Patent Publication Number: US-2012038135-A1

Title: Side airbag device and method for folding bag portion of side airbag device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase of PCT International Patent Application No. PCT/JP2010/056846, filed Apr. 16, 2010, and Japanese Patent Application No. 2009-103530, filed Apr. 22, 2009, in the Japanese Patent Office, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a side airbag device accommodated in a seat back of a seat for a vehicle and expanded and developed at the time of a side crash for protecting a passenger, and a method for folding a bag section thereof. 
     2. Description of the Related Art 
     A conventionally known side airbag device is built in a seat back of a seat for a vehicle. At the time of a side crash of the vehicle, the airbag is supplied with gas from an inflator to be expanded and developed, and is located between the passenger and an invading door trim to protect the chest of the passenger. Recently, airbags tend to be increased in size so as to protect an area including the chest and also the loins of the passenger. 
     The door trim includes an armrest projecting inward in a width direction of the vehicle, and the loins of the passenger project most outward among the parts of his/her body. The side airbag device is desired to have a function of, at the time of a side crash of the vehicle, expanding and developing and thus locating the airbag between the loins of the passenger and the invading door trim so as to urge the loins of the passenger inward in the width direction of the vehicle. 
     Patent Documents 1 and 2 disclose a side airbag device designed to protect an area including the chest and also the loins of the passenger. 
     According to the structure disclosed in Patent Document 1, the inner space of the airbag is partitioned into a lower chamber and an upper chamber by a seam, and an inflator is provided such that a gas ejecting section thereof is directed downward, i.e., toward the lower chamber, so as to expand and develop the lower chamber with priority. 
     According to the structure disclosed in Patent Document 2, the inner space of the airbag is partitioned into an upper airbag chamber corresponding to the chest of the passenger and a lower airbag chamber corresponding to the loins of the passenger. A sewn part is provided for distributing gas generated by the inflator to the upper and lower airbag chambers appropriately so as to provide a desired pressure difference between in the upper and lower airbag chambers. Thus, the loins of the passenger are held quickly by the lower airbag chamber corresponding to the loins of the passenger. 
     However, neither Patent Document 1 nor 2 mentioned above is directed to a structure for folding the airbag. Therefore, the conventional structure disclosed in Patent Document 1 has a problem of requiring a specific inflator having the gas ejecting section directed toward the lower chamber, and the conventional structure disclosed in Patent Document 2 has a problem that the sewing structure for the airbag is complicated. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-41457 
     Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-186891 
     SUMMARY OF THE INVENTION 
     Hence, the present invention has an object of providing a side airbag device and a method for folding a bag section thereof, capable of quickly expanding and developing a loin protection section of the airbag so that, at the time of a side crash of a vehicle, the airbag is located between the loins of the passenger and the door trim with certainty to protect the passenger. 
     A side airbag device according to the present invention is a side airbag device accommodated in a seat back of a seat for a vehicle. The side airbag device includes an airbag; and an inflator for supplying gas to the airbag to develop the airbag at the time of a crash of the vehicle. The airbag includes an airbag upper portion including a chest protection portion for protecting the chest of a passenger sitting on the seat when the airbag is developed and an airbag lower portion including a loin protection portion for protecting the loins of the passenger sitting on the seat when the airbag is developed; and the airbag is accommodated in the state of being folded such that a bottom part of the airbag upper portion and a top part of the airbag lower portion overlap each other in a width direction of the vehicle, and such that a bottom end of the airbag lower portion is located below a top end of the airbag lower portion. 
     According to this structure, the airbag is accommodated in the state of being folded such that the bottom part of the airbag upper portion and the top part of the airbag lower portion overlap each other in the width direction of the vehicle, and such that the bottom end of the airbag lower portion is located below the top end of the airbag lower portion. Therefore, when the airbag is expanded and developed, gas is supplied to the airbag generally right downward, and the airbag lower portion including the loin protection portion is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag can be located between the loins of the passenger and the door trim with certainty to protect the passenger. 
     In summary, the loin protection portion of the airbag is required to have a function of being expanded and developed more quickly than the chest protection portion and, at the time of a side crash, first urging the body of the passenger (especially, the loins) inward in the vehicle chamber. Such a requirement can be fulfilled by the above-described structure. 
     In an embodiment of the present invention, in the state where the airbag is folded, the top end of the airbag lower portion may have a first fold extending generally in a front-rear direction of the seat; the bottom end of the airbag upper portion may have a second fold extending generally in the front-rear direction of the seat; and the first fold may be located above the second fold. 
     According to this structure, since the first fold is located above the second fold, the bottom part of the airbag upper portion and the top part of the airbag lower portion overlap each other in the width direction of the vehicle. The bottom end of the airbag lower portion is located below the top end of the airbag lower portion. Therefore, in the same manner as described above, when the airbag is expanded and developed, gas is supplied to the airbag generally right downward, and the airbag lower portion including the loin protection portion is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash, the airbag can be located between the loins of the passenger and the door trim with certainty. 
     In an embodiment of the present invention, in the state where the airbag is folded, a front edge of the airbag, which is located on the front side when the airbag is developed, may be turned toward the inflator, so that the airbag has a shape lengthy in a top-bottom direction; the lengthy airbag upper portion may include an upper outer portion located on the outer side in the width direction of the vehicle and an upper inner portion located on the inner side in the width direction of the vehicle; the top end of the airbag lower portion may have a first fold extending generally in a front-rear direction of the seat; the bottom end of the airbag upper portion may have a second fold extending generally in the front-rear direction of the seat; a folded portion may be formed between the first fold and the second fold; and the airbag may be folded internally such that the first fold, the second fold, and the folded portion are held between the upper outer portion and the upper inner portion. 
     According to this structure, the airbag is folded internally (put into an internally folded state) in the state where the folds and the folded portion are held between the upper outer portion and the upper inner portion of the airbag upper portion. 
     Even in the case where the airbag is in such an internally folded state, when the airbag is expanded and developed, gas is supplied to the airbag generally right downward, and the airbag lower portion including the loin protection portion is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash, the airbag can be located between the loins of the passenger and the door trim with certainty. 
     In an embodiment of the present invention, in the state where the airbag is folded, a front edge of the airbag, which is located on the front side when the airbag is developed, may be turned toward the inflator, so that the airbag has a shape lengthy in a top-bottom direction; the airbag lower portion having the lengthy shape may include a lower outer portion located on the outer side in the width direction of the vehicle and a lower inner portion located on the inner side in the width direction of the vehicle; the top end of each of the lower outer portion and the lower inner portion may have a first fold extending generally in a front-rear direction of the seat; the bottom end of the airbag upper portion may have a second fold extending generally in the front-rear direction of the seat; a folded portion may be formed between each of the first folds and the second fold; and the airbag may be folded externally such that the folded portion of the lower outer portion and the folded portion of the lower inner portion are located outside the airbag upper portion. 
     According to this structure, the airbag is folded externally (put into an externally folded state) such that the folded portion of the lower outer portion and the folded portion of the lower inner portion are located outside the airbag upper portion. 
     Even in the case where the airbag is in such an externally folded state, when the airbag is expanded and developed, gas is supplied to the airbag generally right downward, and the airbag lower portion including the loin protection portion is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash, the airbag can be located between the loins of the passenger and the door trim with certainty. 
     In an embodiment of the present invention, a third fold located below the first fold and extending generally in the front-rear direction of the seat may be provided in the bottom part of the loin protection portion; and a part of the loin protection portion, which is located below the third fold when the airbag is developed, may be folded internally or externally. 
     According to this structure, the third fold is provided, and the part of the loin protection portion, which is located below the third fold when the airbag is developed, is folded internally or externally. Therefore, the size in the top-bottom direction of the airbag in the folded state can be decreased, and neither the third fold nor the part of the loin protection portion inhibits the airbag from being developed. 
     In addition, the bottom part of the loin protection portion is also folded internally or externally. Therefore, the bottom part of the loin protection portion is expanded and developed generally right downward by the pressure of gas from the inflator. Thus, the loin protection portion is expanded and developed more quickly. 
     In an embodiment of the present invention, the chest protection portion and the loin protection portion of the airbag may be partitioned from each other by a partition extending generally from front to rear in the state where the airbag is developed, except for a gas inlet of the inflator and the vicinity thereof. 
     It is desirable that the volume of the loin protection portion partitioned by the partition is smaller than the volume of the chest protection portion. 
     According to this structure, the chest protection portion and the loin protection portion of the airbag are partitioned from each other by the partition. Therefore, gas from the inflator can be concentratedly supplied to the loin protection portion, and thus the loin protection portion can be expanded and developed more quickly. In other words, the loin protection portion can be expanded and developed with priority. 
     In an embodiment of the present invention, a door trim located to the side of the seat may include an armrest projecting inward in a vehicle chamber; and the airbag may be folded in the state where the overlap area of the airbag upper portion and the airbag lower portion is located below a top surface of the armrest. 
     According to this structure, the loin protection portion of the airbag can be expanded and developed between the armrest and the loins of the passenger. Therefore, even on a side surface of the armrest of the door trim projecting inward in the vehicle chamber, the loin protection portion can be developed quickly. 
     Especially because the overlap area is located below the top surface of the armrest, the loin protection portion is developed below the top surface of the armrest. Thus, the loin protection portion can be developed between the armrest and the seat with certainty. 
     Also, the loin protection portion of the airbag can be expanded and developed between the seat and a space which is below the armrest and projects more outward, in the width direction of the vehicle, with respect to the inner end of the armrest in the width direction of the vehicle. 
     In an embodiment of the present invention, in the state where the side airbag device is accommodated in the seat back, the airbag may be folded such that a rear edge of the airbag upper portion is located rear to a rear edge of the airbag lower portion; and a step may be formed in generally in the front-rear direction of the seat between the rear edge of the airbag upper portion and the rear edge of the airbag lower portion. 
     This structure has the following effects. 
     When the airbag lower portion is merely folded upward, the step is not formed. When the airbag is folded such that the rear edge of the airbag upper portion is located rear to the rear edge of the airbag lower portion as in the above-described structure, the step can be formed between the rear edges. This step can be effectively used. 
     In an embodiment of the present invention, the inflator may be provided in a bottom part of the seat back side frame, and the airbag upper portion may be folded in the state where a top end thereof is turned downward. 
     According to this structure, since the inflator is provided at a lower position of the seat back side frame, an upper part of the seat back side frame can be made thinner and also the inflator having rigidity does not contact the chest of the passenger at the time of a side crash. 
     In addition, since the inflator is provided at a lower position, gas can be provided more quickly from the inflator to the loin protection portion. Since the top end of the airbag upper portion is turned downward, the loin protection portion is expanded and developed more quickly than the chest protection portion with certainty. This allows the loin protection portion to be expanded and developed quickly. By contrast, the chest protection portion is developed after the loin protection portion, and therefore can receive the body of the passenger softly. 
     Since the chest of the passenger is weaker than the loins of the passenger, it is required to alleviate the impact against the chest at the time of the development of the airbag. This requirement can be fulfilled by the above-described structure. 
     A side airbag device according to the present invention is a side airbag device accommodated in a seat back of a seat for a vehicle. The side airbag device includes an airbag; and an inflator for supplying gas to the airbag to develop the airbag at the time of a crash of the vehicle. The airbag includes an airbag upper portion including a chest protection portion for protecting the chest of a passenger sitting on the seat when the airbag is developed and an airbag lower portion including a loin protection portion for protecting the loins of the passenger sitting on the seat when the airbag is developed; and the airbag is folded in the state where an inner path of the airbag upper portion and an inner path of the airbag lower portion communicate to each other generally linearly in a top-bottom direction. 
     According to this structure, the airbag is folded in the state where the inner path of the airbag upper portion and the inner path of the airbag lower portion communicate to each other generally linearly in the top-bottom direction. Therefore, when the airbag is expanded and developed, gas is supplied to the airbag generally right downward, and the airbag lower portion including the loin protection portion is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag can be located between the loins of the passenger and the door trim with certainty to protect the passenger. 
     A method for folding a bag section of a side airbag device according to the present invention is a method for folding a bag section of a side airbag device including an airbag upper portion including a chest protection portion for protecting the chest of a passenger sitting on a seat when the bag section is developed and an airbag lower portion including a loin protection portion for protecting the loins of the passenger sitting on the seat when the bag section is developed. The method includes a first step of turning a front edge of the bag section, which is located on the front side of a vehicle when the bag section is developed, toward an inflator located on the rear side of the vehicle when the bag section is developed, so that the bag section has a lengthy shape; and a second step of folding the bag section such that a bottom part of the airbag upper portion and a top part of the airbag lower portion overlap each other in a width direction of the vehicle, and such that a bottom end of the airbag lower portion is located below a top end of the airbag lower portion. 
     For putting the bag section into a lengthy shape in the first step, the bag section may be folded into a generally roll-like shape, a complete roll shape or may be folded in zigzag. 
     For overlapping the bottom part of the airbag upper portion and the top part of the airbag lower portion on each other, the internally folded structure or alternatively, the externally folded structure may be selected. 
     According to this structure, in the first step, the front edge of the bag section is turned toward the inflator, and thus the bag section has a lengthy shape. In the second step, the bag section is folded such that the bottom part of the airbag upper portion and the top part of the airbag lower portion overlap each other in the width direction of the vehicle, and such that the bottom end of the airbag lower portion is located below the top end of the airbag lower portion. 
     As described above, the bag section is folded such that the bottom part of the airbag upper portion and the top part of the airbag lower portion overlap each other in the width direction of the vehicle, and such that the bottom end of the airbag lower portion is located below the top end of the airbag lower portion. Therefore, when the airbag is expanded and developed, gas is supplied to the airbag generally right downward, and the airbag lower portion including the loin protection portion is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag can be located between the loins of the passenger and the door trim with certainty to protect the passenger. 
     In summary, the loin protection portion of the airbag is required to have a function of being expanded and developed more quickly than the chest protection portion and, at the time of a side crash, first urging the body of the passenger inward in the vehicle chamber. Such a requirement can be fulfilled by the above-described folding method. 
     According to the present invention, when the airbag is expanded and developed, gas is supplied to the airbag generally right downward, and the airbag lower portion including the loin protection portion is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, there is an effect of, at the time of a side crash of the vehicle, locating the airbag between the loins of the passenger and the door trim with certainty to protect the passenger. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a front view of a vehicle having a side airbag device according to the present invention therein. 
         FIG. 2  is a side view of an important part of  FIG. 1  as seen from the right side of  FIG. 1 . 
         FIG. 3  is a side view showing a pre-folded state of an airbag. 
         FIG. 4  is a side view showing an intermediate state of folding of the airbag. 
         FIG. 5  is a side view showing an intermediate state of folding of the airbag. 
         FIG. 6  is a side view showing a state of the airbag folded into a lengthy shape. 
         FIG. 7  is a side view showing an internally folded state of the airbag. 
         FIG. 8  is a rear view of  FIG. 7 . 
         FIG. 9  is a side view showing another example of side airbag device. 
         FIG. 10  is an enlarged view of an important part of  FIG. 9 . 
         FIG. 11  is a side view showing still another example of side airbag device. 
         FIG. 12  is an enlarged view of an important part of  FIG. 11 . 
         FIG. 13  is a rear view of  FIG. 12  showing an externally folded state of the airbag. 
         FIG. 14  is a side view showing still another example of side airbag device. 
         FIG. 15(   a ) is a schematic view showing an internally folded state, and  FIG. 15(   b ) is a schematic view showing an externally folded state. 
         FIG. 16(   a ) is a schematic view showing a folded state having a shape proximate to a roll shape.  FIG. 16(   b ) is a schematic view showing a state of being folded in a zigzag manner, and 
         FIG. 16(   c ) is a schematic view showing a folded state having a roll shape. 
         FIG. 17(   a ) is a schematic view showing an internally folded state with third folds, and 
         FIG. 17(   b ) is a schematic view showing an externally folded state with third folds. 
         FIG. 18  is a side view showing still another example of side airbag device. 
         FIG. 19  is a side view showing a pre-folded state of the airbag. 
         FIG. 20  is a side view showing a state where a top and of an airbag upper portion is turned down. 
         FIG. 21  is a side view showing an intermediate state of folding of the airbag. 
         FIG. 22  is a side view showing a state of the airbag folded into a lengthy shape. 
         FIG. 23  is a side view showing an internally folded state of the airbag. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
     Embodiment 1 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
     The drawings show a side airbag device and a method for folding a bag section thereof. First, with reference to  FIG. 1  and  FIG. 2 , a structure of a vehicle body and a structure of a seat will be described. 
     As shown in  FIG. 1  and  FIG. 2 , a floor panel  2  forming a bottom part of a vehicle chamber  1  is provided. At a central part of the floor panel  2  in a width direction of the vehicle, a tunnel section  3  projecting inward in the vehicle chamber  1  and extending in a front-rear direction of the vehicle. 
     At an outer end of the floor panel  2  in the width direction of the vehicle, a side sill  8  including a side sill inner  4 , a side sill reinforcement  5  and a side sill outer  6  which are joined together and fixed and thus having a side sill closed cross-section  7  extending in the front-rear direction of the vehicle is attached for the purpose of improving the rigidity of a lower part and a side part of the vehicle body. 
     At an outer end of a roof panel  9  forming a roof part in the width direction of the vehicle, a roof side rail  13  including a roof wide rail inner  10  and a roof side rail outer  11  which are joined together and fixed and thus having a roof side closed cross-section  12  extending in the front-rear direction of the vehicle is attached for the purpose of improving the rigidity of an upper part and the side part of the vehicle body. 
     A top ceiling  14  is provided inner to the roof panel  9  in the vehicle chamber  1 . 
     At a boarding opening (door opening) formed between the side sill  8  and the roof side rail  13 , a door  15  (in  FIG. 1 , left front door of the vehicle) for opening and closing the boarding opening is provided. 
     The door  15  is attached via a door hinge provided in a front-side hinge pillar to the hinge pillar so as to be opened and closed, and includes a door outer panel  16 , a door inner panel  17  and a door glass  18 . 
     Inner to the door inner panel  17  in the vehicle chamber  1 , a door trim  19  acting as an interior furnishing member for covering the door inner panel  17  is provided. The door trim  19  includes an armrest  20  projecting inward in the vehicle chamber  1 . 
     As shown in  FIG. 1  and  FIG. 2 , on the floor panel  2 , a pair of, namely, left and right, seat slide rails  22  are attached by use of a plurality of seat blankets  21 . The seat slide rails  22  each include a fixed lower rail  23  and a movable upper rail  24  slidable in the front-rear direction of the vehicle along the lower rail  23 . 
     On the left and right seat slide rails  22 , a seat  25  on which a passenger can sit is provided. 
     In the figures, a front seat (front-row seat) on the left side of the vehicle is shown as an example. 
     The seat  25  includes a seat cushion  26 , a seat back  27  and a head rest  28 . In the seat back  27 , a seat back frame  29  is provided. An upper part of the seat back frame  29  is provided with a pair of, namely, left and right, head rest poles  31 , adjustable in length in a top-bottom direction, via a pair of, namely, left and right, pole guides  30 . The head rest  28  is supported by the head rest poles  31 . 
     On both of the left and right sides of the seat back frame  29 , seat back side frames  32  are provided. The seat cushion  26  and the seat back  27  are coupled to each other by a reclining knuckle  33 , so that the seat back  27  is reclinable. The seat back side frames  32  are provided with a lumbar shaft  34  (see  FIG. 2 ) as a shaft of a lumbar support (not shown) provided in the seat back  27 . 
     In the seat back  27  of the seat  25  (seat for the vehicle) having the above-described structure, a side airbag device  40  is accommodated. In this embodiment, for the convenience of illustration in the figures, the front seat on the left side of the vehicle is shown as an example. Thus, the side airbag device  40  is provided at the left seat back side frame  32  which is located on the outer side in the width direction of the vehicle, among the pair of, namely, left and right seat back side frames  32 . 
       FIG. 3  is a side view of the side airbag device  40  in a pre-folded state. The side airbag device  40  includes an airbag  42  (bag section) formed of two cloth pieces of the same shape which are sewn together along the circumferences thereof by a sewing section  41 , and an inflator  50  for supplying gas to the inside of the airbag  42  to develop the airbag  42  at the time of a crash of the vehicle. 
     The inflator  50  is provided between the two cloth pieces forming the airbag  42  and located on the rear side of the vehicle regardless of whether the airbag  42  is developed or not. As shown in  FIG. 3 , the inflator  50  is located to be directed in the top-bottom direction. A gas inlet (not shown) of the inflator  50  is formed at a position below the center thereof in the top-bottom direction and is directed forward. 
     As shown in  FIG. 3 , the airbag  42  (bag section) includes an airbag upper portion  42 U and an airbag lower portion  42 L integrally formed together. The airbag upper portion  42 U includes a bust guard portion A as a chest protection portion for protecting the chest of the passenger sitting on the seat  25  when the airbag  42  is developed, the airbag lower portion  42 L includes a waist guard portion B as a loin protection portion for protecting the loins of the passenger sitting on the seat  25  when the airbag  42  is developed. 
     In this embodiment, as shown in  FIG. 3 , the bust guard portion A and the waist guard portion B of the airbag  42  are partitioned from each other generally in the top-bottom direction by a seam  43  as a partition extending from front to rear in the state where the airbag is developed as shown in this figure, except for the gas inlet (gas ejecting opening) of the inflator  50  and the vicinity thereof. 
     Also as shown in  FIG. 3 , in the state where the airbag  42  is developed (pre-folded state), length L 3  in the front-rear direction of the airbag lower portion  42 L including the waist protecting portion B is longer than length L 2  in the front-rear direction of the airbag upper portion  42 U including the bust protecting portion A (L 3 &gt;L 2 ). Length L 1  in the front-rear direction of an intermediate portion of the airbag  42  in the top-bottom direction is shortest (L 3 &gt;L 2 &gt;L 1 ). The airbag  42  is formed to have an overall shape generally like a silk cocoon when seen from the side of the vehicle. 
     Between a rear end  43   a  of the seam  43  as a partition and a front surface of the inflator  50 , there is a predetermined gap  44 . 
     Now, a bag section folding method for folding the airbag  42  in the pre-folded state shown in  FIG. 3  into a state shown in  FIG. 7  will be described. 
     As shown in  FIG. 3  and  FIG. 4 , a top end a of the airbag upper portion  42 U is turned downward along a fold line b to form a folded portion c, and a bottom part of the airbag lower portion  42 L is folded along a fold line d and inserted between the two cloth pieces forming the airbag  42 , namely, inserted to the inside of the bag section to form a folded-in portion e. 
     Next, from the state shown in  FIG. 4 , a front edge f of the airbag  42 , which is located on the front side when the airbag  42  is developed, is turned toward the inflator  50  sequentially along fold lines g, h and i extending in the top-bottom direction (see  FIG. 5  and  FIG. 6 ). Thus, the airbag  42  is first put into an intermediate state shown in  FIG. 5  and then is put into a lengthy shape as shown in  FIG. 6  (first step). 
     Next, from the state shown in  FIG. 6 , the airbag  42  is folded as shown in  FIG. 7  such that a bottom part of the airbag upper portion  42 U and a top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle to form an overlap portion OL, and such that a bottom end j of the airbag lower portion  42 L is located below a top end of the airbag lower portion  42 L (second step). 
       FIG. 7  is a side view of the airbag  42  in the folded state, and  FIG. 8  is a rear view of  FIG. 7 . As shown in  FIG. 7  and  FIG. 8 , in the folded state of the airbag  42 , the top end of the airbag lower portion  42 L has two first folds L extending generally in the front-rear direction of the seat  25 , and a bottom end of the airbag upper portion  42 U has two second folds U extending generally in the front-rear direction of the seat  25 . The first folds L are located above the second folds U, and a portion between the first folds L and the second folds U is the overlap portion OL. 
     In this embodiment shown in  FIG. 7  and  FIG. 8 , the airbag  42  is folded internally (put into an internally folded state), and therefore the details of the folded state of the airbag  42  are as follows. 
     As shown in  FIG. 8 , the lengthy airbag upper portion  42 U includes an upper outer portion  42 Uout located on the outer side in the width direction of the vehicle and an upper inner portion  42 Uin located on the inner side in the width direction of the vehicle. Between the first folds L and the second folds U, folded portions Z are formed. As shown in  FIG. 8 , the airbag is folded internally such that the upper and lower folds L and U and the folded portions Z are held between the upper outer portion  42 Uout and the upper inner portion  42 Uin of the airbag upper portion  42 U. 
     As shown in  FIG. 8 , the inflator  50  is integrally provided with a pair of, namely, top and bottom attachment bolts  51  and  52  extending through the upper inner portion  42 Uin of the airbag upper portion  42 U and inward in the width direction of the vehicle. The inflator  50  is attached to the seat back side frame  32  as shown in  FIG. 2  via nuts (not shown) tightened to the pair of attachment bolts  51  and  52 . This way, the side airbag device  40  is built in the seat back  27  of the seat  25  for the vehicle. 
     As shown in the front view of  FIG. 1 , the airbag  42  is folded in the state where the overlap portion OL as an overlap area of the airbag upper portion  42 U and the airbag lower portion  42 L is located below a top surface of the armrest  20  projecting from the door trim  19  inward in the vehicle chamber  1 . 
     The folding structure of the airbag  42  in Embodiment 1 shown in  FIG. 7  and  FIG. 8  is as schematically shown in  FIG. 15(   a ) and  FIG. 16(   a ). As shown in  FIG. 15(   a ), the airbag  42  is folded in the state where a major part of an inner path X of the airbag upper portion  42 U and a major part of an inner path Y of the airbag lower portion  42 L, excluding the folded portions c and the folded-in portions e (see  FIG. 4) , communicate to each other generally linearly in the top-bottom direction. 
     For the purpose of keeping the airbag  42  in the completely folded state shown in  FIG. 7  until a crash occurs, the airbag  42  is held in this state by shape retaining means such as paper tape or the like, which can be easily broken when the airbag  42  is expanded and developed. 
     In the figures, arrow F represents the forward direction of the vehicle and arrow R represents the rearward direction of the vehicle. Arrow OUT represents the outward direction of the vehicle and arrow IN represents the inward direction of the vehicle. 
     As described above, the side airbag device in Embodiment 1 shown in  FIGS. 1 through 8  (corresponding to claims  1 ,  2 ,  3 ,  6 ,  7 ,  10  and  11 ) is a side airbag device accommodated in a seat back  27  of a seat  25  for a vehicle. The side airbag device includes an airbag  42 ; and an inflator  50  for supplying gas to the airbag  42  to develop the airbag  42  at the time of a crash of the vehicle. The airbag  42  includes an airbag upper portion  42 U including a bust guard portion A as a chest protection portion for protecting the chest of a passenger sitting on the seat  25  when the airbag  42  is developed and an airbag lower portion  42 L including a waist guard portion B as a loin protection portion for protecting the loins of the passenger sitting on the seat  25  when the airbag  42  is developed. The airbag is accommodated in the state of being folded such that a bottom part of the airbag upper portion  42 U and a top part of the airbag lower portion  42 L overlap each other (see the overlap portion OL) in the width direction of the vehicle, and such that a bottom end j of the airbag lower portion  42 L is located below a top end of the airbag lower portion  42 L. (See  FIGS. 1 ,  2 ,  3  and  7 .) 
     According to this structure, the airbag is accommodated in the state of being folded such that the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle, and such that the bottom end j of the airbag lower portion  42 L is located below the top end of the airbag lower portion  42 L. Therefore, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (see the waist guard portion B) is expanded generally right downward. This allows the loin protection portion (waist guard portion B) to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag  42  can be located as represented by the phantom line in  FIG. 1  between the loins of the passenger and the door trim  19  with certainty to protect the passenger. 
     In summary, the loin protection portion (waist guard portion B) of the airbag  42  is required to have a function of being expanded and developed more quickly than the chest protection portion (bust guard portion A) and, at the time of a side crash, first urging the body of the passenger inward in the vehicle chamber  1 . Such a requirement can be fulfilled by the above-described structure. 
     In the state where the airbag  42  is folded, the top end of the airbag lower portion  42 L has first folds L extending generally in the front-rear direction of the seat; the bottom end of the airbag upper portion  42 U has second folds U extending generally in the front-rear direction of the seat; and the first folds L are located above the second folds U (see  FIG. 7  and  FIG. 8 ). 
     According to this structure, since the first folds L are located above the second folds U, the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle. The bottom end j of the airbag lower portion  42 L is located below the top end of the airbag lower portion  42 L. Therefore, in the same manner as described above, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion (waist guard portion B) to be expanded and developed quickly. Hence, at the time of a side crash, the airbag  42  can be located between the loins of the passenger and the door trim  19  with certainty. 
     In the state where the airbag  42  is folded, a front edge f of the airbag  42 , which is located on the front side when the airbag  42  is developed, is turned toward the inflator  50 , so that the airbag  42  has a shape lengthy in the top-bottom direction (see  FIG. 6 ); the lengthy airbag upper portion  42 U includes an upper outer portion  42 Uout located on the outer side in the width direction of the vehicle and an upper inner portion  42 Uin located on the inner side in the width direction of the vehicle; the top end of the airbag lower portion  42 L has first folds L extending generally in the front-rear direction of the seat; the bottom end of the airbag upper portion  42 U has second folds U extending generally in the front-rear direction of the seat; folded portions Z are formed between the first folds L and the second folds U; and the airbag  42  is folded internally such that the first folds L, the second folds U, and the folded portions Z are held between the upper outer portion  42 Uout and the upper inner portion  42 Uin (see  FIGS. 3 ,  6 ,  7  and  8 ). 
     According to this structure, the airbag  42  is folded internally (put into an internally folded state) in the state where the folds Land U and the folded portions Z are held between the upper outer portion  42 Uout and the upper inner portion  42 Uin of the airbag upper portion  42 U. 
     Even in the case where the airbag  42  is in such an internally folded state, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion (waist guard portion B) to be expanded and developed quickly. Hence, at the time of a side crash, the airbag  42  can be located between the loins of the passenger and the door trim  19  with certainty. 
     In addition, the chest protection portion (bust guard portion A) and the loin protection portion (waist guard portion B) of the airbag  42  are partitioned from each other by a partition (seam  43 ) extending generally from front to rear in the state where the airbag is developed, except for a gas inlet of the inflator  50  and the vicinity thereof (see  FIG. 3 ). 
     It is desirable that the volume of the loin protection portion (waist guard portion B) partitioned by the partition (seam  43 ) is smaller than the volume of the chest protection portion (bust guard portion A). 
     According to this structure, the chest protection portion (bust guard portion A) and the loin protection portion (waist guard portion B) of the airbag  42  are partitioned from each other by the partition (seam  43 ). Therefore, gas from the inflator  50  can be concentratedly supplied to the loin protection portion (waist guard portion B), and thus the loin protection portion (waist guard portion B) can be expanded and developed more quickly. In other words, the loin protection portion can be expanded and developed with priority. 
     The door trim  19  located to the side of the seat  25  includes an armrest  20  projecting inward in the vehicle chamber  1 ; and the airbag  42  is folded in the state where the overlap area (overlap portion OL) of the airbag upper portion  42 U and the airbag lower portion  42 L is located below a top surface of the armrest  20  (see  FIG. 1 ). 
     According to this structure, the loin protection portion (waist guard portion B) of the airbag  42  can be expanded and developed between the armrest  20  and the loins of the passenger. Therefore, even on a side surface of the armrest  20  of the door trim  19  projecting inward in the vehicle chamber  1 , the loin protection portion (waist guard portion B) can be developed quickly. 
     Especially because the overlap area (overlap portion OL) is located below the top surface of the armrest  20 , the loin protection portion (waist guard portion B) is developed below the top surface of the armrest  20 . Thus, the loin protection portion (waist guard portion B) can be developed between the armrest  20  and the seat  25  with certainty. 
     Also, the loin protection portion (waist guard portion B) of the airbag  42  can be expanded and developed between the seat  25  and a space which is below the armrest  20  and projects more outward, in the width direction of the vehicle, with respect to the inner end of the armrest  20  in the width direction of the vehicle. 
     A side airbag device in Embodiment 1 described above is a side airbag device accommodated in a seat back  27  of a seat  25  for a vehicle. The side airbag device includes an airbag  42 ; and an inflator  50  for supplying gas to the airbag  42  to develop the airbag  42  at the time of a crash of the vehicle. The airbag  42  includes an airbag upper portion  42 U including a chest protection portion (bust guard portion A) for protecting the chest of a passenger sitting on the seat  25  when the airbag  42  is developed and an airbag lower portion  42 L including a loin protection portion (waist guard portion B) for protecting the loins of the passenger sitting on the seat  25  when the airbag  42  is developed. The airbag  42  is folded in the state where an inner path X of the airbag upper portion  42 U and an inner path Y of the airbag lower portion  42 L communicate to each other generally linearly in the top-bottom direction. (See  FIGS. 1 ,  3  and  15 ( a ).) 
     According to this structure, the airbag  42  is folded in the state where the inner path X of the airbag upper portion  42 U and the inner path Y of the airbag lower portion  42 L communicate to each other generally linearly in the top-bottom direction. Therefore, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion (waist guard portion B) to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag  42  can be located between the loins of the passenger and the door trim  19  with certainty to protect the passenger. 
     A method for folding a bag section (airbag  42 ) of a side airbag device in Embodiment 1 described above is a method for folding a bag section of a side airbag device including an airbag upper portion  42 U including a chest protection portion (bust guard portion A) for protecting the chest of a passenger sitting on a seat  25  when the bag section is developed and an airbag lower portion  42 L including a loin protection portion (waist protection portion B) for protecting the loins of the passenger sitting on the seat  25  when the bag section is developed. The method includes a first step (see  FIG. 6 ) of turning a front edge f of the bag section (airbag  42 ), which is located on the front side in the vehicle when the bag section is developed, toward an inflator  50  located on the rear side of the vehicle when the bag section is developed, so that the bag section (airbag  42 ) has a lengthy shape; and a second step (see  FIG. 7 ) of folding the bag section such that a bottom part of the airbag upper portion  42 U and a top part of the airbag lower portion  42 L overlap each other (overlap portion OL) in a width direction of the vehicle, and such that a bottom end j of the airbag lower portion  42 L is located below a top end of the airbag lower portion  42 L. (See  FIGS. 3 ,  6  and  7 .) 
     According to this structure, in the first step (see  FIG. 6 ), the front edge f of the bag section (airbag  42 ) is turned toward the inflator  50 , and thus the bag section (airbag  42 ) has a lengthy shape. In the second step (see  FIG. 7 ), the bag section is folded such that the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other (overlap portion OL) in the width direction of the vehicle, and such that the bottom end j of the airbag lower portion  42 L is located below the top end of the airbag lower portion  42 L. 
     As described above, the bag section is folded such that the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle, and such that the bottom end j of the airbag lower portion  42 L is located below the top end of the airbag lower portion  42 L. Therefore, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (see waist guard portion B) is expanded generally right downward. This allows the loin protection portion (waist guard portion B) to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag  42  can be located between the loins of the passenger and the door trim  19  with certainty to protect the passenger. 
     In summary, the loin protection portion (waist guard portion B) of the airbag  42  is required to have a function of being expanded and developed more quickly than the chest protection portion (bust guard portion A) and, at the time of a side crash, first urging the body of the passenger inward in the vehicle chamber  1 . Such a requirement can be fulfilled by the above-described folding method. 
     In the case where as disclosed in the embodiment (see  FIG. 4 ), the top end a of the airbag upper portion  42 U is turned downward along the fold line b to form the folded portion c and the bottom part of the airbag lower portion  42 L is folded along the fold line d and inserted between the two cloth pieces forming the airbag  42  to form the folded-in portion e, the length in the top-bottom direction of the airbag  42  in the folded state can be made shorter. 
     Regarding the method for folding the bag section (airbag  42 ), in Embodiment 1, the front edge f, of the bag section, which is located on the front side of the vehicle when the bag section is developed, is turned toward the inflator  50  and the bag section is sequentially folded into a generally roll-like shape as shown in  FIG. 16(   a ). Alternatively, the bag section may be folded as shown in  FIG. 16(   b ) or ( c ). 
     Specifically, in  FIG. 16(   b ), the airbag  42  is folded in a zigzag manner. In  FIG. 16(   c ), the front edge f, of the airbag  42 , which is located on the front side of the vehicle when the airbag  42  is developed, is turned toward the inflator  50  so that the airbag  42  has a shape of generally complete roll, namely, a whirlpool-like shape. 
     Instead of the side airbag device shown in  FIG. 2  and  FIG. 7 , a side airbag device shown in  FIG. 9  and  FIG. 10  may be adopted. According to the structure shown in  FIG. 10 , the first folds L provided at the top end of the airbag lower portion  42 L are inclined at an acute angle (e.g., 25 to 35 degrees with respect to the phantom horizontal line) so as to have a higher front end and a lower rear end. When the first folds L are set to be inclined at such an acute angle, a part lower than the first folds L is directed forward in the vehicle. Therefore, as shown in  FIG. 9 , when the airbag  42  is attached to the seat back side frame  32 , the airbag lower portion  42 L does not overlap the lumbar shaft  34 . 
     Therefore, according to the structure shown in  FIG. 9  and  FIG. 10 , the side airbag and a lumbar support device can both be provided. 
     Even with the structure shown in  FIG. 9  and  FIG. 10 , the other elements of the structure, the function and the effect are substantially the same as those in the above-described embodiment. Therefore, in  FIG. 9  and  FIG. 10 , identical elements as those in the previously described figures bear the identical reference signs thereto and detailed descriptions thereof are omitted. 
     Embodiment 2 
       FIG. 11 ,  FIG. 12  and  FIG. 13  show another embodiment of a side airbag device  40 . 
     In Embodiment 1 shown in  FIGS. 1 through 10 , the airbag lower portion  42 L is folded internally; whereas in Embodiment 2 shown in  FIG. 11 ,  FIG. 12  and  FIG. 13 , the airbag lower portion  42 L is folded externally. 
     The method for folding the bag section (airbag  42 ) is the same as that of Embodiment 1 until the bag section is formed into a shape lengthy in the top-bottom direction as shown in  FIG. 6  from the pre-folded state in  FIG. 3  via the intermediate states in  FIG. 4  and  FIG. 5 . 
     In Embodiment 2, after being formed into the shape lengthy in the top-bottom direction shown in  FIG. 6 , the airbag  42  is folded as shown in  FIG. 12  and  FIG. 13 . 
     The lengthy airbag lower portion  42 L includes a lower outer portion  42 Lout located on the outer side in the width direction of the vehicle and a lower inner portion  42 Lin located on the inner side in the width direction of the vehicle. The top end of each of the lower outer portion  42 Lout and the lower inner portion  42 Lin has a first fold L extending generally in the front-rear direction of the seat  25 , and the bottom end of the airbag upper portion  42 U has two second folds U extending generally in the front-rear direction of the seat  25 . Between the first folds L and the second folds U, an outer folded portion Z and an inner folded portion Z are formed. 
     In Embodiment 2 shown in  FIG. 12  and  FIG. 13 , the airbag is folded externally (put into an externally folded state) such that the folded portion Z of the lower outer portion  42 Lout, the folded portion Z of the lower inner portion  42 Lin, and the two first folds L are located outside the airbag upper portion  42 U. In this folded state, a portion between the first folds L located above and the second folds U located below act as the overlap portion OL. 
     In other words, after the first step shown in  FIGS. 3 through 6 , the airbag is folded externally such that the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle to form the overlap portion OL, and such that the bottom end j of the airbag lower portion  42 L is located below the top end (see the first folds L) of the airbag lower portion  42 L (second step). Thus, the state shown in  FIG. 12  and  FIG. 13  is provided. 
       FIG. 12  is an enlarged side view of the side airbag device  40  when being accommodated in the seat back  27 . In the example of Embodiment 2 shown in  FIGS. 11 through 13 , the airbag is folded such that a rear edge  42 Ub of the airbag upper portion  42 U is located rear to a rear edge  42 Lb of the airbag lower portion  42 L in the front-rear direction of the vehicle. Between the rear edge  42 Ub of the airbag upper portion  42 U and the rear edge  42 Lb of the airbag lower portion  42 L, a step  45  is formed generally in the front-rear direction of the seat  25 . 
     The folding structure of the airbag  42  in Embodiment 2 shown in  FIGS. 11 through 13  is as schematically shown in  FIG. 15(   b ) and  FIG. 16(   a ). As shown in  FIG. 15(   b ), the airbag  42  is folded such that a major part of an inner path X of the airbag upper portion  42 U and a major part of an inner path Y of the airbag lower portion  42 L, excluding the folded portions c and the folded-in portions e (see  FIG. 4) , communicate to each other generally linearly in the top-bottom direction. 
     As described above, in Embodiment 2 shown in  FIGS. 11 through 13  (corresponding to claims  1 ,  2 ,  4 ,  6 ,  7 ,  8 ,  10  and  11 ), in the state where the airbag  42  is folded, a front edge f of the airbag (see  FIG. 3 ), which is located on the front side when the airbag is developed, is turned toward the inflator  50 , so that the airbag has a shape lengthy in the top-bottom direction (see  FIG. 6 ); the lengthy airbag lower portion  42 L includes a lower outer portion ( 42 Lout) located on the outer side in the width direction of the vehicle and a lower inner portion ( 42 Lin) located on the inner side in the width direction of the vehicle; the top end of each of the lower outer portion  42 Lout and the lower inner portion  42 Lin has a first fold L extending generally in the front-rear direction of the seat  25 ; the bottom end of the airbag upper portion  42 U has second folds U extending generally in the front-rear direction of the seat  25 ; folded portions Z are formed between the first folds L and the second folds U; and the airbag  42  is folded externally such that the folded portion Z of the lower outer portion  42 Lout and the folded portion Z of the lower inner portion  42 Lin are located outside the airbag upper portion  42 U (see  FIGS. 1 ,  6  and  11  through  13 ). 
     According to this structure, the airbag  42  is folded externally (put into an externally folded state) such that the folded portion Z of the lower outer portion  42 Lout and the folded portion Z of the lower inner portion  42 Lin are located outside the airbag upper portion  42 U. 
     Even in the case where the airbag  42  is in such an externally folded state, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion (waist guard portion B) to be expanded and developed quickly. Hence, at the time of a side crash, the airbag  42  can be located between the loins of the passenger and the door trim  19  with certainty. 
     In addition, in this embodiment shown in  FIGS. 11 through 13 , in the state where the side airbag device  40  is accommodated in the seat back  27 , the airbag  42  is folded such that the rear edge  42 Ub of the airbag upper portion  42 U is located rear to the rear edge  42 Lb of the airbag lower portion  42 L. Thus, the step  45  is formed generally in the front-rear direction of the seat between the rear edge  42 Ub of the airbag upper portion  42 U and the rear edge  42 Lb of the airbag lower portion  42 L. (See  FIG. 11  and  FIG. 12 .) 
     This structure has the following effects. 
     When the airbag lower portion  42 L is merely folded upward, the step  45  is not formed. When the airbag is folded such that the rear edge  42 Ub of the airbag upper portion  42 U is located rear to the rear edge  42 Lb of the airbag lower portion  42 L as in the above-described structure, the step  45  can be formed between the rear edges  42 Ub and  42 Lb. This step  45  can be effectively used. 
     For example, in the case where the lumbar shaft  34  is provided as shown in  FIG. 11  by effective use of the step  45 , the side airbag and the lumbar support device can both be provided. 
     In Embodiment 2 shown in  FIGS. 11 through 13  also, the other elements of the structure, the function and the effect are substantially the same as those in the above-described embodiment. Therefore, in  FIGS. 11 through 13 , identical elements as those in the previously described figures bear the identical reference signs thereto and detailed descriptions thereof are omitted. 
     Instead of the structure shown in  FIG. 11 , the structure shown in  FIG. 14  may be adopted. 
     According to the structure shown in  FIG. 14 , the second folds U formed at the bottom end of the airbag upper portion  42 U and extending generally in the front-rear direction of the seat is lowered to a position close to the lumbar shaft  34 , so that the lumbar shaft  34  is provided by effective use of the step  45 , and also the inflator  50  in the airbag  42  is provided at a position as low as possible of the seat back side frame  32 . 
     In the embodiment shown in  FIG. 14  (corresponding to claim  9 ), the inflator  50  is provided at a lower position of the seat back side frame  32 , and the airbag  42  is folded in the state where the top end a (see  FIG. 4 ) of the airbag upper portion  42 U is turned downward as described above (see  FIG. 4  and  FIG. 14 ). 
     According to this structure, since the inflator  50  is provided at a lower position of the seat back side frame  32 , an upper part of the seat back side frame  32  can be made thinner and also the inflator  50  having rigidity does not contact the chest of the passenger at the time of a side crash. 
     In addition, since the inflator  50  is provided at a lower position, gas can be provided more quickly from the inflator  50  to the loin protection portion (waist guard portion B). Since the top end a of the airbag upper portion  42 U is turned downward, the loin protection portion (waist guard portion B) is expanded and developed more quickly than the chest protection portion (bust guard portion A) with certainty. This allows the loin protection portion (waist guard portion B) to be expanded and developed quickly. By contrast, the chest protection portion (bust guard portion A) is developed after the loin protection portion (waist guard portion B), and therefore can receive the body of the passenger softly. 
     Since the chest of the passenger is weaker than the loins of the passenger, it is required to alleviate the impact against the chest at the time of the development of the airbag  42 . This requirement can be fulfilled by the above-described structure. 
     In the embodiment shown in  FIG. 14  also, the other elements of the structure, the function and the effect are substantially the same as those in  FIGS. 11 through 13 . Therefore, in  FIG. 14 , identical elements as those in  FIGS. 11 through 13  bear the identical reference signs thereto and detailed descriptions thereof are omitted. 
     Embodiment 3 
       FIGS. 17(   a ) and ( b ) show still another embodiment of side airbag device. In both of the examples, in the bottom part of the waist guard portion B as the loin protection portion, third folds T are located below the first folds L and the second folds U and extend generally in the front-rear direction of the seat  25 . Parts α or β of the wait guard portion B, which are located below the third folds T when the airbag  42  is developed, are folded internally (put into an internally folded state) as shown in  FIG. 17(   a ) or externally (put into an externally folded state) as shown in  FIG. 17(   b ). 
     In the internally folded state shown in  FIG. 17(   a ), the airbag  42  is folded such that the second folds U, the folded portions Z, the first folds L, the third folds T and the above-mentioned parts a are located between the upper outer portion  42 Uout and the upper inner portion  42 Uin of the airbag upper portion  42 U. Thus, the airbag  42  is shaped like a bamboo shoot. 
     In the externally folded state shown in  FIG. 17(   b ), the airbag  42  is folded such that the folded portions Z, the first folds L, the third folds T, the above-mentioned parts β, and the lower outer portion  42 Lout and the lower inner portion  42 Lin of the airbag lower portion  42 L are located outside the airbag upper portion  42 U. Thus, the airbag  42  is shaped like a bamboo shoot. 
     As described above, in Embodiment  3  shown in  FIGS. 17(   a ) and ( b ) (corresponding to claim  5 ), third folds T located below the first folds L and extending generally in the front-rear direction of the seat  25  are provided in the bottom part of the loin protection portion (waist guard portion B); and parts α or β. of the loin protection portion, which are located below the third folds T when the airbag is developed, are folded (see  FIG. 17)  internally (see  FIG. 17 ,  a ) or externally (see  FIG. 17 ,  b ). 
     According to this structure, the third folds T are provided, and the parts α or β of the loin protection portion, which are located below the third folds T when the airbag is developed, are folded internally or externally. Therefore, the size in the top-bottom direction of the airbag  42  in the folded state can be decreased, and neither the third folds T nor the parts α and β of the loin protection portion inhibit the airbag from being developed. 
     In addition, the bottom part of the loin protection portion (waist guard portion B) is also folded internally (see  FIG. 17 ,  a ) or externally (see  FIG. 17 ,  b ). Therefore, the bottom part of the loin protection portion (waist guard portion B) is expanded and developed generally right downward by the pressure of gas from the inflator  50 . Thus, the loin protection portion (waist guard portion B) is expanded and developed more quickly. 
     In  FIG. 17 , identical elements as those in  FIG. 15  bear the identical reference signs thereto and detailed descriptions thereof are omitted. 
     Embodiment 4 
       FIGS. 18 through 23  show still another embodiment of side airbag device. 
       FIGS. 18 through 23  also show an internally folded structure, but the structure shown in  FIGS. 18 through 24  is different from that of Embodiment 1 shown in  FIGS. 3 through 7  in the method of folding the airbag  42 . 
     A bag section folding method for folding the airbag  42  in a pre-folded state shown in  FIG. 19  into a state shown in  FIG. 23  will be described. 
     As shown in  FIG. 19  and  FIG. 20 , a top end a of the airbag upper portion  42 U is turned downward along a fold line k to form a folded portion c. The fold line b shown in  FIG. 3  is generally horizontal, whereas the fold line k is inclined so as to have a higher front end and a lower rear end. 
     Next, from the state shown in  FIG. 20 , a front edge f of the airbag  42 , which is located on the front side when the airbag  42  is developed, is turned toward the inflator  50  sequentially along fold lines n, no and p extending in the top-bottom direction (see  FIG. 21  and  FIG. 22 ). Thus, the airbag  42  is first put into an intermediate state shown in  FIG. 21  and then is put into a lengthy shape as shown in  FIG. 22  (first step). 
     Next, from the state shown in  FIG. 22 , the airbag  42  is folded as shown in  FIG. 23  such that a bottom part of the airbag upper portion  42 U and a top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle to form an overlap portion OL, and such that a bottom end r of the airbag lower portion  42 L is located below a top end of the airbag lower portion  42 L (second step). 
       FIG. 23  is a side view of the airbag  42  in the folded state, and  FIG. 15(   a ) is a schematic view thereof. As shown in  FIG. 23  and  FIG. 15(   a ), in the folded state of the airbag  42 , the top end of the airbag lower portion  42 L has two first folds L extending generally in the front-rear direction of the seat  25 , and a bottom end of the airbag upper portion  42 U has two second folds U extending generally in the front-rear direction of the seat  25 . The first folds L are located above the second folds U, and a portion between the first folds L and the second folds U is an overlap portion OL. 
     In Embodiment 4 shown in  FIG. 23  and  FIG. 15(   a ), the airbag  42  is folded internally (put into an internally folded state), and therefore the details of the folded state of the airbag  42  are as follows. 
     As shown in  FIG. 15(   a ), the lengthy airbag upper portion  42 U includes an upper outer portion  42 Uout located on the outer side in the width direction of the vehicle and an upper inner portion  42 Uin located on the inner side in the width direction of the vehicle. Between the first folds L and the second folds U. folded portions Z are formed. As shown in  FIG. 15(   a ), the airbag is folded internally such that the upper and lower folds L and U and the folded portions Z are held between the upper outer portion  42 Uout and the upper inner portion  42 Uin of the airbag upper portion  42 U. 
     The inflator  50  is integrally provided with a pair of, namely, top and bottom attachment bolts (see the figure above) extending through the upper inner portion  42 Uin of the airbag upper portion  42 U and inward in the width direction of the vehicle. The inflator  50  is attached to the seat back side frame  32  as shown in  FIG. 18  via nuts tightened to the pair of attachment bolts. This way, the side airbag device  40  is built in the seat back  27  of the seat  25  for the vehicle. 
     In addition, as shown in  FIG. 23 , the airbag  42  is folded internally such that a rear edge  42 Ub of the airbag upper portion  42 U is located rear to a rear edge  42 Lb of the airbag lower portion  42 L in the front-rear direction of the vehicle. Between the rear edge  42 Ub of the airbag upper portion  42 U and the rear edge  42 Lb of the airbag lower portion  42 L, a step  45  is formed generally in the front-rear direction of the seat  25 . Thus, as shown in  FIG. 18 , the airbag lower portion  42 L is located so as not to interfere with the lumbar shaft  34 . 
     As described above, the side airbag device in Embodiment 4 shown in  FIGS. 18 through 23  and  15 ( a ) (corresponding to claims  1 ,  2 ,  3 ,  6 ,  7 ,  8 ,  9 ,  10  and  11 ) is a side airbag device accommodated in a seat back  27  of a seat  25  for a vehicle. The side airbag device includes an airbag  42 ; and an inflator  50  for supplying gas to the airbag  42  to develop the airbag  42  at the time of a crash of the vehicle. The airbag  42  includes an airbag upper portion  42 U including a chest protection portion (bust guard portion A) for protecting the chest of a passenger sitting on the seat  25  when the airbag  42  is developed and an airbag lower portion  42 L including a loin protection portion (waist guard portion B) for protecting the loins of the passenger sitting on the seat  25  when the airbag  42  is developed. The airbag is accommodated in the state of being folded such that a bottom part of the airbag upper portion  42 U and a top part of the airbag lower portion  42 L overlap each other (overlap portion OL) in the width direction of the vehicle, and such that a bottom end r of the airbag lower portion  42 L is located below a top end of the airbag lower portion  42 L. (See  FIGS. 18 ,  19  and  23 .) 
     According to this structure, the airbag is accommodated in the state of being folded such that the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle, and such that the bottom end r of the airbag lower portion  42 L is located below the top end of the airbag lower portion  42 L. Therefore, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag  42  can be located between the loins of the passenger and the door trim  19  (see  FIG. 1 ) with certainty to protect the passenger. 
     In summary, the loin protection portion (waist guard portion B) of the airbag  42  is required to have a function of being expanded and developed more quickly than the chest protection portion (bust guard portion A) and, at the time of a side crash, first urging the body of the passenger (especially, the loins) inward in the vehicle chamber. Such a requirement can be fulfilled by the above-described structure. 
     In the state where the airbag  42  is folded, the top end of the airbag lower portion  42 L has first folds L extending generally in the front-rear direction of the seat; the bottom end of the airbag upper portion  42 U has second folds U extending generally in the front-rear direction of the seat; and the first folds L are located above the second folds U (see  FIG. 23 ). 
     According to this structure, since the first folds L are located above the second folds U, the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle. The bottom end r of the airbag lower portion  42 L is located below the top and of the airbag lower portion  42 L. Therefore, in the same manner as described above, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash, the airbag can be located between the loins of the passenger and the door trim  19  with certainty. 
     In the state where the airbag  42  is folded, a front edge f of the airbag  42 , which is located on the front side when the airbag  42  is developed, is turned toward the inflator  50 , so that the airbag  42  has a shape lengthy in the top-bottom direction (see  FIG. 22 ); the lengthy airbag upper portion  42 U includes an upper outer portion  42 Uout located on the outer side in the width direction of the vehicle and an upper inner portion  42 Uin located on the inner side in the width direction of the vehicle; the top end of the airbag lower portion  42 L has first folds L extending generally in the front-rear direction of the seat; the bottom end of the airbag upper portion  42 U has second folds U extending generally in the front-rear direction of the seat; folded portions Z are formed between the first folds L and the second folds U; and the airbag  42  is folded internally such that the first folds L. the second folds U. and the folded portions Z are held between the upper outer portion  42 Uout and the upper inner portion  42 Uin (see  FIGS. 19 ,  22  and  15 ( a )). 
     According to this structure, the airbag  42  is folded internally (put into an internally folded state) in the state where the folds Land U and the folded portions Z are held between the upper outer portion  42 Uout and the upper inner portion  42 Uin of the airbag upper portion  42 U. 
     Even in the case where the airbag  42  is in such an internally folded state, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash, the airbag  42  can be located between the loins of the passenger and the door trim  19  with certainty. 
     In addition, in the state where the side airbag device  40  is accommodated in the seat back  27 , the airbag is folded such that the rear edge  42 Ub of the airbag upper portion  42 U is located rear to the rear edge  42 Lb of the airbag lower portion  42 L. Thus, the step  45  is formed generally in the front-rear direction of the seat between the rear edge  42 Ub of the airbag upper portion  42 U and the rear edge  42 Lb of the airbag lower portion  42 L. (See  FIG. 18  and  FIG. 23 .) 
     This structure has the following effects. 
     When the airbag lower portion  42 L is merely folded upward, the step  45  is not formed. When the airbag is folded such that the rear edge  42 Ub of the airbag upper portion  42 U is located rear to the rear edge  42 Lb of the airbag lower portion  42 L as in the above-described structure, the step  45  can be formed between the rear edges  42 Ub and  42 Lb. This step  45  can be effectively used. 
     A side airbag device in Embodiment 4 described above is a side airbag device  40  accommodated in a seat back  27  of a seat  25  for a vehicle. The side airbag device  40  includes an airbag  42 ; and an inflator  50  for supplying gas to the airbag  42  to develop the airbag  42  at the time of a crash of the vehicle. The airbag  42  includes an airbag upper portion  42 U including a chest protection portion (bust guard portion A) for protecting the chest of a passenger sitting on the seat  25  when the airbag  42  is developed and an airbag lower portion  42 L including a loin protection portion (waist guard portion B) for protecting the loins of the passenger sitting on the seat  25  when the airbag  42  is developed. The airbag  42  is folded in the state where an inner path X of the airbag upper portion  42 U and an inner path Y of the airbag lower portion  42 L communicate to each other generally linearly in the top-bottom direction. (See  FIGS. 18 and 15(   a ).) 
     According to this structure, the airbag  42  is folded in the state where the inner path X of the airbag upper portion  42 U and the inner path Y of the airbag lower portion  42 L communicate to each other generally linearly in the top-bottom direction. Therefore, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (waist guard portion B) is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag  42  can be located between the loins of the passenger and the door trim  19  with certainty to protect the passenger. 
     Furthermore, a method for folding a bag section of a side airbag device in Embodiment 4 described above is a method for folding a bag section (airbag  42 ) of a side airbag device including an airbag upper portion  42 U including a chest protection portion (bust guard portion A) for protecting the chest of a passenger sitting on a seat  25  when the bag section is developed and an airbag lower portion  42 L including a loin protection portion (waist protection portion B) for protecting the loins of the passenger sitting on the seat  25  when the bag section is developed. The method includes a first step (see  FIG. 22 ) of turning a front edge f of the bag section, which is located on the front side in the vehicle when the bag section is developed, toward an inflator  50  located on the rear side of the vehicle when the bag section is developed, so that the bag section has a lengthy shape; and a second step (see  FIG. 23 ) of folding the bag section such that a bottom part (see the second folds U) of the airbag upper portion  42 U and a top part (see the first folds L) of the airbag lower portion  42 L overlap each other (see the overlap portion OL) in a width direction of the vehicle, and such that a bottom end r of the airbag lower portion  42 L is located below a top end of the airbag lower portion  42 L. (See  FIGS. 19 ,  22  and  23 .) 
     According to this structure, in the first step, the front edge f of the bag section (see the airbag  42 ) is turned toward the inflator  50 , and thus the bag section has a lengthy shape. In the second step, the bag section is folded such that the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other in the width direction of the vehicle, and such that the bottom end r of the airbag lower portion  42 L is located below the top end of the airbag lower portion  42 L. 
     As described above, the bag section is folded such that the bottom part of the airbag upper portion  42 U and the top part of the airbag lower portion  42 L overlap each other (see the overlap portion OL) in the width direction of the vehicle, and such that the bottom end r of the airbag lower portion  42 L is located below the top end of the airbag lower portion  42 L. Therefore, when the airbag  42  is expanded and developed, gas is supplied to the airbag  42  generally right downward, and the airbag lower portion  42 L including the loin protection portion (see waist guard portion B) is expanded generally right downward. This allows the loin protection portion to be expanded and developed quickly. Hence, at the time of a side crash of the vehicle, the airbag  42  can be located between the loins of the passenger and the door trim  19  (see  FIG. 1 ) with certainty to protect the passenger. 
     In summary, the loin protection portion (waist guard portion B) of the airbag  42  is required to have a function of being expanded and developed more quickly than the chest protection portion (bust guard portion A) and, at the time of a side crash, first urging the body of the passenger inward in the vehicle chamber. Such a requirement can be fulfilled by the above-described folding method. 
     In Embodiment 4 shown in  FIGS. 18 through 23  also, the other elements of the structure, the function and the effect are substantially the same as those in the above-described embodiments. Therefore, in  FIGS. 18 through 23 , identical elements as those in the previously described figures bear the identical reference signs thereto and detailed descriptions thereof are omitted. 
     The elements according to the present invention and the elements in the above-described embodiments correspond to each other as follows. 
     The chest protection portion according to the present invention corresponds to the bust guard portion A in the embodiments; 
     the loin protection portion according to the present invention corresponds to the waist guard portion B in the embodiments; 
     the partition according to the present invention corresponds to the seam  43  in the embodiments; and 
     the overlap area according to the present invention corresponds to the overlap portion OL in the embodiments. 
     However, the present invention is not limited merely to the above-described embodiments. 
     For example, as the partition, partitioning cloth may be used instead of the seam  43 . 
     When the front edge f of the airbag  42  is turned toward the inflator  50 , the front edge f may be turned such that an opening made by the upper outer portion  42 Uout of the airbag upper portion  42 U, the lower outer portion  42 Lout of the airbag lower portion  42 L, the upper inner portion  42 Uin of the airbag upper portion  42 U and the lower inner portion  42 Lin of the airbag lower portion  42 L is directed forward of the vehicle. This is made possible when the zigzag folding structure shown in  FIG. 16(   b ) is adopted. 
     The side airbag device having the above-described structure is also applicable to a front seat located on the right side of the vehicle or a rear seat. 
     INDUSTRIAL APPLICABILITY 
     The present invention is usable for a side airbag device accommodated in a seat back of a seat for a vehicle and expanded and developed at the time of a side crash for protecting a passenger, and a method for folding a bag section thereof. 
     REFERENCE SIGNS LIST 
     
         
           1  . . . Vehicle chamber 
           19  . . . Door trim 
           20  . . . Armrest 
           25  . . . Seat (seat for vehicle) 
           27  . . . Seat back 
           32  . . . Seat back side frame 
           42  . . . Airbag 
           42 L . . . Airbag lower portion 
           42 Lin . . . Lower inner portion 
           42 Lout . . . Lower outer portion 
           42 U . . . Airbag upper portion 
           42 Uin . . . Upper inner portion 
           42 Uout . . . Upper outer portion 
           43  . . . Seam (partition) 
           45  . . . Step 
           50  . . . Inflator 
         A . . . Bust guard portion (chest protection portion) 
         B . . . Waist guard portion (loin protection portion) 
         OL . . . Overlap portion (overlap area) 
         L . . . First fold 
         U . . . Second fold 
         T . . . Third fold 
         X, Y . . . Inner path 
         Z . . . Folded portion 
       
    
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the part that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.