Abstract:
A seat belt retractor of the invention is basically formed of a first reel, a second reel, a rope extending between the first reel and the second reel, and a spiral spring situated in the second reel as an urging force providing device. The first reel is connected coaxially to a rotational axis of a reel shaft for winding a webbing, and has a drum shape on which the rope fixed at one end to the first reel is wound. The second reel has a rotational shaft disposed parallel to and laterally spaced away from the first reel. The second reel includes a small diameter reel portion, a large diameter reel portion, and a transition portion connecting the small diameter reel portion and the large diameter reel portion. The rope attached to the first reel extends to the small diameter reel portion and the transition portion, and is wound around a groove of the large diameter reel portion. The seat belt retractor is improved in assembling the members, and an approximately constant force for withdrawing the webbing can be achieved even if a withdrawal amount of the webbing is increased.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to a seat belt retractor, and more particularly, to a seat belt retractor wherein a force for withdrawing a seat belt is substantially uniform even when a withdrawing amount of the seat belt is increased. 
     Conventionally, in a seat belt retractor equipped in a motor vehicle, a seat belt (hereinafter referred to as a webbing in case of indicating a belt itself to be wound) is wound around a reel by spring driving force of a spiral spring or the like built in the seat belt retractor. In this type of the seat belt retractor, as a withdrawing amount of the webbing is increased, the spiral spring is wound so as to increase an urging-force thereof. Thus, there has been a situation that a force of withdrawing the webbing is increased as the amount of withdrawal of the webbing is increased. 
     To solve the aforementioned situation, a seat belt retractor has already been developed, which includes a tension reducing mechanism for reducing the force of withdrawing the webbing as disclosed in Japanese Patent No. 2,711,428. The tension reducing mechanism of the seat belt retractor is explained hereunder by referring to FIG.  7 . As shown in FIG. 7, a belt reel  51  of a seat belt retractor  50  is axially supported by a base frame (not shown) through a rotational shaft  59 . A webbing  57  is arranged to be wound on an outer peripheral surface of the belt reel  51 . Further, a housing  61  for the tension reducing mechanism is fixed on an outer side wall of the base plate (not shown). A shaft end  59   a  of the rotational shaft  59  extends to an inside of the housing  61  attached adjacent to the base frame. A first rope pulley  52  in a shape of a truncated cone is fixed to the shaft end  59   a  extending inside the housing  61 . A spiral guide groove  55  is incised on an outer peripheral surface of the first rope pulley  52 . 
     Furthermore, a second rope pulley  54  is accommodated in the housing  61  in a state that the second rope pulley  54  is axially supported through a rotational shaft  62 . As in the first rope pulley  52 , the second rope pulley  54  is formed in a shape of a truncated cone and provided with a spiral guide groove  58  on an outer peripheral surface thereof. The rotational shaft  62  is disposed parallel to the rotational shaft  59  at a position laterally spaced away therefrom with a predetermined distance. As shown in FIG. 7, a hollow chamber  63 , which has a cylindrical shape and is coaxial with the rotational shaft  62 , is formed in the second rope pulley  54 . The spiral spring  56  is situated inside the hollow chamber  63 . One end of the spiral spring  56  is fixed to the second rope pulley  54  supported by the rotational shaft  62 , and the other end of the spiral spring  56  is fixed to a bearing boss  65  formed in the hollow chamber  63  of the second rope pulley  54 . 
     Further, a rope  53  extends between the first rope pulley  52  and the second rope pulley  54 . In a condition that the webbing  57  is completely wound, one end of the rope  53  is fixed to a fixing end (not shown) at a largest diameter position of the guide groove  58  of the second rope pulley  54  such that the rope  53  is spirally wound in the entire periphery or length of the guide groove  58 , and the other end of the rope  53  is fixed to a fixing end  67  of a largest diameter portion of the guide groove  55  of the first rope pulley  52 . 
     In the tension reducing mechanism thus structured, an urging force for winding generated in case of winding up the spiral spring  56  is transmitted to the second rope pulley  54  as a rotational force or torque for winding the end portion of the rope  53  wound in the guide groove  58  of the second rope pulley  54 . At this time, as shown in FIG. 7, the rope  53  extends from the smallest diameter portion of the second rope pulley  54  to the largest diameter portion of the guide groove  55  of the first rope pulley  52 . When the webbing  57  in the wound state is withdrawn against a winding force shown by an arrow K in the figure, the first rope pulley  52  of the tension reducing mechanism rotates in a direction of an arrow A in the figure, so that the rope  53  is gradually wound on the first rope pulley  52 . At this time, the rope  53  is wound on the first rope pulley  52  while being held in the guide groove  55 . In accordance therewith, the rope  53  wound in the guide groove  58  is gradually unwound from the second rope pulley  54 . 
     As described above, the rope  53  held in the guide grooves  55  and  58  respectively formed on the outer peripheral surfaces of the first rope pulley  52  and the second rope pulley  54  transmits the rotations of the rope pulleys  52  and  54  therebetween. In this case, by adequately setting the cone shapes of the first rope pulley  52  and the second rope pulley  54 , a rotational ratio of the first rope pulley  52  to the second rope pulley  54  can be controlled, so that the force for withdrawing the webbing can be made constant or reduced regardless of a withdrawal amount of the webbing. 
     In the seat belt retractor shown in FIG. 7, however, since the tension reducing mechanism is formed of two rope pulleys in the shape of the truncated cone having the spiral guide groove, there have been a problem that assembling the rope pulleys with the housing is poor, and a problem that the miniaturizing the entire apparatus is difficult because the roller pulley itself can not be miniaturized. 
     Also, in the seat belt retractor, by winding the rope on the outer peripheral surface of the rope pulley in the shape of the truncated cone, the force for withdrawing the webbing is reduced with a long unwinding stroke. However, since the rope is wound several times on the outer peripheral surface of the truncated cone with a gentle lead angle, a ratio of decreasing a radius of the wound rope is small at the time of unwinding the rope, so that a change in the force of withdrawing the webbing can not be taken greatly. 
     Further, since the rope winding surface is inclined as a result of the shape of the truncated cone, it is necessary to form the deep guide groove so as not to allow the rope to be loosened and disengaged from the guide grooves in case of winding after the seat belt is suddenly withdrawn. 
     Accordingly, an object of the invention is to solve the aforementioned problems in the conventional retractor and to provide a seat belt retractor which is good in assembly and includes a compact tension reducing mechanism to achieve a high reliability in withdrawing the seat belt. 
     Further objects and advantages of the invention will be apparent from the following description of the invention. 
     SUMMARY OF THE INVENTION 
     To achieve the above object, a seat belt retractor of the present invention is formed of a first reel coaxially connected to a rotational axis of a reel shaft for winding a webbing therearound so as not to rotate relatively to the reel shaft; a second reel having a rotational shaft disposed parallel to and spaced away laterally from the first reel which winds a rope member including one end fixed to a predetermined position of the first reel; and urging force providing means accommodated inside the second reel. When the first reel is rotated by withdrawing the webbing, the rope member is wound around the first reel to cause a rotation of the second reel, by which the urging force providing means provides a force of urging the second reel in a direction of winding the webbing against this rotation of the second reel. In the seat belt retractor, by changing a rotational ratio of the first reel to the second reel, a force for withdrawing the webbing is decreased in accordance with an amount of withdrawal of the webbing. 
     The first reel is formed in a shape of a cylindrical drum, and the second reel is provided with a small diameter reel portion, a large diameter reel portion, and a transition portion connecting the small diameter reel portion and the large diameter reel portion. The rope member transferred from the first reel is wound around the large diameter reel portion through the small diameter reel portion and the transition portion. 
     In this case, preferably, the small diameter reel portion and the large diameter reel portion are coaxially provided in a step shape along an axial direction of the rotational shaft of the second reel, and formed integrally to constitute the second reel. 
     It is also preferable that an end surface of the large diameter reel portion is provided with a curved transition groove for connecting the small diameter reel portion and the large diameter reel portion, and the rope member is wound in the transition groove. 
     Further, in the above case, a spiral groove for communicating with the transition groove may be formed on a side surface of the large diameter reel portion, and the rope member is wound in the spiral groove. 
     Preferably, a line shape of the transition groove is a part of an arc having a diameter equivalent to an average of the diameter of the small diameter reel portion and the diameter of the large diameter reel portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a tension reducing mechanism of a seat belt retractor according to the present invention; 
     FIG. 2 is a partial sectional view integrally showing an internal structure of the tension reducing mechanism shown in FIG. 1 in a state that a webbing is wound most; 
     FIG. 3 is a partial sectional view integrally showing an internal structure of the tension reducing mechanism in a state that the webbing is withdrawn most; 
     FIG. 4 is a schematic explanatory view showing positions of a first reel and a second reel, and a relationship of the sizes thereof, in a state that a rope is held between the reels; 
     FIGS.  5 ( a ),  5 ( b ) and  5 ( c ) are schematic explanatory views showing a relationship between a state of withdrawing the webbing and a state of winding the rope wound up on the second reel; 
     FIG. 6 is a diagram of a relationship curve showing a relationship between an amount of withdrawal of the webbing and the force of withdrawal of the webbing in the seat belt retractor; and 
     FIG. 7 is a partial sectional view showing an internal structure of a tension reducing mechanism of a conventional seat belt retractor. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, a structure of a seat belt retractor of the present invention will be explained by referring to the attached drawings. FIG. 1 is an exploded perspective view showing an internal structure of a tension reducing mechanism as an assembly of the seat belt retractor of the invention. 
     The seat belt retractor  1  of the invention is generally formed of a reel shaft  2  on which a webbing W is wound up; a base frame  3 , an outline of which is shown by imaginary lines, for freely rotatably supporting the reel shaft  2 ; a reel lock mechanism  4 , an outline of which is shown by an imaginary line, for preventing a rotation of the reel shaft  2 ; and a tension reducing mechanism  10  which is a feature of the invention. 
     The base frame  3  is a steel plate product having a general U-shape in a plan shape, and supporting holes  3   b  through which end flanges of the reel shaft  2  fit with play are formed in right and left side walls  3   a.  Respective peripheral edges  3   c  of the supporting holes  3   b  are provided with sawtooth-like portions (not shown). An engaging piece (not shown) which swings by a locking operation of the known reel lock mechanism  4  and is attached to one of the side walls  3   a,  engages the peripheral edges  3   c.  This engagement motion locks a rotation of the reel shaft  2  to prevent withdrawal of the webbing in case the webbing is suddenly withdrawn from the seat belt retractor  1 . The tension reducing mechanism  10  which can adjust a force of withdrawal of the webbing W is attached to the other of the side walls  3   a  of the base frame  3 . 
     A structure of the tension reducing mechanism  10  is explained by referring to FIGS. 1 and 2. As shown in FIG. 1, the tension reducing mechanism  10  comprises a housing formed of a retainer plate  11  and a cover plate  12 ; a first reel  20  accommodated inside the housing as urging force transmitting means; a second reel  30  rotatable reciprocally with the first reel  20  through the rope  25  wound around the first reel  20  and extending between the reels, the second reel  30  having a larger diameter than that of the first reel  20 ; and a spiral spring  40  as urging force providing means disposed inside the second reel  30 . The spiral spring  40  provides a predetermined torque to the second reel  30 . 
     Among these members, the retainer plate  11  made of synthetic resin is fixed to the side wall  3   a  of the steel-made base frame  3  through a grommet and an engaging piece (not shown) which are integrally formed at a predetermined position. A bearing opening  13  is formed on an upper projection ll a  of the retainer plate  11 . A reel end of the first reel  20  is rotatably supported in the bearing opening  13 . Also, a bearing boss  14  is integrally projected from an approximately central portion of the retainer plate  11 . A rotational shaft end  31   a  of the second reel  30  is rotatably supported on the bearing boss  14 . Further, an engaging groove  14   a  is formed in a part of the bearing boss  14 . An inner peripheral end  41  of the spiral spring  40  is fixed in the engaging groove  14   a.  Furthermore, around the bearing boss  14 , there is formed an arc-shaped guide  11   b  which is coaxial to an axis of the bearing boss  14  and has a size almost the same as an outer diameter of the second reel  30 . 
     As shown in FIG. 2, the spiral spring  40  as the urging force providing means is accommodated in a hollow portion  39  formed at a side of the retainer plate  11  in the second reel  30 . In the spiral spring  40 , the inner peripheral end  41  is fixed to the bearing boss  14  of the retainer plate  11  as described above, and an outer peripheral end  42  is fixed to an engaging portion  39   a  formed in a part of an inner peripheral surface of the hollow portion  39 . In this case, when the second reel  30  rotates around a rotational shaft  31  supported by the bearing boss  14  upon withdrawal of the webbing, an urging force for rotating the second reel  30  in the direction of winding up the webbing W is stored in the spiral spring  40 . 
     Next, the two reels as the urging force transmitting means are explained. As shown in FIGS. 1 and 2, one end of the first reel  20  is fitted coaxially with an axial end of the rotational axis  5  of the reel shaft  2  not to rotate relative to each other, and is rotatably supported in the bearing opening  13  of the retainer plate  11 . The other end of the first reel  20  fits in the bearing hole  15  formed in the cover plate  12 . The first reel  20  has a length approximately the same as the inner size of the housing, and has a shape of a cylindrical drum with a uniform diameter. Both ends of the cylindrical drum are provided with flanges  21  to prevent a rope  25 , described later, from being disengaged from the drum in case the rope  25  is wound several times. Also, the flanges  21  adequately define a space between the retainer plate  11  and the cover plate  12 , so that the reel is prevented from being shifted in the axial direction in case the first reel  20  rotates. 
     On the other hand, the rotational shaft  31  of the second reel  30  is disposed parallel to the rotational shaft  22  of the first reel  20 , and one end of the rotational shaft  31  is rotatably supported by a bearing hole  14   b  of the bearing boss  14  described above. The other end of the rotational shaft  31  is fitted in a bearing hole  16  formed in the cover plate  12 . In the present embodiment, the proportion of a diameter Φ2 of the second reel  30  to a diameter Φ0 of the first reel  20  is set for 5:1 as shown in FIG.  4 . Although the thickness D of the second reel  30  is determined by a plate width of the spiral spring  40  accommodated therein, the thickness D is set for about 10 mm here. 
     As shown in FIG. 1, a square groove  33  is spirally incised on a peripheral surface  32  having the largest diameter of the second reel  30 , hereinafter referred to as a large diameter reel portion. The square groove  33  is incised in three turns on the peripheral surface of the large diameter reel portion  32  in the present embodiment. On the other hand, on an end surface  32   a  of the large diameter reel portion  32 , a small diameter reel portion  34  having a diameter Φ1 is provided coaxially to the rotational shaft  31  of the second reel  30 , and is formed integrally with the large diameter reel portion  32  in a step shape. The proportion of the diameter Φ1 of the small diameter reel portion  34  to the diameter Φ2 of the large diameter reel portion  32  is set for 2:5. Thus, the proportion of the diameter Φ0 of the first reel to the diameter Φ1 of the small diameter reel portion  34  is 1:2, and the proportion of the diameter Φ0 of the first reel to the diameter Φ2 of the large diameter reel portion  32  is 1:5, so that the rotation urging force by the spiral spring  40  is reduced in accordance with the proportions of the diameters, and transmitted to the first reel  20 . 
     A disc-shaped flange  34   a  is integrally formed on an end surface of the small diameter reel portion  34 , and the small diameter reel portion  34  constitutes a circumferential groove  37 . Further, to communicate between the circumferential groove  37  of the small diameter reel portion  34  and the square groove  33  of the large diameter reel portion  32 , a square groove  38  constituting a diameter transition portion  35  in a semicircular shape having a diameter of R=(Φ1+Φ2)/2 (radius r=(Φ1+Φ2)/4) is formed from a position, in which the square groove  33  contacts the end surface  32   a,  to the end surface  32   a  as shown in FIG.  4 . Incidentally, a line shape of a groove curve of the square groove  38  as the diameter transition portion  35  is not limited to a part of the arc, and can be one of various kinds of smooth curves, such as involute and hyperbola. 
     As described above, the rope  25  with a small diameter is wound around the first reel  20  and the successive square grooves  37 ,  38  and  33  of the second reel  30  as shown in FIG.  2 . 
     Although a rope formed of aramide fibers with a diameter of about 0.5 mm is used as the rope  25  in the present embodiment, various kinds of fiber ropes can be used as long as an elongation amount of the rope is below an allowable value in case of applying a predetermined tension to the rope. In a condition that the webbing W is wound up substantially in the retractor  1 , one end of the rope  25  is fixed to the drum of the first reel  20 , and the rope  25  is wound therearound several times. Then, the rope  25  is extended to the small diameter reel portion  34  of the second reel  30  with a predetermined tension. In the small diameter reel portion  34  of the second reel  30 , the rope, which enables about half of the entire withdrawing amount of the webbing to be withdrawn, is wound in the circumferential groove  37  in advance. Further, the rope  25  is guided to the large diameter reel portion  32  along the square groove  38  of the diameter transition portion  35 , and the rest of the rope  25  is wound around the entire periphery or length of the square groove  33  of the large diameter reel portion  32 . The final end of the rope  25  is fixed to a fixing portion  33   a  at the end of the square groove through fixing means, such as a screw or the like (not shown). 
     Incidentally, FIG. 2 shows a state that the webbing W is wound up most in the seat belt retractor  1 . In this state, the rope  25  extends between the first reel  20  and the small diameter reel portion  34  of the second reel  30 . 
     FIG. 3 shows a state that the webbing W in the webbing wound up state is withdrawn to the maximum withdrawal length. By withdrawing the webbing W in a direction of an arrow B in the figure, the first reel  20  integrally connected to the axial end of the rotational axis  5  of the reel shaft  2  rotates along with the rotation of the reel shaft  2 , and an operation of winding the rope  25  wound around the second reel  30  onto the first reel  20  is started. Then, when the webbing withdrawal length reaches the maximum, the rope  25  extends between the first reel and the large diameter reel portion  32  of the second reel. 
     Here, a state of winding the rope  25  in the second reel  30  in accordance with the amount of withdrawal of the webbing W and a reducing effect of a force for withdrawing the webbing according to the state of winding of the rope are explained with reference to FIG.  5 ( a ) through FIG.  6 . 
     In the present embodiment, the entire length of withdrawal of the webbing W is set for 2,000 mm. By an operation of withdrawal of the webbing W in the length of about 1,000 mm equivalent to a first half of the entire withdrawal length, the rope  25  wound around the circumferential groove  37  in the small diameter reel portion  34  of the second reel  30  is withdrawn therefrom to be wound around the first reel  20  as shown in FIG.  5 ( a ). Meanwhile, a force for urging rotation corresponding to the rotation of the second reel is stored in the spiral spring  40  disposed inside the second reel  30 . Therefore, according to the amount of withdrawal of the webbing W, the force for withdrawing the same is gradually increased as shown as ( 1 ) in FIG.  6 . When the webbing W is withdrawn further, the rope  25  wound around the square groove  38  of the diameter transition portion  35  in the second reel  30  is unwound therefrom, and then the rope  25  wound around the large diameter reel portion  32  is unwound therefrom as shown in FIGS.  5 ( b ) and  5 ( c ). 
     At this time, the force for withdrawing the webbing W is suddenly decreased when unwinding the rope  25  is finished at the diameter transition portion  35  and is transferred to the large diameter reel portion  32  as shown as length ( 2 ) in FIG. 6, because of a wheel and axle effect obtained by a sudden increase in the diameter of the wound rope. Then, the force for withdrawing the webbing W at the large diameter reel portion  32  has a low value, and is gradually increased to the maximum length of withdrawal as shown as length ( 3 ) in FIG.  6 . 
     As described above, in the seat belt retractor of the embodiment, the first reel has a shape of a cylindrical drum with the small diameter, and on the other hand, the second reel is provided with the small diameter portion and the large diameter portion. The small diameter portion and the large diameter portion are connected by the spiral transition portion, and the rope extends between the first reel and the second reel. Therefore, assembly of the respective members is improved. Also, in the operation, regardless of the amount of withdrawal of the webbing, the webbing can be withdrawn by almost a uniform force for withdrawal, and loosening of the rope does not occur even after sudden withdrawal of the webbing. Therefore, high reliability is obtained in the winding and withdrawing operations of the webbing. 
     While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.