Abstract:
A pretensioner  10  for a seat belt  12  utilizes a strap  34  coiled about a clutch assembly  46  wherein the clutch assembly  46  is in normal operation of a vehicle, rotatable about a portion of a webbing reel shaft  16 . The pretensioner  10  contains a first passage  20  and a second passage  22  for the respective containment of the clutch assembly  46  and an actuator  32 , wherein each passage is formed perpendicular and in volumetric intersection with the other. The pretensioner  10  also contains a gas generant composition  30  in combustive communication with the actuator  32 . Upon ignition, the gas pressure produced by the pyrotechnic  30  propels the actuator  32  into the strap  34 , thereby effecting rotary advancement of the clutch assembly  46  and the shaft  16 , and thus pretensioning the seat belt  12  wound about a spool  13  of a seat belt retractor  14 , wherein the spool  13  is rotatably engaged with a second portion of the shaft  16  and thus provided in coaxial relation with the clutch assembly  46.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. Nos. 60/180,270 and 60/180,271, each filed on Feb. 4, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to seat belt pretensioners and, more specifically, to a seat belt pretensioner that incorporates a motion multiplier thereby reducing the actuator stroke requirement. 
     Seat belt pretensioners remove slack from a seat belt in the event of a collision in order to minimize forward movement of the passenger. While it is known to use pyrotechnic gas generators to operate mechanisms which wind up or otherwise pull in slack in the seat belt during a collision, such known pyrotechnic gas generators are often disposed internally of a seat belt retractor. Thus, the vehicle owner is faced with a significant cost penalty in that the entire pretensioner and retractor assembly must be replaced after activation because of the inability to prevent degradation of the retractor. High-temperature gases tend to abrade interior metal surfaces and produce ash and clinkers that bind up the retraction mechanism. 
     Another problem with known pretensioners is that they are designed to activate only in severe accidents, for example, accidents that exhibit “G” forces sufficient to activate the vehicle airbags. Safety system designers generally choose such a relatively high activation threshold due to the expense of replacing the entire seat belt retractor and pretensioner assembly after activation. As a result, seat belt pretensioners do not protect passengers in less severe accidents. 
     A related problem with known pretensioners is that when the pretensioner is activated only in severe accidents, activation is relatively late in the crash sequence. Thus, the pretensioner must rapidly take up slack in the seat belt, sometimes injuring the passenger. 
     U.S. Pat. No. 5,967,440, herein incorporated by reference, describes a pretensioner and a conventional retractor for a safety belt system. When compared to the seatbelt retractor, the pretensioner is relatively complex and large, thus increasing the weight and spatial requirements of the seatbelt assembly. 
     U.S. Pat. No. 5,899,399, herein incorporated by reference, describes a state of the art pretensioner. The pretensioner is relatively complex therefore complicating the manufacturing process. 
     U.S. Pat. No. 5,944,350, herein incorporated by reference, describes a state of the art pretensioner. Again, the pretensioner is relatively large, thus increasing the weight and spatial requirements of the seatbelt assembly. 
     German Patent No. DE 10010379 A1, herein incorporated by reference, describes a pretensioner having an arcuate path for a multi-piece actuator (a plurality of cylinders) that drives a steel cord wrapped around a webbing reel hub and thereby effects pretensioning of an associated seat belt. The manufacturing of the pretensioner is more complex given the multi-piece actuator and given the arcuate channel that the actuator must traverse Uniformity in channel width and effective sealing becomes more difficult with the use of an arcuate channel. Secondly, a noise reduction means is employed to prevent rattling during normal vehicle operation, thereby increasing manufacturing costs. Furthermore, the joint use of the multi-piece actuator with the steel cord results in a friction loss and a reduction in the load applied to the steel cord by the forward-most cylinder of the actuator. As a result, more force is required to drive the multi-piece actuator and pretension the seatbelt. One solution is described by the use of an intermediate element “43” for facilitating the smooth movement of the rotating elements along the arcuate channel. Consequently, without the use of the intermediate element “43”, a more robust pretensioner housing would be necessitated to withstand the relatively greater combustion pressure (produced by a greater amount of propellant) likely necessary to pretension the seatbelt in accordance with customer requirements. 
     SUMMARY OF THE INVENTION 
     The aforesaid problems are solved, in accordance with a preferred constructed embodiment of the present invention, by a seat belt pretensioner that utilizes a flat strap as a motion multiplier and a linear channel for movement of a pretensioner actuator, thereby reducing the load or force lost to friction in certain known pretensioners. Given the ease of sealing, the present design also inhibits retractor and occupant exposure to the gases and solids formed upon combustion of a pyrotechnic composition contained within the pretensioner. 
     In one embodiment of the invention, the pretensioner contains a housing, a clutch assembly that in operation rotatably and constrictively communicates with a webbing reel shaft of a seatbelt retractor, a strap that constrictively exerts a circumferential pressure about the clutch assembly upon pretensioner activation, an actuator that communicates with and is propelled against the strap upon pretensioner activation, a gas generant or pyrotechnic that propels the actuator (or piston), and an initiator that ignites the gas generant upon a signal from an accelerometer or impact sensor, for example. Upon ignition of the gas producing pyrotechnic, the actuator is driven forward in an essentially linear path thereby tightening the strap about the clutch assembly, and thus pretensioning the seat belt wound about a spool in the retractor, wherein the spool and the clutch assembly share the webbing reel retractor shaft as an axis in a preferred embodiment. 
     Stated another way, a pretensioner  10  for a seat belt  12  utilizes a strap  34  coiled about a clutch assembly  46  wherein the clutch assembly  46  is in normal operation of a vehicle, rotatable about a portion of a webbing reel shaft  16 . The pretensioner  10  contains a first passage  20  and a second passage  22  for the respective containment of the clutch assembly  46  and an actuator  32 , wherein each passage is formed perpendicular and in volumetric intersection with the other. The pretensioner  10  also contains a gas generant composition  30  in combustive communication with the actuator  32 . Upon ignition, the gas pressure produced by the pyrotechnic  30  propels the actuator  32  into the strap  34 , thereby effecting rotary advancement of the clutch assembly  46  and the shaft  16 , and thus pretensioning the seat belt  12  wound about a spool  13  of a seat belt retractor  14 , wherein the spool  13  is rotatably engaged with a second portion of the shaft  16  and thus provided in coaxial relation with the clutch assembly  46 . 
     After use, only the pretensioner and perhaps the retractor shaft need be replaced, obviating the expense of also replacing the retractor or other components of the seat belt retraction mechanism. 
     Because expense heretofore associated with activation of a pretensioner is significantly reduced, passengers can be given the protection of seat belt pretensioning in even minor accidents. Moreover, a pretensioner that activates in minor collisions will also activate earlier in a severe crash sequence. Earlier activation of the pretensioner permits the pretensioner to operate less aggressively, reducing the possibility of injury caused by the seat belt pretensioner itself. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic elevational view, partially in cross-section, of a replaceable seat belt pretensioner prior to activation, in accordance with the present invention. 
     FIG. 2 is a second diagrammatic elevational view of a second embodiment, partially in cross-section, of a replaceable seat belt pretensioner after activation; 
     FIG. 3 is a partially exploded view of the embodiment of FIG. 1, in accordance with the present invention. 
     FIG. 4 is a cross-sectional view of the pretensioner of FIG. 1 taken along the line A—A. 
     FIG. 5 is a perspective view of a one-piece injection molded clutch assembly or coupling mechanism, in accordance with the present invention. 
     FIG. 6 is a top view of the clutch assembly of FIG.  5 . 
     FIG. 7 is a side view of the clutch assembly of FIG. 5; 
     FIG. 8 is another embodiment of the present invention wherein a resilient liner is utilized to strengthen a second passage within the housing. 
     FIG. 9 is yet another embodiment of the present invention wherein a resilient sleeve exterior of the housing is fixed to the second passage within the housing, in axial alignment therewith. 
     FIG. 10 is yet another embodiment of the present invention containing a gas generator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the figures, a pretensioner  10  in accordance with a preferred embodiment of the invention is employed to pretension a seat belt  12  wound about a reel  13  of a conventional seat belt retractor  14 . “Pretensioning” is generally defined to take up slack in the seat belt in the event of sudden deceleration, or, a collision. A webbing reel retractor shaft  16  extends from the retractor  14  into the pretensioner  10  and thereby cooperates with the pretensioner  10  to tighten the seatbelt  12  in the event of an accident. The pretensioner of the present invention has generic application, for example only, in seatbelt assemblies having seatbelt retractors as described in U.S. Pat. Nos. 4,558,832 and 4,597,546, herein incorporated by reference. 
     A housing  18  is formed from steel, aluminum, metal alloys, plastic, and/or other known materials suitable for containment of the components described below. The housing  18  is formed with a first circular cylindrical passage  20  volumetrically intersecting a second rectangular cylindrical passage  22 , wherein each passage is formed perpendicular to the other and second passage  22  is substantially coextensive with the length of the housing  18 . The second passage  22  has a first end  24  and a second end  26 . The length of the second passage  22  preferably extends toward second end  26  to permit at least one full turn of the webbing reel shaft  16 . It should be noted that given the sealing means within the first end  24 , for example o-rings and/or a flared actuator  32 , the channel  22  could conceivably extend through the housing  18 . 
     An initiator  28  is fixed (threadedly received, for example) within the first end  24  of the passage  22 . As shown in FIG. 1, the initiator  28  ignitably communicates with a gas generant propellant  30  that may if desired be placed proximate to the initiator  28 , intermediate of the initiator  28  and the second end  26 . An actuator  32 , preferably made of a hard resilient material such as plastic, steel, brass and metal alloys and flared at one end to provide a sealing means, is also placed within the first end  24 , intermediate of the propellant  30  and the second end  26 . The actuator  32  may be formed as a one-piece slug, or as a plurality of segments although other types of actuators may also be implemented. A strap  34 , preferably made from steel or some other hard and ductile metal or alloy, is fixed to the housing  18  and passages through an intermediate portion of the passage  22 . A flat portion  36  of the strap  34  traverses a cross-section of the passage  22  thus providing a relatively large contact area for flush communication of a leading portion  37  of the actuator  32 . 
     In the embodiment shown in FIGS. 1 and 2, a clutch assembly or coupling mechanism  46  is fixed about the retractor shaft  16  whereby the shaft  16  functions as an axis of rotation. As illustrated, the clutch assembly  46  is fixed to a clip  48  that has a circumference roughly equivalent to that of the first passage  20  and a key corresponding to a female counterpart in the channel  20 . Clip  48  is thus key-fitted within the first passage  20  thus preventing rotation upon activation of the clutch assembly  46 . The clutch assembly  46  further includes a first segment  50  and a second segment  52 . A first shear pin  54  is press-fitted through an annulus  56  of the first segment  50 . A second shear pin  58  is press-fitted through an annulus  61  of the second segment  52 , whereby shear pins  54  and  58  form slight protrusions through the respective upper and lower surfaces of segments  50  and  52 . The protrusions  60  thus function to secure each segment to corresponding annuli in the clip  48 . At the other end of the segments, the segments may be held in place by annuli formed within the cover  59 , or, another clip may be used to secure the segments from the coiled energy of the strap  34  during normal vehicular operation. When the segments  50  and  52  are fixed in place, the retractor shaft  16  rotates freely of the segments except when the pretensioner is activated. 
     FIGS. 5-7 illustrate an alternative one-piece injection molding of a clutch assembly  46   a . The clutch assembly includes protrusions  60   a  integral to segments  50   a  and  52   a  thereby securing the clutch  46   a  to clip  48 . Segments  50   a  and  52   a  are joined together by spring members  62  and  64 . 
     In general, the clutch assemblies illustrated herein are merely exemplary of the many known clutches useful in the art. Accordingly, as used herein, the term “clutch assembly” is meant to apply generically to any clutch component or combination of components that are useful in the seatbelt retractor and pretensioner art. Stated another way, a “clutch assembly” is any component or combination of components that exert a rotatable and circumferential force about the retractor shaft  16  and thus produce a pretensioning or tightening of the belt  12  by winding the associated webbing reel  13 . U.S. Pat. Nos. 5,743,480 and 5,222,994, herein incorporated by reference, illustrate known alternatives. 
     The strap  34  is preferably wrapped at least twice around the periphery of segments  50  and  52 , and then hooked in place about an edge of either segment  50  or  52 . As the actuator  32  propels the strap  34  to the second end  26  of passage  22 , the strap  34  tightens about the clutch assembly  46 , shearing pins  54  and  58 , and exerting a peripheral force about the clutch assembly  46 . The forward motion of the actuator  32  thus results in compression and constriction of the clutch segments  50  and  52  about the retractor axle  16 . Rotary movement of the clutch assembly  46  and the webbing reel shaft  16  is thus effected as the associated teeth of the segments  50  and  52  mate with the associated teeth of the shaft  16 . Clutch operation therefore results in rotation of the webbing reel  13  of the retractor  14  and tightening or pretensioning of the associated belt webbing  12 . Cover  59  encloses the passages  20  and  22  and the components contained therein. 
     Thus, in accordance with the present invention, the linear movement of the actuator  32  and the strap  34  thus provides a multiplier effect similar to that provided by geared motion multipliers. The present invention, however, simplifies the design and manufacturing of known motion multipliers and yet retains the multiplier advantage of reducing the strap  34  or actuator  32  stroke requirement. Additionally, the substantially sealed housing largely prevents the release of gas, noise, and flame. 
     In the embodiment shown in FIG. 2, a cup portion  38  may be provided within the actuator  32  and may contain the propellant  30 , if desired. The propellant  30  is simply placed within the actuator  32  prior to installation thereof within passage  22 , and then ignitably connected to the initiator  28 . A spring  39  may if desired be placed between the initiator  28  (or first end  24 ) and the actuator  32 , thereby biasing the leading edge of the actuator  32  against the flat portion  36  of the strap  34 . 
     In one aspect of the invention as shown in FIG. 8, a hard and resilient cylindrical liner  27 , steel for example, may if desired be press fitted within the first end  24  to conform to the inner surface of the passage  22 , wherein the liner  27  extends up to the circumference of the first cylindrical passage  20 , but not in intersecting relationship with the diameter of passage  20 . Liner  27  thus enhances the strength of the combustion or pressure chamber  24  (first end  24 ) therefore permitting the use of lower strength materials for the housing such as aluminum or certain plastics. Material costs may thereby be reduced. 
     In another embodiment shown in FIG. 9, the steel sleeve insert  27  may alternatively be threadedly received or otherwise fixed to an opening in the exterior of housing  10  in axial alignment with passage  22 . The initiator  28  may be threadedly received or otherwise fixed to an opposite end of the sleeve  27 . The propellant  30  and the actuator  32  would then be sequentially positioned within the sleeve  27  between the initiator  28  and the housing  10 . At the second end of the sleeve  27 , the strap  34  and the leading edge of the actuator  32  would again be in flush communication prior to activation of the pretensioner  10 . In contrast to the embodiment shown in FIG. 8, the use of the sleeve  27  outside of the second cylindrical passage  22  permits a reduction in the size of the housing  10  and therefore a reduction in materials and weight. 
     As shown in FIG. 10, a pretensioner of the present invention may also utilize an initiator  28   a  in ignitable communication with a propellant  30   a  within a gas generator  38 , each contained within a third cylindrical chamber  40 . The third cylindrical chamber  40  runs parallel to the first cylindrical chamber  20 , and perpendicular to the second cylindrical chamber  22 . Gas produced from the gas generator is forced through into the second cylindrical passage  22 , thus driving the actuator  32  forward. 
     In operation, electrical contacts  44  on the initiator  28  communicate with a sensor that signals actuation of the pretensioner  10 . Once the initiator  28  receives a signal, from an accelerometer for example, the initiator  28  ignites the pyrotechnic gas generant  30 . The resultant gas pressure propels the piston or actuator  32  against the strap portion  36 , and then forces the actuator  32  and the strap  34  from the first end  24  to the second end  26 . As the strap  34  advances, the clutch assembly  46  engages the webbing reel shaft  16  and pretensions the belt  12 . Linear movement of the actuator  32  thus effects rotary movement of the shaft  16  thereby pretensioning the seatbelt webbing  12  contained and extending from the retractor  14 . 
     The housing  18 , the piston or actuator  32 , and the clutch assembly  46  may be die cast from aluminum, steel or suitable alloys thereof. Or, they may be injection molded if plastic material is used. In a preferred embodiment, the actuator  32  and the clutch assembly  46  are formed from hard resilient plastic thereby reducing the weight of the pretensioner  10 . The gas generator  38 , the gas generant propellant  30 , and the initiator  22  are all manufactured or obtained by methods known to those of ordinary skill. The strap  34  may be cut from steel sheet metal for example. 
     Resultant benefits include simplified manufacturing and retractor preservation. For example, see U.S. Pat. No. 5,899,399 to Brown et al. Also see U.S. Pat. No. 5,397,075 to Behr, herein incorporated by reference, wherein complicated gear driving mechanisms used to actuate the retractor axle can be replaced by the present invention. When utilized in conjunction with the linear movement of the actuator  32 , the strap  34  provides a multiplier effect similar to that provided by geared motion multipliers. The present invention, however, simplifies the design and manufacturing of known motion multipliers and yet retains the multiplier advantage of reducing the strap  34  or actuator stroke requirement. Manufacturing of the present pretensioner is also relatively simple given the straight cylindrical passages. The employment of linear passages not only simplifies manufacturing, but also reduces the friction loss of the applied load on the actuator as compared to arcuate passages, for example. Additionally, the use of linear and cylindrical passages facilitates the sealing of the passages as compared to other geometrically shaped passages. Finally, the essentially sealed housing  10  substantially prevents the release of gas, solids, noise, and flame. After activation of the pretensioner  10 , therefore, only the pretensioner  10  need be replaced. In general, all or most of the retractor mechanism  14  will be salvageable after pretensioner use. 
     Gas generants useful in conjunction with the present invention are gas generants well known to those of ordinary skill in the art. For examples, see U.S. Pat. Nos. 5,035,757, 5,460,668, 5,756,929, and 5,872,329, herein incorporated by reference. These compositions exemplify, but do not limit, useful gas generant compositions. 
     While the foregoing examples illustrate and describe the use of the present invention, they are not intended to limit the invention as disclosed in certain preferred embodiments herein. Therefore, variations and modifications commensurate with the above teachings and the skill and/or knowledge of the relevant art, are within the scope of the present invention.