Valve crosshead assembly with wear-reducing contact pad

Abstract: In accordance with preferred embodiments of the invention, a disc-shaped contact pad (29) of ceramic material is disposed within a recess (32) formed in the top surface of a metal valve crosshead (27). The pad (29) is held in the recess(32) by a retainer (31) that clips onto the crosshead (27). The retainer (31) is designed to maintain only a light pressure on only a narrow shoulder (29c) of the pad (29) to keep it seated, and does not, itself, experience any dynamic loading. Furthermore, by the provision of a circumferential clearance between the pad (29) and both the wall of the crosshead recess (32) and the retainer (31), the need for precision machining of the ceramic material can be avoided. Additionally, a reduction of frictional sliding forces imposed on the crosshead (27), and in turn upon the valves, by the rocker lever is achieved by the ceramic pad (29), so that resultant frictional side loading is not imposed on adjacent valve components to a significant extent.

Description: The present invention relates to valve crosshead assemblies which carry valves of an internal combustion engine, and which are acted upon by the valve rocker levers of the valve drive train. The present invention also relates to the use of ceramic materials to provide wear-resistant contact surfaces for internal combustion engine components. In particular, the present invention relates to a valve crosshead assembly according to the preamble of claim 1 and to an engine cylinder head valve drive train. The harsh operating conditions encountered in an internal combustion engine, particularly the high temperatures and high pressures, cause engine components to wear rapidly. Mechanically driven actuators and actuating components are especially susceptible to wear in this environment. Consequently, the materials used for producing actuating engine components should provide good mechanical strength, thermal stability and wear resistance. While metals have typically been used to form such components, ceramics, such as zirconia, silicon nitride, silicon carbide and the like, have been found to exhibit excellent mechanical strength, thermal stability and improved wear resistance relative to metals. As a result, ceramics are increasingly being used as structural materials for components of gas turbine engines and diesel engines. However, even though ceramics can provide highly wear-resistant engine components, they are generally hard and brittle and lack the formability and workability of the metals which are conventionally applied to low cost precision engine components. Furthermore, while components formed from a ceramic element and a metal element have been proposed to overcome these limitations, and components of this type that can be useful as internal combustion engine components are available, composite components formed of metal and ceramic elements are not without their own problems. The low thermal expansion and tensile strength properties of structural ceramics relative to metals, in general, make formation of a secure connection between these two elements difficult to achieve. Presently, each element is machined to produce tolerances that are not only sufficiently precise to ensure the retention of the ceramic element in the metal element during engine operation, but which also allow for the differential thermal expansion of the ceramic and the metal, and limit tensile stresses in the ceramic. US - A - 5,236,274, which forms the starting point of the present invention. discloses a valve drive train with a crosshead comprising a ceramic pad in a recess. The pad forms a contact surface for a rocker arm. A retainer is disposed in an annular space between the pad and the recess. The retainer holds the pad in the recess in a fixed position by means of frictional forces. Thus, relatively close tolerances are required. Further, side loads of the pad stress the retainer and can decrease the holding ability of the retainer. Object of the present invention is to provide a valve crosshead assembly with a ceramic insert and an engine cylinder head valve drive train with such a valve crosshead assembly, wherein frictional wear and energy loss from contact by a rocker lever as well as friction induced side loads are minimized, while the valve crosshead assembly is relatively simple in its construction and can be produced at low costs. The above object is achieved by a valve crosshead assembly according to claim 1 and an engine cylinder head valve drive train according to claim 19, respectively. Preferred embodiments are subject of the subclaims. An advantage of the present invention is that the ceramic insert is loosely retained in a metal element in a manner enabling the insert to move laterally with respect to the crosshead. These and other objects are obtained preferably by disposing a disc-shaped wear pad of ceramic material within a recess formed in the top surface of the metal valve crosshead. The pad is held in the recess by a retainer that clips onto the crosshead. The retainer is designed to maintain only a light pressure on only a narrow shoulder of the pad to keep it seated, and does not, itself, experience any dynamic loading. Furthermore, by the provision of a circumferential clearance between the pad and both the wall of the crosshead recess and the retainer, the need for precision machining of the ceramic material can be avoided. Additionally, a reduction of frictional sliding forces imposed on the crosshead, and in turn upon the valves, by the rocker lever is achieved by the ceramic pad, so that resultant frictional side loading is not imposed on adjacent valve components to a significant extent. In this regard, it is noted that parasitic horsepower loss due to friction between a conventional metal rocker lever nose and a metal crosshead is significant because an engine uses multiple crossheads, e.g., in a 4-valve per cylinder engine there are two crossheads per cylinder, so that in a 6-cylinder engine there would be twelve crossheads with twelve rocker lever-crosshead frictional interfaces. Moreover, it has been found that the reduced friction achieved at each lever-crosshead interface by the use of a ceramic pad in accordance with the present invention achieves a two-fold frictional horsepower loss reduction. Firstly, parasitic horsepower loss associated with the frictional conversion of energy to heat is reduced at each lever-crosshead interface. Furthermore, the reduced side loading imposed on the valve stems reduces the sliding friction of the valve stem against the valve guide bore as the valve stem slides up and down during valve operation, thereby reducing the parasitic horsepower loss due to frictional conversion of energy to heat at the valve guide bores. In a particularly preferred embodiment, the ceramic pad is given a noncircular shape to prevent it from rotating in its receiving pocket of the crosshead, thereby reducing sliding wear between the crosshead and the ceramic pad as well. These and further objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying figures of the drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention. Brief Description of the Drawings Fig. 1 schematically depicts an engine cylinder head valve drive train of the type in which the valve crosshead assembly of the present invention is used. Figs. 2-5 depict the location of contact between a valve rocker lever and a valve crosshead at various points in the opening and closing cycles of engine exhaust and intake valves; Fig. 6 is an exploded, partial sectional, view of a valve crosshead assembly in accordance with a preferred embodiment of the present invention; Fig. 7 is a cross-sectional view taken along line 7-7 of Fig. 6 but with the parts in their assembled condition; Fig. 8 is a top view of the assembled valve crosshead assembly; Fig. 9 is a bottom view of the assembled valve crosshead assembly. Figs. 10 & 11 are top views corresponding to that of Fig. 8 but of two modified valve crosshead assemblies; Fig. 12. is a cross-sectional view corresponding to that of Fig. 7 but of the crosshead assemblies of Figs. 10 & 11; Figs. 13 & 14 are top and side views of the retainer of the valve crosshead assemblies of Figs. 10 & 11; and Figs. 15 & 16 are top and side views of the contact pad of the valve crosshead assembly of Fig. 11. Throughout the figures, like numerals are used to indicate corresponding components of the various embodiments with prime (') and double-prime (") designations being using to distinguish elements of the second and third embodiments which have been modified relative to the corresponding parts of the first embodiment. Detailed Description of the Preferred Embodiments Fig. 1 depicts an engine cylinder head valve drive train wherein ball and socket joints 11 are provided at each of opposite ends of a push rod 13 that is used to transmit movement produced by a cam 15 to a valve rocker lever 17. Movement of the valve rocker lever 17, produced by the cam 15 and push rod 13, in turn, seats and unseats engine cylinder valves 19 with respect to valve seats 21 by acting on valve crosshead 23. To the extent described so far, this engine cylinder head valve drive train is conventional. As can be seen from Figs. 2-5, which show, for one example, the position of the rocker lever 17 relative to the valve crosshead 23 for intake and exhaust loads at zero lift (Fig. 2), for intake and exhaust loads at one-third lift (Fig. 3), intake loads at full lift (Fig. 4), and exhaust loads at full lift (Fig. 5), except at a position where the valves are at a position of one-third lift (Fig. 3), the dynamic forces imposed by the rocker lever 17 on the crosshead 23 are not centered relative to the crosshead centerline represented by dashed line C. Furthermore, it has been determined that, as can be seen by the line of action of these dynamic forces represented by arrow F, these forces act on the rocker lever 23 at a point that moves back and forth between an inboard side of centerline C (Fig. 2) and an outboard side thereof (Figs. 4 & 5). As a result, bending moments are created due to the frictional forces between rocker lever 17 and the crosshead 23, which forces have been found to increase as the rocker lever 17 wears and are a major factor in fatigue failure of the stem 19a of the valves 19. Thus, to reduce frictional wearing of the rocker lever 17 and friction induced side loading of the crosshead 23 which can lead to fatigue failure of the valve stem 19a, the crosshead 23 of the illustrated drive train has been modified from that of the prior art. In particular, the crosshead 27 in accordance with the invention, comprises an assemblage of metal and ceramic components. In particular, the valve crosshead assembly 25, as illustrated in Fig. 6, includes a valve crosshead 27, a contact pad 29 and a retainer 31. The crosshead 27 is formed of metal, e.g., cold formed and heat treated 8620 steel, the contact pad 29 is manufactured from ceramic powders, e.g., silicon nitride powders that are die pressed and pressureless sintered, and the retainer 31 is formed, e.g., by being stamped from flat stock, of spring steel. To receive the contact pad 29, the crosshead 27 has a recess 32 formed in its top surface. Recess 32 has a diameter which is sufficiently larger than that of the contact pad 29 to insure that the contact pad 29 is loosely received therein despite manufacturing tolerances, i.e., given the usual manufacturing tolerances, a largest possible contact pad 29 within its tolerance range may simply be dropped within a smallest possible recess 32 within its tolerance range without the application of force (e.g., with a 0.1 mm tolerance for the ceramic pad 29 and 0.05 mm tolerance for the diameter of recess 32 in metal crosshead 27, pad 29 would have a design size that is at least 0.15 mm smaller than the diameter of recess 32). Furthermore, instead of providing a radiused curve between the bottom and side walls of the recess 32, a rounded groove 34 is provided at the junction of the bottom and side walls of the recess 32. In this way, it can be assured that the flat bottom of the contact pad 29 will rest flush upon the bottom wall of the recess 32, even if the contact pad 29 adjoins the side wall of the recess 32. In contrast, with a normal radiused curve, it is possible for the contact pad to ride up the radiused curve, resulting in lifting of that portion of the contact pad 29, tilting of the contact pad in a way that could affect valve operation. As can be seen from Figs. 6 and 7, contact pad 29 is disc-shaped having a base portion 29a and a reduced diameter contact portion 29b. Base portion 29a loosely fits within recess 32, as described above, and has a thickness which is at most only slightly larger than the depth of recess 32, and which can be less. The overall height of the contact pad 29 is such that the contact portion 29b projects above the top surface 27a by, for example, approximately 1 mm. Retainer 31 is in the form of a spring clip that can be snapped onto the crosshead 27 in a position overlying the recess 32, and has an opening 31a (Fig. 8) that is smaller than the outer diameter of base portion 29a of the contact pad 29, yet is larger than the diameter of contact portion 29b. In this way, the shoulder 29c of contact pad 29, formed by the top surface of base portion 29a, will be restrained from movement out of the recess 32 by the retainer 31, and the contact portion 29b can extend through the opening 31a with clearance, as apparent from Figs. 7 and 8, to provide a low-friction, wear-resistant contact surface for the valve rocker lever 17. Preferably, retainer 31 maintains a light pressure on shoulder 29c to keep the contact pad 29 seated in recess 32 while allowing some lateral play. Retainer 31, itself, is held onto the crosshead 27 by inwardly bent ends 31b thereof which engage themselves on the bottom of a notch 27b that is formed in the underside of the crosshead 27, as shown in Figs. 7 and 9. Figs. 10-16 show two modified embodiments that are particularly suited for situations where the rocker lever must be scalloped at one side, for example, to accommodate the diameter of a fuel injector spring. In the case of the Fig. 10 embodiment, the valve crosshead assembly 25 has a rocker lever 27' which has a scallop 34 at one side. Contact pad 29' is shaped as shown in Figs. 15 and 16 to accommodate the scallop 34 by being truncated at opposite lateral sides, and the recess 32 in which it is received is similarly modified, to avoid having to substantially reduce the size of the contact portion 29'b. In the case of the Fig. 11 embodiment, to minimize edge stresses by enabling a larger contact portion 29"b to be provided, the side of the contact portion 29"b adjacent the scallop 34 is also scalloped. The use of a rotationally asymmetric contact pad 29', 29" has the added advantage of precluding relative rotation between the contact pad 29', 29" and the crosshead 27' and thereby reducing sliding wear therebetween. To hold the contact pad 29', 29" in place, a modified retainer 31', 31'' is utilized which, unlike retainer 31, is side-mounted instead of being top-mounted, as is particularly apparent from Fig. 12. As can be seen from Fig. 14, the retainer 31', 31'' has an unstressed condition in which the free ends of its legs approach each other, as represented in broken lines. Thus, when the retainer 31', 31" is mounted on the rocker lever 27', its legs are flexed outwardly into their solid-line position, and thus, are resiliently stressed sufficiently to hold the retainer 31', 31" in place. Additionally, as can be seen in Figs. 10, 11 & 13, retainer opening 31'a, 31"a is open in the area where the contact pad 29'b, 29"b adjoins the scallop 34. While various embodiments in accordance with the present invention have been shown and described, it is understood that the invention is not limited thereto, and is susceptible to numerous changes and modifications as known to those skilled in the art. For example, while the preferred ceramic material for the contact pad has been indicated to be silicon nitride, other ceramic materials, such as alumina, zirconia, and zirconium alumina composites may be used. Therefore, this invention is not limited to the details shown and described herein, and includes all such changes and modifications as are encompassed by the scope of the appended claims. Industrial Applicability The valve rocker assembly of the present invention will find utility with respect to a wide variety of engine valve train types, and especially, those where wear and frictional side loading effects due to contact between the rocker lever and valve crosshead are found to be problematic. The present invention will also be found to be helpful in reducing parasitic power loss, thereby offering an opportunity to obtain improved fuel economy.

Claims: Valve crosshead assembly (25, 25', 25'') comprising a valve crosshead (27, 27') formed of metal having a recess (32) in a top surface (27a) thereof, a disc-shaped contact pad (29, 29',29'') formed of a ceramic material, and a resilient retainer (31, 31', 31'') for keeping the contact pad (29, 29', 29'') in the recess (32), characterized in that the contact pad (29, 29', 29'') is loosely disposed in said recess (32) and the resilient retainer (31, 31', 31'') is clipped onto the valve crosshead (27, 27') in a position partially overlying said recess (32) and a portion of a top surface of the contact pad (29, 29', 29'') as a means for retaining the contact pad (29, 29', 29'') in said recess (32). Valve crosshead assembly (25, 25', 25'') according to claim 1, characterized in that the ceramic material of which said contact pad (29, 29', 29'') is formed is silicon nitride. Valve crosshead assembly (25, 25', 25'') according to claim 1 or 2, characterized in that said contact pad (29, 29', 29'') has a base portion (29a) which is disposed in said recess (32) and a contact portion (29b, 29'b, 29''b); the contact portion (29b, 29'b, 29''b) being smaller in area than said base portion (29a) and projecting therefrom above the top surface (27a) of the valve crosshead (27, 27'). Valve crosshead assembly (25, 25', 25'') according to claim 3, characterized in that a shoulder (29c) is formed by a top surface of said base portion (29a) and said contact pad (29, 29', 29'') is retained in said recess (32) by engagement of said resilient retainer (31, 31', 31'') on said shoulder (29c). Valve crosshead assembly (25, 25', 25'') according to claim 4, characterized in that said resilient retainer (31, 31', 31'') has a pair of legs at opposite sides of a connecting portion. Valve crosshead assembly (25, 25', 25'') according to claim 5, characterized in that said connecting portion is disposed on the top surface (27a) of the valve crosshead (27, 27') and has an opening (31a, 31'a, 31''a) through which the contact portion (29b, 29'b, 29''b) of the contact pad (29, 29', 29'') passes. Valve crosshead assembly (25, 25', 25") according to claim 6, characterized in that each of said legs of the resilient retainer (31, 31', 31") is disposed on a respective side of the valve crosshead (27, 27') and has a free end (31b) which is inwardly bent and is engaged on a bottom surface of the valve crosshead (27, 27'). Valve crosshead assembly (25, 25', 25'') according to claim 7, characterized in that the bottom surface of the valve crosshead (27, 27') has a notched area in which the free ends (31b) of the legs of the resilient retainer (31, 31', 31'') are engaged. Valve crosshead assembly (25, 25', 25") according to claim 5, characterized in that said connecting portion is disposed on a side surface of the valve crosshead (27, 27') with said legs on top (27a) and bottom surface of the valve crosshead (27, 27'). Valve crosshead assembly (25, 25', 25'') according to claim 9, characterized in that the leg of said resilient retainer (31, 31', 31") that is disposed on the top surface (27a) of said valve crosshead (27, 27') has an opening (31a, 31'a, 31''a) through which the contact portion (29b, 29'b, 29''b) of the contact pad (29. 29', 29'') passes. Valve crosshead assembly (25, 25', 25'') according to claim 10, characterized in that a side of the valve crosshead (27, 27') opposite that on which said connecting portion is disposed is scalloped between said legs in proximity to said recess (32). Valve crosshead assembly (25, 25', 25'') according to claim 11, characterized in that the base portion (29a) of the contact pad (29, 29', 29'') is truncated so as to eliminate said shoulder (29c) at each of opposite sides thereof. the truncated sides being retained in the recess (32) of the valve crosshead (27, 27') so as to be closest to said sides of the valve crosshead (27, 27'). Valve crosshead assembly (25, 25', 25'') according to claim 12, characterized in that the contact portion (29b, 29'b, 29''b) is scalloped in proximity to the scallop (34) of the valve crosshead (27, 27'). Valve crosshead assembly (25, 25', 25'') according to claim 13, characterized in that the opening (31a, 31'a, 31''a) in the resilient retainer (31, 31', 31'') is laterally open in a direction toward said scallop (34). Valve crosshead assembly (25, 25', 25") according to any one of the preceding claims, characterized in that a circumferential clearance between the contact pad (29, 29', 29'') and the resilient retainer (31, 31', 31'') is provided. Valve crosshead assembly (25, 25', 25") according to any one of the preceding claims, characterized in that the resilient retainer (31, 31', 31'') is externally clipped onto the valve crosshead (27, 27') in a position partially overlying an open side of said recess (32) and surface areas of the contact pad (29, 29', 29'') facing outwardly relative to said open side of the recess (32). Valve crosshead assembly (25, 25', 25") according to any one of the preceding claims, characterized in that the resilient retainer (31, 31', 31') forms a means for retaining the contact pad (29, 29', 29'') in said recess (32) without dynamic loading of the resilient retainer (31, 31', 31'') by the contact pad (29, 29', 29''). Valve crosshead assembly (25, 25', 25") according to any one of the preceding claims, characterized in that the recess (32) is shallow and formed in a top surface (27a) of a solid central portion of the valve crosshead (27, 27'). Engine cylinder head valve drive train, wherein a cam-operated push rod (13) transmits motion to a rocker lever (17) formed of metal and the rocker lever (17) seats and unseats cylinder valves (19) by acting on a valve crosshead (27, 27') according to any one of the preceding claims. Engine cylinder head valve drive train according to claim 19, wherein a valve stem (19a) of a cylinder valve (19) is attached to said valve crosshead (27, 27') at each of opposite sides of the contact pad (29, 29', 29'') of the valve crosshead (27, 27').

Inventors: BENTZ JOSEPH C, CARROLL JOHN T, SHINOSAWA KATSUHIRO, BENTZ, JOSEPH C., CARROLL, JOHN T., SHINOSAWA, KATSUHIRO
Applicants: CUMMINS ENGINE CO INC, TOSHIBA CORP, CUMMINS ENGINE COMPANY, INC., TOSHIBA CORPORATION
Application year: 1995
Main classification: F01L1
IPC codes: F01L1, F02F1, F02B3