Abstract:
Disclosed is a seat belt pretensioner device for moving a moveably supported seat belt buckle in a direction to increase the tension of a seat belt with a power generating device so as to automatically increase the tension of the seat belt in case of an impact situation. such as a vehicle crash. The power generator comprises a bladder-like member that can be pressurized for expansion, and a chamber defined for guiding the direction of the expansion of the bladder-like member. The buckle of the seat belt can be moved in the direction to increase the tension of the seat belt by pushing a member supporting the buckle with the bladder-like member.

Description:
TECHNICAL FIELD 
     The present invention relates to a pretensioner device for automatically increasing the tension of an automotive seat belt. 
     BACKGROUND OF THE INVENTION 
     An automotive scat is typically provided with a seat belt to restrain the vehicle occupant and prevent the occupant from being thrown forward at the time of a vehicle crash. A seat bolt is often incorporated with an emergency locking retractor (ELR) device for locking a winding shaft of the seat belt only at the time of a sudden stop and/or a vehicle crash while permitting free movement of the vehicle occupant under normal conditions. An ELR device locks up the winding shaft in an extremely short time upon detection of deceleration exceeding a prescribed level, and thereby prevents the seat belt from being paid out any further. 
     When the winding shaft is simply locked up, it is unavoidable to have a certain length of the seat belt resulting from the tightening of the length of the seat belt wound on the winding shaft to be paid out. In view of this problem, pretensioner devices of different kinds have been proposed, and one of such pretensioner devices is designed to pull the buckle attached to the seat belt in the direction to increase the tension of the seat belt so as to improve the restraining capability of the seat belt. 
     A typical buckle pretensioner device includes a base plate for attaching the device main body to one side of a seat, an arm having one end pivotally attached to the base plate and another end connected to a base end of a buckle, a power generator attached to the base plate to pull the buckle via a link mechanism and the arm, and a reverse preventing device for preventing the buckle from reversing its motion once it is pulled by the power generator. 
     Because the power generator typically uses the power generated by igniting a propellant received in a cylinder and pushing out a piston member with the resulting combustion gas, it is necessary that the linear motion of the piston is converted into a rotary motion of the arm, and this rotary motion is in turn converted into the linear motion of the buckle. Therefore, the efficiency of power transmission is dictated by the length of the arm, and the size and configuration of the link mechanism. For this reason, it becomes necessary to use a relatively large power generator, a long arm and a complex link mechanism, and these factors contributed to the increase in the size of the device. In the case of a front seat, the buckle is typically provided in a gap defined between a seat slide rail and a center console, and this gap is normally so small that the excessive size of the pretensioner device may require a redesigning of the seat and the entire layout of the passenger compartment. 
     The present invention was made in view of eliminating such problems of the prior art, and its primary object is to provide a seat belt pretensioner device which is compact, requires a small number of component parts and increase the reliability of operation, and is easy to assemble. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the present invention, such an object can be accomplished by providing a seat belt pretensioner device for moving a moveably supported seat belt buckle in a direction to increase the tension of a seat belt with a power generating device, and preventing the reversing of the movement of the buckle with a reverse preventing device, so as to automatically increase the tension of the seat belt in case of an impact situation such as a vehicle crash, characterized by that: the power generator is adapted to produce a pressure by means of a combustion of a propellant; a bladder-like member being expanded under the pressure produced from the power generator inside a chamber defined for guiding a direction of expansion of the bladder-like member to push a member supporting the buckle or, without using a bladder-like member, the member supporting the buckle being moved directly under pressure produced from the combustion of a propellant, to move the buckle in the direction to increase the tension of the seat belt. The reverse preventing device may comprise a plurality of ratchet teeth, and a plurality of pawl members which are adapted to engage the ratchet teeth at different timing. 
     Because the combustion gas pressure obtained from the power generating device can be directly applied to a buckle supporting member to actuate the buckle once the air-tightness of the parts for transmitting the pressure is ensured, a link mechanism is not required, and the number of component parts can be minimized. This contributes to the compact and light-weight design of the overall device. The movement of the buckle is achieved only through a change in the inner pressure of the chamber of the bladder-like member from the start to the end of its motion, and there is no cause for attenuation of power transmission due to the changing of the direction of a force by a link mechanism or other mechanical structures so that the size of the power generator can be minimized. In particular, according to an arrangement where a buckle support member supporting the buckle at a peripheral end thereof is rotatably supported by a seat, it is preferable to define a curved chamber along the rotational direction of the buckle support member for transmission of pressure. 
     Because the exposed parts consist of the buckle and a portion of the buckle support member, the interference with other components or vehicle occupants at the time of operation is minimized, and the layout and design of the device are both simplified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a seat to which the present invention is applied; 
     FIG. 2 is a side view of a seat belt pretensioner device embodying the present invention, with one of the plates being omitted; 
     FIG. 3 a  is a bottom view of the seat belt pretensioner device according to the present invention; 
     FIG. 3 b  is a sectional view taken along line III—III of FIG. 2; 
     FIG. 4 is an exploded perspective view of the seat belt pretensioner device according to the present invention; 
     FIG. 5 a  is a sectional view of the balloon packing in its retracted state; 
     FIG. 5 b  is a sectional view of the balloon packing in its deployed state; 
     FIG. 5 c  and  5   d  are views similar to FIG. 5 a  showing different embodiments of the seat belt pretensioner device; 
     FIG. 6 is a view for showing the mode of operation of the seat belt pretensioner device according to the present invention: 
     FIG. 7 is a view similar to FIG. 2 showing an alternate embodiment of the seat belt pretensioner device according to the present invention; 
     FIGS. 8 a ,  8   b  and  8   c  are views showing the mode of operation of the alternate embodiment of the seat belt pretensioner device according to the present invention; 
     FIG. 9 is a view similar to FIG. 2 showing another alternate embodiment of the seat belt pretensioner device according to the present invention; and 
     FIG. 10 is a sectional view taken along line X—X of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the preferred embodiments of the present invention are described in the following with reference to the appended drawings. 
     FIG. 1 generally shows a vehicle seat to which the present invention is applied. Referring to FIG. 1, a seat belt  3  which extends upward from an ELR device  2  fixedly attached to a lower part of a center pillar  1  of a passenger compartment is passed through a through ring  4  attached to an upper part of the center pillar  1  and then extends downward. An extreme end  6  of the seat belt is attached to a rear part of one side of a seat  5 . The part of the seat belt  3  extending between the through ring  4  and the extreme end  6  is provided with a tongue plate  7  so as to be moveable along the length of the seat belt  3 . The side of the seat remote from the point of attachment of the seat belt end  6  is provided with a buckle  8  via a pretensioner device P embodying the present invention. 
     As the vehicle occupant seated in the seat  5  pulls out the seat belt  3  from the ELR device  2  and engages the tongue plate  7  with the buckle  8 . the seat bolt  3  is passed along the shoulder, chest and waist of the vehicle occupant. 
     The pretensioner device P is designed to automatically increase the tension of the seat belt at the time of a vehicle crash or the like, and as shown in FIGS. 2 to  4 , comprises a base unit  11  combining a pair of plates  12  and  13  one over the other to fixedly attached the device to a side of the seat  5 , an arm  14  having an annular end  14   a  received in the base unit  11  and another end connected to an anchor portion  8   a  of the buckle  8 , a thrust generating device  15  which is substantially fixedly attached to the base unit  11 , and a reverse preventing device  16  for limiting the rotation of the arm  14  only to one direction. 
     The two plates  12  and  13  are integrally attached to each other by placing one of the plates  13  over the other  12 , and crimping bosses projecting from the one plate onto the other plate, after the remaining components are assembled. The component parts can be assembled from only one direction as illustrated in FIG. 4, and this simplifies the assembly work, and makes it suitable for automatic assembly. Because the arm  14  is positioned in a middle with respect to the thickness of the device, and is flanked from both sides, the arm  14  is supported by both of the plates  12  and  13 , and demonstrates a high mechanical strength against a bending load. 
     The base unit  11  receives the annular end  14   a  of the arm  14 , and the other end  14   b  of the arm  14  projects from the base unit  11 . An open end of the base unit  11  which is adapted to abut the other end  14   b  serves as a stopper for defining the limit of angular motion of the arm  14 . This arrangement allows the number of component parts to be reduced. 
     A middle portion of the arm  14  is provided with an opening  14   c  for receiving pins  18   a  and  18   b  projecting form the plates  12  and  13  of the base unit  11 , respectively, so as to form a catch for preventing the inadvertent rotation of the arm  14 . 
     The base unit  11  defines an elongated chamber  19  having the shape of letter-J so as to partially surround the annular end  14   a  of the arm  14 . The thrust generating device  15  is attached to a base end of this chamber  19 . The thrust generating device  15  is adapted to produce a thrust through rapid combustion of a propellant  21  received in the base end of a cylinder  20  according to a signal from a deceleration sensor which is not shown in the drawing. A free end of the cylinder  20  is fitted with a balloon packing  22  consisting of a bladder-like member which is normally compactly folded, but can inflate into a slightly curved rod inside the chamber  19  by virtue of the thrust produced from the thrust generating device  15  (FIG. 5 a  and  5   b ). The free end of the balloon packing  22  is provided with a contact portion  22   a  having a relatively high rigidity for engaging a projection  14   d  provided in the annular portion  14   a  of the arm  14  so that the contact portion  22   a  may push the projection  14   d  and turn the arm  14  in clockwise direction in FIG. 2 upon expansion of the balloon packing  22  owing to the combustion thrust of the propellant  21 . The part of the balloon packing  22  attached to the cylinder  20  is provided with an increased thickness in the manner of an O-ring so that the inadvertent detachment of the balloon packing  22  can be avoided and a favorable sealing can be achieved. The cylinder  20  of the thrust generating device  15  is attached to the base unit  11  via a flange  20   a  formed in a middle part of the cylinder  20 . As this flange  20   a  prevents the movement of the cylinder in both the fore and aft directions. it is possible to place the propellant  21  in the cylinder  20  and close it by crimping to complete the assembling of the pretensioner device after all the remaining component parts including the cylinder  20  are fully assembled to the base unit  11 . Therefore, the safety management of the manufacturing process can be simplified. 
     In practice, it is preferable to reinforce the balloon packing  22  by applying a fabric lining or the like to the inner and/or outer surfaces thereof so that the rupturing of the balloon packing  22  under high pressure may be avoided. It is also possible to insert mold a planar or cup-shaped metallic member in the contact portion at the free end of the balloon packing  22 . In the illustrated embodiment, the balloon packing  22  was folded with the base end out, but it is also possible to fold it with the free end contact portion  22   a  out as shown in FIG. 5 c  or fold it in the manner of a bellows as shown in FIG. 5 d . It is conceivable that enlarging the free end contact portion  22   a  as shown in FIG. 5 c  may be beneficial in improving the efficiency of pressure transmission in an early phase of expansion. 
     The reverse preventing device  16  consists of a ball-type one-way clutch comprising the annular portion  14   a  of the arm  14  serving as an outer race, a fixed inner race  24  which is located inside the annular portion  14   a  and integral with the base unit  11 , a plurality of balls  26  received in corresponding chambers  25  defined between them with each chamber dimensioned so as to get progressively narrower in a circumferential direction, and compression coil springs  27  which resiliently urge the corresponding balls  26  toward the narrower parts of the chambers  25 . 
     The pretensioner device P is normally attached to a side of a seat. Because this pretensioner device P is essentially symmetric with respect to a center line with respect to the thickness thereof, it may be attached to the seat either with one of the plates  12  facing the seat or with the other plate  13  facing the seat. Thus, the same pretensioner device P can be used commonly for the right and left front seats so that the management of the component parts can be simplified, and the assembly work for the pretensioner device P is also simplified. 
     Now the mode of operation of the illustrated embodiment is described in the following. Upon detection of an impact by a deceleration sensor (not shown in the drawings), the propellant  21  of the thrust generating device  15  is ignited. and the pressure of the resulting combustion gas expands the balloon packing  22  into the chamber  19 . As the free end contact portion  22   a  thereof pushes the projection  14   d , the pins  18   a ,  18   b  are broken so as to allow the arm  14  to rotate freely, and tile arm  14  is then turned in a clockwise direction in FIG.  2 . The rotation of the arm  14  causes the buckle  8  to be pulled in (see FIG.  6 ), and the resulting increase in the tension of the seat belt increases the restraining capability thereof. At this time, because the reversal of the movement of the arm  14  is prevented by the reverse preventing device  16 . the slackening of the seat belt can be avoided even after the pressure of the combustion gas has disappeared. The air in the chamber  19  is displaced out of the chamber  19  from an opening in the base unit  11  from which the other end  14   b  of the arm  14  projects. 
     FIG. 7 is a view similar to FIG. 2 showing another embodiment of the seat belt pretensioner device embodying the present invention, and the parts corresponding to those in FIG. 2 are denoted with like numerals without repeating the description of such parts. In this embodiment, the reverse preventing device  30  for preventing the reversing of the movement of the arm  14  consists of a ratchet mechanism instead of the ball-type one-way clutch. This arrangement requires fewer component parts as compared with the arrangement using the ball-type one-way clutch. 
     The annular portion  14   a  of the arm  14  is formed with internal ratchet teeth  31 , and a fixed inner member  32  which is integral with the base unit  11  and located inside the annular portion  14   a  is provided with a plurality (three, in the illustrated embodiment) of ratchet pawl members  34  to  36  each of which is pivotable between two positions for engaging and disengaging the ratchet teeth  31 , respectively. The ratchet pawl members  34  to  36  are each resiliently urged into engagement with the ratchet teeth  31  by a corresponding compression coil spring  37  to  39 . 
     In this embodiment, the ratchet teeth  31  are provided at the pitch of 12 degrees, and are  30  in total number. On the other hand, the ratchet pawl members  34  to  36  are arranged unevenly, instead of being arranged evenly at the pitch of 120 degrees, the pitch between the ratchet pawl members  34  and  35  and between the ratchet pawl members  35  and  36  being 124 degrees while the pitch between the ratchet pawl members  36  and  34  is 112 degrees. In other words, these ratchet pawl members  34  to  36  cannot simultaneously engage the ratchet teeth  31 , and there is an offset of four degrees in the angular interval for the engagement of one pawl member to another. Therefore, the ratchet pawl members  34  to  36  engage the ratchet teeth  31  in turn as sequentially illustrated in FIGS. 8 a  to  8   c , and the arm  14  can be engaged at the pitch of four degrees even though the pitch of the ratchet teeth  31  is 12 degrees without requiring any unduly high precision in the component pans. Thus, even though the ratchet teeth are not required to be formed at any fine pitch, a high resolution power can be achieved, and the backlash at the time of engagement can be minimized. Also, the mechanical strength of engagement can be increased, and the reliability of the locking action can be improved. 
     The mechanical strength of the locking action can also be increased by increasing the material strength of the gear teeth and the ratchet wheel through suitable heat treatment. According to the present invention, because the sizes of the ratchet teeth and pawl members can be increased for a given resolution power, it is possible to achieve the target mechanical strength without requiring any heat treatment Therefore, it is possible to design this arrangement in such a manner that one of the pawl members ensures the target mechanical strength, and when a load exceeding the target mechanical strength is applied to the device, the remaining two pawl members engage the ratchet teeth  31  in a sequential manner with the progress of deformation of the parts involved in the engagement between the ratchet teeth and the pawl member and/or the progress of deformation of the plates  12  and  13  and arm  14 . 
     For instance, if four pawl members are provided at the intervals of 93 degrees, 93 degrees, 93 degrees and 81 degrees, the resolution power of three degrees can be achieved. If six pawl members are provided at the intervals of 62, 62, 62, 62, 62 and 50 degrees, the resolution power of two degrees can be achieved. In short, a desired resolution power can be achieved by proper selection of the pitch of the ratchet teeth and the number of pawl members. In the above described embodiments, the shape of the pawl members  34  to  36  was identical, and an offset was provided in the arrangement of the pawl members, but it is also possible to arrange the pawl members evenly while varying the length and/or shape of each pawl member so as to shift the timing of engagement of the pawl members. 
     Each of the pawl members  34  to  36  is provided with a plurality of pawls which are progressively smaller toward the pivot center. For instance, as shown in FIG. 8 a , the pawl  34   a  most remote from the pivot center has the greatest length, and the pawl  34   d  most adjacent to the pivot center has the smallest length. Therefore, even though there is an offset between the pivot center of the pawl member and the center of the ratchet teeth  31 , the pawls  34   a  to  34   d  can engage with and disengage from the ratchet teeth  31  substantially at the same time. Also, the locking mechanical strength can be increased over the case where the pawls  34   a  to  34   d  are all short. Also, the required stroke of the pawl member can be reduced as compared to the case where all the pawls are as long as the longest pawl  34   a  so that relatively small springs having a smaller spring constant can be used fox resiliently urging the pawl members toward the ratchet teeth at a target spring load. This contributes to the compact design of the device. The same is true with the remaining pawl members  35  and  36 . 
     This embodiment is otherwise similar to the previous embodiment, and the mode of operation of this embodiment is omitted as it is similar to that of the previous embodiment. 
     FIG. 9 is a view similar to FIG. 7 showing yet another embodiment of the seat belt pretensioner device embodying the present invention, and the parts corresponding to those in FIG. 7 are denoted with like numerals without repeating the description of such parts. As shown in FIG. 10 also, in this embodiment, the arm  14  is integrally provided with a packing portion  40 , either by integral forming or by assembling, for directly supporting the pressure of the combustion gas produced from the thrust generating device  15  in the chamber  19 , instead of using a balloon packinting. This allows the number of component parts to be reduced, and simplifies the assembly work. An O-ring is provided between the cylinder  20  of the thrust generating device  15  and the base unit  11 . This embodiment is otherwise similar to the previous embodiments, and the mode of operation is omitted because its mode to operation is not different from those of the previous embodiments. The chamber  19  and packing portion  40  were provided with circular cross sections in the above described embodiments, but the last embodiment permits the use of rectangular and semi-circular cross sections, and simplifies the machining work. 
     INDUSTRIAL APPLICABILITY 
     As can be appreciated from the above description, according to the pretensioner device of the present invention, because the combustion gas pressure obtained from the power generating device is, either directly or only via a bladder-like member, applied to a buckle supporting member to actuate the buckle, a link mechanism is not required, and the number of component parts can be minimized. This contributes to the compact and light-weight design of the overall device. The movement of the buckle is achieved only through a change in the inner pressure of the chamber of the bladder-like member from the start to the end of its motion, and there is no cause for attenuation in power transmission due to the changing of the direction of a force by a link mechanism or other mechanical structures so that the size of the power generator can be minimized. 
     Also, by forming the reverse preventing device with a plurality of ratchet teeth, and a plurality of pawl members which engage the ratchet teeth at different timing, the number of component parts is reduced even further, and a high resolution power can be achieved without unduly increasing the manufacturing precision as compared to the case where the ratchet teeth are formed at a finer pitch. Also, the backlash at the time of engagement can be minimized. Additionally, the mechanical strength of engagement is increased, and the locking reliability is improved.