften” the restraint characteristics of the belt retractor. In yet a further refinement of belt load limiting systems, multilevel load limiting systems have been implemented. These systems may have one, two, or more sections of torsion bar or other deformable elements which can be activated in a controlled manner, depending on a number of factors. For example, it may be desirable to provide a high belt load limiting characteristic when a high severity crash is occurring, or where a large and heavy occupant is involved. On the other hand, in lower severity impacts, lighter weight occupants may optimally be restrained with a less stiff load limiting characteristic. Retractors are presently known which have a pyrotechnic device which can be fired through a controller to select between high and low load limiting conditions, depending on a variety of factors, including those mentioned previously. 
     Providing retractors with increasing features has disadvantages, typically including the cost to provide these features, the complexity of the retractor, and the packaging size in the vehicle required for installation and operation of the retractor. Motor vehicle designers are constantly striving to reduce the mass, cost, and enhance the packaging efficiency of their products, including seat belt retractors. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, an adaptive load limiting seat belt retractor is provided which achieves many enhancements in view of the previously noted desirable characteristics. The seat belt retractor in accordance with an embodiment of this invention features a roto-type pretensioner which is activated by firing a micropyrotechnic gas generator. The retractor spool is coupled with high and low load limiting torsion bars. In one mode of operation, the pretensioner is not fired and in that condition, a relatively low belt load limiting retractor characteristic is provided. If, however, the pretensioner is activated, the pretensioner will lock the spindle assembly in such a manner that a high load limiting characteristic is activated. Accordingly, the retractor in accordance with this invention selects between high and low load limiting characteristics through the operation of the pretensioner. The retractor therefore does not require additional pyrotechnic activation components to provide selection between load limiting characteristics, and importantly, can be fit within the same packaging envelope of currently available seat belt retractors not having adaptive load limiting features. By eliminating the separate pyrotechnic activation device, an additional firing line and control system are not required for this retractor. In addition to the packaging advantages of the present invention, dual load limiting can be provided without significant cost penalties over existing retractors without that feature. This latter benefit is provided since the complexity and number of components are not significantly increased over standard function retractors. 
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial view of a seat belt retractor in accordance with the present invention; 
         FIG. 2  is an exploded pictorial view of the spindle assembly components of the retractor shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  through the retractor shown in  FIG. 1 ; 
         FIG. 4  is a side cross-sectional view taken along line  4 - 4  through the retractor shown in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view similar to  FIG. 3  showing operation in the pretensioning mode; 
         FIG. 6  is a cross-sectional view similar to  FIG. 3  showing operating in a high load limiting condition; 
         FIG. 7  is a cross-sectional view of a retractor in accordance with this invention similar to  FIG. 3  showing the retractor operating in a low load limiting condition; and 
         FIG. 8  is a pictorial view of portions of the spindle assembly of a retractor in accordance with this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates seat belt retractor  10  in accordance with the present invention. Retractor  10  may be employed in a conventional three point active belt system, and can be used as part of single or dual retractor configurations. Seat belt retractor  10  has as major subsystems, including frame assembly  12 , spindle assembly  14 , and pretensioner assembly  16 . Seat belt retractor  10  is used for allowing seat belt webbing (not shown) to be stowed on the spindle and for controlling the forces applied on the seat belt webbing during restraint operation, as will be described in greater detail in the following discussion. 
     Frame assembly  12  includes frame  18  which is generally formed from sheet metal stock and bent in a “U” shape and a pretensioner cover  94  that is connected to frame  18  by fasteners. Frame  18  includes means for mounting the retractor to a motor vehicle structure, principally through tab  19  with a bore for a threaded fastener or other fastening means (not shown). Frame  18  forms mounting surfaces for remaining components of the retractor. Torsion spring cap  20  is affixed to one end of frame  18  and includes an internal torsion spring (not shown) which exerts a torsional compliant force onto spindle assembly  14  for rewinding the belt webbing. Tread head assembly  22  is mounted to the opposite “leg” of frame  18 . Tread head assembly  22  includes internal inertia sensitive components (not shown) of conventional design which cause locking of retractor  10  in response to vehicle deceleration of a predetermined magnitude. This operation of tread head assembly  22  is in accordance with well known prior art principles, and is not described in detail here. Tread head assembly  22  operates to provide an emergency locking retractor (ELR) function. This allows webbing to be freely paid-out from retractor  10 , allowing movement of the occupant and extension of the seat belt webbing except when deceleration forces above a predetermined magnitude are acting on the retractor. The inertia sensors of seat belt retractors will cause tread head assembly  22  to lock, either in response to acceleration forces of a given magnitude and direction acting on the retractor, as well as in conditions where the motor vehicle is in an inclined condition when the inclined angle is greater than a predetermined magnitude. In these cases, tread head assembly  22  locks spindle assembly  14  to a pretensioner cover that is fixed to frame  18  as mentioned previously. Frame assembly  12  further includes a number of additional elements illustrated such as protective caps and other elements not directly related to the novel features of the present invention. 
     Now with specific reference to  FIGS. 2 and 3 , the elements of spindle assembly  14  are illustrated. Spindle  26  (also known as a spool) provides an outer cylindrical surface  28  upon which seat belt webbing is wrapped. Spindle  26  further forms a hollow interior cavity  30  within which other components are installed. Spindle  26  is mounted into bearing cap  32  which allows the spindle to rotate relative to frame element  18 . Spindle  26  further has a protruding post  34  along its longitudinal axis which extends into spring cap  20  and provides a means for a torsion rewind spring to engage with spindle  26 . Spindle hollow interior cavity  30  forms a reduced diameter splined bore  36  at its end adjacent spring cap  20 . Spindle  26  is open on its right hand end, as the parts are illustrated in  FIG. 3 . The open end forms a stepped internal bore, including a bearing bore surface  38  which receives bearing disc  40 . Bearing disc  40  allows for free rotation of spool  26  during load limiting webbing extension. 
     Installed co-axially within hollow interior cavity  30  of spindle  26  is a pair of elongated torsion bars, including high load level torsion bar  42 , and low load level torsion bar  44 . Bar  42  has an enlarged head  46  which is splined to engage with spindle splined bore  36 . The opposite end of torsion bar  42  forms an outer perimeter rim  48  which has external splines as well as a splined end bore  50  which receives a splined headed end  52  of low load level torsion bar  44 . Torsion bar  44  further forms splined end  54 . End  52  and end bore  50  have meshing splines. End  54  is splined into an internal splined bore  62  of tread head hub  24 . Torque transfer tube  56  has one end  57  with internal splines which engage and mesh with torsion bar head  48 , and an opposite end  59  having external splines. 
     Torsion bars  42  and  44  may be separately formed and assembled in a co-linear fashion as described previously. Alternatively, both torsion bars  42  and  44  could be formed in a unitary assembly which would form ends  46 , and  54  and rim  48  and featuring low and high load limiting sections. Such a unitary element could appear as they are shown assembled in  FIG. 2 . 
     Pretensioner pinion  58  is splined or otherwise fastened to torque transfer tube  56  at end  59  and has an outer diameter forming pockets  60  which receive pretensioner ball elements, which will be described in more detail in the following sections. Ball pockets  60  enable pinion  58  to be rotationally driven during pretensioner operation. 
     Torque transfer tube end  59  is mounted over hub tube projection  63 . This connection is preferably not splined and allows some relative rotation between tread head hub  24  and torque tube  56  during load limiting webbing extension. However, is preferable to have some means for limiting the relative rotation between these components for reasons which will be described in more detail when the operation of retractor  10  is described below. One means for providing limited angular rotation between hub  24  and torque tube  56  is to provide the torque tube with protruding pins or posts  65  fit within a circumferential groove  66  formed in the tread head hub  24 . Preferably, groove  66  is formed to sweep an arc which has ends which will limit the possible relative angular rotation between these two components. Angular relative rotation of up to about 330 degrees is practical with this configuration. A degressive bending element  99  is interlocked through bearing disc  40  to torque tube  56  and coupled to spindle  26  by degressive insert  100 . 
     As mentioned above, in some operating circumstances, some relative rotation between torque tube  56  and tread head hub  24  is desirable. However, it may be further desirable to limit such angular rotation until a torque load of given magnitude is exerted between these two components. For example, shear pins (not shown) could be installed between tread head hub  24  and torque tube  56  which would shear when a predetermined torque is applied between them. Another approach could feature a press-fit between these components which would permit relative rotation only after a predetermined torque is applied. 
     As best shown by  FIGS. 1 and 4 , pretensioner assembly  16  includes a formed hollow ball tube  68 . At one end of ball tube  68 , micropyrotechnic gas generator  70  is provided which is connected with an electrical firing line (not shown) and includes an internal chemical gas generating pellet (not shown). Ball tube  68  is loaded with a series of pretensioner balls or spheres  72 . The end  69  of ball tube  68  opposite gas generator  70  exits at a position tangent with pinion  58 . Pretensioner enclosure  74  confines balls  72  to engage with pinion ball pockets  60 . In operation of pretensioner assembly  16 , once a firing signal is sent along the control line, the micro gas generator  70  pyrotechnically generates gas pressure within ball tube  68 . This rapid pressure pulse drives pretensioner balls  72  through the inside of ball tube  68  and forces them into engagement with pinion  58  and ball pockets  60 . This operation, which is illustrated in  FIG. 4 , causes a winding of spindle  26  as the pretensioner pinion  58  is spun. 
     As pretensioner balls  72  are forced into engagement with pinion ball pockets  60  and a winding of pinion  58  occurs, one or more of the pretensioner balls  72  become jammed into position within enclosure  95 . One of balls  72  is confined by pocket  60  and supported by tube wall  96 . This action prevents pinion  58  from reverse rotation or being “back driven”. Accordingly, once pretensioner assembly  16  is fired, spindle  26  undergoes pretensioning rotation, and once this is concluded, pinion  58  becomes locked to frame element  18 . Various ramp surfaces or one-way “clutch” features can be provided to prevent back driving of pinion  58 . One or more “high seal” balls or pistons  97  is provided which has an interference fit with the inside diameter of ball tube  68 . This high seal piston  97  maintains gas pressure in tube  68 . The gas pressure prevents the ball chain formed by balls  72  and  97  from moving in a backwards direction so as to prevent pinion  58  from rotating in a reverse direction. The interference fit of piston  97  also provides friction resistance to help to prevent reverse rotation of pinion  58 . The high seal piston  97  can be spherical or cylindrical in shape and may be provided with various gas sealing features. Ball separator  75  separates balls  72  from pinion pockets  60  and leads the balls into a linear ball trap  98  after pushing the pinion  58 . The linear ball trap  98  confines balls  72  in the same way as tube  68  and does not allow balls  72  to move in two or three dimensional space. The end of ball trap  98  is formed by the pinion or a block stopper (not shown). These features ensure that high seal piston  97  is not ejected so that the gas pressure can be kept in ball tube  68  from being back driven. 
     Operation of seat belt retractor  10  will now be explained in more detail with particular reference to  FIGS. 5 ,  6 , and  7 . These figures are identical to  FIG. 3  except showing the loading paths through retractor  10  in various operational modes. To clarify  FIGS. 5 ,  6 , and  7 , some element numbers of the described features are not shown as they would be repetitive of those shown in  FIG. 3 .  FIG. 5  illustrates the operation of retractor  10  during pretensioning operation. This occurs once a firing signal is sent to gas generator  70  causing engagement of pretensioner balls  72  with pinion  58  as described previously.  FIG. 5  illustrates the transfer of forces through spindle assembly  14  in that mode of operation. Arrow  78  indicates that rotational motion is exerted on pinion  58 . Arrow  79  shows that this rotation is imparted through the splined connection to cause rotation of torque transfer tube  56 . As designated by arrow  80 , this rotation is also exerted on high load level torsion bar rim  48  and through to the opposite end  46  of torsion bar  42  shown by arrow  81 , and then to spindle  26 . Accordingly, activation of pretensioner assembly  16  is coupled to spindle  26  to cause pretensioning rotation which retracts belt webbing, as indicated by arrow  82 . 
       FIG. 6  illustrates operation of seat belt retractor  10  in a high level load limiting mode. This mode is provided after the pretensioner assembly  16  is fired as described previously and as is shown in  FIG. 5 . After this firing, the pressure is kept by high seal piston  97  and the pretensioner balls  72  become jammed between enclosure  95  and pinion  58  as previously described. This action grounds pinion  58  to the retractor frame  18 . After pretensioner firing, in the event that a load is applied on belt webbing wrapped on spindle  26  as indicated by arrow  83 , this belt load produces a torsional load on spindle  26  which is transferred to high load level torsion bar  42  through the path previously described during pretensioner operations. Arrows  84 ,  85 ,  86 , and  87  designate how the spindle torsional forces are coupled to frame  18  through the splined connections described above. Torsional forces are not in a significant magnitude exerted on low load level torsion bar  44 . In this operation mode, torsional deflection and therefore belt load limiting is provided by controlled torsional deflection and deformation of high load level bar  42  as indicated by the double headed arrow heads shown along arrow  84 . 
     The high load limiting mode of operation of retractor  10  may also have degressive load limiting features. When degressive load limiting is desired, for example for large occupants, bending element  99  and degressive insert  100  can be installed. As the degressive bending element  99  is fixed to torque tube  56  through bearing disc  40  and the degressive insert  100  is in a pocket  101  in the spindle  26 , the bending element will be pulled through from bending insert to provide degressive load while high load limiting is activated. These elements are best described with reference to  FIG. 8 . Adjustment of the thickness and length of bending element  99  provides various levels of degressive step down and duration. According, when high load limiting is effective following operation of pretensioner assembly  16 , load limiting torsion is at a higher level than that provided by high load level bar  42  alone as degressive bending element  99  is deformed. Following deformation, load is transferred to high load level bar  42  which provides a step down in load limiting characteristic (albeit to a higher level then provided by low load level bar  44 , as described below). 
       FIG. 7  illustrates an operating condition in which a belt load designated by arrow  83  is exerted on the belt webbing and therefore spindle  26  when the pretensioner assembly  16  has not been activated. In this condition, tread head hub  24  is grounded to retractor frame  18  through internal activation of conventional inertial locking elements. In the case that tread head hub  24  is locked in this manner, belt forces  83  produce torque on spindle  26  which is transferred through torsion bar  42 , shown by arrow  88 , through torsion bar  44  shown by arrow  89 , and to pretensioner cover  94  shown by arrow  90 . 
     In the mode of operation shown in  FIG. 7 , torsion bars  42  and  44  are torsionally loaded in a series manner. Accordingly, torsion loads are also exerted on high load level torsion bar  42 . However, due to the lower torsional stiffness of low load level torsion bar  44 , that torsion bar undergoes more torsional deflection and deformation than bar  42  and predominantly provides belt load limiting in this mode, as designated by the double headed arrow heads along arrow  89 . 
     There could be operational scenarios in which high load level operation is desired even where pretensioner assembly  16  is not activated. The connection between torque tube  56  and tread head hub  24 , as previously described, provides a limited degree of rotational relative motion when tread head  22  is locked. Accordingly, when low load level torsion bar  44  is undergoing deflection, such deflection can occur over a predetermined angle, for example 300 degrees or up to about 330 degrees. This relative rotation will occur between torque transfer tube  56  and hub  24  as low level torsion bar  44  is deformed. Once this predetermined relative rotation angle occurs, the torsional loads can be directly coupled to high load level torsion bar  42  through torque tube  56 . This action is designated by the dotted line arrows  91 ,  92 , and  93  in  FIG. 7 . This operation provides protection against excess deformation of low level torsion bar  44  when pretensioner assembly  16  has not been activated, or where the pretensioner fails to ground forces to frame  18  for any reason while tread head  22  is locked. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.