Abstract:
An inflatable airbag for installation in front of a passenger compartment of a vehicle for protecting an occupant during an emergency includes a rearwardly directed surface positioned to be contacted by the occupant when expanded, an end located at a side opposite to the rearwardly directed surface and having a hole for receiving an inflation gas, and laterally spaced side surfaces extending between the rearwardly directed surface and the end and being oriented substantially vertically in the passenger compartment. Inwardly extending depressions are formed in a middle portion of each side surface of the airbag as viewed in a vertical direction. The depressions are located away from the rearwardly directed surface to restrict movement of the inflation gas inside the airbag when the occupant hits the inflated airbag.

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an airbag for protecting an occupant as a result of expanding during an emergency in a vehicle. 
     In order to protect an occupant during a collision in a vehicle, a driver airbag device, a passenger airbag device, a back-seat airbag device, and a side airbag device are used. Of these various types of airbag devices, the passenger airbag device is accommodated inside an instrument panel disposed at the front side of the vehicle. Of the different types of passenger airbag devices, the type of passenger airbag device which is disposed at a windshield-opposing location of the top portion of the instrument panel is called a top-dash-mount-type passenger airbag device. 
     Hereunder, a description of an airbag device will be given taking a top-dash-mount-type passenger airbag device as an example. 
     FIG.  14 (A) is a schematic side view used to illustrate the form of a conventional passenger airbag device when it has finished spreading. FIG.  14 (B) is a front view thereof. 
     FIG.  15 (A) is a schematic side view used to illustrate the form of the conventional passenger airbag device when a load acts thereupon (that is, when an occupant moves forward). FIG.  15 (B) is a front view thereof. 
     FIG.  16 (A) is a perspective view showing the conventional airbag in an expanded state. FIG.  16 (B) is a perspective view showing the airbag in a squashed state when a load acts thereupon. FIGS.  16 (C) and  16 (D) are schematic views used to illustrate the characteristics of the squashed state of the airbag when a load acts thereupon. 
     The passenger airbag device shown in FIGS.  14 (A) and  15 (A) comprises a retainer R disposed at a windshield-F-opposing location of the top portion of an instrument panel P of a vehicle. Inside the retainer R are disposed an airbag  103  and an inflator I for supplying spread gas into the interior of the airbag  103 . As simply shown in FIG.  16 (A), the airbag  103  is a three-piece bag in which two pieces of side cloths  103   b  are sewed, one at each side of one piece of strip-like central cloth  103   a  interposed therebetween. The airbag  103  has an open end (that is, a gas-circulation hole)  103   c  which is narrowed down thinly at the base of the airbag  103 . The open end  103   c  merges with a space in the inflator I. The airbag  103  is accommodated in a folded state inside the retainer R. 
     A description of the operation of the passenger airbag device will now be given. 
     At the time of a collision of a vehicle, spread gas is supplied into the airbag  103  from the inflator I. This causes the airbag  103  to expand in front of an occupant H, as shown in FIGS.  14 (A) and  14 (B). When the spreading of the airbag  103  is completed, the side cloths  103   b  extend in smooth curved forms or substantially straight lines from top to bottom portions thereof, as shown in FIG.  14 (B). Here, the internal pressure or resistance inside the airbag  103  is substantially uniform at the top and bottom portions thereof. 
     After the airbag  103  has spread, as shown in FIG.  15 (A), the occupant H that moves forward due to inertial force hits the airbag  103 . This causes the airbag  103  to be pushed and squashed as a result of being sandwiched between the instrument panel P and the windshield F and the occupant H. At this time, as shown in FIG.  15 (B), the upper portion of the airbag  103  spreads horizontally by a greater amount than the lower portion thereof. The following factors (1) to (3) cause the airbag  103  to be in a squashed state. 
     (1) As simply shown in FIG.  15 (B), regarding the areas of the portions of the upper part of the body (from the waist upward) of the occupant H that hits the airbag  103 , the area of the upper portion of the upper body (from the neck upwards) is smaller than the area of the lower portion of the upper body (from the shoulders downward). 
     (2) Regarding the masses of the portions of the upper body of the occupant H, the mass of the upper portion of the upper body is smaller than that of the lower portion of the upper body. 
     (3) While the lower portion of the airbag  103  is pushed upward by the knees of the occupant H, the upper portion of the airbag  103  is relatively not pushed. 
     When an attempt is made to correct the characteristics of the squashed state of the airbag  103 , the output of the inflator I must be set relatively high. 
     A description of the resistance on the airbag  103  when a load is exerted thereupon will now be given with reference to FIG.  16 . 
     When the occupant hits the airbag  103  shown in FIG.  16 (A) from the front surface of the airbag  103 , an external force a shown in FIG.  16 (B) acts upon the airbag  103 . As shown in the same figure, this causes the airbag  103  to become squashed while spreading flatly. Here, as shown schematically in FIGS.  16 (C) and  16 (D), the airbag  103  escapes towards a region of lower resistance (that is, in the directions of empty arrows inside the bag  103 ), so that the resistance against a load body f becomes smaller, thereby making it easier to squash the bag. 
     In view of the above-described problems, it is an object of the present invention to provide an airbag which has a more preferable spread form without increasing the output of an inflator. 
     SUMMARY OF THE INVENTION 
     To overcome the above-described problems, according to the present invention, there is provided an airbag for protecting an occupant as a result of expanding during an emergency in a vehicle, wherein, in a front form of the airbag at the time of expansion thereof as viewed from the occupant, an inwardly extending depression is formed in a middle portion of a side surface of the airbag as viewed in a vertical direction. 
     When the occupant collides against the airbag, the depression at the middle portion of the airbag makes it difficult for the gas in the bottom portion of the bag to move upward. Consequently, the resistance at the bottom portion of the bag effectively acts upon the occupant. Therefore, it is possible to provide an airbag which has a more preferable spread form without increasing the output of the inflator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 (A) is a schematic side view showing the form of a passenger airbag device of an embodiment of the present invention when it has finished spreading; 
     FIG.  1 (B) is a front view thereof; 
     FIG.  2 (A) is a schematic side view showing the form of the passenger airbag device of the embodiment of the present invention when a load acts thereupon (that is, when an occupant moves forward); 
     FIG.  2 (B) is a front view thereof; 
     FIG. 3 (A) is a perspective view showing the state of the airbag of the present invention when it is expanded; 
     FIG.  3 (B) is a perspective view showing the squashed state of the airbag when a load acts thereupon; 
     FIGS.  3 (C) and  3 (D) are schematic views used to illustrate the characteristics of the squashed state of the airbag when a load acts thereupon; 
     FIG.  4 (A) is a perspective view showing a structural example (that is, a partition-type structure) of the airbag of the present invention; 
     FIG.  4 (B) is a vertical sectional view of FIG.  4 (A); 
     FIG.  4 (C) is a vertical sectional view of another example of the airbag; 
     FIGS.  5 (A) to  5 (E) are schematic plan views of the forms of the front surface of the air bag when it has finished spreading; 
     FIG.  6 (A) is an exploded perspective view of a structural example of an airbag using one tether strap; 
     FIG.  6 (B) is a side view of the airbag shown in FIG.  6 (A); 
     FIG.  6 (C) is a front view of the airbag shown in FIG.  6 (B); 
     FIG.  7 (A) is a schematic side view of a top-dash-mount-type passenger airbag device using two tether straps; 
     FIG.  7 (B) is a front view of the airbag shown in FIG.  7 (A); 
     FIG.  7 (C) is a sectional view taken along line  7 (C)— 7 (C) of FIG.  7 (A); 
     FIG.  7 (D) is a sectional view taken along line  7 (D)— 7 (D) of FIG.  7 (A); 
     FIG.  8 (A) is a schematic side view of a top-dash-mount-type passenger airbag device using a tucked seam; 
     FIG.  8 (B) is a front view of the airbag shown in FIG.  8 (A); 
     FIG.  8 (C) is a sectional view taken along line  8 (C)— 8 (C) of FIG.  8 (A); 
     FIG.  9 (A) is a perspective view of an example of an airbag having a two-piece structure; 
     FIG.  9 (B) is an exploded perspective view of the airbag shown in FIG.  9 (A); 
     FIG.  9 (C) is an exploded plan view of the airbag shown in FIG.  9 (A); 
     FIGS.  9 (D) and  9 (E) are perspective views used to illustrate the procedure of assembling the airbag shown in FIG.  9 (A); 
     FIG.  10 (A) is an exploded plan view of another example of the airbag having a two-piece structure; 
     FIG.  10 (B) is a perspective view used to illustrate the procedure of assembling the airbag shown in FIG.  10 (A); 
     FIGS.  11 (A)- 11 (H) are used to illustrate the procedure or assembling the airbag having a two-piece structure; 
     FIG.  12 (A) is a plan view of an example of an airbag having a one-piece structure; 
     FIG.  12 (B) is a perspective view of the airbag formed from the one-piece structure; 
     FIG.  12 (C) is a front view (in the direction of arrow  12 (C) in FIG.  12 (B)) when the airbag has finished spreading; 
     FIG.  13 (A) is a plan view of another example of the airbag having a one-piece structure; 
     FIG.  13 (B) is a perspective view of the airbag shown in FIG.  13 (A); 
     FIG.  13 (C) is a front view (in the direction of arrow  13 (C) in FIG.  13 (B)) when the airbag has finished spreading; 
     FIG.  14 (A) is a schematic side view used to illustrate the form of a conventional passenger airbag device when it has finished spreading; 
     FIG.  14 (B) is a front view thereof; 
     FIG.  15 (A) is a schematic side view used to illustrate the form of the conventional passenger airbag device when a load acts thereupon (that is, when an occupant moves forward); 
     FIG.  15 (B) is a front view thereof; 
     FIG.  16 (A) is a perspective view showing the conventional airbag in an expanded state; 
     FIG.  16 (B) is a perspective view showing the airbag in a squashed state when a load acts thereupon; 
     FIGS.  16 (C) and  16 (D) are schematic views used to illustrate the characteristics of the squashed state of the airbag when a load acts thereupon; 
     FIG.  17 (A) is a perspective view of another example of the partition-type airbag of the invention; 
     FIG.  17 (B) is a plan view of a partition of the airbag shown in FIG.  17 (A); 
     FIG.  17 (C) is a vertical sectional view of FIG.  17 (A); 
     FIG.  17 (D) is a plan view of another example of the partition wall; and 
     FIG.  17 (E) is a vertical sectional view of the airbag using the partition shown in FIG.  17 (D). 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereunder, a description will be given with reference to the drawings. 
     In the description of the following embodiments, an airbag of the present invention is described as being a top-dash-mount-type passenger airbag device. 
     FIG.  1 (A) is a schematic side view showing the form of a passenger airbag device of an embodiment of the present invention when it has finished spreading. FIG.  1 (B) is a front view thereof. 
     FIG.  2 (A) is a schematic side view showing the form of the passenger airbag device of the embodiment of the present invention when a load acts thereupon (that is, when an occupant moves forward). FIG.  2 (B) is a front view thereof. 
     FIG.  3 (A) is a perspective view showing the state of the airbag of the present invention when it is expanded. FIG.  3 (B) is a perspective view showing the squashed state of the airbag when a load acts thereupon. FIGS.  3 (C) and  3 (D) are schematic views used to illustrate the characteristics of the squashed state of the airbag when a load acts thereupon. 
     FIG.  4 (A) is a perspective view showing a structural example (that is, a partition-type structure) of the airbag of the present invention. FIG.  4 (B) is a vertical sectional view of FIG.  4 (A). FIG.  4 (C) is a vertical sectional view of another example of the airbag. 
     The airbag device shown in FIGS.  1 (A) and  2 (A) comprises a retainer R disposed at a windshield-F-opposing location of the top portion of an instrument panel P of a vehicle. Inside the retainer R are disposed an airbag  11  made of cloth, and an inflator I for supplying spread gas into the airbag  11 . Ordinarily, the airbag  11  is accommodated in a folded state inside the retainer R. 
     As simply shown in FIG.  4 (A), the airbag  11  is a three-piece bag which is formed by sewing together one piece of strip-like central cloth  12  and two side cloths  13  ( 13   a  and  13   b ) one at each side of the central cloth  12 . The airbag  11  has an open end (that is, a gas circulation hole)  14  which is narrowed down thinly at the base thereof. The gas circulation hole  14  merges with a space of the inflator I. 
     A partition  15  is mounted inside the airbag  11 . The partition  15  is disposed at the middle portion of the airbag  11  as viewed in the vertical direction, and divides the interior of the airbag  11  into an upper portion  11 A and a lower portion  11 B. The partition  15  is formed of cloth or is a knitted product, and the material or the form thereof is such as to allow passage of gas between the two divided chambers (that is, the upper portion  11 A and the lower portion  11 B). For example, as shown in FIG.  4 (A), the partition  15  is formed by making the portion thereof disposed towards the open end  14  of the airbag  11  short. In this case, as simply shown in FIG.  4 (B), a gas flow path S which connects the upper portion  11 A and the lower portion  11 B is formed inside the airbag  11 . In another example, the partition  15  is formed by also making the portion thereof towards the front side of the airbag  11  short, in which case, as shown in FIG.  4 (C), two gas flow paths S and S′ can be formed. Cutaway portions  15   a  are formed, one in each edge of the partition  15 . The outer peripheral edges of the partition  15  are attached to the inside surface of the airbag  11  by, for example, sewing or welding. The cutaway portions  15   a  of the partition  15  allow the substantially middle locations of both side cloths  13  of the airbag  11  (portions where the cutaway portions  15   a  of the partition wall  15  are attached) to be brought towards each other, thereby forming inwardly extending depressions d. 
     A description of a modification of the partition  15  will be given later. 
     A description of the operation of the airbag device having the above-described structure will be given. 
     In the usual state of a vehicle, the airbag  11  is accommodated in a folded state in the retainer R. When the vehicle collides, a sensor (not shown) detects the collision, and sends an ignition signal to an initiator of the inflator I. The initiator is ignited, and a propellant is ignited, thereby producing spread gas from the inflator I. There is also a type of airbag device which is spread using accumulated pressure of inactive gas. 
     The spread gas that has been produced flows inside the airbag  11  after passing through the gas circulation hole  14 . As shown in FIGS.  1 (A) and  1 (B), the airbag  11  expands and spreads in front of the occupant H. As shown in FIG.  1 (B), at the time of completion of the spreading of the airbag  11 , the depressions d, disposed at substantially the central portions of the side cloths  13   a  and  13   b , are depressed inward, so that the front surface of the airbag  11  is shaped like a package. 
     As shown in FIGS.  2 (A) and  2 (B), the occupant H moves forward due to inertial force and hits the spread airbag  11 . This causes the airbag  11  to be pushed and squashed as a result of being sandwiched among the instrument panel P and the windshield F and the occupant H. At this time, the depressions d make it difficult for the gas at the lower portion inside the airbag  11  to escape towards the upper portion, so that the resistance of the lower portion  11 B of the bag acts uniformly upon the lower portion of the upper body (from the waist to the chest) of the occupant H. As shown in FIG.  2 (B), the upper portion  11 A and the lower portion  11 B of the airbag  11  are substantially equally horizontally spread. 
     The resistance acting in the airbag a load is applied will be described in more detail with reference to FIGS. 3 to  16 . 
     An external force α equivalent to the force produced by the body of the occupant acts upon the airbag  11  shown in FIG.  3 (B) from the front surface thereof. Here, as long as the portions where the depressions d of the airbag  11  are formed are not spread to the sizes of other portions, the depressions d limit the movement of the gas inside the bag. As described above and as shown schematically in FIGS.  16 (C) and  16 (D), in the conventional airbag, the airbag  103  escapes towards a region of lower resistance in the directions of empty arrows inside the bag  103 , so that the resistance with respect to the load body f becomes small, thereby making it easy for the bag to become squashed. On the other hand, as shown schematically in FIGS.  3 (C) and  3 (D), air in the airbag  11  of the present invention cannot easily escape in the direction of the upper portion  11 A of the bag, thereby making it possible to produce a large resistance with respect to a load body f at the lower portion  11 B. This causes the lower portion of the airbag  11  to have sufficient resistance. 
     Although the embodiment has been described by taking as an example the case where the depressions d are formed in substantially the central portions of the airbag  11  by placing one partition inside the bag, various modifications may be made as described below. 
     Hereunder, modifications will be given in terms of the front surface shapes of the airbag during expansion thereof with reference to FIG.  5 . 
     FIGS.  5 (A) to  5 (E) are schematic plan views of the forms of the front surface of the air bag when it has finished spreading. 
     The form shown in FIG.  5 (A) is the same as that shown in FIGS.  1 (B) and  2 (B). More specifically, in this case, the depressions d are formed in substantially the central portions of the side surfaces of the airbag, and a maximum width B 1  of the upper portion of the bag and a maximum width B 2  of the lower portion thereof are substantially equal to each other. 
     In the form shown in FIG.  5 (B), the depressions d are formed closer to the bottom portion of the airbag, and a maximum width B 1  of the upper portion of the bag is greater than a maximum width B 2  of the lower portion of the bag. 
     In the form shown in FIG.  5 (C), which is the reverse form of that shown in FIG.  5 (B), the depressions d are formed closer to the top portion of the airbag, and a width B 1  of the upper portion of the bag is less than a width B 2  of the lower portion of the bag. 
     In the form shown in FIG.  5 (D), depressions d 1  and depressions d 2  are formed in two levels in the bag. 
     In the form shown in FIG.  5 (E), three levels of depressions d 1 , depressions d 2 , and depressions d 3  are formed in the bag. More than three levels of such depressions may also be formed. 
     Modifications of the structure of the airbag will be given. 
     Specific Examples of Airbags Having Three-Piece Structures Using a Tether Strap or Tether Straps 
     Specific examples of airbags using tether straps will be described with reference to FIGS.  6 (A)- 7 (D). The characteristic of this type of airbag is that a tether strap or tether straps are used instead of the partition used in the above-described airbag  11 . 
     FIG.  6 (A) is an exploded perspective view of a structural example of an airbag using one tether strap. FIG.  6 (B) is a side view of the airbag. FIG.  6 (C) is a front view of the airbag. 
     FIG.  7 (A) is a schematic side view of a top-dash-mount-type passenger airbag device using two tether straps. FIG.  7 (B) is a front view of the airbag shown in FIG.  7 (A). FIG.  7 (C) is a sectional view taken along line  7 (C)— 7 (C) of FIG.  7 (A). FIG.  7 (D) is a sectional view taken along line  7 (D)— 7 (D) of FIG.  7 (A). 
     In an airbag  21  shown in FIGS.  6 (A)- 6 (C), a tether strap  22  is provided in a tensioned state between both side surface cloths  13   a  and  13   b  of the airbag having a three-piece structure. The tether strap  22  is a string-like or strip-like member which is formed of cloth or which is a knitted product. The ends of the tether strap  22  are sewed to the corresponding side surface cloths  13   a  and  13   b  through corresponding reinforcing cloths  23 . The side surface cloths  13   a  and  13   b  are brought inwardly towards each other by the tether strap  22 , and depressions d (see FIG.  6 (C)) are formed where the ends of the tether strap  22  are sewed. 
     In an airbag  25  shown in FIGS.  7 (A)- 7 (D), two tether straps  22  of the same type as that used in the airbag  21  shown in FIGS.  6 (A)- 6 (C) are sewed in two levels. In this case, two levels of depressions d 1  and d 2  are formed in the side surfaces of the airbag  25  in correspondence with the two tether straps  22 . The form of the front surface of the airbag  22  at the time of expansion thereof is that shown in FIG.  7 (B). It is the same as the form of the front surface of the airbag shown in FIG.  5 (D). 
     Specific Example of Airbag Having a Three-Piece Structure Using a Tucked Seam 
     A specific example of an airbag using a tucked seam will be described with reference to FIGS.  8 (A)- 8 (C). The characteristic of this type of airbag is that depressions are formed by a tucked seam without using a tether strap or tether straps or the partition used in the above-described airbag  11 . 
     FIG.  8 (A) is a schematic side view or a top-dash-mount-type passenger airbag device using a tucked seam. FIG.  8 (B) is a front view of the airbag shown in FIG.  8 (A). FIG.  8 (C) is a sectional view taken along line  8 (C)— 8 (C) of FIG.  8 (A). 
     In an airbag  28  shown in FIGS.  8 (A)- 8 (C), a tucked seam  29  is formed at portions of both side surface cloths  13   a  and  13   b  of the airbag having a three-piece structure. The side surface cloths  13   a  and  13   b  are brought towards each other and joined at the tucked seam  29 , and a depression d is formed at the sewed portion thereof. The locations and number of depressions can be increased by forming tucked seams  29  at a plurality of locations. 
     Specific Examples of Airbags Having Two-Piece Structures 
     Hereunder, a description of specific examples of airbags having two-piece structures will be given. 
     FIG.  9 (A) is a perspective view of an example of an airbag having a two-piece structure. FIG.  9 (B) is an exploded perspective view of the airbag. FIG.  9 (C) is an exploded plan view of the airbag. FIGS.  9 (D) and  9 (E) are perspective views used to illustrate the procedure of assembling the airbag. 
     FIG.  10 (A) is an exploded plan view of another example of the airbag having a two-piece structure. FIG.  10 (B) is a perspective view used to illustrate the procedure of assembling the airbag. 
     FIGS.  11 (A)- 11 (H) are used to illustrate the procedure of assembling the airbag having a two-piece structure. 
     In an airbag  31  shown in FIGS.  9 (A)- 9 (E), a cloth front panel  31  (at the side of an occupant) and a back panel  33  (at the side of an instrument panel) are integrally sewed together into the shape of a bag. As simply shown in FIG.  9 (C), both of the panels  32  and  33  are elliptical. As most simply shown in FIGS.  9 (B) and  9 (C), protruding ears  32   a  are formed, one on each of the two sides of the front panel  32 . A tether strap  34  is provided in a tensioned state between both ears  32   a.    
     In an airbag  31  shown in FIG. 9, a cloth front panel  31  (at the side of an occupant) and a back panel  33  (at the side of an instrument panel) are integrally sewed together into the shape of a bag. As simply shown in FIG.  9 (C), both of the panels  32  and  33  are elliptical. As most simply shown in FIGS.  9 (B) and  9 (C), protruding ears  32   a  are formed, one on each of the two sides of the front panel  32 . A tether strap  34  is provided in a tensioned state between both ears  32   a.    
     As most simply shown in FIGS.  9 (B) and  9 (C), a rectangular gas circulation hole  33   a  is formed in the center of the back panel  33 . In addition, two circular vent holes  33 b are formed in the back panel  33 . A reinforcing cloth  33   c  is sewed along the periphery of the gas circulation hole  33   a  of the back panel  33  and reinforcing cloths  33   d  are sewed along the peripheries of the corresponding vent holes  33   b . Holes  33   e  are formed in the reinforcing cloth  33   c , sewed along the periphery of the gas circulation hole  33   a.    
     The ears  32   a  used for mounting the tether strap  34  may be formed at the back panel  33 . 
     In an airbag  35  shown in FIGS.  10 (A) and  10 (B), tether straps of the type used in the airbag  31  shown in FIGS.  9 (A)- 9 (E) are integrally provided at a front panel. More specifically, as shown in FIG.  10 (A), the airbag  35  comprises tether straps  34 A and  34 B, one extending from each side of a front panel  32 . 
     In this case, the tether straps  34 A and  34 B maybe provided at a back panel  33 . 
     An airbag  38  shown in FIGS.  11 (A)- 11 (H) comprises a package-shaped front panel  32 ′ and a package-shaped back panel  33 ′, each of which has a cutaway portion  39 , and has a two-piece structure without a tether strap. In the airbag  38 , depressions are formed where the cutaway portions  39  are formed. 
     The airbags  31 ,  35 , and  38  are assembled using the following procedure. 
     (1) The reinforcing cloth  33   c  and the reinforcing cloths  33   d  are aligned with positioning holes that are previously formed in the back panel  33  ( 33 ′), and are sewed to the back panel  33  ( 33 ′). Then, the gas circulation hole  33   a  and the vent holes  33   b  are formed by cutting operations. (See FIGS.  11 (A) to  11 (C).) 
     (2) Outer surfaces (as viewed in the state shown in FIG.  9 (A)) of the front panel  32  ( 32 ′) and the back panel  33  ( 33 ′) are positioned so as to oppose each other, and are placed upon each other in order to sew them together along their outer peripheries. (See FIGS.  11 (D) and  11 (E).) 
     ( 3 ) (For the airbag  31  shown in FIG.  9 (A)) 
     As shown in FIG.  9 (D), the tether strap  34  is sewed to both ears  32   a  of the front panel  32 . 
     ( 3 ′) (For the airbag  35  shown in FIG.  10 (A)) 
     As shown in FIG.  10 (B), the ends of the tether straps  34 A and  34 B that protrude from the front panel  32  are placed upon each other and are sewed to ether. 
     These steps are not required for the airbag  38  shown in FIGS.  11 (A)- 11 (H). 
     (4) As shown in FIGS.  9 (D) and  11 (E), the front panel  32  ( 32 ′) is pulled out from the gas circulation hole of the back panel  33  ( 33 ′), and the inside and outside surfaces of both panels  32  and  33  ( 32 ′ and  33 ′) that have been sewed together are reversed. 
     In the airbags  31 ,  35 , and  38 , depressions are formed where the tether strap  34 , the tether straps  34 A and  34 B, and the cutaway portions  39  are formed, respectively. The shapes of the front surfaces of the airbags  31 ,  35 , and  38  when they have finished spreading are as shown in FIG.  5 (A). By moving the tether strap or tether straps or the cutaway portions vertically in the upward direction or the downward direction, the airbags  31 ,  35 , and  38  can take the form shown in FIGS.  5 (B) or  5 (C). In addition, by providing the tether straps or the cutaway portions in two or three levels, they can take the form shown in FIGS.  5 (D) or  5 (E). 
     Specific Examples of Airbags Having One-Piece Structures 
     Hereunder, a description of specific examples of cases where the present invention is applied to airbags having one-piece structures will be given. 
     FIG.  12 (A) is a plan view of an example of an airbag having a one-piece structure. FIG.  12 (B) is a perspective view of the airbag. FIG.  12 (C) is a front view (in the direction of arrow  12 (C) in FIG.  12 (B)) when the airbag has finished spreading. 
     FIG.  13 (A) is a plan view of another example of the airbag having a one-piece structure. FIG.  13 (B) is a perspective view of the airbag. FIG.  13 (C) is a front view (in the direction of arrow  13 (C) in FIG.  13 (B)) when the airbag has finished spreading. 
     An airbag  41  shown in FIGS.  12 (A)- 12 (C) is previously formed into the shape of a bag. Triangular protruding portions  41 A to  41 F are formed, three on each side of the airbag  41 . The center protruding portions  41 B and  41 E have ears  42 . A tether strap  45  is provided in a tensioned state between both ears  42 . 
     As shown in FIG.  12 (A), a gas circulation hole  43  is formed in the illustrated right end of the airbag  41 , and two vent holes  44  are formed towards the left end thereof. A reinforcing cloth  43   a  and reinforcing cloths  44   a  are sewed along the periphery of the gas circulation hole  43  and the peripheries of the vent holes  44 , respectively. Holes  43   b  are formed in the reinforcing cloth  43   a  formed along the periphery of the gas circulation hole  43 . 
     As shown in FIG.  12 (B), the airbag  41  is formed into a three-dimensional form by sewing together, as shown in FIG.  12 (B), sewing lines, formed along the outer periphery thereof, having the same reference numerals (X 1 , X 2 , Y 1 , Y 2 , Z 1 , Z 2 , and W), and by accommodating a tether strap  45  inside the airbag  41 . Reference numeral T denotes edge lines. In this case, the front surface has the shape of a package as shown in FIG.  12 (C). 
     In an airbag  48  shown in FIGS.  13 (A)— 13 (C), protruding portions  41 B′ and  41 E′ such as those used in the airbag  41  shown in FIGS.  12 (A)- 12 (C) are formed smaller than the other protruding portions  41 A,  41 C,  41 D, and  41 F, and no tether straps are used. By forming the protruding portions  41 B′ and  41 E′ smaller, arcuate edge lines L are formed in the state shown in FIG.  13 (B) after the sewing operation. In the airbag  48 , the front surface also has the shape of a package as shown in FIG.  13 (C). 
     Modification of Partition-Type Airbag 
     Hereunder, a description of a modification of a partition-type airbag will be given. 
     FIG.  17 (A) is a perspective view of another example of the partition-type airbag. FIG.  17 (B) is a plan view of a partition of the airbag shown in FIG.  17 (A). FIG.  17 (C) is a vertical sectional view of FIG.  17 (A). FIG.  17 (D) is a plan view of another example of the partition wall. FIG.  17 (E) is a vertical sectional view of the airbag using the partition shown in FIG.  17 (D). 
     A partition  55  is mounted inside an airbag  50  shown in FIG.  17 (A). The partition  55  is disposed at the middle portion of the airbag  50  as viewed in the vertical direction, and divides the inside of the airbag  50  into an upper portion  50 A and a lower portion  50 B. As simply shown in FIG.  17 (B), the partition  55  has two holes  55   x  and  55   y  formed therein. As shown in FIG.  17 ( 0 ), these holes  55   x  and  55   y  make it possible to connect the upper portion  50 A and the lower portion  50 B inside the airbag. Cutaway portions  55   a  are formed, one at each side edge of the partition  15 . The outer peripheral edges of the partition  55  are attached to the inside surface of the airbag  50  by, for example, sewing or welding. As in the airbag  11  shown in FIGS.  4 (A)- 4 (C), the cutaway portions  55   a  of the partition  55  form inwardly extending depressions d at substantially the middle portions thereof as viewed in the vertical direction. 
     The external shape of a partition  65  shown in FIG.  17 (D) is the same as that of the partition  55  shown in FIG.  17 (B) and does not have holes. The partition  65  is made of cloth or is a knitted product having high air permeability. In this case, as shown in FIG.  17 (E), gas circulates almost uniformly over the entire surface of the partition  65 . 
     As is clear from the foregoing description, according to the present invention, it is possible to provide an airbag having a more preferable spread form without increasing the output of an inflator.