Patent Publication Number: US-7581755-B2

Title: Airbag device

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a divisional of prior U.S. patent application Ser. No. 10/932,875, filed 2 Sep. 2004, which claims priority under 35 USC 119 based on Japanese patent application Nos. 2003-317761, filed 10 Sep. 2003, and 2003-411899, filed on 10 Dec. 2003, and 2003-412200, filed on 10 Dec. 2003 and 2003-417862, filed on 16 Dec. 2003 and 2004-188825, filed on 25 Jun. 2004 and 2004-188826, filed on 25 Jun. 2004. The subject matter of each of these priority documents is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to an airbag device having an airbag, the airbag comprising first and second base cloths sewn at their outer peripheries, and inflated by supplying gas generated by an inflator to the inside of the airbag through a central portion of one base cloth upon collision of a vehicle. 
     2) Description of the Related Art 
     Airbags for driver&#39;s seats, which are deployed from a central portion of a steering wheel into a vehicle compartment, generally comprises two circular base cloths sewn together at their outer peripheries. Japanese Patent Application Laid-open No. 7-125586 discloses an airbag comprising two base cloths sewn together in V-letter-shaped seams which are spread out radially from the inside to the outside in such a manner that the seams are broken by load generated upon inflation of the airbag, thus preventing the airbag to suddenly expand in fore and aft directions, and allowing the airbag to deploy in fore and aft directions in an overall flat shape in order to softly restrain passengers. 
     Moreover, Japanese Patent Application Laid-open No. 8-119052 discloses an airbag comprising two base cloths sewn together in breakable circular seams, with a gas intake hole and a gas discharge hole respectively formed radially from the inside to the outside of the seam, thus preventing the airbag to expand at once in fore and aft directions during the initial stage of deployment, and preventing excessive buildup of the internal pressure of the airbag after breaking of the seam by allowing the gas intake hole and the gas discharge hole to communicate with each other. 
     Incidentally, the airbags described in Japanese Patent Application Laid-open No. 7-125586 and Japanese Patent Application Laid-open No. 8-119052 both include their starting points of break of seams sewn in a V-letter shape, so that the threads on either side of the vertex of the V-letter shape disperse the load of gas pressure which attempts to push and separate the two base cloths, thus inhibiting the smooth break of seams. 
     Moreover, in the airbag described in Japanese Patent Application Laid-open No. 8-119052, since the seam is formed in a generally annular shape, the airbag inflates in two stages, namely before and after the break of the seam, causing an abrupt change in the airbag&#39;s internal pressure and making a smooth deployment difficult. 
     BRIEF SUMMARY OF THE INVENTION 
     In consideration of the above, it is an object of the present invention to provide an airbag which ensures the separation of a separable bonding portion joining first and second base cloths by means of stress by gas pressure, and which enhances passenger-restraining capability by arbitrarily controlling the internal pressure of the airbag during deployment. 
     In order to achieve the above objects, according to a first feature of the invention, there is provided an airbag device having an airbag, the airbag comprising first and second base cloths sewn at their outer peripheries, and inflated by supplying gas generated by an inflator to the inside of the airbag through a central portion of one base cloth upon collision of a vehicle, wherein the first and second base cloths are joined by a bonding portion continuously extending in a generally circumferential direction from the central portion towards the outer periphery, and wherein load caused by inflation of the airbag breaks the bonding portion from the central portion end towards the outer periphery end thereof. 
     With this arrangement, the first and second base cloths are bonded together by the bonding portions continuously extending in generally a circumferential direction from the central portion to outer peripheries, and the bonding portion is sequentially broken from the central portion to the outer periphery by means of load generated during inflation of the airbag. Therefore, sudden deployment of the airbag in fore and aft directions during the initial stage of deployment is prevented, and soft restraint of passengers is possible by appropriately controlling change in the internal pressure of the airbag to gradually increase the volume of the airbag. Moreover, upon completion of the breaking of the bonding portion, a sufficient restraining force is achieved by deploying the airbag into a final shape that is flat in fore and aft directions. In addition, the linear shape of the bonding portion allows reliable breaking by concentrating a large stress on the ends thereof, and the bonding portion extending in a circumferential direction does not obstruct folding of the base cloths, thus reducing the size of airbag after folding. 
     According to a second feature of the invention, in addition to the first feature, the bonding portion is vortical. 
     With this arrangement, the vortical shape of the bonding portions eliminates abrupt change in the breaking direction, thus providing smooth breaking of the bonding portion and easy control of the internal pressure and deployment speed of the airbag. 
     According to a third feature of the invention, in addition to the second feature, the bonding portion is divided into a plurality of parts in a radial direction of the airbag. 
     With this arrangement, the division of the bonding portion into multiple parts in a radial direction allows variation in the separation strength of the bonding portion and in distance between the adjacent parts, thus facilitating control of the internal pressure and deployment speed of the airbag. 
     According to a fourth feature of the invention, in addition to the second feature, the bonding portion has separation portions where bondage is interrupted, and the separation portions are disposed at different locations in a circumferential direction in relation to a radially inner end of the bonding portion. 
     With this arrangement, since the bonding portion has the separation portions where the bondage is interrupted, even when a section of the bonding portion is pressed down and breaking is terminated there, the breaking restarts from an end where the bonding portion leading a separation portion, thus deploying the airbag without hindrance. Further, since the separation portions are displaced in regard to the radial inner end of the bonding portion in a circumferential direction, to lower the probability that the radial inner end of the bonding portion and separation portion are simultaneously pressed down, thus enabling further reliable deployment of the airbag. 
     According to a fifth feature of the invention, in addition to the second feature, the bonding portion has separation portions where bondage is interrupted, and a radially inner end of the bonding portion and/or an inner end of the separation portion are oriented towards the central portion of the airbag. 
     With this arrangement, by placing separation portions at bonding portions where the bondage is interrupted, even if a section of the bonding portion is pressed down and breaking is stopped there, the breaking restarts from an end where the bonding portion leading to a separation portion, thus deploying the airbag without hindrance. Further, since the radially inner end of the bonding portion and/or the inner end the separation portion is oriented towards the central portion of the airbag, load of the pressure of gas generated by an inflator provided at the central portion of the airbag is effectively concentrated on the radially inner end of the bonding portion and/or the inner end the separation portion, thus providing a reliable breaking. 
     According to a sixth feature of the invention, in addition to the first feature, the bonding portion is divided into a plurality of parts in a switchbacking shape and disposed in a circumferential direction. 
     With this arrangement, by dividing the bonding portion into a plurality of switchbacking pars and disposing them in a circumferential direction, separation strength can be set or tuned separately for each divided part, thus allowing a more favorable control of airbag deployment. Further, even if some divided parts of the bonding portion are pressed down and breaking is stopped there, since the probability that the other divided parts are simultaneously pressed down is low, the other divided parts will break to ensure a reliable airbag deployment. 
     According to a seventh feature of the invention, in addition to the first feature, the radial inner ends of the bonding portion have a separation strength higher than that of the other parts. 
     With this arrangement, since the radial inner ends of the bonding portion have a separation strength higher than that of the other parts, the internal pressure of the airbag during the initial stage of deployment is rapidly increased, thus providing a favorable passenger-restraining capability. 
     According to an eighth feature of the invention, in addition to the first feature, sewing forms the bonding portion, and the first and second base cloths are bonded together by adhesion or welding at parts corresponding to the bonding portion. 
     With this arrangement, by adhesion or welding of the base cloths at parts corresponding to the breakable bonding portion while superimposing the base cloths constituting the airbag, not only gas leakage is prevented by adhesion or welding filling the thread holes of the bonding portion, but also the internal pressure of the airbag is decreased by delaying the breaking of the bonding portion while alleviating damage received by the base cloths upon breaking by lowering the breaking strength of the bonding portion. Although it is difficult to set the breaking strength and breaking start point only by adhesion or welding, combination of adhesion or welding with sewing facilitates the setting of the breaking strength and breaking start point. 
     According to a ninth feature of the invention, in addition to the first feature, at least a portion of a periphery of a vent hole formed on at least one of the first and second base cloths is surrounded by an auxiliary bonding portion which continues to the bonding portion, and the vent hole is opened by breaking of the auxiliary bonding portion accompanying breaking of the bonding portion. 
     With this arrangement, at least a portion of a periphery of a vent hole formed on at least one of the first and second base cloths is surrounded by an auxiliary bonding portion which continues to the bonding portion, and the vent hole is opened by breaking of the auxiliary bonding portion accompanying breaking of the bonding portion. Therefore, the vent hole is open at an arbitrary timing during deployment of the airbag, thereby enabling a further precise control of the internal pressure of the airbag. 
     According to a tenth feature of the invention, in addition to the first feature, an end of a vent hole cover which closes a vent hole formed on at least one of the first and second base cloths is joined to the bonding portion in the vicinity of the outer periphery of the airbag, and the end of the vent hole cover is separated from the first and second base cloths by breaking of the bonding portion to open the vent hole. 
     With this arrangement, by joining the end of the vent hole cover to the bonding portion in the vicinity of the outer periphery of the airbag, the end of the vent hole cover is separated from the base cloths by breaking of the bonding portion during the final stage of airbag deployment, to thereby open the vent hole. Therefore, the vent hole is maintained in a closed state until the airbag deployment is substantially completed, thereby promoting the increase of the internal pressure. Further, upon completion of airbag deployment, the gas is discharged through the vent hole, thereby suppressing excessive increase in the internal pressure. In addition, the timing of opening the vent hole is easily controlled by adjusting the location at which the end of the vent hole cover is joined to the bonding portion. 
     According to an eleventh feature of the invention, in addition to the tenth feature, the vent hole cover is joined at a portion of the periphery of the vent hole. 
     With this arrangement, by joining the vent hole cover at a portion of the periphery of the vent hole, the direction of the gas discharged through the vent hole is regulated by the vent hole cover. 
     According to a twelfth feature of the invention, in addition to the first feature, a duct with one end communicating with a vent hole formed on at least one of the first and second base cloths, and the other end joined to the bonding portion in the vicinity of the outer periphery of the airbag to be closed, is placed along inner faces of the first and second base cloths, and the vent hole is opened by opening the closed other end of the duct by breaking of the bonding portion. 
     With this arrangement, by placing a duct along the inner faces of the base cloths, with one end communicating with a vent hole and the other end joined to the bonding portion to be closed, the bonding portion breaks at the final stage of deployment of the airbag to open the closed other end of the vent hole cover thus opening the vent hole. Therefore, the vent hole is maintained in a closed state until the airbag deployment is substantially completed, thereby promoting the increase of the internal pressure. Further, upon completion of airbag deployment, the gas is discharged through the vent hole, thereby suppressing excessive increase in the internal pressure. In addition, the timing of opening the vent hole is easily controlled by adjusting the location at which the end of the vent hole cover is joined to the bonding portion, and discharge volume of the gas is controlled by changing the duct diameter. 
     According to a thirteenth feature of the invention, in addition to the first feature, a duct with one end communicating with a vent hole formed on at least one of the first and second base cloths, and the other end joined to the bonding portion in the vicinity of the outer periphery of the airbag to be closed, is placed outside the airbag, and the vent hole is opened by opening the closed other end of the duct by breaking of the bonding portion. 
     With this arrangement, by placing a duct at the exterior of the airbag, with one end communicating with a vent hole and the other end joined to the bonding portion to be closed, the bonding portion breaks at the final stage of deployment of the airbag to open the closed other end of the vent hole cover thus opening the vent hole. Therefore, the vent hole is maintained in a closed state until the airbag deployment is substantially completed, thereby promoting the increase of the internal pressure. Further, upon completion of airbag deployment, the gas is discharged through the vent hole, thereby suppressing excessive increase in the internal pressure. In addition, the timing of opening the vent hole is easily controlled by adjusting the location at which the end of the vent hole cover is joined to the bonding portion, and discharge volume of the gas is controlled by changing the duct diameter. Further, by placing the duct in a direction going away from the passenger, discharged gas can be prevented from exerting on the passenger. Additionally, by placing the duct outside the airbag, the duct length can be freely set, thus allowing a further appropriate control of the gas discharge volume and discharge duration. 
     According to a fourteenth feature of the invention, in addition to any of the ninth to thirteenth features, at least one of the first and second base cloths has a gas discharge portion which is opened when the vent hole fails to function and the internal pressure of the airbag reaches or exceeds a predetermined level, thereby depressurizing the airbag. 
     With this arrangement, when the vent hole formed on at least one of the first and second base cloths fails to function for some reason and the internal pressure of the airbag reaches or exceeds a predetermined level, the gas discharge portion is opened by such a internal pressure, thereby depressurizing the airbag. Therefore, the gas discharge portion can function as a vent hole to prevent excessive increase of the internal pressure of the airbag. 
     The seam  39  of the embodiments corresponds to the bonding portion in the present invention, the non-sewn portions  39   a  and  39   b  of the embodiments correspond to the non-bonding portion in the present invention, the auxiliary seam  60  of the embodiments corresponds to the auxiliary bonding portion in the present invention, and the fragile portions  38   a ,  53  and  61  of the embodiments correspond to the gas discharge portion of the present invention. 
     The above and other objects, features and advantages of this invention will be more clearly understood from the following detailed description of the preferred embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 to 6  show a first embodiment of the present invention. 
         FIG. 1  is a perspective view of a front portion of a vehicle compartment of a motor vehicle. 
         FIG. 2  is an enlarged sectional view taken along line  2 - 2  in  FIG. 1 . 
         FIG. 3  is an exploded perspective view of an airbag. 
         FIG. 4  is a rear view of the airbag. 
         FIG. 5  is a front view of the airbag. 
         FIG. 6  is a sectional view taken along line  6 - 6  in  FIG. 4 . 
         FIG. 7  is a rear view of an airbag according to a second embodiment of the present invention. 
         FIG. 8  is a rear view of an airbag according to a third embodiment of the present invention. 
         FIG. 9  is a rear view of an airbag according to a fourth embodiment of the present invention. 
         FIG. 10  is a rear view of an airbag according to a fifth embodiment of the present invention. 
         FIG. 11  is a rear view of an airbag according to a sixth embodiment of the present invention. 
         FIG. 12  is a rear view of an airbag according to a seventh embodiment of the present invention. 
         FIG. 13  is a front view of an airbag according to an eighth embodiment of the present invention. 
         FIG. 14  is a front view of an airbag according to a ninth embodiment of the present invention. 
         FIG. 15  is a rear view of an airbag according to a tenth embodiment of the present invention. 
         FIG. 16  is a front view of an airbag according to an eleventh embodiment of the present invention. 
         FIG. 17  is a front view of an airbag according to a twelfth embodiment of the present invention. 
         FIGS. 18 to 20  show a thirteenth embodiment of the present invention. 
         FIG. 18  is a front view of the airbag. 
         FIG. 19  is a rear view of the airbag. 
         FIG. 20  is an enlarged sectional view taken along line  20 - 20  in  FIG. 18 . 
         FIG. 21  is a rear view of a fourteenth embodiment. 
         FIG. 22  is an exploded perspective view of an airbag according to a fifteenth embodiment. 
         FIG. 23  is an exploded perspective view of an airbag according to a sixteenth embodiment. 
         FIGS. 24 to 28B  show a seventeenth embodiment of the present invention. 
         FIG. 24  is an exploded perspective view of an airbag. 
         FIG. 25  is a front view of the airbag. 
         FIG. 26  is a rear view of the airbag. 
         FIG. 27  is an enlarged sectional view taken along line  27 - 27  in  FIG. 26 . 
         FIGS. 28A and 28B  are effect explanatory diagrams of the airbag during deployment. 
         FIG. 29  is a drawing according to an eighteenth embodiment of the present invention, corresponding to  FIG. 26 . 
         FIG. 30  is a drawing according to a nineteenth embodiment of the present invention, corresponding to  FIG. 26 . 
         FIG. 31  is a drawing according to a twentieth embodiment of the present invention, corresponding to  FIG. 26 . 
         FIGS. 32 and 33  show a twenty-first embodiment of the present invention. 
         FIG. 32  is a view corresponding to  FIG. 24 . 
         FIG. 33  is an effect explanatory diagram of the airbag during deployment. 
         FIGS. 34 to 35B  show a twenty-second embodiment of the present invention. 
         FIG. 34  is a drawing corresponding to  FIG. 24 . 
         FIGS. 35A and 35B  are effect explanatory diagrams of the airbag during deployment. 
         FIGS. 36 to 37B  show a twenty-third embodiment of the present invention. 
         FIG. 36  is a drawing corresponding to  FIG. 26 . 
         FIGS. 37A and 37B  are enlarged sectional views taken along line  37 - 37  in  FIG. 36 . 
         FIGS. 38A and 38B  show a twenty-fourth embodiment of the present invention, and respectively corresponding to  FIGS. 37A and 37B . 
         FIG. 39  is a drawing according to a twenty-fifth embodiment of the present invention, corresponding to  FIG. 26 . 
         FIG. 40  is a drawing according to a twenty-sixth embodiment of the present invention, corresponding to  FIG. 26 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 to 6 , a first embodiment of the present invention will be described below. 
     As shown in  FIG. 1 , an airbag module  13  for a driver&#39;s seat is stored inside a steering wheel  12  disposed in front of a driver&#39;s seat  11 . 
     As shown in  FIG. 2 , the steering wheel  12  comprises a boss portion  16  fixed to the rear end of a steering shaft  14  by a nut  15 , a front cover  17  fixed to the boss portion  16 , a rear cover  19  fixed to a back side of the front cover  17  by bolts  18  . . . , a plurality of spoke portions  20  . . . radially extending from the front cover  17 , and a steering wheel main body  21  which leads to the outer peripheries of the spoke portions  20  . . . . A retainer  22 , bolted together with bolts  18  . . . to an inner periphery of the rear cover  19 , supports the airbag module  13 . A thin-walled tear line  19   a , which breaks during inflation of the airbag  32 , is formed on the inside of the rear cover  19 . 
     The airbag module  13  comprises an inflator  31  filled with a propellant to generate high-pressure gas by combustion, an airbag  32  composed of sewn base cloths, and a fixing ring  33  which fixes a base portion of the airbag  32 . A flange  31   a  of the inflator  31  and the fixing ring  33  are superimposed onto the front and rear sides of the retainer  22 , and bolted by bolts  34  . . . and nuts  35  . . . , while fixing a base portion of the airbag  32  which is sandwiched between the rear side of the retainer  22  and the front side of the fixing ring  33 . 
     As shown in  FIG. 3 , the circular airbag  32  comprises a rear first base cloth  36  which faces the passenger, and a front second base cloth  37  which is superimposed onto the front of the rear base cloth  36 . The first and second base cloths  36  and  37  are integrally sewn at an outer peripheral seam  38 . A circular aperture  37   a  which encircles the inflator  31 , and two vent holes  37   b  and  37   b  which release a portion of the gas during inflation of airbag  32 , and four bolt holes  37   c  . . . through which the bolts  34  . . . pass are formed on the second base cloth  37  of airbag  32 . Therefore, the airbag  32  is supported at the central portion of second base cloth  37  by a retainer  22 , and the gas generated by the inflator  31  is supplied to the interior of the airbag  32  through the aperture  37   a  formed in the center of second base cloth  37 . 
     As shown in  FIGS. 4 and 5 , the superimposed first and second base cloths  36  and  37  are sewn together by one vortical seam  39  which starts from point a near the center, to point b in the outer periphery. While the outer peripheral seam  38  is firmly sewn by using a thicker thread to prevent breaking during inflation of airbag  32 , vortical seam  39  is fragilely sewed using a finer thread to encourage breaking during inflation of the airbag  32 . Point a at the radial inner end of the vortical seam  39  is oriented towards the central portion of airbag  32 , i.e., the inflator  31 . 
     Thus, when an acceleration exceeding a predetermined value is detected upon collision of the vehicle, the inflator  31  is ignited to generate gas, which starts the inflation of folded airbag  32 . The tear line  19   a  of the rear cover  19  breaks when receiving pressure from the inflating airbag  32 , to form an aperture through which the airbag  32  deploys itself inside the vehicle compartment. 
     As shown in  FIGS. 4 to 6 , because the first and second base cloths  36  and  37  are integrally joined by the vortical seam  39 , sudden inflation of the airbag  32  in fore and aft directions is prevented during the inflation process of the airbag  32 , thereby enabling a soft restraining of the passenger. During inflation, because a strong stress is concentrated on the inner end point a of the vortical seam  39 , the break of the vortical seam  39  starts at point a and sequentially proceeds towards point b. As the break proceeds, the bondage between the first and second base cloths  36  and  37  is broken, and the airbag  32  deploys into a final shape that is flat in fore and aft directions, thereby exerting maximum restraining capability as indicated by the chained line shown in  FIG. 6 . 
     Additionally, the seam  39  shaped into one line instead of a V-letter shape allows a strong stress to concentrate on its inner end point a to provide a reliable breaking. In particular, since the inner end of the vortical seam  39  is oriented towards the inflator  31  in the central portion of the airbag  32 , stress of the pressure of gas generated by inflator  31  can be effectively concentrated on point a thereby further ensuring a reliable breaking of seam  39 . Moreover, since the seam  39  extends in a circumferential direction so as not to hinder the folding of the first and second base cloths  36  and  37 , resulting in a reduced size of airbag  32  after being folded. Furthermore, the seam  39  is vortical without direction-changing points, thus enabling a smooth breaking of the seam  39  and an accurate control of the internal pressure and deployment speed of the airbag  32 . 
     As described above, the internal pressure, deployment speed, shape as well as the other aspects of the airbag  32  during deployment can be arbitrarily controlled to effectively restrain passengers, by sewing the first and second base cloths  36  and  37  along the vortical seam  39 , and using the stress upon inflation to sequentially break the seam  39  radially from the inside to the outside. In particular, the vortical seam  39  has a generally uniform curvature without portions where the curvature abruptly changes, thus providing a smooth breaking of the seam  39 , as well as an easy control of the internal pressure, deployment speed, shape and the other aspects of the airbag  32  during deployment. 
     Next, second and third embodiments of the present invention will be described with reference to  FIGS. 7 and 8 . 
     In the second embodiment shown in  FIG. 7 , two vortical seams  39  and  39  are set 180 degrees out of phase with each other, while in the third embodiment shown in  FIG. 8 , four vortical seams  39  . . . are set 90 degrees out of phase with each other. Also in the second and third embodiments, the radially inner ends points a the vortical seams  39  are oriented towards the central portion of the airbag  32 , thus allowing a strong stress to concentrate on the points a so that they function as starting points of breaking. 
     The second and third embodiments also achieve the same effects as in the first embodiment. In addition, the internal pressure, deployment speed, shape and the other aspects of airbag  32  during deployment can be further minutely controlled, because setting or tuning of the breaking strength of each seam  39  can be individually performed. 
     Next, a fourth embodiment of the present invention will be described with reference to  FIG. 9 . 
     The fourth embodiment is a variation of the first embodiment. While the vortical seam  39  is formed by a one line of sewing in the first embodiment, four vortical seams  39  having a length slightly exceeding 360 degrees are radially provided in the fourth embodiment. Although the functions of each seam  39  is the same as in the seam  39  of the first embodiment, the fourth embodiment achieve an effect in addition to those of the first embodiment, that is, effect of enhancing freedom in controlling the internal pressure, deployment speed, shape of the other aspects of the airbag  32  during deployment, by giving different seam strengths to the four seams  39 , or by varying the radial distances between the four seams  39 . 
     Next, fifth to seventh embodiments of the present invention will be described with reference to  FIGS. 10 to 12 . 
     In the first to fourth embodiments, the seams  39  are formed into a vortical shape, while in the fifth to seventh embodiments the first and second base cloths  36  and  37  are divided into a plurality (in these embodiments, three or four) of fan-like sections. In each section, switchbacking seams  39  are disposed to radially extend from the inside to the outside. In the fifth embodiment shown in  FIG. 10 , the three seams  39  . . . comprise alternately-disposed circular arc portions extending in a circumferential direction and straight portions extending in a radial direction. In the sixth embodiment shown in  FIG. 11 , the four seams  39  . . . comprise circular arc portions extending in a circumferential direction and semicircular portions connecting adjacent circular arc portions. In the seventh embodiment shown in  FIG. 12 , the four seams  39  . . . comprise a plurality of zigzagging lines. 
     The fifth to seventh embodiments also achieve the same effects as in the first embodiment. In addition, the internal pressure, deployment speed, shape and the other aspects of airbag  32  during deployment can be further minutely controlled, because setting or tuning of the breaking strength of each seam  39  can be individually performed. Further, since the seam  39  . . . is divided into three or four parts in a circumferential direction, even when something is pressing down on one part to preventing it from breaking, the remaining parts will break to deploy the airbag  32 . 
     Next, an eighth and ninth embodiments of the present invention will be described with reference to  FIGS. 13 and 14 . 
     The eighth and ninth embodiments are variations of the second and third embodiments. Three vent holes  37   d  . . . are formed in the vicinity of the outer periphery of the second base cloth  37  disposed in front of the airbag  32 . These vent holes  37   d  . . . are disposed at a location surrounded by the outer peripheral seam  38  of the first and second base cloths  36  and  37 , the three vortical seams  39  . . . , and the fragile auxiliary seams  60  . . . which connect both the seams  38  and  39  . . . . The only difference between the eighth and ninth embodiments is the shape of the auxiliary seams  60  . . . added for surrounding the vent holes  37   d  . . . , and the effects are the same. 
     In addition to the effects of the second and third embodiments, the eighth and ninth embodiments achieve the following effects. The vent holes  37   d  remain closed during the initial and middle stages of deployment of the airbag  32  where the seams  39  . . . are radially broken from the inside to the outside; and are opened at the final stage of deployment when the auxiliary seams  60  . . . are broken accompanying the breaking of the seam  39  . . . , thus discharging gas to prevent an excessive increase of the internal pressure of the airbag  32 . In addition, the timing of opening the vent holes  37   d  . . . can be arbitrarily controlled by placing the vent holes  37   d  . . . either radially inside or outside the seam  39  . . . , thereby further minutely controlling the internal pressure of the airbag  32 . 
     In the eighth and ninth embodiments, when the inflator  31  is ignited in two stages, the internal pressure of the airbag  32  . . . can be further precisely controlled by reducing the strength of seam  39  . . . at a radially inner portion A and increasing the strength of seam  39  . . . at a radially outer portion B, and breaking the portion A of the seam  39  . . . in front of the vent hole  27   d  . . . with the first ignition of the inflator  31 , and breaking the portion B of seam  39  . . . which includes vent hole  27   d  . . . with the second ignition of the inflator  31 . 
     For instance, even if there is an error in the timing of the breaking of portion A of seam  39  . . . with the first ignition of the inflator  31 , the error in the timing for opening vent holes  27   d  . . . can be eliminated by timely breaking the portion B of the seam  39  . . . with the second ignition of the inflator  31  so that the error is compensated for. Of course, the inflator  31  can be ignited in three or more stages. 
     Next, a tenth embodiment of the present invention will be described with reference to  FIG. 15 . 
     The tenth embodiment is a variation of the first embodiment shown in  FIG. 4 , and comprises three seams  39  . . . which sew together the superimposed first and second base cloths  36  and  37  in a vortical shape. Each seam  39  has a plurality (two in this embodiment) of non-sewn portions  39   a  and  39   b  at which sewing is interrupted. The end of seam  39  . . . , point c, which continues into the non-sewn portions  39   a  and  39   b  is oriented towards the central portion of the airbag  32 . The circumferential position of at least one of the two non-sewn portions  39   a  and  39   b  (in this embodiment the radially inner non-sewn portion  39   a ) is misaligned in a circumferential direction from the radially inner end portion point a of the seam  39  . . . . 
     Therefore, even if a unbreakable portion encircled by a chained line is pressed by something during the deployment of the airbag  32 , and breaking which has started at the radially inner end point a of vortical seam  39  . . . terminates at a non-sewn portion, a new breaking starts at point c facing the two non-sewn portions  39   a  and  39   b  of seam  39  . . . in clockwise and counterclockwise directions, ultimately breaking the entire seam  39  . . . except the non-sewn portions, thus deploying most parts of the airbag  32  to exhibit passenger-restraining capability. In this process, since the point c facing the two non-sewn portions  39   a  and  39   b  of seam  39  . . . is oriented towards the central portion of the airbag  32  as is the radially inner end point a of seam  39  . . . , a strong stress can be concentrated thereon to ensure a reliable start of breaking. 
     Also, since the circumferential position of at least one of the two non-sewn portions  39   a  and  39   b  (in this embodiment the radially inner non-sewn portion  39   a ) is misaligned in a circumferential direction from the radially inner end point a corresponding to the seam  39  . . . , it is possible to prevent situations where the point a and the plurality of the points c are all included in the unbreakable portions, thus disabling breaking of the seam  39  . . . . 
     Next, an eleventh embodiment of the present invention will be described with reference to  FIG. 16 . 
     The eleventh embodiment is a variation of the second embodiment shown in  FIG. 7 , wherein two vent holes  37   d  and  37   d  are formed on the second base cloth  37  among the superimposed first and second base cloths  36  and  37 , and a predetermined number (two in this embodiment) of fragile portions  38   a  and  38   a  are formed at the outer peripheral seam  38  of the first and second base cloths  36  and  37 . The fragile portions  38   a  and  38   a  are sewn using a thread finer than the other portions of the seam  38  for easier breaking. 
     Thus, even if the two vent holes  37   b  and  37   b  remain closed for some reason and are unable to exhibit their functions at the final stage of deployment of the airbag  32 , when the internal pressure of the airbag  32  reaches or exceeds a predetermined level, the fragile portions  38   a  and  38   a  of the seam  38  break and an aperture formed thereon functions as a vent hole to discharge the gas, thus preventing an excessive increase of the internal pressure of the airbag  32 . 
     Next, a twelfth embodiment of the present invention will be described with reference to  FIG. 17 . 
     The twelfth embodiment is further a variation of the eleventh embodiment shown in  FIG. 16 . While the fragile portions  38   a  and  38   a  are formed on the seam  38  in the eleventh embodiment, a fragile portion  61  sewn so as to close a slit linearly cut into the second base cloth  37  is formed in the twelfth embodiment. The breaking strength of the fragile portion  61  is set to be higher than that of the seams  39  and  39 , at the same time lower than that of the seam  38 . 
     Thus, even if the two vent holes  37   b  and  37   b  remain closed for some reason and are unable to exhibit their functions at the final stage of deployment of the airbag  32 , when the internal pressure of the airbag  32  reaches or exceeds a predetermined level, the fragile portion  61  breaks and an aperture formed thereon functions as a vent hole to discharge the gas, thus preventing an excessive increase of the internal pressure of the airbag  32 . 
     Next, a thirteenth embodiment of the present invention will be described with reference to  FIGS. 18 to 20 . 
     The superimposed first and second base cloths  36  and  37  are sewn together by three vortical seams  39  . . . which start from point a near the center, to point b in the outer periphery, and set 120 degrees out of phase with each other. While the outer peripheral seam  38  is firmly sewn by using a thicker thread to prevent breaking upon inflation of the airbag  32 , the vortical seam  39  . . . are fragilely sewn by using a finer thread to facilitate breaking upon inflation of the airbag  32 . In addition, in the section where the first and second base cloths  36  and  37  are sewn by three vortical seams  39 , that is, the annular section having a predetermined width at the outer periphery of the airbag  32 , the first and second base cloths  36  and  37  are adhered to each other by an adhesive  70  (refer to  FIG. 20 ). In  FIG. 18 , hatching indicates the section where the first and second base cloths  36  and  37  are bonded together by adhesion. 
     Thus, when an acceleration exceeding a predetermined value is detected upon collision of the vehicle, the inflator  31  is ignited to generate gas that starts the inflation of the folded airbag  32 . The tear line  19   a  of the rear cover  19  breaks when receiving pressure from the inflating airbag  32  to form an aperture, through which the airbag  32  deploys into the vehicle compartment. 
     As shown in  FIGS. 19 and 20 , because the first and second base cloths  36  and  37  are integrally bonded by the vortical seam  39  . . . , sudden inflation of the airbag  32  in fore and aft directions during inflation of the airbag  32  is prevented, thus enabling a soft restraining of the passenger. During inflation, because a strong stress is concentrated on the inner end point a of the vortical seam  39  . . . , the breaking of the seam  39  . . . starts at point a and sequentially proceeds towards point b. Accompanying the breaking proceeds, the bondage between the first and second base cloths  36  and  37  are broken, so that the airbag  32  deploys into a final shape that is flat in fore and aft directions, thereby exerting the maximum restraining force. 
     Since the first and second base cloths  36  and  37  are adhered together at the annular section corresponding to the seam  39  . . . , not only gas leakage is prevented by the adhesive  70  filling the thread holes of the seam  39  . . . , but also damage received by the first and second base cloths  36  and  37  at time of break is alleviated by lowering the breaking strength of the seam  39  . . . , while delaying the breaking at the seam  39  by adhesion force of the adhesive  70  to increases the internal pressure of the airbag  32 . Although it is difficult to set the breaking strength and breaking start point only by the adhesive  70 , combination of the adhesive  70  with the seam  39  facilitates the setting of the breaking strength and breaking start point, thus enabling a precise control of the internal pressure in the process of deployment of the airbag  32 . 
     Moreover, in the thirteenth embodiment, the outer peripheral seams  38  of the first and second base cloths  36  and  37  are simultaneously adhered together at the annular section, thus effectively preventing gas leakage through the thread holes of the seam  38 . 
     Next, a fourteenth embodiment of the present invention will be described with reference to  FIG. 21 . 
     While the first and second base cloths  36  and  37  are adhered together at the annular section covering the three seams  39  . . . in the thirteenth embodiment, adhesion is performed only to the periphery of three seams  39  . . . (see hatched portion) in the fourteenth embodiment. 
     As described above, by adhesion only to the periphery of the three seams  39  . . . , the gas penetrates into the gaps between the seam  39  . . . (i.e. bonding portion) and attempts to push and separate the first and second base cloths  36  and  37 , thereby ensuring a further reliable breaking of the seam  39  . . . . 
     Next, a fifteenth embodiment of the present invention will be described with reference to  FIG. 22 . 
     While the airbag  32  consists of first and second base cloths  36  and  37  in the thirteenth and fourteenth embodiments, the airbag  32  in the fifteenth embodiment comprises annular third and fourth base cloths  36 ′ and  37 ′ that are sandwiched between the outer peripheral portions of the first and second base cloths  36  and  37 . The third base cloth  36 ′ is firmly adhered to the inner face of first base cloth  36  so as to avoid detachment, and the fourth base cloth  37 ′ is firmly adhered to the inner face of second base cloth  37  so as to avoid detachment. Then, the first base cloth  36 , third base cloth  36 ′, fourth base cloth  37 ′ and second base cloth  37  are superimposed and sewn along the seams  38 ,  39  and  39 . In this process, the third base cloth  36 ′ and the fourth base cloth  37 ′ are adhered together by a relatively detachable adhesive. 
     According to this fifteenth embodiment, in addition to the effects of the thirteenth embodiment, there is provided a further enhanced gas leakage prevention effect by reinforcement of the first base cloth  36  by the third base cloth  36 ′ and reinforcement of the second base cloth  37  by fourth base cloth  37 ′ to further reliably prevent damage to the seams of the first and second base cloths  36  and  37 . 
     Although the seams  39  and  39  are concentrically formed in the fifteenth embodiment, they may be shaped into a vortex as in the thirteenth and fourteenth embodiments. Also, while the third and fourth base cloths  36 ′ and  37 ′ are adhered to the inner faces of the first and second base cloths  36  and  37  in the fifteenth embodiment, they may be adhered to the outer sides of the first and second base cloths  36  and  37 . 
     Next, a sixteenth embodiment of the present invention will be described with reference to  FIG. 23 . 
     The sixteenth embodiment is a variation of the fifteenth embodiment. While the first and second base cloths  36  and  37  are reinforced by adhesion on the third and fourth base cloths  36 ′ and  37 ′ in the fifteenth embodiment, in the sixteenth embodiment, an annular section (the densely hatched area in the enlarged sectional view) is woven in advance for reinforcement along the outer periphery of the first and second base cloths  36  and  37 . 
     According to the sixteenth embodiment, the same effects as in the fifteenth embodiment can be achieved without increasing the number of components. Also in the sixteenth embodiment, the seams  39  and  39  are concentrically formed, but they may be shaped into a vortex as in the thirteenth and fourteenth embodiments. 
     Next, a seventeenth embodiment of the present invention will be described with reference to  FIGS. 24 to 28B . 
     As apparent from  FIGS. 24 to 27 , a mounting portion  40   a  of an airbag cover  40  is superimposed on the outer face of the central portion of second base cloth  37  and integrally sewn at two seams  41  and  41 ; and two band-like portions  40   b  and  40   b  of the airbag cover  40  are inserted into the two vent holes  37   b  and  37   b  of the second base cloth  37  from the outside to the inside. Since the diameters of vent holes  37   b  and  37   b  are the same as the widths of the band-like portions  40   b  and  40   b , the vent holes  37   b  and  37   b  are closed by the band-like portions  40   b  and  40   b . In addition, the superimposed second base cloth  37  and the mounting portion  40   a  of the vent hole cover  40  are sandwiched between the rear face of the retainer  22  and the front face of the fixing ring  33  and bolted together with a bolt  34 . Therefore, gas generated by the inflator  31  is supplied to the interior of airbag  32  through the aperture  37   a  at the center of second base cloth  37  and the aperture  40   c  of the vent hole cover  40 . 
     The superimposed first and second base cloths  36  and  37  are sewn at locations close to their outer peripheries by three concentric seams  39  . . . . While the outer periphery seam  38  is firmly sewn by using a thicker thread to prevent breaking upon inflation of airbag  32 , a finer thread fragilely sews the inner three concentric seams  39  . . . to facilitate breaking upon inflation of the airbag  32 . In this process, the tip ends of the two band-like portions  40   b  and  40   b  extending into airbag  32  are sewn together at the three seams  39  . . . . 
     Thus, when an acceleration exceeding a predetermined value is detected upon collision of the vehicle, the inflator  31  is ignited to generating gas that starts the inflation of the folded airbag  32 . The tear line  19   a  of the rear cover  19  breaks when receiving pressure from the inflating airbag  32  to form an aperture, through which the airbag  32  deploys into the vehicle compartment. 
     As shown in  FIG. 28A , since the three seams  39  . . . integrally bond together the first and second base cloths  36  and  37 , the sudden inflation of airbag  32  is prevented when the airbag  32  inflates, thus enabling a soft restraining of passengers. As the internal pressure of the airbag  32  increases, breaking of the three seams  39  . . . sequentially proceeds radially from the inside to the outside, allowing the airbag  32  to gradually increase its volume while maintaining an appropriate internal pressure and ultimately deploying into a final shape that is flat in fore and aft directions, thereby exerting a maximum restraining force. 
     Since the other ends of the two band-like portions  40   b  and  40   b  of the vent hole cover  40  are restrained by the first and second base cloths  36  and  37  until all the three seams  39  . . . are broken, the vent holes  37   b  and  37   b  are closed by the band-like portions  40   b  and  40   b  to prevent gas leakage, thus enabling an immediate increase of the internal pressure of the airbag  32  and maintaining the internal pressure at an appropriate level. When all the three seams  39  . . . are broken at the final stage of deployment of the airbag  32 , the restraints on the other ends of the two band-like portions  40   b  and  40   b  of the vent hole cover  40  are released as shown in  FIG. 28B , to push portions of the band-like portions  40   b  and  40   b  of the vent hole cover  40  out from the vent holes  37   b  and  37   b , thus opening the vent holes  37   b  and  37   b . As a result, excess gas is discharged through the vent holes  37   b  and  37   b , thus preventing an excessive increase of the internal pressure of the airbag  32 . 
     While the tip ends of the two band-like portions  40   b  and  40   b  of the vent hole cover  40  are sewn together at all the three seams  39  . . . in this embodiment, the timing of the opening of vent holes  37   b  and  37   b  can be arbitrarily adjusted by sewing them to the two inner seams  39  and  39  or to the one innermost seam  39 . Also, since the band-like portions  40   b  and  40   b  are arranged so that they pass through the vent holes  37   b  and  37   b , thereby preventing misalignment of the band-like portions  40   b  and  40   b  from the vent holes  37   b  and  37   b  to thus prevent gas leakage. 
     Next, an eighteenth and nineteenth embodiments of the present invention will be described with reference to  FIGS. 29 and 30 . 
     In the seventeenth embodiment, the mounting portion  40   a  of the vent hole cover  40  is disposed outside the second base cloth  37 , and the band-like portions  40   b  and  40   b  pass through the vent holes  37   b  and  37   b  to enter the inner face of second base cloth  37 , while the vent hole cover  40  in the eighteenth embodiment shown in  FIG. 29  is generally arranged so as to conform to the shape of the inner face of the second base cloth  37 . To prevent misalignment of the band-like portions  40   b  and  40   b  of the vent hole cover  40  from the vent holes  37   b  and  37   b , a guide member  42  that allows longitudinal sliding of the band-like portions  40   b  and  40   b  is fixed to the inner face of the second base cloth  37  by the seams  43  and  43 . 
     Further, the vent hole cover  40  in the nineteenth embodiment shown in  FIG. 30  is disposed so that it generally conforms to the shape of the outer face of the second base cloth  37 , and guide members  42  that allow longitudinal sliding of the band-like portions  40   b  and  40   b  are fixed to the outer face of the second base cloth  37  by the seams  43  and  43  in order to prevent misalignment of the band-like portions  40   b  and  40   b  from the vent holes  37   b  and  37   b.    
     Thus, the eighteenth and nineteenth embodiments achieve the same effects as in the seventeenth embodiment. 
     Next, a twentieth embodiment of the present invention will be described with reference to  FIG. 31 . 
     While the seventeenth to nineteenth embodiments include a mounting portion  40   a  of the vent hole cover  40  fixed to the central portion of the second base cloth  37 , and tip ends of the band-like portions  40   b  and  40   b  are fixed to the outer periphery of the second base cloth  37 , the twentieth embodiment features two vent hole covers  40  and  40  formed generally into a band-shape disposed parallel to each other and sandwiching the aperture  37   a  from opposite sides at the outer face of the second base cloth  37 . Each vent hole cover  40  slidably penetrate through two guide members  42  and  42  which are fixed to the second base cloth  37  by seams  43  . . . so as to cover vent hole  37   b  of the second base cloth  37 , while the opposite ends are sewn onto the second base cloth  37  by three breakable concentric seams  39  . . . . 
     Thus, this twentieth embodiment achieves the same effects as in the seventeenth embodiment described above. Moreover, the vent hole covers  40  and  40  may be disposed at the inner face of the second base cloth  37 , or may be disposed to pass through the vent holes  37   b  and  37   b  with one end positioned outside and the other end positioned inside. 
     Next, a twenty-first embodiment of the present invention will be described with reference to  FIGS. 32 and 33 . 
     The twenty-first embodiment comprises a duct component  44  that is fixed to the inner face of the second base cloth  37 . The duct component  44  comprises an annular mounting portion  44   a , and two duct portions  44   b  and  44   b  extend therefrom in directions going away from each other. A circular aperture  44   c  and four bolt holes  44   d  . . . through which bolts  34  . . . pass are formed on mounting portion  44   a . The mounting portion  44   a  is superimposed onto the inner face of the central portion of the second base cloth  37  and sewn together at two seams  45  and  45 , while the duct portions  44   b  and  44   b  are sewn onto the inner face of the second base cloth  37  by U-shaped seams  46  and  46 . Thus, cylindrical ducts  47  and  47  are formed between the duct portions  44   b  and  44   b  and the second base cloth  37 , with their radially inner ends communicating with the vent holes  37   b  and  37   b , and portions near the radially outer ends are sewn by three seams  39  . . . to prevent communication with the inner space of the airbag  32 . 
     Therefore, at the initial and middle stages of deployment of the airbag  32 , the seam  39  . . . closes the ducts  47  and  47  to prevent the vent holes  37   b  and  37   b  from functioning. At the final stage of deployment of the airbag  32 , the seam  39  . . . breaks to allow the inner space of the airbag  32  to communicate with the vent holes  37   b  and  37   b  through the ducts  47  and  47 , thereby discharging excess gas to prevent an excess increase of the internal pressure of the airbag  32 . 
     According to this twenty-first embodiment, in addition to the effects of the seventeenth to twentieth embodiments, there is provided an additional effect of adjusting gas discharge through the vent holes  37   b  and  37   b  by adjusting the diameters of the ducts  47  and  47 . 
     Next, a twenty-second embodiment of the present invention will be described with reference to  FIGS. 34 to 35B . 
     While the twenty-first embodiment comprises two ducts  47  and  47  inside the airbag  32 , the twenty-second embodiment comprises three ducts  48  . . . outside the airbag  32 . That is, three vent holes  37   b  . . . are formed at the outer peripheral portion of the second base cloth  37  of the airbag  32 , and a base end of a cylindrical duct component  49  . . . formed by the same material as that of the first and second base cloths  36  and  37  are sewn by seam  50  . . . around these vent holes  37   b.    
     The three breakable seams  39 ,  39  and  39  integral with the first and second base cloths  36  and  37  extend radially into a vortical shape from the inside to the outside, and an end of the folded duct component  49  . . . is integrally sewn thereto by the breakable seam  39  . . . . Moreover, the radially inner end point a of the seam  39  . . . is oriented towards the center of the airbag  32 , to promote the start of breaking of the seam  39  . . . by concentrating stress on the point a upon deployment of airbag  32 . 
     Therefore, at the initial and middle stages of deployment of the airbag  32 , the seam  39  . . . closes the ducts  48  to prevent the vent holes  37   b  . . . from functioning. At the final stage of deployment of the airbag  32 , the seam  39  . . . breaks to allow the inner space of the airbag  32  to communicate with external air through the vent holes  37   b  . . . and the duct  48  . . . , thereby discharging excess gas and preventing an excess increase of the internal pressure of the airbag  32 . 
     According to this twenty-second embodiment, in addition to the effects of the seventeenth to twentieth embodiments, there is provided additional effects of adjusting gas discharged through the vent holes  37   b  . . . by adjusting the diameter of duct  48  . . . , and of preventing gas discharged from the vent hole  37   b  . . . from hitting the passenger by orienting the duct component  49  . . . towards the front side of the vehicle, i.e., a side opposite from the passenger. Further, by placing the duct  48  . . . outside the airbag  32 , the length of the duct  48  . . . can be freely adjusted, thus a still additional effect achieving a further appropriate adjustment of the discharge volume and discharge duration of the gas. 
     Furthermore, by sewing the first and second base cloths  36  and  37  of the airbag  32  at the vortical seam  39  . . . , and using the stress of the inflation to sequentially break the seam  39  . . . radially from the inside to the outside, the internal pressure, deployment speed, shape and the other aspects of the airbag  32  upon deployment can be arbitrarily controlled, to thereby effectively restrain passengers. In particular, the vortical seam  39  . . . has a generally uniform curvature without portions where the curvature abruptly changes, thus providing a smooth breaking of the seam and a further easier control of the internal pressure, deployment speed, shape and the other aspects of airbag  32  upon deployment. 
     In addition, in the twenty-second embodiment, when the inflator  31  is ignited in two stages, the internal pressure of the airbag  32  can be further precisely controlled by reducing the strength of seam  39  . . . between the radially inner points a and b, and increasing the strength between the radially outer points b and c; and breaking the seam  39  . . . between points a and b in front of the vent hole  37   b  . . . with the first ignition of the inflator  31 , and breaking the seam  39  . . . between points b and c which includes vent hole  37   b  . . . with the second ignition of the inflator  31 . 
     For instance, even if there is an error in the timing of the breaking of seam  39  . . . between points a and b by the first ignition of the inflator  31 , the error in the timing for opening vent holes  37   d  . . . can be eliminated by timely breaking the seam  39  . . . between points b and c with the second ignition of the inflator  31  so that the error is compensated for. Of course, the inflator  31  can be ignited in three or more stages. 
     Next, a twenty-third embodiment of the present invention will be described with reference to  FIGS. 36 to 37B . 
     The twenty-third embodiment comprises three seams  39  . . . which sew together the superimposed first and second base cloths  36  and  37  into a vortical shape. Opposite ends of a vent hole cover  40  which covers a single vent hole  37   b  placed on the second base cloth  37  is integrally sewn by the three seams  39  . . . . In the vicinity of the vent hole  37 , a portion located at the outer periphery of the airbag  32  is integrally sewn onto the vent hole cover  40  by a seam  51 . 
     In addition, each seam  39  . . . has a plurality (two in this embodiment) of non-sewn portions  39   a  and  39   b  at which sewing is interrupted. The end of the seam  39  . . . , point d which leads to the non-sewn portions  39   a  and  39   b  is oriented towards the central portion of airbag  32 . The circumferential position of at least one of the two non-sewn portions  39   a  and  39   b  (in this embodiment the radially inner non-sewn portion  39   a ) is misaligned in a circumferential direction from the radially inner end portion point a of the seam  39  . . . . 
     Therefore, even if a unbreakable portion encircled by a chained line is pressed by something during the deployment of the airbag  32 , and breaking which has started at the radially inner end point a of the vortical seam  39  . . . terminates at the unbreakable portion, a new breaking starts at point d facing the two non-sewn portions  39   a  and  39   b  of seam  39  . . . in clockwise and counterclockwise directions, ultimately breaking the entire seam  39  . . . except the unbreakable portions, thus deploying most parts of the airbag  32  to exhibit passenger-restraining capability. In this process, since point d facing the two non-sewn portions  39   a  and  39   b  of the seam  39  . . . is oriented towards the central portion of the airbag  32  as is the radially inner end point a of the seam  39  . . . , a strong stress can be concentrated thereon to ensure a reliable start of breaking. 
     Further, since the circumferential position of at least one of the two non-sewn portions  39   a  and  39   b  (in this embodiment the radially inside non-sewn portion  39   a ) is misaligned in a circumferential direction from the radially inner end point a of seam  39  . . . , it is possible to prevent situations where point a and the plurality of points d are all included in the unbreakable portions, thus disabling breaking of seam  39  . . . . 
     Furthermore, since the vicinity of the vent hole  37   b  on the side of the outer periphery of the airbag  32  is obstructed by the vent hole cover  40  (refer to  FIG. 37B ), when the restraints at the opposite ends of the vent hole cover  40  are released to open the vent hole  37   b , excess gas discharged from vent hole  37   b  can be deflected to the center side of the airbag  32 . 
     Next, a twenty-fourth embodiment of the present invention will be described with reference to  FIGS. 38A and 38B . 
     In the twenty-third embodiment, the single vent hole cover  40  is sewn by the seam  51  at one side of the vicinity of the vent hole  37   b , whereas the twenty-fourth embodiment comprises two vent hole covers  40  and  40  which are superimposed, with one vent hole cover  40  sewn by seam  51  at one side of the vicinity of the vent hole  37   b , and the other vent hole cover  40  sewn by seam  52  at one side of the vicinity of the vent hole  39   b.    
     Therefore, the two vent hole covers  40  and  40  cooperate to guide the excess gas by surrounding the vent hole  37   b  from opposite sides, thus discharging the excess gas straightforward. 
     Next, a twenty-fifth embodiment of the present invention will be described with reference to  FIG. 39 . 
     The twenty-fifth embodiment is a variation of the seventeenth embodiment shown in  FIG. 26 , wherein two vent holes  37   b  and  37   b  are formed on the second base cloth  37  among the superimposed first and second base cloths  36  and  37 , and a predetermined number (three in this embodiment) of fragile portions  38   a  . . . are formed at the outer peripheral seam  38  of the first and second base cloths  36  and  37 . The fragile portions  38   a  . . . are sewn using a thread finer than the other portions of seam  38  for easier breaking. 
     Thus, even if the two vent holes  37   b  and  37   b  remain closed for some reason and are unable to exhibit their functions during the final stage of deployment of airbag  32 , when the internal pressure of the airbag reaches or exceeds a predetermined level, the fragile portions  38   a  of the seam  38  break, and an aperture formed thereon functions as a vent hole to discharge gas, thus preventing an excessive increase of the internal pressure of the airbag  32 . 
     Next, a twenty-sixth embodiment of the present invention will be described with reference to  FIG. 40 . 
     The twenty-sixth embodiment is a variation of the twenty-fifth embodiment shown in  FIG. 39 . While the fragile portions  38   a  . . . were formed on the seam  38  in the twenty-fifth embodiment, a fragile portion  53  sewn so as to close a slit cut linearly into the second base cloth  37  is formed in the twenty-sixth embodiment. The breaking strength of the fragile portion  53  is set to be higher than the seams  39 , at the same time lower than the seam  38 . 
     Thus, even if the two vent holes  37   b  and  37   b  remain closed for some reason and are unable to exhibit their functions at the final stage of deployment of airbag  32 , when the internal pressure of the airbag  32  reaches or exceeds a predetermined level, the fragile portion  53  breaks and an aperture formed thereon functions as a vent hole to discharge gas, thus preventing an excessive increase of the internal pressure of the airbag  32 . 
     Having described the embodiments of the present invention, it is understood that various design changes can be made without departing from the subject matter of the present invention. 
     For instance, while the first and second base cloths  36  and  37  are sewn by the seam  39  . . . in the above embodiments, sewing may be replaced with an arbitrary joining means such as adhesive. 
     Further, the internal pressure of airbag  32  can be controlled by changing the seam strength (bonding strength of the bonding portion) of the seam  39 , thread thickness, thread material, seam density, length of seam  39  . . . , number of lap-seams etc. Generally, increasing the breaking strength or lengthening the seam  39  . . . makes it difficult for the airbag  32  to increase its volume, resulting in an increased internal pressure, while the opposite will allow the airbag  32  to increase its volume more easily, resulting in a decreased internal pressure. 
     Additionally, in the embodiments with a plurality of the vortical seams  39  . . . or the switchbacking seams  39  . . . , by narrowing the distance between seams  39  . . . , i.e., by decreasing the cross section of the influx passage of the pressurized gas, it will become difficult for the airbag  32  to increase its volume, resulting in an increased internal pressure, while the opposite allows the airbag  32  to increase its volume more easily, resulting in a decreased internal pressure. For example, in the first or third embodiment, the cross section of the influx passage of the pressurized gas can be arbitrarily adjusted by widening or narrowing the distance between the seams  39  . . . by making the vortical seams  39  . . . partially serpentine. 
     Further, by combining portions of high and low breaking strengths in a single seam  39 , the internal pressure of the airbag  32  can be more minutely controlled. Particularly, strengthening the breaking strength of breaking starting point a of the seam  39  does not make the seam  39  susceptible to breaking at the initial stage of deployment, the internal pressure of the airbag  32  is quickly increased at the early stage of deployment to secure a required passenger restraining force, and thereafter by breaking seam  39 , the internal pressure of the airbag  32  can be kept substantially uniform to enable an optimum performance in restraining passengers. 
     Furthermore, in the embodiments wherein starting point a of seam  39  . . . is not oriented towards the central portion of the airbag  32 , such points may be oriented towards the central portion of airbag  32 . 
     Moreover, arbitrary combination of the various features of the embodiments described above shall also be included in the embodiments of the present invention.