Abstract:
The present disclosure relates to shims  10, 24  adapted for placement between a rocker shaft  42  and rocker shaft mounts  62, 64 . The rocker shaft mounts  62, 64  project from the cylinder head  60  of an internal combustion engine. The shims  10, 24  are composed of a copper alloy, preferably bronze.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to shims, more particularly the present disclosure relates to shims for use in mounting rocker shafts to rocker shaft mounts and a method of reducing fretting wear in engines. 
       BACKGROUND 
       [0002]    Fretting is a type of wear caused by two contact surfaces undergoing relative motion under load. The relative motion is often not intended and may be caused by vibration or part deflection under load. The ASM Handbook on Fatigue and Fracture defines fretting as “a special wear process that occurs at the contact area between two materials under load and subject to minute relative motion by vibration or some other force.” 
         [0003]    One example of this is rocker shafts in internal combustion engines. Rocker shafts are used to mount rockers. Rockers are activated by camshafts and control valve and injector motion. They oscillate on rocker shafts. The rocker shafts are mounted on rocker shaft mounts which are mounted on the cylinder head of an internal combustion engine. 
         [0004]    Fretting can be a problem on the contact surfaces of rocker shaft and rocker shaft mounts, because of rocker shaft deflections. 
         [0005]    Prior art solutions have been proposed that minimize movement by providing a close fit between the rocker shaft and rocker shaft mount. For example, U.S. Pat. No. 6,230,676B1 describes that the semi-circular recess of a rocker shaft mount should be dimensioned just slightly larger than the rocker shaft to permit assembly but prevent unwanted looseness or play. 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect, the present invention is directed to a shim for use in mounting a rocker shaft to a rocker shaft mount. The shim comprises a body having a first surface portion adapted to contact a rocker shaft in use and a second surface portion adapted to contact a rocker shaft mount in use, wherein at least one of said first surface portion and said second surface portion is composed of a copper alloy. 
         [0007]    In another aspect, the present invention is directed to an engine comprising a cylinder block, a cylinder head and a rocker shaft. The cylinder head includes a rocker shaft mount, and a shim, the shim comprising a body having a first surface portion contacting the rocker shaft and a second surface portion contacting the rocker shaft mount, wherein the shim is composed of a shim material with a lower modulus of elasticity than the material of the rocker shaft and rocker shaft mount. 
         [0008]    In another aspect, the present invention is directed to a method of reducing fretting wear in an engine. The engine comprises a cylinder block, a cylinder head, and a rocker shaft, with the cylinder head including a rocker shaft mount. The method includes the step of mounting a shim composed of a shim material with a lower modulus of elasticity than the material of the rocker shaft and rocker shaft mount. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The accompanying drawings illustrate presently preferred exemplary embodiments of the disclosure, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain, by way of example, the principles of the disclosure. 
           [0010]      FIG. 1  is a perspective view of a shim according to an exemplary embodiment of the present disclosure; 
           [0011]      FIG. 2  is a perspective view of a further shim according to an exemplary embodiment of the present disclosure; 
           [0012]      FIG. 3  is a perspective view of a rocker shaft for use with the shims of  FIGS. 1 &amp; 2 ; 
           [0013]      FIG. 4  is perspective view of the rocker shaft of  FIG. 3  in situ on a cylinder head showing placement of the shims of  FIGS. 1 &amp; 2 ; 
           [0014]      FIG. 5  is a perspective view of cylinder head of an engine according to an exemplary embodiment of the present disclosure; 
           [0015]      FIG. 6  is a detailed perspective view of the rocker shaft shims of  FIGS. 1 &amp; 2  in situ on the cylinder head of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A shim  10  is shown in  FIG. 1 . The shim  10  comprises a body  12 , a first surface  14  and a second surface  16 . The shim  10  is a mid-shaft shim  10 . The mid-shaft shim  10  is arcuately shaped. The body  12 , first surface  14  and second surface  16  are also arcuately shaped. Three apertures  18  pass through the body  12  of the mid-shaft shim  10 . The three apertures  18  are orientated along a longitudinal axis A-A of the mid-shaft shim  10  approximately halfway between a first longitudinal edge  20  and a second longitudinal edge  22  of the mid-shaft shim  10 . The mid-shaft shim  10  is composed from a copper alloy. The copper alloy is a bronze. The copper alloy contains manganese and silicon. The mid-shaft shim  10  is approximately 1 mm thick. The distance between the first surface  14  and the second surface  16  is therefore approximately 1 mm. In other embodiments, however, the mid-shaft shim may be thicker or thinner than approximately 1 mm. For example, in other embodiments, the mid-shaft shim may be in the range of approximately 0.5 mm to approximately 1.5 mm thick. 
         [0017]    A second shim  24  is shown in  FIG. 2 . The second shim  24  comprises a body  26 , a first surface  28  and a second surface  30 . The second shim  24  is an end-shaft shim  24 . An aperture  32  pass through the body  26  of the end-shaft shim  24 . A tab  34  projects from a first edge  36  of the body  26  of the end-shaft shim  24 . The tab  34  and aperture  32  are orientated along the longitudinal axis B-B of the end-shaft shim  24  approximately halfway between a first longitudinal edge  38  and a second longitudinal edge  40  of the end-shaft shim  24 . The end-shaft shim  24  is composed from a copper alloy. The copper alloy is a bronze. The copper alloy contains manganese and silicon. The end-shaft shim  24  is approximately 1 mm thick. The distance between the first surface  28  and the second surface  30  is therefore approximately 1 mm. In other embodiments, however, the end-shaft shim may be thicker or thinner than approximately 1 mm. For example, in other embodiments, the end-shaft shim may be in the range of approximately 0.5 mm to approximately 1.5 mm thick. 
         [0018]    A rocker shaft  42  is shown in  FIG. 3 . The rocker shaft  42  comprises a rocker shaft body  44 . The rocker shaft body  44  is substantially cylindrical. A rocker shaft central bore  46  passes through the centre of the rocker shaft body  44 . A first shaft face  48  and second shaft face  50  are provided on either end of the rocker shaft body  44 . 
         [0019]    Two end-shaft flat portions  52  are provided on the circumference of the rocker shaft  42  adjacent the first shaft face  48  and second shaft face  50 . The end-shaft flat portions  52  are squared surfaces around the otherwise circular cross-section of the rocker shaft  42 . 
         [0020]    A mid-shaft flat portion  54  is provided on the circumference of the rocker shaft  42  approximately half-way along the length of the rocker shaft  42 . The mid-shaft flat portion  54  is a squared surface around the otherwise circular cross-section of the rocker shaft  42 . 
         [0021]    The end-shaft flat portions  52  and mid-shaft flat portion  54  share a common plane. 
         [0022]    One end-shaft flat portion aperture  56  is provided on each end-shaft flat portion  52 , and is perpendicular to the rocker shaft central bore  46  passing radially through the rocker shaft body  44 . 
         [0023]    Two mid-shaft flat portion apertures  58  are provided on the mid-shaft flat portion  54 , and are perpendicular to the rocker shaft central bore  46  passing radially through the rocker shaft body  44 . 
         [0024]    A cylinder head  60  is shown in  FIG. 5 . Five main rocker shaft mounts  62  project from the cylinder head  60 . Two perimeter rocker shaft mounts  64  also project from the cylinder head  60 . The main rocker shaft mounts  62  and perimeter rocker shaft mounts  64  project in a common plane. A main mount cylindrical mounting surface  66  is provided on each main rocker shaft mount  62 . A perimeter mount mounting surface  68  is provided on each perimeter rocker shaft mounts  64 . The perimeter mount mounting surface  68  is formed from a cylindrical perimeter mount mounting surface portion  70  joined to a hemispherical perimeter mount mounting surface portion  72 . 
         [0025]    Threaded mounting bores  74  are provided on the main mount cylindrical mounting surface  66  and the perimeter mount mounting surface  68 . A lubricant bore  76  is provided on the main mount cylindrical mounting surface  66 , between two threaded mounting bores  74 , one located longitudinally either side of the lubricant bore  76 . 
         [0026]      FIG. 4  shows detail of the rocker shaft  42  mounted onto the cylinder head  60  using the mid-shaft shim  10  and the end-shaft shim  24 . 
         [0027]    The mid-shaft shim  10  locates around the outer surface of the rocker shaft  42 . The mid-shaft shim  10  is mounted around the portion of the outer surface of the rocker shaft  42  longitudinally adjacent the mid-shaft flat portion  54 . The mid-shaft shim  10  is mounted circumferentially opposite the mid-shaft flat portion  54 . 
         [0028]    The end-shaft shim  24  locates around the outer surface of the rocker shaft  42 . The end-shaft shim  24  is mounted around the portion of the outer surface of the rocker shaft  42  longitudinally adjacent the end-shaft flat portion  52 . The end-shaft shim  24  is mounted circumferentially opposite the end-shaft flat portion  52 . 
         [0029]    Neither mid-shaft shim  10  nor end-shaft shim  24  cover the mid-shaft flat portion  54  or the end-shaft flat portion  52 . 
         [0030]    The rocker shaft  42  is then mounted upon the main rocker shaft mounts  62  and the perimeter rocker shaft mounts  64 . The mid-shaft shim  10  is mounted into the main mount cylindrical mounting surface  66  of a main rocker shaft mount  62 . The mid-shaft shim  10  is therefore sandwiched between the rocker shaft  42  and the main rocker shaft mount  62 . 
         [0031]    The end-shaft shim  24  adjacent the first shaft face  48  is mounted into the perimeter mount mounting surface  68  of a perimeter rocker shaft mount  64 . The end-shaft shim  24  adjacent the second shaft face  50  is mounted into the main mount cylindrical mounting surface  66  of a main rocker shaft mount  62 . 
         [0032]    Both of the shims (mid-shaft shim  10  and end-shaft shim  24 ) may be sized such that they clasp the rocker shaft  42 . For example, in the depicted embodiment of the mid-shaft shim  10 , the first longitudinal edge  20  and the second longitudinal edge  22  are, in a free state, spaced apart a distance that is less than the diameter of the rocker shaft  42 . Thus, when being mounted onto the rocker shaft  42 , the first longitudinal edge  20  and the second longitudinal edge  22  flex apart at the widest part of the rocker shaft  42 . The resilient nature of the shim material results in a gripping force onto the rocker shaft  42  to retain the mid-shaft shim  10  on the shaft. In the depicted embodiment, the mid-shaft shim  10  and the end-shaft shim  24  extend around the outer surface of the rocker shaft  42  greater than half of the circumference of the shaft. In other embodiments, the mid-shaft shim  10  and the end-shaft shim  24  may not be configured to clasp the rocker shaft  42  and may not extend around the outer surface of the rocker shaft  42  greater than half of the circumference of the shaft. 
         [0033]    The tab  34  of the end-shaft shim  24  provides an orientation aid for mounting the end-shaft shim  24 . 
         [0034]    Bolts (not shown) or other suitable mechanical fasteners are used to secure the rocker shaft  42 , mid-shaft shim  10  and end-shaft shims  24  to the cylinder head  60  via the main rocker shaft mounts  62  and the perimeter rocker shaft mounts  64 . The bolts (not shown) pass through the end-shaft flat portion apertures  56  and mid-shaft flat portion apertures  58  of the rocker shaft  42 , through the apertures  18  of the mid-shaft shim  10  and the apertures  32  of the end-shaft shim  24  and into the threaded mounting bores  74 . Washers (not shown) may be provided between the bolt head and the mid-shaft flat portion  54  and/or the end-shaft flat portion  52  to mitigate wear or potential damage. 
         [0035]    The cylinder head  60  may then be mounted upon a cylinder block  61  shown schematically in  FIG. 5 . 
       INDUSTRIAL APPLICABILITY 
       [0036]    During engine operation, undesirable and unintended relative movement of the rocker shaft  42  with respect to the main rocker shaft mounts  62  and the perimeter rocker shaft mounts  64  may occur. Undesirable relative movement may be caused by, for example, vibration from the reciprocation of the various engine parts or part deflection under load, such as the deflection of the rocker shaft due to injector actuation loading. This relative motion may cause fretting of the rocker shaft  42 , the mounts or both. 
         [0037]    The shims (both mid-shaft shim  10  and end-shaft shim  24 ) will provide two surfaces for relative slip to occur over. The first surfaces  14 ,  28  will contact the mounting surfaces  66 ,  68  and the second surfaces  16 ,  30  will contact the rocker shaft  42 . 
         [0038]    The low elastic modulus copper alloy material, in this embodiment bronze, reduces the contact pressure since the ability of the material to deflect allows a larger contact surface to develop between the shims (both mid-shaft shim  10  and end-shaft shim  24 ), the mounting surfaces  66 ,  68  and the rocker shaft  42 . 
         [0039]    The bronze layer provided by the shims  10 ,  24  also disperses the contact pressure developed at the interface between the mounting surfaces  66 ,  68  and the rocker shaft  42  such that the mounting surfaces  66 ,  68  experience less contact pressure than the rocker shaft  42 . 
         [0040]    The bronze contains silicon and manganese that offer improved surface lubrication to further reduce the shear stress at the interface. 
         [0041]    Suitable engine lubricant (not shown) may be pumped through the cylinder head  60 , through the lubricant bore  76 , through the mid-shaft shim  10  and into the rocker shaft central bore  46 . This provides lubrication to the various components. 
         [0042]    The shims (both mid-shaft shim  10  and end-shaft shim  24 ) serve as sacrificial wear parts that can be replaced instead of having to replace the rocker shaft  42  or cylinder head  60  if they were subject to fretting wear. 
         [0043]    A method is also provided for reducing fretting wear by fitting shims composed of a shim material with a lower modulus of elasticity than the material of the rocker shaft  42  and rocker shaft mounts  62 ,  64 . 
         [0044]    It will be apparent to those skilled in the art that various modifications and variations can be made to the apparatus and method. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed apparatus and method. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents. 
         [0045]    For example, although described with particular reference to copper alloys, and in particular bronze containing silicon and manganese alloying elements, different elements, substances or alloys may be used. These may include other copper alloys such as, but not limited to, brass. Furthermore, materials which have the desired property of having a lower elastic modulus than the material from which the cylinder head  60  is made may be considered, such as, for example, mild steel.