Patent Publication Number: US-2023160385-A1

Title: Pump actuator with stamp-aligned anti-rotation feature

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation under 35 U.S.C. § 120 of U.S. Nonprovisional patent application Ser. No. 16/477,944, filed Jul. 15, 2019, which is a National State Entry of PCT/US2018/014864, filed Jan. 23, 2018, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/451,495, filed Jan. 27, 2017. All of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to tappets for internal combustion engines, especially pump actuators for high pressure fuel pumps. 
     BACKGROUND 
     A tappet translates the rotational motion of a cam into a reciprocating motion. A tappet body generally includes a cylinder-conforming bore-running surface that guides the tappet as it reciprocates within a bore. A roller may be mounted at a drive input end of the tappet body to follow a cam that drives the tappet. High-pressure fuel pump actuation is one of the more demanding tappet applications. Pump actuator tappets generally require hardened ferrous metal to meet operating life requirements. 
     Alignment between the roller and the cam is critical to keeping friction and noise within acceptable limits. The tappet may have an anti-rotation guide feature to keep the roller axis aligned in a plane with the cam axis. The mounting of the roller to the tappet body may be relied on to keep the roller axis perpendicular to the bore axis and parallel to the cam axis. 
     SUMMARY 
     One aspect of the present teachings is a tappet that includes a body that is a contiguous piece of case-hardened ferrous metal. The body has a cylinder-conforming bore-running surface and an anti-rotation guide feature that has been made by stamping to project outwardly from the bore-running surface. A cam follower is mounted to the body. A tappet according to the present teachings provides superior alignment of the roller to the cam, which reduces friction and noise. 
     Another aspect of the present teachings is a tappet formed by a process that includes forming a body out of ferrous metal with a surface that includes a cylinder-conforming bore-running surface, stamping the body to form an anti-rotation guide feature projecting outward from the bore-running surface, case hardening the body by a process that adds nitrogen to the ferrous metal at sub-critical temperatures, and attaching a cam follower to the body. 
     Another aspect of the present teachings is a method of manufacturing a tappet. The method includes forming ferrous sheet metal to provide a cylinder-conforming bore-running surface, stamping by which there is formed an anti-rotation guide feature projecting outward from the cylinder-conforming bore-running surface, case hardening the body by a process that adds nitrogen to the ferrous metal while maintaining the ferrous metal in a ferritic state, and mounting a cam follower at one end of the body. 
     The alignment of the roller is largely determined by the geometric relationships between the roller mounting, the bore-running surface, and the anti-rotation guide. In a tappet according to the present teachings, these relationships may be controlled through stamping processes. Stamping forms the anti-rotation guide feature and features on the body that relate to the relative location of the roller. In some of these teachings, the body includes two parallel planar surfaces that are formed by stamping and are proximate a drive-input end of the body. Axle holes may be formed in these planar surfaces and an axial support pin for the cam follower may be mounted through those axle holes. The orientation of those planar surfaces relative to the anti-rotation guide feature contributes to the roller alignment. In some of these teachings axle holes for the cam follower are formed by stamping. A stamping process that forms axle holes may include piercing and shaving. Forming the axle holes by stamping improves the roller alignment. 
     The cylinder-conforming bore-running surface is operative to engage a first cylindrical bore to guide translation of the tappet within the bore. The axis of the bore-running surface becomes coaligned with the bore axis. A cam is arranged with its contact surface perpendicular to the bore. In some of these teachings, the anti-rotation guide feature is operative to engage a second cylindrical bore having a smaller diameter than first cylindrical bore and intersecting the first cylindrical bore. In this configuration, the anti-rotation guide feature restricts rotation of the tappet within the first cylindrical bore. 
     According to some of the present teachings, the body is not subjected to any hardening process that heats the metal above the critical temperature. The critical temperature is the temperature at which the metal transition from a ferritic phase to an austenitic phase. For example, the body is not subjected to carbonitriding, which is a conventional case hardening process. Carbonitriding involves heating the metal above the critical temperature. If the body were subjected to carbonitriding before stamping, the metal would have insufficient malleability for the stamping process. If the body were subjected to carbonitriding after stamping, the cylinder-conforming bore-running surface would be distorted and the anti-rotation guide feature would interfere with processing to restore circularity to the bore-running surface. 
     The body of a prior art tappet is subjected to carbonitriding. The hardening process results in shape distortion. The outer surface of the body is returned to a cylinder-conforming shape by a process such as OD grinding, which removes metal from the surface. It was found, however, that carbonitriding and OD grinding can alter the geometric relationship between the roller mounting and the bore-running surface. In the present teachings, these processes may be avoided. In some of the present teachings, the body lacks distortions of the type that would be produced by a hardening process that involves heating the body above the critical temperature. 
     In some of the present teachings, the bore-running surface does not bear evidence of any operation that has contributed to determining its outer diameter and that has not also been applied to the surface of the anti-rotation guide. In some of the present teachings, a final outer diameter for the cylinder-conforming bore-running surface is produced without any grinding, milling, or abrading that affects the outer diameter. The outer diameter may be largely determined prior to stamping, although case hardening may have a measurable effect on the outer diameter. In some of the present teachings, the body is formed from sheet metal by deep drawing. In some of the present teachings, the outer diameter of the body is determined by processes consisting essentially of deep drawing, stamping, and case hardening. In some of the present teachings, the metal that provides the bore-running surface is present at the surface of the body prior to stamping. The body is case hardened, but in accordance with some of the present teachings, the body has an interior that is comparatively malleable. 
     In some aspects of the present teachings, the tappet is a pump actuator. In some of these teachings, the tappet is a high-pressure fuel pump actuator. The pump actuator application requires high fatigue resistance. In the present teachings, the body is case hardened by a process that adds nitrogen to the ferrous metal while maintaining the ferrous metal in a ferritic state. In some of these teachings, the case hardening process is ferritic nitrocarburizing. A case hardening process is one that modifies the metal proximate the surface of a part to provide a hardened shell. 
     A crossmember may be installed within the body. In some of these teachings, the crossmember is ferrous metal hardened through its full thickness whereas the body has an interior that is malleable. Hardening the crossmember through its full thickness includes heating the crossmember to temperatures at which the ferrous metal enters an austenitic phase. In some of these teachings, the crossmember is mounted within the body by a process that includes crimping to secure the crossmember within the body. 
     Because the anti-rotation guide feature is formed by stamping the metal that also provides the bore-running surface, the anti-rotation guide feature is contiguous with the bore-running surface. In some of these teachings, the anti-rotation guide feature has a length extending along an axis of the cylinder-conforming bore-running surface and the anti-rotation guide feature meets the cylinder-conforming bore-running surface along two opposite sides of the anti-rotation guide feature both of which extend along the length. In some of these teachings, an interface between the anti-rotation guide feature and the cylinder-conforming bore-running surface forms a perimeter about the anti-rotation guide feature. This means that the anti-rotation guide is continuous with the bore-running surface on all sides. 
     In some of these teachings, the body comprises two parallel planar surfaces at its drive-input end, an axle hole is formed in each of the two planar surfaces, an axial support pin for the cam follower is mounted through the axle holes, and the body further comprises two additional surfaces that are substantially planar. The two additional surfaces are within transition regions between the cylinder-conforming bore-running surface and the two parallel planar surfaces. The additional surfaces are adjacent the parallel planar surfaces at ends of the parallel planar surfaces that are distal from a drive-input end of the body. In some of these teachings, the additional surfaces are inclined relative to an axis of the cylinder-conforming bore-running surface and the angle of inclination is in the range from 15 to 75 degrees. Having those surfaces so inclined reduces the weight of the tappet while maintaining or increasing its fatigue resistance. 
     The primary purpose of this summary has been to present certain of the inventors&#39; concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventors&#39; concepts or every combination of the inventors&#39; concepts that can be considered “invention”. Other concepts of the inventors will be conveyed to one of ordinary skill in the art by the following detailed description together with the drawings. The specifics disclosed herein may be generalized, narrowed, and combined in various ways with the ultimate statement of what the inventors claim as their invention being reserved for the claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a tappet according to some aspects of the present teachings. 
         FIG.  2    is a perspective view of the body of the tappet of  FIG.  1    prior to assembly. 
         FIG.  3    is a perspective view of a crossmember of the tappet of  FIG.  1   . 
         FIG.  4    is a perspective view of the body of  FIG.  2    and the crossmember of  FIG.  3    after assembly. 
         FIG.  5    is a sketch illustrating the measurement of perpendicularity. 
         FIG.  6    is a cross-section taken through the line  6 - 6  of  FIG.  4   . 
         FIG.  7    is an illustration of the tappet of  FIG.  1    installed in an engine to operate as a fuel pump actuator in accordance with some aspects of the present teachings. 
         FIG.  8    is a cross-section taken through the line  8 - 8  of  FIG.  4   , but showing the tappet as installed in the engine of  FIG.  7   . 
         FIG.  9    is a flow chart of a process according to some aspects of the present teachings. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a perspective view of a tappet  100 , which is an example according to some of the present teachings. Tappet  100  includes body  101 , crossmember  121  (not visible in  FIG.  1   ), and cam follower  131 .  FIG.  2    is a perspective view of body  101 .  FIG.  3    is a perspective view of crossmember  121 . Crossmember  121  is mounted within body  101  as shown in  FIGS.  4  and  6   .  FIG.  4    is a perspective of body  101  and crossmember  121  and  FIG.  6    is a cross-sectional view corresponding to  FIG.  4   . 
     Cam follower  131  include axial support pin  133 , bearings  137 , and roller  135 . Roller  135  is mounted on axial support pin  133  through bearings  137 . Cam follower  131  is mounted to body  101  proximate a drive input end  103 . Crossmember  121  may rest on a ledge  125  formed on an inner side of body  101 . Crossmember  121  may be secured against ledge  125  by dimples  123 , which may be formed in body  101  by crimping. Body  101  and crossmember  121  are both formed out of a ferrous metal, which is steel. Crossmember  121  is hardened throughout its thickness, whereas body  101  is only case hardened and has an interior that is malleable. The hardened material, which includes the shell of body  101  and the interior of crossmember  121 , has a hardness greater than 500 HV. The malleable material has a hardness less than 500 HV. 500 HV is a Vickers Pyramid Number based on the Vickers hardness test. 
     Case hardening may harden only the metal within 100 microns of the surface. In some of these teachings, the hardening is limited to within 50 microns of the surface. In some of these teachings, the hardening is limited to within 30 microns of the surface. The thickness of the hardened layer may be between about 10 and 15 microns. The distribution of hardening may be determined by forming sections and taking hardness traces. 
     Body  101  has a cylinder-conforming bore-running surface  109 . Surface  109  is an outer surface of body  101 . It is cylinder-conforming in that it follows the shape of a cylinder having axis  151 . While surface  109  conforms to the shape of a cylinder, it need not in itself form any complete cylinder. Surface  109  is a bore-running surface in that it is operative to guide translation of tappet  100  when installed in a matching bore and will limit rocking within that bore. 
     Body  101  is a contiguous piece of ferrous metal that includes anti-rotation guide feature  115 . Body  101  has been stamped to form anti-rotation guide feature  115  as an outward protrusion from cylinder-conforming bore-running surface  109 . The formation of anti-rotation guide feature  115  by stamping is evident from its continuity with the metal that forms bore-running surface  109 . Anti-rotation guide feature  115  has a length  153  extending parallel to axis  151  and meets bore-running surface  109  on a first side  117 A and a second side  117 B, each of which extends along length  153 . Preferably, anti-rotation guide feature  115  meets bore-running surface  109  through most of length  153 . More preferably, anti-rotation guide feature  115  meets bore-running surface  109  through its entire length  153 . Still more preferably, anti-rotation guide feature  115  meets bore-running surface  109  about its entire perimeter, as is the case for tappet  100  as shown in the figures. These continuity features may be achieved by forming anti-rotation guide feature  115  in a stamping process. 
     Body  101  includes two parallel planar surfaces  105  proximate drive input end  103 . Cam follower  131  includes axial support pin  133 , which is mounted to body  101  through axle holes  111  formed in surfaces  105 . Surfaces  105  are stamped into body  101 . The orientation of cam follower  131  relative to anti-rotation guide feature  115  is related to the orientation of surfaces  105  relative to anti-rotation guide feature  115 . Forming both surfaces  105  and anti-rotation guide feature  115  by stamping improves the orientation of cam follower  131  relative to anti-rotation guide feature  115 . Moreover, axles holes  111  are also formed by stamping, which further improves their orientation with respect to anti-rotation guide feature  115  and with respect to the bore-running surface  109 . 
     A high degree of perpendicularity is achieved between bore-running surface  109  and cam follower  131 .  FIG.  5    illustrates the measurement of perpendicularity. The perpendicularity is measured as the end-to-end variation of roller  135 &#39;s distance from a plane  163  that is perpendicular to the axis  151  of bore-running surface  109 . That variation is the difference between distance  165  and distance  167 . Roller  135  typically has a length in the range from about 5 mm to about 20 mm, with 11 mm being the length in this example. For a roller of this size, it is desirably to maintain a perpendicularity below 45 microns. The present teachings allow a perpendicularity below 30 microns to be achieved. For tappet  100 , the perpendicularity is about 20 microns. To relate these perpendicularities to rollers of other sizes, they may be normalized in terms of the 11 mm roller length to give dimensionless perpendicularities of 0.0041, 0.0027, and 0.0018. 
     The perpendicularity is partially the result of what has not been done to body  101 . Bore-running surface  109  has not been subjected to a heat treatment process that would distort its shape. Bore-running surface  109  has not been subjected to OD grinding or any other grinding, milling, or abrading operation that would be suitable for restoring the surface  109  of body  101  to a cylinder-conforming shape following a shape-distorting hardening operation such as carbonitriding. OD grinding leaves behind traces such as grind lines and marks. Bore-running surface  109  does not bear the traces of OD grinding or any other grinding, milling, or abrading operation that would determine its outer diameter  157 . 
     Body  101  also includes planar surfaces  113 . Planar surfaces  113  are within transition regions between bore-running surface  109  and parallel planar surfaces  105 . Planar surfaces  113  come adjacent parallel planar surfaces  105  proximate ends  107  of parallel planar surfaces  105 , which are distal from the drive-input end  103  of body  101 . Planar surfaces  113  are inclined relative to axis  151  of the cylinder-conforming bore-running surface  109 . The angle of inclination is 40 degrees away from axis  151  which is an angle in the range from 15 to 75 degrees. 
     Body  101  is case-hardened by a process that diffuses nitrogen into the metal while maintaining the metal in a ferritic phase. The arrangement of the nitrogen atoms within the metal is distinct from the case where nitrogen is added while the metal is in an austenitic phase. The metal is not heated above the critical temperature during case hardening, or afterward. Accordingly, an analysis of the distribution of nitrogen and its structure within the metal lattice will reveal that the parts have been case-hardened by a process that diffuses nitrogen into the metal while maintaining the metal in a ferritic phase. The analysis may be carried out with methods such as X-ray crystallography and scanning electron microscopy. 
     Tappet  100  is a bucket tappet. Tappet  100  is a high-pressure fuel pump actuator, although the same construction may be used in other tappet applications, as in a roller lifter.  FIG.  7    illustrates tappet  100  installed in an engine  150 . Engine  150  includes a cylinder head  141  having a bore  143 . Tappet  100  is installed within bore  143  and its axis  151  is coaligned with and axis of bore  143 . A smaller bore  145  that is parallel to and overlaps bore  143  is also formed in cylinder head  141 . A guide groove may be used in place of bore  145 . Anti-rotation guide  115  rides within bore  145 . A spring  171  within bore  145  biases cam follower  131  against cam  147 . Cam  147  has three lobes. Three-lobed and four-lobed cams are typical for high-pressure fuel pumps. Cams with other numbers of lobes can also be used. 
     An electronically controlled metering valve  177  is configured to selectively admit low pressure fuel from inlet  179  into pumping chamber  175 . As cam  147  rotates, it drives tappet  100  upward. Tappet  100  compresses spring  171  and drives piston  173  into pumping chamber  175 . Tappet  100  interfaces with piston  173  through crossmember  121 . Crossmember  121  transmits force from body  101  to piston  173 . Crossmember  121  may be hardened to resist fatigue while performing this function. The fuel within pumping chamber  175  is compressed by piston  173  until it reaches a critical pressure at which check-valve  181  opens to release pressurized fuel to the outlet  183 . A high-pressure relief valve  185  may be provided to allow a return flow of fuel to pumping chamber  175  once the pressure at outlet  183  is sufficiently high. 
       FIG.  8    provides a cross-sectional view of tappet  100  in bore  143 . The cross-section corresponds to the tappet cross-section  8 - 8  identified in  FIG.  4   . Cam follower  131  is removed from this view to provide greater clarity. As shown in this view, bore-running surface  109  mates with the wall of bore  143 . Diameter  157  may be referred to as the nominal outer diameter of tappet  100 . Tappet  100  is a high-pressure fuel pump actuator. Diameter  157  may be any of the standard sizes, which include 26 mm, 31 mm, and 32 mm. Accordingly, diameter  157  may be in the range from 26 mm to 32 mm. For the high-pressure fuel pump application, diameter  157  is generally in the range from about 10 mm to about 50 mm. Tappet  100  may alternatively have either a larger or smaller diameter. 
     The diameter  159  of bore  143  is very slightly larger than diameter  157  of bore-running surface  109  to provide a running clearance. The clearance may be in the range from 10 μm to 40 μm. Anti-rotation guide feature  115  extends out of bore  143  into the space of bore  145 . Anti-rotation guide feature  115  mates with the walls of bore  145  to narrowly limit rotation of tappet  100  within bore  143 . The diameter  161  of bore  145  may be much smaller than the diameter  159  of bore  143 . The diameter  161  is typically in the range from about 2 mm to about 8 mm. The diameter  161  is about 4 mm in this example. The cylinder conforming bore-running surface  109  has a diameter variance less than 50 μm. For example, the variance may be 15 μm. 
       FIG.  9    provides a flow chart of a process  200  that may be used to manufacture the tappet  100 . Process  200  begins with a strip of sheet metal, which may be taken from a coil. In act  201 , a piece of sheet metal is subjected to deep drawing to produce a cylindrical form. In act  203 , the cylindrical form is subjected to a series of stamping operations to produce body  101 . These operations may include act  205 , which forms anti-rotation guide feature  115 , act  207 , which forms parallel planar surfaces  105 , and act  209 , which forms axle holes  111 . Act  209  includes piecing and shaving. 
     Acts  211  through  215  produce and process crossmember  121  independently from body  101 . Act  211  is stamping to form crossmember  121 . Act  213  is neutral hardening. Neutral hardening includes heating crossmember  121  above the critical temperature and quenching. Act  215  is tempering, a heat treatment process that relieves internal stress developed during the hardening process. 
     Act  217  is mounting crossmember  121  within body  101  and crimping to hold it against ledge  125 . Crimping forms dimples  123 . Crossmember  121  may be described as a transverse web and is mounted within body  101 . Act  219  is ferritic nitrocarburizing (FNC), which is a case hardening process. FNC is a process that adds nitrogen to a ferrous metal by diffusion while the metal is below a critical temperature. The critical temperature is the temperature at which the metal begins to transition from a ferritic phase to an austenitic phase temperature. The critical temperature is generally around 733° C. The FNC process is preferably carried out between 525° C. and 625° C. The FNC may be a gas FNC process, a salt bath FNC process, or a plasma FNC process. 
     Act  221  is mounting cam follower  131  to body  101 . Roller  135  is mounted on bearings  137  which are mounted on axial support pin  133 . Mounting cam follower  131  to body  101  includes fitting axial support pin  133  through axle holes  111 . The assembled tappet  100  may be installed in engine  150 , in which tappet  100  is operative as a fuel pump actuator. 
     Although modified by FNC, the metal exposed at bore-running surface  109  of body  101  is essentially metal that is present at the outer surface of the sheet metal following act  201 , deep drawing. The stamping operations  203  have little or no effect on the outer diameter  157 . The outer diameter  157  is essentially determined by act  201 , deep drawing, act  203 , stamping, and act  219 , FNC. Outer diameter  157  may be essentially determined by act  201 , deep drawing, alone. 
     The components and features of the present disclosure have been shown and/or described in terms of certain embodiments and examples. While a particular component or feature, or a broad or narrow formulation of that component or feature, may have been described in relation to only one embodiment or one example, all components and features in either their broad or narrow formulations may be combined with other components or features to the extent such combinations would be recognized as logical by one of ordinary skill in the art.