Abstract:
In a tappet-driven single piston pump, piston side loads caused by return spring out-of-squareness are eliminated by effectively piloting the piston return spring, preferably the associated spring seat, by the tappet, thereby allowing the tappet to bear the spring side load. The piston engages and is returned by the spring seat, but radial clearance between the piston and spring seat is greater than radial clearance between the spring seat and tappet, thus eliminating side loading imparted to the piston. The spring seat can be considered a piston retainer, in which the piston is not closely attached to the retainer but instead exhibits a predefined radial clearance greater than the piloting clearance between the retainer and the tappet.

Description:
BACKGROUND 
       [0001]    Single piston, cam driven high pressure pumps have become a common solution for generating high pressure fuel in today&#39;s common rail, direct injection, gasoline engines. These pumps are typically driven via a tappet and cam with multiple lobes. In order to keep the tappet in contact with the cam and pump piston in contact with the tappet at high speeds, a coil spring is positioned between the pump body and a spring seat affixed to the pump piston. This execution has proven robust in regions of the world with well controlled fuel quality. In regions of the world with poor fuel quality, pump piston seizures have been a problem due to fluid film breakdown and poor lubricating qualities of those fuels. It is advantageous for these applications to reduce pump piston side loads in order to minimize the fluid film breakdown. One significant source of these side loads is the out-of-squareness of the piston/tappet return spring positioned between the pump body and plunger spring seats. When both ends are constrained by each spring seat to radially align the spring, the spring must be deflected to do so, and in the installed state a significant side load will be imparted to the pump piston. 
       SUMMARY 
       [0002]    The primary purpose of this invention is to eliminate pump piston side loads caused by spring out-of-squareness, making the pump resistant to seizures when run on poor quality fuels. 
         [0003]    The invention accomplishes this by effectively piloting the piston return spring, preferably the associated spring seat, by the tappet, thereby allowing the tappet to bear the spring side load. The piston engages and is returned by the spring seat, but radial clearance between the piston and spring seat is greater than radial clearance between the spring seat and tappet, thus eliminating side loading imparted to the piston. 
         [0004]    The spring seat can be considered a piston retainer that features a novel relationship between the piston and the piston retainer, in that the piston is not closely attached to the retainer but instead exhibits a predefined radial clearance greater than the piloting clearance between the retainer and the tappet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0005]    The invention will be disclosed in greater detail with reference to the accompanying drawing, in which: 
           [0006]      FIG. 1  is a longitudinal section view of a pump that incorporates one embodiment of the present invention; 
           [0007]      FIG. 2  is a detailed view of a portion of  FIG. 1 , showing the region of the pump where the tappet drives the pumping piston; 
           [0008]      FIG. 3  is a detailed view similar to  FIG. 2 , showing a second embodiment of the invention; 
           [0009]      FIG. 4  is a detailed view similar to  FIG. 2 , showing a third embodiment of the invention; 
           [0010]      FIG. 5  is a detailed view similar to  FIG. 2 , showing a fourth embodiment of the invention; and 
           [0011]      FIG. 6  is a detailed view similar to  FIG. 2 , showing a fifth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIGS. 1 and 2  show a cam-driven high pressure single piston fuel pump  10  having a pump housing  12 , a pumping chamber  14  within the pump housing, a piston  16  with one end  16   a  in the pumping chamber and another end  16   b  outside the pump housing. A piston sleeve  18  is mounted in the pump housing and has a bore  20  in which the piston reciprocates with specified clearance along a pumping axis  22  between a retracting motion during which fuel is delivered to the pumping chamber and a pumping motion during which the piston pressurizes fuel in the pumping chamber. A tappet  24  is coaxially aligned with the piston  16 , having one end  24   a  adapted to be reciprocally driven by a rotating cam and another end  24   b  operatively associated with the other end  16   b  of the piston for reciprocating the piston. A coil return spring  26  is seated between the housing  12  and a generally disc-like tappet spring and piston retainer  28  (hereinafter, “piston retainer”) with the return spring and piston retainer coaxially aligned with the piston and operatively associated at  28   b  with the piston, for biasing the other end of the piston away from the pumping chamber via the engagement of the piston end  16   b  with the portion  28   b  of the piston retainer  28 . Preferably, the end of the spring  26  closer to the housing  12  seats in the outer portion of a sleeve retainer  30 , with the inner end of the sleeve retainer acting through a load ring  32  to urge the upper end of the sleeve  18  into sealing engagement around the periphery of the pumping chamber at the upper end of bore  20 . The piston  16  is also fluidly sealed at  34 , within the sleeve retainer  30 . 
         [0013]    The tappet  24  is forced upward by rotation of an engine camshaft. The tappet forces the piston  16 , retainer  28 , and piston  16  upward to compress fluid in the pumping chamber  14 . The high pressure fluid from the pumping chamber is then forced through a check valve and via connections into a common rail. 
         [0014]    In a key aspect of the present disclosure, the tappet  24  and the piston retainer  28  have radially overlapping concentric walls  24   c ,  28   c  with a radial gap A that accommodates side loads on the spring  26 . In order to prevent side load imparted by spring out-of-squareness within normal tolerances against the piston  16  the piston retainer  28  is guided on its OD within the ID of tappet  24 . This is guaranteed by assuring that gap A is always smaller than gaps B and C. The piston retainer  28  is positioned axially against the tappet  24  at interface E and is preloaded by spring  26 . Because gap A is smaller than gaps B and C, the tappet  24  bears all side loads imparted by the spring  26 . 
         [0015]    In the illustrated embodiment, the other end  16   b  of the piston is operatively associated with the retainer at  28   a  by a profiled tip of the piston, such a neck or shank  36 , that is captured in a recess of the retainer, such as  38 , and head or flange portion  40  captured by shoulder  42 , with a radial gaps B and C that are each greater than the radial gap A between the retainer and the tappet. An axial gap D is also provided as a lash feature at the shoulder  42 . This lash prevents the load of the spring  26  from bearing directly against the piston  16  in the axial direction. In this embodiment, the central portion of the axial end  44  of the piston retainer  28  protrudes and bears on the surface  46  of the tappet drive element  48 . In essence, the profiled tip  16   b  of the piston has a smaller diameter shank portion passing through a central opening defining a recess of the piston retainer and a larger diameter flange portion captured by a shoulder within the retainer. To facilitate assembly, the piston retainer has a slot from the circumference to the whereby the piston end  16   b  can be slid radially into position in the recess. 
         [0016]    Preferably, Gap A should be at least 2 microns, Gaps B and C should be at least 10 microns, and gap D should be at least 2 microns. Generally, the radial Gap C should be at least five times the radial Gap A. 
         [0017]    It should be appreciated that the present invention can be employed with a wide variety of tappet and piston connections. In  FIGS. 1 and 2 , the tappet is a so-called “bucket” tappet wherein the main tappet shaft  50  has a drive element  48  and together with a substantially cylindrical collar  24   b  engaging and extending axially from the main shaft, define a generally cup or bucket shaped collar with cylindrical wall portion  24   c  concentrically overlapping the outer circumference of  28   c  of the piston retainer  28 , with nominal gap A. 
         [0018]      FIG. 3  depicts an alternate embodiment  100 , which also eliminates spring induced side loads on piston  102 . The piston retainer  104  has a radially inner retainer element.  106  that is affixed to the piston and flares  108  radially outwardly. A radially outer retainer element  110  has stepped inner portion with edges  112 ,  114  that are radially spaced from the cylindrical portion of the inner element and flange, respectively. The outer element is also axially spaced above the flange  108  of the inner retainer element, providing axial lash D and radial clearances B and C. The outer element  110  includes a radially outer surface  116  that provides a seat for the spring  118 . The tappet extension  120  is a cylindrical collar and the outer element  110  pilots within the collar  120  with a radial clearance A less than the radial clearances between the outer retainer and inner retainer. 
         [0019]    In this execution, the outer retainer  110  has a depending rim that bears on a shoulder  124  at the inside base of the collar. The piston retainer element  110  is guided on its OD within the ID of the tappet collar  120 , but bears axially against the tappet along a peripheral edge at interface E. Inner retainer  106  is fastened to the piston via a press-fit. Gaps A, B, C, and D correspond to and have the same function as the similarly labelled gaps in  FIGS. 1-2 . In this execution the axial pumping loads are transmitted directly from the tappet  126  to the piston end surface  128 . 
         [0020]      FIG. 4  depicts another embodiment  200 , which also eliminates spring induced piston side loads. Spring retainer  202  is at the housing, seating one end of spring  204 , with the other end seated in a different kind of piston and tappet spring retainer  206 . The piston  208  has a body portion and an insert or extension portion  210 , with the extension portion coaxially secured to the body portion and defining the profiled tip of the piston. The tappet  212  has a body portion and an extension or insert portion  214 , in the form of a plug that is surrounded by the retainer and thus defines the wall that pilots the retainer. The piston extension enters the tappet extension through a hole  216  at the top. 
         [0021]    The piston retainer  206  has an outwardly flared bottom  218  that bears on drive surface  220  of the tappet  212 . The tappet extension can be connected to the drive surface  220  with a reduced diameter boss or the like  222  passing through a hole  224  in that surface. 
         [0022]    The piston profile includes a narrowed shank and enlarged flange  226 , which cooperate with the inwardly turned flange  228  at the top of the piston retainer. In this execution the tappet is inside the piston retainer and provides OD  230  to the ID  232  of the piston retainer with radial gap A In this embodiment interface E is shown as a surface normal to the pump axis. Gaps B, C, and D are functional equivalents to corresponding gaps previously described. 
         [0023]    It should be appreciated that the piston extension  210  and tappet extension  214  are shown as inserts, but these could be integral with the main bodies  208 ,  212  to provide equivalent functionality. 
         [0024]      FIG. 5  depicts an another embodiment  300  similar to  FIG. 4  in which Gap D has been eliminated, thus eliminating all axial lash of the piston and piston insert  302  relative to tappet and tappet insert  304 . Gaps A, B, and C maintain the same function as previously described. The load bearing interface between the tappet body  306  and the bottom  308  of the piston reatiner has also been eliminated (at  310 ).  FIG. 6  depicts an alternative embodiment  400  to  FIGS. 1 and 2  in which Gap D has been eliminated, thus eliminating all axial lash of the piston  402  relative to the piston retainer  404 . and tappet  406 . Gaps A, B, and C maintain the same function as previously described. The load bearing interface between the tappet  406  and piston retainer  404  has also been eliminated. The tappet  406  bears directly against the lower tip  408  of the piston.