Abstract:
A vibration isolator assembly, such as an isolator bushing or cradle mount, includes a housing and an isolator connected to the housing. A shaft assembly includes first and second mating components, the first component being connected to the elastomer and having a cavity of a first dimension for receiving the second component having a different, second dimension therein. The differing dimensions alters the stress characteristics of the vibration isolator assembly. According to a preferred method of altering stress characteristics of a vibration isolator assembly, the method includes the steps of forming a cavity in the shaft and varying a dimension of the shaft to alter the spring rate characteristics. In the preferred arrangement, the shaft assembly includes a split, first component and a second component is inserted between the split portions to relieve tensile stress in the isolator and, if desired, to impart a compressive stress in the isolator.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a vibration isolator assembly, that generically refers to a device that absorbs vibrations and dampens relative movement between two structures, such as an isolator mount, bushing assembly, cradle mount assembly, etc. 
   A typical vibration isolator includes an external housing and an internal mounting shaft joined by an isolator such as a molded elastomer (e.g., rubber). The elastomer provides isolation between the housing and the mounting shaft. Typically, the elastomer is molded to the housing shaft in a high-temperature molding operation. This provides a desirable bond between the elastomer and the housing, as well as between the elastomer and the mounting shaft. After the molding operation, the elastomer experiences shrinkage as the part cools. Depending on the design, an undesirable effect of this shrinkage is to impart tensile stress to the elastomer. In such cases, fatigue performance of the vibration isolator assembly is generally improved by relieving the stress. One common way of relieving the stress is to pass the external housing through a funnel or reduced diameter opening to permanently reduce the diameter of the assembly. Such an arrangement is shown and described in the prior art representation of FIGS. 11–13 of U.S. Pat. No. 6,094,818, and is also well known to one skilled in the art. 
   Also, due to general poor fatigue performance of an elastomer under tensile loading, further enhancements to fatigue life may be realized by going beyond the simple relief of imparted tensile stresses and imparting compressive stress to the elastomer. Unfortunately, conventional methods for reducing the outer diameter of the isolator are limited in their effectiveness when there is a desire to impart compressive stress to the elastomer. This is due, for example, to negative effects on the bonding between the isolator and housing materials, i.e., the adhesive layer bonding between the isolator and housing, and isolator and the mounting shaft. There are also limits on the extent of deformation that the housing material can undergo. 
   It will also be appreciated that a substantial amount of time and money are required to design, redesign, tool, and retool a product. The development cycle requires significant design and engineering time to be sure that the final product meets the final product specifications. If the specifications are altered during the development process, a need exists to remove, modify, and reinstall the assembly in a short time frame. With regard to producing manufactured mounting and vibration isolators for power trains, i.e., engines/transmissions in various consumer and commercial vehicles, as well as a development tool for engineering purposes that allows optimization and tuning of a power train mounting system for improved isolation and performance, a need exists to address tuning and durability issues. If such issues surface late in a program cycle, it is necessary to implement changes without a major redesign or a long retooling time, even though the basic characteristics of the isolator are being modified. 
   Thus a need exists to enhance the durability and tuning ability of a vibration isolator assembly. Means to relieve residual tensile stress, as well as vary the level of precompression of the elastomer portion, are desired. A need also exists to vary the travel limits of the elastomer portion. Lastly, there is a need to overcome these problems without major reworking of prototype or production tooling. 
   SUMMARY OF THE INVENTION 
   The present invention discloses a vibration isolator assembly having a housing and a shaft assembly interconnected by an isolator. The shaft assembly includes first and second mating components, the first component of which is connected to the elastomer. The first shaft component forms a cavity of a first dimension to receive the second component having a second dimension slightly greater than the first dimension for altering stress characteristics of the vibration isolator assembly. 
   In a preferred embodiment of the invention, the isolator is an elastomer, and the housing may be either metal or non-metal. 
   The second mention of the second component of the shaft assembly is pre-selected to relieve molded in tensile stress, or impart compressive stress to the isolator. 
   In one embodiment, the first component of the shaft assembly is a split member and the second component is received along the split plane. 
   The first and second components of the shaft assembly have a keyed contour relationship to selectively alter spring rate build-up characteristics. 
   A method of altering spring rate characteristics in a vibration isolator assembly comprises the steps of forming a cavity in the shaft and varying a dimension of the shaft to alter spring rate characteristics of the isolator. 
   The varying steps include inserting a component of the shaft into the cavity to relieve tensile stress in the isolator and/or impart a compressive stress in the isolator. 
   The varying step is accomplished without affecting a bond formed between the isolator and the housing, or between the isolator and shaft assembly. 
   A primary benefit of the invention resides in the ability to relieve molded in tensile stress and, if desired, impart compressive stress without affecting the housing or bonded layer between the housing and the elastomer material. 
   Another benefit of the invention resides in the ability to manipulate spring rate magnitude, rate ratios, rate build-up, and component fatigue characteristics of the vibration isolator assembly. 
   Still another benefit resides in reduced tooling costs and reduced lead times associated with different designs so that more options can be made available within the constraints of a fixed or predetermined development budget. 
   Yet another benefit is the ability to make changes to a design and tuning specifications without impacting a launch schedule of the product. 
   Still other benefits and advantages of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of its advantages may be obtained by referring to the following detailed description in conjunction with the accompanying drawings. 
       FIG. 1  is a cross-sectional view illustrating a conventional vibration isolator assembly or suspension bushing. 
       FIG. 2  is an elevational view of a preferred embodiment of the present invention. 
       FIG. 3  is a cross-sectional view taken generally along the lines  3 — 3  of  FIG. 2 . 
       FIG. 4  is a perspective view of a second component of the shaft assembly separated from the remainder of the vibration isolator assembly. 
       FIG. 5  illustrates a separation of a first component of the shaft assembly along a split plane for receipt of the second component. 
       FIG. 6  illustrates partial insertion of the second component into the remainder of the vibration isolator assembly. 
       FIG. 7  is a perspective view of the final assembly. 
       FIG. 8  is an elevational view of another embodiment of the present invention, namely, a cradle mount assembly. 
       FIG. 9  is a cross-sectional view taken generally along the lines  9 — 9  of  FIG. 8 . 
       FIG. 10  is an exploded perspective view of individual components of the assembly. 
       FIG. 11  illustrates a further step in the assembly of the vibration isolator assembly or cradle mount. 
       FIG. 12  shows enlarging the cavity of a first component of the shaft assembly. 
       FIG. 13  illustrates partial insertion of the second component of the shaft assembly. 
       FIG. 14  illustrates the completed assembly of the second preferred embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   By way of brief description, a conventional vibration isolator assembly A is shown in  FIG. 1 . The assembly includes a housing A and a shaft B that are interconnected by an isolator such as elastomer C. As noted above, the elastomer is typically mold bonded to the shaft and the housing and provides the vibration damping between the shaft secured to a first structure or component that moves relative to the housing which is secured to a second structure or component. 
   A first preferred embodiment of the present invention is shown in  FIGS. 2–7 . Vibration isolator assembly  20  includes a housing  22 . Typically, the housing is a metal structure, although as will be appreciated in accordance with the present invention, alternative materials including non-metallic materials such as nylon can be used to form the housing. As illustrated in  FIGS. 2 and 3 , when used as a mount bushing, the housing typically adopts a generally cylindrical conformation, i.e., circular in cross section. Secured to an inner surface  24  of the housing is an isolator  30 . The isolator is often an elastomer or rubber construction because of the ability to isolate and reduce noise and vibration from being transmitted therethrough because of the elastic nature of the material. Rubber is generally an incompressible material that is displaced during loading and exhibits the desirable property that the more the rubber is compressed, the higher the stiffness. This rate build-up controls noise, vibration, and harshness associated with, for example, the vehicle environment. The elastomer is preferably mold bonded to the inner surface  24 , although it will be appreciated that other bonding arrangements such as an adhesive bond, can be used without departing from the scope and intent of the present invention. Likewise, an inner diameter portion  34  of the isolator is secured to a shaft assembly  40 , which is preferably comprised of first and second components  42 ,  44 . The isolator is secured to a first or outer surface  46  of the first component and again is illustrated as being mold bonded thereto. As will become more apparent below, the first component  42  in this embodiment is defined by first and second portions  42   a ,  42   b  that are separated and have outer surfaces  46   a ,  46   b  that are bonded to the isolator, respectively. Inner, facing surfaces  48   a ,  48   b  of the first component are separated along a split plane and slidably engage the second component  44  of the shaft assembly. As perhaps best illustrated in  FIG. 2 , and  FIGS. 4–7 , the first and second components  42 ,  44  of the shaft assembly have mating, keyed configurations that provide for slidable insertion of the second component into a cavity defined along the split plane between the first and second portions  42   a ,  42   b  of the first component, and likewise resist relative movement between the first and second component in other directions. This mating, keyed configuration provides an interlocking feature that allows the separate components (here, three separate pieces) to merge and function as a single component. For example, as illustrated in  FIG. 2 , the first and second portions  42   a ,  42   b  of the first component have dove-tailed keyways  60  formed on opposite axial sides of central axis  62 . The keyways are dimensioned to receive mating, dovetailed keys or projections  64  that extend outwardly from the second component  44  of the shaft assembly. Thus, with continued reference to  FIG. 2 , and additional reference to  FIGS. 4 and 5 , it is seen that the first component  42  includes first and second mirror-image portions  42   a ,  42   b  having a central rounded portion  66  that merges into generally planar, outer radial portions  68 . The first and second portions  42   a ,  42   b  are separable or split along the plane  70  that passes through the longitudinal axis  62 . The second component is similarly contoured, having a central rounded portion  76  that merges into generally planar, outer radial portions  78  disposed on diametrically opposite sides of the rounded portion. As will be appreciated, although this contour has met with success, still other variations or contours may be used without departing from the scope and intent of the present invention. 
   As represented in  FIG. 4 , the mirror image portions  42   a ,  42   b  of the first component of the shaft assembly are integrally bonded to the isolator. The portions are separable along the facing, mating surface along split plane  70 . Thus, as shown in  FIG. 4 , the facing dovetailed key ways  60  define an outer radial cavity portion and the rounded portions  66   a ,  66   b define a central cavity portion  80 . When the portions  42   a ,  42   b  are disposed in abutting relation as shown in  FIG. 4 , the cavities  80  defined by the keyways present a first dimension of the first component of the shaft assembly that is smaller than a second dimension provided by the preselected, mating contour of the second component. Thus, and as will be appreciated from a comparison of  FIGS. 4 and 5 , the first and second portions  42   a ,  42   b  of the first component of the shaft assembly are spread apart or separated along the split plane  70  to enlarge the cavity or opening  80  and allow selective sliding receipt of the second component  44  of the shaft assembly therein. This sliding receipt is represented in  FIG. 6 , where the second component is partially received into the first component of the shaft assembly. Ultimately, complete insertion of the second component into the first component is achieved and is represented in  FIG. 7 . 
   As noted in the Background discussion above, the isolator is typically an elastomer molded between the housing and shaft, here between the housing and the first component  42  of the shaft assembly. When it cures, the elastomer ends up with residual tensile stress. As noted, the prior art has addressed this in a different manner. Here, using a two-part shaft assembly alters the stress characteristics of the vibration isolator assembly. At a predetermined dimension of the second component  44 , the residual tensile stress in the elastomer is removed upon complete insertion of the second component into the first component. If the second component is made even larger, then a predetermined compressive stress is formed in the isolator/elastomer. All of this is achieved without affecting or adversely impacting on the bond between the housing and elastomer, and likewise between the elastomer and shaft assembly. 
   By varying the thickness of the second component, variable compressive stresses can be introduced into the elastomer. This has two basic effects. The first effect is associated with spring rate. The compressive rate characteristics of the main elastomer elements will increase in magnitude, roughly in proportion to the degree of precompression, while the shear rate characteristics of the same elastomer elements will remain relatively unaffected. This results in a change in the compressive-to-shear rate ratio of the final assembly. 
   The second effect deals with fatigue life. In general, an elastomer or rubber can tolerate increasing levels of compressive stress under cyclic loading much better than it tolerates smaller amounts of tensile stress. Thus, by introducing compressive stress via this method, fatigue life of the assembly can be enhanced. The forced separation of the molded split shaft assembly relieves the tensile stress resulting from part shrinkage, and imparts or introduces a desired compressive stress to the molded rubber, if desired. 
   It is also known in these isolation mount assemblies or bushing assemblies to incorporate one or more openings in the elastomer. Thus, as shown in  FIGS. 2–7 , the elastomer is radially continuous between the housing and the shaft assembly in one diametrical dimension (e.g., in the vertical direction as shown) and is discontinuous in another diametrical direction (e.g., in the horizontal direction as shown) through the inclusion of openings  90 ,  92 . Here, the openings  90 ,  92  are generally symmetrical in the vertical and horizontal directions, but need not necessarily be symmetrical or similarly shaped. Likewise, boundary rubber  94 ,  96  define snubbing rubber extending inwardly from the housing in the horizontal direction. By selectively altering the width dimension of the second component of the shaft assembly, i.e., by extending the planar portions  78  radially outward, the gap between the shaft assembly and the boundary rubber is selectively altered. As the shaft assembly width is increased, the amount of travel that the shaft assembly proceeds through before contacting the boundary rubber on the housing will decrease. This transfers loading and stress more rapidly from the main central rubber elements to the exterior boundary or snubbing elements, and effectively alters the spring rate build-up characteristics. This has the effect of providing additional tuning options and improved fatigue durability. 
   The present invention is also useful in other vibration isolator assemblies such as the cradle mount shown in  FIGS. 8–14 . Since much of the structure and function is substantially identical, reference numerals with a primed suffix refer to like components (e.g., housing is referred to by reference numeral  22 ′), and new numerals identify new components. The primary distinction relates to the frame  100  of the vehicle in which the housing  22 ′ of the cradle mount assembly  20 ′ is also molded to the first component portions  42   a ′,  42   b ′ of the shaft assembly. The subassembly, i.e., housing  22 ′, is initially inserted into the frame  100 , namely opening  102 . Rather than encountering high insertion forces as is typical with conventional structures, smaller friction forces can be provided during insertion of the housing into the frame in accordance with the present invention. Thereafter, the first component portions of the shaft assembly are spread apart ( FIG. 12 ) to accommodate the second component  44 ′. The second component is slidably received therein ( FIG. 13 ), thus altering the stress characteristics of the isolator/elastomer, and also enhancing the abutting engagement force between the housing  22 ′ and the frame  100 . 
   The housing  26 ′ is preferably metal, but is should be noted that other composite or hard materials may be used. Likewise, the outer dimension of the housing can relate to any shape such as rectangular, square, circular, triangular, or any other shape or surface depending on the size and requirements needed for the end use. Similarly, the shaft assembly is shown as being located at or near the center point of the housing, although that could be varied. Likewise, although it is preferred to extrude the shaft assembly components from aluminum, occasionally other materials, e.g., cast, extruded, powdered metal, forgings, cold headed steel, or still other materials of construction may be used without departing from the scope and intent of the invention. The number and/or shape of the openings can also be varied, and the elastomer isolator that is bonded to the housing components may be secured thereto with any other type of bonding. 
   In summary, the manipulation of spring rate magnitude, rate ratios, rate build-up, and component fatigue characteristics is accomplished using a common molded isolator or elastomer and interchangeable shaft assemblies of different designs. This is important since higher tooling costs and longer lead times are typically associated with the isolator/elastomer, while the inserted shaft can be modified quickly and economically. The economical aspect of the vibration isolator assembly means that more options can also be made available within the constraints of a development budget. This highlights yet another feature of the invention wherein the ability to make changes to design and tuning specifications later in the program can be achieved without impacting the launch schedule of the product. 
   The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon reading and understanding this specification. For example, various other manufacturing steps may be employed or in a different sequence. Likewise, different materials may be used or alternative processes without departing from the present invention. It is intended to include all such modifications and alterations in so far as they come within the scope of the appended claims or the equivalents thereof.