Abstract:
A vibration-sensitive module having a mounting flange is mounted to a vibrational base with a bushing assembly secured to the base, where the mounting flange has cammed surfaces radially adjacent to the bushing assembly that variably engage the radial peripheries of the bushings in response to vibrational movement of the base. The cammed surfaces produce a desired force vs. deflection characteristic of the mounting apparatus, and the bushing material retains its compressibility under load to minimize cross-coupling of vibrational force impulses.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to apparatus for mounting a vibration-sensitive module on a vibrational base such as an internal combustion engine, and more particularly to a mounting geometry that more effectively isolates the module from vibrational movements of the base. 
       BACKGROUND OF THE INVENTION 
       [0002]    When a vibration-sensitive module such as an electronic control unit is mounted on a vibrational base such as an internal combustion engine, elastomeric bushings are interposed between the module and the base to dampen the amount of vibrational energy transmitted from the base to the module. If too much vibrational energy is transmitted to the module, electronic components and solder joints in the module can suffer fatigue failures that degrade the performance of the module or render it inoperative. 
         [0003]      FIG. 1  depicts a prior art arrangement for mounting an electronic module on a vibrational base  10 . The module housing  12  is typically a cast or stamped metal part, but in some cases may be formed of plastic. In any case, the housing  12  has several integrally formed mounting plates or flanges, one of which is designated in  FIG. 1  by the reference numeral  12   a . The flange  12   a  is provided with an internal through-hole  12   b  for receiving a bushing assembly  14 , and a mounting bolt  16  passing through the center of the bushing assembly  14  is threaded into a tapped opening in the vibrational base  10 . The bushing assembly  14  includes upper and lower annular rings of elastomeric material (i.e., bushings)  18  and  20 , an upper washer  22  disposed between the upper bushing  18  and the head  16   a  of bolt  16 , a lower washer  24  disposed between the lower bushing  20  and the vibrational base  10 , and a tubular sleeve  26  disposed about the bolt  16  between the upper and lower washers  22  and  24  that limits the bolt travel and determines the preset compression force applied to bushings  18  and  20 . As seen in  FIG. 1 , the faces of bushings  18  and  20  that seat on the flange  12   a  are undercut as indicated by the reference numerals  18   a  and  20   a , allowing the bushings  18  and  20  to partially extend into the flange through-hole  12   b . This centers the bushings  18  and  20  in the flange through-hole  12   b  during installation and mounting, and fills part of the through-hole  12   b  with bushing material that acts to absorb vibrational movement of the base  10  in a direction transverse to bolt  16 . 
         [0004]    The elastomeric material of bushings  18  and  20  is designed to absorb vibrational movements of the base  10 . However, the configuration of bushing assembly  14  places most of the elastomeric material between the flange  12   a  and the upper and lower washers  22  and  24 , and only a small amount of the elastomeric material (i.e., the bushing portions  18   a  and  20   a ) within the flange through-hole  12   b . Consequently, the bushing assembly  14  exhibits good absorption of vibrational impulses parallel to the longitudinal axis of bolt  16  (referred to herein as z-axis impulses), but only limited absorption of impulses in directions transverse to the z-axis before the inner ring of elastomeric material within the through-hole  12   b  locally collapses or bottoms-out. When a transverse impulse collapses the bushing material between the flange  12   a  and the sleeve  26 , the bushing assembly  14  can no longer isolate the module  12 , and the force transmitted to module  12  from the base  10  rises sharply. Even in a best-case scenario where the vibrational force impulse is predominantly along the z-axis, the elastomeric material of bushings  18  and  20  resonantly couple the vibrational movement into the transverse plane, resulting in transmitted vibrational vectors that can even exceed the z-axis force impulse. Of course vibrational force impulses are rarely unidirectional in nature, and the bushing assembly  14  is frequently incapable of adequately isolating module  12  in the transverse plane. 
         [0005]    In view of the above, what is needed is an improved but cost-effective mounting arrangement that more effectively isolates a vibration sensitive module from both z-axis and transverse plane movements of a vibrational base. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is directed to an improved vibration isolating mounting apparatus for vibration-sensitive module having a mounting flange sandwiched between axial faces of upper and lower elastomeric bushings that are secured to a vibrational base, where the mounting flange has cammed surfaces radially adjacent to the bushings that variably engages an outer radial periphery of the bushings in response to vibrational movement of the base. The cammed surfaces produce a desired force vs. deflection characteristic of the mounting apparatus, and the bushing material retains its compressibility under load to minimize cross-coupling of vibrational force impulses. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a diagram of a prior art mounting apparatus for a vibration-sensitive module including a mounting flange and a bushing assembly for securing the mounting flange to a vibrational base; 
           [0008]      FIG. 2  is a diagram of a mounting apparatus for a vibration-sensitive module according to the present invention; 
           [0009]      FIG. 3  is a diagram of the mounting apparatus of  FIG. 2  when subjected to a z-axis vibrational impulse; 
           [0010]      FIG. 4  is a diagram of the mounting apparatus of  FIG. 2  when subjected to a transverse-axis vibrational impulse; and 
           [0011]      FIG. 5  is a diagram of an alternate mounting arrangement for a vibration-sensitive module according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    Referring to  FIG. 2 , the reference numeral  30  generally designates a mounting apparatus for a vibration-sensitive module  32  according to this invention. The mounting apparatus  30  includes a mostly conventional bushing assembly  34  and a unique module flange  36  having an internal through-hole  36   a . Similar to the prior art bushing assembly  14  of  FIG. 1 , the bushing assembly  34  includes upper and lower annular elastomeric bushings  38  and  40 , upper and lower washers  42  and  44 , and a tubular sleeve  46  surrounding a mounting bolt  48 . The bolt  48 , washers  42  and  44 , and sleeve  46  are identical to the prior art mounting apparatus depicted in  FIG. 1 . However, the upper and lower bushings  38  and  40  are not undercut as in the prior art bushing assembly  14 . Instead, the bushings  38  and  40  are purely cylindrical in profile, and rest against the annulus  36   b  of flange  36  bordering through-hole  36   a , leaving the core of annulus  36   b  free of elastomeric material. Preferably, the flange  36  has a pair of small circular shoulders  36   c  and  36   d  radially outward of the annulus  36   b  and extending above and below the annulus  36   b  to engage the radial periphery of the bushings  38  and  40 ; this aids in centering the bushings  38  and  40  in the through-hole  36   a  during installation and mounting. Radially outboard of the annulus  36   b  and the shoulders  36   c  and  36   d , the upper and lower peripheries of the flange  36  are contoured or cammed as indicated by the reference numerals  36   e  and  36   f . When the bolt  48  is tightened during mounting of the module  12 , the bushings  38  and  40  bulge radially as shown, but do not substantially engage the cammed surfaces  36   e  and  36   f  of flange  36 . 
         [0013]      FIG. 3  depicts the mounting apparatus of  FIG. 2  when subjected to an outward z-axis (upward in  FIG. 3 ) vibrational movement of the base  10 . The force impulse associated with the movement is transmitted to the module  12  via the lower bushing  40  and the flange  36 . The elastomeric material of bushing  40  bulges as shown as it absorbs a portion of the vibrational movement. The inboard axial face of the bushing  40  remains in contact with the inboard side of the annulus  36   b  of flange  36 , and its radial periphery increasingly engages the lower cammed surface  36   f  of flange  36  as the elastomeric material bulges in response to the force. The z-axis force transmitted to module  12  is effectively damped by the relatively large bulk of active bushing material. Moreover, cross-coupling of the force to the transverse plane of flange  36  is minimized because the elastomeric material retains it transverse absorption capability (i.e., it does not collapse or bottom-out as with the prior art mounting apparatus). 
         [0014]      FIG. 4  depicts the mounting apparatus of  FIG. 2  when subjected to an outward transverse (rightward in  FIG. 4 ) vibrational movement of the base  10 . The force impulse associated with the movement is transmitted to the module  12  via flange  36  and the outboard half of upper and lower bushings  38  and  40 . The bushings  38  and  40  bulge against the outboard side of the flange  36  as shown as they absorb a portion of the vibrational movement. The radial peripheries of the bushings  38  and  40  remain in contact with the upper and lower shoulders  36   c  and  36   d  of flange  36 , and increasingly engage the upper and lower cammed surfaces  36   e  and  36   f  of flange  36  as the elastomeric material bulges in response to the force. The transverse force transmitted to module  12  is effectively damped by the relatively large bulk of active bushing material. Also, cross-coupling of the force to the z-axis domain is minimized because the elastomeric material retains it transverse absorption capability and does not collapse or bottom-out. 
         [0015]    The profile of the cammed surfaces  36   e  and  36   f  can be selected to provide a desired force vs. deflection characteristic for the mounting apparatus  30 . The concave profile depicted in the preferred embodiment of  FIGS. 2-4  tends to produce a linear force vs. deflection characteristic.  FIG. 5  depicts an alternative approach in which a flange  50  having an internal through-hole  50   a  bordered by an annulus  50   b  has cammed surfaces  50   c  and  50   d  that are linear in profile and slope radially outward from the annulus  50   b  adjacent the radial peripheries of the upper and lower bushings  38  and  40 . Convex flange profiles are also possible. 
         [0016]    In summary, the mounting arrangement of the present invention provides a simple way of significantly improving the vibration isolation capability of a bushing assembly without impacting manufacturing or assembly costs. As demonstrated, isolation capability in the transverse plane has been substantially improved, and cross-coupling of force vectors has been greatly diminished by virtually eliminating bottoming-out of bushing material between the flange annulus  36   b  or  50   b  and the sleeve  46 . While the present invention has been described with respect to the illustrated embodiments, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the flange shoulders  36   c  and  36   d  may be omitted, the flange profiles may be asymmetrical, and so on. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.