Air bag system and method of folding an air bag

An air bag system is provided that limits the forward projection of the air bag and reduces production costs wherein folded portions are formed by folding the peripheral portion of a flatly spread air bag along folding lines which are provided at specified locations. The formed folded portions are constrained by stitched portions which are formed by stitching folded-down portions with thread. The air bag is then folded into a size so as to be constrained in a cover. When gas is injected from an inflator, the air bag tears the cover and projects forward and at the same time the stitched portions limit the distance by which the air bag projects forward. As the internal pressure in the air bag increases, the stitched portions break and the folded portions are unfolded so that the air bag expands radially.

FIELD OF THE INVENTION 
The present invention relates to an air bag system for protecting a 
passenger of a vehicle or the like from a shock resulting from a collision 
of same. The invention also relates to a method of folding such an air 
bag. 
BACKGROUND OF THE INVENTION 
An example of conventional air bag systems is an air bag system for 
protecting a driver, which is incorporated in the center pad of a boss 
portion of the steering wheel of a vehicle This air bag system essentially 
comprises a base plate fastened to the body of the steering wheel, a 
basically cylindrical gas generator (generally called an inflator) mounted 
on the base plate, a pouch-shaped air bag so folded as to cover the 
inflator, and a cover so attached to the base plate as to cover the air 
bag. With the structure as above, when the control unit of the system 
detects a shock which may result from a collision of the vehicle, the air 
bag system actuates the inflator to cause a large quantity of inert gas to 
be instantaneously ejected from the inflator into the air bag, thereby 
unfolding and inflating the air bag. The pressure of the expansion of the 
air bag tears the cover and projects the air bag towards the front of the 
driver, where the air bag continues to be unfolded and inflated at the 
same time to protect the driver by thus reducing the shock received by the 
driver. 
A need exists for the air bag of an air bag system described above to be so 
constructed and arranged in the system as to project from the cover 
speedily by tearing the cover instantaneously and to be inflated into a 
specified shape, such as a wide, flat shape spreading in front of the 
driver. 
An example of structures dealing with the above problems is disclosed in 
Japanese Patent Laid-open No. 127330/1994, which uses straps having a 
specified length and sewed at the two ends of each strap to the inner face 
of the front and the bottom fabric materials of the air bag in order to 
limit the height (the distance) by which the air bag projects forward, 
towards the person seated in front of the air bag system (hereinafter 
simply referred to as a passenger or the passenger) and allow the air bag 
to smoothly expand in the radial direction (in the direction of the 
circumferential edge of the air bag). However, the above structure that 
uses straps bridging the inner surface of the fabric materials 
necessitates a complicated sewing operation to sew the fabric materials as 
above and consequently increases production costs. As the straps receive a 
great load when the air bag is inflated, the straps have to be ensured to 
have a sufficient strength. This, too, increases production costs. 
Furthermore, as the straps inside the air bag increase the volume of the 
whole folded air bag, it is difficult to provide a compact air bag system. 
Another example of air bag systems is disclosed in Japanese Patent 
Laid-open No. 305387/1994. An air bag disclosed therein has a stitched 
portion where the front fabric material and the back fabric material of 
the air bag are sewed together at a specified location or locations in the 
state that the bag is spread out in a circular state. Thereafter, tie air 
bag is folded up into a compact shape and disposed in a housing. According 
to the above structure, when gas is ejected into the air bag, the stitched 
portions limit the forward inflation of the air bag, thereby allowing the 
air bag to smoothly expand in the radial direction, towards its 
circumferential edge. Then, at a specified stage during the process of the 
inflation of the air bag, the threads of the stitched portions are tom so 
that the air bag expands forward, in the direction of the passenger. 
However, the structure that calls for folding an air bag after sewing 
together the front and the back fabric materials presents a problem in 
that it is difficult to make use of gas pressure effectively to tear the 
cover, because expansion of the air bag is limited until the threads are 
broken. Furthermore, as the air bag expands forward when the increased 
internal pressure in the air bag has broken the threads, it is necessary 
to conduct thorough research and investigations concerning various factors 
such as the radial dimension of the air bag, how to fold the air bag and 
the breaking strength of the stitching thread in order to appropriately 
control the distance which the air bag is projected forward so as to 
expand the air bag widely and evenly. This means that reduction of 
production costs is difficult according to the above structure. 
Another example structure wherein the fabric materials of an air bag are 
sewed together is disclosed in Japanese Patent Laid-open No. 329664/1995, 
which calls for folding the peripheral edge of an air bag inward and 
stitching this folded portion at a stitched portion. This structure, 
however, has a drawback wherein reduction of production costs is difficult 
because not only is a complicated procedure of turning the air bag inside 
out necessary but it is also difficult to smooth out the stitched portions 
to eliminate wrinkles. 
Another example is disclosed in Japanese Patent Laid-open No. 277129/1995, 
which calls for providing breakable ear portions around the 
circumferential edge of an air bag, bending these ear portions towards the 
bottom face of the air bag and secure them by means of bolts which fasten 
the air bag and the inflator. According to this structure, however the ear 
portions formed at the peripheral edge of the air bag are fastened to the 
bottom of the system. Therefore, when the air bag is inflated, the ear 
portions and the portions of the air bag fabric which are rolled together 
with the ear portions are pushed against the base plate or the inner 
surface of the cover, preventing the ear portions from breaking smoothly, 
and consequently making it difficult for the air bag to be unfolded 
smoothly. 
As described above, the above structure that uses straps bridging the inner 
surface of the fabric material at two locations necessitates a complicated 
sewing operation to sew the fabric in such a manner and consequently 
increases production costs. As the straps receive a great load when the 
air bag is inflated, the straps have to be ensured to have a sufficient 
strength, which, too, increases production costs, Furthermore, as the 
provision of the straps inside the air bag increase the volume of the 
whole folded air bag, the above structure presents another problem in that 
it is difficult to provide a compact air bag system. The structure that 
calls for sewing the fabric materials at the front and the back sides of 
the air bag together and subsequently folding the air bag, too, has a 
drawback wherein making use of gas pressure effectively to tear the cover 
is difficult, because expansion of the air bag is limited until the thread 
is broken. In addition, as the air bag expands forward when the increased 
internal pressure in the air bag has broken the thread, it is necessary to 
conduct thorough research and investigations concerning various factors 
such as the radial dimension of the air bag, how to fold the air bag and 
the breaking strength of the stitching thread in order to appropriately 
control the distance which the air bag is projected forward so as to 
expand the air bag widthwise and evenly, and such criteria makes reduction 
of production costs difficult. Furthermore, the structure that calls for 
folding the peripheral edge of an air bag inward and stitching the folded 
portion at a stitched portion makes it difficult to reduce production 
costs because not only is a complicated procedure of turning the air bag 
inside out necessary but it is also difficult to smooth out the stitched 
portion to eliminate wrinkles According to the structure that calls for 
providing breakable eat portions at the circumferential edge of an air 
bag, bending these ear portions towards the bottom of the air bag and 
fastening them by means of bolts which secure the air bag and the 
inflator, the ear portions and the portions of the air bag fabric which 
are rolled together with the ear portions are pushed against the base 
plate or the inner surface of the cover when the air bag is inflated. As a 
result the ear portions are prevented from breaking smoothly, and it is 
therefore difficult for the air bag to be unfolded smoothly. 
In order to solve the above problems, an object of the present invention is 
to provide an air bag system and a method of folding an air bag which are 
capable of rapidly unfolding and inflating an air bag. 
SUMMARY OF THE INVENTION 
An air bag system of the invention includes a pouch-shaped air bag to be 
unfolded and inflated by gas introduced thereinto, the air bag having 
folded portions formed by folding the peripheral portion of the flatly 
spread air bag towards the center of the air bag, and each folded portion 
stitched to the flat portion of the air bag at a constraint portion 
located within the folded portion. 
According to the above structure, when gas flows into the air bag, the part 
surrounded by the stitched portions projects forward by a specified 
distance, and, when the internal pressure in the air bag reaches a 
specified value, the stitched portions are released and the folded 
portions are unfolded and inflated outward. As an air bag according to the 
invention can be produced simply by folding a conventional air bag at 
specified positions and stitching the folded portions, the production 
process can be simplified. 
An air bag system of the invention includes a pouch-shaped air bag to be 
unfolded and inflated by gas introduced thereinto, a cover for containing 
the air bag in a folded state, and a supporting member for supporting the 
air bag, wherein the air bag includes folded portions formed by folding 
the peripheral portion of the air bag towards the side where the 
supporting member is located, constraint portions for so constraining the 
respective folded portions as to release the folded portions at a 
specified pressure, a first-stage inflated portion located within the area 
defined by the constraint portions and to be inflated by gas introduced 
thereinto, and a second-stage inflated portion to be inflated outside the 
constraint portions. 
According to the above structure, when gas flows into an air bag, the 
first-stage inflated portion is inflated as a smaller air bag which 
projects forward by a specified distance and projects from the cover by 
tearing or otherwise opening the cover. The second-stage inflated portion 
expands in the radial direction around the first-stage inflated portion. 
Then, when the internal pressure in the air bag reaches a specified value, 
the constraint portions are released and the folded portions are unfolded 
and inflated outward so that the first-stage and second-stage inflated 
portions expand together into a specified shape. 
An air bag system is disclosed, wherein the folded portions are so arranged 
as to expand in a specified direction or directions when the constraint 
portions are released. 
According to the above structure, as the folded portions expand in a 
specified direction or directions when the constraint portions are 
released, the direction or directions of expansion of the air bag can 
easily be controlled. 
An air bag system is disclosed, wherein the constraint portions comprise 
stitch lines formed by stitching together the fabric materials that 
constitute the air bag along the outline of the folded air bag, each 
stitch line having a length not exceeding a half of the length of the 
folded portion on which the stitch line is formed. 
According to the above structure, the constraint portions can be easily 
formed by stitching together the fabric materials of an air bag which has 
been flatly spread and then folded. Furthermore, each constraint portion 
can easily be broken by so limiting its length as not to exceed a half of 
the length of the folded portion. 
An air bag system is disclosed, wherein in the state that the first-stage 
inflated portion of the air bag has expanded and projected forward from 
the opening that has been formed in the cover due to the expansion of the 
first-stage inflated portion, the constraint portions are located outside 
the aforementioned opening. 
According to the above structure, unfolding and inflation of the 
second-stage inflated portions take place outside the cover, and, when the 
constraint portions are broken, the second-stage inflated portions 
smoothly expand in the radial direction. 
A method of folding an air bag of the invention includes folding a 
pouch-shaped air bag to be unfolded and inflated by gas introduced 
thereinto, said method calling for flatly spreading the air bag, folding 
the peripheral portion of the air bag towards the portion through which 
gas is introduced, constraining the folded portions in such a manner as to 
be capable of being released, and folding up the air bag into an 
appropriate shape to be contained in the space serving as its housing. 
According to the above method, when gas flows into the air bag, at first 
the air bag except for the parts where it is folded and constrained is 
unfolded and expand and rapidly projects from a cover placed over the air 
bag by breaking or otherwise opening the cover. However, the distance by 
which the air bag projects forward at that time is limited because the 
folded portions are constrained. Then, when the internal pressure in the 
air bag reaches a specified value, the constraint portions are released so 
that the folded portions are unfolded and inflated outward. 
The above, and other objects, feature and advantages of the present 
invention will become apparent from the following description read in 
conjunction with the accompanying drawings in which like reference 
numerals designate the same element and the scope of the invention will be 
indicated in the claims.

DETAILED DESCRIPTION OF THE INVENTION 
Next, an embodiment of an air bag system according to the invention and a 
method of folding the air bag of said air bag system are explained 
hereunder, referring to the drawings. 
Referring to FIG. 2, numeral 1 denotes an air bag system, which is mounted 
on a boss portion of tie body of the steering wheel (not shown) of a 
vehicle so as to protect a passenger of the vehicle (the driver in case of 
this embodiment) from the shock of a collision. The body of the steering 
wheel is normally used at an angled position, being attached to a steering 
shaft whose inclination can be adjusted to an appropriate angle. In the 
explanation hereunder, the side of the steering wheel facing the driver, 
in other words the side where air bag system 1 is provided, is referred to 
as the upper side or the front side, while the side facing the body of the 
vehicle is referred to as the lower side or the bottom side. 
An air bag system 1 essentially comprises a base plate 11 serving as a 
supporting member, a cover 12 to be attached to base plate 11, an air bag 
15, and an inflator 16. 
Base plate 11 may be formed by pressing a metal plate and consists of a 
generally rectangular flat base portion 21 (a flat, generally square shape 
in case of the present embodiment) and a side plate portion 22 which is 
formed as an integral body with base portion 21 by bending the peripheral 
edge of the base portion downward. Base portion 21 is provided with a 
circular through hole 23 to accept the inflator (hereinafter referred to 
as inflator through hole 23) and a plurality of through holes 24 which are 
provided around inflator through hole 23 as shown in, for example, FIG. 3. 
Side plate portion 22 has a bracket portion (not shown) which is fastened 
to the body of the steering wheel. 
Cover 12 is formed of a synthetic resin as an integral body that comprises 
a curved cover portion 26 and a mounting portion 27, which is in the shape 
of a square tube projecting downward from the lower surface of cover 
portion 26. Cover portion 26 covers the boss portion and a part of the rim 
portion of the body of the steering wheel The space defined by the 
underside of cover portion 26 and the inner wall of mounting portion 27 
serves as the housing of an air bag 15. Facing this housing, a weak tear 
line 28, which may be H-shaped or in any other suitable shape, is formed 
in the back surface of cover portion 26. Mounting portion 27 is fitted 
around the outer surface of base plate 11 and affixed thereto by means of 
a plurality of rivets or the like. 
Air bag 15 is formed into a flat bag by sewing upper and lower fabric 
materials 31,32 together around the edge. Of the two fabric materials that 
are in a flat, circular shape, lower fabric material 32 has at the center 
a circular hole, which is a gas inlet opening 34 serving as the gas inlet, 
and a plurality of through holes 35 bored around gas inlet opening 34 as 
shown in, for example, FIG. 3. Lower fabric material 32 also has a 
plurality of vent holes 37 to discharge gas out of air bag 15. In the 
state where gas inlet opening 34 is aligned with inflator through hole 23 
of base plate 11, air bag 15 is pressed against base plate 11 by means of 
an annular retainer 38 disposed in air bag 15 with bolts 39 projecting 
downward from the underside of retainer 38 through respective through 
holes 24,35. In this state, air bag 15 is folded into a specified shape 
and contained in the housing in cover 12. 
Inflator 16 comprises a generally cylindrical main body 41, which is 
provided with a flange portion 42 projecting outward and a plurality of 
gas injection holes 43 for injecting gas, flange portion 42 formed around 
the cylindrical wall of main body 41. Inflator 16 structured as above is 
secured by pushing main body 41 from underneath base plate 11 through 
inflator through hole 23 into gas inlet opening 34 of air bag 15 until 
flange portion 42 comes into contact with base portion 21 of base plate 
11, and fastening flange portion 42 in this state to base plate 11 with 
nuts (not shown) tightened around bolts 39. 
Next, the procedure of folding air bag 15 is explained referring to FIGS. 3 
to 6. 
First of all, as shown in FIG. 3, air bag 15 is flatly spread with upper 
and lower fabric materials 31,32 overlapping each other in close contact. 
In this state, the edge of air bag is folded downward along four folding 
lines 51 consisting of two pairs of opposing lines that constitute the 
four sides of the square inscribed in the peripheral edge of air bag 15, 
thereby forming four folded portions 53. The folded air bag 15 is in the 
shape shown as the first configuration 55 in FIG. 4, whose outline is a 
square similar to the plane of base plate 11. Then, stitched portions 56 
(or stitch lines 56) that serve as constraint portions are formed by 
stitching each folded portion 53 along folding lines 51, each folding line 
51 corresponding to each side of first configuration 55. Stitch lines 56 
extend parallel to respective folding lines 51 at a specified distance 
therefrom. The portion inside of these stitched portions 56 will be the 
first-stage inflated portion, while folded portions 53 and the outer 
portion surrounding these stitched portions 56 will serve as the 
second-stage inflated portion. 
Then, first configuration 55 is folded inward at the two lateral ends as 
shown in FIG. 5 (hereinafter referred to as the lateral ends), and 
subsequently at the upper and lower ends as shown in FIG. 6 (hereinafter 
referred to as the vertical ends or the upper and the lower ends). By thus 
folding air bag 15 compact so that it can be placed on base portion 21 of 
base plate 11, the procedure of folding air bag 15 is completed. 
Next, the operation of air bag 15 is explained referring to FIG. 1, in 
which the illustration of cover 12 is omitted in order to explain how air 
bag 15 is spread and inflated. 
When a vehicle equipped with said air bag system 1 receives a shock 
resulting from a collision of the vehicle, a control unit (not shown) 
incorporated in air bag system 1 or in the body of the vehicle detects the 
shock and feeds the power to inflator 16 to be actuated so that gas is 
suddenly injected through gas injection holes 43 of inflator 16 into 
folded air bag 15. As a result, air bag 15 is inflated and unfolded, 
thereby tearing cover 12 along tear line 28 and opening cover portion 26 
of cover 12 like doors. As a breakthrough opening has thus been formed, 
air bag 15 is projected from cover 12 through this opening and continues 
to be unfolded and inflated at the same the in front of the driver. 
At the first stage of the aforementioned process of being unfolded and 
inflated, air bag 15 begins to be unfolded from the folded portions at the 
four ends while it begins to be inflated from the portion near gas inlet 
opening 34, in other words the first-stage inflated portion, in the 
forward direction. During this first unfolding stage, airbag 15 is 
constrained with folded portions 53 sewed down at respective stitched 
portions 56, which have a specified strength. Therefore, air bag 15 
functions in a sense as a smaller air bag whose top view is the first 
configuration 55; as shown in FIG. 1 (a), it is inflated only to specified 
dimensions of width D1 and height h1. As a result, the pressure is 
concentrated to the middle portion of air bag 15, allowing air bag 15 to 
smoothly tear line 28 and rapidly project out of cover 12. 
Then, when the internal pressure in air bag 15 exceeds a specified value 
due to the gas supplied from inflator 16, threads of stitched portions 
break, thereby releasing air bag 15. This initiates the second stage of 
inflation, wherein folded portions 53, i.e. the second-stage inflated 
portions, instantly rotate, allowing the gas to flow in the radial 
direction towards the sides of the air bag (in the directions represented 
by allows X in FIG. 1 (b), in other words towards the peripheral edge of 
the air bag) so that air bag 15 expands to the maximum dimensions of width 
D2 and height h2. At that time, expansion of folded portions 53 projects 
air bag 15 more radially than forward, because the tensile force of the 
radial expansion limits the expansion in the forward direction. 
Furthermore, as the second-stage inflated portion is sewed together at the 
middle, the air bag smoothly expands towards the circumferential edge of 
the air bag as if being divided into a plurality of chambers. 
As described above, according to the structure of the present embodiment of 
air bag system 1, wherein air bag 15 is folded and sewed together at the 
circumferential end and subsequently folded to be placed in the housing, 
air bag 15 is capable of rapidly breaking cover 12 and projecting 
therefrom in the initial stage of expansion. When stitched portions 56 
constraining air bag 15 are broken, as air bag 15 expands mainly towards 
its circumferential edge, thereby limiting the forward projection, air bag 
15 is prevented from projecting prominently and narrowly forward. 
Other benefits of tie present embodiment lie in that as forward projection 
of air bag 15 can be limited without requiring a particular member such as 
a strap to bind a fabric material of the air bag, it is capable of 
reducing production costs by reducing parts (materials), simplifying the 
production process, increasing yield and so forth, and also reducing the 
weight and the volume of the folded air bag. 
Another benefit of the embodiment lies in that air bag 15 in the 
constrained state is formed into the shape shown as the first 
configuration 55 in FIG. 4, which resembles the top view of base plate 11. 
This structure not only permits the air bag to function effectively as a 
smaller air bag when unfolded but also to be folded smoothly and easily, 
because there is no need of starting the folding operation from the state 
where the air bag is spread to its maximum size. 
Furthermore, as an air bag having the structure according to the embodiment 
can be produced simply by folding a conventional air bag at specified 
positions and stitching the folded portions, the structure of the present 
embodiment is capable of reducing production costs by the simplified 
production process. 
As an air bag 15 explained above is formed by sewing together flat, 
circular fabric materials 31,32, it expands into a shape resembling a 
flattened ball so as to cover the steering wheel and effectively protect 
the driver when completely expanded. However, in cases where the air bag 
is to be attached to a different seat such as the passenger scat, it may 
be of a different shape. 
Although the invention is explained referring to the above embodiment, 
wherein air bag 15 is folded along four folding lines 51 so as to be 
formed into a flat, rectangular shape shown as the first configuration 55, 
air bag 15 may be in various shapes, such as a triangle, a pentagon or any 
one of other polygons which can be fitted in the cover. For example, as 
shown in FIG. 7, it may be a triangle having folding lines 51 and stitched 
portions 56 at three locations, or a hexagon shown in FIG. 8 which has 
folding lines 51 and stitched portions 56 at six locations. 
Although folded portions 53 and stitched portions 56 are provided generally 
in the manner of central symmetry along the entire circumference of air 
bag 15 according to the above embodiment, air bag 15 may be folded and 
stitched at a location or locations biased in a specified direction or 
directions with respect to the neutral steering position. For example, 
taking into consideration where the horizontal extension of tear line 28 
of cover 12 is located, air bag 15 may be provided with two full-size 
folded portions and constrained at stitched portions respectively formed 
at said folded portions, which are located near the vertical ends of the 
air bag. In this case, the two lateral ends may have no folded portions or 
have small folded portions and short stitches formed thereat. The above 
structure permits the air bag to not only unfold itself more easily, 
thereby breaking tear line 28 smoothly as if opening doors, but also 
expand laterally rather than in the vertical direction (in the 
fore-and-aft direction with respect to the body of the vehicle while 
limiting the forward projection (projection towards the driver) of the air 
bag. In another structure, air bag 15 may be constrained either at a total 
of three folded portions that are respectively formed near both lateral 
ends and the upper end as shown in FIG. 7 (the end closer to the 
windshield) or at these three folded portions in addition to a smaller 
folded portion which is so formed near the lower end (the end facing the 
lower front part of the driver) as to be smaller than the folded portions 
at the two lateral ends. This structure permits the air bag formed into 
the first configuration to expand smoothly towards the abdomen of the 
driver. 
Furthermore, although stitched portions 56 that use breakable threads 
constitute the constraint portions in the embodiments described above, 
constraint portions which can be released may be formed by using other 
means such as various bonding agents or a Hook-and-Loop fastener. For 
example, in a structure using a bonding agent, a constraint portion may be 
formed simply by applying the bonding agent to specified locations on 
fabric material 31 and applying pressure to those locations. In cases 
where a Hook-and-Loop fastener or fasteners are used, the fastener or 
fasteners may be attached to specified parts of fabric material 31 of air 
bag 15. As constraint portions can be easily formed by folding flatly 
spread fabric material 31 and pressing the folded portions, this structure 
is effective in reducing production costs. 
Furthermore, according to the embodiment described above, air bag 15 is 
first folded and stitched to be formed into the first configuration 55, 
and both sides are subsequently folded and then the two vertical ends are 
folded so that air bag 15 can be placed on inflator 16. However, air bag 
15 can be folded in other ways, an example of which is shown in FIGS. 9 
and 10, wherein air bag 15 is first formed into the first configuration 55 
and subsequently folded into a wave-like shape over inflator 16, and then 
folded into a shape resembling flower petals by wrapping overhanging 
portions 61 around the portion that has been folded. As there is virtually 
one layer of fabric material 31 right in front of inflator 16, tear line 
28 of cover 12 can be torn instantly by gas generated by inflator 16. 
Furthermore, by means of stitched portions or otherwise formed constraint 
portions, air bag 15 can be prevented from projecting forward in a narrow 
shape as the one represented by imaginary lines 63, and can instead by 
smoothly inflated in the radial direction as shown in solid lines 64. 
Next, another embodiment of the invention is explained hereunder, referring 
to FIGS. 11 to 15. The elements similar to those in the embodiment 
described above are identified with the same reference numerals, with the 
explanation thereof being omitted. According to the second embodiment, as 
shown in FIG. 11 (a), an air bag system 1 comprises a base plate 11, an 
air bag 15 folded into a flower-like shape, an inflator 16, and a cover 12 
mounted on case plate 11 by means of rivets R, air bag 15 and inflator 16 
fastened to base plate 11 by means of bolts of a retainer 38. 
The procedure of folding air bag 15 described above is shown in FIGS. 13 
and 14, wherein air bag 15 which is flatly spread out in a circle is 
folded down towards the bottom along four folding lines 51 into a 
generally rectangular shape pointed at the top and bottom as well as at 
the right and left ends. Each folding line 51 forms an arc-shaped folded 
portion 53 connected to the adjacent folding lines at the respective two 
ends. Then, each folded portion 53 is sewed to the flat portion of the air 
bag fabric at a stitched portion or stitch line 56 at a specified location 
and serves as a constraint portion as referred to the claims. As shown in 
FIGS. 14 and 15, each stitched portion 56 is formed by sewing together 
fabric materials 31,32 with a thread, the fabric materials overlapping 
each other in a total of four layers. Each stitched portion 56 consists of 
3 to 5 stitches, with a total length a of approximately several 
millimeters but not exceeding a half of the length of folded portion 53, 
and extends along the outer edge of folded portion 53, at some distance 
from the outer edge. Air bag 15 is stitched by machine upside down, with 
the front side facing downward. The needle thread has to be stronger than 
the bobbin thread; for example, the needle thread may be of 1260 deniers 
while the bobbin thread may be of 840 deniers. 
Each stitched portion 56 extends nearly parallel to the adjacent folding 
line 51 at a distance shorter than distance c between folding line 51 and 
the end of the folded portion formed by folding the fabric along said 
folding line 51. In other words, each stitched portion 56 is formed 
between folding line 51 and the end of its folded portion. The inner part 
surrounded by these stitched portions 56, i. e. the part gas inlet opening 
34 is located, serves as the first-stage inflated portion 71, and the part 
surrounding these stitched portions 56 serves as the second-stage inflated 
portion 72. 
Air bag 15 formed as above is then affixed to base plate 11 and folded into 
a wave-like shape by means of, for example, pressing each folded portion 
53 against the middle portion or base plate 11. By wrapping the 
overhanging portions around the body of the air bag that has thus been 
folded and placing cover 12 over air bag 15, the procedure of folding air 
bag 15 is completed. 
Next, the inflation operation of air bag system 1 described above is 
explained hereunder referring to FIGS. 11 and 12, in which (a) to (d) of 
FIG. 11 respectively illustrate the air bag at the same stages as those 
depicted in (a) to (d) of FIG. 12. 
When air bag system 1 is actuated in the state shown in FIG. 11 (a) and 
FIGS. 12 (a), gas injected from inflator 16 flows into air bag 15 so that 
the first-stage inflated portion 71 expands while each space between two 
adjacent stitched portions 56 is inflated radially as shown by arrows f1. 
As shown in FIG. 11 (b) and FIG. 12 (b), the air bag is thus inflated into 
the first-stage unfolded shape that resembles a flattened concave-sided 
diamond. Meanwhile, the gas flows between stitched portions 56 towards the 
outer part of the air bag, thereby inflating the outer part as shown by 
arrows f2 and forming the second-stage inflated portion 72. The 
second-stage inflated portion 72 consists of a first sub chamber 73 
communicating with the first-stage inflated portion 71 and having a 
U-shaped cross section, and second sub chambers 74 which are formed by the 
gas flowing between stitched portions 56 to the end of each folded portion 
53. First sub chamber 73 and second sub chambers 74 can be identified in 
the sectional view shown in FIG. 11. In the top view in FIG. 12, however, 
they appear as a single donut-like shape surrounding stitched portions 56. 
In the state where first-stage inflated portion 71 is inflated, cover 
portion 26 of cover 12 is tom along tear line 28 and opened like doors in 
the fore-and-aft direction with respect to the body of the vehicle, so 
that an opening 76 ranging tom both side portions of tear line 28 to the 
portions that serve as the hinges of the doors is formed. This opening 76 
has virtually the same dimensions as the part surrounded by mounting 
portion 27. Air bag 15 is so arranged that when the first-stage and 
second-stage inflated portions 71,72 are in the expanded state prior to 
breakage of stitched portions 56, stitched portion 56 and second-stage 
inflated portion 72 are located outside opening 76, at the front side of 
the system. 
As the gas supply continues, the pressure of the gas breaks stitched 
portions 56, so that air bag 15 is unfolded and inflated from the shape 
shown in two-dot chain line in FIG. 11 (c) to the shape shown in solid 
line in FIG. 11 (c) and FIG. 12 (c). At that time, as folded portions 53 
are unrolled in the direction represented by arrow f3, allowing high 
pressure gas to rush in the radial direction into folded portions 53, air 
bag 15 is unfolded and expands in the radial direction with virtually no 
change in height h. When the gas supply continues further, air bag 15 
expands to a somewhat flattened circular shape shown in FIG. 11 (d) and 
FIG. 12 (d) so that it is capable of snugly cushioning the driver over a 
wide area. 
As described above, while having the same effect as those of an air bag 
system according to the embodiments described above, the present 
embodiment is capable of inflating air bag widely while limiting the 
forward projection of air bag 15, which is the projection towards the 
driver, because air bag 15 is constrained at stitched potions 56 in such a 
manner as to be released whenever it is necessary. Especially because each 
stitched portion 56 is formed by stitching together four layers of fabric 
materials 31,32 at the middle of a folded portion 53 where the fabric 
materials are folded towards the bottom side, a plurality of sub chambers 
73,74 can easily be formed adjoiningly to stitched portions 56. By means 
of inflating these sub chambers 73,74, the forward projection of the air 
bag can be effectively limited. As shapes and volumes of sub chambers 
73,74 can easily be adjusted by changing length a and/or the locations of 
respective stitched portions 56, the inflation characteristics can easily 
be changed by means of such adjustment to obtain the appropriate 
characteristics. As appropriate inflation characteristics can be obtained 
by a simple means such as providing stitched portions 56 described above, 
there is no need of a complex structure such as those requiring straps. 
Therefore, the structure according to the above embodiment is capable of 
reducing production costs and the dimensions of a system. 
As a result of experiments, it has been found that limiting length a of 
each stitched portion 56 up to a half of length b of folding line 51 
adjacent thereto enables the smooth expansion of sub chambers 73,74 as 
shown in FIG. 14 and other drawings, effectively limits the forward 
projection of the air bag and also permits stitched portions 56 to break 
properly, thereby smoothly inflating air bag 15 to its fullest extent. 
According to the embodiment shown in FIG. 14, each stitched portion 56 is 
formed virtually a half way between folding line 51 and the end of its 
folded portion. However, a stitched portion 56 may be formed near the end 
of each folded portion as shown in FIG. 16 or near folding line 51 as 
shown in FIG. 17. 
Furthermore, four stitched portions 56 need not necessarily be formed in an 
identical manner; various stitching conditions may be employed for 
respective stitched portions 56 in accordance with whether there is a 
hinge part at a part of cover 12 to be opened like a door or, in an 
alternative form, stitched portions 56 may be formed only at the upper and 
the lower ends where the hinge parts are provided. 
As the forward projection of air bag 15 can be limited by stitched portions 
56, a part of second-stage inflated portion 72 stay be still inside 
opening 76 of cover 12, in other words inside the housing of air bag 15, 
when first-stage inflated portion 71 is formed. This means that, for 
example, a part of or the entire second sub chambers 74 may be inside 
cover 12. 
Furthermore, there is no need for stitched portions 56 to be always formed 
in straight lines along folding lines 51; they may be formed in the radial 
direction on air bag 15 or in a shape other than a straight line, such as 
a wave-like shape. 
In another alternative form, a stitched portion 56 may consist of a 
plurality of stitched lines. With such a structure, the process of 
inflating air bag 15 can be controlled in a number of stages. 
In addition to the arrangement of the stitched portions and the cover shown 
in FIG. 12 and other drawings according to the above embodiment, other 
arrangements are also possible, including the one shown in FIG. 18, 
wherein stitched portions 56 are provided to the front and back and left 
and right of inflator through hole 23 so that the air bag expands into an 
X-like shape at the first stage of inflation. 
Furthermore, an air by system according to the present invention can be 
used not only as an air bag system mounted on the steering wheel of a 
vehicle but also as an air bag system to be installed in other parts, 
including and not limited to the passenger seat, a seat of another type or 
a door. In addition, the invention is also applicable to reducing a shock 
received by a moving body which may not necessarily be a vehicle. 
According to the structure of an air bag system of the invention, when gas 
flows into an air bag, the middle part of the air bag surrounded by the 
portions which are stitched together projects forward by a specified 
distance, and, when the internal pressure in the air bag reaches a 
specified value, the stitched portions are released and the folded 
portions are unfolded and inflated outward. Thus, the structure is capable 
of limiting the forward expansion of the air bag and allowing the air bag 
to smoothly expand in the radial direction. As an air bag having the 
structure according to the invention can be produced simply by folding a 
conventional air bag at specified positions and stitching the folded 
portions, the invention provides a simplified production process and 
therefore is capable of reducing production costs. 
According to the structure of an air bag system of the invention, when gas 
flows into an air bag, the first-stage inflated portion is inflated and 
functions in a sense as a smaller air bag, which projects forward by a 
specified distance and projects from the cover by, for example, tearing a 
part of the cover, and the second-stage inflated portion expands radially 
around the first-stage inflated portion. Then, when the internal pressure 
in the air bag reaches a specified value, the constraint portions are 
released and the folded portions are unfolded and inflated outward so that 
the first-stage and second-stage inflated portions expand together into a 
specified shape. As described above, the air bag is inflated forward 
effectively so as to project forward from the cover rapidly and by a 
specified distance in the initial stage of expansion, and subsequently 
inflated into its final inflated shape with the folded portions unfolded 
and inflated outward when the constraint portions are released. Therefore, 
the invention provides at less production costs an air bag system whose 
air bag can be folded into a compact shape than those provided by 
conventional structures which call for attaching another member such as a 
strap by means of sewing. Furthermore, compared with conventional 
structures such as one calls for stitching together the fabrics of a 
flatly spread air bag at a specified position or positions and 
subsequently folding up the stitched air bag, another which calls for 
tucking in the hem of an air bag and stitching the tucked portion, or yet 
another structure which calls for providing breakable ear portions around 
the circumferential edge of an air bag, bending these ear portions towards 
the bottom of the air bag and securing them, an air bag system according 
to the invention is capable of unfolding and inflating an air bag more 
smoothly, and also capable of expanding folded portions smoothly in the 
radial direction, towards the circumferential edge of the air bag, when 
constraint portions are released. As an air bag having the structure 
according to the invention can be produced simply by folding a 
conventional air bag at specified positions and stitching the folded 
portions, the invention provides a simplified production process and 
therefore is capable of reducing production costs. 
According to the invention, the folded portions expand in a specified 
direction or directions when the constraint portions are released. 
Therefore, an air bag system is capable of easily controlling the 
direction or directions of expansion of the air bag is provided. 
According to the invention, the constraint portions are comprised of stitch 
lines formed by stitching together fabric materials that constitute the 
air bag. Therefore, an air bag system that the constraint portions can be 
easily formed simply by folding the fabric materials of a flatly spread 
air bag and stitching the fabrics together is provided. Furthermore, by 
forming each stitch line along the outline of the folded air bag and 
limiting the length of each stitch line to a half or less of the length of 
the folded portion on which the stitch line is formed, the stitch lines 
can be ensured to break smoothly. 
According to the invention, when the first-stage inflated portion has 
expanded and projected forward from the opening that has been formed in 
the cover due to the expansion of the first-stage inflated portion, the 
constraint portions are located outside said opening. Therefore, an air 
bag system which is capable of inflating the second-stage inflated 
portions outside the cover so that the second-stage inflated portions 
smoothly expand in thee radial direction when the constraint portions are 
broken is provided. 
According to the method of folding an air bag of the invention, when gas 
flows into an air bag, at first the air bag except for the parts where it 
is folded and constrained is unfolded and expand and rapidly projects from 
a cover placed over the air bag by tearing or otherwise opening the cover. 
The distance by which the air bag projects forward at that time is limited 
because the folded portions are constrained in that stage. Then, when the 
internal pressure in the air bag reaches a specified value, the constraint 
portions are released and the folded portions are unfolded and inflated 
outward so that the entire air bag is unfolded and expands into its final 
inflated shape. Therefore, the method as claimed in the invention is 
capable of reducing production costs of an air bag and folding the air bag 
into a compact shape compared with those provided by conventional arts 
which call for attaching another member such as a strap by means of 
sewing. Furthermore, compared with conventional methods such as one calls 
for stitching together the fabrics of a flatly spread air bag at a 
specified position or positions and subsequently folding up the stitched 
air bag, another which calls for tucking in the hem of an air bag and 
stitching the tucked portion, or yet another method which calls for 
providing breakable ear portions around the circumferential edge of an air 
bag, bending these eat portions towards the bottom of the air bag and 
securing them, the method according to the invention permits the air bag 
to rapidly project out of its cover and is also capable of limiting the 
forward projection of the air bag when the constraint portions are 
released.