Abstract:
Seat belt retractors for spooling seat belts (webbings) include multiple independent torque transmission or absorbing systems. The seatbelt retractor comprises a webbing, a spool, a pinion, a pretensioner, at least one coupler pawl, and a locking mechanism including a lock base and a lock pawl. The at least one coupler pawl is positioned in a cavity in the spool. During a predetermined low acceleration event, the at least one coupler pawl pivots so that a portion of the at least one coupler pawl leaves the cavity and engages the lock base to load the lock base which is then prevented from rotating in a spool extracting direction by the locking pawl. When the pretensioner activates during a predetermined higher acceleration event, the pretensioner rotates the pinion in the webbing retraction direction to rotate the spool in the webbing retraction direction.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/193,253, filed Nov. 12, 2008, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to the field of seat belt retractors for spooling seat belt (webbings) for restraining an occupant of a seat system of vehicles and the like. More specifically, this disclosure relates to a retractor having multiple independent torque transmission or absorbing systems. 
     A seatbelt device for use within a vehicle provides safety to an occupant by restraining the movement of the occupant during a sudden acceleration, typically resulting from a dynamic impact event of the vehicle. A typical seatbelt device includes a webbing or belt, a buckle, a tongue member to engage the buckle, a retractor, and an anchor member. Retractors include a spool and through the use of a force, often generated by a spring, wind the webbing around the spool. During a dynamic impact event of the vehicle, the retractor locks the webbing from extracting or unwinding, which restricts movement of the occupant. 
     It has been known to construct a retractor which includes a pretensioner and a locking mechanism. The pretensioner includes an explosive charge that rapidly generates gas during a vehicle dynamic event to create pressure to move a piston that may drive a rack, ball bearings, or any other member that may be coupled to a pinion gear through a teeth mesh. The pinion may be coupled directly or indirectly, through a member or hub, to a torsion shaft coupled to the spool, whereby rotation of the pinion transmits torque through the torsion shaft into the spool, creating torque to retract the webbing. This pretension applied to the seatbelt removes the slack between the webbing and the occupant, therefore reducing the movement that the occupant may be undergo during the dynamic impact event. The pretensioner may be deployed when sensors on the vehicle detect an impact event and are typically designed to deploy at high speed impacts. The locking mechanism may include a locking pawl driven by a force, from a spring or inertia, and having teeth to mesh with teeth of the frame member under low speed impacts, thus preventing the rotation of the spool and preventing the seatbelt from extracting. A locking pawl may be coupled to the torsion bar indirectly through other members. The torsion bar is designed to deform torsionally when subjected to a predetermined torque to absorb energy during loading, imparted by the mass of an occupant during acceleration of the vehicle, to reduce the restraint force exerted on the occupant during the dynamic impact event, thereby providing improved safety to the occupant. 
     When traditional retractors have a pretensioner and a spool dependently coupled, the automatic locking retractor (ALR) zone may shift due to yielding of the torsion bar from the high torque resulting from the acceleration of the occupant during a low speed dynamic impact event, when the pretensioner does not fire. This results in the ALR zone being no longer useable. This ALR zone shift may prohibit the locking mechanism from locking the retractor, allowing for extraction of the belt webbing, which reduces the ability of the seat belt system to restrain a child seat or an occupant. Additionally, the spool of these traditional retractors remains coupled to the pretensioner following deployment. This coupling creates an undesirable effect of having a delay or variable performance of energy management of the seat belt system, since to transmit torque through the torsion bar, the torque induced from restraining the occupant must overcome the energy (i.e., the torque) of the pretensioner. 
     Accordingly, an object of the present disclosure is to provide a cost effective retractor mechanism which includes a pretensioner and locking mechanism that are independently coupled to eliminate ALR zone shifting and additionally provide efficient energy management of the seat belt system. 
     SUMMARY 
     One exemplary embodiment relates to a seatbelt retractor for a seat belt device in a vehicle. The retractor comprises a frame and a spool configured to be fixed to one end of a webbing so that the webbing may be wound around the spool. The spool is rotatably mounted at both ends to the frame and is configured to rotate in a webbing extraction direction and a webbing retraction direction. The retractor further comprises a torsion bar positioned in the spool. A first end of the torsion bar is coupled to a torsion bar cam and a second end of the torsion bar is connected to a pinion. The torsion bar cam is configured to engage the spool. The retractor further comprises a pretensioner coupled to the second end of the torsion bar via the pinion. The pretensioner is configured to rotate the pinion in a webbing retraction direction when the pretensioner activates in response to an acceleration of the vehicle greater than a predetermined high acceleration. The retractor also comprises at least one coupler pawl positioned in a cavity in the spool and connecting the spool to a locking mechanism to prevent rotation of the spool in response to an acceleration of the vehicle greater than a low acceleration, but less than the predetermined high acceleration. The locking mechanism is configured to prevent rotation of the spool. The at least one coupler pawl is configured to pivot so that when the vehicle acceleration exceeds the predetermined high acceleration and the pretensioner is activated, the spool is not connected to the locking mechanism thereby allowing the spool to rotate relative to the locking mechanism. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below. 
         FIG. 1  is a side view of a vehicle showing a seat belt system including a retractor according to an exemplary embodiment. 
         FIG. 2  is a section view of an exemplary embodiment of a retractor with pretensioned spool, for use within a vehicle. 
         FIG. 3  is an exploded view of the retractor of  FIG. 2 . 
         FIG. 4  is an exploded view of a portion of the retractor of  FIG. 2 . 
         FIG. 5  is an exploded view of a portion of the retractor of  FIG. 2 . 
         FIG. 6  is a perspective view of the spool assembly for use within the retractor of  FIG. 2 . 
         FIG. 7  is a perspective section view of a retractor with pretensioned spool illustrating the load path when the pretensioner does not fire. 
         FIG. 8  is a perspective section view, taken along line  8 - 8  of  FIG. 7 , illustrating the coupler pawls engaged between the spool and the lock base. 
         FIG. 9  is a perspective section view, taken along line  9 - 9  of  FIG. 7 , illustrating the torsion bar cam allowing the coupler pawls to engage the spool. 
         FIG. 10  is a perspective section view of a retractor with pretensioned spool illustrating the load path when the pretensioner does fire. 
         FIG. 11  is a perspective section view, taken along line  11 - 11  of  FIG. 10 , illustrating the coupler pawls disengaged from the lock base. 
         FIG. 12  is a perspective section view, taken along line  12 - 12  of  FIG. 11 , illustrating the torsion bar cam contacting the spool. 
         FIG. 13  is a perspective section view illustrating the shear pins of the lock base, which engage the torsion bar cam, prior to shearing. 
         FIG. 14  is a perspective section view illustrating the shear pin of each the coupler pawl, which engage the spool, prior to shearing. 
         FIG. 15  is a cross-section view of a spool assembly for a retractor according to another exemplary embodiment taken through sheer pins extending from the torsion bar cam. 
         FIG. 16  is a cross-section view of the spool assembly of  FIG. 15  taken through the coupler pawls. 
         FIG. 17  is an illustration of a mechanism that causes a lock base pawl to engage a frame due to vehicle acceleration. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment of the disclosure, the retractor comprises two independent mechanisms to transfer or absorb torque. For a low acceleration event, the torque is absorbed to prohibit extraction of the webbing to mitigate the movement of the occupant to improve safety. Webbing extraction is prohibited by locking rotation of the spool in the extraction direction by loading at least one coupler pawl between a spool and a locking mechanism. The locking mechanism includes a lock base, which contacts the coupler pawls, and a locking pawl, having teeth that engage teeth of the frame, thus preventing rotation of the locking mechanism. For a high acceleration event, a torque is generated by a pretensioner to retract the webbing of the seat belt system to remove clearance between the webbing and the occupant to mitigate movement of the occupant to improve safety. The pretensioner may be pivotally coupled to a pinion and thereby rotates the pinion (in the webbing retraction direction), which is also pivotally coupled to a torsion bar thereby transferring rotation to the torsion bar. The torsion bar is also pivotally coupled, thereby transferring the torque, to a torsion bar cam, which contacts the coupler pawls, which also contact the spool, prohibiting rotation of the spool in the webbing extraction direction. The pretensioner is non-reversible, so after firing it is prevented from rotating in the webbing extraction direction and serves to lock the second end of the torsion bar. The first end of the torsion bar is locked to the spool, through the torsion bar cam, and then is subjected to a torque in the webbing extraction direction resulting from the force of the occupant being decelerated into the webbing. The torsion bar thereby absorbs this torque, from the occupant, and is designed to deform elastically and plastically at a predetermined torque to manage the energy from the occupant to mitigate the resultant force being transmitted back into the occupant. This mitigation of the force on the occupant through the torsion bar improves safety. 
     Referring to  FIG. 1 , a seat belt system  10  is shown according to an exemplary embodiment. The seat belt system  10  is used within a vehicle to help restrain the movement of an occupant  11  during a sudden acceleration, typically resulting from a dynamic impact event of the vehicle. The term acceleration refers to the absolute value of the acceleration that the vehicle experiences, whether negative (e.g., deceleration) or positive. The seat belt system  10  includes a webbing or belt  12 , a buckle  14 , a tongue member  16  to engage the buckle  14 , an anchor member  18 , and a retractor  20 . During a dynamic impact event of the vehicle, the retractor  20  locks the webbing from extracting or unwinding, which restricts movement of the occupant. The seat belt system  10  includes one or more sensors (not shown) configured to detect a sudden acceleration of the vehicle. The sensor(s) is configured to send a signal to a controller (not shown) for the retractor  20  to activate the retractor  20  as appropriate depending on whether the sensor detects a low or high acceleration event. 
     Referring to  FIGS. 2-6 , a retractor  20  is shown that includes a pretensioner  22 , a frame  24 , and a spool assembly  28 . The spool assembly  28  comprises a pinion  30 , a spool  40 , at least one coupler pawl  50 , a torsion bar  60 , a torsion bar cam  70 , a lock base  80 , and a lock pawl  90 . The pinion  30 , torsion bar  60 , torsion bar cam  70 , locking base  80 , and spool  40  share a substantially common pivot axis  100  ( FIG. 6 ). This pivot axis  100  is about which the webbing  12  of the seat belt system  10  may be extracted or retracted. The pretensioner  22  may be constructed according to known methods, and includes a coupling member to drive rotational motion into the pinion  30  during pretensioner deployment. The pretensioner  22  is also non-reversible, so that once the pretensioner  22  deploys to retract the webbing  12  of the seat belt system  10 , the pretensioner  22  may not be rotated in the webbing extracting direction D 1  (see  FIG. 8 ). 
     The pinion  30  may be made from steel or any other material strong enough to transmit the high torque generated by the pretensioner  22  and may be made by conventional methods (e.g., forging, broaching, machining). The pinion  30  includes a first and a second end, whereby the first end may be pivotally coupled to the pretensioner  22  by a tooth mesh  32 , which transfers the torque generated by the pretensioner  22  during deployment into the pinion  30 . The second end of the pinion, having a female key-way  34 , may be pivotally coupled to the second end of the torsion bar  60 , having a male key-way  64 . The key-way  34  may be any shape (e.g., star shape, polygon) that transmits the predetermined torque. The second end of the pinion  60  may also include a bearing surface  36  on its outer diameter that may couple to the inner surface  42  of the second end of the spool to provide substantial concentricity between the pinion  30 , torsion bar  60  and spool  40 , for smooth rotation of the spool assembly. The construction of the pinion  30  is not limited to that disclosed above, and may be constructed from any geometry which transmits the required torque to another member. For example, according to other embodiments, the second end of the pinion  30  may have a male key-way that may be pivotally coupled to the second end of the torsion bar  60 , having a female key-way or other useful shape to transmit torque. 
     The torsion bar  60  may be made from steel or other material strong enough to transmit the high torque generated by the pretensioner  22  and may be made through conventional methods (e.g., forging, broaching, machining). The torsion bar  60  includes a first and a second end, whereby the first end, having a male key-way  62 , may be pivotally coupled to first end of the torsion bar cam  70 , having a female key-way  72 . The second end of the torsion bar  60 , having a male key way  64 , may be coupled to the second end of the pinion  30 , having a female key-way  34 . The torsion bar  60  is pivotally coupled at both ends to transfer and to absorb a predetermined torque, which provides energy management through first elastic deformation, then by plastic deformation, as it yields under the torque generated by the extracting belt force resulting from the force of the occupant being decelerated during a vehicle impact event. The construction of the torsion bar  60  is not limited to that disclosed above, and may be constructed from any geometry which transmits the required torque to another member. For example, according to other embodiments, the first end of the torsion bar  60  may have a female key-way that may be pivotally coupled to the first end of the torsion bar cam  70 , having a male key-way or other useful shape to transmit torque. 
     The torsion bar cam  70  may be made from steel or other material (e.g., zinc) strong enough to transmit the high torque transferred through the torsion bar  60  from the pretensioner  22  and may be made through conventional methods (e.g., cast, forged then broached, machined). The torsion bar cam  70  includes a first and a second end, whereby the first end having a female key-way  72 , may be pivotally coupled to the first end of the torsion bar  60 , having a male key-way  62 . The second end of the torsion bar cam  70  may include a bearing surface  74  on its outer diameter, which contacts the inner bearing surface  84  of the first end of the lock base  80 , and may further include at least one shear pin hole  75 , to interface with a shear pin  86  of the lock base  80 . The shear pin(s)  86  are constructed to shear at a low torque, thereby allowing relative rotation between the torsion bar cam  70  and the lock base  80  during pretensioner  22  deployment. Additionally, the torsion bar cam  70  includes a plurality of protrusions  76 , which may extend outwards in the radial direction from the bearing surface  74  of the first end. Each protrusion  76  may be constructed to begin flush with the leading edge of the first end of the torsion bar cam  70 , or it may be positioned between its first and second ends of the torsion bar cam  70 . Each protrusion  76  may include a contact surface  77 , which during pretensioner deployment is rotated into contact with a mating contact surface  44  on the spool  40 , thereby transferring torque through the contact surfaces  77  and  44 . The torsion bar cam  70  also includes at least one protrusion  76  containing a cam face  78 , and according to the exemplary embodiment shown contains two cam faces  78 , each provided to rotate one coupler pawl  50 . 
     According to the exemplary embodiment shown, torque input into the torsion bar cam  70 , as generated by deployment of the pretensioner  22 , shears the shear pin(s)  86  from the lock base  80 , allowing counter-clockwise rotation (webbing retraction direction D 1 ), as shown in  FIG. 5 , of the torsion bar cam  70  until the contact surfaces  77  touch the contact surfaces  44  of the spool  40 . During this counter-clockwise rotation, the cam faces  78  of the torsion bar cam  70  drive the coupler pawls  50  outward in a radial direction, such that the contact surfaces  54  of the coupler pawls  50  may disengage from the mating contact surfaces  85  of the lock base  80 . The disengagement of the coupler pawls  50  allows the spool  40  to move independently of the lock base  80  when the pretensioner  22  is fired, and creates a smooth and controlled energy management by carrying the load through the torsion bar  60 . 
     The lock base  80  may be made from steel or other material (e.g., zinc) strong enough to transmit the torque transferred through the lock base  80  during low acceleration events, where the pretensioner  22  does not fire, and may be made through conventional methods (e.g., cast, cold forged, machined). The lock base  80  includes a first and a second end, whereby the first end further includes a protruded section  82 , having an inner bearing surface  84  and also having contact surfaces  85 , which contact the coupler pawls  50  to prevent rotation of the spool  40  in the webbing extraction direction D 1  during low acceleration events. The first end of the lock base  80  includes the at least one shear pin  86 , which is designed to shear at a predetermined torque to allow rotation of the torsion bar cam  70  relative to the lock base  80  during a high acceleration event in which the pretensioner  22  is fired. The second end includes a pivot surface  88  for the locking pawl  90  to attach and pivot about, and further includes guide surfaces  89  which the locking pawl  90  rotates within ( FIG. 6 ). 
     The lock or locking pawl  90  may be made from steel or other material (e.g., zinc) strong enough to transmit the torque transferred through the locking pawl  90  from low acceleration events and may be made through conventional methods (e.g., cast, forged then broached, machined). The locking pawl  90  comprises a pivot  93  that pivotally couples the locking pawl to the lock base  80 . The locking pawl further includes teeth  92  formed on the exterior to engage the teeth  26  of the frame  24  during low acceleration events to prevent extraction of the webbing of the seat belt system  10 . When the teeth  92  of the locking pawl  90  engage the teeth  26  of the frame  24 , rotation of the lock base  80  in the webbing extraction direction D 1  is prohibited, thus prohibiting rotation of the spool  40  in the webbing extraction direction D 1 . According to another embodiment, the locking pawl  90  may be pivotally coupled to the frame  24 , having teeth formed on the end opposite to the pivot that may engage teeth on the lock base during low acceleration events to prevent extraction of the webbing  12  of the seat belt system  10 . When the teeth of the locking pawl  90  engage the teeth of the lock base  80 , rotation of the lock base  80  in the webbing extraction direction D 1  is prohibited, which, in turn, prohibits rotation of the spool  40  in the webbing extraction direction D 1 . The locking pawl  90  rotates to engage or disengage the teeth  92  with the teeth  26  of the frame  24  based upon information received by an acceleration sensor. The locking pawl  90  moves due to vehicle acceleration.  FIG. 17  also illustrates the movement of the locking pawl  90  to engage the frame  24 . 
     Referring to  FIGS. 7-9 , a retractor  10  having a pretensioned spool  40  is shown. The load path P taken by forces exerted on the seat belt webbing  12  by an occupant  11  during a low acceleration impact whereby the pretensioner  22  is not fired is illustrated. This load path P follows from the belt  12 , into the spool  40 , into the coupler pawls  50 , into the lock base  80 , into the lock pawl  90 , then into the frame  24 . The webbing  12  is fixed to the spool  40 , and then wound around the spool  40  as the spool rotates about its pivot axis. The load is transferred from the spool  40  through the coupler pawls  50  and into the lock base  80 , as shown in  FIG. 8 . The spool  40  comprises cavities  46  to house each coupler pawl  50 , whereby the coupler pawl  50  may pivot so that the nose  52  of the coupler pawl  50  leaves the cavity  46  so that the contact surface  54  of the coupler pawl  50  engages the contact surface  85  of the lock base  80 , as shown in  FIG. 8 . A force (e.g., spring, inertia, etc.) may bias the coupler pawls  50  out of the cavities  46  to engage or load the lock base  80 . The lock base  80  is prevented from rotating in the spool extracting direction D 1 , because the lock base  80  is held fixed by the lock pawl  90 . The lock pawl  90  may have teeth  92  that extend outward in the radial direction to engage teeth  26  of the frame  24 . The frame teeth  26  are fixed and thereby the lock pawl  90  is fixed through the engagement of teeth  92  with teeth  26 , which in turn fixes the lock base  80 . Therefore, during low acceleration impacts, in which the pretensioner  22  does not fire, the webbing  12  of the seat belt assembly  10  is prevented from extracting, which limits the movement of the occupant  11  being restrained. Under conditions in which the pretensioner  22  does not fire (such as a low acceleration impact), the torsion bar cam  70  does not rotate, thereby allowing the coupler pawls  50  to remain in the engaged position with the lock base  80 , as shown in  FIG. 9 . 
       FIGS. 10-12  illustrate a retractor  20  having a pretensioned spool  40 , showing the load path P taken by forces exerted on the seat belt webbing  12  by an occupant  11 , during a high acceleration impact whereby the pretensioner  22  is fired. This load path P follows from the belt  12 , into the spool  40 , into the torsion bar cam  70 , into the torsion bar  60 , into the pinion  30 , then into the pretensioner  22 . The webbing  12  is fixed to the spool  40 , and then wound around the spool  40  as the spool  40  rotates about the pivot axis  100 . The load is transferred from the spool  40  directly into the torsion bar cam  70  through the contact surfaces  44  and  77 , as shown in  FIG. 12 . The torque generated by the pretensioner  22  rotates the pinion  30  in a direction D 2  opposite to the extracting direction D 1  (e.g. clockwise direction, relative to  FIG. 12 ) which in turn rotates the torsion bar in a direction opposite to the extracting direction D 1 . This torque between the torsion bar  60  and torsion bar cam  70  shears the shear pin(s)  86  from the lock base  80 , allowing rotation of the torsion bar cam  70  with respect to the lock base  80 , until the contact surfaces  77  of the torsion bar cam  70  contact the contact surfaces  44  of the spool (this condition is illustrated in  FIG. 12 ). This torque then induces rotation of the spool  40  in a direction to retract the belt (opposite to the extracting direction D 1 ), which removes slack between the belt  12  and occupant  11 , thereby mitigating the initial allowable deflection or movement of the occupant  11  during a high acceleration event. After the initial deployment of the pretensioner  22 , the force generated by the decelerating occupant  11  imparts a force on the webbing  12  of the seat belt system  10  in the spool extracting direction D 1 . This force generates a torque transferred from the spool  40 , through the torsion bar cam  70 , and into the torsion bar  60 . The second end of the torsion bar  60  is held fixed by the pinion  30  (e.g., through the engagement of key-ways  34  and  64 ), which is held fixed by the pretensioner  22 , since the pretensioner  22  is non-reversible. The first end of the torsion bar  60  may rotate, with respect to its fixed second end, at a predetermined torque, from elastic deformation and then from plastic deformation, after yielding, of the torsion bar  60 . This deformation allows the spool  40  to rotate along with the torsion bar cam  70 , thus allowing for the webbing  12  to extract a certain amount and mitigate the forces exerted on the occupant  11  during the acceleration event. This mitigation of forces exerted on the occupant  11  provides a smooth energy management method and improves safety to the occupant  11 . 
     Also referring to  FIGS. 11 and 12 , the rotation of the torsion bar cam  70 , in the clockwise direction D 2 , drives the coupler pawls  50  into the cavities  46  and out of engagement with the lock base  80 . Each coupler pawl  50  may be driven by a mating cam face  78  on the torsion bar cam  70 , which acts like a ramp whereby the pawl  50  rides up the ramp until the cam  78  runs out of surface, which corresponds to the pawl  50  being contained within the cavity  46  of the spool  40 . The torsion bar cam  70  also includes a flat surface  79  after the cam surface  78  to hold the pawl  50  in the cavity  46  during loading between the spool  40  and the torsion bar cam  70 . With the pawls  50  disengaged from the lock base  80  and with the torsion bar cam  70  contacting the spool  40 , the spool  40  may rotate in the webbing extracting direction D 1 , as the torsion bar  60  deforms. This configuration improves occupant safety by providing a smooth energy management method by loading directly through the torsion bar  60  without the effects from the lock pawl  90  and lock base  80 . 
     Referring to  FIG. 13 , shear pins  86  of the lock base  80  are shown, according to an exemplary embodiment, engaging the shear pin holes  75  in the torsion bar cam  70 . Shear pins  86  of the lock base  80  serve two primary purposes. First, the shear pins  86  improve manufacturability and aid functionality by providing a method of assembly which ensures correct orientation of the torsion bar cam  70  within the retractor  20 , such that the each cam face  78  of the torsion bar cam  70  is in position to contact and rotate a corresponding coupler pawl  50  out of engagement with the lock base  80  when the pretensioner  22  fires. Second, the shear pins  86  substantially eliminate relative motion between the torsion bar cam  70  and the lock base  80 , prior to the pretensioner  22  firing, which mitigates the potential for noise, which may be perceived by end users or customers as unwanted and annoying. Shear pins  86  of the lock base  80  are designed to shear at a predetermined torque lower than that which the torsion bar cam  70  is subjected to when the pretensioner  22  fires. 
     Referring to  FIG. 14 , according to another exemplary embodiment, the coupler pawls  50  may comprise shear pins  58  that engage corresponding shear pin holes  48  in the spool  40 . Shear pins  58  of the coupler pawls  50  serve two primarily purposes. First, the shear pins  58  improve manufacturability by providing a positive engagement feature between the coupler pawl  50  and spool  40 , which holds the coupler pawl  50  in place properly during assembly of the retractor  20 . Second, the shear pins  58  improve function, by having an axis of rotation about which the coupler pawl  50  rotates about when the torsion bar cam  70  drives the coupler pawl  50  out of engagement with the lock base  80 . According to another embodiment, shear pins may protrude from the spool  40 , engaging corresponding shear pins holes in the coupler pawls  50 , having the size of each pin tailored to a predetermined shear stress. 
     Referring now to  FIGS. 15 and 16 , cross-sections of a spool assembly  128  are shown according to another exemplary embodiment. The spool assembly  128  is similar to spool assembly  28  in function, but comprises a lock base  180  with a protruded section  182  that is a solid boss, rather than a generally cylindrical wall (i.e., protruded section  82  of lock base  80 , shown in  FIG. 4 ). The torsion bar cam  170 , therefore, lies against the surface of the protruded section  182  rather than being at least partially surrounded by protruded section  182 . The torsion bar cam  170  comprises one or more shear pins  175  that protrude from the second end of the torsion bar cam  170  and engage corresponding shear pin holes  186  formed in the lock base  180 . The shear pins  175  and shear pin holes  186  are analogous in function to the shear pins  86  and shear pin holes  75  of the previously described embodiment, having the a size that is tailored so that the shear pins  175  shear at a predetermined shear stress (e.g., a predetermined torque lower than that which the torsion bar cam  170  is subjected to when the pretensioner  22  fires). 
     As utilized herein, the terms “approximately,” “about,” “substantially”, and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims. 
     It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples). 
     The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. 
     References herein to the positions of elements (e.g., “top,” “bottom,” “above,” “below,” etc.) are merely used to describe the orientation of various elements in the FIGURES. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure. 
     It is important to note that the construction and arrangement of the retractor with pretensioned spool as shown in the various exemplary embodiments is illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention.