Cover for air bag installation

Abstract: An automotive air bag assembly includes a sheet metal housing (12) having a cover (28) formed of a homogeneous plastic. The cover is formed with locking arms for engaging a rim (26) of the housing. Two of the locking arms (32,34) are clamped onto the rim by a clamping member (24) and two orthogonal locking arms are not so clamped. The cover defines tear lines whose thickness varies to control the sequence and direction of tear whereby doors are created whose hinges form in thickened regions of the cover material.

Description: This invention relates to air bags for automotive crash protection. More specifically, it relates to an improved cover assembly for an air bag installation. Automotive air bags are commonly installed to protect both the driver and the front seat passenger from injury in the event of a head-on collision. The term "air bag" is something of a misnomer because, typically, the bag is filled with a rapidly evolving gas produced by an ignitor. The bag is designed to inflate in no more than approximately 50 milliseconds. In its uninflated and folded condition it is mounted within a housing having a cover which is designed to tear along predetermined lines to provide an opening for the escape of the air bag. In view of the explosive nature of the air bag inflation, one or the major problems with prior art installations has been the tendency of the air bag cover to part from its housing or to break into pieces. The air bag cover or its pieces thereby become projectiles capable of inflicting damage on the occupants. Various approaches have been employed in the prior art to overcome this problem. In one such approach, the plastic cover member is strengthened by embedding therein a fabric band made of a highly tear resistant material. Such an approach, for example, is disclosed in U.S. Patent 4,334,699 of Patzelt et al. In another approach disclosed in U.S. Patent 5,092,627 of Igawa, the plastic cover is riveted to a metal plate which deforms as the cover breaks open. U.S. Patent 4,968,057 of Rafferty discloses metal sheet inserts which are also riveted to the housing. Other approaches to the solving of these problems will be found in U.S. Patents 4,148,503 of Shiratori et al., 4,852,907 of Shiraki et al., and 5,013,065 of Kreuzer. One of the major problems with all of the prior art solutions is that they are expensive to implement and result in an undesirably heavy air bag installation. Furthermore, the covers lack flexibility with the result that they are not easily assembled. Accordingly, it is a primary object of the present invention to provide an air bag installation comprising a molded cover which requires no reinforcements. Another object is to provide such a cover which is sufficiently flexible that it may be readily assembled to the air bag housing. Another object is to provide such an assembly wherein the energy of the expanding air bag is dissipated in a managed way to prevent the cover from breaking up. Another object is to provide a cover which is designed so that, as it opens, it flexes in such a manner that it becomes more firmly secured to the housing. According to a first aspect of the invention there is provided an air bag assembly for use in a motor vehicle, of the type including a rigid box shaped housing having a bottom and a side wall defining a substantially open top, a collapsed air bag within said housing, an inflator connected to inflate said air bag, and a cover closing said housing and defining tear lines adapted to be ruptured by the inflating air bag, the cover being of a substantially homogeneous material, said tear lines being regions of reduced thickness of the cover material forming a generally H shaped pattern having portions of differing thickness, wherein an outwardly extending rim on said side wall is disposed around at least a portion of said open top, a plurality of locking arms being formed integral with said cover and engaging said rim, and means being secured to the bottom of said housing for retaining at least two said locking arms in engagement with said rim during inflation of said air bag, whereby upon deployment of the air bag the thinnest tear line portions will be the first to rupture and form doors releasing the air bag, energy from the air bag being absorbed during the formation of said doors. An air bag assembly of this general type is disclosed in GB-A-2244682. The assembly of the present invention is characterised in that said tear lines comprise at least three said portions of differing thickness of which the first and thinnest tear line portions are the crossbar of the H and the regions of the vertical bars immediately adjacent the crossbar, the succeeding second portions of the vertical bars being thicker, with the end of each vertical bar terminating in a third portion defining a curve leading to a thickened edge region of the cover, the third portion having a thickness intermediate that of the first and second portions, said thickened edge regions form hinges for said doors. According to a further aspect of the invention there is provided a homogeneous moulded plastics cover for an open housing of an automotive air bag assembly which comprises: a substantially flat face portion defining therein weakened tear lines forming a generally H shaped pattern, first and second locking arms extending rearwardly from said face portion for engagement with opposed portions of an outwardly-turned rim of an air bag housing and adapted to be retained in said engagement by an external retaining member; and third and fourth locking arms extending rearwardly from said face portion and orthogonal to said first and second arms for engagement with opposed portions of said housing rim, whereby the tear line portions are arranged to rupture under the influence of an inflating air bag and absorb energy during the formation of doors releasing the air bag, energy from the air bag being absorbed during the formation of said doors, characterised in that the tear lines have at least three portions of differing thickness, of which the first and thinnest tear line portions are the crossbar of the H and the regions of the vertical bars immediately adjacent the crossbar, the succeeding second portions of the vertical bars being thicker, with the end of each vertical bar terminating in a third portion defining a curve leading to a thickened edge region of the cover, the third portion having a thickness intermediate that of the first and second portions, and in that said thickened edge regions form hinges for said doors. The thickness of the tear lines is adjusted so that the tear begins in the cross bar of the H. In a preferred embodiment, as the tear progresses, the cover deforms so that the unretained locking arms more tightly engage the housing rim. The tear line thickness beyond the cross bar increases to dissipate energy, but then thins again to direct the tear into a hinge region at a thicker portion of the molded cover. In this manner, the doors formed by the tearing of the cover are prevented from separating from the cover. Preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein: Fig. 1 is a cross-section illustrating the air bag assembly of the invention mounted within an automobile steering wheel; Fig. 2 is a front view of the assembly of the invention; Fig. 3 is a cross-section taken substantially along the line 3-3 of Fig. 2; Fig. 4 is a cross-section taken substantially along the line 4-4 of Fig. 2; Fig. 5 is a side view, on a reduced scale, of the assembly of the invention; Fig. 6 is a rear view of the assembly of Figs. 2-5; and Fig. 7 is a rear view of another version of a cover in accordance with the present invention. Referring first to Fig. 1, there is illustrated an air bag assembly 10 in accordance with the present invention, mounted within an automobile steering wheel S. The air bag assembly 10 is illustrated in detail in Figs. 2 - 4. It comprises a metal housing 12 having a circular aperture 14 in its base. An annular retaining ring 16 on the inside of the housing 12 surrounds the aperture 14 and is connected to the mouth of an air bag 18. A cup 20 is suspended beneath the aperture 14 and encloses an ignitor assembly 22. As the latter does not form a part of this invention, it will not be further described. Mounted to the bottom of the housing 12 and surrounding the cup 20 is a spider 24. As will be apparent from Figs. 2 and 6, the housing 12 is roughly rectangular in shape and its upper edge carries a recurved rim 26. The cover 28 of the housing of this invention is a molded homogeneous member in that no reinforcements are molded into it. A high pressure injection molding material such as a thermoplastic is preferred. An especially preferred material would be a terpolymer elastomer made from ethylene-propylene diene monomer such as Santoprene (a trademark of Monsanto Company). The cover 28 includes a solid face 30 which serves to enclose the housing 12 and airbag 18. Integrally molded with the cover 28 and extending rearwardly from its face 30 are four locking arms. The four locking arms are independently movable for ease of mounting to the housing. In some embodiments, the locking arms are connected by flexible sidewalls which permit such desired independent movement. One set of locking arms 32, 34 will be seen in Fig. 4. They are on opposite sides of the housing 12 and each includes a notch 36 for receiving the rim 26 and a reentrant recess 38 as will be most apparent in Figs. 4 and 6. The spider 24 which is mounted on the bottom of the housing 12 includes a pair of retaining clips 40, 42 which enter the recesses 38 and serve to retain the cover 28 in position. It will be noted that the spider 24 also includes a pair of mounting plates 44, 46. However, as these do not form a part of this invention, they will not be further described. A second pair of locking arms 48, 50 projects from the rear of the cover 28 substantially orthogonal to the retained locking arms 32, 34. Each of the locking arms 48, 50 defines a notch 52 which engages the rim 26 of the housing 12. These locking arms 48, 50 may be further retained by the spider 24, if desired. Particularly referring now to Figs. 2 and 3, it will be noted that the inner surface of the cover face 30 defines an array of tear lines. These are thin portions of the face 30 which split when the air bag 18 inflates to thereby form hinged doors 54, 56 as shown in Fig. 1. The tear lines are generally in the form of the letter H. The thinnest 58 of the tear lines represents the cross bar of the H as viewed in Fig. 2. Side tear lines 60, 62 represent the sides of the H. At each end of each side tear line 60, 62 a curved portion 64 leads into a hinge region 66, 68 adjacent one of the locking arms 32, 34. The tear lines are designed to open in a preselected sequence which serves the multiple purposes of: (1) increasing the retention force between the cover 28 and the housing 12; (2) dissipating the energy of the expanding air bag 18; and (3) preventing the newly formed doors 54, 56 from detaching from the main body of the cover 28. The tear lines within the region labelled "A" in Fig. 2 are the thinnest. The material underlying the lines in this region may be, for example, .5 mm thick. It will be seen that this includes all of the cross bar 58 and a small portion of each of the side tear lines 60, 62 adjacent the cross bar. As the bag expands, the cross bar tear line 58 begins to split at its center as the face 30 begins to bulge upwardly. The top and bottom of the cover 28, as viewed in Fig. 2, are firmly retained against the parallel edges of the housing rim 26 by the retaining clips 40, 42 of the spider 24. However, the absence of any stiffening of the cover 28 at the corners of the housing 12 causes the cover 28 to flex inwardly at its sides, thereby causing the locking arms 48, 50 seen in Fig. 3 to be forced inwardly against the housing 12. In this manner, the cover 28 resists any tendency to separate from the housing 12. The cross bar tear line 58 continues to tear along its entire length and the tear continues into the thinner regions of side tear lines 60, 62. At that point, however, the tear enters the region "B" and in this region the thickness of the tear lines essentially doubles to 1.1 - 2 mm, thereby dissipating more of the air bag energy. At the end of each of the thicker tear line portions, it is desired to channel the tear into a thick hinge area which includes the locking arms 32, 34. This is achieved by means of a tuned tensile energy dissipation line comprising the return portion 64 at each end of side tear lines 60, 62 in the region "C" shown in Fig. 3. The portions 64 tear away in tension to dissipate energy from the opening door. Within this region, the tear line thickness is reduced to approximately .7 - .9 mm and the line is approximately perpendicular to the direction of travel of the doors 54, 56. In this manner the tear is directed into the desired hinge region of extra thickness at the locking arms 32, 34. Fig. 7 illustrates a modified cover 70. Except for shape, the modified cover 70 closely resembles that of Figs. 1 - 6 and includes, for example, retained locking arms 72, 74 and non-retained locking arms 76, 78. The tear line array differs slightly in that the cross bar tear line 80 is more centrally positioned within the face 82 of the cover 70. Otherwise, however, the cover 70 is quite similar to the previously described cover 28.

Claims: An air bag assembly (10) for use in a motor vehicle, including a rigid box shaped housing (12) having a bottom and a side wall defining a substantially open top, a collapsed air bag (18) within said housing, an inflator connected to inflate said air bag, and a cover (28) closing said housing and defining tear lines (58, 60, 62) adapted to be ruptured by the inflating air bag, the cover (28) being of a substantially homogeneous material, said tear lines being regions of reduced thickness of the cover material forming a generally H shaped pattern having portions of differing thickness, wherein an outwardly extending rim (26) on said side wall is disposed around at least a portion of said open top, a plurality of locking arms (32, 34) being formed integral with said cover and engaging said rim, and means (24, 40, 42) being secured to the bottom of said housing for retaining at least two said locking arms in engagement with said rim during inflation of said air bag, whereby upon deployment of the air bag the thinnest tear line portions will be the first to rupture and form doors releasing the air bag, energy from the air bag being absorbed during the formation of said doors, characterised in that said tear lines comprise at least three said portions of differing thickness of which the first and thinnest tear line portions are the crossbar (58) of the H and the regions of the vertical bars (60) immediately adjacent the crossbar, the succeeding second portions (B) of the vertical bars being thicker, with the end of each vertical bar terminating in a third portion (64) defining a curve leading to a thickened edge region of the cover, the third portion having a thickness intermediate that of the first and second portions, said thickened edge regions form hinges for said doors. An air bag assembly according to claim 1 wherein said two retained locking arms (32, 34) are substantially diametrically opposed relative to said open top. An air bag assembly according to claim 2 further comprising at least two additional locking arms (48, 50) which are substantially diametrically opposed relative to said open top. An air bag assembly according to claim 3 wherein said two retained locking arms (32, 34) and said two additional locking arms (48, 50) lie along substantially perpendicular axes. An air bag assembly according to any preceding claim wherein each of said thickened cover regions includes one of said retained locking arms (32, 34). A homogeneous moulded plastics cover (28) for an open housing of an automotive air bag assembly which comprises: a substantially flat face portion (30) defining therein weakened tear lines (58, 60, 62) forming a generally H shaped pattern, first and second locking arms (32, 34) extending rearwardly from said face portion for engagement with opposed portions of an outwardly-turned rim of an air bag housing and adapted to be retained in said engagement by an external retaining member (24); and third and fourth locking arms (48, 50) extending rearwardly from said face portion and orthogonal to said first and second arms for engagement with opposed portions of said housing rim, whereby the tear line portions are arranged to rupture under the influence of an inflating air bag and absorb energy during the formation of doors releasing the air bag, energy from the air bag being absorbed during the formation of said doors, characterised in that the tear lines have at least three portions of differing thickness, of which the first and thinnest tear line portions (A) are the crossbar (58) of the H and the regions of the vertical bars (60) immediately adjacent the crossbar, the succeeding second portions (B) of the vertical bars being thicker, with the end of each vertical bar terminating in a third portion (64) defining a curve leading to a thickened edge region of the cover, the third portion having a thickness intermediate that of the first and second portions, and in that said thickened edge regions form hinges for said doors. A cover according to claim 6 wherein said plastics is sufficiently resilient that, upon rupture and formation of said release doors, the third and fourth locking arms (48, 50) are forced into tighter engagement with said housing rim. A cover according to claim 6 or claim 7 wherein each of said thickened cover regions includes one of said first and second locking arms (32, 34).

Inventors: ADAMS GARY, HARRIS BRADLEY D, SADERHOLM DAVIN, STRAHL PAUL, ADAMS, GARY, HARRIS, BRADLEY D., SADERHOLM, DAVIN, STRAHL, PAUL
Applicants: MORTON INT INC, MORTON INTERNATIONAL, INC.
Application year: 1993
Main classification: B60R21
IPC codes: B60R21