Abstract:
A bush assembly is seated in a housing, having an outer sleeve ( 10 ), an inner tube ( 12 ) and a resilient body ( 14 ) connecting the inner tube ( 12 ) and the outer sleeve ( 10 ). The outer sleeve ( 10 ) has projections ( 20 ) from its outer surface that abut the inner surface of the housing ( 40 ) and cause local deformations on the sleeve around the projections ( 20 ). The local deformations around the projections ( 20 ) cause the radial gap ( 16 ) between the inner tube ( 12 ) and the outer sleeve ( 10 ) to change circumferentially and thus bush has different radial stiffness in different directions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bush assembly, to a method of manufacturing such a bush assembly and to a bush for use in such an assembly. It is particularly, but not exclusively, concerned with a bush assembly for use in a sub frame of an automobile. 
     2. Summary of the Prior Art 
     One known bush for use in a automobile between the chambers and the sub frame has an outer sleeve and a tube in the sleeve, the sleeve and the tube being interconnected by a resilient body. The sleeve is then mounted in a housing such as a bracket. Then the bracket may be attached to, or embedded in, the sub frame and the tube attached to the chassis, or vice versa. Thus active movement of the sleeve and the tube, and hence the chassis and the relevant part of the sub frame, are resisted by the resilience of the resilient body. 
     In such a bush assembly, the stiffness of the assembly, and hence its effect on vibrations is determined by the resilient body. However, it is been found desirable to have different radial stiffnesses in different directions. 
     SUMMARY OF THE INVENTION 
     At its most general, the present invention proposes that at least one projection be provided on the sleeve, which projection(s) will abut the wall of the bore of the housing into which the sleeve is inserted, thereby locally deforming the sleeve. 
     Thus, at the location of the projection(s) the gap between the sleeve and the tube is reduced as compared with other radial positions. This locally compresses the resilient body, permitting the desired radial stiffness characteristics to be achieved. 
     In the present invention, the deformation of the sleeve due to the presence of the projection(s) is achieved when the sleeve is inserted in the housing, rather than being achieved by the initial shaping of the sleeve. 
     It would be more difficult to pre-mould the sleeve to that shape, or to provide an additional component between the sleeve and the tube to achieve the same effect. 
     Thus, a first aspect of the present invention may provide a bush assembly comprising: 
     a sleeve formed of a deformable nylon material and having an outer surface that has an outer diameter that is formed with at least one projection; 
     a tube within the sleeve, there being a gap between the sleeve and the tube; 
     a resilient body formed of a rubber material interconnecting the tube and the sleeve; and 
     a housing having a bore with an inner diameter into which the sleeve is received, the at least one projection abutting the wall of the bore; 
     wherein the sleeve outer diameter is of a size to provide a good grip between the sleeve and the housing bore and further is adapted to locally deform radially inwardly adjacent the projection when the sleeve is mounted within the bore such that the radial width of the gap between the sleeve and the tube radially inward of the projection is less than at least one other circumferential point of the tube. 
     The sleeve needs to be of a material suitable to be deformed under the effect of the interaction of the projection and the wall of the bore of the housing. 
     Preferably, the sleeve is therefore of nylon or other plastics material which may provide sufficient rigidity to maintain the resilient body in place, but be sufficiently deformable for the effects of the present invention to be achieved. 
     To improve the deformability, the thickness of the sleeve in the radial direction may be reduced adjacent to the position of the projection, e.g. by providing a groove in the outer surface of the sleeve. 
     It should also be noted that, prior to insertion of the bush into the housing, the diameter of the sleeve will be slightly greater than the diameter of the bore, to provide an overall compressor effect which is desirable to eliminate and stresses in the resilient body, and also to provide good grip between the sleeve and the housing. 
     The projections then provide an additional reduction in diameter at their radial positions. 
     Preferably, two projections are provided at opposite ends of a diameter, and those projections may then be at a mid point of the sleeve, in the axial direction of the sleeve. 
     Moreover, whilst it would be usual for the projections to have the same height, it is not essential and they may be different to give different stiffness effects on opposite sides of the bush. 
     According to a second aspect of the present invention there may be provided a method of manufacturing a bush assembly, comprising inserting a bush into a bore in a housing, the bush comprising: 
     a sleeve formed of a deformable nylon material and having an outer surface that has an outer diameter that is formed with at least one projection; 
     a tube within the sleeve, there being a gap between the sleeve and the tube; 
     a resilient body formed of a rubber material interconnecting the tube and the sleeve; and 
     a housing having a bore with an inner diameter into which the sleeve is received, the at least one projection abutting the wall of the bore; 
     wherein the sleeve outer diameter is of a size to provide a good grip between the sleeve and the housing bore and further is adapted to locally deform radially inwardly adjacent the projection such that the radial width of the gap between the sleeve and the tube radially inward of the projection is less than at least one other circumferential point of the tube. 
     According to a third aspect of the present invention there may be provided a bush comprising: 
     a sleeve formed of a deformable nylon material and having an outer surface that has an outer diameter and that is formed with at least one projection; 
     a tube within the sleeve, there being a gap between the sleeve and the tube; 
     a resilient body formed of a rubber material interconnecting the tube and the sleeve; wherein radial depression of at least one projection relative to the rest of the sleeve causes local deformation of the sleeve such that the radial width of the gap between the sleeve and the tube radially inward of the projection is less than at least one other circumferential point of the tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described in detail by way of example, with reference to the accompanying drawings in which: 
         FIGS. 1   a  and  1   b  are transverse and axial sectional views through a bush for use in an embodiment of the present invention,  FIG. 1   b  being taken along the line B to B in  FIG. 1   a;    
         FIG. 2  is a perspective view of the sleeve of the bush of  FIGS. 1   a  and  1   b;    
         FIG. 3  is a perspective view of the bush shown in  FIGS. 1   a  and  1   b;    
         FIGS. 4   a  and  4   b  are transverse and axial sectional views through a bush assembly being an embodiment of the present invention, and incorporating the bush of  FIGS. 1   a  and  1   b.    
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention, being a sub frame bush assembly for use in an automobile, will now be described. 
     The assembly comprises a bush which is mounted in a housing, and  FIGS. 1   a  and  1   b  show the bush prior to its mounting in the housing, the bush comprises an outer sleeve  10  and an inner tube  12  connected by a resilient body  14 . In the axial sectional of view of  FIG. 1   b , the resilient body appears to have two parts  14   a  and  14   b , and attached to the sleeve  10  and one attached to tube  12 . However, as the transverse sectional view of  FIG. 1   a  shows, space  16  does not extend for the whole circumference of the bush. 
     Instead, the part  14   a  and  14   b  are interconnected at circumferential positions perpendicular to sectional line of  FIG. 1   b , by interconnections known as struts  18 . The struts  18  determine the radial stiffness of the bush in a direction perpendicular to the line B to B. 
     It can be seen that the inner part  14   b  of the resilient body has projections  20  thereon at a circumferential position on such that the radial width of the gap between the sleeve and the tube radially inward of the projection is less than at least one other circumferential point of the tube. 
     These projections locally reduce the radial width of the space  16  between the parts  14   a  and  14   b  of the resilient body, but there remains a gap  22  between the parts  14   a  and  14   b  prior to the mounting of the bush in a housing. 
       FIG. 1   b  also shows that the sleeve  10  has a projection  24  thereon, projecting radially outwards of the sleeve. The sleeve is locally thinned by grooves  26  at the periphery of the projection  24  and the projection has a recess  28  in it outer surface. As illustrated in  FIG. 1   b , the projection  24  and the sleeve  10  are made of the same material and are integrally formed. 
     The sleeve  10  is shown in perspective view in  FIG. 2  which illustrates that the sleeve  10  has a rim  30  at the axial ends thereof, 
       FIG. 3  then shows a perspective view similar to that of  FIG. 2 , but showing the whole of the bush illustrated in  FIGS. 1   a  and  1   b . The struts  18  can be seen more clearly in that view. 
     It should also be noted that the inner tube  12  has an inner bore  32 , which will receive a bolt to attach it to an automobile chassis (not shown). 
     The bush of  FIGS. 1 to 3  is then mounted in a bore of a housing embedded in an automobile sub-frame. The resulting structure is shown in  FIGS. 4   a  and  4   b , in which the housing is shown schematically by reference numeral  40 . 
     Other features of the bush are the same as in  FIGS. 1   a  and  1   b  and shown by the same reference numerals. Some reference numerals are omitted for the sake of clarity. 
     As is conventional, the bush, when manufactured has a slightly greater diameter than the diameter of the bore of the housing  40  in to which the bush is mounted. This causes overall compression of the bush by e.g. about 3%, and this compresses the resilient material of the body (comprising parts  14   a  and  14   b ) to eliminate post-moulding shrinkage stresses and also to cause the outer surface of the sleeve  10  to press against the inner wall  42  of the housing  40  so that the sleeve is gripped by the housing. 
     However, since the projections  24  project outwardly of the sleeve, they abut against the wall  42  of the bore of the housing  40 , locally deforming the sleeve  10  inwardly. 
     This reduces the space  16  to bring the parts  14   a  to  14   b  of the resilient body into abutment along the line A to A. In that direction the radial stiffness of the bush is determined by the abutment of parts  14   a  and  14   b  of the resilient body. 
     Typically, the additional deformation of the sleeve  10  at the position of the projections  24  is of the order of 1% to 5%, preferably around 2.7%. 
     With such a bush, the radial stiffness in the direction perpendicular to the line A to A is of the order of 1650N/mm, as previously mentioned, and the radial stiffness in the direction along the line A to A is of the order of 500N/mm. 
     Thus, significantly different stiffnesses can be achieved in different radial directions, and the value of the radial stiffness in the direction A to A can be determined by the size and configuration of the projections  24 . 
     Without the projections, it is generally found that the stiffnesses are too low. 
     The sleeve is preferably made of nylon, preferably glass filled nylon 6.6 the glass filled range of which is 0% to 30%. 
     The resilient material of the body  14  formed by parts  14   a  and  14   b  is normally of rubber. 
     The projections  24  may be circular or oval, and preferably are positioned at the axial mid point of the bush as illustrated in the embodiment discussed above. Although the embodiment discussed above shows the projections at opposite ends of a diameter, this is not essential and it could be at any circumferential positions on the sleeve. 
     There are preferably two projections, but more may be provided if more complicated stiffness characteristics were to be needed. It would also be possible to have projections of different heights at different circumferential positions.