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Title:
PUMP ASSEMBLIES WITH STATOR JOINT SEALS
Document Type and Number:
WIPO Patent Application WO/2018/138487
Kind Code:
A1
Abstract:
A pump assembly is disclosed comprising: two half shell stators defining one or more pumping chambers; end pieces mounted at either end of said two half shell stators; longitudinal seals for sealing between longitudinal contact faces of said two half shell stators on either side of said pumping chamber; and at least one further seal for sealing between one of said end pieces and said stator half shells. The longitudinal seals have end portions that abut against said at least one annular seal; and an aspect ratio of width to height of said longitudinal seals and said further seal is between 1:1 and 2:1.

Inventors:
HOLBROOK, Alan Ernest Kinnaird (Edwards Limited, Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
SCHOFIELD, Nigel Paul (Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
MIRZA, Ikram Murtaza (Edwards Limited, Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
Application Number:
GB2018/050192
Publication Date:
August 02, 2018
Filing Date:
January 23, 2018
Export Citation:
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Assignee:
EDWARDS LIMITED (Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
International Classes:
F01C19/00; F01C21/02; F04C25/02; F04C27/00
Domestic Patent References:
WO2009044197A22009-04-09
Foreign References:
JP2011185224A2011-09-22
US5516122A1996-05-14
GB2489248A2012-09-26
Attorney, Agent or Firm:
RAWLINS, Kate (Edwards Limited, Innovation Drive, Burgess Hill Sussex RH15 9TW, RH15 9TW, GB)
Download PDF:
Claims:
CLAIMS

1 . A vacuum pump assembly comprising: two half shell stators defining one or more vacuum pumping chambers; end pieces mounted at either end of said two half shell stators;

longitudinal seals for sealing between longitudinal contact faces of said two half shell stators on either side of said vacuum pumping chamber; and at least one further seal for sealing between one of said end pieces and said stator half shells; wherein

the longitudinal seals have end portions that abut against said at least one annular seal; and

an aspect ratio of width to height of said longitudinal seals and said further seal is between 1 :1 and 2:1 .

2. A vacuum pump assembly according to claim 1 , wherein said further seal is an annular seal and comprises an O-ring with an aspect ratio of 1 :1 and said longitudinal seal has a rectangular cross section.

3. A vacuum pump assembly according to claim 1 , wherein said further seal comprises an annular rectangular seal and said longitudinal seal and said annular seal each have an aspect ratio of width to height of between 1 :1 and 2:1 , preferably each between 1 .1 :1 and 1 .3:1 .

4. A vacuum pump assembly according to any preceding claim, wherein said longitudinal seal has an aspect ratio of width to height of between 1 .1 :1 and 1 .3:1 .

5. A vacuum pump assembly according to any preceding claim, wherein said further seal and said longitudinal seal have cross sectional areas of a similar order, differing from each other by less than 50%.

6. A vacuum pump assembly according to claim 5, wherein said further seal and said longitudinal seal have cross sectional areas, differing from each other by less than 30%.

7. A vacuum pump assembly according to any preceding claim, wherein said longitudinal seal comprises a flat end surface when not compressed for abutting with said further seal.

8. A vacuum pump assembly according to any preceding claim, wherein said longitudinal seal is manufactured to a height of more than 2mm with a tolerance of 0.07mm and said groove is manufactured to have a depth that is 20% smaller than said height of said gasket with a tolerance of 0.05mm, said compression variation due to said tolerances being below 7%.

9. A vacuum pump assembly according to any preceding claim, wherein said longitudinal seal is manufactured to have a height of more than 2.5mm with a tolerance of 0.07mm and said groove is manufactured to have a depth that is 20% smaller than said height of said gasket with a tolerance of 0.05mm, said compression variation due to said tolerances being below 5.5%.

10. A vacuum pump assembly according to any preceding claim, wherein said longitudinal seal is manufactured to have a height of more than 3mm with a tolerance of 0.07mm and said groove is manufactured to have a depth that is 20% smaller than said height of said gasket with a tolerance of 0.05mm, said compression variation due to said tolerances being below 4.7%.

1 1 . A vacuum pump assembly according to any one of claims 8 to 10, wherein said longitudinal seal comprises a width of between 2.5 and 3.5 mm, preferably 3mm.

Description:
PUMP ASSEMBLIES WITH STATOR JOINT SEALS

FIELD OF THE INVENTION

The field of the invention relates to pumps and in particular to seals for a stator of a pump.

BACKGROUND

The mounting of a rotor within a stator of some pump assemblies is made simpler by the use of half shell stators. This allows the rotor to be placed within one half shell and the other half shell to be fitted on top. Head plates or end pieces are then used at either end of the stator to support bearings and drive systems. A seal is required between the stator half shells and between the end pieces and stator. The point at which the split line between the half shell stators reaches the seal that seals between the head plates and stator at is called the T-joint. It is difficult to provide an effective or reliable seal at this T-joint particularly where the pump assembly is operating at high temperatures. GB2489248 discloses a vacuum pump with half shells stators. It has longitudinal stator joint seals for sealing between the stator half shells and annular stator seals for sealing between the stator and head pieces. The stator half shells have formations for holding the longitudinal seals in place against the annular seals, thereby improving the seal at this T-junction.

SUMMARY

A first aspect of the present invention provides a vacuum pump assembly comprising: two half shell stators defining one or more vacuum pumping chambers; end pieces mounted at either end of said two half shell stators;

longitudinal seals for sealing between longitudinal contact faces of said two half shell stators on either side of said pumping chamber; and at least one further seal for sealing between one of said end pieces and said stator half shells; wherein said longitudinal seals have end portions that abut against said at least one further seal; and an aspect ratio of width to height of each of said longitudinal seals and said further seal is between 1 :1 and 2:1 . The inventors of the present invention recognised that an interface between two different seals will be affected by the stress and compression on each seal, variations in the stress and compression on each seal causing corresponding changes in their elastomeric properties and in their distortion. Furthermore, they recognised that stress in a seal when the seal is under compression increases as the width to height ratio increases due to increased distortion which changes the sealing geometry leading to an increased risk of failure at seal interfaces. High stresses in a seal also accelerate the onset of compression set which shortens the life of the seal. The geometry of a seal to seal interface is particularly affected where the stress change experienced by the seals due to changes in operating conditions, such as an increase in temperature, is different in each seal. If the stress on one seal increases differently to the stress on the other then it will result in a stress mismatch which will affect the geometry of the seal interface and therefore the sealing properties of the interface will change. This problem has been addressed by providing a similar, relatively low width to height aspect ratio for each of the seals. This results in any mismatch in stress experienced by each seal being small, with correspondingly small changes in the interface geometry between the seals as the temperature of operation changes, providing effective operation across a wide temperature range.

In some embodiments, said further seal is an annular seal and comprises an O- ring with an aspect ratio of 1 :1 and said longitudinal seal has a rectangular cross section.

Owing to the geometry of the pump assembly, the seal between the end pieces and the stator half shells may have an annular form and in some cases may comprise an O-ring. O-rings are effective seals and are readily available. They generally have a circular cross-section giving them an aspect ratio of 1 :1 . The longitudinal seal, by contrast, will have a rectangular cross-section that may be a square and will have an aspect ratio of between 1 :1 and 2:1 . The longitudinal seal may be in the form of a gasket and gaskets are generally formed with a rectangular cross section having a large width to height aspect ratio. Providing the seal with an aspect ratio that is similar to the aspect ratio of the O-ring provides for effective sealing across large temperature ranges due to the similar stress experienced within the seals. Furthermore, the low width to height aspect ratio in the longitudinal seal leads to reduced stress when compared to a conventional gasket, which has a width-to-height aspect ratio of 3:1 or higher, leading to longer seal life.

In other embodiments, said further seal comprises an annular rectangular seal and said longitudinal seal and said annular seal each have an aspect ratio of width to height of between 1 :1 and 2:1 .

Although O-rings are readily available and effective seals and thus, often used as annular seals, in some embodiments a rectangular cross section seal is used as the further seal. Where a rectangular seal is used, then the cross section of the two seals may be very closely matched.

In some embodiments, said longitudinal seal has an aspect ratio of width to height of between 1 .1 :1 and 1 .3:1 .

An aspect ratio of the longitudinal seal that is slightly greater than 1 makes it easy to manipulate and place in the groove. However, making it close to 1 provides for a closer match where the further seal is an O-ring, providing for more uniform stress across the junction. Furthermore, although having an aspect ratio of slightly greater than 1 makes it easy to manipulate, locate and keep within a groove, making it close to 1 provides for a thicker seal which provides a more uniform stress profile particularly towards the end of the seal, where the stress profile is important. A lower stress also reduces chemical susceptibility of the seal which increases with stress.

In some embodiments said further seal also has an aspect ratio of width to height of between 1 .1 :1 and 1 .3:1 .

In some embodiments, said further seal and said longitudinal seal have cross sectional areas of a similar order, differing from each other by less than 50%, preferably by less than 30% and in some embodiments by less than 1 0%.

The maximum possible area of the interface between the seals is limited by the cross sectional area of the seal with the smallest cross-section. Thus, providing similar cross-sectional areas avoids the interface area being unduly limited by one particularly low cross-sectional area. Furthermore, where there is good stress matching in the two different seals this provides a balance in the pressures within each seal, allowing for the interface to maintain its shape, and where it is well matched to provide a flat sealing surface.

In some embodiments, said longitudinal seal comprises a flat end surface when not compressed for abutting with said further seal.

Although the end surface of the longitudinal seal may have a contoured shape for matching with, for example, an O-ring, in some embodiments it comprises a flat surface. A flat surface is easier to manufacture and easier to manipulate. In this regard, when assembling the pump the end of the longitudinal seal may extend further than the ends of the stator half shells and be pushed back to align with the end surface of the stator half-shells using a tool. Where the end surface is flat then this procedure and the manufacture of the tool is made simpler.

Furthermore, where the O-ring is compressed then this will provide a relatively flat surface on which the flat surface of the longitudinal seal can mate and seal effectively. In some embodiments, said longitudinal seal is manufactured to a height of more than 2mm with a tolerance of 0.07mm and said groove is manufactured to have a depth that is 20% smaller than said height of said gasket with a tolerance of 0.05mm, said compression variation due to said tolerances being below 7%.

In some embodiments, said longitudinal seal is manufactured to have a height of more than 2.5mm with a tolerance of 0.07mm and said groove is manufactured to have a depth that is 20% smaller than said height of said gasket with a tolerance of 0.05mm, said compression variation due to said tolerances being below 5.5%.

As is noted above, where the minimum height of the seal is set to 2 millimetres and the tolerance is 0.07 millimetres and the groove is manufactured to have a depth that is 20% smaller than this height with a tolerance of 0.05 millimetres, compression variations due to the tolerances are limited to being below 7 percent. However, where the longitudinal seal is manufactured to have a height of more than 2.5 millimetres with similar tolerances then the compression variations due to the tolerances fall below 5.5 percent. Furthermore, where the height is increased still further to 3 millimetres with the same tolerances then the compression variation falls to below 4.7 percent. In this way, one can see that increasing the height of the seal, where tolerances remain the same, decreases the compression variations due to these tolerances and this leads to increased predictability and a better ability to match the stress and strain felt by the seals at their interface. For the above seals a width of between 2.5 millimetres and 3.5 millimetres, preferably 3 millimetres provides the desired aspect ratio.

When selecting the relative width of the groove compared to that of the seal it should be selected to be wide enough so that the seal does not run out of space or overfill the groove when compressed or expanding due to increases in temperature. However, it should not be much wider than the requirements above dictate as otherwise the seal could stray or become misaligned. Vacuum pumps have particularly high pressure differences and require

particularly effective seals. Therefore, embodiments of this invention are particularly applicable to such vacuum pumps.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 illustrates sections through gaskets of the prior art that have a width of 3mm and height of 1 mm in a free and a compressed state;

Figure 2 illustrates sections through gaskets according to an embodiment in the free and in the compressed state;

Figure 3 shows the effect of tolerances on gaskets of the prior art and gaskets according to embodiments;

Figures 4a and 4b shows a cross sectional view of the interface between the gasket and the O-ring for longitudinal seals of different aspect ratios;

Figure 5 shows a pump assembly with a horizontal split line;

Figure 6 shows an isometric view of a pump assembly having seals according to an embodiment; and

Figure 7 shows the profile of a longitudinal seal according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided.

Embodiments propose longitudinal seals or gaskets that are suitable for use across a large temperature range and have a low width to height aspect ratio providing low seal distortion and low seal stress. This low aspect ratio is provided by providing a longitudinal seal with an increased height or thickness compared to conventional stator shell gaskets, and not only does this decrease the stress within the gasket but it requires a deeper groove and this leads to easier placement of the seal within the groove.

The reduced compression variation leads to more predictable surface pressure on the end surfaces which abut, or mate, with the end piece seal, allowing better stress matching and a seal interface geometry which varies less with variations in operational condition variations such as variations in temperature.

The reduction in internal stress due to the reduced width to height aspect ratio also reduces chemical susceptibility and increases the lifetime of the seal in that permanent set, i.e. irreversible deformation, occurs more quickly as stress increases.

By providing aspect ratios that are similar for the two seals, some degree of stress matching is achieved for these seals and variations in the interface geometry with changes in temperature are reduced.

Where the cross sectional area of the two seals is similar this also provides improved stress matching and reduced variations in interface geometry leading to improved seal effectiveness over a greater temperature range. Furthermore, a similar sized cross sectional area of the two seals also provides for an increased interface area as the maximum size of this is limited by the cross sectional area of the seal with the smaller cross sectional area. A seal with a low width to height aspect ratio leads to an increase in height of the longitudinal seal compared to many conventional seals. An increase in height in the seal leads to a corresponding increase in groove depth, and not only does this make the seal easier to place and retain in the groove but it reduces variations that arise due to manufacturing tolerances. For example, where the tolerances are 0.05 millimetres then for a seal of 1 millimetre height this relates to a 5 percent variation, whereas for a 3 millimetre high seal the variation is a third of this. Smaller percentage variations in the size of the seal lead to smaller variations in compression range and more predictability on the surface pressure exerted by the end surface of the seal.

Figure 1 diagrammatically shows a cross section through gaskets having an uncompressed height of 1 mm and width of 3mm according to the prior art. As can be seen when such longitudinal gaskets are compressed as occurs when mounted between two stator shells of a pump assembly, then they expand within the width of the groove and experience relatively high distortion and stress. In particular, as can be seen the edge portions of the seal have a relatively sharp cross section. The end portions of the seal that abuts with the annular seal will be distorted in a similar way and this sharp cross section may dig into the corresponding annular end piece seal particularly, where the end piece seal is not experiencing similar compression and stress, and this leads to distortions in the interface which in turn can lead to seal leakage.

Figure 2 shows cross sections through a gasket for a pump assembly according to an embodiment of the present invention. In this case, at rest the width of the gasket is 3mm while the height is 2.5mm. This shape provides a much lower width to height aspect ratio when compared to the gasket of Figure 1 , and this leads to considerably lower distortion and stress when the gasket is compressed. Furthermore, the stress is more uniform across the gasket leading to edge and end surfaces that are flatter and not so sharp. Figure 3 provides a table showing the effect of manufacturing tolerances on variations in the compression ratio as the thicknesses of the gasket increases. In particular, a prior art gasket of width 3mm and thickness 1 mm within a gasket groove of depth 0.8 mm is shown to have variations in the compression ratio, that is the percentage that the gasket is compressed by when the stator is assembled, of between 7.5% and 34.4% due to manufacturing tolerances of 0.07mm in the gasket thickness and 0.05mm in the groove depth.

The effect of tolerances on the compression ratio of gaskets according to different embodiments is then considered. These gaskets have the same 3mm width but an increased height or thickness. The manufacturing tolerances for the gaskets and grooves are considered to be the same as for the prior art, that is 0.07mm in the gasket thickness and 0.05mm in the groove depth. Firstly a gasket that is 2mm thick and within a groove of 1 .60 mm is considered. In this instance the tolerances produce variations in the compression ratio of between 6.9 and 6.5, that is below 7%.

Increasing the gasket thickness to 2.5mm reduces the variations in compression ratio to 5.5% or below. Further increases in thickness to 3mm which provides a gasket with a square cross section reduces the variation in compression ratio to 4.6% or below.

The table of Figure 3 clearly shows that variations in the compression ratio caused by manufacturing tolerances reduce considerably as the gasket thickness increases. This reduction in variations leads to more predictable stress within the gasket which makes it possible to more consistently manufacture pumps with better stress matching between the seals. In summary a low thickness of 1 mm results in a wide variation in possible compression range, and the high compression ratio at the upper end of this range accelerates the onset of compression set in a gasket which leads to an early seal failure. High gasket compression also leads to the gasket cutting into the O-ring at the interface.

In preferred embodiments the gaskets are 3mm wide and 2.5 mm high and have substantially the same cross sectional area as the annular seal. The low aspect ratio 1 .2:1 results in low seal distortion and low seal stress. The 2.5 mm thickness results in a narrow compression range and the lower compression ratio at the upper end of this range reduces the onset of compression set in the gasket which extends the seal life. Finite element analysis has been used to optimise the balance between compression on the O-ring and the gasket such that the extremes of compression and temperature do not cause a loss of interface pressure or damage to the O-ring. Furthermore, the 2.5mm thick gasket engages much more positively in a housing than a 1 mm thick gasket and is very unlikely to come out of the groove. This helps to avoid pinching of the gasket when it is compressed by the securing screws which has been observed on occasion in a 1 mm thick gasket.

Figures 4a and 4b schematically shows how the interface between the annular seal 40 and the end pieces in the longitudinal gasket 20 between the stator half shells changes in shape with an increased thickness gasket.

Figure 4a shows the interface between an increased thickness gasket 20 according to an embodiment and an O-ring 40 when under compression. As can be seen the gasket 20 and the O-ring exert fairly equal pressure on each other leading to a substantially flat surface for much of the area of the interface. The substantially equal pressure from each side is due to the substantially equal stress experienced by each seal. Furthermore, this matching is maintained across a large temperature range.

Figure 4b shows a thinner gasket and in this case the stress and strain

experienced by the gasket is considerably higher than that experienced by the O- ring and the end of the gasket therefore pushes into the O-ring causing distortion of the interface and the possibility of leakage between the seals is increased.

In summary, for a seal to work well the contact pressure between the gasket and the O-ring should be maintained across operating temperatures. The interface between the gasket and the O-ring will be shaped according to the relative compressions on the gasket and O-ring. With balanced compression on the gasket and O-ring, the interface is a substantially straight line (as shown in Figure 4a). Providing there are sufficient compressions on the gasket and O-ring, there will be no issues with sealing in the balanced case shown here.

However, if the O-ring compression and the gasket compression are not matched one will bulge into the other. If, for example the gasket compression is larger than the O-ring compression, due perhaps to its shape, the gasket will bulge into the O-ring as shown in Figure 4b.

Figures 5 to 7 show specific examples of the longitudinal seal and annular seal and the vacuum pump assemblies that they are used in. These seals benefit from the low width to height aspect ratio of embodiments of the invention. The seals have additional features for centring the longitudinal seal to improve the interface with the annular seal, and for providing axial flexibility of the longitudinal seal. Some embodiments also comprise features for biasing of the longitudinal seal against the inner surface of the groove closest to the pumping chamber to inhibit leakage of fluid along the groove.

Figure 5 schematically shows a multiple chamber rotary vacuum pump assembly, with two stator half shells and end pieces, which assembly may be

advantageously sealed using seals according to embodiments. The pump assembly is formed of two stator half shells 1 04 and 102 between which a rotor (not shown) is mounted. The two shells are fixed together to form the pump chambers. Each of the chambers 1 06, 1 08, 1 1 0, 1 1 2, 1 14 & 1 1 6 are separated by pump chamber walls 1 34. End pieces 1 22 and 1 24 are mounted on the half shells of the stators to complete the pump assembly.

Figure 6 shows an isometric view of the seals that are arranged between the stator half shells and between the end faces. In this embodiment, there are longitudinal seals 20 and 22 on either side of pumping chamber 30. There are also O-ring seals 40 and 42 between the end pieces and the stator half shells.

In this embodiment, the annular seal 40, 42 is a standard O-ring and is mounted in a rectangular groove of a constant cross section that can be machined using a plain cylindrical tool. An effective seal between the longitudinal seal 20, 22 and the O-rings 40, 42 can be difficult to maintain. In particular, longitudinal seals 20, 22 are mounted within a groove that is wider that the seal to provide freedom for some lateral movement and the ability for the gasket to expand under

compression and under an increased temperature. However, this provides a degree of freedom for the end surface of the longitudinal seal which means that its position is not exactly determined and it may not mate accurately with the O- ring seal. Figure 7 shows features that provide centring of the longitudinal seal in the groove to address the above issue. In particular, the gasket 20 comprises an end loop 25 which is within groove 50 and has curved longitudinal arms that form a bow shape. Owing to the symmetrical nature of the loop the bow shape acts against the outer surface of the groove on either side of the loop, each side generating a force of substantially the same magnitude but in opposite directions, thereby centring the seal and in particular centring the end portion 29 of the seal that extends from the loop and mates with the O-ring.

The bow shape of the loop preserves axial rigidity but allows lateral flexibility to ensure an interference fit is possible. The section/width of the gasket is

maintained in the bow shaped centering features to avoid excessive expansion at high temperatures. Space is provided inside the bow sides and straight members to provide freedom for spreading during compression and expansion at higher temperatures.

In addition to providing this centring feature the loop also provides some axial stability and axial flexibility. The axial stability is provided by the axial alignment faces 52 of the groove which form the outer surfaces of the arms extending from the longitudinal groove in either direction. The loop portion of the seal contact the axial alignment faces and this holds the longitudinal seal axially in position. The length of the groove between the axial alignment faces at each end of the stator is configured so that the longitudinal seal is mounted under a slight tension and is thus, held more securely in the groove. Axial flexibility is provided by the flexing of the lateral arms that allow some axial movement of the seal thereby inhibiting it becoming over tensioned with the corresponding thinning in the seal that this might trigger.

In the embodiment of Figure 7, the loop 25 has pips 23 on the outer surface which abut the axial alignment surface 52 of stator half shell. These pips 23 provide a known contact position for the loop with the axial alignment faces 52 which locate the gasket for axial tension. They are located towards an outer side of loop allowing for increased axial flexion to occur due to the bending of the portion of the loop between the pips 23. Further axial flexion is provided in this embodiment by the longitudinal deviation or bump 27 which also provides biasing of the gasket against the inner surface of the groove. This bump can contract and expand to allow for the axial flexion.

Each end of the gasket comprises an end portion 29 extending from loop 25. This end portion 29 is aligned with the end of the stator half shells and contacts the O-ring. It is centred by the bow shape of the loop in this embodiment. When the pump assembly is assembled the lower stator has two gaskets mounted on either side of the pump within grooves 50. They are mounted under tension against the axial alignment faces 52 at either end of the stator. The protruding end 29 is then held against the end of the stator half shell by a flat ended tool and the upper stator half shell is lowered on to the lower stator half shell and they are fixed together such that the gasket is held in place under compression. The end pieces 1 22, 1 24 which contain the O-ring seals 40, 42 can then be mounted against the stator end faces.

As can be seen the groove 50 is of a constant width and thus can be machined in one pass with one tool. This provides an advantage over grooves which contain pips to maintain the seal in place. Furthermore, the absence of these pips reduces the chances of there being pinch points for the gasket as it expands under compression and temperature changes.

The protrusion 29 is pushed back flush with the end faces of the stator during assembly. This is possible due to the flexibility of loop 25 and in particular to the lateral arms of the loop which can flex in and out and provide this axial flexibility. The groove 50 deviates towards the pumping chamber as it nears the axial alignment faces 52. This deviation is provided so that the biasing of the gasket towards the pumping chamber(s) does not bias the gasket against the inner side of the groove close to its ends. Centring of the gasket within the groove allows for lateral movement of the gasket 20 towards the loop 25 in response to lateral forces. This helps in the centring of the end protrusion 29 when opposing lateral forces are applied from either side.

In summary the above gasket design uses a simplified geometry. The end regions have a One box' form that provides the required functions. The flat ended gasket is formed with the width to height aspect ratio set out above to interface to a end seal, in this case standard round section O-ring, according to the present invention. Axial alignment of the gasket with the O-ring grooves is important for a good quality seal. This centring is achieved by ensuring that when the gaskets are placed in the housing they protrude beyond the O-ring grooves and then by pushing the gaskets back to the O-ring grooves with bespoke tooling.

Axial flexibility is required in the ends of the gasket to provide a protruding end that can be pushed back. This flexibility is provided by the transverse or lateral member that supports the end sealing surface, see Figure 7.

Axial flexibility in the centre of the gasket helps ensure it is in tension and does not buckle. The gasket is stretched and located against the alignment faces 52, in some embodiments on the alignment pips 23. The flexibility is provided by the transverse flexible members that supports the alignment pips. The bumps 27 in the central region of the gasket provide additional axial flexibility.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCE SIGNS

20, 22 longitudinal seal

23 pips

25 loop

27 deviation

29 end protrusion

30,106, 108, 1 10, 1 12, 1 14 & 1 16 pump chambers

40, 42 O-ring seal

50 groove

52 axial alignment face

102, 104 stator half shell

120 contact surface between stator half shells

124, 126 mating end surface of stator half shells

130, 132 mating surface of end pieces

134 pump chamber walls