Abstract:
A rocker arm assembly has a first arm for following a first or opening camshaft lobe, a second arm for following a second or closing camshaft lobe, and a pivot axis of the rocker arm therebetween. A captive roller follower on the first arm follows the first lobe, and a slider on the second arm follows the second lobe. The position of the slider with respect to the roller and to the pivot axis is mechanically and controllably adjustable to optimally set the mechanical lash among these components after installation of the rocker arm assembly into a variable valve mechanism of an internal combustion engine.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a Continuation-In-Part of a pending U.S. patent application, Ser. No. 09/755,345 filed Jan. 5, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to valve train systems for use on internal combustion engines; more particularly, to devices for controllably varying the lift and/or timing of valves in such engines; and most particularly, to means for controlling the lash between the camshaft lobes and a two-arm rocker arm in a valve train system having cam lobes for both opening and closing an individual valve.  
         BACKGROUND OF THE INVENTION  
         [0003]    Devices for controllably varying the degree of lift and the timing of opening and closing valves in internal combustion engines are well known. See, for example, U.S. Pat. No. 5,937,809 issued Aug. 17, 1999 to Pierik et al., and U.S. Pat. No. 6,019,076 issued Feb. 1, 2000 to Pierik et al., the relevant disclosures of which are herein incorporated by reference. Such a device is referred to in the art as a Variable Valve Mechanism (WM). Such devices commonly employ a rocker arm which pivots with or about a shaft or pin as a part of the apparatus train. Typically, the rocker arm has a first bearing element, for example a roller, which follows the profile of a cam lobe during rotation of a camshaft.  
           [0004]    Conventional variable valve mechanisms typically include many component parts, such as link arms, joints, pins, and return springs, and are thus relatively complex mechanically. Return springs are used typically to maintain the roller in contact with the input cam lobe and to reduce mechanical lash as the input cam lobe rotates from a high lift position to a low lift position. The use of such return springs negatively impacts the durability of the WM and also may limit the operating range of the mechanisms, thereby limiting the operation of the intake valve throttle control system to a correspondingly-limited range of engine operation.  
           [0005]    It is known to provide a second cam lobe per valve in place of return springs, and to employ a two-armed rocker arm sub-assembly having appendages in contact with both the opening lobe and the closing lobe at all times. The angular orientation between the eccentrics of the opening and closing lobes on the camshaft defines the rotational angle through which the valve is open. Typically, the surface of the opening lobe is followed by a roller mounted on the first rocker arm, and the surface of the closing lobe is followed by a slider mounted on the second rocker arm. Such an arrangement provides positive control of the rocker arm sub-assembly, and thus of the associated valve, at all positions of the camshaft and obviates the need for return springs.  
           [0006]    A practical problem can arise in manufacturing and assembling such a two-lobe system. The stack-up of machining and mounting tolerances among the rocker, the roller, the pivot shaft for the rocker, the two cam lobes, and the camshaft mounting in the engine head can be formidable. Ideally, the roller and slider are just lightly in contact with the base circles of their respective cam lobes during the non-lift portions of the rotational cycle. If this lash relationship is too tight, i.e., zero or negative clearance, the valve may not open or close properly, or the rocker arm assembly may be stressed and distorted. If the lash relationship is too loose, the rocker arm assembly may clatter or chatter undesirably against the cam lobes, and the valve may not open fully or precisely in time.  
           [0007]    What is needed is a simple means whereby the valve train components may be manufactured and assembled with loose tolerances and then the lash relationship of the cam followers to the cam lobes may be easily and precisely adjusted and retained after the valve train is assembled.  
           [0008]    It is a principal object of the present invention to provide improved apparatus and method for setting the lash relationship of cam followers to cam lobes in a two-cam, two-follower valve train.  
           [0009]    It is a further object of the invention to provide such a system wherein the setting may be conveniently and accurately done after the valve train is assembled.  
         SUMMARY OF THE INVENTION  
         [0010]    Briefly described, a rocker arm assembly in accordance with the invention has a first arm for following a first or opening camshaft lobe and a second arm for following a second or closing camshaft lobe, the arms being designated with respect to a pivot axis of the rocker arm therebetween. Preferably, the first arm is provided with a captive roller follower and the second arm is provided with a captive sliding follower or slider. These elements are so selected for economy because opening of the valve is more mechanically demanding than is closing it. The pivot axis of the rocker arm, the surface of the roller at the contact point with the opening lobe, and the surface of the slider at the contact point with the closing lobe, taken together define a triangle in space which must fit exactly into the space requirements of the valve train assembly of each valve in a multi-cylinder engine. In accordance with the invention, the shape of the triangle is mechanically and controllably adjustable to change the location of the slider with respect to the other two points of the triangle and to the camshaft axis of rotation, to adjustably control the mechanical lash in the system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is an elevational view of a rocker arm assembly for a double-lobe camshaft substantially as disclosed in the parent application, Ser. No. 09/755,345, having some components omitted for clarity, wherein the relative positions of the assembly pivot axis, roller, and slider are fixed and not adjustable;  
         [0013]    [0013]FIGS. 2 through 9 are elevational views of various embodiments of a rocker arm assembly in accordance with the invention, illustrating various means for controllably varying the spacing of the closing-lobe slider to the closing lobe. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    Referring to FIG. 1, there is shown a portion of a variable valve mechanism  10 , including an input camshaft  12  on which are mounted valve-opening cam lobe  14  and valve-closing cam lobe  16 . The cam lobes are disposed in a predetermined angular relation relative to each other and relative to central axis A of input shaft  12 . The paired lobes  14 , 16  (only one pair shown) are spaced along the length of input shaft  12 . Each respective pair of cam lobes  14 , 16  is associated with a corresponding variable valve mechanism  10  and with a corresponding cylinder of an internal combustion engine (not shown). For purposes of clarity, a single variable valve mechanism  10  is discussed hereafter.  
         [0015]    Opening cam lobe  14  and closing cam lobe  16  rotate as substantially one body with input shaft  12 . The lobes are, for example, affixed to or integral with shaft  12  which is received within and extends through bearing mounts disposed on the head of the engine.  
         [0016]    Output cam  18  is oscillatably disposed on shaft  12  for actuation of a valve stem, tappet, or roller finger follower (none shown) in known fashion via contact with eccentric surface  20 . Cam  18  is pivotably connected to link  19  which is an elongate arm member pivotably coupled at a first end to output cam  18  and at a second and opposite end to rocker assembly  22 . Rocker assembly  22  is coupled, for example, by pins  24 , to link  19  and to a frame member (omitted for clarity) about which it pivots upon axis B. The frame member may be independently rotated to various positions about shaft  12  to advance or retard the timing of valve opening, as disclosed in the incorporated reference patents. Rocker arm assembly  22  may be thought of as comprising two arms  26 , 28 . First arm  26  carries roller  30  which followingly engages valve-opening cam lobe  14  along eccentric surface  32  and is pivotably pinned to link as discussed above. As shaft  12  and lobe  14  rotate, roller  30  causes assembly  22  to pivot about axis b, thus causing, via link  19 , output cam  18  to oscillate about shaft  12 .  
         [0017]    Rocker assembly  22  further includes a following slider pad  34  disposed on second arm  28  which slidingly engages valve-closing cam lobe  16  along eccentric surface  36 . Lobes  14 , 16  are so shaped and oriented, and arms  26 , 28  are so oriented with respect to axis B that followers  30 , 34  are in contact with eccentric surfaces  32 , 36 , respectively at all times during rotation of shaft  12 . Thus the action of rocker assembly  22  is fully controlled at all times and does not require use of return springs to assure proper motion. (In practice, the lash adjustment of the system optimally provides for a rest clearance of about 0.001″ between slider  34  and surface  36 .)  
         [0018]    As noted above, there is little room for error in the manufacture and installation of the components shown in FIG. 1, or of the receiving engine head and WM mounting components as well. Assembly  22  must have roller  30  and slider  34  positioned accurately with respect to axis B such that they just touch surfaces  32 , 36 , respectively. It is a principal object of the invention to provide apparatus and method whereby the position of the slider may be adjusted post-assembly to achieve the required degree of accuracy and optimal amount of lash.  
         [0019]    Referring to FIG. 2, an improved rocker arm assembly  22  is shown wherein slider  34  is disposed close-fittingly in a well  38  formed in arm  28  and is guided by the walls of the well in motion into and out of the well. A smooth portion of adjustment screw  40  is retained in a smooth bore  42  through arm  28  into well  38  and is retained therein by flange  44 . A threaded portion of screw  40  extends into a mating threaded bore in slider  34 . Slider  34  may be advanced or retarded with respect to surface  36  by rotation of screw  40 . Once the proper position of slider  34  is set, screw  40  may be locked from further rotation by any of various well-known locking means.  
         [0020]    Referring to FIG. 3, in this embodiment, a stepped well  46  is provided in arm  28 . Slider  34  has a head portion  48  for engaging surface  36  and a shaft portion  50  extending through bore  42 . Head portion  48  is provided with a boss  52  extending into well  46  and engaging belleville washer  54  in the bottom of the well. Shaft portion  50  is threaded where it exits bore  42  opposite well  46 . Nut  56  may be rotated to vary the axial position of shaft  50  in bore  42 , and thus head portion  48  with respect to surface  36 , by using washer  54  as a resistance spring. Once adjusted, the position of nut  56  may be fixed by lock nut  58 . An advantage of this embodiment is that the belleville washer can also function as a load-leveling or load-relieving spring as required.  
         [0021]    Referring to FIG. 4, in this embodiment, arm  28  is formed having a tapered longitudinal slot  60  having inner  61  and outer  63  jaws receivable of a wedge  62  having a threaded longitudinal bore  64 . Preferably, inner jaw  61  is thinner and more flexible than outer jaw  63 . A smooth bore  66  extends through the longitudinal remainder of arm  28 , as well as through pin  24 . A threaded adjustment screw  68  extends through bore  66  and engages wedge  62  along threaded bore  64 . Pin  24  is thus rotatably attached to assembly  22  and turns with it. In addition, the screw through the pin retains the pin in the rocker assembly, so no other retaining feature is needed. Rotation of screw  68  acts to drive wedge  62  into or out of slot  60 , thereby adjusting the spacing of slider  34  in relation to eccentric surface  36 .  
         [0022]    Referring to FIG. 5, this embodiment is similar to that shown in FIG. 4 except that screw  68  is threaded throughout its length. Again, pin  24  is rotationally coupled to rocker assembly  22 .  
         [0023]    Referring to FIG. 6, this embodiment is similar to the embodiments shown in FIGS. 4 and 5. However, outer jaw  63  is provided with a load-leveling and load-relieving system similar to that shown in FIG. 3. A well  70  in outer jaw  63  is receivable of a piston  72  backed by a belleville washer  54  such that unexpected or transient loads placed on slider  34  may be absorbed by axial compression or extension of washer  54 . The overload compensating features are adjustable at assembly by the choice of size of the belleville washer and by insertion of spacers  71  under the washer. Preferably, the belleville washer is preloaded to a predetermined degree by depression of the piston, and the piston is then retained at that degree of preload by a retaining clip  73  disposed in an annular groove in the wall of well  70  above piston  72 .  
         [0024]    Referring to FIG. 7, in this embodiment, second arm  28  is separate from first arm  26 , and both are pivotable on pin  24  about axis B. Arm  26  is provided with an extension  74  extending beyond pin  24  and generally parallel to arm  28  and having a threaded bore  76  therethrough in the direction of arm  28 . A well  77  in arm  28  is receivable of a piston  78  and a belleville washer  54  for cushioning the axial motion of piston  78  in well  76 , similar to the mechanism disclosed in FIG. 6 and described above. Preferably, piston  78  has a recess  80  in an outer surface thereof for receiving an end of an adjusting screw  68 . Rotation of adjusting screw  68  changes the included angle between arms  26  and  28 , and thus changes the relation between slider  34  and the valve-closing cam.  
         [0025]    Referring to FIGS. 8 through 10, arm  28  is provided with inner and outer jaws  61 , 63 , respectively, separated by a tapered slot  60 .  
         [0026]    In the embodiment shown in FIG. 8, a wedge  62  has a longitudinal threaded bore  64 , and pin  24  has a corresponding threaded bore  64 a such that the wedge is drawn into or out of the jaws upon rotation of screw  68 , thus moving slider  34  toward or away from cam lobe  16  (not shown). Pin  24  is thus rotationally coupled to rocker assembly  22 , as in the embodiments shown in FIGS. 4 and 5.  
         [0027]    In FIG. 9, screw  68  is engaged in a threaded portion  76  of arm  28 , and rocker assembly  22  is thus free to rotate about pin  24 .  
         [0028]    In FIG. 10, wedge  62  is disposed in tapered slot  60  and screw  68  is disposed in an adjacent threaded bore, the head of screw  68  overlapping the butt end of wedge  62 . The wedge may be driven into or brought out of the tapered slot via rotation of the screw.  
         [0029]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.