Patent Publication Number: US-7584993-B2

Title: Airbag device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Divisional of application Ser. No. 10/562,432 filed Dec. 28, 2005. The entire disclosure of the prior application, application Ser. No. 10/562,432 is considered part of the disclosure of the accompanying Divisional Application and is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an airbag device whose airbag is deployable at the time of collision of a vehicle to protect an occupant, more particularly to an airbag device capable of precisely controlling an expansion speed of an airbag in the midway of the expansion. 
     BACKGROUND OF THE INVENTION 
     An airbag device fitted to a driver seat and a passenger seat of a vehicle is to protect an occupant by rapidly expanding an airbag by means of a gas pressure let out from an inflator at the time of collision of the vehicle. In the airbag device described above, it is necessary to ease influence of impact given to the occupant when the airbag is expanded, thereby receiving the occupant softly by the airbag. To that end, it is preferable to avoid a situation where the airbag is abruptly expanded to protrude toward the occupant when the inflator operates. Particularly, since the occupant comes into contact with the airbag in a short time in the case where the occupant sits down adjacent to the airbag device, it is preferable to avoid a situation where the airbag is abruptly expanded to protrude toward the occupant. 
     For a device for controlling the expansion speed of the airbag described above, various technologies for adjusting opening and closing degree of a vent hole of the airbag are developed. For example, JP 11-334521A (patent reference 1) proposes a device for opening and closing the vent hole by an actuator. In this example, there are provided an actuator made of a plate-like piezoelectric element and a vent hole openable and closable by the actuator, respectively, formed on a retainer for supporting an inflator and an airbag, wherein an internal pressure at the beginning of the expansion of the airbag is detected by a pressure sensor, and it is decided that the occupant approaches the airbag when the internal pressure is higher than a pressure at normal time, thereby energizing the actuator to operate it, so that the vent hole is opened through which gas inside the airbag is discharged outward. 
     Further, JP 11-105664A (patent reference 2) proposes a device for closing a vent hole by a valve for regulating volume of gas to be let out which is formed of the same base cloth as an airbag. In this example, when the valve for regulating volume of gas to be discharged is temporarily stopped by a tacking thread and the like, a state where the vent hole keeps open is maintained, thereby controlling an expansion speed at the beginning of the expansion of the airbag. Thereafter, when the airbag is expanded, and a stretching force of the airbag becomes larger than a tacking force of the tacking thread, the tacking thread is cut so that the vent hole is throttled by the valve for regulating volume of gas to be discharged. As a result, the gas inside the airbag is restrained from being discharged outward so that the airbag is expanded until it becomes a suitable size to protect the occupant. 
     However, since the device in the patent reference 1 has a structure to close the vent hole by operating the actuator made of the plate-like piezoelectric element and the like, there is a drawback in that a mechanism to close the vent hole is complex and expensive, and also a response speed is slow. Further, since the patent reference 2 has a system to close the vent hole by cutting the tacking thread by the stretching force of the airbag, there is a drawback in that timing for closing the vent hole is varied largely due to the variations of a breaking force of the tacking thread and the stretching force of the airbag, and also the timing for closing the vent hole can not be changed in response to a driving posture of the occupant. 
     SUMMARY OF THE INVENTION 
     The invention has been made in view of the technical problems of the prior arts described above and it is a first object of the invention to provide an airbag device having a closing mechanism of a vent hole which is simple and manufactured at low cost and the closing of the vent hole can be effected with reliability. 
     It is a second object of the invention to provide an airbag device capable of discharging gas from the vent hole with reliability. 
     It is a third object of the invention to provide an airbag device capable of protecting an occupant with reliability by closing the vent hole in a short time. 
     It is a fourth object of the invention to provide an airbag device capable of closing the vent hole and switching of the developing height of an airbag with good timing. 
     (1) An airbag device in the first aspect of the invention comprising an inflator for generating gas and an airbag expandable by the gas, said airbag device is provided with a gas inflow port provided in the airbag through which the gas flows in, a vent hole provided in the airbag for discharging the gas flown in the airbag, a cloth-like piece connected to a vicinity of the vent hole at one end thereof, and to a vicinity of the gas inflow port at the other end thereof releasably, wherein said cloth-like piece does not close the vent hole in a state where the other end is retained, and closes the vent hole due to an internal pressure of the airbag in a state where the other end is released, a retaining device for retaining the other end of the cloth-like piece, and releasing means for releasing retention of the retaining device. 
     (2) The airbag device described under item (1) as above, wherein a length of the cloth-like piece from one end to the other end is shorter than a length of the airbag from one end to the other end. 
     (3) The airbag device described under item (1) or (2) as above, wherein the cloth-like piece is connected to an inside of the airbag at one end at the position in the vicinity of the vent hole and remote from the gas inflow port. 
     (4) An airbag device comprising an inflator for generating gas and an airbag expandable by the gas, said airbag device is provided with a gas inflow port provided in the airbag through which the gas flows in, a vent hole provided in the airbag for discharging the gas flown in the airbag, a cloth-like piece connected to an inside of the airbag at both ends thereof and spaced apart from an edge of the vent hole, an extension cloth connected to the cloth-like piece at one end, to the vicinity of the gas inflow port at the other end to be releasable, wherein said extension cloth does not close the vent hole in a state where the other end is retained, and closes the vent hole due to an internal pressure of the airbag in a state where the other end is released, a retaining device for retaining the other end of the cloth-like piece, and releasing means for releasing retention of the retaining device. 
     (5) The airbag device described under any one of items (1) to (4), wherein the retaining device has a retaining plate, and the releasing means implements releasing by forcibly deforming the retaining plate. 
     (6) The airbag device described under item (5) as above, wherein the releasing means implement releasing by forcibly deforming the retaining plate by a pressure generated by an explosive. 
     (7) The airbag device described under any one of items (1) to (6) as above, wherein the airbag is provided with a tether belt for restraining expansion of the airbag, and one end of the tether belt is retained by the retaining device. 
     According to the invention as set forth in the above items (1), (2), (3) and (4), the vent hole can be closed by merely releasing retention of the other end of the cloth-like piece, so that a closing mechanism of the vent hole is simple and low in cost and the closing of the vent hole can be carried out with reliability. Further, since gas can be discharged through the vent hole by way of a pocket functioning as a gas reservoir, impact to be applied to the occupant at the time of expansion of the airbag can be eased with reliability. 
     According to the invention as set forth in the above item (5), since the vent hole can be closed in a short time by releasing retention of the cloth-like piece by forcibly deforming the retaining plate, the occupant can be protected with reliability. 
     According to the invention as set forth in the above item (6), since the retention of the cloth-like piece is released by forcibly deforming the retaining plate owing to explosion by the explosive, an actuator for releasing retention is simple and the vent hole can be closed in a short time. 
     According to the invention as set forth in the above item (7), since the tether belt and the cloth-like piece are releasably retained by a common retaining device, a structure of the retaining device is simple, and the closing of the vent hole and the switching of the developing height of the airbag can be carried out with good timing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an airbag device according to a first embodiment of the invention: 
         FIG. 2  is a sectional view taken along line A-A in  FIG. 1  showing a state where a vent hole is opened; 
         FIG. 3  is a sectional view taken along line A-A in  FIG. 1  showing a state where the vent hole is closed; 
         FIG. 4  is an enlarged sectional view taken along line B-B 
         FIG. 1 ; 
         FIG. 5  is a view as viewed from an arrow Z in  FIG. 4 ; 
         FIG. 6  is a sectional view taken along line C-C in  FIG. 5 ; 
         FIG. 7  is a plan view of an airbag device according to a second embodiment of the invention; 
         FIG. 8  is a sectional view taken along line D-D in  FIG. 7  showing a state where a vent hole is opened; 
         FIG. 9  is a sectional view taken along line D-D in  FIG. 7  showing a state where the vent hole is closed; 
         FIG. 10  is an enlarged sectional view taken along line E-E in  FIG. 7 ; 
         FIG. 11  is a view as viewed from an arrow Y in  FIG. 10 ; 
         FIG. 12  is a sectional view taken along line F-F in  FIG. 11 ; 
         FIG. 13  is a perspective view of a cloth-like piece of an airbag device according to a third embodiment of the invention; 
         FIG. 14  is a sectional view of  FIG. 13  showing a state where a vent hole is opened; and 
         FIG. 15  is a sectional view of  FIG. 13  showing a state where the vent hole is closed. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An airbag device according to the first embodiment of the invention is described hereinafter with reference to the accompanying drawings.  FIG. 1  is a plan view showing a state where an airbag device  1  is expanded, which shows a state where an inflator is removed.  FIG. 2  is a sectional view taken along line A-A in  FIG. 1  showing a state where the inflator is fitted. As shown in  FIG. 1  and  FIG. 2 , the airbag device  1  is formed of the airbag  2  comprised of an attachment side base cloth  3  having a circular shape and an opposed side base cloth  4  having also the circular shape wherein peripheries of the attachment side base cloth  3  and opposed side base cloth  4  are sewn together and reversed to form the airbag  2  having a bag-like shape. The attachment side base cloth  3  is located at a side to be fitted to a vehicle together with the inflator  6  while the opposed side base cloth  4  is located at a side to face the occupant and receives the occupant when the airbag  2  is expanded. The inflator  6  is fitted to a circular gas inflow port  5  formed on the center of the attachment side base cloth  3 , and gas generated in the inflator  6  flows in the airbag  2  through the gas inflow port  5 , to expand the airbag  2  while developing the airbag  2  in a folded state. There are provided four bolt insertion holes  11 A,  11 B,  11 C and  11 D in the attachment side base cloth  3  each spaced at 90 degrees at the peripheral portion of the gas inflow port  5  of the attachment side base cloth  3 , and the airbag  2  is fitted to the vehicle using these bolt insertion holes  11 A- 11 D. 
       FIG. 4-FIG .  5  show a detailed structure of a metal fitting portion by means of which the airbag  2  is fitted to the vehicle, wherein  FIG. 4  is an enlarged sectional view taken along line B-B in  FIG. 1  and  FIG. 5  is a view as viewed from an arrow Z in  FIG. 4 . As shown in  FIG. 4-FIG .  5 , the attachment side base cloth  3  is clamped between a substantially disc-like presser plate  13  and a metal fitting  12 , and it is fixed to the metal fitting  12  together with a flange  6 A of the inflator  6  by mans of four bolts  14 A- 14 D inserted into four bolt insertion holes  11 A- 11 D and nuts  15  screwed into the bolts  14 A- 14 D respectively. In the manner described above, the folded airbag  2  and the metal fitting  12  to which the inflator  6  is fitted are fixed to a central portion of a steering wheel of the vehicle (not shown). 
     As shown in  FIG. 1 , three vent holes  7 A,  7 B and  8  are provided in the attachment side base cloth  3  for discharging gas flown in the airbag  2  outward. Two vent holes  7 A,  7 B are formed at positions which are equally spaced from a center  5 A of the gas inflow port  5  in a radial direction, while the gas vent hole  8  is formed at a position spaced outwardly from the center  5 A of the gas inflow port  5  in a radial direction, wherein a distance from the vent hole  8  to the center  5 A is longer than that from the vent holes  7 A,  7 B to the center  5 A. As shown by dotted lines in  FIG. 1 , a cloth-like piece  9  for closing the vent hole  8  is fitted to the inside of the attachment side base cloth  3 . The cloth-like piece  9  has a length extending outwardly from the peripheral portion of the gas inflow port  5  and exceeding the outer end of the vent hole  8 , and it is comprised of a parallel portion  9 B extending outwardly in the radial direction from the peripheral portion of the gas inflow port  5  and a semicircular portion  9 A having a radius curvature which is larger than that of the vent hole  8 , and made of a cloth which is the same material as the airbag  2 . The cloth-like piece  9  may be made of a cloth which is different from the material of the airbag  2 . Further, the cloth-like piece  9  is not limited to a cloth and may be formed of a sheet-like material such as plastics and the like if the material has a flexibility capable of closing the vent hole  8 . The cloth-like piece  9  is sewn and fixed to the attachment side base cloth  3  at coordinate axis position X 2  which is spaced by a distance d in an outward radial direction from a coordinate axis position X 1  extending in the direction of X from the center  5 A of the gas inflow port  5  to the edge of the vent hole  8  in the radial direction. A sewn portion  10  of the cloth-like piece  9  has a circular-arc shape and spaced from the edge of the vent hole  8 . A width W of the parallel portion  9 B of the cloth-like piece  9  is set to be larger than a diameter of the vent hole and has a width which is sufficient to completely close the vent hole  8 . It is also possible to control the vent hole  8  so as to discharge gas slightly, by making the width W of the parallel portion  9 B of the cloth-like piece  9  smaller than the diameter of the vent hole  8 , without the vent hole  8  completely closed. 
     The parallel portion  9 B is detachably fitted to a presser plate  13  at the peripheral portion side of the gas inflow port  5 . That is, as shown in  FIG. 1 , there are provided two retaining holes  16 A,  16 B in the attachment side base cloth  3  between the bolt insertion holes  11 D,  11 C along the same circumference of the bolt insertion holes  11 D,  11 C. An ignitor insertion hole  23  through which an ignitor  20 , described later, is inserted, is defined in the attachment side base cloth  3  between two retaining holes  16 A,  16 B while a U-shaped notch  24  is formed on the parallel portion  9 B so as to surround the ignitor insertion hole  23 . As shown in  FIG. 4-FIG .  5 , the parallel portion  9 B is inserted in a gap between the presser plate  13  and the attachment side base cloth  3  while the two retaining holes  16 A,  16 B are positioned so as to align with two holes  21 A,  21 B defined in the presser plate  13 . Then a retaining plate  17  is fixed to the presser plate  13  by way of bolts  14 C,  14 D, while retaining projections  18 A,  18 B of the retaining plate  17  are inserted into the retaining holes  16 A,  16 B through two holes  21 A,  21 B, thereby restraining the parallel portion  9 B from not being come off the presser plate  13 . 
     An explosive chamber  19  protruding cylindrically toward the inside of the airbag  2  is formed on the retaining plate  17  at the intermediate portion between the retaining projections  18 A and  18 B, while the explosive  19 A is sealed in the explosive chamber  19 . The ignitor  20  for igniting the explosive  19 A is fitted to the flange  6 A of the inflator  6 , and the tip end of the ignitor  20  extends to a portion in the vicinity of the explosive  19 A. Since necking portions  17 A,  17 A are provided on the retaining plate  17 , when the explosive  19 A in the explosive chamber  19  is exploded, the retaining plate  17  is bent along the necking portions  17 A due to gas pressure caused by explosion, as shown by two dotted chain lines in  FIG. 6 , to be subjected to plastic deformation, so that the retaining projections  18 A,  18 B come off the retaining holes  16 A,  16 B of the parallel portion  9 B, enabling the parallel portion  9 B to come off the presser plate  13 . Accordingly, when the explosive  19 A is exploded in a state where the airbag  2  is expanded, the cloth-like piece  9  is pulled toward the outer peripheral side of the airbag  2  due to a tensile force acting on the attachment side base cloth  3  and the parallel portion  9 B comes off the presser plate  13 . Igniting timing of the ignitor  20  is determined by detecting whether the occupant sits down on the seat in the vicinity of the airbag device or not. There are concrete examples, as a detection method, such as a method of detecting the occupant by means of a CCD camera, and an infrared sensor, a method of detecting a magnitude or a distribution of a load of the occupant applied to the seat by a load sensor, and the like. 
     As shown in  FIG. 2 , as for a length from the sewn portion  10  to the right periphery  13 A of the presser plate  13  is set such that L 1  is shorter than L 2  where L 2  indicates a length at the attachment side base cloth  3  side while L 1  indicates a length at the cloth-like piece  9  side. Accordingly, when the airbag  2  is expanded due to a pressure of gas flown in, the cloth-like piece  9  having the short length is stretched linearly while the attachment side base cloth  3  side having the long length is expanded substantially in circular-arc to move away from the cloth-like piece  9  outward, so that the vent hole  8  keeps open. Particularly an arc-like pocket  22  defined between the sewn portion  10  and the vent hole  8  functions as a gas reservoir for interrupting a flow of gas directing from the gas inflow port  5  toward the opposed side base cloth  4  to function to guide the interrupted gas into the vent hole  8 , thereby discharging the gas efficiently outward from the vent hole  8  so that the airbag  2  is expanded while a speed of the gas directing toward the occupant is reduced. By changing the length L 2  at the attachment side base cloth  3  side, the length L 1  at the cloth-like piece  9  side, a distance d from the outer end of the vent hole  8  to the sewn portion  10 , and a width W of the parallel portion  9 B appropriately, discharge amount of the gas from the vent hole  8  can be changed and an expanding speed can be suitably adjusted depending on the size of the airbag  2  and the like. 
     Now operation of the airbag device according to the first embodiment of the invention is described as follows. That is, in the case where the vehicle comes into collision while traveling, when an acceleration sensor detects that an acceleration of collision exceeds a set value, gas is discharged from the inflator  6  and the gas flows in the airbag  2  through the gas inflow port  5 , thereby developing the folded airbag  2  to expand. When the airbag  2  is expanded as shown in  FIG. 2 , the cloth-like piece  9  stretches linearly due to the tensile force of the airbag  2 , while the attachment side base cloth  3  at the vent hole  8  side moves away from the cloth-like piece  9  outwardly to expand in generally circular-arc, thereby keeping the vent hole  8  in an open state. At this point in time, since the arc-like pocket  22  formed between the sewn portion  10  and the vent hole  8  functions as a gas reservoir for interrupting the flow of the gas directing toward the opposed side base cloth  4  side so as to guide the interrupted gas into the vent hole  8 , the gas flown in the airbag  2  is efficiently discharged outward through the vent hole  8 , the airbag  2  is expanded while an impactive expansion directing toward the occupant side is eased. 
     At the time when the airbag  2  is sufficiently developed up and down and left and right relative to the occupant to become a sufficient size to protect the occupant, the ignitor  20  is ignited to ignite the explosive  19 A in the explosive chamber  19  so that the necking portions  17 A,  17 A of the retaining plate  17  are instantaneously bent due to a gas pressure caused by the explosion of the explosive  19 A. The retaining plate  17  is bent up to the position as shown by two dotted chain lines in  FIG. 6 , so that the retaining projections  18 A,  18 B come off the retaining holes  16 A,  16 B of the cloth-like piece  9 . Since a tensile force is applied to the cloth-like piece  9  by an expansion force of the attachment side base cloth  3  caused by gas pressure, the parallel portion  9 B comes off the presser plate  13  so that the semi-circular portion  9 A is brought into intimate contact with the attachment side base cloth  3  due to a gas pressure as shown in  FIG. 3 , thereby closing the vent hole  8  by the semicircular portion  9 A to stop the discharge of the gas through the vent hole  8 . At this stage, the opposed side base cloth  4  of the airbag  2  contacts the occupant so that the opposed side base cloth  4  is slightly expanded by the flowing gas, thereby receiving the occupant softly. 
     An airbag device according to the second embodiment of the invention is next described hereinafter with reference to the accompanying drawings. The second embodiment is structured such that the closing of a vent hole  8  and the switching of the developing height of an airbag are carried out by a common retaining device. Parts which are the same as those in the first embodiment are denoted by and described with the same reference numerals.  FIG. 7  is a plan view showing a state where an airbag device  1  is expanded, and an inflator  6  is removed.  FIG. 8  is a sectional view taken along line D-D in  FIG. 7  showing a state where the inflator  6  is fitted. As shown in  FIG. 7-FIG .  8 , the airbag device  1  is formed of the airbag  2  comprised of an attachment side base cloth  3  having a circular shape and an opposed side base cloth  4  having also the circular shape and peripheries of the attachment side base cloth  3  and opposed side base cloth  4  are sewn together and reversed to form the airbag  2  having a bag-like shape. The attachment side base cloth  3  is located at a side to be fitted to a vehicle together with the inflator  6  while the opposed side base cloth  4  is located at a side to face the occupant and receives the occupant when the airbag  2  is expanded. The inflator  6  is fitted to a circular gas inflow port  5  formed on the center of the attachment side base cloth  3 , and gas generated in the inflator  6  flows in the airbag  2  through the gas inflow port  5 , to expand the airbag  2  while developing the airbag  2  in a folded state. There are defined four bolt insertion holes  11 A,  11 B,  11 C and  11 D in the attachment side base cloth  3  each spaced at 90 degrees at the peripheral portion of the inflow port  5  of the attachment side base cloth  3 , and the airbag  2  is fitted to the vehicle using these bolt insertion holes  11 A- 11 D. 
       FIG. 10-FIG .  11  show a detailed structure of a metal fitting portion by means of which the airbag  2  is fitted to the vehicle, wherein  FIG. 10  is an enlarged sectional view taken along line E-E in  FIG. 7 ,  FIG. 11  is a view as viewed from an arrow Y in  FIG. 10 , and  FIG. 12  is a sectional view taken along line F-F in  FIG. 11 . As shown in  FIG. 10  to  FIG. 12 , the attachment side base cloth  3  is clamped between a substantially disc-like presser plate  13  and a metal fitting  12 , and it is fixed to the metal fitting  12  together with a flange  6 A of the inflator  6  by means of four bolts  14 A- 14 D inserted into the bolt insertion holes  11 A- 11 D and nuts  15  screwed into the bolts  14 A- 14 D respectively. In the manner described above, the folded airbag  2  and the metal fitting  12  to which the inflator  6  is fitted are fixed to a central portion of a steering wheel of the vehicle (not shown). 
     As shown in  FIG. 8 ,  FIG. 10  and  FIG. 11 , a first tether belt  28  is stretched between an inside end face  6 A of the inflator  6  and a central portion  4 A of the opposed side base cloth  4 . That is, an upper end of the first tether belt  28  is put in the inside end face  6 A of the inflator  6  by a presser plate  26 , and the presser plate  26  is fixed to the inside end face  6 A by two bolts  25 ,  25 , thereby fixing the first tether belt  28  to the inside end face  6 A. The lower end of the first tether belt  28  is sewn and fixed to the central portion  4 A of the opposed side base cloth  4 . Further, a lower end of a second tether belt  27  is sewn and fixed to the opposed side base cloth  4  at a substantially intermediate position  28 A of the first tether belt  28  in the longitudinal direction thereof. Further, an upper end  27 A of the second tether belt  27  is detachably fitted to the presser plate  13  together with the cloth-like piece  9 , described later. In a state where the upper end  27 A of the second tether belt  27  is fitted to the presser plate  13 , the opposed side base cloth  4  is restricted in its developing height to H 1  by the second tether belt  27 , and at this point in time, the first tether belt  28  is rendered in a slackened state at the side over the intermediate position  28 A. That is, as shown in  FIG. 9 , when the upper end  27 A of the second tether belt  27  comes off the presser plate  13 , the slackness of the first tether belt  28  at the side over the intermediate position  28 A is removed owing to a gas pressure operating on the opposed side base cloth  4 , so that the airbag  2  is expanded up to the developing height H 2  which is determined by the full length of the first tether belt  28 . According to the second embodiment, although the first tether belt  28  go slacks at the side over the intermediate position  28 A when the developing height is restricted to H 1  by the second tether belt  27 , the first tether belt  28  may not go slacks at the side over the intermediate position  28 A. That is, in a state where the opposed side base cloth  4  is restricted in developing height to H 1  by the second tether belt  27 , the first tether belt  28  is bent in an L-shape at the intermediate position  28 A and stretches linearly from the intermediate position  28 A to the upper end of the first tether belt  28 . When the upper end  27 A of the second tether belt  27  comes off the presser plate  13 , the first tether belt  28  stretches linearly from the upper end to the lower end thereof, rendering the developing height to H 2 . 
     In the second embodiment shown in  FIG. 8  and  FIG. 9 , although the lower end of the second tether belt  27  is sewn to the first tether belt  28  at substantially the intermediate position  28 A of the first tether belt  28  in the longitudinal direction, it may be sewn to the opposed side base cloth  4  (at the position in the vicinity of the central portion  4 A to which the lower end of the first tether belt  28  is sewn). In this case, it is sufficient to set the length of the second tether belt  27  from the upper end to the lower end to be smaller than the length of the first tether belt  28  from the upper end to the lower end thereof. In the manner described above, in a state where the upper end  27 A of the second tether belt  27  is fitted to the presser plate  13 , the opposed side base cloth  4  is restricted to be low in the developing height by the length of the second tether belt  27  from the upper end to the lower end, and at this point in time, the first tether belt  28  is caused to go slack at the entire length. Then, when the upper end  27 A of the second tether belt  27  comes off the presser plate  13 , the first tether belt  28  has no slackness due to gas pressure applied to the opposed side base cloth  4 , so that the airbag  2  is expanded up to the developing height which is determined by the entire length of the first tether belt  28 . 
     As shown in  FIG. 7 , three vent holes  7 A,  7 B and  8  are defined in the attachment side base cloth  3  for discharging gas flowing in the airbag  2  outward. Two vent holes  7 A,  7 B are formed at positions which are equally spaced from a center  5 A of the gas inflow port  5  in a radial direction, while the gas vent hole  8  is formed at a position spaced outwardly from the center  5 A of the gas inflow port  5  in a radial direction while opposing the vent holes  7 A,  7 B, wherein a distance from the vent hole  8  to the center  5 A is longer than that from the vent holes  7 A,  7 B to the center  5 A. As shown by dotted lines in  FIG. 7 , a cloth-like piece  9  for closing the vent hole  8  is fitted to the inside of the attachment side base cloth  3 . The cloth-like piece  9  has a length extending outwardly from the peripheral portion of the gas inflow port  5  and exceeding the outer end of the vent hole  8 , and it is comprised of a parallel portion  9 B extending outwardly in the radial direction from the peripheral portion of the gas inflow port  5  and a semicircular portion  9 A having a curvature radius which is larger than that of the vent hole  8 , and made of a cloth which is the same material as the airbag  2 . The cloth-like piece  9  may be made of a cloth which is different from the material of the airbag  2 . Further, the cloth-like piece  9  is not limited to a cloth and may be formed of a sheet-like material such as plastics and the like if the material has a flexibility capable of closing the vent hole  8 . The cloth-like piece  9  is sewn and fixed to the attachment side base cloth  3  at a coordinate axis position X 2  which is spaced by a distance d in an outward radial direction from a coordinate axis position X 1  extending in the direction of X from the center  5 A of the gas inflow port  5  to the edge of the vent hole  8  in the radial direction. A sewn portion  10  of the cloth-like piece  9  has a circular-arc shape and spaced from the edge of the vent hole  8 . A width W of the parallel portion  9 B of the cloth-like piece  9  is set to be larger than a diameter of the vent hole  8  and has a width which is sufficient to completely close the vent hole  8 . When the width W of the parallel portion  9 B of the cloth-like piece  9  is made smaller than the diameter of the vent hole  8 , the vent hole  8  is not completely closed but it is possible to control the vent hole  8  so as to discharge gas slightly. 
     The parallel portion  9 B and the upper end  27 A of the second tether belt  27  are detachably fitted to a presser plate  13  at the peripheral portion side of the gas inflow port  5 . That is, as shown in  FIG. 7 , there are defined two retaining holes  16 A,  16 B in the parallel portion  9 B, the upper end  27 A of the second tether belt  27 , and the attachment side base cloth  3  between the bolt insertion holes  11 D,  11 C along the same circumference of the bolt insertion holes  11 D,  11 C. An ignitor insertion hole  23  through which an ignitor  20  is inserted, is defined in the attachment side base cloth  3  between two retaining holes  16 A,  16 B while U-shaped notches  24  are formed on the parallel portion  9 B and the upper end  27 A of the second tether belt  27 , respectively, so as to surround the ignitor insertion hole  23 . As shown in  FIG. 10  and  FIG. 11 , the parallel portion  9 B and the upper end  27 A of the second tether belt  27  are inserted in a gap defined between the presser plate  13  and the attachment side base cloth  3  while the two retaining holes  16 A,  16 B are positioned so as to align with two holes  21 A,  21 B defined in the presser plate  13 . Then, the retaining plate  17  is fixed to the presser plate  13  by way of bolts  14 C,  14 D, while retaining projections  18 A,  18 B of the retaining plate  17  are inserted into the retaining holes  16 A,  16 B of the parallel portion  9 B and the upper end  27 A of the second tether belt  27  through two holes  21 A,  21 B, thereby restraining the parallel portion  9 B and the upper end  27 A of the second tether belt  27  from not being come off the presser plate  13 . 
     An explosive chamber  19  protruding cylindrically toward the inside of the airbag  2  is formed on the retaining plate  17  at the intermediate portion between the retaining projections  18 A,  18 B, while the explosive  19 A is sealed in the explosive chamber  19 . The ignitor  20  for igniting the explosive  19 A is fitted to the flange  6 A of the inflator  6 , and the tip end of the ignitor  20  extends to a portion in the vicinity of the explosive  19 A. Since necking portions  17 A,  17 A are provided on the retaining plate  17 , when the explosive  19 A in the explosive chamber  19  is exploded, the retaining plate  17  is bent along the necking portions  17 A,  17 A due to gas pressure caused by explosion, as shown by two dotted chain lines in  FIG. 12 , to be subjected to plastic deformation, the retaining projections  18 A,  18 B come off the retaining holes  16 A,  16 B of the parallel portion  9 B and the upper end  27 A of the second tether belt  27 , enabling the parallel portion  9 B and the upper end  27 A of the second tether belt  27  to come off the presser plate  13 . Accordingly, when the explosive  19 A is exploded in a state where the airbag  2  is expanded, the cloth-like piece  9  is pulled out toward the outer peripheral side of the airbag  2  due to a tensile force operating on the attachment side base cloth  3 , and the parallel portion  9 B comes off the presser plate  13 . At the same time, the upper end  27 A of the second tether belt  27  is pulled out toward the opposed side base cloth  4  side owing to a tensile force operating on the first tether belt  28 , to thereby come off the presser plate  13 . Igniting timing of the ignitor  20  is determined by detecting as to whether the occupant sits down on the seat in the vicinity of the airbag device or not. There are concrete examples, as a detection method, such as a method of detecting the occupant by means of a CCD camera, and an infrared sensor, a method of detecting a magnitude or a distribution of a load of the occupant applied to the seat by a load sensor, and the like. 
     As shown in  FIG. 8 , as for a length from the sewn portion  10  to the right periphery  13 A of the presser plate  13  is set such that L 1  is shorter than L 2  where L 2  indicates a length at the attachment side base cloth  3  side while L 1  indicates a length at the cloth-like piece  9  side. Accordingly, when the airbag  2  is expanded due to a pressure of gas introduced in the airbag  2 , the cloth-like piece  9  having the short length is stretched linearly while the attachment side base cloth  3  side having the long length is expanded substantially in circular-arc to move away from the cloth-like piece  9  outward, so that the vent hole  8  keeps open. Particularly an arc-like pocket  22  defined between the sewn portion  10  and the vent hole  8  functions as a gas reservoir for interrupting a flow of gas directing from the gas inflow port  5  toward the opposed side base cloth  4  to function to guide the interrupted gas into the vent hole  8 , thereby discharging the gas efficiently outward from the vent hole  8  so that the airbag  2  is slowly expanded while a speed of the airbag  2  gas directing toward the occupant is reduced. If the length L 2  at the attachment side base cloth  3  side, the length L 1  at the cloth-like piece  9  side, a distance d from the outer end of the vent hole  8  to the sewn portion  10 , and a width W of the parallel portion  9 B are changed appropriately, respectively, discharge amount of the gas from the vent hole  8  is changed so that an expanding speed of the airbag  2  can be suitably adjusted depending on the size of the airbag  2  and the like. 
     An airbag device according to the third embodiment of the invention is next described hereinafter with reference to the accompanying drawings. The third embodiment is structured such that the shape of the cloth-like piece of the first embodiment is changed to a simple shape and parts which are the same as those in the first embodiment are denoted by and described with the same reference numerals. 
       FIG. 13  is a perspective view of a cloth-like piece of an airbag device according to the third embodiment of the invention. As shown in  FIG. 13 , a rectangular cloth-like piece  30  which is long in right and left direction for closing a vent hole  8  is fitted to an inside of an attachment side base cloth  3 . The cloth-like piece  30  is sewn to and fixed to the attachment side base cloth  3  at sewing portion  32 A,  32 B at both ends spaced from an edge of the vent hole  8 . A width W of the cloth-like piece  30  is set to be larger than a diameter of the vent hole  8  and is sufficient to completely close the vent hole  8 . A lower end of an extension cloth  31  is fixed to the cloth-like piece  30  at substantially central portion thereof in the right and left longitudinal direction, and two retaining holes  16 A,  16 B are defined in the extension cloth  31  at the upper end. A material of the extension cloth  31  may be the same as the airbag  2  or may be different from the airbag  2 . Further, the extension cloth  31  is not limited to be made of a cloth but may be made of a material such as plastics and the like if it has flexibility. The extension cloth  31  may be integrally formed with the cloth-like piece  30 . The retaining holes  16 A,  16 B of the extension cloth  31  at the upper end are retained by a retaining device  17  as explained in the first embodiment, and the retention is released at given timing, so that the cloth-like piece  30  is pressed toward the attachment side base cloth  3  by a gas pressure to close the vent hole  8 . That is, when retaining projections  18 A,  18 B of the retaining plate  17  (not shown), as explained in the first embodiment, is inserted into the retaining holes  16 A,  16 B defined in the upper end of the extension cloth  31 , the extension cloth  31  is restrained not to be come off the presser plate  13 . 
     When the airbag  2  is expanded owing to gas let out from an inflator  6  at the time of collision of the vehicle, the extension cloth  31  is stretched linearly by a tensile force of the airbag  2 , so that the cloth-like piece  30  is pulled toward the inside of the airbag  2 . As a result, the cloth-like piece  30  is moved away from the attachment side base cloth  3 , and expanded in a substantially circular arc to form a gap  33  having a substantially circular arc between the cloth-like piece  30  and the attachment side base cloth  3 , thereby keeping a state where the vent hole  8  is opened ( FIG. 14 ). Accordingly, gas flown in the airbag  2  is discharged efficiently outward from the vent hole  8 , so that the airbag  2  is expanded while an impactive expansion directing toward the occupant is eased. When the airbag  2  becomes a sufficient size to protect the occupant, the explosive  19 A is ignited to generate a gas pressure, which causes the retaining plate  17  to be instantaneously bent at the necking portions  17 A. The retaining projections  18 A,  18 B come off the retaining holes  16 A,  16 B of the extension cloth  31 . Since a tensile force based on the expansion force of the attachment side base cloth  3  caused by the gas pressure is added to the extension cloth  31 , the extension cloth  31  comes off the presser plate  13 , so that the cloth-like piece  30  is brought into intimate contact with the attachment side base cloth  3  owing to the gas pressure as shown in  FIG. 15 , thereby closing the vent hole  8  to stop the discharge of gas from the vent hole  8 . At this stage, the opposed side base cloth  4  of the airbag  2  contacts the occupant, so that the opposed side base cloth  4  somewhat expands owing to inflowing gas to receive the occupant softly.