WO2012120383A1

WO2012120383A1 - A sealing arrangement, hydraulic system comprising the same and a back up ring for the sealing arrangement

Info

Publication number: WO2012120383A1
Application number: PCT/IB2012/000888
Authority: WO
Inventors: David Green; Javedkhan Nijam TAMBOLI; Parag S. JADHAV
Original assignee: Eaton Limited
Priority date: 2011-03-04
Filing date: 2012-05-04
Publication date: 2012-09-13
Also published as: GB2549366A; GB2549366B; GB201701820D0; GB2488588A; GB201103664D0; GB2488588B; WO2012120383A8

Abstract

There is described a sealing arrangement for inserting in a groove in a hydraulic system to form a seal between two surfaces in the system. The sealing arrangement comprises an outer ring and an inner ring arrangeable co-planarly and concentrically in the groove. The inner ring comprises a resiliently deformable material and the outer ring is more rigid than the inner ring and is for supporting the inner ring in the groove. The outer ring comprises an inner surface facing substantially radially inwardly of the outer ring and the inner ring comprises an outer surface facing substantially radially outwardly of the inner ring. The inner surface of the outer ring describes a surface recess that accepts at least a portion of the outer surface of the inner ring.

Description

A SEALING ARRANGEMENT, HYDRAULIC SYSTEM COMPRISING THE SAME AND A BACK UP RING FOR THE SEALING ARRANGEMENT

Field of the Invention The present invention relates to a sealing arrangement for sealing an interface between two components in a hydraulic system, for example, a sealing arrangement that forms a face seal in a two way cartridge valve.

Background of the Invention

Two way cartridge valves are well known and are typically used as direction control valves, flow control valves or pressure control valves. Known two way cartridge valves comprise an insert that is inserted into a standard bore hole in a manifold, a flat cover for covering the insert in the manifold and a face seal provided between the insert and the cover to prevent leakage of hydraulic fluid during the operation of the valve. Ideally, the height of the insert and the depth of the bore hole are an exact match but in practice manufacturing tolerances in the dimensions of these components often result in the height of the insert being less than the depth of the bore hole. If the height of the insert is less than the depth of the bore hole, the insert is able to oscillate up and down in the bore in response to pressure changes in the valve. This movement causes dynamic compression of the face seal which may induce premature failure of the face seal. Currently, this problem is dealt with by inserting an additional component, for example a shim, into the bore hole to make up the difference between the height of the insert and the depth of the bore so that the insert is maintained stationary in the bore hole and so the face seal is subjected only to compression caused by internal hydraulic fluid pressure.

Embodiments of the present invention aim to provide a sealing arrangement that is durable and robust. The sealing arrangement is particularly, but not exclusively, suitable for forming a seal between a cover and an insert of a two way cartridge valve and negates the need to use a shim in instances when the height of the insert is less than the depth of the bore hole. Embodiments of the invention also aim to provide an improved back up ring for use in such a sealing arrangement.

Summary of the Invention According to the invention there is provided a sealing arrangement for inserting in a groove in a hydraulic system to form a seal between two surfaces in the system, the sealing arrangement comprising: an outer ring and an inner ring arrangeable co-planarly and concentrically in the groove, wherein the inner ring comprises a resiliently deformable material and the outer ring is more rigid than the inner ring and is for supporting the inner ring in the groove, wherein the outer ring comprises an inner surface facing substantially radially inwardly of the outer ring and the inner ring comprises an outer surface facing substantially radially outwardly of the inner ring, wherein the inner surface of the outer ring describes a surface recess that accepts at least a portion of the outer surface of the inner ring.

The recess is preferably concave.

According to the invention there is also provided a sealing arrangement for inserting in a groove in a hydraulic system to form a seal between two surfaces in the system, the sealing arrangement comprising: an outer ring and an inner ring arrangeable co-planarly and concentrically in the groove, wherein the inner ring comprises a resiliently deformable material and the outer ring is more rigid than the inner ring and is for supporting the inner ring in the groove, wherein the outer ring comprises an inner surface facing substantially radially inwardly of the outer ring and the inner ring comprises an outer surface facing substantially radially outwardly of the inner ring, wherein the outer ring compresses the inner ring radially.

These arrangements are particularly effective at maintaining a seal between two surfaces in a hydraulic system where the sealing arrangement is exposed to both a varying dynamic compression resulting from the sealing arrangement being squeezed between the two surfaces because of relative movement of the two surfaces and forces resulting from hydraulic fluid pressure in the system.

In either arrangement, the outer ring may comprise an outer circumferential surface facing substantially radially outwards of the outer ring and which slants, so that in cross section the outer ring appears roughly trapezoidal.

According to the invention there is also provided a back up ring for supporting an elastomeric ring seal in a groove of a hydraulic system, the back up ring comprising an inner circumferential surface that extends around an inner circumference of the back up ring and faces generally radially inwardly of the ring, the inner circumferential surface describing a surface recess for receiving an outer circumferential surface portion of the elastomeric ring seal.

According to the invention there is also provided a hydraulic system comprising a back up ring for supporting an elastomeric seal in the system, wherein the back up ring is in a pre-compressed state in the system.

Description of the Drawings

The invention will be described in more detail below, with reference to the accompanying drawings, in which:

Figure 1 is a schematic axial sectional view of a known valve;

Figure 2 is a schematic axial sectional view of a valve system embodying the present invention;

Figure 3 is a schematic plan view of the valve system of Figure 2 with a valve cover removed;

Figure 4 is a highly schematic enlarged view of part of the valve system of Figure 2; and

Figure 5 is a schematic axial section view showing part of the valve system embodying the invention;

Figure 6 is a schematic view of part of a cross section of a back up ring used in the valve system of Figure 2. Detailed Description

Figure 1 illustrates a typical known two port cartridge valve 1. The valve 1 comprises a generally tubular valve sleeve 4, extending along an axial axis A of the valve 1 and which is inserted into a stepped bore 2 of a manifold 3. The valve 1 further comprises a poppet 5 mounted within the valve sleeve 4 for sliding reciprocal movement along the axis A, and a closure spring 6 that biases the poppet 5 into a sealing engagement with a valve seat 7 formed in valve sleeve 4.

The stepped bore 2 comprises a first bore step 8 and a second bore step 9 of larger diameter than the first bore step 8. The second bore step 9 has its mouth in an upper surface 10 of the manifold 3. A first flow channel 11 for hydraulic fluid in the manifold 3 opens coaxially into the first bore step 8 and a second flow channel 12 for the hydraulic fluid opens laterally into the second bore step 9. A valve cover 13, which covers the valve sleeve 4 in the manifold 3 is connected to the upper surface 10 by suitable fixing means 14, for example bolts. The valve sleeve 4 comprises an upper section 15 having a cross sectional diameter that corresponds with that of the second bore step 9 and a lower section 16 having a cross sectional diameter that corresponds with that of the first bore step 8. The valve sleeve 4 further comprises a middle section 17, between the upper 15 and lower 16 sections, the middle section 17 and the manifold 3 defining, in the second bore step 9, an annular chamber 18 that surrounds the valve sleeve 4. The valve sleeve 4 defines an internal axial guide bore 19 within which the poppet 5 is mounted for reciprocal movement between the valve seat 7 and the cover 13. Immediately below the valve seat 7, the valve sleeve 4 defines an axial fluid port 20 that communicates with the first flow channel 1 1 in the manifold 3. Immediately above the valve seat 7, the valve sleeve 4 defines in its middle section 17 a plurality of lateral fluid ports 21, spaced apart around the circumference of the middle section 17, which open into the annular chamber 18.

The poppet 5 comprises at its lower end a closure member 22 for sealingly engaging the valve seat 7 to close the axial fluid port 20. The poppet 5 defines an axial spring chamber 23 containing the closure spring 6. The closure spring 6 has a free end supported on the underside of the valve cover 13 and is biased so as to urge the poppet 5 towards the valve seat 7. An axial pilot oil chamber 24 is defined within the guide bore 19, which is connected to a pilot oil source (not shown) via a pilot oil bore 25 in the cover 13.

The lower section 16 of the valve sleeve 4 comprises, part way down its length, a groove 26 formed around its circumference, containing a first sealing ring arrangement 27 forming a seal between the lower section 16 and the manifold 3. Likewise, the upper section 15 of the valve sleeve 4 comprises, part way down its length, a groove 28 formed around its circumference, containing a second sealing ring arrangement 29 forming a seal between the upper section 15 and the manifold 3. Furthermore, at its upper circular rim 30, the upper section 15 of the valve sleeve 4 comprises another groove 31 formed around its circumference, containing a third sealing ring arrangement 32, forming a seal between the upper section 15 and the cover 13. These seals may typically be comprised of a suitable elastomer, for example, PolyUtherene. In use, and as is well known, a flow of liquid (generally oil), between the manifold's 3 first flow channel 11 and second flow channel 12 is controlled by the pressure in the pilot chamber 24. For example, by reducing the fluid pressure in the pilot chamber 24, the poppet 5 may be lifted under the pressure of fluid in the first flow channel 11 , lifting the closure element 22 away from the valve seat 7 to enable fluid to flow from the first flow channel 11 to the second flow channel 12 via the axial fluid port 20, the lateral fluid ports 21 and the annular chamber 18. By increasing the fluid pressure in the pilot chamber 24, the poppet 5 may be lowered against the pressure of fluid in the first flow channel 1 1 , bringing the closure element 22 into sealing engagement with the valve seat 7 and preventing fluid flow from the first flow channel 1 1 to the second flow channel 12.

To ensure that such two port cartridge valves are interchangeable, both they and the stepped manifold bores must conform to defined standards, for example, the ISO 7368 standard in the UK and elsewhere which defines allowed characteristics including lengths and diameters. Ideally, the height of the valve sleeve 4 matches exactly the depth of the stepped bore 2. However, in practice, the manufacturing tolerances associated with these dimensions often results in a valve sleeve that is shorter than the stepped bore into which it is to be inserted. When such a valve sleeve is inserted into such a stepped bore and the valve is assembled as per normal, a small gap exists between the valve sleeve and the cover. In use, this gap enables the valve sleeve to move up and down in response to the prevailing fluid pressure conditions in the valve/manifold system. This motion varies the cavity volume between the upper section of the valve sleeve 15 and the cover 13 which is available to the sealing arrangement 32. This results in the sealing arrangement 32 operating under so called dynamic loading conditions in which the squeeze applied to it is different from that applied when the valve sleeve 4 is static. These conditions often result in the premature failure of the sealing arrangement 32, for example, by seal extrusion. Currently, as is illustrated in Figure 1, this issue is addressed by inserting an annular shim 33 between the bottom of the first bore step 8 and the valve sleeve 4 to eliminate any gap between the valve sleeve 4 and the cover 13 and hence prevent any movement of the valve sleeve 4. This approach is inconvenient because the cost of the shim 33 adds to the cost of the valve 1. Additionally, the steps of measuring the gap in order to select a shim of matching thickness and fitting the shim in the stepped bore are time consuming. Sometimes, oversized or undersized shims are mistakenly selected for use, which again may result in the premature failure of the sealing arrangement 32.

Referring now to Figures 2 to 6, there is described a two port cartridge valve 40, embodying the present invention, which is inserted in a stepped bore of a manifold 3\ Features illustrated in Figures 2 to 6 which are identical to features illustrated in Figure 1 have the same reference numerals as in Figure 1 followed by a \ In the interests of brevity, these features will not be described in detail again. An important difference between the prior art valve design shown in Figure 1 and the valve design described with respect to Figures 2 to 6 is in the sealing arrangement 41 in the groove 3 which forms a face seal between the upper section 15^x of the valve sleeve 4^" and the cover 13^'. As is perhaps best illustrated in Figure 3, which shows a plan view of the valve 40 with the cover 13' removed, and Figure 4 which shows an enlarged sectional view of part of the valve 40, the sealing arrangement 41 comprises an elastomeric seal 42, in this example an O-ring, which in use forms a seal between the valve insert 4^' and the cover 13' (not shown in Figure 3), and a back up ring 43, formed from a more rigid material than that of the elastomeric seal 42, which acts to retain the elastomeric seal 42 in position in the groove 3Tto prevent its extrusion. Both the elastomeric seal 42 and the back up ring 43 are ring shaped and lie co-axially and concentrically in the groove 3 , with in plan view, the inner circumference of the back up ring 43 arranged around the outer circumference of the elastomeric seal 42 so that the back up ring 43 circumscribes the elastomeric seal 42. The elastomeric seal 42 may therefore be thought of as an inner ring and the back up ring 43 as an outer ring of the sealing arrangement 41.

Referring now to Figures 4 to 6, the back up ring 43 comprises an inner circumferential surface 44 which faces generally radially inwardly of the back up ring 43 towards the centre of the ring 43 and defines an inner circumference of the ring 43, and an outer circumferential surface 45 which faces generally radially outwardly of the back up ring 43 and defines an outer circumference of the ring 43. The back up ring 43 further comprises an upper surface 46 and a lower surface 47 which is parallel with the upper surface 46. It can be seen that in cross section, the back up ring 43 appears roughly trapezoidal. When the back up ring 43 is in situ in the groove 3 , the lower surface 47 rests against the bottom surface 31 a of the groove 31 ' and the upper surface 46 faces the underside of the cover 13\

Importantly, at least around part of its length, the inner circumferential surface 44 describes a recess 44a for receiving at least a portion of an outer facing side 48 of the elastomeric seal 42. Preferably, the recess 44a is substantially concave and extends substantially across the whole width of the inner circumferential surface 44 and substantially around the whole length of the inner circumferential surface 44 (i.e. around the inner circumference of the back up ring 43). Preferably, the shape of the recess 44a formed in the inner circumferential surface 44 complements the shape of the outer facing side 48 of the elastomeric seal 42. That is to say, the two shapes are the inverse of each other, so that as in case of the described preferred example where the shape of the recess 44a is concave, the outer facing side 48 of the elastomeric seal 42 is convex. The inner circumferential surface 44 is thus not shaped like the internal surface of a regular cylinder, which would not curve inwardly as does surface 44 as shown in Figures 4 and 5 but instead would appear as a straight vertical line. Referring again to Figures 4 to 6, it can be seen that the outer circumferential surface 45 of the back up ring 43 slants inwardly of the ring 43 from the upper surface 46 to the lower surface 47. Accordingly, the outer circumferential surface 45 has the same shape as does the side surface of a right frusto - cone having its base in the plane of the upper surface 46. As best seen in Figure 4, the outer circumferential surface 45 faces around it length an upper surface section 49 of the second bore step 8. In accordance with the ISO 7368 standard, the upper surface section 49 slants inwardly from the rim of the mouth of the second bore step 8 to a depth approximately a little more than the depth of the groove 3 Γ. The slant angle of the upper surface section 49 is very slightly less than that of the outer circumferential surface 45 so that a small wedge shaped gap 50 exists between the two. Advantageously, this provides a space to accommodate slight tilting movements of the back up ring 43 when in use, which helps prevent the back up ring fracturing. A somewhat wider gap 51 exists between an inner facing side 50 of the elastomeric seal 42 and a side wall 31b of the groove 3 .

The outer circumferential surface 45 thus has a different shape than the corresponding outer circumferential surfaces of conventional back up rings which surfaces are shaped like the external surface of a regular cylinder. Advantageously, the sealing arrangement 41 forms a particularly robust and durable seal between the valve sleeve 4' and the cover 12\ This is at least in part because the arrangement of having the outer facing side 48 of the elastomeric seal 42 received in the recess defined in the inner surface 44 of the back up ring 43 helps prevent relative movement between the back up ring 43 and the elastomeric seal 42. This enhances the ability of the back up ring 43 to retain the elastomeric seal 42 in place and prevent its extrusion.

Additionally, it is believed that because the outer surface 45 is slanted rather than vertical this assists in preventing the back up ring 43 from fracturing.

Preferably, the back up ring 43 is arranged to compress the elastomeric seal 42 radially (e.g. generally transverse to the longitudinal axis), which helps maintain the two in close contact. This may be achieved by the back up ring having a maximum internal diameter (indicated as di in Figure 5 ) which is slightly smaller than the outer diameter of the elastomeric seal 42 (as measured when the seal 42 is not under any stress) so that the elastomeric seal 42 must be compressed radially in order to fit around the inside of the back up ring. In a preferred arrangement, the back up ring 43 compresses the outer diameter of the elastomeric seal by about 1% to 5%.

Preferably, the elastomeric seal 42 has an inner diameter (indicated as d₂ in Figure 5) which is greater than the inner diameter of the groove 31 ' (indicated as d₃ in Figure 5) so that there is a space between the elastomeric seal 42 and the surface 31b" of the groove which can accommodate changes in the shape of the elastomeric seal 42 induced by pressure changes when the valve is in use. This arrangement contrasts with known arrangements in which the inner diameter of the elastomeric seal 42 is slightly smaller than the inner diameter of the groove 31 so that the elastomeric seal 42 has to be stretched to fit into the groove and remains in tight contact with the inner side of the groove.

Preferably, the ratio of the volume of the elastomeric seal 42 and the volume of the space available to the elastomeric seal 42 in the groove (i.e. the volume between the inner facing side 50 of the seal 42 and the inner wall 31b of the groove 3 ) is less than a pre-determined value. This is advantageous because we have found that if the elastomeric seal 42 occupies too much of the volume available to it in the groove 3 , in use, the forces exerted on the sealing arrangement 41 cause crimping of the back up ring 43 and/or the O-ring 42. We find in particular that such crimping is minimised if the aforementioned ratio is less than about 0.85.

Preferably, the valve 40 is assembled such that the sealing arrangement 41 is located in the groove 3 in a pre-compressed state (i.e. the sealing arrangement is permanently under compression even when the assembled valve 40 is inactive and there is no internal fluid pressure in the valve). This may be achieved by using a back up ring 43 of a length or height along its longitudinal axial direction (indicated as d₄ in Figure 5) and an elastomeric seal 42 comprising a longitudinal axial cross sectional diameter (indicated as d₅ in Figure 5) which are both greater than the perpendicular distance (indicated as d₆ in Figure 5) between the underside of the cover 13\ when fixed in place, and the bottom surface 31 a of the groove 31 ^' when the insert is fully inserted into the bore. In this way, fixing the cover 13' in place compresses the back up ring 43 and the elastomeric seal 42 in the axial direction (i.e. along the longitudinal axis) down to the distance between the underside of the coverl3\ and the bottom surface 31a of the groove 31 " when the insert is fully inserted into the bore. . Advantageously, we have found that the novel feature of a pre-compressed back up ring 43 assists to some extent in reducing the stresses that act to elongate the back up ring 43 when the valve 43 is active and is internally pressurised by hydraulic fluid.

In a preferred arrangement, the elastomeric seal 42 is pre-compressed along the longitudinal axis by 30% or less and the back up ring 43 by 20% or less.

Preferably, the minimum thickness of the back up ring 43 along the radial direction (indicated as d₇ in Figure 5) is about equal to half the difference between the outer diameter of the back up ring 43 (indicated as d₈ in Figure 5) and the inner diameter of the back up ring 43. This minimum thickness requirement helps prevent the back up ring 43 fracturing.

As is best seen in Figure 6, the inner upper edge 44b, the inner lower edge 44c, the upper outer edge 45a and the lower outer edge 45b are slightly curved so as not to form sharp edges. In the illustrated example, the radii of curvature of the outer upper 45a and outer lower 45b edges are (to within manufacturing tolerances) the same and are greater than the radii of curvature of the inner upper 44b and inner lower 44c edges which are (to within manufacturing tolerances) also the same. The radii of curvature of the outer edges may be for example twice that of the inner edges. It is believed that the curved edges, as opposed to sharp edges enhance the lifetime of the sealing arrangement.

We have found that the sealing arrangement 41 is so effective that it negates the need to use shims in valves in which manufacturing tolerances have resulted in a valve sleeve that is shorter than the stepped bore. Accordingly, as can be seen by comparing Figures 1 and 2, the valve 40, unlike the valve 1, does not make use of a shim 33 and as is illustrated in Figure 4, a small gap 53 exists between the valve sleeve 3' and the cover 13^' allowing movement of the valve 1 back and forth along the axial direction. Despite this axial movement of the valve 40 and the dynamic squeezing it induces, the effective lifetime of the sealing arrangement 41 compares favourably with that of known conventional sealing arrangements as used in valves where axial movement is inhibited by the presence of a shim. The back up ring 43 may be made out of any suitably robust material, for example a thermoplastic material such as Teflon, optionally containing a filler for example glass fibres. The elastomeric seal 42 may be made from any suitable resiliently deformable material, for example Nitrile with 90 shore hardness. Many modifications or variations may be made to the described embodiment, without departing from the scope of the claims. For example, the sealing arrangement may be used as a seal, in particular a face seal, in many types of hydraulic systems not just the cartridge valve of the described embodiment.

Claims

1. A sealing arrangement for inserting in a groove in a hydraulic system to form a seal between two surfaces in the system, the sealing arrangement comprising:

an outer ring and an inner ring arrangeable co-planarly and concentrically in the groove, wherein the inner ring comprises a resiliently deformable material and the outer ring is more rigid than the inner ring and is for supporting the inner ring in the groove, wherein the outer ring comprises an inner surface facing substantially radially inwardly of the outer ring and the inner ring comprises an outer surface facing substantially radially outwardly of the inner ring, wherein the inner surface of the outer ring describes a surface recess that accepts at least a portion of the outer surface of the inner ring.

2. A sealing arrangement according to claim 1, wherein, the outer ring compresses the inner ring radially.

3. A sealing arrangement according to claim 1 or claim 2, wherein, the inner surface of the outer ring defines an inner diameter of the outer ring, and the outer surface of the inner ring defines a outer diameter of the inner ring and wherein the inner diameter of the outer ring is smaller than the outer diameter of the inner ring.

4. A sealing arrangement according to any preceding claim, wherein the surface recess is substantially convex.

5. A sealing arrangement for inserting in a groove in a hydraulic system to form a seal between two surfaces in the system, the sealing arrangement comprising:

an outer ring and an inner ring arrangeable co-planarly and concentrically in the groove, wherein the inner ring comprises a resiliently deformable material and the outer ring is more rigid than the inner ring and is for supporting the inner ring in the groove, wherein the outer ring comprises an inner surface facing substantially radially inwardly of the outer ring and the inner ring comprises an outer surface facing substantially radially outwardly of the inner ring, wherein the outer ring compresses the inner ring radially.

6. A sealing arrangement according to claim 5 wherein, the inner surface of the outer ring defines an inner diameter of the outer ring, and the outer surface of the inner ring defines an outer diameter of the inner ring and wherein the inner diameter of the outer ring is smaller than the outer diameter of the inner ring

7. A sealing arrangement according to claim 5 or claim 6 wherein the inner surface of the outer ring describes a surface recess that accepts at least a portion of the outer surface of the inner ring.

8. A sealing arrangement according to claim 7, wherein the surface recess is substantially convex.

9. A sealing arrangement according to any preceding claim, wherein the outer ring further comprises an outer circumferential surface facing substantially radially outwards of the outer ring and which slants.

10. A sealing arrangement according to any preceding claim wherein the inner ring is an O - ring.

1 1. A sealing arrangement according to any preceding claim wherein the outer ring comprises a thermo-plastic material and/or the first ring comprises an elastomer.

12. A hydraulic system comprising a groove and the sealing arrangement of any preceding claim arranged in the groove.

13. An hydraulic system according to claim 12 wherein the groove is annular and comprises a groove inner diameter, and the inner ring comprises an inner surface facing substantially radially inwardly of the inner ring and defining an inner diameter of the inner ring, and wherein, the inner diameter of the inner ring is greater than the groove inner diameter.

14. A hydraulic system according to claim 13 wherein the outer ring is pre- compressed axially.

15. A hydraulic system according to claim 14, whereon the system comprises a cover for covering the sealing arrangement in the groove and wherein the outer ring is pre-compressed axially when the cover is fixed in place.

16. A hydraulic system according to any of claims 12 to 15 wherein the outer ring further comprises an outer circumferential surface facing substantially radially outwardly of the outer ring and which slants and wherein the outer circumferential surface is closely spaced apart from a slanting surface of the hydraulic system and wherein the angle of slant of the outer circumferential surface is greater than the angle of slant of the slanting surface.

17. A hydraulic system according to any of claims 12 to 16 wherein, in use relative motion of the two surfaces causes the sealing arrangement to be dynamically compressed between them.

18. The hydraulic system according to any of claims 12 to 17 wherein the elastomeric seal has a volume that is equal to or less than that of 85% of the volume available to the elastomeric seal in the groove.

19. A hydraulic system according to any of claims 12 to 18, wherein the system is a valve.

20. The hydraulic system of any of claims 12 to 19 comprising:

a manifold comprising a stepped bore;

a cartridge valve inserted into the stepped bore, the cartridge valve comprising: a valve sleeve containing a valve poppet;

a valve cover for covering the valve sleeve; and wherein

the sealing arrangement is inserted in an annular groove defined by the valve sleeve and forms a seal between the valve sleeve and the valve cover.

21. A back up ring for supporting an elastomeric ring seal in a groove of a hydraulic system, the back up ring comprising an inner circumferential surface that extends around an inner circumference of the back up ring and faces generally radially inwardly of the ring, the inner circumferential surface describing a surface recess for receiving an outer circumferential surface portion of the elastomeric ring seal.

22. A back up ring according to claim 21 wherein the surface recess is generally concave in an axial cross section.

23. A back up ring according to claim 21 or 22 wherein the back up ring comprises an outer circumferential surface that extends around an outer circumference of the back up ring and faces generally radially outwardly of the ring, wherein the outer

circumferential surface is slanted.

24. A hydraulic system comprising a back up ring for supporting an elastomeric seal in the system, wherein the back up ring is in a pre-compressed state in the system.

25. A system according to claim 24 wherein the back up ring and the elastomeric seal are co-axial.

26. A system according to claim 25 wherein the back up ring and the elastomeric seal are co-planar.

27. A system according to any of claims 24 wherein the back up ring is an outer ring and the elastomeric seal is an inner ring.

28. A system according to any of claims 24 to 27 wherein the system is a valve.

29. A system according to claim 28 wherein the valve is a cartridge valve.