Patent Publication Number: US-8967037-B2

Title: Thrust lubrication strategy for roller lifters of a common rail fuel pump

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to common rail fuel pumps that supply pressurized fuel to fuel injectors of an internal combustion engine, and more particularly to a thrust lubrication strategy for roller lifters of a common rail fuel pump. 
     BACKGROUND 
     Many modern engines, including compression ignition engines, utilize a common rail fuel system for supplying fuel to each individual cylinder of the engine. In a common rail fuel system, a common rail fuel pump takes in low pressure fuel and supplies high pressure fuel to a common rail. Fuel injectors associated with each individual cylinder are fluidly connected to the common rail via individual branch passages. Over the years, the industry has demanded ever higher injection pressures, and hence ever higher common rail fuel pressures. As these rail pressures have exceeded 200 MPa and quickly approach 300 MPa, new problems have emerged in common rail fuel systems. 
     Common rail fuel pumps typically include two or more pump assemblies in a pump housing that are driven by a rotating cam shaft that includes one or more cams, each having one or more lobes. The different pump assemblies are typically out of phase so that the common rail can receive intermittent doses of high pressure fuel throughout the engine cycle to compensate for intermittent fuel injection from individual fuel injectors around the same engine cycle. In one particular example, a common rail fuel pump might include a cam shaft mounted for rotation in a pump housing. Rotational motion of the cam is translated into reciprocating motion of pump pistons by way of two or more individual tappet assemblies. Each tappet assembly includes a tappet that carries an axle about which a roller is rotationally mounted. The roller maintains contact with the rotating cam, and causes a reciprocating motion with each passage of a cam lobe. In order to function properly over an extensive working life, the good lubrication must be maintained for the roller, or premature wear and potential failure of the pump can occur. 
     The present disclosure is directed toward one or more problems set forth above. 
     SUMMARY 
     In one aspect, a common rail fuel pump includes a cam shaft with at least one cam rotatably supported in a pump housing. A plurality of tappet assemblies are each reciprocatingly movable in the pump housing, and include an axle pin mounted in a tappet, and a roller mounted in contact for rotation about the axle pin. The roller includes a bearing surface and a cam contact surface extending between a first thrust surface and a second thrust surface. The roller is trapped to move along an axis of the axle pin in a tappet pocket of the tappet between a first thrust contact position and a second thrust contact position. The first thrust surface of the roller being in contact with a first thrust face of the tappet at the first thrust contact position, and the second thrust surface of the roller being in contact with a second thrust face of the tappet at the second thrust contact position. The axle pin defines a lubrication passage that opens through a roller bearing surface of the axle pin to the bearing surface of the roller. The first thrust surface include a plurality of the non-contiguous first planar surfaces separated by first lubrication grooves. The second thrust surface includes a plurality of non-contiguous second planar surfaces separated by second lubrication grooves. 
     In another aspect, a tappet assembly includes a tappet with a first thrust face and a second thrust face that partially define a tappet pocket. The tappet defines a lubrication supply passage. An axle pin in affixed to the tappet and includes an annular roller bearing surface extending between the first thrust face and the second thrust face, and defines a roller lubrication passage that connects on one end to the lubrication supply passage and opens at an opposite end through the roller bearing surface. A roller includes a roll bearing surface and a cam contact surface extending between a first thrust surface and a second thrust surface, and is mounted in contact for rotation about the axle pin. The first thrust surface includes a plurality of non-contiguous first planar surfaces separated by first lubrication grooves, and the second thrust surface includes a plurality of non-contiguous second planar surfaces separated by second lubrication grooves. A portion of lubrication fluid moves from the lubrication supply passage, along the roller bearing surface, into the first and second lubrication grooves, and then between the first thrust face and the first thrust surface, and between the second thrust face and the second thrust surface when the roller is rotating. 
     In still another aspect, a method of operating a common rail fuel pump includes reciprocating a plurality of tappet assemblies in a pump housing by rotating a cam shaft. The reciprocating step includes rotating a roller on an axle pin of each of the tappet assemblies, and contacting the roller with a cam of the cam shaft. A roll interaction between the roller and the axle pin is lubricated from a lubrication passage that open through a roller bearing surface of the axle pin. A thrust interaction between the roller and thrust faces of the tappet of the tappet assembly is lubricated by moving lubrication fluid from lubrication grooves separating planar thrust surfaces of the roller to between the thrust surfaces and thrust faces of the tappet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a common rail fuel pump according to the present disclosure; 
         FIG. 2  is a sectioned side view of the one pumping element of the common rail fuel pump of  FIG. 1 ; 
         FIG. 3  is a front sectioned view through the common rail fuel pump of  FIG. 1 ; 
         FIG. 4  is an enlarged sectioned front view of one of the tappet assemblies shown in  FIG. 3 ; 
         FIG. 5  is an enlarged sectioned side view of the tappet assembly shown in  FIG. 2 ; and 
         FIG. 6  is a perspective end view of a roller according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIG. 1 , a common rail fuel pump  10  is shown schematically in a common rail fuel system such that fuel arrives at low pressure inlet  12 , fuel pressure is raised in pump housing  11  and exits at fuel outlet  13 . Thereafter, a common rail supplies individual fuel injectors, which may be located for direct injection in the case of a compression ignition engine. Common rail fuel pump  10  may be directly driven by an engine via a gear train that includes gear  16 . Common rail fuel pump  10  may be internally lubricated with lubrication oil that arrives at inlet  14 , lubricates the interior moving parts, and exits pump housing  11  at lubrication oil outlet  15  for recirculation. 
     Referring in addition to  FIGS. 2 and 3 , common rail fuel pump  10  includes a cam shaft  18  that is rotatably supported in pump housing  11 , and driven to rotate by gear  16 . Pump shaft  18  is shown as including four cams  19  that each include two lobes  20 . Thus, in the illustrated example, common rail fuel pump  10  includes four pumping assemblies  21  that are each associated with an individual tappet assembly  30 . Tappet assembly  30  converts the rotational motion of cam lobes  20  into reciprocating motion that is transferred to pump pistons  23  that reciprocate to pressurized fuel in a pump chamber  22 . The coupling between pump assemblies  21  and cam shaft  18  is maintained by the pre-load of a biasing spring  24  in a known manner. 
     Referring now in addition to  FIGS. 4 and 5 , each tappet assembly  30  includes an axle pin  40  affixed to and mounted in a tappet pocket  33  defined by a tappet  31 . A roller  50  is mounted in contact for rotation about axle pin  40 . The pump piston  23  may contact a top surface  32  of tappet assembly  30 , while a cam contact surface  53  rolls on cam  19  under the action of spring  24 . The roller  40  includes a bearing surface  52  that bears against roller bearing surface  43  of axle pin  40 . Roller  50  rotates about axis  51  responsive to rotation of cam shaft  18 . Bearing surface  52  and cam surface  53  extend between a first thrust surface  54  and a second thrust surface  55 . The roller  50  is trapped to move along axis  51  in tappet pocket  33  of tappet  31  between a first thrust contact position and, in the opposite direction, a second thrust contact position. The first thrust surface  54  of roller  50  is in contact with a first thrust face  34  of tappet  31  at the first thrust contact position. When the roller  50  moves in an opposite direction, the second thrust surface  55  is in contact with a second thrust face  35  of tappet  31  at the second thrust contact position. 
     Lubrication of the roller interaction between roller  50  and axle pin  40 , as well as the thrust interaction of roller  50  with tappet  31  is facilitated by a lubrication pathway  44  that extends between lubrication oil inlet  14  and lubrication oil outlet  15 , with the segment associated with tappet assembly  30  shown in  FIG. 4 . The lubrication pathway  44  includes in sequence a lubrication supply passage  36  that is defined by tappet  31 , and then into a roller lubrication passage  41  defined by axle pin  40 . In particular, roller lubrication passage  41  opens at one end  42  to the lubrication supply passage  36 , and at its opposite end  45  opens through roller bearing surface  43 . Opposite opening end  45  may be located at about the center of axle pin  40  and roller  50 . After exiting at opposite end  45 , the lubrication fluid moves in opposite directions along roller bearing surface  43  parallel to axle  51  to lubricate the roll interaction between roller  50  and axle pin  40 . 
     After moving along roller bearing surface  43 , the lubrication fluid moves into lubrication grooves  56  that separate a plurality of planar surfaces  57 , that together make up first and second thrust surfaces  54  and  55  at opposite ends of roller  50 . As roller  50  rotates, the lubrication fluid in lubrication grooves  56  may be urged along ramps  58  that terminate at the planar surfaces  57 . Although not necessary, the shape of each lubrication groove  56  may be symmetrical on either side of its centerline  60  so that roller  50  may be symmetrical about a plane  59  perpendicular to axis  51 . With this symmetry, roller  50  may be mounted in either direction on axle pin  40  at time of assembly so that mis-assembly is not possible. Each of the centerlines  60  of the individual lubrication grooves  56  may coincide with a radius extending from rotation axis  51 . In the illustrated embodiment, each roller  50  includes six separate planar surfaces  57  separated by six individual lubrication grooves  56  on each end of the roller. Nevertheless, those skilled in the art will appreciate that any number of planar surfaces and lubrication grooves would also fall within the scope of the present disclosure. Thus, the planar surfaces  57  can be considered as non-contiguous due to their separation by lubrication grooves  56 . 
     INDUSTRIAL APPLICABILITY 
     The common rail fuel pump  10  of the present disclosure finds potential application in any fuel system for internal combustion engines that utilize a common rail fueling system. Although the common rail fuel pump has been illustrated as including a cam shaft with four cam lobes and associated with four individual pump assemblies  21 , those skilled in the art will appreciate that each cam  19  could power two or more pump assemblies and the pump may have only a single cam. The common rail fuel pump of the present disclosure finds specific application in association with compression ignition engines that utilize extremely high injection pressures, such as to facilitate cleaner combustion cycles to produce better emissions. These extremely high pressures have resulted in new lubrication problems emerging. In some circumstances there may be an inability to maneuver sufficient quantities of lubrication fluid between a thrust surface  54 ,  55  of a roller coming in contact with a counterpart thrust face  34 ,  35  of a tappet  31 . 
     When in operation, an engine, not shown, drives gear  16  and cam shaft  18  to rotate. The tappet assemblies  30  reciprocate in the pump housing  11  responsive to rotation of cam shaft  18 . The roller  50  rotates on axle pin  40  responsive to rotation of the individual cams  19  via the contact interaction therewith. The roller interaction between the roller  50  and the axle pin  40  is lubricated from lubrication oil emerging from a lubrication passage at an opening through roller bearing surface  43  of axle pin  40 . The thrust interaction between roller  50  and tappet  31  is lubricated by moving lubrication oil into lubrication grooves  56  that separate the planar thrust surfaces  57  of roller  50 . The lubrication oil moves out of the lubrication grooves  56  into the space between thrust surface  54 ,  55  and thrust faces  34 ,  35  of tappet  31 . Each of the lubrication grooves  56  may include a ramp  58  that terminates at one of the planar surfaces  57  for urging the lubrication fluid along the ramp and into the thrust lubrication area. By orienting the lubrication grooves  56  to coincide with a radius from the rotation axis  51  of roller  50 , centrifugal force may tend to help move lubrication fluid into the individual lubrication grooves  56 , and the symmetry may allow the rollers  50  to be mounted in either direction with equal performance. Due to geometry of the individual components, potential mounting orientation of common rail fuel pump  10 , and other known and unknown factors, the roller  50  can be expected to move along axis  51  between contact with thrust faces  34  and  35  of tappet  31 . By ensuring an adequate supply of lubrication fluid between the thrust surfaces  54 ,  55  of roller  50  with the counterpart thrust faces  34 ,  35  of tappet  31 , premature wear and potential failure of common rail fuel pump  10  can be reduced. 
     It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Thus, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims.