Abstract:
A seat belt retractor assembly has a seat belt retractor for generating a retraction force to rewind a seat belt on a reel. A coupling device selectively couples and decouples the retractor from the reel. A tensioning device generates a tensioning force in the seat belt when in an activated state. The coupling device decouples the retractor from the reel when the tensioning device is activated.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a seat belt retractor assembly. 
     A seat belt for a passenger vehicle often includes a seat belt retractor that retracts the seat belt to remove slack when drawn over a passenger or, alternatively, to withdraw the seat belt when no longer in use. A coil spring generally provides the necessary rewinding or retraction force for the retractor. When the seat belt is drawn over a passenger, the rewind force from the retractor typically increases. In some systems, the amount of force may be objectionable to the vehicle occupant. 
     Applicant has developed a tension reducing feature that reduces this rewinding force from the seat belt retractor. The details of this seat belt retractor are disclosed and hereby incorporated into this disclosure by U.S. Pat. No. 5,934,595 to Kohlndoffer, et al. Essentially, this tension reducing feature allows for a variable output of the retraction force from the seat belt retractor. One embodiment of the invention uses a cable to couple the seat belt retractor to a reel on which the seat belt is rewound. The cable itself is rewound upon a cone having varying diameters. The cone is linked to the retractor. As the seat belt is drawn out, the cable draws out the spring of the seat belt retractor. Because the cable is wound around the cone of the retractor, the retraction force may also vary. 
     Current seat belt systems may also include a so-called “pretensioner.” The pretensioner serves to remove slack in a seat belt in the event of a vehicle accident. The pretensioner removes the slack more quickly than a seat belt retractor. A pretensioner may be located either at the belt buckle or the seat belt retractor. For example, upon the detection of a crash, a buckle pretensioner has a gas cylinder that pushes a piston. The piston is linked to the seat belt and quickly applies a tensioning force to the seat belt in the direction of the belt buckle so as to remove slack in the seat belt across the passenger. 
     When a pretensioner is used with a tension reducing feature, a design challenge arises. When used with a buckle pretensioner, the rapid acceleration of the retractor can cause high stress and breakage in the cable of the tension reducing feature. Moreover, when the retractor locks up during the buckle pretensioner event, the rotational momentum of the tension reducing cone may cause it to over travel and thereby induce slack in the cable, which may foul the retractor. For a retractor pretensioner, the rapid acceleration of the spool may cause the tension reducing cone to create slack in the cable and also foul the seat belt retractor. 
     A need therefore exists for a seat belt retractor assembly that permits tension reduction but avoids the problems that may arise when the pretensioner is activated. 
     SUMMARY OF THE INVENTION 
     Like existing seat belt retractor assemblies, the invention has a seat belt retractor that rewinds a seat belt on a reel. A tensioning device, such as a pretensioner, generates a tensioning force in the seat belt in the event of a vehicle accident. In contrast to other seat belt retractor assemblies, however, the inventive assembly has a coupling device that selectively couples and decouples the retractor from the reel. In this way, in the event of a vehicle accident, the seat belt retractor is decoupled from the seat belt system so that only the tensioning device controls movement of the seat belt. In so doing, problems of line fouling and line breakage are avoided. 
     The retractor has a tension reducing feature that reduces the amount of retraction force on the seat belt from the seat belt retractor. This tension reducing device may be linked to the reel for the seat belt by a cable. In addition, the tension reducing device may reduce the retraction force on the seat belt in a variable manner. 
     The coupling device may comprise a clutch that couples and decouples the retractor from the reel. The clutch may have a first clutch plate that selectively engages a second clutch plate. The first clutch plate is linked to the retractor while the second clutch plate is linked to the reel. A spring may bias the first clutch plate into contact with the second clutch plate. 
     The clutch may also comprise an overrunning clutch. The clutch decouples the retractor from the reel when the reel reaches a speed faster than the speed of the retractor. The overrunning clutch may have a first clutch plate that has teeth and a second clutch plate that has grooves to receive the teeth. The teeth decouple from the grooves when the speed of the reel exceeds the speed of the retractor. A spring may bias the first clutch plate into contact with the second clutch plate. 
     The invention further includes a method of controlling retraction of a seat belt. A seat belt retractor is coupled to the seat belt. The seat belt retractor generates a retraction force on the seat belt. In addition, a tensioning device is connected to the seat belt for generating a tensioning force on the seat belt in the event of a vehicle accident. The seat belt retractor is decoupled from the seat belt when the tensioning device is activated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
         FIG. 1  illustrates the inventive seat belt retractor in a vehicle cabin. 
         FIG. 2  illustrates the inventive seat belt assembly, showing the clutch in relation to the seat belt reel and the seat belt retractor. 
         FIG. 3  illustrates a cross-sectional view of one type of clutch of the inventive seat belt retractor assembly. 
         FIG. 4  illustrates clutch plates of the clutch of  FIG. 3 . 
         FIG. 5  illustrates a side cross-sectional view of another type of clutch of the inventive seat belt retractor assembly. 
         FIG. 6  illustrates a close up view of clutch plates of the clutch of  FIG. 5 . 
         FIG. 7  illustrates an exploded perspective view of the inventive clutch plates of  FIGS. 5 and 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates seat belt retractor assembly  10 . Like existing seat belt retractor assemblies, seat belt retractor assembly  10  comprises seat belt retractor  22 , which rewinds seat belt  18  on reel  14  as shown in  FIG. 2 . As shown in  FIG. 1 , seat belt retractor  22  is mounted to B-pillar  21  of a vehicle. As known, seat belt  18  is connected to seat belt retractor  22 , turning loop  19 , and seat belt buckle  17 . In the event of a vehicle accident, a tensioning device, or pretensioner, removes slack between seat belt retractor  22  and seat belt buckle  17 . Either buckle pretensioner  23  or seat belt retractor pretensioner  25  may be used as a tensioning device. If buckle pretensioner  23  is employed, buckle pretensioner  23  creates tensioning force BT to draw seat belt  18  in the direction shown toward seat belt buckle  17 . Alternatively, in the event a retractor pretensioner is employed, retractor pretensioner  25  creates tension in the direction of R T  as shown in  FIG. 1  to remove slack in seat belt  18 . In either case, the movement of belt  18  will be very rapid and exceed the speed that retractor  22  operates. As shown in  FIG. 2 , to avoid damaging retractor  22 , seat belt retractor assembly  10  employs clutch  30 , which decouples retractor  22  from reel  14  in the event tensioning device, either buckle pretensioner  23  or retractor pretensioner  25 , is activated. 
     The details of the functioning of seat belt retractor assembly  10  will now be explained. With reference to  FIG. 2 , seat belt  18  is wound around reel  14 , which is connected to pin  27  so that pin  27  rotates with reel  14 . Pin  27  is linked to clutch  30 . Clutch  30  is linked to retraction spring  29  through line  42 . When seat belt  18  is drawn out from reel  14  in the direction of arrow A, spring  29  is compressed to create retraction force F. Retraction force F moves seat belt  18  in the direction of arrow B around reel  14  to take up seat belt slack or to return seat belt  18  to its housing. Retraction force-reducing device  38 , a tension reducing device, may be used to reduce retraction force F on seat belt  18 . As shown in  FIG. 2 , retraction force-reducing device has cone  39 , which allows for variable output force of retraction spring  29  as taught by U.S. Pat. No. 5,934,595. 
     In the event of a vehicle accident, seat belt  18  experiences tensioning force from the pretensioner that greatly exceeds any retraction force from retraction spring  29 . If the retractor pretensioner  25  is employed, seat belt  18  is accelerated with force R T  in the direction of arrow B. Alternatively, in the event a buckle pretensioner is used, seat belt  18  experiences tensioning force B T  in the direction of arrow A. In either instance, clutch  30  decouples movement of reel  14  from seat belt retractor  22 . In this way, movement of reel  14  caused by pretensioning force, either R T  or B T , will not affect line  42  or retractor spring  29 . 
     The functioning of clutch  30  will now be explained with reference to  FIGS. 3–7 . For buckle pretensioner  23 , clutch  30  may comprise first clutch plate  46 , which is mechanically linked to retractor  22  through line  42 . As shown in  FIG. 3 , retractor clutch plates  46  are part of spindle  49 , upon which line  42  is wound and linked to spring  29  through cone  39 . Reel clutch plates  50  are rotationally linked to pin  27 , which is rotationally linked to reel  14  as shown in  FIG. 2 . Reel clutch plates  50  are compressed onto retractor clutch plates  46  by spring  54 . 
     As shown in  FIG. 4 , during normal seat belt retraction operation, retractor clutch plates  46  will rotate at the same speed and same direction T as reel clutch plate  50 , thereby coupling reel  14  to retractor  22 . However, in the event buckle pretensioner  23  activates, reel clutch plates  50  will rotate in direction S while retractor clutch plates  46  will rotate in direction T, a direction opposite of S. This is so because buckle pretensioner  23  will pull seat belt  18  in an opposite direction of retractor  22 . Consequently, reel clutch plates  50  will slip with respect to retractor clutch plates  46  to thereby decouple reel  14  from retractor  22 . Accordingly, line  42  will not foul and cone  39  will not over spin. 
     Alternatively, in the event retractor pretensioner  25  is employed, it is preferable to use one way overrunning clutch  58  as shown in  FIGS. 5–7 . Overrunning clutch  58  comprises first clutch plate  62  and second clutch plate  66 . First clutch plate  62  has teeth  70 , shaped like ramps extending around its periphery as shown in  FIG. 7 . In addition, for each tooth  70 , second clutch plate  66  has ramp-shaped grooves  74 , which are sized to receive teeth  70 . Spring  54  applies spring force S to put first clutch plate  62  into contact with second clutch plate  66 . 
     During normal operation, both first clutch plate  62  and second clutch plate  66  rotate in the direction of arrow V as shown in  FIGS. 6 and 7  at the same speed, Q 1 . In the event pretensioner  25  is activated, belt  18  is drawn in the direction of arrow B and causes reel  14  as well as second clutch plate  66  to rotate at speed, Q 2 , a speed greater than Q 1 . As shown in  FIGS. 6 and 7 , second clutch plate  66  rotates at speed Q 2  while first clutch plate  62  tends to rotate at a lower speed, G 1 . Consequently, first clutch plate  62  slips from second clutch plate  66  to thereby decouple seat belt retractor  22  from reel  14 . 
     The aforementioned description is exemplary rather that limiting. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed. However, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. Hence, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For this reason the following claims should be studied to determine the true scope and content of this invention.