Patent Publication Number: US-6901844-B2

Title: Guided shoe for radial piston pump

Description:
This application is a continuation of application Ser. No. 10/187,823, filed Jul. 2, 2002, now U.S. Pat. No. 6,694,950, under the provisions of 35 U.S.C.§120. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to radial piston pumps and more particularly, to radial piston pumps of the type used in fuel supply systems for internal combustion engines. 
     Radial piston pumps, particularly the type used for pressurizing fuel for delivery to the combustion chambers of internal combustion engines, typically have a housing defining a central cavity and a drive member mounted about a drive axis for rotation in the cavity. At least one piston bore extends radially relative to the axis, through the housing to the cavity. A piston oriented radially within the piston bore has a radially outer pumping end and a radially inner driven end cooperating with the drive member for reciprocal movement in the piston bore between top dead center and bottom dead center travel limits. A sliding shoe engages the driven end of the piston and bears on the drive member, for providing the cooperation whereby the rotary movement of the drive member is converted to the reciprocal movement of the piston. A return spring urges the driven end of the piston toward the shoe and the drive member. For the type of pump to which the present invention is especially directed, the drive member is eccentric, i.e., it has an outer circular surface with a center that is offset with respect to the drive axis. The driven end of the piston bears pivotally against, without being rigidly attached to, the shoe, to accommodate the eccentric path of the drive member. 
     Particularly in operational modes where the pumping chamber of the piston is not fully charged before the pressurization, or discharge, stroke of the piston, unbalanced forces can act on the shoe with potentially detrimental, if not disastrous, results. Such unbalanced forces can result in separation of the shoe from both the driven end of the piston and the drive member while the piston is at or near the top dead center position such that, despite the restorative forces of the return spring, the piston does not seat properly in the shoe, or in the worst scenario, the shoe is carried into the cavity, resulting in catastrophic damage to the pump. 
     One type of pump and associated control scheme in which this problem can arise, and for which the invention is particularly suited, is described in U.S. application Ser. No. 10/187,823, filed on Jul. 2, 2002, entitled “Hybrid Control Method in Fuel Pump Using Intermittent Recirculation at Low and High Engine Speeds”. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to improve the performance and reliability of the sliding shoe associated with the conversion of rotating motion of a drive member, to the reciprocal motion of the pumping pistons, in a radial piston pump. 
     According to one aspect, the invention is directed to a radial piston pump in which the sliding shoe has a central region that engages the driven end of the piston and an outer region or rim surrounding the driven end of the piston and projecting into the pumping chamber bore a distance such that for all positions of the piston relative to the drive member, at least a portion of the outer region of the shoe remains within the pumping chamber bore. 
     In another aspect, the invention is directed to a sliding shoe for a radial piston pump, wherein the shoe has a concave bottom side, a socket portion projecting centrally on the top side, and a plurality of guide arms projecting upwardly on the top side and spaced laterally from the socket portion. 
     Preferably, the projecting rim is in the form of guide arms that are spaced apart to form a castellated, substantially annular rim around the socket portion, such that in the event of separation of the driven end of the piston from the socket and the bottom side of the shoe from the drive member, with a resulting “floating” and misorientation of the shoe, at least a portion of one and preferably two of the guide arms, remains within the mounting bore of the piston, thereby preventing the shoe from experiencing excessive misorientation or displacement into the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The preferred embodiment will be described below with reference to the accompanying drawing, in which: 
         FIG. 1  is a sectional view of a portion of a radial piston pump, showing the preferred form of the sliding shoe according to the invention, as well as the preferred relationship of the outer region of the sliding shoe to the pumping chamber mounting bore, when the piston is in the bottom dead center position; 
         FIGS. 2A and 2B  show a typical prior art sliding shoe in relation to the driven end of the piston and the drive member, for illustrating the unbalanced forces that give rise to the problem solved by the present invention; 
         FIG. 3  is a section view similar to  FIG. 1 , showing the relationship of the sliding shoe, the pumping chamber mounting bore, and the drive member, during the normal top dead center position of the piston, but with momentary separation between the driven end of the piston and the sliding shoe socket; 
         FIG. 4  is a section view similar to  FIG. 3 , showing the piston in the top dead center position, with the drive member in a different position resulting an even greater separation between the driven end of the piston and the socket of the sliding shoe, but with the sliding show retained within the pumping chamber mounting bore in accordance with the invention; 
         FIG. 5  is a perspective view of the preferred shape of the inventive sliding shoe; 
         FIG. 6  is an elevation view of the sliding shoe of  FIG. 5  as seen along line V 6 ; 
         FIG. 7  is a section view along line  7 — 7  of  FIG. 6 ; and 
         FIG. 8  is a bottom view of the shoe of FIG.  5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a pump housing  10  having a bore  12  in which is mounted a pumping chamber assembly  14 , for example, via a threaded connection  16  with associated seals  18  and cap  20 . The assembly includes a generally cylindrical piston chamber wall  22 , in which the pumping piston or plunger  24  is oriented for reciprocal motion. In a retracted or bottom dead center position, the portion  26  of the chamber is filled, or partially filled with relatively low pressure feed fuel. Upon driving of the piston toward the top dead center position, the fuel in the pumping chamber  26  is highly pressurized and discharged for ultimately delivery, such as by injection, to the engine cylinders. 
     The piston mounting bore  12  opens to a cavity  28  of the housing where feed fuel is maintained at a relatively low pressure and where a rotating drive member, especially an eccentric drive member  30 , is mounted for rotation about a drive axis. Thus, the piston bore extends radially, relative to the drive axis, through the housing to the cavity, and the piston  24  is oriented radially within the piston bore. The piston has a radially inner, driven end  32 , preferably in the form of a bulb or portion of a sphere, and a radially outer pumping end  34 . A sliding shoe  36  is provided for pivotally engaging the driven end  32  of the piston while sliding on the outer surface of the drive member  30 , to convert the radial motion of the drive member to the reciprocal motion of the piston. 
     In the illustrated embodiment, one or more charging orifices  38  are situated at the driven end, adjacent the spherical head  32 , for fluid communication with the low-pressure fuel and cavity  28 . This orifice  38  can be formed in a notch or neck  40 , from which the head  32  extends downwardly. A charging passage  42  extends from the charging orifice  38  in fluid communication with the pumping chamber  26 , through the center of the piston  24 . A check valve  44  with associated spring  46  are mounted in the charging passage  42 , for permitting fuel flow therein during charging from the cavity, but preventing fuel from flowing back into the cavity  28  during pressurization of the fuel in the pumping chamber  26 . A piston return spring, such as a coil spring, is mounted at one end  48  to a shoulder on the cylinder wall  22 , concentrically but exterior to the lower portion of piston  24 , and has another end  50  bearing on a rim or flange portion  52  of a spring seat which has a inner portion  54  bearing on a shoulder of the notch  40  associated with head  32 . 
     The sliding shoe  36  has an upper or top side on which a socket  56  is formed for the pivotal engagement via complementary concave surface to the convex surface formed by driven end  32  of the piston. The socket  56  and the spherical end  32  are both preferably formed at the surfaces of rotation about a common axis, e.g., the piston reciprocation axis. The shoe has an outer region  58  surrounding the driven end  32  of the piston and projecting into the piston bore  12 . As will be described in greater detail below, all or some of such projection remains in the piston bore  12  during all positions of the piston  24  relative to the drive member  30 . The return spring  50  extends longitudinally along a portion of the piston bore  12  externally of the piston  24  and acts on the driven end of the piston, and the outer region of shoe  58  projects into the piston bore in overlapped relation to the return spring when the piston bears against the central region of the shoe, as shown. In other words, the outer region  58  of the shoe overlaps the return spring  50  when the piston is at the bottom dead center position. As will be described in greater detail below, the outer region  58  overlaps the return spring  50  when the piston is at the top dead center position as well. This is preferably implemented by configuring the piston  24  and shoe  36  in relation to the drive member  30 , such that when the complementary surfaces or formations of the piston head  32  and the shoe socket  56  are engaged, these formation are in the piston bore  12  and the outer region  58  on the shoe extends into the piston bore  12  a greater distance than the engagement of the complementary formations. 
     It can be appreciated from  FIG. 1 , that the outer region  58  of the shoe is annularly spaced about the socket  56  formed in the central region of the shoe, thereby defining an annular space between the central region and the outer region of the shoe. The rim portion  52  of the return spring seat, and the radially inner end  50  of the return spring, are situated in the space when the head  32  is fully engaged with the socket  56 . 
     As may be further appreciated with reference to  FIGS. 1 ,  5 ,  6 ,  7  and  8 , the sliding shoe  36  preferably has top  60  and bottom  62  sides for cooperatively connecting the piston at the top with the drive member at the bottom. The sliding shoe  36  can be considered as having a base  64  having a concave bottom surface  66 , a socket portion  56  projecting centrally on the top side, and a plurality of guide arms  58  projecting upwardly on the top side and spaced laterally from the socket portion. The plurality of guide arms preferably form a castellated, substantially annular rim around the socket portion. The upper surface  70  of the base is generally convex, and the arms project obliquely away from each other from the upper surface  70 . 
     The plurality of guide arms preferably consists of four spaced apart arms  58   a ,  58   b ,  58   c  and  58   d  which together span a total of between 180 and 270 degs. of the rim circumference, with the spaces  76   a ,  76   b ,  76   c  and  76   d  between the arms together spanning a total of about 90 to 180 degs. of the circumference. Preferably each arm has substantially the same span. At least two of the arms can project from the top surface a greater distance than the projection of the socket. The relative length of the arms depends on the maximum piston travel. The shoes as shown on  FIG. 3  allow for larger eccentricity and by that for higher pump output, without the danger of ever leaving the bore. For smaller eccentricity the arms can be made shorter. 
     In the embodiment illustrated in  FIGS. 5-8 , the castellated arms  58  define U-shaped spaces  76   a ,  76   b ,  76   c  and  76   d  between adjacent arms, where the horizontal portion  78  of the U is defined by the top surface  70  of a step that projects a relatively shorter distance from the top surface of the base, and two facing side walls  82  and  84  of adjacent arms  58   a ,  58   b  that project a relatively longer distance from the base. 
     In order to reduce the shoe mass and flow restriction of the shoe while it moves up and down through the surrounding fuel, two of the four castellated guide arms could be of lesser height, or could be eliminated. Preferably, the bottom surface  66  of the shoe has plural grooves  86   a,b  to facilitate lubrication at the sliding interface, 
     It can be appreciated that the drive member  30  may be a cylinder having a drive member axis that is offset from the drive shaft axis (not shown), such that the drive member has an outer surface that is not circular with respect to the drive axis.  FIGS. 6 ,  7  and  8  show that the bottom surface  66  of the shoe is also not circularly symmetric. Thus, neither the drive member surface nor the concave bottom surface  60  of the shoe  36  is circularly symmetric about the axis of socket  56 . Moreover, it is not essential that the rim or projections of outer region  58  be circularly symmetric about the socket axis. Rather, in the broadest embodiment, the outer region  58  would have two diametrically oppose projecting arms that together could span less than 180 deg. of the circular arc around the socket axis. 
       FIGS. 1 ,  2 A,  2 B and  3  illustrate the problem solved by the present invention whereby in a control scheme where the inlet flow through the feed orifice  38  such as shown in  FIG. 1  or in some other manner, a smaller quantity of fuel is charged into the pumping chamber  26 , relative to the full available charging volume defined by the difference in the top dead center and bottom dead center positions of the piston. During the partial filling of the pumping chamber  26 , there will always be a force component F 1 , originating from the pressure drop across the piston inlet (metering orifice plus opening pressure of the inlet check valve) acting over the effective area of the piston, trying to counter act the piston return spring force F 2 . Although a small separation between the sliding shoe and the surface of the actuating eccentric drive member is beneficial because it helps regenerate the layer of lubricating fuel that was “squeezed out” during the previous pumping event as a consequence of the high pumping forces. However if the separation becomes too large, the pumping reaction forces as shown in  FIGS. 2A and 2B  will lead very quickly to even larger separation. If the sliding shoe is not retained in some manner, the shoe socket will lose engagement and slide in the direction of rotation, driven by the fictional forces F 3  as well as hydraulic forces F 4  generated by the flow of fuel displaced by the eccentric rotating inside of the pump cavity into the gap between the pump housing and the shaft. The shoe will subsequently be crushed by the eccentric, resulting in a catastrophic failure of the pump. 
     The projecting arms of the sliding shoe according to applicant&#39;s invention, not only physically retain the shoe within the piston mounting bore in the event of such misalign mart or displacement of the shoe, but furthermore, the castillation of the arms by which spaces are present between adjacent arms, significantly reduces the hydraulic forces caused by the axial motion of the shoe through the liquid, which would otherwise further aggravate the problem described with respect to  FIGS. 2A and 2B . 
       FIGS. 3 and 4  further illustrate the effect of the operation of the invention. These figures show that even in the unlikely event of the maximum possible separation (piston in top dead center position and drive member in bottom dead center position) the outer region of the shoe not only prevents the shoe from leaving the piston mounting bore, but also ensures that the ball or similar formation at the driven end of the piston finds its socket in the shoe as the eccentric again moves into what should be the top dead center position for the piston. 
     As a further explanation, the phantom line for the drive member in  FIG. 3  represents the maximum pumping position corresponding to top dead center of the piston, where the shoe would be engaged with the piston and the drive member would be engaged with the shoe. However, if the return spring is not fully effective as the eccentric continues rotating to the position corresponding to the solid eccentric surface in  FIG. 3 , the shoe may remain on the surface of the eccentric while detaching from the piston. If this continues, the worst scenario the eccentric reaches the position corresponding to the bottom dead center of the piston, but the piston is still in its top dead center position. The phantom line show the position of the shoe if it continues to be carried away from the piston by the eccentric. It is evident that the outer regions of the shoe as shoe as shown in phantom in  FIG. 4 , are still within the piston mounting bore and therefore cannot, even if the shoe continues to slide relative the drive member, before hydraulic or other forces out of the piston mounting bore. The solid line rendition of the mounting shoe in  FIG. 4 , illustrates the worst case misorientation when the drive member is returning again toward approaching the position corresponding to top dead center of the piston (i.e., approaching the phantom line shown in  FIG. 3  for the drive member). The direction of this rotation would tilt the shoe but, in accordance with the present invention, the at least one arm of the shoe is restrained from moving excessively out of alignment, by contact with the spring seat for the return spring. Thus, as the eccentric continues rotation the driven end of the piston has an opportunity to reengage the socket and continue normal operation. 
     Although the invention has been described with respect to a radial piston pump in which the charging is controlled or restricted, it is also very beneficial for all other radial piston pumps actuated radially outwardly, particular by an eccentric drive member, inasmuch as even a miniscule amount of debris slowing down the motion of the piston, could also result in excessive separation resulting in catastrophic damage.