Abstract:
A strut top mount utilizing a mount dome including lightweight polymeric material for the primary structural member as a replacement for the steel material that is traditionally used. The utilization of the polymeric material is possible because the factors associated with strut top mount stresses are successfully managed by a strategic geometric arrangement of the components thereof and the body structure to which it interfaces, as for example by the mount dome being configured to seat in abutting relation to a support shell of the strut tower, wherein the polymeric material is placed under compression. A load bypass is provided in the event of a maximum jounce event.

Description:
TECHNICAL FIELD 
     The present invention relates to Macpherson strut type motor vehicle suspension systems, and particularly to single fastener MacPherson strut top mounts, and more particularly to a strut top mount having its primary structural member composed of a polymeric material. 
     BACKGROUND OF THE INVENTION 
     Motor vehicle suspension systems are configured so that the wheels are able to follow elevational changes in the road surface as the vehicle travels therealong. When a rise in the road surface is encountered, the suspension responds in “jounce” in which the wheel is able to move upwardly relative to the frame of the vehicle. On the other hand, when a dip in the road surface is encountered, the suspension responds in “rebound” in which the wheel is able to move downwardly relative to the integrated body/frame structure of the vehicle. In either jounce or rebound, a spring (i.e., coil, leaf, torsion, etc.) is incorporated with the body structure in order to provide a resilient response to the respective vertical movements of the wheel with regard to the vehicle body structure. However, in order to prevent wheel bouncing and excessive vehicle body motion, a shock absorber or strut is placed at the wheel to dampen wheel and body motion. An example of a MacPherson strut is disclosed in U.S. Pat. No. 5,467,971. 
     Due to the high stresses encountered during operation of a strut top mount, it is conventional practice in the art to fabricate strut top mounts having the primary structural element made of steel. In this regard, an example of an innovative strut top mount is exemplified in U.S. patent application Ser. No. 11/677,070, filed Feb. 21, 2007, to Paul A. Winocur, and assigned to the assignee hereof, the disclosure of which is hereby incorporated herein by reference. 
     One of the goals sought after in the automotive arts is increasing fuel economy based upon decreasing weight of the motor vehicle. Accordingly, it would be very desirable if somehow a strut top mount could be made lighter by somehow overcoming the stress factors that require the primary structural member be made of metal, such that a lighter polymeric material may be utilized as the primary structural member. 
     SUMMARY OF THE INVENTION 
     The present invention is a strut top mount which utilizes a lightweight polymeric material for the primary structural member, as a replacement for the steel material that is traditionally used, wherein the utilization of the polymeric material is possible because the factors associated with strut top mount stresses are successfully managed by a strategic geometric arrangement of the components thereof and the body structure to which it interfaces. 
     The single path strut top mount according to the present invention is configured to supportably seat with respect to a support shell of a strut tower which, at its lower end, is connected to the body structure of the motor vehicle. The strut top mount of the present invention utilizes a mount dome composed of a lightweight polymeric primary structural member, as for example a glass reinforced nylon, and of a resilient body, as for example rubber, which is bonded to the polymeric primary structural member, wherein the shape of the mount dome is complementary to the shape of the support shell of the strut tower, such that the mount dome seats into the support shell in a nestled, embraced manner. 
     The polymeric primary structural member is annular, having a central polymeric wall, an outer polymeric wall spaced from the central polymeric wall, and a top polymeric wall spanning the central and outer polymeric walls. The resilient body partly overmolds the polymeric structural member, wherein the resilient body bondingly covers the outer and top polymeric walls so as to serve as a resilient interface with the strut tower, the resilient body further bondingly covers the central polymeric wall so as to provide a central resilient element having a cup-shaped metallic insert, and the resilient body bondingly covers the central polymeric wall at a bottom surface thereof so as to provide an abutment element. 
     A bearing adjoins the central resilient element, adjacent the metallic insert. A jounce bumper plate has an inner plate flange abutting the lower race of the bearing, wherein the jounce bumper plate carries a jounce bumper interface and a spring seat. A strut shaft has a reduced diameter, threaded end portion defined by a shaft shoulder, wherein the inner plate flange abuts the shaft shoulder. A first nut is threaded onto the threaded end portion of the shaft and presses the lower race against the inner plate flange. A mount retainer includes a retention washer which is secured to the threaded end portion by a second nut. 
     The significant advantage of the strut top mount according to the present invention is that it utilizes a lightweight polymeric material as its primary structural member, which offers significant mass reduction at a lower cost as compared to a conventional strut top mount having steel as the material of the primary structural member. Yet, while it is to be understood that the utilization of a polymeric material as the primary structural member of a strut top mount is desirable for mass savings, such a substitution for a steel material of a traditional strut mount is not straightforward and is fraught with difficulties due to the high operational stresses placed upon it, and the high stiffness requirement from, the structure of a strut top mount which is hard fastened to the vehicle body structure. 
     In this regard, therefore, several insights of the present invention need first to be understood: 1) use of a dome shaped mount to strut tower interface with a specific geometric arrangement would allow the steel structure of the vehicle body to support, and therefore greatly reduce, the working stress of the dome shaped mount; and 2) polymeric materials offer the most resistance to damage and the least deformation when stressed in a compressive manner, while minimizing tension and bending stresses. 
     With the aforementioned inventive insights in mind, there are five main enablers according to the present invention by which a polymeric material can be substituted for the steel material in a single path strut top mount: 1) the outer diameter of the polymeric primary structural member is enlarged as compared to a conventional steel primary structural member, wherein the cross-sectional thickness of the polymeric primary structural member is chosen to provide adequate strength and stiffness; 2) the mount dome is seated in floating, nestled abutment at the support shell of strut tower which serves to embracingly back the main mount component and thereby reduces operational stress in the polymeric primary structural member; 3) a provision is made for maximum jounce loads that ensures that the operational stresses in the rubber and its bonded interface to the polymeric structure do not exceed a tolerable level, whereduring, the jounce bumper plate strikes an abutment element of the resilient body superposed the central polymeric wall of the polymeric primary structural element, whereby the load is transmitted therethrough to the strut tower; 4) the mount dome has an inner diameter which is less than the inner diameter of the polymeric primary structural member; and 5) the resilient body interface of the mount dome to the support shell of the strut tower at the outer polymeric wall is angled to create, under vehicle curb weight, compressive stress in the polymeric material of the polymeric primary structural member, wherein, as mentioned, polymeric materials, such as glass reinforced nylon, react much more robustly to compressive stress than to shear or tension. 
     The low mass polymeric material in the strut top mount of the present invention provides significant mass reduction, for example around a 25% weight reduction over a conventional strut top mount, and this weight reduction is being provided for a motor vehicle component which is positioned high and forward in the motor vehicle. As such, not only achieved is an overall reduction in vehicle mass, but also a lowering of the vehicle center of gravity height and an improvement in front to rear mass distribution. 
     Accordingly, it is an object of the present invention to provide a strut top mount which utilizes a lightweight polymeric material for the primary structural member, as a replacement for the steel material that is traditionally used, wherein the utilization of the polymeric material is possible because the factors associated with strut top mount stresses are successfully managed by a strategic geometric arrangement of the components thereof and the body structure to which it interfaces. 
     This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partly sectional side view of a strut top mount according to the present invention. 
         FIG. 2  is a perspective, lower view of the mount dome according to the present invention. 
         FIG. 3  is a detail sectional view of the strut top mount, seen at circle  3  of  FIG. 1 . 
         FIG. 4  is a left side, sectional partly view of the strut top mount of  FIG. 2 , now shown under a maximum jounce load. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the Drawing,  FIGS. 1 through 4  depict various aspects, by way of exemplification and not limitation, of the strut top mount according to the present invention. 
     The single path strut top mount  100  according to the present invention is configured to nestingly interface with a support shell  102  of a strut tower  104  which, at its lower end, is connected (not shown) to the body structure of the motor vehicle. The support shell  102  has a top shell wall  102   a  and an annular shell sidewall  102   b  having an acute angle α with respect to a normal of the top wall (see also  FIG. 3 ). The strut top mount  100  of the present invention utilizes a mount dome  106  composed of a lightweight polymeric primary structural member  108 , as for example a glass reinforced nylon, and of a resilient body  110 , as for example rubber, which is bonded to the polymeric primary structural member. With respect to the nestling, the dome shape of the mount dome  106 , defined by a top dome wall  106   b  and an annular dome sidewall  106   a  (oriented at an acute angle α′ as shown at  FIG. 3 ), is complementary to the shape of the support shell  102  of the strut tower  104 , such that the mount dome seats into the support shell  102  in a nestled, abuttingly embraced manner, wherein the polymeric primary structural member  108  is under compression and the resilient body floats in resilient abutment to the support shell, as further discussed in detail hereinbelow. 
     The polymeric primary structural member  108  is annular, having a central polymeric wall  108   a , an outer polymeric wall  108   b , and a top polymeric wall  108   c  spanning the central and outer polymeric walls. An example of a suitable polymeric material is a glass reinforced nylon, as for example a 30% to 50% glass reinforced fiber within the nylon, available, for example, through BASF Corporation of Mount Olive, N.J. 07828. 
     The resilient body  110  partly overmolds the polymeric primary structural member  108 , wherein the resilient body is bondingly affixed to the outer surfaces  108   b ′,  108   c ′ of the outer and top polymeric walls  108   b ,  108   c , respectively. In this regard, as best shown at  FIGS. 2 and 3 , the resilient body  110 , serves as a resilient outer element  110   a  in abutting interface with the top shell wall  102   a  and shell sidewall  102   b  of the support shell  102  of the strut tower  104 . Further in this regard, the resilient body  110  is bondingly affixed to the inner surface  108   a ′ of the central polymeric wall  108   a  so as to provide a central resilient element  110   b . An annular, generally inverted L-shape metallic insert  112  is located inside the central resilient element  110   b , and provides ample bonding surface to the central resilient element so that shear load is widely distributed across the bonding surface thereof. An abutment element  122  of the resilient body  110  is bonded to the central polymeric wall  108   a  at the bottom surface  108   a ″ thereof. A suitable material for the resilient body is a 55 shore hardness rubber, for example available through Hawthorne Rubber Mfg. Corp., of Hawthorne, N.J. 07507. 
     A bearing  114  is composed of an upper race  114   a  abuttingly interfaced with a bearing seat  110   c  of the central resilient element  110   b  adjacent the metallic insert  112 , and is further composed of a lower race  114   b . A jounce bumper plate  116  has an inner plate flange  116   a  abutting the lower race  114   b  of the bearing  114 , wherein the jounce bumper plate carries a spring seat  116   b  and a jounce bumper interface  116   c . A strut shaft  118  has a reduced diameter, threaded end portion  118   a  defined by a shaft shoulder  118   b , wherein the inner plate flange  116   a  abuts the shaft shoulder. A first nut  120 , as for example a tube nut, is threaded onto the threaded end portion  118   a  of the strut shaft  118  and presses the lower race  114   b  of the bearing  114  against the inner plate flange  116   a.    
     A mount retainer  124  includes a retention washer  126  and a retention washer rubber element  126   a . The retention mount retainer  124  is secured to the threaded end portion  118   a  of the strut shaft  118  by a second nut  128 . 
     As shown at  FIG. 2 , the mount dome  106  is, as described above, a bonded composite of the polymeric primary structural member  108  (shown without shading in  FIG. 2 ) and the resilient body  110  (shown shaded in  FIG. 2 ). The polymeric primary structural member  106  features a recess  130 , wherein a plurality of buttresses  132  span the recess, wherein the recess minimizes the mass of the polymeric material, and the buttresses provide strength thereto, particularly as regards the compressive load supplied by the shell sidewall  102   b  to the outer polymeric wall  108   b.    
     How the compressive load is applied by the shell sidewall  102   b  of the support shell  102  to the outer polymeric wall  108   b  of the polymeric primary structural member  108  will now be detailed with additional reference being directed to  FIG. 3 . 
     As mentioned hereinabove, the shell sidewall  102   b  has an acute angle α with respect to a vertical, the vertical being a normal of the shell top wall  102   a , which is generally flatly disposed in the horizontal (i.e., the horizontal being perpendicular to the vertical). The mount dome  106  has a dome sidewall  106   a  (the largest diameter of which is defined by the flutes  110   f ) which is at an acute angle of essentially also α. To allow for build variation and compression of the polymeric primary structural member  108 , the cross-section of the dome sidewall  106   a  at the flutes  1 I Of exceeds the cross-section of the interior surface  102   b ′ of the shell sidewall  102 . Accordingly, as shown at  FIG. 3 , when the mount dome  106  is seated in the support shell  102 , the flutes  110   f  compress to position  110   f ′ and the dome sidewall compresses to position  106   a ′. The resulting compression of the resilient outer element  110   a  is applied to the outer polymeric wall  108   b , whereby the polymeric primary structural member is under a state of compression. 
     Operational aspects of the strut top mount  100  will now be discussed. 
     The support shell  102  of the strut tower  104  provides a steel backing structure of the vehicle body to support the mount dome  106  when nestled therein, whereby greatly reduced is the working stress of the polymeric primary structural member  108 . The outer diameter of the outer polymeric wall  108   b  is enlarged as compared to a conventional steel primary structural member, wherein the cross-sectional thickness is chosen to provide adequate strength and stiffness. The top shell wall  102   a  of the support shell  102  has an area which generally superposes the top dome wall  106   b  in juxtaposed relation to the top polymeric wall  108   c , wherein the top shell wall and the top dome wall are in mutually parallel disposition. 
     The dome sidewall and the shell sidewall have a mutual predetermined size relationship and a mutual predetermined angular relationship such that when the dome top surface abuts the shell top surface, the polymeric primary structural member is placed under compression by a resilient compression of the outer resilient element between the polymeric outer sidewall and the shell sidewall. In this regard, the dimensions and angle of dome sidewall  106   a , inclusive of the flutes  110   f  with respect to the interior surface  102   b ′ of the shell sidewall  102   b  create, under vehicle curb weight, compressive stress in the polymeric material of the polymeric primary structural member, resulting in compression to the polymeric primary structural member  108 , wherein, as mentioned, polymeric materials, such as glass reinforced nylon, react much more robustly to compressive stress than to shear or tension. 
     The case of operation in a maximum jounce event is shown at  FIG. 4 . In a maximum jounce load event, it is important that the operational stresses in the resilient body  110  and its bonded interface to the polymeric material of the polymeric primary structural member  108  do not exceed a predetermined tolerable level. In this regard, in a maximum jounce, the jounce bumper plate  116  at the spring seat  116   b  thereof strikes the abutment element  122  of the resilient body at a location superposed the central polymeric wall  108   a  of the polymeric primary structural element  108 . As such, the jounce load is thereupon bypassed by being transmitted through the central polymeric wall  108   a  to the top shell wall  102   a  of the support shell  102  of the strut tower  104 . It is preferred for the abutment element  122  to be castellated. In a maximum jounce event, the abutment element  122  serves to snub and arrest vertical displacement without abruptness. 
     The strut top mount  100  has two operational regions, a low stiffness region of approximately 500 N/mm which affects normal operation and a high stiffness region of approximately 5 kN/mm that affects large jounce load inputs, the two regions being defined by a response “knee” corresponding to when load bypass is engaged; that is, when the jounce bumper plate  116  strikes the abutment element  122 . The strut top mount  100  is molded in its full rebound position and when loaded by the corner weight of the vehicle, it shifts up to a position corresponding to approximately half way between the as molded position and the response knee on the load deflection curve. 
     From the foregoing description, it is seen that the strut top mount  100  according to the present invention provides a significant mass reduction at a lower cost as compared to a conventional strut top mount having steel as the material of the primary structural member. 
     To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.