Abstract:
An airbag device includes an outer shell forming an airbag and having a vent hole, a restriction member for restricting gas flow through the vent hole, a first chamber having the vent hole, a second chamber, the inner panel partitioning the airbag into the first and second chambers and having a communication portion, and a tether member for connecting the restriction member and the inner panel. Gas is introduced into the first chamber and then into the second chamber. When the first chamber is filled with gas and the inner panel is tensioned, the tension exceeding a predetermined value is applied to the restriction member so that the vent hole is substantially closed. When the first chamber is not filled with gas and the inner panel loosens, the tension applied to the restriction member does not reach the predetermined value so that the vent hole is opened.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT  
       [0001]    The present invention relates to an airbag which is structured to restrict gas flow from the airbag through a vent hole. Further, the present invention relates to an airbag apparatus comprising the airbag. 
         [0002]    It is well known to form a vent hole in an airbag to release gas from the inside of an inflated airbag, whereby the vehicle occupant is softly received by the airbag. 
         [0003]    Japanese Unexamined Patent Application Publication No. 2005-199987 (Patent Document 1) discloses an airbag in which a vent hole is covered by a lid member. In this publication, a proximal end portion, near an inflator (the center of the airbag) in relation to the vent hole, of the lid member is unreleasably connected to the airbag. In addition, a distal end portion, on the other side of the vent hole from the inflator, of the lid member is caught at the periphery of the airbag between a front panel (occupant side surface) and a rear panel (non-occupant side surface) of the airbag and is releasably sewn together by a tear seam. The tear seam is formed by a sewing yarn which is designed to break when the inner pressure of the airbag exceeds a predetermined value. 
         [0004]    According to the airbag of the publication, at an initial stage of inflation of the airbag with gas from the inflator, the connection of the front panel and the rear panel of the airbag and the lid member by the tear seam is not released so that the lid member overlaps with the vent hole. Therefore, the gas flow through the vent hole is restricted, thereby achieving rapid inflation of the airbag. 
         [0005]    When the inner pressure of the airbag then reaches the predetermined value, the tear seam breaks so that the front panel and the rear panel of the airbag are separated and the lid member is released from the front panel and the rear panel. Because of the inner gas pressure of the airbag, the lid member is pushed out of the airbag through the vent hole so that the vent hole is opened. Therefore, when the vehicle occupant collides with the inflated airbag, the gas flows from the inside of the airbag to the outside of the airbag through the vent hole. As a result, the vehicle occupant is softly received by the airbag. 
         [0006]    According to the airbag disclosed in the Patent Document 1, the vent hole can be opened only by break of the tear seam. 
         [0007]    A threshold value of strength for the tear seam to break strongly depends on temperature and is thus affected by heat of gas spouted from the inflator. According to the location, the output, and the like of the inflator, the timing of the break of the tear seam tends to vary. 
         [0008]    It is an object of the present invention to provide an airbag which does not use a tear seam, while the airbag is still capable of maintaining a vent hole in the closed or slightly opened state at an early stage of inflation of the airbag and is also capable of opening or largely opening the vent hole when gas generation of the inflator is decreased or ended after the early stage and also provide an airbag apparatus comprising the airbag. 
         [0009]    Further objects and advantages of the invention will be apparent from the following description of the invention. 
       SUMMARY OF THE INVENTION  
       [0010]    According to a first aspect of the invention, an airbag is inflated with gas from an inflator and comprises a vent hole, and a restriction member for restricting gas flow through the vent hole, wherein said restriction member maintains the vent hole in the closed or slightly opened state when tension exceeding a predetermined value is applied, and opens or largely opens the vent hole when the tension is equal to or lower than the predetermined value. The airbag further comprises an inner panel which partitions the inside of the airbag into a first chamber on the non-occupant side and a second chamber on the occupant side, wherein said vent hole is provided in said first chamber. The peripheral edge of said inner panel is connected to a side peripheral surface of said airbag. Said inner panel is provided with a communication portion for allowing communication between the first chamber and the second chamber. 
         [0011]    As the inflator starts its operation, gas is first introduced into the first chamber and is then introduced into the second chamber from the first chamber through the communication portion. A tether member is provided for joining said restriction member and the inner panel. When the first chamber is filled with gas and the inner panel is tensioned, tension exceeding the predetermined value is applied to said restriction member via the tether member so that said vent hole is closed or slightly opened, and when the first chamber is not filled with gas and the inner panel loosens, the tension applied to said restriction member does not reach the predetermined value so that said vent hole is opened or largely opened. 
         [0012]    According to a second aspect of the invention, at an early stage of inflation of the airbag according to the first aspect of the invention where the inner pressure of said first chamber is higher than the inner pressure of said second chamber, the restriction member is overlapped on the vent hole by the tension applied to the restriction member via said tether member so as to maintain the vent hole in the closed or slightly opened state. Subsequently, as the gas generation of the inflator is decreased or ended, the inner panel loosens and the tension becomes equal to or lower than the predetermined value so as to open or largely open the vent hole. 
         [0013]    As to a third aspect of the invention, when an occupant in the vicinity of the airbag according to the first aspect collides with the airbag in a state that said inner panel is loose before the first chamber is filled with gas, the tension applied to said restriction member does not reach the predetermined value so that the vent hole is opened or largely opened. 
         [0014]    As to a fourth aspect of the invention, said restriction member is disposed on the outer surface of the airbag according to any one of the first to the third aspects of the invention and opens or largely opens the vent hole by separating from the vent hole, wherein said tether member is disposed to extend through a through hole formed in the airbag. 
         [0015]    As to a fifth aspect of the invention, said restriction member is disposed on the inner surface of the airbag according to any one of the first to the third aspects and opens or largely opens the vent hole by coming out of the airbag through said vent hole. 
         [0016]    As to a sixth aspect of the invention, said tether member in the airbag according to any one of the first to the fifth aspects is connected to said inner panel. 
         [0017]    With respect to a seventh aspect of the invention, a tether is provided to join the non-occupant-side surface of said airbag according to any one of the first to the fifth aspects and said inner panel, and said tether member is connected to a midway portion in the longitudinal direction of said tether. 
         [0018]    With respect to an eighth aspect of the invention, said restriction member has a sheet-like shape in which one end portion across said vent hole is connected to said airbag according to any one of the first to the seventh aspects and the other end across said vent hole is connected to said tether member. 
         [0019]    An airbag according to a ninth aspect of the invention has an inner panel which partitions the inside of the airbag into a first chamber on the non-occupant side and a second chamber on the occupant side, wherein the peripheral edge of said inner panel is connected to a side peripheral surface of the airbag. The inner panel is provided with a communication portion for allowing the communication between the first chamber and the second chamber. When the inflator starts its operation, gas is first introduced into the first chamber and is then introduced into the second chamber from the first chamber through the communication portion. 
         [0020]    The airbag comprises a vent portion which includes a bulging portion, which bulges outwards from the first chamber, and a vent hole formed in the bulging portion, wherein a tether member extends through a through hole formed in said airbag to join said bulging portion and the inner panel, and wherein when the first chamber is filled with gas and the inner panel is tensioned, tension exceeding the predetermined value is applied to said bulging portion via the tether member so that said bulging portion is folded by squash fold so as to maintain said vent portion in the closed or slightly opened state. Also, when the first chamber is not filled with gas and the inner panel loosens, the tension applied to said bulging portion does not reach the predetermined value so that said bulging portion stands so as to open or largely open said vent portion. 
         [0021]    With respect to a tenth aspect of the invention, when said first chamber of the airbag according to the ninth aspect of the invention is filled with gas and tension exceeding said predetermined value is applied to said bulging portion, the bulging portion is folded by squash fold such that the bulging portion is overlaid on an outer surface of the airbag near the bulging portion, and said vent hole is formed in a surface of the bulging portion so as to be overlaid on the outer surface of the airbag. 
         [0022]    With respect to an eleventh aspect of the invention, wherein said tether member in the airbag according to the ninth or tenth aspect is connected to said inner panel. 
         [0023]    With respect to a twelfth aspect of the invention, a tether is provided to join the non-occupant-side surface of said airbag according to the ninth or tenth aspect and said inner panel, and said tether member is connected to a midway portion in the longitudinal direction of said tether. 
         [0024]    With respect to a thirteenth aspect of the invention, the volume of said first chamber in the airbag according to any one of the first to twelfth aspects is in a range of from 30 to 45 liters. 
         [0025]    With respect to a fourteenth aspect of the invention, the diameter of the airbag according to any one of the first to thirteenth aspects when being inflated is in a range of from 630 to 700 mm, and the diameter of the first chamber when being inflated is in a range of from 550 to 600 mm. 
         [0026]    With respect to a fifteenth aspect of the invention, an airbag comprises an airbag according to any one of the first through fourteenth aspects and an inflator for inflating said airbag. 
         [0027]    In an airbag of any one of the first through the eighth aspects and an airbag apparatus comprising the airbag, at an early stage of inflation of the airbag in which the inner pressure of the first chamber is higher than the inner pressure of the second chamber, the first chamber is filled with gas and the inner panel is tensioned so that tension exceeding the predetermined value is applied to the restriction member via the tether member, thereby maintaining the vent hole in the closed or slightly opened state. Since the gas flow through the vent hole is thus restricted, the airbag is promptly inflated. 
         [0028]    Subsequently, as the gas generation of the inflator is decreased or ended, the first chamber and the second chamber become nearly the same inner pressure so that the inner panel loosens and the tension applied to the restriction member is thus reduced. Accordingly, when an occupant collides with the inflated airbag, the vent hole is opened or largely opened so that gas flows out from the inside of-the airbag through the vent hole. As a result, the occupant is softly received by the airbag. 
         [0029]    When the inflator is actuated in a state that the occupant is out-of-position in the vicinity of the airbag apparatus and the occupant collides with the airbag before the first chamber is filled with gas, the inner panel does not reach the tensioned state so that the tension of the restriction member does not reach the predetermined value. Therefore, the vent hole is opened or largely opened so that gas inside the airbag flows out through the vent hole. 
         [0030]    The airbag and the airbag apparatus do not use a tear seam, while the airbag and the airbag apparatus are still capable of maintaining a vent hole in the closed or slightly opened state at an early stage of inflation of the airbag and are also capable of opening or largely opening the vent hole when gas generation of the inflator is decreased or ended after the early stage, thereby preventing timing of opening the vent hole from being affected by the influence of heat of gas spouted from the inflator. 
         [0031]    In one embodiment of the present invention, the restriction member is disposed on the outer surface of the airbag and the tether member extends through the through hole formed in the airbag. In this case, when the tension applied to the tether member is larger than the predetermined value, the restriction member is overlaid on the vent hole from the outside of the airbag, thereby maintaining the vent hole in the closed or slightly opened state. When the tension applied to the tether member becomes equal to or smaller than the predetermined value, the restriction member is pushed out by gas inside the airbag so as to separate from the vent hole, thereby opening or largely opening the vent hole. 
         [0032]    In another embodiment of the present invention, the restriction member is disposed on the inner surface of the airbag. When the tension applied to the tether member is larger than the predetermined value, the restriction member withstands the inner pressure of the airbag to continuously cover the vent hole, thereby maintaining the vent hole in the closed or slightly opened state. When the tension applied to the tether member becomes equal to or smaller than the predetermined value, the restriction member is pushed by the gas pressure inside the airbag so as to come out of the airbag through the vent hole, thereby opening or largely opening the vent hole. 
         [0033]    In an airbag according to any one of the ninth through twelfth aspects and an airbag apparatus comprising the airbag, at an early stage of inflation of the airbag in which the inner pressure of the first chamber is higher than the second chamber, the first chamber is filled with gas and the inner panel is tensioned so that the bulging portion is strongly pulled by the tether member and is thus folded by squash fold, thereby maintaining the vent hole in the closed or slightly opened state. Accordingly, gas flow through the vent hole is prevented, thereby promptly inflating the airbag. 
         [0034]    After that, as the gas generation of the inflator is decreased or ended, the first chamber and the second chamber become to have nearly the same inner pressure so that the inner panel loosens. Accordingly, the bulging portion stands upward relative to the airbag, thereby opening or largely opening the vent portion. When the occupant collides with the inflated airbag, gas flows out from inside of the airbag through the vent hole so that the occupant is softly received by the airbag. 
         [0035]    In the present invention, according to the sixth or eleventh aspect, the tether member may be connected directly to the inner panel. In this case, easier structure is achieved. 
         [0036]    In the present invention, according to the seventh or twelfth aspect, a tether may be provided to join the non-occupant-side surface of the airbag and the inner panel. By the tether, the configuration of the inner panel when filled with gas can be restricted. In this case, the tether member may be connected to a midway portion in the longitudinal direction of the tether. 
         [0037]    If the vent hole is formed in a surface of the bulging portion which is folded by squash fold so as to be overlaid on the outer surface of the airbag according to the tenth aspect, gas flow through the vent hole is sufficiently restricted in the state that the bulging portion is folded by squash fold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0038]      FIG. 1  is a sectional view of an airbag and an airbag apparatus according to an embodiment, showing an early stage of inflation of the airbag. 
           [0039]      FIG. 2  is a sectional view of the airbag and the airbag apparatus of  FIG. 1 , showing a stage just before gas generation of an inflator is ended. 
           [0040]      FIG. 3  is a sectional view of the airbag and the airbag apparatus of  FIG. 1 , showing a stage when the vent hole is opened. 
           [0041]      FIG. 4  is a sectional view of the airbag and the airbag apparatus of  FIG. 1 , showing a state when an out-of-position occupant collides with the airbag. 
           [0042]      FIG. 5  is a sectional view of an airbag and an airbag apparatus according to another embodiment. 
           [0043]      FIG. 6  is a sectional view of an airbag and an airbag apparatus according to still another embodiment, showing a stage just before gas generation of an inflator is ended. 
           [0044]      FIG. 7  is a sectional view of the airbag and the airbag apparatus of  FIG. 6 , showing a stage when the vent hole is opened. 
           [0045]      FIG. 8  is a sectional view showing a vent portion of the airbag of  FIG. 7 . 
           [0046]      FIG. 9  is a perspective view showing the vent portion of  FIG. 8 . 
           [0047]      FIG. 10  is a perspective sectional view showing the periphery of an inner panel inside an airbag according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]    Hereinafter, embodiments of the present invention will be described with reference to the attached drawings. 
         [0049]      FIGS. 1 through 4  are sectional views of an airbag and an airbag apparatus comprising the same according to an embodiment corresponding to the first aspect through sixth aspect and the eighth aspect of the invention, showing respective states during the inflation.  FIG. 1  shows an early stage of the inflation in which the inner pressure of a first chamber is higher than the inner pressure of a second chamber.  FIG. 2  shows a stage just before gas generation of an inflator is ended, and  FIG. 3  shows a stage when the vent hole is opened. In addition,  FIG. 4  shows a state when an out-of-position occupant collides with the airbag during inflation. 
         [0050]    In this embodiment, the airbag  10  is a driver-side airbag which is inflated and deployed from a steering wheel  50  toward an occupant. 
         [0051]    The airbag  10  comprises a front panel  12  composing an occupant-side surface, a rear panel  14  composing a surface opposite to the occupant-side surface (hereinafter, referred to “non-occupant-side surface”) when the airbag  10  is inflated, an inner panel  16  disposed inside of the airbag  10 , a vent hole  20  which can be closed and opened by a lid member  60  as a restriction member, and a tether member  70  having a band-like shape which connects the lid member  60  to the inner panel  16 . In this embodiment, the lid member  60  covers the vent hole  20  from the outside of the airbag  10 . 
         [0052]    The front panel  12 , the rear panel  14 , and the inner panel  16  are circular woven fabrics, respectively. The front panel  12  and the rear panel  14  have substantially the same diameter and their peripheries are sewn together by seam  15  so as to make an envelope shape. The seam  15  is provided circularly in an annular shape along the outer periphery of the front panel  12  and the rear panel  14 . The seam  15  is formed with sewing yarns, but not limited thereto. 
         [0053]    The inner panel  16  has a diameter smaller than that of the rear panel  14 . The inner panel  16  is arranged coaxially with the rear panel  14  and is sewn along its outer periphery to the rear panel  14  by seam  17 . The inner panel  16  partitions the inside of the airbag  10  into a first chamber  1  on the non-occupant side and a second chamber  2  on the occupant side. The inner panel  16  is provided with a plurality of communication holes  18  for allowing communication between the first chamber  1  and the second chamber  2 . It is preferable that the number of communication holes  18  is two or more and the communication holes  18  are formed near the periphery of the inner panel  16  to have space therebetween in the circumferential direction. 
         [0054]    The rear panel  14  has an inflator-receiving opening (without numeral) formed at its center. Formed around the inflator-receiving opening are bolt through holes (without numeral) through which stud bolts  44  of a retention ring  42  as will be described later are inserted. 
         [0055]    In this embodiment, as shown in  FIG. 2  and  FIG. 3 , the vent hole  20  is located in a region between the steering wheel  50  and the center of the rear panel  14  when the airbag  10  is inflated. 
         [0056]    The rear panel  14  has a through hole  22  which is formed adjacent to the vent hole  20  and through which the tether member  70  is inserted. In this embodiment, the through hole  22  is located between the vent hole  20  and the center of the rear panel  14 . The locations of the vent hole  20  and the through hole  22  are not limited thereto. 
         [0057]    In this embodiment, the size of the through hole  22  is smaller than that of the vent hole  20 . The vent hole  20  and the through hole  22  may have slit-like shape. 
         [0058]    In this embodiment, the aforementioned lid member  60  is made of a substantially fan-shaped fabric cloth. The lid member  60  is overlaid on the vent hole  20  from the outside of the airbag  10  such that an arc edge portion thereof is located toward the outer periphery of the rear panel  14  and a convex portion facing the arc edge portion is located toward the center of the rear panel  14 . The lid member  60  is sewn to the rear panel  14  along its arc edge portion on the side of the outer periphery of the rear panel  14  by seam  61 . That is, the lid member  60  has a fixed end on the side of the outer periphery of the rear panel  14  and has a free end on the side of the center of the rear panel  14 . 
         [0059]    Connected to the convex portion, on the side of the center of the rear panel  14 , of the lid member  60  is an end of the tether member  70 . In this embodiment, the lid member  60  and the tether member  70  are separately formed and the end of the tether member  70  is sewn to the convex portion, on the side of the center of the rear panel  14 , of the lid member  60  by the seam  71 . However, the lid member  60  and the tether member  70  may be uniformly made of a common fabric cloth. 
         [0060]    The other end of the tether member  70  is drawn into the airbag  10  through the through hole  22  and is sewn to a portion near the center of the inner panel  16  by seam  75 . 
         [0061]    The tether member  70  has such a length that, in the early stage of inflation of the airbag  10  before the gas generation from the inflator  36  is decreased to a predetermined level or less or is ended, the tether member  70  is tensioned (strained) between the inner panel  16  and the lid member  60  according to the movement of the side periphery of the airbag  10  away from the inner panel  16  so as to prevent the lid member  60  from separating from the vent hole  20 . 
         [0062]    A retainer  30  for mounting the airbag  10  has an inflator mounting opening (without numeral) formed at the center thereof and bolt through holes (without numeral) formed around the opening. 
         [0063]    The inflator  36  is substantially cylindrical in shape and has gas exhaust holes  36   a  formed in outer side face of a distal end portion in the axial direction of the cylinder. The inflator  36  is structured so as to spout gas through gas exhaust holes  36   a  in radial directions. A flange  38  for securing the inflator is formed to project from the side periphery of a midway portion (on the rear end side with regard to the gas exhaust holes  36   a ) in the axial direction of the cylinder of the inflator  36 . The flange  38  is provided with bolt through holes (without numeral) formed therein. The distal end portion of the inflator  36  is fitted into the inflator mounting opening of the retainer  30 . 
         [0064]    For mounting the airbag  10  to the retainer  30 , the peripheral portion of the inflator mounting opening of the retainer  30  is superposed on the peripheral portion of the inflator-receiving opening of the rear panel  14 . Then, stud bolts  44  of the retention ring  42  are inserted into the respective bolt through holes of the rear panel  14 , the retainer  30 , and the flange  38  of the inflator  36 , and nuts  46  are screwed onto distal ends of the stud bolts  44 , thereby fixing the rear panel  14  and the inflator  36  to the retainer  30 . 
         [0065]    After that, the airbag  10  is folded and a module cover  48  is attached to the retainer  30  to cover the airbag  10  in the folded state, thereby composing the airbag apparatus. The airbag  10  maybe previously folded prior to the fixing of the airbag  10  to the retainer  30 . 
         [0066]    The airbag  10  is folded in a state that the lid member  60  is overlaid on the vent hole  20  even though this state is not clearly shown in the drawings. By folding the airbag  10  in this manner, the vent hole  20  can be closed by the lid member  60  from the start of inflation of the airbag  10 . 
         [0067]    The airbag apparatus is mounted to a steering wheel  50  of a vehicle. 
         [0068]    In the event of a collision of a vehicle in which the airbag apparatus having the aforementioned structure is mounted, the inflator  36  is actuated to spout gas. The airbag  10  is inflated by the gas to push open a module cover  48  and is thus deployed into the vehicle cabin. 
         [0069]    During the deployment, as shown in  FIG. 1 , the first chamber  1  is first inflated with gas from the inflator  36 . Then, the gas flows from the first chamber  1  into the second chamber  2  through the communication holes  18  so that the second chamber  2  is inflated. 
         [0070]    In this embodiment, the lid member  60  is overlaid on the vent hole  20  at the start of inflation of the airbag  10 . Therefore, the inside of the airbag  10  is quickly filled with gas. 
         [0071]    At the early stage of inflation of the airbag  10  in which the inner pressure of the first chamber  1  is higher than the inner pressure of the airbag  10 , that is, before the gas generation from the inflator  36  is decreased to the predetermined level or less or is ended, the first chamber  1  is filled with gas so that the inner panel  16  is tensioned as shown in  FIG. 2 . 
         [0072]    In this state, since the tension of the inner panel  16  is high, the configuration of the inner panel  16  absolutely or substantially stays constant even though the tether member  70  is pulled outside of the airbag  10 . If the lid member  60  is about to separate from the vent hole  20  because of the inner gas pressure of the airbag  10 , the lid member  60  is caught by the inner panel  16  via the tether member  70  so that the lid member  60  does not separate from the vent hole  20 . Accordingly, the closed state of the vent hole  20  by the lid member  60  is maintained, thereby promptly increasing the inner pressure of the airbag  10  and thus quickly inflating and deploying the airbag  10 . 
         [0073]    Subsequently, as the gas generation of the inflator  36  is decreased to the predetermined level or less, or is ended, the first chamber  1  and the second chamber  2  which communicate with each other via the communication holes  18  become nearly the same inner pressure so that the inner panel  16  loosens. Accordingly, as shown in  FIG. 3 , the inner panel  16  moves toward the through hole  22  because of the tension of the tether member  70  so that the lid member  60  moves outside the airbag  10  to separate from the vent hole  20  by the gas pressure inside the airbag  10 . Therefore, the vent hole  20  is opened. 
         [0074]    When, the occupant subsequently collides with the airbag  10  which is inflated, gas flows out of the airbag  10  through the vent hole  20  so that the occupant is softly received by the airbag  10 . 
         [0075]    As mentioned above, the airbag  10  mentioned above achieves such a structure without using tear seam that the vent hole  20  is closed at the early stage of inflation of the airbag  10  and the vent hole  20  is opened when the gas generation of the inflator  36  is decreased to the predetermined level or less, or is ended after the early stage, thereby preventing timing of opening the vent hole  20  from being affected by the influence of heat of gas spouted from the inflator  36 . 
         [0076]    When the vehicle is subjected to a vehicle collision in a state that the occupant is positioned forward relative to the usual sitting position, that is, the occupant is out-of-position, the occupant may collide with the airbag  10  at a relatively early stage from the start of actuation of the inflator  36 . In this case, the first chamber  1  is quickly inflated because the volume of the first chamber  1  is smaller than that of the airbag  10 , and gas from the inflator  36  is directly introduced into the first chamber  1 , whereby the occupant can be received by the first chamber  1  at an early stage. 
         [0077]    Also in this case, as shown in  FIG. 4 , even before the gas generation of the inflator  36  is decreased to the predetermined level or less, or is ended, the inner panel  16  receiving the occupant is pressed by the occupant and is thus deformed to move backward, thereby moving the lid member  60  outside the airbag  10  to separate from the vent hole  20 . Therefore, the vent hole  20  is opened to allow gas to flow out of the airbag  10  through the vent hole  20 , thereby softly receiving the occupant. 
         [0078]    In case that the diameter of the airbag  10  when inflated is approximately 630-700 mm, the diameter of the first chamber  1  when inflated, that is, the diameter of a circle formed by the seam  17  is preferably approximately 550-600 mm. 
         [0079]    It should be noted that the shape of the airbag  10  or the inner panel  16  is not limited to a circle and may be a quadrangle such as a square and a trapezoid, or a similar shape. In addition, the shape maybe a polygon such as a pentagon or a hexagon. In case of non-circular airbag or inner panel, the “diameter” used above means an average diameter. 
         [0080]    The volume of the first chamber  1  is preferably 30-45 liters, more preferably 35-40 liters. 
         [0081]    The airbag of the present invention is not limited to the aforementioned dimensions. 
         [0082]    Though the lid member  60  closes the whole vent hole  20  in the aforementioned embodiment, the lid member  60  may adjust the vent hole  20  from the largely opened state to the slightly opened state. For example, even when the lid member  60  is overlaid on the vent hole  20 , the vent hole  20  may partially protrude from the lid member  60  so that the deviating portion allows gas to flow out. Alternatively, the lid member  60  may be provided at a position overlaid on the vent hole  20  with a small hole which is smaller than the vent hole  20 , whereby the small hole allows gas to flow out even when the vent hole  20  is closed. 
         [0083]    Though the lid member  60  covers the vent hole  20  from the outside of the airbag in the aforementioned embodiment, the lid member  60  may cover the vent hole  20  from the inside of the airbag in the present invention. 
         [0084]    In the present invention, the restriction member for restricting the gas flow through the vent hole  20  may have a structure other than the lid member  60  covering the vent hole  20 . For example, the restriction member may be a member which is arranged around the vent hole  20  and is pulled inside the airbag to bind up the vent hole  20  so as to close the vent hole  20 . 
         [0085]    Though the other end of the tether member  70  is directly connected to the inner panel  16  in the aforementioned embodiment, a tether may be disposed within the airbag to join the inner panel and the non-occupant-side surface of the airbag so as to adjust the bulging amount of the inner panel toward the occupant when the airbag is inflated and the rear end of the tether member  70  may be connected to a midway of the tether according to the seventh aspect of the present invention. That is, the tether member may be indirectly connected to the inner panel. 
         [0086]      FIG. 5  is a sectional view of an example of an airbag and an airbag apparatus having the structure mentioned above. 
         [0087]    The airbag  10 A shown in  FIG. 5  is provided inside thereof with tethers  90  connecting the inner panel  16  and the rear panel  14 . Numeral  91  designates seam  91  sewing one end of each tether  90  to the inner panel  16 . Each tether  90  is provided at the other end thereof with a bolt through hole (not shown). The other end is sandwiched between the retention ring  42  and the rear panel  14 . A bolt  44  is inserted through the bolt through hole formed in the other end of the tether  90 , thereby connecting the other end of the tether  90  to the retainer  30 . The other end of the tether  90  may be sewn to the rear panel  14 . In this embodiment, a plurality of tethers  90  are arranged to be spaced from each other in the circumferential direction of the airbag  10 A. By setting the length of the tethers  90 , the bulging amount of the inner panel  16  toward the occupant when the airbag  10 A is inflated can be adjusted. 
         [0088]    The other end of the tether member  70  is connected to a midway in the longitudinal direction of the tether  90  by seam  73 . 
         [0089]    The other components of the airbag  10 A are the same as those of the airbag  10  shown in  FIGS. 1-4  and the same numerals in  FIG. 5  designate the same components as those in  FIGS. 1-4 . 
         [0090]    The actions of the airbag apparatus comprising the airbag  10 A having the aforementioned structure are the same as the aforementioned airbag apparatus. That is, in the event of a vehicle collision or the like, the inflator  36  is actuated to spout gas. The airbag  10 A is inflated by the gas to push open the module cover  48  and is thus deployed into the vehicle cabin. 
         [0091]    At the early stage of inflation of the airbag  10 A in which the inner pressure of the first chamber  1  is higher than the inner pressure of the airbag  10 A, that is, before the gas generation from the inflator  36  is decreased to the predetermined level or less, or is ended, the first chamber  1  is filled with gas so that the inner panel  16  is tensioned. 
         [0092]    Since the lid member  60  is restrained by the inner panel  16  via the tether  90  and the tether member  70  so that the closed state of the vent hole  20  is maintained, the inner pressure of the airbag  10 A is promptly increased and the airbag  10 A is thus quickly inflated and deployed. 
         [0093]    Subsequently, as the gas generation of the inflator  36  is decreased to the predetermined level or less, or is ended, the first chamber  1  and the second chamber  2  which communicate with each other via the communication holes  18  become to have nearly the same inner pressure so that the inner panel  16  loosens. Accordingly, similarly to the case shown in  FIG. 3 , the inner panel  16  moves toward the through hole  22  because of the tension of the tether member  70  so that the tether  90  loosens and the lid member  60  moves outside the airbag  10 A to separate from the vent hole  20  by the gas pressure inside the airbag  10 A. Therefore, the vent hole  20  is opened. 
         [0094]    When the occupant subsequently collides with the airbag  10 A which is inflated, gas flows out of the airbag  10 A through the vent hole  20  so that the occupant is softly received by the airbag  10 A. 
         [0095]    When the vehicle is subjected to a vehicle collision in a state that the occupant is positioned in front of the usual sitting position, that is, the occupant is out-of-position, the occupant may collide with the airbag  10 A at a relatively early stage from the actuation of the inflator  36 . In this case, the first chamber  1  is quickly inflated because the volume of the first chamber  1  is smaller than that of the airbag  10 A, and gas from the inflator  36  is directly introduced into the first chamber  1 , whereby the occupant can be received by the first chamber  1  at an early stage. 
         [0096]    Also in this case, similarly to the case shown in  FIG. 4 , even before the gas generation of the inflator  36  is decreased to the predetermined level or less, or is ended, the inner panel  16  receiving the occupant is pressed by the occupant and is thus deformed to move backward, thereby moving the lid member  60  outside the airbag  10 A to separate from the vent hole  20 . Therefore, the vent hole  20  is opened to allow gas from flowing out of the airbag  10 A through the vent hole  20 , thereby softly receiving the occupant. 
         [0097]      FIGS. 6 through 9  are sectional views of an airbag and an airbag apparatus comprising the same according to an embodiment corresponding to the ninth aspect through the twelfth aspects of the invention, showing respective states during inflation.  FIG. 6  shows a stage just before gas generation of an inflator is ended, and  FIG. 7  shows a stage when the vent hole is opened.  FIG. 8  is a sectional view of a vent portion and  FIG. 9  is a perspective view of the vent portion. 
         [0098]    An airbag  10 B of this embodiment is different form the airbag  10  shown in  FIGS. 1-4  by a structure around the vent hole. 
         [0099]    In this embodiment, the airbag  10 B is provided with a bulging portion  100  and a vent hole  110  is formed in the bulging portion  100 . The bulging portion  100  and the vent hole  110  compose a vent portion  120 . 
         [0100]    In this embodiment, an opening  125  is formed in the airbag  10 B and a proximal end portion of the bulging portion  100  is sewn to a peripheral edge of the opening  125  by seam  121 . The distal end of the bulging portion  100  is closed, and the proximal end thereof is opened so that the bulging portion  100  has an envelope shape. The through hole  22  is formed at a position near the bulging portion  100 . The tether member  70  is inserted through the through hole  22 . The vent hole  110  is formed in a surface, facing the through hole  22 , of the side periphery of the bulging portion  100 . One end of the tether member  70  is connected to the distal end portion in the bulging direction of the bulging portion  100  by seam  71 . 
         [0101]    For folding the airbag  10 B, the bulging portion  100  is folded by squash fold toward the through hole  22  such that the vent hole  110  is overlaid on an outer surface, near the bulging portion  100 , of the airbag  10 B. 
         [0102]    The other components of the airbag  10 B are the same as those of the airbag  10  shown in  FIGS. 1-4  and the same numerals designate the same components as those in  FIGS. 1-4 . 
         [0103]    In the event of a collision or the like of a vehicle in which an airbag apparatus comprising the airbag  10 B having the structure as mentioned above is installed, the inflator  36  is actuated to spout gas. The airbag  10 B is inflated by the gas to push open the module cover  48  and is thus deployed into the vehicle cabin. 
         [0104]    During the deployment, similarly to the case shown in  FIG. 1 , the first chamber  1  is first inflated with gas from the inflator  36 . Then, gas flows from the first chamber  1  into the second chamber  2  through the communication holes  18  so that the second chamber  2  is inflated. 
         [0105]    In this embodiment, the bulging portion  100  is folded by the squash fold and the vent hole  110  is overlaid on the outer surface of the airbag  10 B at the start of inflation of the airbag  10 B. Accordingly, gas barely flows through the vent portion  120  so that the airbag  10  is promptly filled with gas. 
         [0106]    At the early stage of inflation of the airbag  10 B in which the inner pressure of the first chamber  1  is higher than the inner pressure of the airbag  10 B, that is, before the gas generation from the inflator  36  is decreased to the predetermined level or less, or is ended, the first chamber  1  is filled with gas so that the inner panel  16  is tensioned as shown in  FIG. 6 . 
         [0107]    In this state, the tension of the inner panel  16  is high. Therefore, even though the bulging portion  100  is about to stand upward and the tether member  70  is thus pulled outside the airbag  10 B, the configuration of the inner panel  16  slightly varies or does not vary at all. That is, even though the bulging portion  100  is about to stand because of the inner gas pressure of the airbag  10 B, the bulging portion  100  is restrained by the inner panel  16  via the tether member  70  and thus does not stand upward, thereby maintaining the vent portion  120  composed of the bulging portion  100  and the vent hole  110  in the closed or slightly opened state. Therefore, the inner pressure of the airbag  10 B is promptly increased, and the airbag  10  is thus quickly inflated and deployed. 
         [0108]    After that, as the gas generation of the inflator  36 is decreased to the predetermined level or less, or is ended, the first chamber  1  and the second chamber  2  which communicate with each other via the communication holes  18  become to have nearly the same inner pressure so that the inner panel  16  loosens. Accordingly, as shown in  FIGS. 7 and 8 , the inner panel  16  moves toward the through hole  22  because of the tension of the tether member  70  so that the bulging portion  100  stands upward. Therefore, the vent portion  120  composed of the bulging portion  100  and the vent hole  110  is opened or largely opened. 
         [0109]    When the occupant subsequently collides with the airbag  10 B which is inflated, gas flows out of the airbag  10 B through the vent hole  110  so that the occupant is softly received by the airbag  10 B. 
         [0110]    When the vehicle is subjected to a vehicle collision in a state that the occupant is positioned in front of the usual sitting position, that is, the occupant is out-of-position, the occupant may collide with the airbag  10 B at a relatively early stage from the actuation of the inflator  36 . In this case, the first chamber  1  is quickly inflated because the volume of the first chamber  1  is smaller than that of the airbag  10 B, and gas from the inflator  36  is directly introduced into the first chamber  1 , whereby the occupant can be received by the first chamber  1  at an early stage. 
         [0111]    Also in this case, similarly to the case shown in  FIG. 4 , even before the gas generation of the inflator  36  is decreased to the predetermined level or less, or is ended, the inner panel  16  receiving the occupant is pressed by the occupant and is thus deformed to move backward, thereby allowing the bulging portion  100  to stand. Therefore, the vent portion composed of the bulging portion  100  and the vent hole  110  is opened or largely opened to allow gas to flow out of the airbag  10 B through the vent hole  110 , thereby softly receiving the occupant. 
         [0112]    Since any of the aforementioned embodiments are an example of the present invention, the present invention is not limited to the aforementioned embodiments. 
         [0113]    Though all of the vent hole  20 , the through hole  22 , and the opening  125  are circular openings in any of the aforementioned embodiments, the shapes of the vent hole  20 , the through hole  22 , and the opening  125  are not limited thereto and may be another shape such as a square and a slit-like shape. 
         [0114]    Though the inner panel  16  is provided with communication holes  18  in any of the aforementioned embodiments, communication portions  18   a  may be formed between the inner panel  16  and the rear panel by making portions without seam  17  in the periphery of the inner panel  16  as shown in  FIG. 10 . 
         [0115]    Though any of the aforementioned embodiments is an example that the present invention is applied to an airbag and an airbag apparatus to be installed for a driver&#39;s seat of a vehicle, the present invention can be applied to other various airbags and airbag apparatus, such as airbags and airbag apparatuses to be installed for a front passenger&#39;s seat and a rear seats. 
         [0116]    The disclosure of Japanese Patent Application No. 2007-043107, filed on Feb. 25, 2008, is incorporated in the application. 
         [0117]    While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.