The present invention relates to a pump assembly for a fluid delivery system for an internal combustion engine. BACKGROUND

It is known to provide fluid delivery systems for conveying fuel, such as diesel, to an internal combustion engine (ICE) of a vehicle. Such fluid delivery systems typically comprise a low pressure fuel pump, which draws fuel from a fuel tank and delivers it to a high pressure pump, which then delivers the fuel to the ICE via a common rail injection assembly.

A known low pressure fuel pump comprises a mechanical pump having a pair of meshed gears arranged in a pump housing. The pump housing defines an internal fluid pathway between an inlet and an outlet of the fuel pump. During use, the rotation of the gears creates a positive flow of fuel through the pump housing. The positive flow of fuel increases the pressure at the outlet of the fuel pump causing fuel to be pumped along the fuel line towards the engine. Leakage losses from the pump assembly cause a back flow of fluid within the pump housing, which thereby affects the performance of the pump. Leakage losses can be minimised by configuring the pump such that clearances between the moving components of the pump are very close. Of particular interest are the clearances between the housing and the teeth and the end faces of each of the gears, and also between the interlocking teeth of the respective gears. During certain operating conditions of the pump leakage losses are still observed, despite efforts to reduce the clearances between the pump components. Such leakage losses can represent a significant energy loss over the operational life of the low pressure fuel pump.

It is an aim of the present invention to address the disadvantages associated with the known pump assemblies. SUMMARY OF INVENTION

According to a first aspect of the invention there is provided a pump assembly for a fluid delivery system for an internal combustion engine, the pump assembly comprises; a pump housing which defines an internal fluid pathway between an inlet and an outlet of the pump housing; a first gear and a second gear arranged at least partially within the pump housing, wherein the first gear is arranged to rotatably engage the second gear to pump fluid along the fluid pathway towards the outlet; the first gear comprises a first gear portion and a second gear portion which are both rotatable about a rotation axis of the first gear, the first gear portion being rotatable with respect to the second portion, and a biasing element which is configured to bias the second gear portion in a direction which opposes a rotational direction of the first gear portion. The biasing element is advantageously configured to inhibit back flow of fluid between the interlocking gears which thereby increases the operating efficiency of the pump assembly. The biasing element also prevents harsh rattling between the first and second gears thereby reducing noise and vibrations associated with the operation of the pump assembly. In this way, the biasing element is configured to apply an opposing torque to the second gear portion in order to maintain contact between the teeth of the second gear portion of the first gear and the teeth of the second gear.

The biasing element may comprise a spring having a first end which is configured to engage the first gear portion and a second end which is configured to engage the second gear portion.

The spring may be configured such that a biasing force of the spring acts in a direction that is tangential to the rotational direction of the first gear.

The spring may be a helical spring which is arranged such that a central axis of the helical spring is substantially perpendicular to the rotation axis of the first gear. The first gear may comprise a spring housing arranged to house the biasing element. The spring housing may be provided in at least one of the first and second gear portions at an intersection therebetween.

The first gear may be rotatably coupled to a drive shaft of the pump assembly such that, in use, the first gear is configured to rotatably drive the rotation of the second gear.

The first gear portion may comprise a first retaining element which is configured to rotatably couple the first gear portion to the drive shaft, and the second gear portion may comprise a second retaining element which is configured to allow rotation of the second gear portion, relative to the drive shaft.

The second retaining element may comprise a key socket arranged on a shaft facing surface of the second gear portion. The key socket may be configured to receive a key which is arranged on an outer surface of the drive shaft. The key socket may be dimensioned to allow relative rotational movement of the key within the key socket of the second retaining element.

The key socket may extend from at least one end of the second gear portion. The key socket may be configured to receive the key when the second gear portion is mounted to the drive shaft.

The key socket may be configured to allow greater rotational movement between the first and second gear portions compared to the maximum rotational movement which is afforded by the rotatable engagement between the first and second gears.

The first gear portion may comprise a plurality of teeth which are configured to enmesh with a plurality of teeth spaces of the second gear. The maximum rotational movement between the first and second gear portions may be determined by the relative play between the teeth of the first gear and the teeth spacing of the second gear when the first and second gears are rotatably engaged. The first retaining element may comprise a key socket arranged on a shaft facing surface of the first gear portion. The key socket may be configured to receive the key which is arranged on an outer surface of the drive shaft. The socket may be dimensioned to inhibit relative rotational movement of the key within the socket of the first retaining element. The key may extend in a longitudinal direction along the outer surface of the drive shaft The key may be configured to be received within the key sockets of the first and second gear portions.

The first gear may be configured to be driven by the second gear. The first gear may comprise a sleeve which is arranged between the first gear and a driven shaft of the pump assembly to allow relative rotation therebetween. The first gear portion may comprise a retaining means which is configured to rotatably couple the first gear portion to the sleeve, and allow rotation of the second gear portion, relative to the sleeve.

The first gear may have a length which is measured in a direction that is substantially parallel with the rotation axis of the first gear. The length of the first gear portion of the first gear may be substantially greater than the length of the second gear portion.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of a pump assembly according to an embodiment of the present invention, showing a first and second gear arranged in a pump housing of the pump assembly; Figure 2 is a perspective view of the first gear of the pump assembly shown in Fig. 1;

Figures 3 and 4 show an end view and a side view, respectively, of the first gear shown in Fig. 2;

Figure 5 is a cross-sectional view of the first gear taken along the line A shown in Fig. 4; and Figure 6 shows an enlarged part of the cross-sectional view of the pump assembly shown in Fig. 1, illustrating the engagement between the teeth of the first and second gears.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and which illustrate specific embodiments of the invention. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them.

Fig. 1 illustrates the basic architecture of a pump assembly 10 for use in a fluid delivery system which is arranged to deliver fuel to an internal combustion engine of a vehicle. To achieve this, the pump assembly 10 is fluidly connected to a fluid conduit, or feed line, which is connected at one end to a fuel tank and at another end to a high pressure fuel pump of the fluid delivery system. During operation of the fluid delivery system, the pump assembly 10 is configured to draw fuel from the fuel tank and pump it to the high-pressure pump assembly, which then delivers the fuel to the engine via a common-rail injection assembly. Accordingly, the pump assembly 10 is configured to operate as a low pressure pump of the fluid delivery system, which is arranged to operate within a pressure range of between 1 and 15 bar.

The pump assembly 10 comprises a pair of interlocking gears which rotatably engage with each other to pump fluid through the pump assembly 10. According to an embodiment of the invention, the pump assembly 10 is advantageously configured to inhibit the back flow of fluid between the interlocking gears which thereby increases the operating efficiency of the pump assembly 10. The pump assembly 10 is also configured to reduce noise and vibration corresponding to its operation. The pump assembly 10 is an external mechanical gear pump which has a pump housing 12 arranged to house a pair of interlocking gears which rotatably engage with each other to pump fluid between a fluid inlet 16 - or inlet 16 - and a fluid outlet 18 - or outlet 18 - of the pump housing 12. The interlocking, or intermeshing, gears are at least partially arranged within the pump housing 12. In particular, the interlocking gears are arranged within a pumping chamber 14 which forms an interior of the pump housing 12. The pump housing 12 defines an internal fluid pathway between the inlet 16 and the outlet 18 of the pump assembly 10. During operation of the pump assembly 10, the inlet 16 receives fuel from the fuel tank via a first section of the feed line of the fluid delivery system. A first gear 20 of the pump assembly 10 is arranged to rotatably engage a second gear 22 in order to pump fuel through the pumping chamber 14 from the inlet 16 towards the outlet 18. The outlet 18 is arranged to enable the expulsion of fuel from the pump assembly 10 into a second section of the feed line, from which it is then delivered to the high pressure fuel pump of the fluid delivery system.

The rotation of the first and second gears 20, 22 causes a reduction in the fluid pressure at the input 16, which causes the fuel to be drawn into the pump assembly 10. The input 16 therefore defines a suction connection at the inlet side of the pumping chamber 14. The rotation of the gears 20, 22 also increases the pressure at the outlet 18, which causes the fuel to be expulsed from the pump assembly 10. The outlet 18 therefore defines an expulsion connection at the outlet side of the pumping chamber 14

A cross-sectional view of the pump assembly 10 is shown in Fig. 1, in which the section has been made perpendicular to the rotation axes of the first and second gears 20, 22 to reveal an end face of each of the first and second gears 20, 22. Each gear comprises a cylindrical body 34 - or gear hub 34 - upon which there is provided a plurality of teeth 32. The plurality of teeth 32 are equally spaced around the circumference of the gear hub 34 to form a plurality of depressions 38 - or tooth spaces 38, which are dimensioned to receive the teeth 32 of the opposing gear when the gears are enmeshed.

Each gear hub 34 comprises an axial bore 36 - or bore 36 - which is centred about a rotational axis R of the respective gear, as shown in Fig. 2. The bore 36 is configured to receive a shaft which enables the first and second gears 20, 22 to be rotatably mounted within the pump housing 12. The first and second gears 20, 22 are mounted on separate shafts which are arranged in parallel with each other, which thereby enables the first and second gears 20, 22 to mesh together. For each of the first and second gears 20, 22, the bore 36 extends through the respective gear hub 34, from one end face of the gear to the other. In this way, each of the gears 20, 22 is configured to allow its respective shaft to protrude out of the bore 36 at either side of the gear. The first gear 20 is rotatably coupled to the first shaft 24 by way of a retaining means which is configured to ensure that rotation of the first shaft 24 causes rotation of the first gear 20. The retaining means comprises a machined groove which extends along the length of the bore 36 to define a keyway 40 of the first gear 20. The first shaft 24 is configured with a key 42 which engages with the keyway 40 when the first gear 20 is installed on the first shaft 24.

The key 42 comprises a ridge which protrudes from a circumferential outer surface of the first shaft 24. The ridge is configured to extend in a longitudinal direction along the length of a portion of the first shaft 24. In particular, the ridge is arranged such that its length is at least as long as the length of the first gear 20. In this way, the ridge can engage with the bore 36 of the first gear 20 across its entire length, when the first gear 20 is installed on the first shaft 24. Both the key 42 and keyway 40 have a substantially curved profile when viewed through an axial cross-section of the first gear 20, as shown in Fig. 1. In an alternative arrangement, the key 42 comprises a separate pin which is mounted to the circumferential outer surface of the shaft. The pin is fixably attached to the shaft prior to the gear being installed thereon.

The first shaft 24 is supported by the side walls of the pump assembly 10. In particular, the first shaft 24 is received within a pair of openings which are provided in the side walls of the housing 12, on either side of the first gear 20. Each opening comprises a bearing to allow the first shaft 24 to rotate freely with respect to the housing 12.

The second gear 22 is rotatably mounted to a second shaft 26 such that the second gear 22 can freely rotate with respect to the second shaft 26. A bearing 44 is arranged between the second shaft 26 and the bore 36 of the second gear 22. The second shaft 26 is fixedly mounted to at least one of the side walls of the pump assembly 10 on either side of the gear to prevent rotation of the shaft relative to the pump housing 12.

The first shaft 24 extends through a side wall of the housing 12 and is coupled thereafter to an actuator (not shown) of the pump assembly 10. The actuator is configured to mechanically drive the rotation of the first shaft 24 which causes the first gear 20 to rotate in a counter-clockwise direction, as indicated by the arrow 28 in Fig. 1. The actuator is driven by a high-pressure pump of the internal combustion engine. Alternatively, the actuator may be driven by another component of the engine, or by a separate electric motor, for example.

The rotation of the first gear 20 causes the second gear 22 to rotate in a clockwise direction, as indicated by the arrow 30. Thus, the first gear 20 defines a drive gear - or driving gear - of the pump assembly 10, and the second gear 22 defines an idler gear - or driven gear - of the pump assembly 10. Accordingly, the first shaft 24 and the second shaft 26 define a drive shaft and a static shaft of the pump assembly 10, respectively.

The operation of the first and second gears 20, 22 will now be described with reference to Fig. 1. The rotation of the first and second gears 20, 22 causes the teeth 32 which are positioned at the inlet side of the pumping chamber 14 to be moved in opposite directions away from the inlet 16. As the gears become uncoupled from each other, their respective teeth openings 38 create an expanded volume which reduces the pressure at the inlet side of the pumping housing 14.

The rotation of the teeth 32 also draws fluid away from the inlet 16, collecting the fluid in the teeth openings 38 between the teeth 32. The collected fluid is trapped between the teeth 32 and an interior wall of the housing 12 as the gears continue to rotate. The teeth 32 convey the trapped fluid around the outer regions of the pumping chamber 14, as indicated by the arrows labelled 46 in Fig. 1. As the teeth 32 approach the outlet side of the pump assembly 10 from opposite directions, the conveyed fluid is directed towards the outlet 18.

As the rotation of the gears 20, 22 continues, the teeth 32 of the first gear 20 engage with the corresponding tooth spaces 38 of the second gear 22 in a central region of the pumping chamber 14. This enmeshing of the gears reduces the volume at the outlet side of the pumping chamber 14 as the teeth spaces 38 are occupied by the teeth 32 of the opposing gear. The increase in pressure at the inlet side of the pumping chamber 14 forces the fluid through the outlet 18 under pressure. The rotation of the first gear 20 then continues to drive the teeth 32 of the first and second gears 20, 22 towards the inlet side of the pump assembly 10 to complete a single rotation.

It is known to configure the gears of an external gear pump to prevent fluid from being transferred back through the central region of the pumping chamber 14, i.e. between the interlocked teeth 32 of the first and second gears 20, 22. In particular, the gears 20, 22 are dimensioned to minimise the tolerances between the intermeshing teeth in order to disrupt the flow path by which the pressurised fluid can flow back towards the inlet side of the pumping chamber 14.

Despite efforts to minimise the back flow of fluid through the pump assembly 10, a certain degree of ‘play’ between the teeth is always present in order to allow the gears to intermesh correctly. During operation of the pump assembly 10, the localised pressure fluctuations can cause the gears to vibrate relative to each other causing gear-to-gear leakage from the outlet side to the inlet side of the pumping chamber 14. The pressurised fluid is able to zig-zag through the intermeshed teeth as they rotate through the central region of the pumping chamber 13.

According to the present invention, at least one of the gears 20, 22 is configured to inhibit the back flow of fluid through the pump assembly 10, as will now be explained with reference to Fig. 2 to 6. According to an embodiment of the invention, the first gear 20 comprises a pair of unequally sized gear portions 50, 52, as shown in Fig. 2. The gear portions 50, 52 are both rotatable about the rotation axis of the first gear 20, as indicated by the dashed line R.

The first gear 20 comprises a first gear portion 50 and a second gear portion 52. The first and second gear portions 50, 52 have an equal number of teeth 32 arranged around the circumference of the gear hub 34 of each respective gear portion.

The first and second gear portions 50, 52 each have a length which is measured in a longitudinal direction that is substantially parallel with the rotation axis of the first gear 20, wherein the length of the first gear portion is substantially greater than the length of the second gear portion. In particular, the first gear portion 50 has a length L1 which is measured along a radial plane of the gear in a direction that is parallel with the rotation axis of the first gear 20. The lengths L1 , L2 of the first and second gear portions 50, 52 are configured according to a ratio which is determined by dividing the length of the first gear portion L1 by the length of the second gear portion L2 - (i.e. L1:L2). The lengths are configured such that the determined ratio L1:L2 is within a range of 5 and 10. According to an exemplary arrangement of the first gear 20, the first and second gear portion lengths are configured such that the determined ratio is 6.

The second gear portion 52 has a length L2 which is substantially less than the length L1 of the first gear portion 50. The teeth of the first and second portions 50, 52 extend across the respective lengths L1, L2 (i.e. from a first end face to a second end face of the gear portion), such that the teeth of the first and second gear portions 50, 52 engage with the teeth of the second gear 22 across its entire length.

The teeth 32 of the first and second gear portions 50, 52 are arranged such that they can be aligned in a neutral configuration so that the teeth define a substantially unitary tooth profile. Each gear portion also comprises an axial bore 36 through its respective gear hub 34. The axial bores 36 of the gear portions 50, 52, together define the bore 36 of the first gear 20, when the gear portions 50, 52 are arranged adjacent to each other on the first shaft 24. The first gear portion 50 is rotatably coupled to the first shaft 24 such that the rotation of the first shaft 24 drives the rotation of the first gear portion 50 in the same direction as the shaft. In this way, the first gear portion 50 defines a drive gear portion of the first gear 20 being coupled to the drive shaft and configured, therefore, to drive the rotation of the second gear 22, which defines a driven gear portion of the first gear 20.

The first gear portion 50 includes a first retaining element which is configured to rotatably couple the first gear portion 50 to the first shaft 24. The first retaining element comprises a key socket 60 - or socket 60 - which is arranged on a shaft facing surface of the first gear portion 50. The socket 60 is configured to receive the key 42 provided on the outer surface of the first shaft 24 in order to provide a substantially fixed coupling between the first gear 20 and the drive shaft. In this way, the first retaining element defines at least a portion of the keyway 40 of the first gear 20, and thus forms part of the retaining means of the first gear 20.

The socket 60 comprises a machined groove which extends along the length of the bore running through the centre of the first gear portion 50. The machined groove terminates at either end of the bore of the first gear portion 50. The first gear portion 50 is able to slide therefore onto the first shaft 24 when the machined groove is aligned with the key 42 of the first shaft 24. Accordingly, the length of the socket 60 is configured to be substantially the same as the length L1 of the second gear portion 51. The socket 60 further comprises a depth which is measured radially in an axial plane of the first gear 20 from a shaft facing surface 66 of the bore of the first gear portion 50. The depth may also be defined as the radial distance between the shaft facing surface 66 and a point of the socket 60 which is located furthest from the rotational axis of the first gear portion 50. The machined groove further comprises a width which is measured in an axial plane of the first gear portion 50 in a direction which is substantially perpendicular to the rotation axis of the first gear 20. In particular, the width of the socket 60 represents the size of the circumferential opening in the shaft facing surface 66 of the bore. The key 42 has a height which is measured from the point on the ridge which has the furthest radial distance from the circumferential outer surface - or outer surface - of the first shaft 24. The key 42 also comprises a width which is defined as the widest point of the ridge when measured in a circumferential direction along the outer surface of the first shaft 24. The machined groove of the first retaining element is configured to provide a close fit around the key 42 in order to inhibit any forward and backward rotational movement between the key 42 and the socket 60. To achieve this, the width and depth of the socket 60 are dimensioned so that they substantially match the corresponding width and height of the key 42. The socket 60 is also shaped to match the curved profile of the key 42 in order to ensure a close fit therebetween, and to also enable the sliding of the first gear portion 50 onto the first shaft 24. Accordingly, the first retaining element is configured to rotatably couple the drive shaft to the first gear portion 50 of the first gear 20. The second gear portion 52 is configured to be mounted to the first shaft 24 such that it can rotate relative to the shaft, i.e. it is not directly coupled to the first shaft 24 in the same manner as the first gear portion 50. The second gear portion 52 comprises a second retaining element which is configured to allow a small degree of relative rotation between the second gear portion 52 and the first shaft 24.

The second retaining element also comprises a socket 62, which is configured to receive the key 42 when the first gear 20 is mounted to the drive shaft. The socket 62 comprises a machined groove which extends from one end of the second gear portion 52 to the other, i.e. from an outwardly facing end face 64 of the second gear portion 52 to an inwardly facing end face of the second gear portion 52 (not shown). The inwardly facing end of the second gear portion 52 is defined as the end face which abuts the end face of the first gear portion 50 when the first gear 20 is installed on the first gear shaft 24. Put another way, the length of the socket 62 of the second retaining element is arranged to extend across the entire length L2 of the second gear portion 52, from one end face to the other.

The socket 62 of the second retaining element is configured to provide a loose fit around the key 42 in order to allow relative rotational movement between the key 42 and the socket 62. To achieve this, the width of the socket 62 is substantially greater than the width of the key 42. The height of the socket 62 of the second gear portion 52 is substantially the same as the height of the socket 60 of the first gear portion 50, as shown in Fig. 3. The socket 62 is configured to allow the second gear portion 52 to at least partially rotate relative to the drive shaft. Advantageously, the socket 62 performs this function without having to modify the key 42 along the length of the first gear shaft 24. Thus, the key 42 can be configured to have substantially uniform dimensions along the length of the portion of the shaft which engages with the first gear 20. In an alternative arrangement, the key 24 is configured with a non- uniform width, such that the sockets 60, 62 may be configured with substantially the same width, whilst still allowing some rotational movement of the second gear portion 52 relative to the first gear shaft 24. A biasing element 54 is configured to bias the first and second gear portions 50, 52 in opposing rotation directions. In particular, the biasing element 54 traverses the interface at the mating faces of the first and second gear portions 50, 52 and is configured to exert a biasing force on both the first and second gear portions 50, 52.

The biasing element 54 is configured to bias the second gear portion 52 in a direction which opposes a rotational direction of the first gear portion 50. The biasing element 54 comprises a spring which has a first end 70 that is configured to engage with the first gear portion 50 and a second end 72 which is configured to engage with the second gear portion 52, as shown in Fig 5.

The biasing force of the biasing element 54 acts in a direction that is tangential to the rotational direction of the first gear 20. In this way, the biasing element 54 is configured to rotationally offset the first and second gear portions 50, 52, as shown in Figs. 2 to 6. It is noted that the rotational offset between the first and second gear portions 50, 52 is exaggerated in Figs. 2 to 5. During normal operation of the pump assembly 10, the first and second gear portions 50, 52 are substantially aligned with each other and the biasing element 54 is configured to accommodate only small rotational movements between the respective gear portions 50, 52. During operation of the pump assembly 10, the teeth 32 of the first gear portion 50 are rotatably engaged with the corresponding tooth spaces 38 of the second gear 22. The teeth 32 and tooth spaces 38 are configured with some play between them in order to enable the enmeshing of the respective gears. The biasing element 54 is configured to bias the first and second gear portions 50, 52, in opposing directions, against the teeth which define the corresponding teeth space 38 of the second gear 22. In this way, the biasing element 54 and the sprung second gear portion 52 are advantageously arranged to prevent the teeth 32 of the first gear portion 50 from rattling in the corresponding tooth space 38 of the second gear 22.

Furthermore, the rotational offset between teeth of the first and second gear portions 50, 52 creates a more tortuous flow path through the intermeshed teeth 32 of the gears 20, 22 for the fluid to pass through. By establishing a more tortuous flow path through the central region of the pumping chamber 14, the gear-to-gear leakage from the outlet side to the inlet side of the pump housing 12 is reduced, which thereby increases the operating efficiency of the pump assembly 10. Owing to the configuration of the biasing element 54 and the first and second retaining elements, as described above, the first gear portion 50 is defined as an un-sprung portion of the first gear 20. This is because the first gear portion 50 is directly coupled to the drive shaft of the pump assembly 10. By contrast, the second gear portion 52 is defined as the sprung portion of the first gear 20, since it is coupled to the drive shaft by the biasing element 54 which enables it to exhibit a certain degree of rotational movement, relative to the first gear portion 50.

In the exemplary arrangement of the first gear 20 shown in Figs. 2 to 5, the first gear 20 is configured to rotate in a counter-clockwise direction, as described above with reference to Fig. 1. The width of the socket 62 of the second retaining element is configured to allow the second gear portion 52 to rotate ahead of the first gear portion 50 in a relative counter-clockwise direction, or behind the first gear portion 50 in a relative clockwise direction, such that the teeth 32 of the respective gear portions can become misaligned. Put another way, the socket 62 is configured to enable the teeth of the second gear portion 52 to become substantially misaligned with the teeth of the first gear portion 50. Thus, the second gear portion 52 can rotate away from a neutral position in which the teeth of the first and second gear portions 50, 52 are substantially aligned with each other. In this way, the socket 62 defines a limit of rotation for the second gear portion 52 relative to the first gear portion 50.

During the operation of the pump assembly 10, the second gear portion 52 will lag behind the first gear portion 50, as shown in Fig. 1. This lag corresponds to the relative rotational movement between the first and second gear portions 50, 52, which corresponds to less than +/- one degree of rotation.

The first gear 20 is configured such that the rotational displacement between the first and second gear portions 50, 52 is able to take up the play between teeth 32 of the first gear 20 and the corresponding tooth spaces 38 of the second gear 22. In an alternative arrangement, the first gear 20 is configured to allow relative rotation of the gear portions 50, 52 by +/- two degrees, in order to account for the manufacturing tolerances of the gears 20, 22.

In an alternative arrangement, the maximum rotational movement between the first and second gear portions 50, 52, in the lag direction, is configured to be greater than the amount of play between the teeth 32 of the first gear 20 and the tooth space 38 of the second gear 22.

Referring again to the biasing element 54, the biasing element takes the form of a spring, typically a helical spring, which is arranged such that its central axis is substantially perpendicular to the rotation axis R of the first gear 20. Accordingly, the central axis of the helical spring is arranged in a direction that is perpendicular to the rotation axis of the first and second gear portions 50, 52 of the first gear 20.

The first gear 20 comprises a spring housing 74 which is configured to house the biasing element 54. The spring housing 74 is arranged at the intersection between the first and second gear portions 50, 52 to allow the spring to engage with both gear portions. With reference to Fig. 5 in particular, a first spring housing is provided in the gear hub of the first gear portion 50 facing the second gear portion 52. A second spring housing is provided in the gear hub of the second gear portion facing the first gear portion 50. The first and second spring housings are configured to receive at least a portion of the spring, and as such they each define a portion of the spring housing 74. The first and second spring housings each comprise substantially cuboidal opening in their respective portions of the first gear 20. Each opening comprises two side walls, a lower wall and an upper wall. Accordingly, each opening has a substantially rectangular shape when viewed through an axial cross section or a longitudinal section of the first gear 20, wherein the longitudinal section corresponds to a plane taken along the rotation axis R of the first gear 20.

Each opening comprises a height, a depth and a width. The opening height is measured in a radial plane of the gear in a direction which is perpendicular to the rotation axis R of the first gear 20. The opening depth is measured in the radial plane of the gear in a direction which is parallel to the rotation axis R of the first gear 20. The opening width is measured in the radial plane of the gear in a direction which is perpendicular to that of both the height and the depth of the opening. The first end 70 of the spring is mounted to a side wall of the opening of the first spring housing which defines a first spring seat 76 of the first gear 20. A second spring seat 78 is provided by a side wall of the opening of the second spring housing which substantially opposes the first spring seat 76. Thus, as the first and second gear portions 50, 52 are rotated towards each other the spring is compressed between the opposing side walls of the spring seats 76, 78, as indicated by the arrow 90 shown in Fig. 5

The first and second spring seats 76, 78 are arranged so that the spring is retained in the spring housing 74 under compression during the operation of the pumping assembly 10. To achieve this, the first and second spring seats 76, 78 are configured to be arranged a distance apart from each other that is less than the un-compressed length of the spring at all times during the operation of the pump assembly 10. For example, when the second gear portion 52 is rotated forward of the first gear portion 50, the spring is held in compression by the spring seats 76, 78. The compressive forces acting upon the spring decrease as the second gear portion 52 rotates back towards a neutral alignment with the first gear portion 50, and decreases further as the second gear portion 52 rotates behind the first gear portion 50.

The spring and the spring housing 74 are dimensioned so that the spring remains at least partially compressed throughout the range of rotational movement between the first and second gear portions 50, 52. Furthermore, the relative rotational movement between the first and second gear portions 50, 52 is limited by the relative play between the teeth 32 of the first gear 20 and teeth spaces 38 of the second gear 22. Thus, the spring and spring housing 74 are configured to prevent the spring from fully decompressing during operation of the pump assembly 10, which thereby reduces the risk of the spring moving around within the spring housing 74.

Maintaining the spring in an at least partially compressed state ensures that the first and second gear portions 50, 52 are misaligned with each other when the first gear 20 is stationary, i.e. when the first gear 20 is not driven by the motor of the pump assembly 10. By installing the spring in position in a state of compression, the spring is conveniently held in position within the spring housing 74 during the assembly of the pump assembly 10. For example, even if the second gear portion 52 is allowed to rotate forward of the first gear portion 10 then the spacing between the teeth 32 of the first gear 20 and the teeth spaces 38 of the second gear 22 is such that it prevents the rotation of the second gear portion 52 from fully decompressing the spring. The openings of the spring housing 74 are dimensioned to prevent the spring from deforming in an uncontrolled manner during the relative rotation of the first and second gear portions 50, 52. The height of each opening is configured to provide a close fitting at the upper and lower sides of the helical spring when it is arranged within the spring housing 74, as shown in Fig. 3. All other faces of the spring housing 74 can arranged to optimise spring support with low risk of rubbing wear, since the relative rotational movement between the first and second gear portions 50, 52 is small.

The opening of the second spring housing extends through the gear hub of the second gear portion 52. In this way, the depth of the opening is equal to the length L2 of the second spring portion 52, as shown in Fig. 5. This allows access to the spring when it is installed between the first and second gear portions 50, 52. It thereby enables the spring to be mounted within the first gear 20 once the first and second gear portions 50, 52 are arranged together in the pump assembly 10.

The second gear portion 52 is configured so that it forms only a thin slice of the first gear 20, as shown in Fig. 2. The reduced thickness of the second gear portion 52, relative to the first gear portion 50, conveniently reduces the force exerted upon the biasing element 54 due to the relative rotation of the gear portions, which means that a smaller spring can be used to bias the second gear portion 52 in an opposing direction to the rotational direction of the drive gear.

The operation of the pump assembly 10 will now be described with particular reference to Fig. 6. Upon rotation of the drive shaft, the first gear 20 is urged to rotate in a counter clockwise rotation, as indicated by the arrows 28. The first gear portion 50, which is shown in the background of Fig. 6, is directly coupled to the drive shaft such that it immediately rotates in the counter-clockwise direction.

The second gear portion 52 is not directly coupled to the drive shaft. Instead, the second gear portion 52 is coupled to the first gear portion 50 by the biasing element 54 such that there is a delay in the initial rotation of the second gear portion 52. Accordingly, the teeth 32 of the second gear portion 52 are defined as sprung teeth of the first gear 20, whereas the teeth 32 of the first gear portion 50 are defined as un-sprung teeth of the first gear 20.

The biasing element 54 is further arranged to bias the second gear portion 52 in the opposite rotational direction to the movement of the first gear portion 50. The inertia of the second gear portion 52 applies a tensional force on the biasing element 54 which is absorbed by the spring in order to maintain the relative separation between the teeth of the first and second gear portions 50, 52.

As the first gear 20 rotates, an un-sprung tooth 80 of the first gear portion 50 is urged into the tooth space 82 which is formed between a first and a second tooth 84, 86 of the second gear 22. As the first gear 20 continues to rotate, a lower surface of the un-sprung tooth 80 engages an upper surface of the first tooth 84 of the second gear 22, causing the second gear 22 to rotate in a clockwise direction. At the same time, a sprung tooth 88 of the second gear portion 52 of the first gear 20 is urged by the biasing element against a lower surface of the second tooth 86 of the second gear 22. The engagement between the sprung tooth 88 of the first gear 20 and the second tooth 86 of the second gear 22 thereby eliminates the ‘partial backlash’ which is formed between the un-sprung tooth 80 of the first gear 20 and the tooth space 84 of the second gear 22. The backlash defines the amount by which the width of the tooth space 84 exceeds the width of the un-sprung tooth 80. In this way, the sprung tooth 88 of the first gear 20 closes off the backlash by bridging the connection between the space which opens up between the un-sprung tooth 80 of the first gear 20 and the second tooth 86 of the second gear 22.

The biasing element 54 is configured to maintain the contact between the sprung tooth 88 of the first gear 20 and the second tooth 86 of the second gear 22 as they rotate through the central region of the pumping housing 14. The biasing element 54 thereby maintains the seal which prevents fluid from zig-zagging back through the interlocked teeth of the gears during the operation of the pump assembly 10. Put another way, the first gear portion 50 drives the second gear 22 in a conventional manner forming a seal between the un-sprung tooth 80 and the tooth space 84 of the second gear 20. In case of extraneous vibrational or pressure pulsation inputs, the second gear portion 52 and the biasing element 54 are configured to prevent the second gear 22 from “overrunning” the first gear portion 50 and breaking the seal between the un-sprung tooth 80 and the tooth space 84.

In an alternative exemplary embodiment of the pump assembly 10, the second gear portion 52 and the biasing element 54 are configured to enable the pump assembly 10 to continue to pump against an extraneous back pressure. Such a back pressure may be caused by a downstream blockage, for example. According to this embodiment, the operation of the first and second gear portions 50, 52 are reversed compared to the previously described embodiment such that the second gear portion 52 is arranged to lead the first gear portion 50 during rotation of the first gear 20. In particular, the biasing element 54 is configured to bias the second gear portion 52 to lead the first gear portion 50 so that second gear portion 52 drives the second gear 22. During operation of the pump assembly 10, torque is transmitted from the first shaft 24 to the first gear portion 50 via the first retaining element. The torque is then transmitted from the first gear portion 50, through the biasing element 54, to the second gear portion 52 which then drives the second gear 22.

The biasing element 54 is configured so that, at a certain pressure, the back pressure acting upon the second gear 22 will overcome the biasing element 54 allowing the first gear portion 50 to over-run the second gear 22; thereby permitting regulated leakage between the teeth of the first gear portion 50 and the teeth of the second gear 22. Thus, the biasing element 54 may be arranged to provide a pressure limitation or regulation function of the pump assembly 10. In particular, the spring of the biasing element 54 is configured to allow movement of the sprung portion of the first gear relative to the un-sprung portion, once the back pressure exceeds a threshold value. In this way, the relative movement of the portions of the first gear enables regulated fluid flow through the intermeshed teeth of the gears 20, 22. Thus, the spring 54 enables back pressure to be at least partially released from the pump housing 12 without the need for a separate pressure release valve to be provided in the pump assembly 10.

In an alternative arrangement of the pump assembly 10, the second gear shaft 26 is rotatably coupled to the second gear 22 such that they cannot rotate independently of each other. The second shaft 26 is supported by a set of bearings which are mounted to the side walls of the pump housing 12. The bearings enable the second shaft 26, and therefore the second gear 22, to rotate freely relative to the pump housing 12.

In an alternative exemplary arrangement of the first gear 20, the key socket 62 of the second retaining element is configured to only permit the second gear portion 52 to rotate behind the first gear portion 50. Put another way, the key socket 62 is configured to prevent rotation of the second gear portion 52 up to and/or forward of a neutral arrangement with the first gear portion 50. According to an alternative embodiment of the pump assembly 10, the first gear is rotatably mounted to the first shaft of the pump assembly 10 such that, in use, the first gear 20 is driven by the second gear 22. The second gear 22 is rotatably coupled to the second shaft 26 such that it can be rotatably driven by the second shaft 26. The second shaft 26 is coupled at its distal end to an actuator which is thereby configured to drive the rotation of the second gear 22. According to this exemplary arrangement, the second gear 22 defines the drive gear and the first gear 20 defines the idler gear of the pump assembly 10. The first gear 20 comprises a sleeve which is arranged between the first gear 20 and the first shaft 24 to allow relative rotation therebetween. To achieve this, the sleeve comprises at least one bearing which is arranged between the first shaft and a shaft facing surface of the sleeve. A retaining means is provided to rotatably couple the first gear portion 50 to the sleeve whilst at the same time allowing relative rotational movement between the sleeve and the second gear portion 52. To achieve this, the first gear portion 50 is provided with a boss which provides a support for the bore of the second gear portion 52. The boss is further configured to provide a limit to the relative movement between the first and second gear portions 50, 52. In this way, the boss is arranged to perform a similar function to the key 42 and key sockets 60, 62 of the previously described embodiment.

In all other respects, the pump assembly 10 according to the above described embodiment is configured to operate in a similar manner to the previously described embodiment. For example, the first gear 20 comprises a biasing element 54 which is configured to offset the first and second gear portions 50, 52 to produce a tortuous flow path to prevent fluid from passing through the intermeshed teeth of the first and second gears 20, 22, which thereby prevents back flow through the pumping chamber 12 during operation of the pump assembly 10.

Reference List Pump assembly 10 Pump housing 12

Pumping chamber 14 Pump assembly inlet 16 Pump assembly inlet 18 First gear 20 Second gear 22 First shaft 24 Second shaft 26 Rotation arrows 28, 30 T eeth 32 Gear hub 34 Bore 36

Tooth Space 38 Keyway 40 Key 42 Bearing 44 Fluid flow arrows 46 First gear portion 50 Second gear portion 52 Biasing element 54

Key socket of the first retaining element 60 Key socket of the second retaining element 62

Outwardly facing end of the second gear portion 64 Shaft facing surface of the gear 66 Deepest part of the key socket 68 First end of spring 70 Second end of the spring 72 Spring housing 74 First spring seat 76 Second spring seat 78 Un-sprung tooth of the first gear portion 80 Tooth spacing 82

First tooth of the second gear 84 Second tooth of the second gear 86 Sprung tooth of the second gear portion 88 Spring compression arrows 90