Abstract:
The rocker arm ( 2 ), obtained from a shaped element ( 30 ) made of metallic material, comprises surfaces ( 15,18 ) for engagement with a tappet ( 3 ) and with a stem ( 20 ) of a valve, respectively, and a portion ( 7 ) for mounting of a rotatable roller ( 8 ) able to co-operate with a cam-type actuating member ( 4 ). According to the invention, the surface ( 15 ) is formed in an insert ( 25 ) produced separately and fixed to the rocker arm. This constructional solution is particularly advantageous when the design constraints, determined by the position of the other elements of the valve train, require a substantial alignment between the center (A) of the surface ( 15 ) engaging the tappet, the center (B) of the roller and the contact (C) surface engaging the valve stem ( 20 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority, under 35 U.S.C. 119, of earlier-filed Italian Application TO2001A000133, filed Feb. 15, 2001. 
     
    
     
       BACKGROUND OF THE DISCLOSURE  
         [0002]    The present invention relates to a rocker arm for valve trains of internal-combustion engines, and more particularly, to rocker arms for use in valve gear trains of the “end-pivot” rocker arm type.  
           [0003]    For a better understanding of the state of the art regarding the subject in question and the problems relating thereto, firstly a rocker arm of known design will be described, with reference to FIGS. 6, 7A and  7 B of the accompanying drawings.  
           [0004]    [0004]FIG. 6 is a view, partially sectioned longitudinally, of a valve train, generally designated  1 , which is able to cause the alternating rectilinear movement of an engine poppet valve (only a stem  20  of the valve being shown in FIG. 6) in accordance with a predetermined opening sequence. The valve train comprises a rocker arm  2 , a hydraulic tappet  3  and a cam-type actuating member  4 .  
           [0005]    The mutual arrangement of the above-described components may vary depending on the type of engine and the type of distribution chosen. In particular the present invention relates to rocker arms of the type comprising end portions  5  and  6  able to engage the tappet  3  and the valve, respectively, and an intermediate portion  7  intended to receive a roller  8  co-operating with the cam-type actuating member  4 . An example of an embodiment of a rocker arm of this type is illustrated in detail in FIGS. 7A and 7B which show longitudinally sectioned and cross sectional views thereof, respectively.  
           [0006]    The operating principle of a valve train of the above-mentioned type is well-known to a person skilled in the art: the rotational movement of a cam shaft (which is not shown, but which rotates the cam-type actuating member  4 ) is converted into the alternating rectilinear movement of the valve. Such rectilinear movement is the result of the interaction between the cam member  4 , having a base circle portion  9  and an eccentric profile (lift portion)  10 , and the roller  8  of the rocker arm, said interaction acting so as to cause oscillation of the rocker arm in its own longitudinal plane of symmetry (coinciding with the plane of the sheet showing FIG. 6), about a fulcrum point located in the zone of contact between the rocker arm  2  and the tappet  3 .  
           [0007]    At present this type of rocker arm is advantageously produced by means of the operations of:  
           [0008]    (a) shearing of a shaped element  30  (see FIG. 8), from a sheet of steel with a low carbon content, the element  30  having a form symmetrical with respect to a longitudinal axis  35  and being provided with an opening  36 , in a substantially intermediate position, and with two holes  21  and  22  situated laterally with respect to the opening  36 ;  
           [0009]    (b) pressing the above-mentioned shaped element  30  in order to perform bending upwards (or downwards) of lateral portions  31  and  32 , along bending lines  33  and  34 , respectively, so as to provide the part with a substantially U-shaped cross section (see FIG. 7B), having a horizontal plate portion  11  which connects two vertical side walls  12  and  13 ;  
           [0010]    (c) forming the horizontal plate portion  11 , at the end  5  of the rocker arm, so as to produce a partly spherical portion  14  having a concave surface of revolution  15  with an essentially ogive-shaped section able to engage with an essentially hemispherical convex outer surface  16  at the top of the hydraulic tappet  3 ;  
           [0011]    (d) forming the above-mentioned horizontal plate portion  11 , at the end  6  of the rocker arm, so as to produce a shoe element  17  having a surface  18  with its concavity directed downwards and an arched cross section (in the plane of oscillation of the rocker arm), able to interact with the top (tip)  19  of the stem  20  of the engine poppet valve; and  
           [0012]    (e) inserting and locking a cylindrical pin  23  in the two seats defined by the above-mentioned holes  21  and  22 , the roller  8  being rotatably mounted on the pin  23  by means of rolling elements  24  so as to project partially from the opening  36  in order to engage with the cam member  4 .  
           [0013]    The shape and dimensions of the rocker arm  2  are dictated by the design requirements of the engine manufacturer and must therefore be able to satisfy precise geometrical constraints associated with predetermined positions, in the engine cylinder head, of the other valve train elements with which the rocker arm  2  must co-operate. The geometrical constraints determine the arrangement, in the plane of longitudinal symmetry of the rocker arm (coinciding with its plane of oscillation), of three significant points A, B and C, indicated in FIG. 6, as follows:  
           [0014]    A is a center of the theoretical circumference (or hemisphere) of contact between the engaging surfaces  15  and  16  of the rocker arm  3  and the tappet  4 , respectively;  
           [0015]    B is a center of the pin  23  of the roller  8 ; and  
           [0016]    C is a theoretical point of contact between the contact surface  18  of the rocker arm  2  and the contact surface on the valve tip  19  of the valve stem  20 .  
           [0017]    If the design requirements of the engine cylinder head result in the positioning of the above-mentioned point B at a sufficiently large lateral distance from the straight line passing through the other two points A and C at the opposite end zones  5  and  6  of the rocker arm  2 , respectively, the rocker arm may be manufactured by means of simple shearing and bending operations, with low production costs.  
           [0018]    In order to clarify this point, it should be noted, with reference to FIG. 8, how the central opening  36  of the semi-finished product  30  has an elongated shape in the longitudinal direction, with an intermediate section  37  having a transverse dimension, or width, which is smaller than that of two longitudinal end sections  38  and  39  and how the two holes  21  and  22  are positioned opposite the above-mentioned intermediate section  37 . The width of the intermediate section  37  of the opening  36  cannot be less than a certain minimum value imposed by the technological constraints associated with the feasibility of the shearing operation. Consequently, the width of internal flanges  41  and  42  located between the intermediate section  37  and the holes  21  and  22 , respectively, has an upper limit value, once the dimensions of the above-mentioned holes and their distance from the axis  35  have been fixed.  
           [0019]    In the design situation where the center B of the pin  23  is located underneath the straight line joining the end points A and C, the operation of bending of the lateral portions  31  and  32  of the semi-finished product  30  into a “U” is performed downwards, along the bending lines  33  and  34 . These bending lines, viewed in the longitudinal plane of symmetry of the rocker arm, are substantially parallel to the straight line passing through the points A and C. Observing, in FIG. 8, the geometry of the shaped element  30 , it can be easily understood that, if the distance of the point B from the straight line passing through the points A and C is fairly large, then the maximum width of the flanges  41  and  42  is sufficient to perform the function of laterally containing the rolling elements  24  (usually rollers) of the roller  8 .  
           [0020]    When, on the other hand, the center of the pin  23  (point B) must be located above the straight line joining the end points A and C, the above-mentioned operation of bending into a “U” shape is performed upwards, again along the lines  33  and  34 . The two internal flanges  41  and  42  are thus positioned, at the end of bending, underneath the holes  21  and  22  of the pin and therefore must no longer perform the function of laterally containing the rolling elements of the roller, but must ensure the necessary flexural stiffness of the rocker-arm body. If, therefore, the distance of the point B from the straight line A-C, i.e. the distance of the centers of the holes  21  and  22  from the bending lines  33  and  34 , respectively, is too small, the maximum width of the flanges  41  and  42  may not be sufficient to provide the rocker arm with the required rigidity.  
           [0021]    The problem of how to produce a rocker arm by means of pressing therefore arises, in particular, when the design constraints require a substantially aligned position of the three above-mentioned points A, B and C, i.e. essentially the top of the tappet, center of the roller and top of the valve stem.  
           [0022]    Known prior art solutions envisage in this case the bending, for example downwards, of the lateral portions  31  and  32  of a shaped element  30  similar to that of FIG. 8, along bending lines which, viewed in the plane of longitudinal symmetry of the rocker arm, no longer substantially coincide with the straight line passing through the end points A and C, but are inclined upwards through an angle such as to ensure a width of the flanges  41  and  42  sufficient for performing the function of containing the rolling elements of the roller. It is therefore necessary to perform a further operation involving plastic deformation in order to displace the end portion  5  of the horizontal plate portion  11  of the rocker arm, which at the end of this first operation is still located aligned with the bending line, downwards as far as the level of the point C. This process is costly since it requires the use of presses capable of generating very high forces.  
         BRIEF SUMMARY OF THE INVENTION  
         [0023]    Accordingly, it is an object of the present invention to provide a rocker arm which, even in the case of substantially aligned positioning of the above-mentioned three points A, B and C, may be produced through simple bending into a “U” shape by means of pressing without the need for further plastic deformation operations which produce a relative displacement of the end portions  5 ,  6  and the intermediate portion  7  of the rocker arm.  
           [0024]    These and other objects and advantages, which will emerge more clearly from the following description, are achieved by providing an improved rocker arm of the type constructed by means of deformation of a shaped element made of metallic material, comprising surfaces for engagement with a tappet and with a stem of a valve, respectively, and a portion for mounting of a rotatable roller, able to co-operate with a cam-type actuating member.  
           [0025]    The improved rocker arm is characterized by the fact that the surface of engagement with the hydraulic tappet is formed in an insert fixed to the rocker arm. In accordance with a more limited aspect of the invention, the insert is formed, dimensioned, and positioned so that the surface for engagement with the tappet is essentially aligned with the roller and with the surface for engagement with the poppet valve. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The invention will be described in greater detail below, purely by way of a non-limiting example, with reference to the accompanying drawings in which:  
         [0027]    [0027]FIG. 1 is a partially longitudinally sectioned view of a valve train provided with a rocker arm according to the invention;  
         [0028]    [0028]FIGS. 2 a ,  2   b  and  2   c  are, respectively, a longitudinally sectioned view, a laterally sectioned view, and a top plan view of the rocker arm according to FIG. 1;  
         [0029]    [0029]FIG. 3 is a bottom perspective view of the rocker arm of the present invention according to FIG. 1;  
         [0030]    [0030]FIG. 4 is a bottom perspective view of the body of the rocker arm according to FIG. 1, without the roller or the insert;  
         [0031]    [0031]FIG. 5 is a sectioned perspective view of an insert able to be fastened to the rocker arm according to FIG. 1;  
         [0032]    [0032]FIG. 6 is a partially longitudinally sectioned view of a valve train provided with a rocker arm of the conventional type;  
         [0033]    [0033]FIGS. 7A and 7B are, respectively, a longitudinally sectioned view and a cross sectional view of the rocker arm according to FIG. 6; and  
         [0034]    [0034]FIG. 8 is a plan view of a semi-finished article for production of the body of a rocker arm made in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]    In the following description, only the elements and parts of specific importance and interest for the purposes of understanding the invention will be illustrated in detail; as regards, however, elements and parts not mentioned or considered, reference should be made to the solutions of the known type.  
         [0036]    [0036]FIG. 1 shows a valve train  1  in which the geometric arrangement of its elements is such as to involve a substantial alignment of the three significant points A, B and C previously defined with reference to a known example of a prior art embodiment. The valve train comprises a rocker arm  2  of the same type as that shown in FIG. 6, but with the difference that, according to the invention, the concave surface  15  with an ogive-shaped cross section, able to engage with the respective surface  16  of the hydraulic tappet  3 , is formed in an insert  25  produced separately and fixed to the end portion  5  of the rocker arm, instead of being formed directly on this portion by means of a plastic deformation process.  
         [0037]    The point A at the center of the theoretical circumference (or hemisphere) of contact between the concave surface  15 , formed in the insert  25 , and the surface  16  of the top of the tappet is substantially aligned with the center of the roller  8  and with the point C of theoretical contact between the surface  18  of the rocker arm and the top  19  of the valve stem  20 .  
         [0038]    The insert  25 , which may be advantageously made by means of cold-pressing (cold forming) from a material which is not necessarily identical to that used for the body of the rocker arm  2 , has preferably a parallelepiped shape. The insert  25  has two flat side faces  26  and  27  (FIG. 5) able to mate with respective inner faces  12   a  and  13   a  (visible in FIG. 4) of the side walls  12  and  13  in the end portion  5  of the rocker arm.  
         [0039]    With reference still to FIG. 5, in which a preferred embodiment of the insert  25  is shown, it can be seen how, in order to facilitate assembly and engagement with the rocker arm, the insert has, on its upper side  28 , a lug  29 . The lug  29  is able to be inserted into, and mate with, a hole  45  formed in the horizontal plate portion  11  of the rocker arm in the zone where the insert  25  is housed. The lug  29  may either have only a positioning and centering function during assembly of the insert or form an actual fastening element co-operating with the body of the rocker arm. In this latter case, the lug  29  has a height preferably greater than the thickness of the sheet metal of the rocker arm body, so that once inserted inside the hole  45 , an upper end portion extends beyond the upper wall of the horizontal plate portion  11  and is thus capable of being fixed to the wall. This fixing operation may be performed in conventional ways, known to a person skilled in the art, which are simple to perform and not costly, for example, by means of crushing or riveting.  
         [0040]    It can therefore be understood how, with a rocker arm according to the invention, it is possible to satisfy fully the design specifications imposed by the engine manufacturer, in terms of geometrical and mechanical characteristics, by means of simple and high-productivity machining operations which can be performed using machines which do not require a high initial outlay. In particular, the adoption of an insert produced separately from the body of the rocker arm means that it is no longer required to perform further plastic deformation operations which require the use of presses capable of generating high pressing forces and therefore having a correspondingly high cost. According to the present invention, in fact, these plastic deformation operations are replaced by cold-pressing of the insert, with the formation of its concave surface having an ogive-shaped cross section (although those skilled in the art will understand that such a shape is by way of example only), and by fixing the insert to the rocker arm body. Such fixing may be advantageously performed, in the case of an insert provided with the mating lug  29 , by means of the same type of operation used for locking the pin of the roller in the two holes of the rocker arm body, for example by riveting the top end of the above-mentioned lug  29  in order to produce wedging thereof by means of interference inside the corresponding hole  45 .  
         [0041]    Another advantage of the rocker arm of the present invention is the “modular” design, because it is possible to use the same type of insert for rocker arms with different shapes and/or dimensions or, vice versa, use inserts with different shapes and/or dimensions for the same type of rocker arm.  
         [0042]    It is also possible to manufacture the insert  25  from a material which is different or is treated differently from the shoe  17 , so as to satisfy the opposing requirements arising from the two couplings, i.e. the insert/tappet coupling and the shoe/stem coupling. In the first case, in fact, the coupling between the concave ogive-shaped surface of revolution  15  of the insert and the substantially hemispherical surface  16  of the tappet produces low contact stresses, but high frictional wear; on the other hand, the coupling between the convex surface  18  of the shoe with an arched cross section and the generally flat surface at the top  19  of the valve stem  20  produces high contact stresses, but low frictional wear.  
         [0043]    Another advantageous feature of the invention consists of the possibility of implementing a simple but effective system for lubricating the roller, based on the use of the oil which is supplied from the hydraulic tappet and which has the function of lubricating the surfaces  15  and  16  of contact between the tappet  3  and the rocker arm  2 . With reference to FIG. 5, a through-hole  43  of small diameter is formed at the top of the ogive-shaped surface  15  of the insert, the hole  43  allowing the lubricating oil to flow from the zone of engagement with the top end of the tappet towards the upper surface of the insert around the lug  29 . The oil may thus be collected in an impression  44  (FIG. 4) formed in the bottom wall of the horizontal plate portion  11  of the rocker arm around the hole  45  and distributed towards the roller by means of a channel  46 . The channel  46  is also formed on the bottom wall of the plate portion  11  so as to perform lubrication of the contact surfaces of the roller  8  and the cam member  4 . In addition to the channel  46  which can be seen in FIG. 4, arranged longitudinally, other channels may also be formed, for example two channels arranged laterally on opposite sides of the above-mentioned channel  46  so as to convey part of the oil collected in the impression  44  towards the two zones of frictional contact between the side surfaces of the roller and the inner faces of the side walls  12  and  13  of the rocker arm body.  
         [0044]    Obviously, without modifying the principle of the invention, the embodiments and the constructional details may be greatly varied from that described and illustrated purely by way of a non-limiting example, without thereby departing from the scope of the invention as defined in the accompanying claims.  
         [0045]    The invention has been described in great detail in the foregoing specification, and it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the appended claims.