Abstract:
A novel apparatus and method for attaching a rounded member to another member or surface, particularly a flat surface, is disclosed. A spacer, which may take the form of a split ring, is positioned around an attachment surface of the rounded member. Then, while the spacer is supported against further motion along the inflator, a flange having a shank with a tapered inside diameter may be passed around the inflator and spacer. As the inside diameter of the flange narrows to contact the spacer, the flange exerts inward pressure tending to frictionally engage the inflator with the spacer, and the spacer with the flange. Thus, an interference fit having a high retention strength may be comparatively easily and rapidly formed. The spacer may be hardened so as to indent the inflator and/or the flange to enhance the retention strength of the assembly. The spacer may be reconfigured in a number of ways to provide the appropriate combination of resistance against axial pressure and torsion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. The Field of the Invention  
           [0002]    The present invention relates to systems and methods for attaching two or more members together. More specifically, the present invention relates to a novel system and method for securely and easily mounting a rounded member, for example, an airbag inflator, to an interior surface of a vehicle.  
           [0003]    2. The Relevant Technology  
           [0004]    Many methods are currently available for fastening two or more parts together as part of an assembly. Flat parts, such as steel beams, struts, and the like can typically be comparatively easily fastened together through the use of fasteners, adhesives, welding, or a similar method. Adjoining flat surfaces provide an even, simple interface for the attachment.  
           [0005]    However, rounded members, such as bars, pipes, pressure vessels, and the like present greater attachment problems. Round, convex surfaces often require the use of a corresponding concave surface to provide an attachment interface. Thus, many implements, such as conventional clamps and the like, that are useful for attaching two flat parts together, cannot be used for rounded parts.  
           [0006]    The attachment problem is further multiplied when the rounded member is subject to high stress. In the case of pressure vessels, for example, a comparatively thin wall is subject to high stress from a pressurized internal fluid. The walls of such vessels are typically manufactured to have a uniform thickness and a comparatively uniform curvature so that stresses are evenly distributed throughout the wall. Thus, significant deformation or piercing of the wall is to be avoided. Holes, in particular, are problematic even if they do not extend fully through the wall of the pressure vessel, because stresses tend to concentrate at holes. As a result, cracks often begin forming at holes, and propagate outward from the hole. Although thinner wall sections are not as critical as holes, they are also often failure points because of stress concentration.  
           [0007]    As a result, the number of methods that can be used to attach a rounded member under considerable stress to another member is very limited. Fasteners that require holes, such as screws, bolts, rivets, and the like, are clearly undesirable. Welding also has a tendency to weaken the underlying material, and requires that the joint to be welded be accessible to the welding equipment.  
           [0008]    Conventional press fitting, or “interference” fitting, is an attachment process by which a member is attached to another member or a fixture by friction. “Frictional engagement” refers to two surfaces that are pressed together such that friction keeps them from sliding relative to each other. “Interference” refers to a geometric state in which one part blocks motion of another part; in an interference fit, one or both parts are deflected to make the relative motion possible.  
           [0009]    In order to provide an interference fit, a protrusion in one member is typically inserted into a cavity in another, and the cavity is dimensioned slightly smaller than the protrusion. The cavity must then be stretched, and the protrusion compressed, in order to fit together. A considerable amount of radial pressure between the protrusion and the cavity results, so that the protrusion is held within the cavity by frictional force. Often, the protrusion, the cavity, or both may be tapered so that the protrusion can be gradually forced into the cavity.  
           [0010]    The force required to force the protrusion into the cavity is generally proportional to the force required to withdraw it. In order to create an attachment that will withstand a desired axial (along the axis of symmetry of the rounded member) tension, a commensurate degree of compression may need to be applied to insert the protrusion into the cavity. However, in circumstances in which torsion, or rotational force, is to be coupled with the tension, a lower amount of tension may be required to withdraw the protrusion.  
           [0011]    One such application in which it is desirable to rigidly mount a rounded member is for automotive safety restraint devices. The inclusion of inflatable safety restraint devices, or airbags, is now a legal requirement for many new vehicles. Airbags are typically installed in the steering wheel and in the dashboard on the passenger side of a car. In the event of an accident, an accelerometer within the vehicle measures the abnormal deceleration and triggers the ignition of an explosive charge. Expanding gases from the charge fill the airbags, which immediately inflate in front of the driver and passenger to protect them from impact against the windshield. Side impact airbags have also been developed in response to the need for similar protection from impacts in a lateral direction, or against the side of the vehicle.  
           [0012]    The explosive charge is typically located in an inflator, which often takes the form of a cylindrical metal pressure vessel designed to contain the explosion and channel the resulting gases into the airbag. Since the inflator contains explosive materials, it is very important that it be firmly fastened to an interior surface of the vehicle. The inflator typically has a cylindrical central portion with roughly hemispherical end caps. Thus, the problems described above in connection with attachment of rounded members generally, apply to inflators. Additionally, Department of Transportation regulations restrict the use of any welded joints in motor vehicles. Generally, attachments in motor vehicles, particularly attachments related to safety systems, must be strong enough to withstand the operating vibrations of the vehicle as well as potential impacts.  
           [0013]    Furthermore, known attachments are typically not adaptable to inflators of different sizes. Airbag sizes may vary from one vehicle to the next; consequently, an airbag manufacturer may need to be able to make and install several different inflator sizes. With most known attachment systems, each inflator size would require its own specially-sized attachment. The need to pair each size with an associated attachment assembly has increased the time and expense required for inflator installation.  
           [0014]    Consequently, it would be an advancement in the art to provide a method and apparatus for attaching a rounded member to another member without welding. More specifically, it would be an advancement in the art to enable the attachment of a rounded member such as an inflator to a comparatively flat surface such as a vehicle surface.  
           [0015]    The method and apparatus should preferably be easily carried out with a minimum of equipment. Thus, the method and apparatus should preferably provide a comparatively large holding force with a comparatively small attachment force. Preferably, the method and apparatus should be capable of maintaining attachment even under combined axial and torsional loads. The method and apparatus should also be usable to attach rounded members with a wide range of sizes, without the need to design and use different attachment hardware with each size. Furthermore, the method and apparatus should be simple and inexpensive to implement.  
         BRIEF SUMMARY OF THE INVENTION  
         [0016]    The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available systems and methods for attaching rounded members. Consequently, the present invention provides a novel system and method for attaching a rounded member, particularly to a flat surface. Although the following disclosure provides the example of an airbag inflator, the method and system disclosed herein may be used with any rounded member.  
           [0017]    An inflator attachment may comprise an inflator, a flange, and a spacer. The inflator typically takes the form of a cylindrical pressure vessel with roughly hemispherical ends. The inflator is located within a suitable compartment of a vehicle, such as a passenger side dashboard compartment. The inflator provides pressurized gas to an airbag, either through a conduit, or through direct passage of air into an opening of the airbag from an opening, or diffuser, positioned in a first end of the inflator. The inflator may be attached to a vehicle surface within the compartment at a second end of the inflator, so as to maintain the inflator in a cantilevered, suspended position within the compartment.  
           [0018]    In the alternative to the cantilevered configuration, the first end of the inflator may be attached in similar fashion to a bolt. Thus, the diffuser may have threads sized to engage an opening of the vehicle surface, or a nut used in combination with an opening to secure the first end. Other attachment methods such as crimping may also be used to secure the first end. If desired, the airbag can then be folded into the compartment with the opening of the airbag facing the inflator.  
           [0019]    Either form of attachment may be facilitated by attaching the flange to the second end of the inflator. In the case of a cantilevered attachment, the second end of the inflator may actually be attached to a vehicle surface through the use of the flange. If the first end is attached, the flange may simply be used as a shoulder to maintain the second end properly positioned with respect to the first, and to support the inflator against axial and torsional forces that may be applied during installation.  
           [0020]    Preferably, the flange has a shank and a web. The shank has a tubular configuration large enough to fit with clearance around the inflator. The web may then comprise a perpendicular, disc-like extension with a flat mounting surface that can be mated to the vehicle surface within the compartment. The web may be affixed to the vehicle surface through fastening, adhesive or chemical bonding, welding, or any other suitable method.  
           [0021]    The spacer may comprise a compressible, split-ring design that can slide relatively easily over the inflator in its uncompressed configuration. The spacer is positioned between the inflator and the flange, and is preferably dimensioned so as to be compressed between the spacer and the inflator. If desired, the spacer may be constructed of a material harder than the inflator and/or the flange, so as to create indentations in the inflator and/or flange during assembly. The indentations then serve to keep the spacer in position with respect to the inflator and the flange. The edges of the spacer are effectively held by interference within the indentations so that no axial motion of the spacer is possible. If desired, the spacer may be formed with indentations or other shaped features to increase the amount of interference and thereby increase the resistance of the interference fit to axial force.  
           [0022]    In addition, to the extent that the spacer comprises any radial irregularities, such as gaps (as in a split ring), protrusions, or the like, the indentation is shaped accordingly. Thus, the interference of the indentation with the radial irregularities effectively precludes rotation of the ring in response to torsional forces acting on the inflator during installation of the inflator or operation of the vehicle. In order to enhance resistance against torsional force, the spacer may have a configuration different from a split ring. For example, a series of curved blocks, separate or connected by ring sections, may be utilized. Alternatively, ridges parallel to the axis of symmetry of the spacer may be formed on the inside and/or the outside of the spacer.  
           [0023]    In addition to the enhanced resistance to axial and torsional force, the present invention provides a number of distinct assembly advantages. Notably, in certain embodiments, a comparatively small assembly force may be used, even though the force required to remove the flange from the inflator remains large.  
           [0024]    Initially, the spacer maybe slid into position around a circumferential portion of the second end of the inflator. A tubular support designed to fit around the inflator with clearance, and within the flange with clearance, may then be positioned in abutting relation to the ring. The flange is preferably constructed with a tapered inside diameter, in which a larger portion is sized to fit over the spacer with clearance, and a smaller portion is small enough to interfere with the spacer, while still fitting over the inflator with clearance. The larger portion may be positioned proximate the web.  
           [0025]    The flange may thus be slid over the inflator, with the web and the larger portion of the tapered inside diameter leading, from the first end of the inflator toward the second end. The larger portion of the tapered inside diameter slides over the spacer, and the smaller portion of the tapered inside diameter comes into contact with the spacer. The support and inflator may then be held firmly in place while the flange is forced further toward the second end of the inflator. The narrowing inside diameter of the flange effectively forces the spacer inward, so that the spacer firmly engages the inflator, and the flange firmly engages the spacer.  
           [0026]    Alternatively, in embodiments in which the spacer is harder than the inflator and the flange, the spacer may simply be pressed into the inflator to form the indentation prior to inclusion of the flange. This may be accomplished through the use of an external press, thermal contraction, or any other known method. The flange may then be assembled onto the spacer and inflator as described above, or by another method. For example, the flange may be created in modular portions and assembled around the spacer/inflator arrangement, or the flange may be heated, positioned around the spacer and inflator, and then allowed to contract and cool.  
           [0027]    Such a method of assembly provides numerous advantages over known interference fit operations. Deformation of the inflator is limited to a comparatively narrow, circumferential portion of the inflator. Additionally, in certain embodiments, a comparatively small force must be applied over only a comparatively small distance to bring about secure engagement of the inflator, spacer, and flange. The tapered inside diameter of the flange strongly resists outward motion of the inflator from the vehicle surface because outward motion of the inflator tightens the interference fit.  
           [0028]    Additionally, particularly where the spacer is an expandable structure, such as a split ring, the same spacer and flange can be fitted on inflators with different diameters. The tapered interior diameter of the flange permits the flange to be installed over spacers with a range of sizes; the flange simply engages the spacer at a different location within the flange.  
           [0029]    These and other advantages of the present invention will become more fully apparent from the following description and appended claims, or maybe learned by the practice of the invention as set forth hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    In order that the manner in which the above-recited and other advantages and objects of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0031]    [0031]FIG. 1 is a perspective view of one embodiment of an inflator assembly according to the present invention;  
         [0032]    [0032]FIG. 2 is a cross-sectioned, side view of the inflator assembly of FIG. 1;  
         [0033]    [0033]FIG. 3 is a perspective view of a method of assembly suitable for the inflator assembly of FIG. 1; and  
         [0034]    [0034]FIG. 4 is a cross-sectioned, side view of an alternative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    The presently preferred embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the apparatus, system, and method of the present invention, as represented in FIGS. 1 through 4, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.  
         [0036]    In determining the strength of an interference fit, several factors are relevant. Generally, the retention strength of a fit is the force required to induce slippage between two attached members. The strength of a conventional cylindrical press fit is typically proportional to the frictional coefficient between materials that make up the two members, the contact area between the members, the modulus of elasticity of the two materials, and the amount of interference (i.e., overlap between the parts in their undeformed state). The strength of the fit is also typically inversely proportional to the radius of the interface between the two parts.  
         [0037]    In certain applications, the degree to which these factors can be changed to alter the strength of the fit is somewhat limited. For example, when an inflator having a standard shape, size, and material is to be used, there can be little control over the frictional coefficient or the modulus of elasticity because changing these values would require using different materials or property-changing treatments. Increasing the amount of interference between the inflator and its attachment also increases deformation of the inflator, which is undesirable for the reasons mentioned above in relation to pressure vessels. Due to the limited space available, it may also be difficult to increase the contact area between the inflator and its attachment. If the contact area is increased, assembly is made more difficult because the inflator and the attachment must be pressed together over a longer distance.  
         [0038]    Additionally, even to the extent that relevant factors can be varied to obtain a sufficient retention strength, conventional press fits require a compressive attachment force that is generally similar to the retention strength. As a general rule, the stronger the retention strength, the more difficult the assembly.  
         [0039]    The present invention provides an apparatus and method with which a high strength press fit may be obtained without having to apply a comparatively large attachment force over a large distance. Yet, the interference fit still has a high retention strength. This high strength is obtained, in part, by providing an interference fit in which members are attached and fitted in the same direction for which continued relative motion is to be restrained by the interference fit. Thus, attempts to draw the inflator out from the assembly simply tighten the fitting. In certain embodiments, additional retention strength may be obtained by compressing parts of the assembly tightly enough to indent softer parts, so as to increase the disassembly interference.  
         [0040]    These concepts will be described in further detail in the following disclosure. Although the example of an airbag inflator is used in the description and drawings, those skilled in the art will recognize that the apparatus and method described may easily be adapted for attachment of other types of rounded members. A “rounded member” need not be cylindrical, but may have any shape in which a curved surface forms a significant portion of the surface of the member.  
         [0041]    Referring to FIG. 1, an inflator assembly  10  is provided for firmly attaching an inflator  12  inside a vehicle. The inflator  12  may be a standard airbag inflator containing an igniter and propellant designed to produce gas to inflate an airbag (not shown). The inflator  12  may thus be a pressure vessel with a substantially cylindrical portion capped by roughly hemispherical end caps. The inflator  12  may have a first end  14  and a second end  16 , with an outlet  18  positioned at the first end  14  to provide an escape path for gases leaving the inflator  12 . The outlet  18  may be exposed to expel gas directly into an open end of the airbag, or may be connected to the airbag by a suitable conduit (not shown).  
         [0042]    The inflator assembly  10  may be installed inside a vehicle compartment  24 , such as a compartment in the passenger side of a vehicle. The compartment  24  may be dimensioned to accommodate an airbag folded around the inflator  12 , in position for rapid deployment through a cover (not shown) facing the passenger compartment. The inflator  12  may be mounted with an attachment member  26 , or flange  26 , in a cantilever arrangement with a vehicle surface  28  comprising one wall of the compartment  24 . The vehicle surface  28  may be substantially flat, as shown, or may be rounded or otherwise contoured to suit the space available for the compartment  24 . Likewise, the flange  26  need not be configured as shown, but may have any geometry that captures the inflator  12 , and yet provides a suitable interface with a vehicle surface  28  of the compartment  24  for mounting.  
         [0043]    As mentioned previously, the cantilevered attachment is only one possible configuration. According to other attachment methods, the first end  14  may be affixed to a vehicle surface (not shown), in addition to, or in place of, rigid attachment of the flange  26  to the vehicle surface  28 . The outlet  18  is depicted with threads that can be used to accomplish such attachment.  
         [0044]    The flange  26  may have a shank  40  designed to fit around the inflator  12 , preferably with a modest amount of clearance. The shank  40  may thus be tubular in shape, as depicted in FIG. 1, but may also be differently shaped if it is desirable to mount the inflator  12  from an angle different from that depicted in FIG. 1. The shank  40  also need not necessarily comprise a full tubular member, but may have suitable gaps to cut down on material costs, weight, or stiffness. A web  42  is then provided, in substantially perpendicular arrangement with the axis of the shank. The web  42  is preferably shaped to fit the vehicle surface  28 ; the web  42  may thus be flat. The web  42  is affixed to the vehicle surface  28  by a suitable method, such as fastening, chemical or adhesive bonding, thermal, frictional, vibrational, or radio frequency welding, or the like. Fasteners  44  are depicted in FIG. 1 by way of example.  
         [0045]    Referring to FIG. 2, a cross section of one possible embodiment of the inflator assembly  10  is depicted. The web  42  may have a mounting surface  46  facing the vehicle surface  28 , and shaped to engage the vehicle surface  28 . The inflator  12  may have a plateau  47  protruding from the second end  16  of the inflator  12 . The plateau  47  may rest against the vehicle surface  28 , or may be disposed within a suitable alcove in the vehicle surface  28  when the inflator assembly  12  is affixed within the compartment  24 .  
         [0046]    The inflator  12  may have an outer membrane  48  designed to contain the pressurized gases inside the inflator  12 . The outer membrane  48  is preferably constructed of a comparatively ductile material, such as steel or aluminum. Preferably, the outer membrane  48  is devoid of cracks, holes, or other features that may create stress concentrations, or stress risers, in the outer membrane  48 . The outer membrane  48  may be formed around an ignitor and propellant within the inflator  12 . Preferably, the outer membrane comprises a substantially uniform thickness.  
         [0047]    A spacer  50  may be positioned between the second end  16  of the inflator  12  and the flange  26 . Preferably, the flange  26  substantially encircles the spacer  50 , or surrounds it to the extent necessary to provide inward pressure. Thus, the flange  26  need not entirely cover or surround the spacer  50 . According to certain embodiments, the spacer  50  may have a substantially ring-like shape substantially encircling an attachment surface  52 , or circumferential portion  52 , of the inflator  12 . A substantially ring-like shape need not extend full circle, and need not have a uniform cross section.  
         [0048]    Preferably, the attachment surface  52  comprises a cylindrical band, or section, of the inflator  12 . However, the attachment surface  52  may comprise any path around the inflator  12 . The spacer  50  may comprise a split ring, as shown, that can be expanded to fit with clearance around the inflator  12 , or compressed to grip the inflator  12 . Alternatively, the spacer  50  may take a number of different forms, depending on the type of loading the inflator  12  will be most subject to.  
         [0049]    If the inflator  12  is to bear a comparatively high degree of tension along its axis, the spacer  50  may advantageously comprise a plurality of split rings, so as to provide greater gripping force against axial motion. Alternatively, if a high torsional loading pattern is expected, the spacer  50  may comprise a series of ring sections separated by gaps or smaller connecting portions, or may simply comprise a split or whole ring with a plurality of ridges parallel to the axis of the spacer  50  to grip the inflator  12  and/or the flange  26  for rotational stability.  
         [0050]    The spacer  50  may also have a peaked portion  54  configured to provide enhanced gripping reinforcement against axial motion. The flange  26  may have a tapered inside diameter  60  including a larger portion  62  proximate the web  42  and a smaller portion  64  further from the web  42 . The tapered inside diameter  60  may thus have a continuously tapering configuration, or may have one or more portions with no taper adjoining tapered portions. In the embodiment depicted in FIG. 2, the peaked portion  54  engages an indentation  66  formed in the tapered inside diameter  60 . The indentation  66  may be preformed in the tapered inside diameter  60 , or may be formed in the course of the assembly process. The indentation  66  provides a comparatively severe interference with the peaked portion  54 . In other words, the indentation  60  would have to expand considerably over a comparatively small distance in order to release the peaked portion  54  of the spacer  50 .  
         [0051]    Referring to FIG. 3, an exploded view of the inflator assembly  10  is depicted, in the process of assembly. One advantage of the inflator assembly  10 , and particularly the use of the spacer  50 , is that the flange  26  need not be pressed onto the inflator  12  from the second end  16 . If the flange  26  were pressed on from the second end  16 , in order to obtain a strong interference fit, a high attachment force would have to be applied, because the attachment force is proportional to the retention strength. If the flange  26  were to be drawn over the first end  14  of the inflator  12  and onto the attachment surface  52 , without the use of a spacer  50 , the flange  26  would have to be dimensioned to interfere with the inflator  26 , and would have to be forced with considerable pressure along nearly the entire length of the inflator  12 .  
         [0052]    The inflator assembly  10  of the present invention overcomes these limitations through the use of the spacer  50  in combination with the flange  26 . The spacer  50  may advantageously have a split ring configuration, or a ring with only a comparatively small gap  82 . The gap  82  permits the spacer  50  to flex somewhat, so that the spacer  50  is able to slide over the inflator  12 , and yet grip the attachment surface  52  once inward pressure is applied against the spacer  50 .  
         [0053]    A tube-shaped support  92  may then be provided for purposes of assembly. The support  92  is preferably constructed of a comparatively stiff material, such as a metal. The support  92  is then fixed in position with respect to the inflator  12 , by a method such as clamping or placing the second end  16  of the inflator  12  and the support  92  against a support plate (not shown). The spacer  50  may thus be slid into position abutting a shoulder  94  of the support  92 , so that the support  92  holds the spacer  50  in position over the attachment surface  52 .  
         [0054]    Preferably, the support  92  is made with an outside diameter small enough to fit within the larger portion  62  of the tapered inside diameter  60  of the flange  26 . The support  92  need not be tube-shaped, but may comprise any configuration suitable for holding the spacer  50  in place under pressure. If the spacer  50  does not comprise a ring shape, the shoulder  94  may be shaped to removably abut the spacer  50  to provide the necessary registering force.  
         [0055]    When the spacer  50  is in position, the flange  26  may then be inserted around the first end  14  of the inflator  12 , as depicted in FIG. 3, and slid toward the attachment surface  52  and the spacer  50 . As shown in FIG. 2, the larger and smaller portions  62 ,  64  of the tapered inside diameter  60  of the flange  26  preferably fit with clearance over the inflator  12 . Consequently, the flange  26  may be moved with relative ease over the inflator  12  until the flange  26  reaches the spacer  50 . The larger portion  62  of the tapered inside diameter  60  of the flange  26  also preferably fits with clearance over the spacer  50 .  
         [0056]    Between the larger portion  62  and the smaller portion  64 , the tapered inside diameter  60  begins to interfere with the spacer  50 . Thus, pressure must be applied to the flange  26  to continue moving the flange  26  over the spacer  50 . Pressure may be applied by any suitable means. In certain embodiments, the flange and spacer may be dimensioned so that adequate pressure may be applied by hand to obtain the fully assembled configuration depicted in FIG. 2. Alternatively, the inflator assembly  10  may be fixtured within a press or other machine that applies linear force, so that greater relative pressure may be applied between the flange  26  and the spacer  50 , inflator  12 , and support  92 .  
         [0057]    The point at which the tapered inside diameter  60  begins to interfere with the spacer  50  varies somewhat, depending on the size of the inflator  12 . If the inflator  12  is comparatively large, the spacer  50  may be expanded to a high degree, so that the gap  82  is comparatively large. In such a case, the inside diameter  60  engages the spacer  50  toward the larger portion  62 . Conversely, where the inflator  12  is comparatively small, the spacer  50  need not expand as much, and the inside diameter  60  does not begin to interfere with the spacer  50  until the spacer  50  is positioned further toward the smaller portion  64  of the inside diameter.  
         [0058]    In either case, the flange  26  may obtain the proper position relative to the inflator  12  through the application of the appropriate amount of pressure. If desired, differently sized spacers  50  may be used in conjunction with a single size of flange  26  to keep the applied pressure constant between different inflator sizes.  
         [0059]    As the portion of the tapered inside diameter  60  in contact with the spacer  50  becomes smaller, more pressure must be applied to draw the flange  26  toward the second end  16  of the inflator, over the support  92 . The spacer  50  is pressed inward against the attachment surface  52  by the effective narrowing of the tapered inside diameter  60  of the flange  26 . If configured as a split ring, the spacer  50  may accommodate compression by virtue of the gap  82 , which may shrink under inward radial pressure against the spacer  50 . The sloping shape of the peaked portion  54  of the spacer  50  may help to ensure that no sharp edge of the spacer  50  is able to prematurely dig into any portion of the tapered inside diameter  60  of the flange  26 , and thereby arrest the motion of the flange  26  around the spacer  50 .  
         [0060]    Ultimately, the flange  26  reaches the position depicted in FIG. 2, and pressure need no longer be applied. The flange  26  is under radial tension, and the spacer  50  is radially compressed by the flange  26  against the inflator  12 . As a result, the friction between the spacer  50  and the inflator  12  firmly attaches the spacer  50  to the inflator  12 , and friction between the spacer  50  and the flange  26  firmly attaches the flange  26  to the spacer  50 . Thus, the flange  26  is held firmly in position around the inflator  12  by virtue of the spacer  50 , even though the flange  26  is dimensioned to fit around the inflator  12  with clearance.  
         [0061]    Referring to FIG. 4, a cross-sectional view of an alternative embodiment of an inflator assembly  110  according to the invention is depicted. The inflator assembly  110  may also comprise a standard inflator  12  like that of FIGS.  1 - 3 . Like the flange  26  depicted in FIG. 2, the flange  126  of the inflator assembly  110  may comprise a shank  140  and a web  142  with a mounting surface  146  configured to interface with the vehicle surface  28 .  
         [0062]    A spacer  150  may also be provided. However, for this embodiment, the spacer  150  is preferably harder than the inflator  12 , the flange  126 , or both. The spacer  150  may thus be formed of a harder material, or may be treated through a process such as case hardening, alloying, heating and quenching, or the like. Thus, the spacer  150  may be configured to create a somewhat more severe indentation in the inflator  12  or the flange  126 , or both.  
         [0063]    As depicted, the spacer  150  has a peaked portion  154 , which may be similar in configuration to the peaked portion  54  depicted in FIG. 2. Alternatively, the peaked portion  154  may be somewhat truncated so as to avoid creating a stress-concentrated, point loading condition in the shank  140  of the flange  126 . The spacer  150  may also have one or more ridges  156  interiorly formed on the spacer  150 . The effect of the ridges,  150  and the peaked portion  154  is to create somewhat severe indentations  158 ,  159  in the inflator  12  and the shank  140  of the flange  126 , respectively. These indentations  158 ,  159  provide additional resistance against axial relative motion of the inflator  12 , spacer  150 , and flange  126 .  
         [0064]    As with the spacer  50 , additional features may be used in addition to or in place of the peaked portion  154  and the ridges  156 . For example, ridges may be added in the direction of the axis of symmetry of the inflator  12  to shape the indentations  158 ,  159  for support against relative rotation of the inflator  12 , spacer  150 , and flange  126  in response to torsional force. As with the spacer  50 , the spacer  150  may also comprise multiple rings or ring sections, depending on whether additional resistance is needed to axial or torsional force.  
         [0065]    The inflator assembly  110  may be assembled in the same manner described in connection with FIG. 3, if desired. Thus, the flange  126  may be formed with a tapered inside diameter  160 , with a larger portion  162  and a smaller portion  164 . The flange  126  may then be pressed over the spacer  150  and support  92  to push the spacer  150  into engagement with the inflator  12 , thereby forming the indentation  158 .  
         [0066]    In the alternative, the spacer  150  may be pressed into the inflator  12  prior to installation of the flange  126 . This may be performed by simply applying radial pressure inward against the spacer  150  with a machine press or other tool configured to apply the appropriate pressure. The process depicted in FIG. 3 may then be performed to attach the flange  126  to the spacer  150 . If the spacer  150  has already been pressed into position, it may be unnecessary to use the support  92  to keep the spacer  150  in place during installation of the flange  126 .  
         [0067]    However, if the spacer  150  is already pressed into the inflator  12 , the method depicted in FIG. 3 need not be used to attach the flange  126 . The flange  126  may, for example, be heated into expansion to fit over the spacer  150  with clearance, and then permitted to cool and contract. Alternatively, the flange  126  may be made large enough to fit over the installed and compressed spacer  150  with clearance, and may be externally pressed inward to engage the spacer  150  by a method such as that used to press the spacer  150  into engagement with the inflator  12 .  
         [0068]    As with the embodiment of FIGS. 1 through 3, the inflator assembly  110  may include a range of different inflator sizes, without necessarily requiring the use of a different flange  126  and/or spacer  150 . Thus, the inflator assembly  110  may provide cost and installation time advantages over previously known methods.  
         [0069]    Consequently, the present invention provides an apparatus and method whereby a rounded member may be tightly attached to resistant to axial and torsional forces. The geometry of the spacer  50  or  150 , and the associated indentations  66  or  158 ,  1159  provide a more severe interference to prevent rotational or linear slippage of the inflator  12  from its mounting. Additionally, the method of assembly disclosed by the present invention permits a considerable amount of radial pressure to be applied to further strengthen the interference fit. Furthermore, in certain embodiments, the attachment assemblies  10 ,  110  may be used with different inflator sizes, with a minimal required change of attachment hardware, thereby making inflator installation generally faster and less expensive.  
         [0070]    Through the method disclosed, disassembly of the inflator assembly  10  or  110  may not be carried out by reversing the process steps used to assemble the inflator assembly  10  or  110 . Thus, in certain embodiments, a high retention strength may be obtained without applying a similarly high assembly force. Additionally, the flange  26  or  126  and spacer  50  or  150  may be easily and inexpensively manufactured. No welded joints are present, so regulations concerning welded joints in vehicles need not be dealt with.  
         [0071]    The present invention may be embodied in other specific forms without departing from its structures, methods, or other essential characteristics as broadly described herein and claimed hereinafter. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.