Abstract:
A fluid pump assembly comprising a driven cam and a reciprocating member reciprocal within a bore provided in a pump housing as the cam is driven, in use, to cause pressurisation of fluid within a pump chamber. The pump assembly further comprises an interface between the cam and the reciprocating member, for example in the form of bevelled surfaces of the cam and the reciprocating member, which serve to drive the reciprocating member (i) to translate in a first, axial direction within the bore and (ii) to rotate within the bore in a second, rotational direction. An optional feature of the fluid pump assembly is that the pump housing defines a bearing for the cam which is provided with a recess to define a region of weakness to allow the bearing to deflect, in use, thereby to provide an increased lubrication volume between the cam and the bearing. The reciprocating member may take the form of a tappet which cooperates with a pumping plunger to pressurise fluid within the pump chamber.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a fluid pump assembly and, in particular, but not exclusively, to a pump assembly for fuel. The pump assembly is suitable for use in a common rail fuel injection system for supplying high pressure fuel to a compression ignition (diesel) internal combustion engine. In particular, the invention has application in a pump assembly of the type in which an engine driven cam imparts reciprocating, pumping motion to a drive member. 
       BACKGROUND TO THE INVENTION 
       [0002]    One known common rail fuel pump is of radial pump design and includes three pumping plungers arranged at equi-angularly spaced locations around an engine driven cam—such a pump is described in, for example, WO 2004/104409. In this pump, each plunger is mounted within a plunger bore provided in a pump head mounted to a main pump housing. As the cam is driven in use, the plungers are caused to reciprocate within their bores in a phased, cyclical manner. As the plungers reciprocate, each causes pressurisation of fuel within a pump chamber defined at one end of the associated plunger bore. Fuel that is pressurised within the pump chambers is delivered to a common high pressure supply line and, from there, is supplied to a common rail or other accumulator volume, for delivery to the downstream injectors of the common rail fuel system. The fuel pump has an inlet valve for admitting fuel under low pressure and an outlet valve for letting out the pressurised fuel. 
         [0003]    In this pump assembly, the cam carries a cam rider that extends co-axially with the drive shaft for the cam. The cam rider is provided with a plurality of flat surfaces (“flats”), one for each of the plungers. An intermediate drive member in the form of a tappet co-operates with the flat on the cam rider and couples to the plunger so that, as the tappet is driven upon rotation of the cam, drive is imparted to the plunger. 
         [0004]    A fuel pump of radial pump design necessarily occupies a relatively high volume and, for some engine applications, this can be a disadvantage. Furthermore, the tappets are prone to wear due to the side loads experienced as they reciprocate, in use, and there can be significant damage to the tappet face that cooperates with the cam rider due to inadequate lubrication. 
         [0005]    It is an object of the present invention to provide a fluid pump assembly which alleviates these problems when used to pump fuel in a fuel injection system. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the present invention, there is provided a fluid pump assembly comprising a driven cam and a reciprocating member reciprocal within a bore provided in a pump housing as the cam is driven, in use, to cause pressurisation of fluid within a pump chamber. The fluid pump assembly further includes interface means between the cam and the reciprocating member which cause the reciprocating member (i) to translate in a first, axial direction within the bore and (ii) to rotate within the bore in a second, rotational direction, as the cam is driven. The reciprocating member is arranged to rotate about its own axis within the bore. 
         [0007]    In one embodiment, the reciprocating member is an intermediate drive member, typically in the form of a tappet, which is cooperable with a pumping plunger to cause pressurisation of fluid within the pump chamber as the pumping plunger is driven by the intermediate drive member. 
         [0008]    In another embodiment, the reciprocating member is a pumping plunger which interfaces directly with the cam. 
         [0009]    The invention is particularly applicable to fuel injection systems for internal combustion engines in which a fuel pump assembly pressurises fuel to a relatively high pressure suitable for injection. Such a fuel pump assembly is particularly suitable for use in a common rail fuel injection system. However, the invention has wider application than fuel pumps for engines, and may be used as a pump for any other type of fluid also. 
         [0010]    In one embodiment, the interface means includes a bevelled face of the reciprocating member and a correspondingly bevelled face of the cam which cooperate so as to cause axial and rotational motion of the reciprocating member as the cam rotates. As the reciprocating member rotates about its own axis within its bore, the constant relative velocity between the parts aids lubrication so as to reduce the effects of wear due to friction. 
         [0011]    The reciprocating member may be arranged to rotate at substantially the same angular velocity as the cam. 
         [0012]    The fluid pump assembly may comprise an axial bearing for the cam which is defined by an axially-facing internal surface of the pump housing. The fluid pump assembly may further comprise, in addition or as an alternative, a radial bearing for the cam which is defined by a radially-facing internal surface of the pump housing. 
         [0013]    The cam may be provided with a low friction coating, for example a soft phosphate or PTFE coating, which deforms, in use, to the profile of the radial bearing. The profile of the coating on the cam being matched with the profile of the radial bearing provides good conditions for promotion of a hydrodynamic film. 
         [0014]    The axial bearing may be provided with at least one recess to provide a volume for receiving lubricating fluid. The recess therefore provides for a supply of lubricating fluid to the axial bearing to aid lubrication between the rotating cam and the axial bearing. 
         [0015]    Furthermore, the axial bearing may include an un-recessed area which defines a load bearing surface for the cam. 
         [0016]    In one particular embodiment, the axial bearing is provided with a region of weakness to allow the axial bearing to deflect, in use, thereby to create an increased volume for lubricating fluid between the axial bearing and the facing surface of the cam. Deflection of the axial bearing in this way opens up an enlarged gap between the cam and the axial bearing to encourage lubricating fluid to be drawn between the parts. Optionally, the region of weakness is defined by forming a recess in the bearing. 
         [0017]    In another embodiment the axial bearing is further provided with a cut-away section to define a lead-in edge for lubricant drawn between the axial bearing and the facing surface of the cam. 
         [0018]    The fluid pump assembly may comprise at least two intermediate drive members (e.g. tappets) and at least two pumping plungers, each of the intermediate drive members being cooperable with a respective one of the plungers and each of the intermediate drive members being cooperable with a cam common to all intermediate drive members. In one embodiment, for example, the fluid pump assembly includes three intermediate drive members and three pumping plungers, associated pairs of the drive members and the pumping plungers being arranged at equi-angularly spaced locations about a central pump axis. In an embodiment in which the reciprocating members are pumping plungers which interface directly with the cam, the pumping plungers are arranged at equi-angularly spaced locations about the central pump axis. 
         [0019]    In one embodiment, the pump chambers are defined within the pump housing and are closed by a plate mounted to the pump housing. Alternatively, the pump chambers may be defined entirely within the pump housing. 
         [0020]    Depending on the nature of the drive through which the engine is coupled to the drive shaft, an output end of the drive shaft may extend rearward of the cam and act against a bearing defined by the pump housing so as to counter side loads applied to an input end of the drive shaft. Such an arrangement is particularly suitable for belt, chain or gear drive applications where the nature of the input drive causes side loads to be imparted to the drive shaft. 
         [0021]    According to a second aspect of the invention, there is provided a fluid pump assembly comprising a driven cam and a reciprocating member reciprocal within a bore provided in a pump housing as the cam is driven, in use, so as to cause pressurisation of fluid within a pump chamber. The reciprocating member includes a bevelled face which cooperates with a correspondingly bevelled face of the cam so as to impart drive to the reciprocating member as the cam rotates. 
         [0022]    In the second aspect of the invention, the reciprocating member may be driven both axially and rotationally within the bore. 
         [0023]    According to a third aspect of the invention, a fluid pump assembly comprises a driven cam and a reciprocating member reciprocal within a bore provided in a pump housing as the cam is driven, in use, so as to cause pressurisation of fluid within a pump chamber. The pump housing defines a bearing for the cam which is provided with a region of weakness to allow the bearing to deflect, in use, thereby to provide an increased lubrication volume between the cam and the bearing. Optionally, it is an axial bearing defined by the pump housing that is provided with the region of weakness. Such an arrangement provides the aforementioned advantages for lubrication between the rotating cam and the axial bearing. 
         [0024]    It will be appreciated that optional features of the first aspect of the invention, as set out above and in the dependent claims, may be included in the second or third aspects of the invention also, alone or in appropriate combination. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will now be described, by way of example only, within reference to the following drawings in which: 
           [0026]      FIG. 1  is a cross sectional view of a fuel pump assembly of a first embodiment of the invention, having two pumping plungers; 
           [0027]      FIG. 2  is a cross sectional view of a part of the fuel pump assembly in  FIG. 1  to illustrate a spring seat for a return spring; 
           [0028]      FIG. 3   a  is a cross sectional view of a cam and a pump housing of the fuel pump assembly in  FIG. 1 ; 
           [0029]      FIG. 3   b  is an end view of an axial bearing defined by the pump housing in  FIG. 3   a;    
           [0030]      FIG. 4   a  is a cross sectional view of the pump housing in  FIG. 3   a  to illustrate an area of weakness on the external surface; 
           [0031]      FIG. 4   b  is an end view of the internal surface of the pump housing in  FIG. 4   a;    
           [0032]      FIG. 5  is cross sectional view of a fuel pump assembly of a second embodiment of the invention having two pumping plungers; 
           [0033]      FIG. 6  is a cross sectional view of a fuel pump assembly of a third embodiment of the invention having two pumping plungers; 
           [0034]      FIG. 7  is a cross sectional view of a fuel pump assembly of a fourth embodiment of the invention having a single pumping plunger; 
           [0035]      FIG. 8  is a perspective view of a part of a fuel pump assembly of a fourth embodiment of the invention having three intermediate drive members for three pumping plungers; and 
           [0036]      FIG. 9  is a cross sectional view of a fuel pump assembly of a fifth embodiment of the invention in which the intermediate drive members of previous embodiments are removed. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0037]    Referring to  FIG. 1 , a first embodiment of the fuel pump assembly  10  of the invention includes a pump housing having a first housing part  12  which is provided with a central bore for receiving a drive shaft  16  (only a part of which is shown). The first housing part includes a front plate  12   a  of the pump housing and a cylindrical body  12   b  towards the rear. The rear end of the drive shaft  16  carries a cam  18  which rotates with the drive shaft  16 , in use. Typically, the front or input end of the drive shaft is driven by the engine through an Oldham coupling, as would be familiar to a person skilled in the art. 
         [0038]    The cam  18  is wedge-shaped and so has a thin end  18   a  and a thick end  18   b  with a bevelled contact surface  18   c  on its front face. The back face of the cam  18  is planar and acts against an axially-facing internal surface of the pump housing  12 , which therefore acts as an axial bearing  22  for the cam  18  as it rotates. The outer surface of the cam, at its thick end  18   b , bears against a radially-facing internal surface of the first housing part  12 , which therefore acts as a radial bearing  24  for the cam  18  as it rotates. 
         [0039]    The pump assembly includes first and second reciprocating members, in the form of tappets  26 ,  28 , each of which has a bevelled surface  26   a ,  28   a , respectively, for contact with the correspondingly bevelled surface  18   c  of the cam  18 . Each tappet  26 ,  28  is received within an associated tappet bore provided in a second housing part  30  mounted to the first housing part  12 , and is coupled to an associated pumping plunger,  32 ,  34  respectively, in axial alignment with its tappet  26 ,  28 . The tappets  26 ,  28  therefore form an intermediate drive member between the cam  18  and the associated plunger  32 ,  34 . 
         [0040]    Each pumping plunger  32 ,  34  is received within an associated plunger bore provided in the second housing part  30 . An end of the pumping plunger  32 ,  34  remote from the tappet  26 ,  28  defines an internal surface of a pump chamber  33 ,  35  which receives fuel to be pressurised during a plunger pumping stroke, in use, as described in further detail below. 
         [0041]    Referring also to  FIG. 2  (which shows only the first tappet  26 ), each tappet  26  takes the form of a bucket tappet of generally U-shaped cross section having a base  26   b , which defines the bevelled contact surface  26   a , and a cylindrical upper body  26   c . Within the internal volume of the tappet  26 , on the side of the base  26   b  opposed to the bevelled contact surface  26   a , the tappet includes a projection  26   d  which defines a contact surface for the associated plunger  32 . A spring seat assembly for a plunger return spring  37  is received within the internal volume of the tappet  26  defined within the cylindrical upper body  26   c . The return spring  37  serves to provide a return load to the plunger  30  and the tappet  26  to effect a return stroke of the plunger, as described in further detail below. 
         [0042]    The spring seat assembly has two parts. A first part  36  is of top-hat construction and is located at the base of the plunger  32 , the plunger  32  extending through a central bore of the first part  36 . The first part  36  defines an abutment surface for one end of the return spring  37 , the other end of the return spring  37  remote from the spring seat assembly  36 ,  38  abutting an internal surface  41  of the second housing part  30 . A second part  38  of the spring seat assembly is an annular piece forms a push-fit on the base end of the plunger  32  and serves to retain the first part  36  of the assembly in place. 
         [0043]    In an alternative embodiment (not shown), the return spring  37  may be a smaller component located within the pump chamber  33 ,  35 , rather than surrounding the plunger  32 ,  34 . 
         [0044]    Referring again to  FIG. 1 , the pump chambers  33 ,  35  are closed by a plate  39  at the rear end of the pump assembly  10 . The closure plate  39  is provided with a plurality of drillings to allow relatively low pressure fuel to be conveyed into the pump chambers  33 ,  35  and to allow pressurised fuel to be conveyed from the pump chambers  33 ,  35  to a pump outlet (not shown). An inlet drilling is provided for each of the pump chambers  33 ,  35 , each inlet drilling having a respective spring-biased inlet valve  40 ,  42  through which relatively low pressure fuel passes to enter the associated pump chamber  33 ,  35 , prior to pressurisation. An outlet drilling is provided for each of the pump chambers  33 ,  35 , each outlet drilling having a respective spring-biased outlet valve  44 ,  46  through which pressurised fuel is delivered to the common outlet of the pump assembly when the pressure level in the pump chambers  33 ,  55  reaches a predetermined amount. The common outlet is connected to a common rail or accumulator volume of the fuel injection system, from where fuel is delivered to the fuel injectors of the engine. 
         [0045]    Operation of the fuel pump assembly will now be described in further detail. 
         [0046]    Considering the first tappet  26  and its associated plunger  32 , as the drive shaft  16  rotates, in use, cooperation between the rotating bevelled surface  18   c  of the cam  18  and the bevelled surface  26   a  of the tappet  26  results in the tappet  26  reciprocating axially within its tappet bore and, thus, the plunger  32  is caused to reciprocate within its plunger bore also. As the plunger  32  is driven it performs the pumping stroke, in which fuel within the associated pump chamber  33 ,  35  is pressurised to a high level suitable for injection, followed by the return stroke which is effected by means of the associated return spring  37 . 
         [0047]    At the start of the return stroke, the outlet valve  44  is closed under its spring force. As the plunger  32  moves outwardly from its bore to expand the volume of the pump chamber  33 , the pump chamber  33  fills with fuel at relatively low pressure from a supply pump (e.g. transfer pump) through the inlet valve  40  which is open. As the cam  18  continues to rotate and the plunger  32  completes its return stroke, cooperation between the bevelled surfaces  18   c ,  26   a  of the cam and the tappet causes the tappet, and hence the plunger, to move inwardly within their bores to reduce the volume of the pump chamber  33 . Soon after the volume of the pump chamber  33  starts to decrease, fuel pressure in the pump chamber  33  starts to increase and the force due to fuel pressure acting on the inlet valve  40  causes it to close. The pressure within the pump chamber  33  continues to rise as the plunger  32  continues through its pumping stroke, until such time as the pressure in the pump chamber  33  is sufficient to overcome the closing force of the outlet valve  44 , which is then urged open to allow pressurised fuel to be delivered through the pump outlet. 
         [0048]    As a result of the rotating bevelled surface  18   c  of the cam  18  interacting with the correspondingly bevelled surface  26   a  of the tappet  26 , the tappet is driven to move axially within its bore, hence driving axial motion of the plunger. Importantly, cooperation between the rotating bevelled surface  18   c  of the cam  18  and the correspondingly bevelled surface  26   a  of tappet  26  also means that the tappet is driven to rotate within its bore at the same angular velocity at which the cam  18  is driven by the drive shaft  16 . The interface between the cam and the tappet therefore results in a deliberately driven, continuous rotation of the tappet about its axis. 
         [0049]    Due to the nature of the two-part spring seat assembly, rotation of the tappet  26  also causes the plunger  32  to rotate within the plunger bore as it reciprocates. The spring seat assembly is configured such that the frictional force between the return spring  37  and the first part  36  of the spring seat assembly is greater than the frictional force between the second and first parts  38 ,  36  of the spring seat assembly. Hence, as the tappet  26  rotates, the plunger  32  may also rotate, whereas the first part  36  of the spring seat assembly and the return spring  37  remain static. In this way relative movement between the end of the return spring  37  and the internal surface  41  of the second housing part  30  is prevented, to avoid unwanted wear, whilst the plunger  32  is allowed to rotate. Unwanted relative movement between the first part  36  of the spring seat assembly and the return spring  37  is also avoided. 
         [0050]    The second tappet  28  and the second plunger  34  are driven in a similar manner to operate in phased, cyclical motion with the first tappet/plunger  26 / 32 , with both pump chambers  33 ,  35  filling a common rail with pressurised fuel through the respective outlet valves  44 ,  46 . 
         [0051]    A clearance between each tappet and its tappet bore provides a volume for lubricating fluid and so, due to the relative motion between the rotating tappet and its bore, lubrication of parts is promoted to reduce wear. 
         [0052]    As illustrated in  FIG. 3(   a ), the return load on the cam  18  due to pressurised fuel within the pump chamber  33  is exerted on the cam  18  in a direction perpendicular to the bevel angle of the cam surface  18   c . The thick end  18   b  of the cam  18  bears against the radially-facing internal surface of the first housing part  12 , which therefore acts as a radial bearing  24  for the cam  18  as it rotates. The rear face  18   d  of the cam (i.e. the face opposed to the bevelled surface  18   c ) bears against the axially-facing internal surface of the pump housing  12 , which therefore acts as an axial bearing  22  for the cam  18  as it rotates. 
         [0053]      FIG. 3(   b ) illustrates a coating that is applied to the radially-facing surface of the cam  18 . The surfaces of the cam  18  which bear against the axial and radial bearings  22 ,  24  may be provided with a soft lubricating coating, for example phosphate or PTFE. The dashed line illustrates the profile of the coating  25  on the cam  18 , in use. As the coating  25  is soft, the coating deforms as the cam  18  rotates so as to conform to the profile of the bearing surface  24 , hence providing good conditions for promotion of a hydrodynamic film. The soft phosphate coating is also applied to the bevelled face  18   c  of the cam  18  which cooperates with the bevelled surface  26   a  of the tappet. 
         [0054]    Referring to  FIGS. 4(   a ) and  4 ( b ), the axial bearing  22  is modified, on its front and rear faces, so as to aid lubrication between the parts  12 , 18 . Firstly, as shown in  FIG. 4(   b ), the axial bearing  22  includes first and second raised segments  46   a ,  46   b , or pads, separated by first and second recessed segments  48   a ,  48   b . The raised segments  46   a ,  46   b  are positioned so as to be axially aligned with a respective one of the tappets  26 ,  28  so as to absorb the tappet return load. The recesses  48   a ,  48   b  define an enlarged volume for lubricating fluid to aid lubrication between the cam  18  and the bearing  22  as the cam rotates. 
         [0055]    In addition, and as can be seen in  FIG. 4(   a ), the opposite face  12   c  of the first housing part  12  to the axial bearing  22  is provided with a further recess  50  to define a weakened region of the first housing part  12 . As the pump is driven and the cam  18  is loaded by the tappet and bears on the axial bearing  22 , the weakened region of the pump housing  12  allows the housing to deflect causing a wedge-shaped gap (not shown) to open between the axial bearing  22  and the facing surface  18   d  of the cam  18 . In particular, the weakened region  50  allows approximately one half of the pad to bend to provide a hydrodynamic wedge. This provides a lead-in edge for lubricating fluid and allows fluid to be drawn between the parts  12 ,  18 , allowing a hydrodynamic bearing to be generated between them as the cam  18  rotates. 
         [0056]    In addition to the lead-in edge provided by deflection of the pump housing  12 , the axial bearing  22  may also be provided with a chamfer, radius or bevel (not shown) at the lead-in edge to further encourage lubricating fluid to be drawn between the parts  12 ,  18  as the cam rotates. 
         [0057]    In  FIG. 1 , where an Oldham coupling is provided between the engine and the drive shaft  16 , there are only an insignificant side loads on the drive shaft  16 . However, where a belt, chain or gear drive is used between the engine and the drive shaft  16 , a significant side load is exerted on the drive shaft which causes unwanted tilt and translation forces to act on the cam  18 . For belt, chain or gear drive applications it is therefore necessary to counter these side loads to prevent unwanted translation and/or tilt of the cam  18  by providing a different bearing arrangement to that shown in  FIG. 1 . 
         [0058]      FIG. 5  shows an embodiment of the invention which is appropriate for a belt, chain or gear drive coupling (not shown) between the engine and the drive shaft  16 . Similar parts to those shown in  FIGS. 1 to 4  are denoted with like reference numerals. In this embodiment a rear or output end of the drive shaft  16  extends further rearward into the pump assembly  10 , and beyond the bevelled contact face  18   c  of the cam  18 , to be received within a central bore provided in the second housing part  30 . At the rearmost end of the drive shaft  16  the internal surface of this central bore defines a radial bearing  52  which counters the tilting force acting on the front end of the drive shaft  16  and, hence, prevents unwanted tilt of the cam  18  as it rotates. In addition, the thin end  18   a  of the cam  18  is provided with an axially-extending flange  54 , the outer surface of which bears on the radially-facing internal surface  56  of the pump housing  12 . The axially-extending flange  54  bearing against the radially-facing internal surface  56  of the pump housing  12  counters the translation force acting on the front end of the drive shaft  16  and, hence, prevents unwanted translation of the cam  18  as it rotates. The bearing arrangement  52 ,  54 ,  56  of  FIG. 5  therefore prevents unwanted tilting and translation of the cam  18  due to side loading of the drive shaft  16  at its front end. 
         [0059]    It will be appreciated that the pump assembly in  FIG. 5  is of greater width than that in  FIG. 1  due to the need for the drive shaft  16  to extend further rearward into the pump housing  12 ,  30 , and beyond the cam  18 , to define the rear bearing  52 , and hence the need for separation between the tappets  26 ,  28  to be greater. The width is also increased due to the provision of the flange  54  on the cam  18 . 
         [0060]    In the  FIG. 5  embodiment, the radially-facing internal surface of the pump housing provides a bearing surface  24  for the wide end  18   b  of the cam  18  and the axially-facing internal surface  22  of the pump housing  12  defines a bearing surface for the front face of the cam  18 , as in the  FIG. 1  embodiment. 
         [0061]    Another alternative bearing arrangement suitable for use with a belt, chain or gear drive is shown in  FIG. 6 . As in  FIG. 5 , the drive shaft  16  extends further into the second housing part  30  and beyond the cam  18  so as to define a radial bearing  52  at the rear end of the drive shaft  16  which counters the tilting force applied to the cam  18  due to the side loads at the front end of the drive shaft  16 . In this case, however, the flange on the thin end  18   a  of the cam  18  is removed, and instead the cam  18  is made of increased thickness in this region  18   a ′ (i.e. the length of the cam along the axis of the drive shaft is increased at its thinnest end). The thin end  18   a ′ of the cam  18  is therefore of greater thickness than in  FIG. 5  and bears against the radially-facing internal surface  56  of the first housing part  12  to counter the translation force acting on the cam  18  due to the side loading at the front end of the drive shaft  16 . This arrangement results in a pump assembly of longer axial length than the  FIG. 5  embodiment due to the increased thickness of the cam  18  at region  18   a ′, but one of reduced width due to the removal of the flange  54  in the  FIG. 5  embodiment. 
         [0062]    A further alternative embodiment is shown in  FIG. 7  which, again, is appropriate for use with a belt, chain or gear drive. Here, the drive shaft  16  does not extend rearward beyond the cam  18 , but instead the wide end  18   b  of the cam is provided with a radially-extending flange  58  which engages with a radially-extending, axially-facing surface  60  of the second housing part  30 . The bearing provided by the radially-extending surface  60  of the second housing part  30  counters both the tilting and translation forces exerted on the cam  18  due to the side loads at the input end of the drive shaft  16 . 
         [0063]    Although the pump housing in  FIG. 7  is still of two-part construction, the first housing part  12  which defines the axial bearing  22  is a much smaller component than in previous embodiments, with the second housing part  30  extending further towards the front end of the pump assembly  10  to define the bearing surface  60 . The radially-extending flange  58  bears against the bearing surface  60  which counters the side loads on the input end of the drive shaft. The bearing surface  60  therefore takes the place of the bearing surfaces  52 ,  56  in  FIGS. 1 ,  5  and  6 . 
         [0064]    Another difference between the embodiment in  FIG. 7  and those described previously is that in  FIG. 7  there is only a single pumping plunger  32  having a single associated tappet  26  cooperating with the bevelled cam  18 . In practice, the pump assembly may include any number of plungers/tappets, depending on delivery requirements. As shown in  FIG. 8 , for example, the pump assembly may include three plungers (not shown), each having an associated tappet  126 ,  226 ,  326  which cooperates with a common bevelled cam  18 . In a tri-tappet assembly the tappets  126 ,  226 ,  326 , and their associated plungers, are arranged at equi-angularly spaced locations around a central axis of the pump assembly which is aligned with the drive shaft axis. 
         [0065]    Another arrangement of the bearings (not shown) involves removing the flange  58  in the  FIG. 7  embodiment, and creating a bearing between a flat portion  18   e  of the front face of the cam (i.e. a portion that isn&#39;t bevelled) and the facing surface of the second housing part  30 . In this embodiment, the pump assembly has two axial bearings (at  22  and  18   e ), facing in opposite directions, to counter the side loads on the input end of the drive shaft  16 . 
         [0066]    In another example, as shown in  FIG. 9 , the tappets  26 ,  26 ,  126 ,  226 ,  326  of previous embodiments may be removed altogether and the bevelled surface  18   c  of the cam  18  may act directly on a correspondingly bevelled surface  32   a ,  34   a  of the reciprocating plungers  32 ,  34 . In other arrangements a single plunger, or more than two plungers, may be provided to interface directly with the bevelled cam  18 , again avoiding the need for an intermediate drive member. 
         [0067]    Other embodiments of the invention are also envisaged without departing from the scope of the invention as set out in the claims. For example, the rear closure plate  39  in  FIGS. 1 ,  5 ,  6 ,  7  and  9  may be replaced by a housing part (not shown) which includes regions extending into the second housing part  30  so as to define the plunger sealing lengths and the pump chambers  33 ,  35 . In this way the main pump housing  30  does not have to have the required material strength to accommodate the high pressures of fuel within the pump chambers  33 ,  35 , and only the closure plate  39  needs to be made from high-strength, expensive material. It is also envisaged that an intermediate part may be provided between the cam and the tappet e.g. to provide additional hardness.