Patent Publication Number: US-2006005797-A1

Title: Roller valve lifter

Description:
The present application claims priority from U.S. Provisional Patent Application Ser. No. 60/587,627, filed on Jul. 8, 2004. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to valve lifters for actuating push rods in internal combustion engines. More particularly, the present invention relates to an improved roller valve lifter that requires no conventional ball bearing for the roller.  
      2. Related Art  
      Internal combustion engines typically comprise a pair or more of valves associated with each cylinder for intake and exhaust, which open and close to take in fuel and air, and to allow exhaust gasses to escape following combustion. These valves are typically opened and closed by the rotation of a camshaft comprising a plurality of eccentric cams disposed on a shaft which rotates synchronously with the engine. Associated with each valve is a valve lifter, which is in contact with one cam of the camshaft and aligned perpendicular thereto, so as to translate rotational motion of the cam into axial reciprocal motion of the valve lifter. The lifter in turn is connected to a push rod, which is connected to a hinged rocker arm assembly, which acts directly on the valve. The entire valve lifter/push rod/rocker arm/valve assembly is typically biased in a closed position by a spring, to keep the valve lifter in contact with its associated cam.  
      One of the major sources of friction in such a driven valve train is created at the interface between the valve lifter and the cam. In very early internal combustion engines, valve trains were frequently actuated by flat-ended, solid valve lifters, which were placed in sliding contact with each cam. This construction was simple, but with production of engines designed for higher operating speeds and having higher valve spring biasing forces, flat lifters became inadequate. In spite of the use of sufficient lubricating oil, flat lifters experienced extremely high wear of the cam lobe and lifter, and generated substantial heat from the friction. The cam lobe and lifter surface would be quickly worn down, introducing abrasive debris into the engine, and eventually wearing the lifter to the point that the valve action would no longer allow the engine to run properly.  
      To address some of these problems, attempts were made many years ago to employ valve lifters having a radius formed on the bearing end surface. However, even when hardened steel or chrome steel were used, the friction-induced wear on the cam and lifter were found to be excessive. This condition was exacerbated by the difficulty of providing sufficient lubricating oil to the sliding surface. Consequently, these attempts were generally unsuccessful, and radius lifters were long ago abandoned in favor of roller valve lifters.  
      Roller valve lifters employ a roller disposed at the bearing end of the valve lifter that is in contact with the cam lobe. The current method of producing roller type valve lifters uses steel rollers centered around an axle, with a needle bearing, having an array of thin steel needles set laterally and parallel around the axle, separating the axle from the wheel. Thus instead of sliding friction between the valve lifter and the cam, the roller valve lifter rolls over the cam lobe with far less friction. Unfortunately, the needle bearings and axles are frequently unable to withstand the high frequency, high impact load of the rotating cam, and tend to fracture, spreading sharp metal debris throughout the engine. This is a main cause of frequent failures in roller bearing valve lifters used in high-performance engines. Additionally, the wheel, axle, and other portions of typical roller valve lifters make them relatively complicated, costly, and heavy.  
     SUMMARY OF THE INVENTION  
      It has been recognized that it would be advantageous to develop a simple, economical, and lightweight roller valve lifter that can reduce friction without experiencing excessive wear or causing excessive wear to the cam.  
      It has also been recognized that it would be desirable to have a roller valve lifter that avoids the drawbacks associated with needle bearings in typical roller valve lifters.  
      It has also been recognized that it would be desirable to have a roller valve lifter which incorporates an improved system for transmitting lubricating oil to regions where it is needed.  
      In one embodiment, the invention provides a valve lifter apparatus for following an eccentric cam in an internal combustion engine. The valve lifter includes a cylindrical roller, configured to roll upon the eccentric cam, and a lifter body, having a curved roller socket configured to retain the roller and to allow rotation of the roller in the roller socket upon a layer of lubricating fluid trapped between the roller surface and a bearing surface of the roller socket. The roller socket includes tapered end portions defining distal gaps between the roller surface and the bearing surface, for allowing entry of splash oil therebetween.  
      In accordance with another embodiment thereof, the invention provides a valve lifter apparatus for following an eccentric cam in an internal combustion engine. The valve lifter apparatus includes a lifter body, having a roller socket, a cylindrical roller, disposed in the roller socket, and means for laterally retaining the roller in the roller socket. The roller socket has a curved bearing surface defining an arc of greater than 180 degrees, and the bearing surface includes a circular central portion with a constant radius. The cylindrical roller includes a roller surface configured to roll upon the eccentric cam, and has a radius substantially equal to the radius of the circular central portion of the roller socket. The roller is configured for sliding bearing in the roller socket upon lubricating fluid disposed between the roller surface and the bearing surface.  
      In accordance with another more detailed embodiment thereof, the valve lifter includes a lubricant passageway in the lifter body, having an outlet in the bearing surface, configured to allow a flow of lubricating fluid to the region between the roller surface and the bearing surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention, and wherein:  
       FIG. 1  is a partial cross-sectional fragmented view of a typical valve lifter shown in operational relationship to the valve assembly of a typical internal combustion engine;  
       FIG. 2  is a side view of a typical prior art roller valve lifter;  
       FIG. 3  is a perspective view of one embodiment of a roller valve lifter in accordance with the present invention;  
       FIG. 4  is an exploded perspective view of the valve lifter assembly of  FIG. 3 ;  
       FIG. 5  is a perspective view of the internal lifter body;  
       FIG. 6  is a longitudinal cross-sectional view of the internal lifter body of  FIG. 5 ;  
       FIG. 7A  is a cross-sectional view of the valve lifter assembly of  FIG. 3 , taken along a plane that is perpendicular to the rotational axis of the roller;  
       FIG. 7B  is a cross-sectional view of the valve lifter assembly of  FIG. 3 , taken along a plane that is parallel to the rotational axis of the roller;  
       FIG. 8  is a close-up cross-sectional view showing the geometry of the roller and roller socket;  
       FIG. 9  is an exploded perspective view of another embodiment of a lifter body and roller in accordance with the present invention; and  
       FIG. 10  is a perspective view of a roller valve lifter having an alternative outer shell. 
    
    
     DETAILED DESCRIPTION  
      Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.  
       FIG. 1  is a partial cross-sectional fragmented view of a typical valve lifter shown in operational relationship within the valve assembly of a typical internal combustion engine  10 . The valve lifter  12  is disposed within a cylindrical valve lifter bore  14  with its lower end held in contact with a cam  16 . The top of the valve lifter is connected to a push rod  18  which is connected to a rocker arm  20  which acts directly on a valve  22  which governs the ingress and egress of combustion reagents and products with respect to a cylinder  24 . As the cam rotates on its shaft, its eccentric lobe  17  cyclically moves toward and then away from the end of the valve lifter. The contact of the end of the valve lifter on the surface of the rotating cam converts the rotational motion of the cam into axial reciprocal motion of the valve lifter, and consequently the valve. The entire valve lifter/push rod/rocker arm/valve assembly is biased by coil spring  26 , which acts to keep the bottom of the valve lifter in contact with the cam, and keeps the valve closed when the cam lobe is rotated away from the end of the lifter.  
       FIG. 2  provides a more detailed side view of a prior art valve lifter  12  comprising a roller assembly  30  at its lower end. The roller assembly is designed to engage the surface of the cam lobe  17  so as to convert the rotational motion of the cam into axial reciprocal motion of the valve lifter, thence imparting the axial reciprocation to the push rod  18 . The push rod is typically fitted into a socket  19  formed in a conical or other shaped recess  21  formed in the top of the valve lifter. Extending from the center of the socket is a bleed oil conduit  23  which leads to a bleed oil hole  25  formed in the side of the valve lifter. A small partition  27  is provided in the opening of the bleed oil hole to separate it from the main oil flow. The bleed oil conduit is in fluid communication with a similar oil conduit  29  provided in the center of the push rod. This configuration advantageously feeds a relatively small quantity of lubricating oil from the high oil pressure zone present in the very small annular space between the outside surface  33  of the lifter and the valve lifter bore, up to the push rod/rocker arm assembly.  
      Typical valve lifters  12  are also frequently provided with oil transmission passages for allowing engine galley oil to flow from one valve lifter to another along the valve train. Viewing  FIG. 1 , the oil galley would typically extend perpendicular to the plane of the page (parallel to the cam shaft  16 ), and would intersect each valve lifter bore approximately at the average midpoint of each valve lifter. To allow the galley oil to flow along the valve train, each valve lifter is provided with oil transmission means. This may comprise an annular recess or slot around the middle of each valve lifter, or may be a transverse conduit of various configurations formed through the middle of the valve lifter body. The prior art valve lifter shown in  FIG. 2  incorporates an oil transmission means comprising an annular recess or groove  35  formed about its midsection. This allows the oil to flow between valve lifters, and also provides oil for each valve lifter and associated components, including the cam and the valve lifter/push rod/rocker arm/valve assembly.  
      The roller lifter assembly  30  typically comprises a wheel  32  having an axle and needle bearings  34  attached to the valve lifter by a cradle or frame  36 . Such rollers are well known, and have represented the state of the art for many years. Unfortunately, this configuration is relatively complicated and expensive because it involves many parts, and it tends to be heavy because of the relatively stiff connection required for the roller assembly. Additionally, the wheel, axle, and needle bearings are prone to wear out, especially in high performance engines. The needle bearings and axles are frequently unable to withstand the high frequency, high impact load of the rotating cam, and tend to fracture, spreading sharp metal debris throughout the engine. This is a main cause of frequent failures in roller bearing valve lifters used in high performance engines.  
      Advantageously, the inventor has devised a roller valve lifter that overcomes some of the problems of prior roller valve lifters. One embodiment of a roller valve lifter  100  in accordance with the present invention is shown in  FIGS. 3-4 . The valve lifter comprises three general parts: an internal lifter body  110 , a roller  112 , and an outer sleeve or shell  114 . The roller is held within a roller well or socket  116  that is transversely bored in the lower end of the internal lifter body. The lower end of the roller extends through a roller opening  118  located in the lower end of the outer shell.  
      The inventor&#39;s approach is a departure from prior valve lifters in that it uses a solid roller having no center hole to accommodate an axle and supporting needle bearings. Instead, the roller  112  is suspended on a film of oil trapped within the roller socket  116 . The roller floats as it spins on the trapped oil, making no direct contact with the corresponding contour of the socket or roller well. This operation of the roller is made possible because liquids are incompressible, following the same principle that is applied in connecting rod bearings, which float on oil around a crankshaft journal, while making no direct contact with the crank journal.  
      The inner lifter body  110  is preferably lightweight, and includes features for transmitting lubricating fluid. In one embodiment, the inner lifter body is made of hardened aluminum, which helps allow it to resist wear and high temperatures. Alternatively, the inner lifter body can be of steel, such as hardened steel. The inner lifter body can be configured in a variety of ways. One possible configuration is shown in  FIGS. 4-7 . Viewing  FIGS. 5 and 6 , the upper end of the inner lifter body includes a conical recess  120  with a socket  122  that is configured to receive a push rod with a radiused end. The cross-sectional views of  FIGS. 7A and 7B  show a push rod socket  124  and push rod  126  (in dashed lines) installed in the socket. A vertical push rod lubricant passageway  130  is provided in the center of the internal lifter body to allow lubricating oil to flow to the push rod via the socket. Those skilled in the art will recognize that the push rod socket can be supported by a hydraulic piston assembly (not shown) that cushions the lifter impact and reduces the need for lash adjustment in the valve lifter/push rod/rocker arm/valve assembly.  
      As shown in  FIG. 7B , a horizontal push rod lubricant passageway  132  interconnects the vertical push rod lubricant passageway with an external opening  134 , that allows lubricating oil to enter. The external opening of the horizontal lubricant passageway is disposed in an annular groove  136  formed in the inner lifter body. This annular groove provides an oil transmission passage for allowing engine galley oil to flow from one valve lifter to another along the valve train. As noted above, the oil galley would typically intersect each valve lifter bore approximately at the average midpoint of each valve lifter. The annular groove is provided to allow the galley oil to flow along the valve train to each valve lifter. As shown in  FIGS. 3, 4 , and  7 B, the outer sleeve or shell  114  includes oil flow openings  138  on each side that coincide with the annular groove, and allow oil to flow from the galley oil passages into the groove. This allows the oil to flow between and among the entire bank of valve lifters, and also provides oil for each valve lifter and associated components, including the cam and the valve lifter/push rod/rocker arm/valve assembly. It will be apparent that other configurations for transmitting lubricating oil can also be provided.  
      The inner lifter body  110  also includes two lubricating oil passageways  140  that extend downward from the annular groove  136  to openings  142  in the roller well/socket  116 . The longitudinal portion  144  of these passageways comprises a groove formed in the outer surface of the inner lifter body. It will be apparent that when covered by the outer sleeve  114  this groove becomes a closed passageway. The transverse portion  146  of this passageway leads to the openings in the roller socket. These passageways provide a path for lubricating oil to travel from the annular groove to the inside surface of the roller socket to lubricate the roller.  
      Another embodiment of an inner lifter body  210  is shown in  FIG. 9 . This embodiment is configured to be less massive than that described above, while still including all essential functional elements and providing sufficient strength to withstand the forces it will experience. As with the embodiment of  FIGS. 3-7 , this lifter body includes a roller well or socket  216  that is transversely bored in the lower end of the internal lifter body, in which the roller is configured to be suspended on a film of trapped oil. The inner lifter body includes features for transmitting lubricating fluid. It includes a conical recess  220  and socket  222  that are configured to receive a push rod bearing, with a vertical push rod lubricant passageway (not shown) to allow lubricating oil to flow to the push rod bearing.  
      The inner lifter body  210  includes a very large annular groove  236  which provides an oil transmission passage for allowing engine galley oil to flow from one valve lifter to another along the valve train, in the manner discussed above, and can also include a passageway for transmitting oil to the vertical push rod lubricant passageway. The inner lifter body also includes lubricating oil passageways  240  that extend downward from the annular groove to openings  242  in each side of the roller well/socket  216 , and are generally configured like the comparable structure of the lifter body embodiment described above. These passageways provide a path for lubricating oil to travel from the annular groove to the inside surface of the roller socket to lubricate the roller. The following discussion of the roller and roller socket, though specifically referring to the inner lifter body  110  of  FIGS. 4-8 , applies equally to the inner lifter body of  FIG. 9 .  
      Referring back to  FIGS. 3-8 , the roller  112  is a substantially solid cylindrical body of a suitable material that is configured to rotate within the roller socket  116 . As shown in  FIG. 8 , the roller socket defines an arc a of greater than 1800, so that the bearing surface  148  of the socket wraps around the roller to hold it in place. The roller socket has a curvature that allows the roller to rotate in the socket against a thin film of lubricating oil trapped between the roller surface and the bearing surface of the socket, creating a lubrication zone. Advantageously, the trapped lubricating oil directly supports the spinning roller with very little friction, and is not forced out during the rapid high pressure impact of the roller within the socket with each rotation of the cam. This is due in part to the fact that liquids are incompressible, and because each impact is so rapid that the lubricating oil simply cannot flow out of the way fast enough.  
      This solid roller configuration provides advantages over some prior art. Roller valve lifters having a hollowed steel roller running against a saddle of plastic bearing material have been proposed. Unfortunately, these lightweight materials do not provide sufficient rigidity to prevent loss of conformity between the roller and the saddle. This can allow the elimination of the hydrostatic film of oil separating the roller and the saddle under high loads, causing undesirable contact of the two surfaces. In contrast, testing has demonstrated that the solid roller of the present valve lifter, saddled in the hardened inner lifter body, does not experience substantial deformation and a resultant loss of the hydrostatic film of lubricating oil, thus maintaining separation of the two surfaces even under extremely heavy loads.  
      One aspect of the present invention is the use of a roller having a very high modulus of elasticity so as to have very little flexure. In one embodiment, the roller  112  is made of Silicon Nitride (SiNi), a very hard and lightweight composite ceramic material that is commercially available from a variety of sources. The solid composite roller is advantageous in comparison with many steel rollers. A steel roller of similar material and similar hardness to a cam lobe has a tendency to gall and/or pick up metal as the roller is accelerated onto the opening ramp ( 150  in  FIG. 7A ) of a cam lobe. This galling of the surfaces can transfer small particles of metal to the surface of the bearing that the roller rides in, thus scratching or scoring this surface, and providing escape pathways for the lubricating oil. This hinders the operation of the “trapped oil” concept.  
      Advantageously, a composite roller, having a much greater hardness than the cam lobe, maintains a higher degree of its polished finish for longer lasting, non-galling relationship with the cam lobe. The hardness of the composite ceramic roller material also allows it to accept and maintain an extremely smooth finish to prevent wear to both the cam lobe and the lifter&#39;s bearing surface, thus helping to prevent scoring. A highly polished silicon nitride surface will not scratch like most grades of steel. Contact between silicon nitride and steel or cast iron will tend to simply polish the metal, without picking up particles that can scratch or score the roller well, which can be of aluminum.  
      Additionally, a composite roller, having considerably greater stiffness than a steel roller of comparable configuration, retains its shape better, and thus conforms better to the precision matching radius of the bearing surface  148  of the roller well  116  without distortion or burnishing from the extreme pressure brought on by the valve springs of high performance engines. The hard, polished ceramic roller retains its shape better, and thus the roller and the roller well maintain their ability to trap oil.  
      The light weight of the composite roller  112  also aids in the acceleration of the roller against the spinning camshaft lobe. That is, the roller will speed up and slow down significantly with each rotation of the cam shaft. Advantageously, the lighter weight composite roller presents lower inertial forces, and thus requires less energy to change its velocity with each revolution. This helps make the engine more efficient.  
      While silicon nitride is a very suitable material for the roller, it is believed that other materials can also be used in the present invention. For example, steel of suitable hardness (e.g. heat-treated steel) or other metals could also be used, so long as the modulus of elasticity and shape are such as to resist flexure or deformation that could hinder conformity of the roller and roller bearing socket. One grade of steel that may be suitable is 52100 steel.  
      In some applications, a steel roller may be preferred for its lower cost where the operating conditions are not so intense and the superior characteristics of the ceramic material are not required. Alternatively, for example, in more severe operating conditions, a steel roller might be used where a racing association&#39;s rules require the use of a roller lifter, but prohibit the use of ceramic materials. In such cases, the shortened life may be acceptable since it need last only for the duration of the race, and would normally be replaced when the engine is rebuilt for the next event. In short, while a ceramic roller with a hardened steel lifter body is one desirable configuration, other combinations of materials can be used where other considerations such as cost or association rules make this desirable. This would allow the advantages of the invention—reduced friction without the use of unreliable axles and needle bearings—to be retained even when, for other reasons, the ceramic roller could or would not be used.  
      The outer shell or sleeve  114  can be made of hardened steel, and can be fabricated in various ways. In on embodiment, the inventor has machined the outer shell from a solid piece of bar stock. Alternatively, the inventor has manufactured the outer shell from tubing of a suitable size and grade. Other suitable manufacturing techniques can also be applied. The outer shell can be attached or affixed to the inner lifter body in various ways. For example, the outer shell can be bonded to the inner lifter body with a suitable adhesive, such as an epoxy. Alternatively, dowel pins can be provided to mechanically affix the sleeve to the inner lifter body.  
      The outer shell or sleeve  114  performs several functions in the roller lifter assembly. First, the shell is fixed permanently as a cover for the inner lifter body  110 , and provides a wear-resistant outer surface for the lifter in its lifter bore. For example, where the inner lifter body is of aluminum, a hardened steel outer shell will provide greater wear resistance. Second, the outer shell also operates as a retainer, to enclose and center the roller  112 , restricting the roller&#39;s lateral movement. The shell can also include flattened side faces or facets  115  on opposing sides near the push rod end, for mating with a non-circular shape in a portion of the valve lifter bore. Such a configuration is sometimes used for preventing rotation of lifters within their lifter bores, to ensure that the axis of rotation of the roller remains parallel to the axis of rotation of the camshaft.  
      In the embodiment shown in  FIGS. 3-7 , the roller opening  118  of the outer shell includes side surfaces  152  that are disposed adjacent to the vertical sides  154  of the roller. If the roller drifts laterally as it rotates within the socket  116 , its sides will contact the sides of the roller opening of the hardened steel outer shell, and thus be prevented from further drift. This centers and retains the roller in the inner lifter body, and also prevents the outer corner radius surface  156  of the roller from making contact with the inside wall of the steel shell, while still keeping the roller centered on the cam lobe.  
      The steel shell  114  also helps support the integrity of the roller socket  116  in the inner lifter body  10 . That is, because the inner lifter body and the bore surrounding the roller leaves a relatively small thickness of material for holding the roller, the added structure of the steel outer shell helps provide strength and stability to the roller socket. Advantageously, the roller opening  118  in the steel outer body is smaller in length L than the diameter D of the roller  112 , and thus provides backup structure to prevent the roller from escaping from the roller well. By having a length dimension that is shorter than the diameter of the protruding roller, the roller cannot escape, even if the inner body surrounding the roller socket were to crack or break.  
      In the embodiment depicted in  FIGS. 3-4  and  7 , the outer shell  114  includes flattened side portions  161  at the roller end on either side of the roller, with windows  162  in these side portions. This configuration provides several functions. The flattened side portions on each lateral side of the roller are provided to reduce the overall width of the lifter on the roller end, so as to provide clearance for neighboring cam lobes and other obstructions that may exist in the cylinder block. Given that the shell is a hollow cylinder, machining these flattened side portions has the effect of creating the windows. Advantageously, the windows provide an escape route for oil that is flushed from the roller well. That is, when the roller well is provided with pressurized oil from the lifter galley via the lubricating oil passageways  140 , this pressurized flow will generally create an outflow of hot oil from the roller well region. The windows help provide a pathway for the escape of this oil.  
      In one aspect, the roller valve lifter  100  can function with only splash oil. This is particularly desirable because some engines do not have engine oil pressure feeding the lifter galley. Those of skill in the art will recognize that splash oil is flung about by centrifugal force from the rotating elements of the crankshaft and camshaft assemblies. Using only this splash oil is possible because the valve lifter provides an entry path at the bottom of the lifter, facing the crankshaft, for splash oil to be channeled into the space between the roller and the roller well. Specifically, the shape of the roller socket  116  is configured to provide a gap  160  between the roller  112  and the socket at the lower extremity of the roller. This gap allows splash oil to be drawn between the roller and the bearing surface  148  of the roller socket, to provide a cushion or boundary layer and to lubricate and cool during rotation of the roller. Additionally, the windows  162  in the outer shell  114  can also provide a pathway to allow splash oil to enter the roller well region, though this is not believed to be a major contributor to the introduction of splash oil.  
      The oil gap  160  is created by the shape of the roller well  116 , shown in an exaggerated view in  FIG. 8 . The bearing surface  148  of the roller well has three distinct sections. First, it has a circular central portion  164  with a constant radius that substantially matches the radius of the roller  112 . This matching radius is substantially the same, within machining tolerances, as the radius of the roller, so that a thin film of lubricating oil can be trapped between the two surfaces. This constant radius portion of the bearing surface provides a compression zone, wherein most of the force exerted by the cam on the roller is transferred to the inner lifter body  110 . In the embodiment depicted in  FIG. 8 , the compression zone defines an arc P of about 130° to 140°, though the arc length can vary from this.  
      The roller well  116  also includes two tapered end portions  166  that provide the oil gaps  160  at their lower extremities between the roller  112  and the bearing surface  148 . The gap shown in  FIG. 8  is greatly exaggerated in size in order to show its configuration. In one embodiment, the tapered end portions define spiral or non-circular curves, having curved surfaces of gradually increasing radius from the ends of the compression zone to the lower end of the lifter body. An approximate spiral curve can be created by boring one or more larger diameter bores, each slightly larger (by only a few thousands of an inch) than the previous hole, but centered below the previous bore, with small tangent surfaces between each adjacent curve. Because they have a lower center, the larger bores will not conflict with the main bore (the one that defines the circular central portion of the roller well), but will widen the lower region of the roller well. Other curve shapes, such as parabolic, semi-elliptical, etc., can also be used to shape the roller well to provide the oil gap.  
      This spiral shape creates the gap  160  that allows splash oil to be channeled into the compression zone  164 . This is a great advantage in comparison with some other prior art valve lifters. Some prior art valve lifters have closed edge openings that tend to scrape the oil from the roller. This design, in contrast, allows splash oil to be drawn between the roller and the bearing surface by the rotation of the roller, to form a boundary layer acting as a cushion between the roller and the bearing. Because the liquid oil is incompressible, like fluids generally, it will not be able to squeeze out of the compression zone between the roller and the bearing surface in response to the rapid impact of the cam lobe  150  against the roller  112 , thus maintaining the needed separation and a highly lubricated condition.  
      However, splash oil lubrication is not the only lubrication method anticipated by this roller valve lifter. As noted above, the inner lifter body  110  includes lubricating oil passageways  140  that extend downward from the annular groove  136  to openings  142  in the roller well/socket  116 . The openings in the roller socket provide a path for lubricating oil to travel from the annular groove to the inside surface of the roller socket to lubricate the roller. As shown in  FIG. 8 , the openings of these passageways are located in the tapered end portions  166  of the roller well, outside the compression zone  164 . It will be apparent that if the openings were in the compression zone, pressure from the roller would tend to force oil out of the compression zone and back into the oil passageways, rather than allowing the oil passageways to supply oil to be trapped between the roller and the bearing surface  148 , as intended.  
      Advantageously, the oil passageways  140  and the gaps  160  at the bottom of the roller well  116  also provide a pathway for hot oil to escape from the compression zone. This allows oil to enter the compression zone, to cushion the impact, lubricate, and cool the roller for a time, and then escape to be recirculated. This prevents oil from remaining in the compression zone too long and becoming too hot. Hot oil does not lubricate as well, and can chemically break down, thus allowing undesired contact and wear of the surfaces. However, when oil is properly circulated, it is allowed to cool properly and perform its intended function much better.  
      Another embodiment of an outer shell  214  is shown in  FIG. 10 . This embodiment of the outer shell can be used with any of the embodiments of the inner lifter body shown and described herein. This embodiment of the sleeve or shell includes features comparable to those described above with respect to the embodiment of  FIGS. 3-4  and  7 . It includes oil flow openings  238  on each side to allow oil to flow between and among an entire bank of valve lifters, and also to provide oil internally for each valve lifter and associated components, including the cam and the valve lifter/push rod/rocker arm/valve assembly. It also includes a drilled hole  262  for allowing the sleeve to be mechanically dowel-pinned to the inner lifter body. The portions of the shell that are disposed adjacent to the ends of the roller socket (that is, near the oil gap  260  adjacent the bearing surface of the roller) are also configured substantially like the shell embodiment of  FIGS. 3-4  and  7  and function in substantially the same way.  
      Advantageously, the shell  214  of  FIG. 10  includes tapered side portions  252  that converge into roller socket side panels  254 . This tapered shape provides several advantages. First, it reduces the overall width of the lifter at the roller end, so as to provide clearance for neighboring cam lobes and other obstructions that may exist in the cylinder block. Additionally, the side panels are located adjacent to the vertical sides of the roller, and operate to center the roller and laterally keep it in its socket. The clearance between the side panels of the shell and the side surfaces of the roller can be large enough to allow the escape of oil from the roller well, and yet small enough to keep the roller wheel properly centered. In one embodiment, the inventor has provided a clearance of about 0.010″ between the side panels of the shell and the sides of the roller. Because it provides larger surfaces and is of continuous material, this configuration gives the shell greater side thrust strength for keeping the roller centered than does the embodiment of  FIGS. 3-4  and  7 . Specifically, rather than a very small edge surface that contacts the sides of the roller (e.g. side surfaces  152  of the roller opening  118  in  FIG. 4 ), this configuration places the entire side panel against the side of the roller, providing additional surface area and more material to resist lateral forces. Additionally, the absence of windows in the shell (like the windows  162  in  FIGS. 3-4  and  7 ) in the region of the roller well also provides greater strength.  
      Advantageously, the shell embodiment  214  of  FIG. 10  can be fabricated from tube stock, and requires less labor to produce than the embodiment of  FIGS. 3-4  and  7 . The tube stock, which can be less expensive to machine than bar stock, is tapered at the roller end in a punch press operation to form the desired shape, then, after assembly of the complete lifter, is ground on its outer surface to ensure the proper diameter and roundness and to provide a smooth outer surface. This process involves fewer steps and produces a stronger part with less waste than the use of bar stock, and can be significantly less expensive overall.  
      By way of example, and without limitation, the invention can be described as providing a valve lifter apparatus for following an eccentric cam in an internal combustion engine. The valve lifter includes a solid cylindrical roller configured to roll upon the eccentric cam, a lifter body having a roller socket configured to contain the roller, and a retainer configured to laterally retain the roller in the roller socket. The roller socket has a bearing surface defining an arc of greater than 180 degrees, having a circular central portion with a constant radius substantially equal to the radius of the roller, and tapered end portions defining distal gaps between the roller and the bearing surface. The distal gaps are configured for allowing entry of lubricating fluid between the roller and the bearing surface. The roller is configured for sliding bearing in the roller socket upon a layer of lubricating fluid disposed between the roller surface and the bearing surface of the roller socket. The retainer can be configured to allow splash oil from a cam shaft associated with the eccentric cam to reach the distal gaps between the roller and the bearing surface, for allowing entry of lubricating fluid therebetween.  
      As a more detailed example, the valve lifter can include a lubricant passageway in the lifter body, having an outlet in the bearing surface of the roller socket outside the circular central portion, configured to allow a flow of lubricating fluid to the region between the roller surface and the bearing surface. More particularly, the lubricant passageway can extend to an inlet in fluid communication with an oil galley configured for transmitting lubricating oil between a plurality of valve lifters.  
      While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.