WO1989012191A1

WO1989012191A1 - Sealing rings

Info

Publication number: WO1989012191A1
Application number: PCT/GB1989/000639
Authority: WO
Inventors: John Kettle; Alex Phelps
Original assignee: Dowty Seals Limited
Priority date: 1988-06-10
Filing date: 1989-06-08
Publication date: 1989-12-14
Also published as: GB8913255D0; ES2020811A6; YU116489A; EP0379536A1; PT90781A; GB2219636A; CN1040667A; AU3747389A; GB2219636B; GR890100363A; JPH02504665A; PL279852A1; IL90455A0; GB8813784D0; ZA894312B

Abstract

A sealing ring (4) comprises a sealing member (5) having a contact surface (17) for engagement with a surface (3) to be sealed and extending over at least 50 per cent of the axial extent (W) of the sealing ring (4), and an elastomeric energising member (6) for radially pressing the contact surface (17) into engagement with the surface to be sealed, characterised in that the sealing member (5) comprises a polymeric material possessing good wear resistance and a higher modulus of elasticity than the energising member (6) at the operating temperature of the sealing ring, and in that the sealing member (5) is bonded to the energising member (6) preferably over the whole of the area of contact (7) therebetween. Preferably, the sealing member (5) and the sealing ring (4) have rectangular cross-section. Further, the sealing member (5) has a radial thickness between 10 and 70 per cent of that of the energising member (6).

Description

SEALING RINGS Technical Field

This invention relates to sealing rings, especially the kind suitable for use between reciprocating components including pistons and cylinders and also piston rods, cylinder heads and the like.

There are many problems associated with effecting a good seal between components which reciprocate relative to each other such as pistons and cylinders; in particular it is generally difficult to achieve an effective seal with good wear characteristics over a prolonged period, especially in dry, i.e. "no leak", conditions. In many heavy duty applications, it has been found essential to provide complex seals having- a plurality of components. The use of such complex seal assemblies is not without problems in respect of both performance and cost even in the case of unidirectional seals; and in the case of bi-directional seals, such problems are exacerbated.

A known type of sealing ring is that of the lip seal in which a sealing member has a contact surface of minimum axial extent that is pressed radially into engagement with a moving surface to be sealed. The narrow contact surface of this type of seal makes it an effective high contact pressure, low friction seal, but it is vulnerable to the intrusion of solid contaminants that can damage the sealing lip and cause leakage.

Another known type of sealing ring is that of the broad band, face contact type which may be rectangular in cross-section and has a broad annular face pressed radially into engagement with the moving surface, to be sealed. This type of sealing ring is less vulnerable to damage by solid contaminants because of its broader contact surface, but the contact pressure is lower and leakage can be produced by hydrodynamic effects between the contact surface and moving surface.

British Patent No. 1438619 discloses a lip type seal comprising a polytetrafluoroethylene (PTFE) sealing member in which the lip is formed, and an elastomeric energising member that presses radially on the sealing member to urge the lip into contact with the moving surface to be sealed.

A recess is formed in the axial face of the seal at the high pressure end, and a wedge-like gap is provided between the sealing lip and the low pressure end of the seal so as to improve sealing and reduce leakage. The sealing member and energising member are shown as separate components or in one instance as an integral one-piece structure.

Disclosure of the Invention

An object of the present invention is to provide ah improved sealing ring of the broad band, face contact type.

According to the invention there is provided a sealing ring comprising a sealing member having a contact surface for engagement with a surface to be sealed and extending over at least 50 per cent of the axial extent of the sealing ring, and an elastomeric energising member for radially pressing the contact surface into engagement with the surface to be sealed, characterised in that the sealing member comprises a polymeric material possessing good wear resistance and a higher modulus of elasticity than the energising member at the operating temperature of the sealing ring, and in that the sealing member is bonded to the energising member over the area of contact therebetween.

The presence of a bond between the sealing member and the energising member is essential and this is preferably effected across the whole of the area of contact between the members. The effect of the bond is to vary the stress distribution within the body of the sealing ring so that a peak contact pressure is produced between the sealing member and surface to be sealed at the low pressure end of the sealing ring.

The manner in which the bond is achieved between the sealing member and energising member will vary depending on the respective materials of the members. It is envisaged that the bond may be formed either physically and/or chemically by methods known in the art.

The cross-section of the sealing ring may vary depending on the use to which it is to be put in practice. In its simplest and preferred form, the overall cross section is rectangular or square and advantageously the sealing and energising members are rectangular or square also, thereby providing a flat sealing surface to contact the body being sealed and with the bond being parallel to that surface. Clearly, the sealing member is generally arranged in a coaxial configuration with respect to the energising member with the sealing member on the inner diameter in the case of an internal seal and on the outer diameter in the case of an external seal.

In such cases, the respective radial thicknesses of sealing member and energising member may vary depending on a number of parameters. However, it is preferred that the sealing member has a radial thickness of at least 10%, but no more than 70% of that of the energising member; most preferably this figure does not exceed 50%, for example from 20 to 35%.

With regard to the choice of material for the sealing member and energising member for any 6 particular seal, it is essential that the former has both good wear resistance and a higher modulus of elasticity than the latter at the operating temperature; it is preferred that the difference in modulus between the members is substantial.

In particular, it has been found advantageous to define the distinction between the elasticity of the sealing member and that of the energising member by use of a modulus based on the decrease in height of a standard cylindrical specimen of the sealing member/energising member material under an applied load between the ends of the cylinder, as described hereinafter.

In addition, as stated above, good wear resistance of the sealing member is also important. In addition to the determination of a preferred modulus of elasticity, it is also preferred to determine good wear resistance of the sealing member material in accordance with a standard test and equate this with the load of the modulus test above.

The preferred material for the energising member is natural or synthetic rubber in general; nitrile rubber such as nitrile butadiene rubber having been found to be especially useful. Preferred materials for the sealing member include polyurethanes.

Description of the Drawings

The invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic axial section of a sealing ring according to one embodiment of the invention,

Figure 2 is a similar section to Figure 1 showing a sealing ring according to a second embodiment of the invention,

Figure 3 is a similar section to Figure 1 showing a sealing according to a third embodiment of the invention,

Figure 4 is a graph of load (L) against deformation produced from a modulus test to select materials for a sealing ring according to the invention, and

Figure 5 is a graph showing the load distribution across the axial width of the sealing ring of Figure 1.

Best Mode of Carrying Out the Invention With reference to Figure 1, there is shown an internal sealing ring of the invention positioned in an annular groove 1 of a housing 2 and positioned about a rod 3 of circular cross section which in use reciprocates within the housing.

The sealing ring 4 is of overall rectangular cross section and comprises two components, a sealing member 5 and an energising member 6, both also of rectangular cross section. The sealing member is made of polyether urethane and the energising member is made of nitrile butadiene rubber. The sealing and energising members have a good bond formed across the whole of the area of contact 7 therebetween, this bond being formed in situ during the manufacturing process.

The size of the groove cross section is such that the sealing ring as a whole fills substantially the whole of the groove in a radial direction but leaves gaps 8, 9 on either side of the sealing ring in the longitudinal direction. Such a gap on at least one side of the sealing ring is generally necessary to cause the seals to operate at their optimum manner. Prior to insertion of the rod 3, the inner surface of the sealing member is slightly proud of the groove so that some deformation and thus loading of the sealing ring occurs on insertion. After insertion, the inner contact surface 17 of the sealing member 5 is loaded into sealing engagement with the rod 3.

In selecting the materials for the sealing member 5 and energising member 6, it has been found advantageous to define the distinction be'tween the elasticity of the materials of these members by the use of a modulus based on the decrease in height of a standard cylindrical specimen of the sealing member/energising member composition under an applied load (L) between the ends of the cylinder.

In tests to determine this modulus, a specimen in the form of a cylinder is used having a diameter of 16mm (with no positive variance allowed but a 0.2mm negative variance possible) and a height of 11mm (+ 0.2mm) and in increasing load applied to the top end of the cylinder to produce a strain at a rate of lmm/minute.

Results of such tests, and in particular plots of the applied load (L) versus strain (percentage deformation) (H) , have shown that it is preferred for the results for the sealing member material to fall mainly above a line drawn between a point of 25 Kilograms Force (kgf) load, 5% deformation and a point of 120 kgf load, 30% deformation, as shown by X and Y in Figure 4; and that the results for the energising member should fall mainly beneath this line. Most preferably, the results for the sealing member and energising member are wholly above and below the line respectively, at least over the operating temperature range of the sealing ring.

Typical modulus test results for the polyether urethane of the sealing member 5 are shown plotted as line A in Figure 4, and typical modulus test results for the nitrile butadiene rubber of the energising member 6, are shown plotted as line B in Figure 4, these results being obtained at room temperature. In order to illustrate the variation of the modulus with temperature, the test results for the same sample of polyether urethane as measured at 50 degrees C and 100 degrees C are also shown in Figure 4 as lines A' and A' ' . From this it is clear that the modulus approaches the standard line X - Y with increasing temperature but that it still lies above this line.

Another characteristic of the elastomeric materials that is important in the context of the present invention, is that of stress relaxation or compression set. This is a well known phenomenon with elastomeric seals and involves a loss of seal contact pressure over a period of time following initial installation. Stress relaxation is dependent on material, deformation, time, temperature, and fluid environment, and has to be allowed for in selecting the material of the sealing member so as to ensure that in the aforesaid modulus tests, the results fall above said load - deformation line X - Y. As far as the energising member 6 is concerned, stress relaxation is less of a problem because of the lower modulus of elasticity of this member.

By way of example, the stress relaxation for the polyether urethane used in the modulus tests of Figure 4, is shown in Figure 4 as measured for 10 per cent deformation and 25 per cent deformation at both 50 degrees C and 100 degrees C. In each case, the specimen was compressed by 10 per cent or 25 per cent of its height and held compressed for three days, after which time the compression was removed and the height measured. The decrease in height as a percentage of the initial compressed height is a measure of compression set, which, assuming compression set is proportional to load, is taken as a measure of stress relaxation. At 50 degrees C, a deformation of 10 per cent produces a compression set of 37.9 per cent, shown as a load reduction to point Cl, and a deformation of 25 per cent produces a compression set of 31.3 per cent, shown as a load reduction to point C2. Both of these points still lie above the standard line X - Y and thus indicate that the polyether urethane material of the sample is suitable for operation at this temperature of 50 degrees C.

Similar compression set/stress relaxation test results are shown in Figure 4 for the same polyether urethane material at 100 degrees C. A deformation of 10 per cent produces a compression set of 93.5 per cent, shown as a load reduction to point Dl, and a deformation of 25 per cent produces a compression set of 90.7 per cent, shown as a load reduction to point D2. Both of these points Dl and D2 lie below the standard line X - Y and thus indicate that this particular polyether urethane material is unsuitable at temperatures of 100 degrees C and above. Operating temperatures between 50 degrees C and 100 degrees C may be acceptable and can be simply determined using the test material described above.

In addition, good wear resistance of the sealing member 5 is also important. In addition to the determination of a preferred modulus of elasticity as described above, it is also preferred to determine good wear resistance of the sealing member material in accordance with a standard test and equate this with the load (L) of the modulus test above.

The particular wear resistance test is that of DIN Standard 53516 which determines the weight loss of a given sample when moved in contact with a predetermined surface; hence a percentage volume loss (Wf) for this sample can be calculated to provide a load to wear ratio (L/Wf).

In such preferred embodiments, the load to wear ratio (L/Wf) and the percentage strain should be in accordance with one or more and most preferably all the following equations for the sealing member:

15 < L/Wf < 30 at 5% strain

25 < L/Wf < 55 at 10% strain

40 < L/Wf < 80 at 20% strain

55 < L/Wf <100 at 30% strain.

For the polyether urethane material used for the sealing member 5 in the illustrated embodiment, the load to wear ratio (L/Wf) and the percentage strain are as follows:

21.3 at 5% strain

35.3 at 10% strain

47.3 at 20% strain

62.7 at 30% strain.

A typical sealing ring such as illustrated in Figure 1 has a cross-section 8.05mm by 8.10mm, has the radial thickness of the sealing member 5 as 34 per cent of that of the energising member 6, has an internal bore adapted to fit a rod 3 of 60mm diameter, and has the sealing member 5 composed of the polyether urethane and the energising member 6 composed of the nitrile butadiene rubber for which test results are quoted above. This sealing ring was installed as a gland seal in a hydraulic piston employing the mineral hydraulic oil DT26 and was subject to an endurance test involving repeated operation of the piston rod in a cycle with pressures from 0 to 206 bar and an operating temperature of 59 degrees C. During a test period involving 289,000 cycles, equivalent to 266km, over a period of 440 hours, no leaks developed. The load distribution across the axial width (W) of the sealing ring 4 in Figure 1 is shown in Figure 5, and illustrates how a peak load (L) is produced between the sealing member 5 and the surface of the piston rod 3 adjacent to the low pressure end of the sealing ring. This characteristic is found to be typical of sealing rings according to the invention having the sealing member and energising member bonded together over their area of contact 7, and contributes directly to their improved performance.

Figure 2 shows an external sealing ring of the invention positioned in an annular groove 10 on the outside surface of a piston 11 and in contact with the internal surface of a cylinder 12.

The sealing member 13 and energising member 14, are made of the same materials as the sealing ring shown in Figure 1 and its mode of operation is largely identical.

Figure 3 shows a sealing ring similar to that of

Figure 1 (the same reference numerals being used for equivalent components) except that a recess 15 is formed in the axial end face of the sealing ring 4 at the low pressure end and accommodates an anti-extrusion ring 16.

Although the embodiments illustrated in Figures 1, 2 and 3 all involve the sealing ring contacting a moving surface to be sealed, it will be appreciated that sealing rings according to the invention can also be used as static seals between two relatively fixed members.

It will be appreciated that many different types of polyurethane can be used for the sealing member in sealing rings according to the invention, including those prepared by the reaction of a polyol and an isocyanate together with a chain extender, and those prepared from prepolymers or by a "one shot" manufacture. Examples of typical polyols are polyethers, polyesters, polycaprolactones, polybutadienes and polycarbonates of various molecular weights. Examples of isocyanates are well known in the art and include toluene diisocyanate (TDI), diphenyl ethane diisocyanate (MDI), naphthalene diisocyanate (NDI), xylylene diisocyanate (XDI), dicyclohexylmethane diisocyanate (H MDI), hexa ethylene diisocyanate (HDI), cyclohexyl diisocyanate (CHDI) , para-phenylene diisocyanate

(PPDI) and di ethyldiphenyl diisocyanate (TODI).

Examples of chain extenders are 1, 4 butane diol and trimethylol propane. Mixtures of any of the above substances may be employed when appropriate in the manner known in the art. Also the polyol isocyanate, and the chain extender, can be of varying functionality.

Alternative compositions for the sealing member, especially those suitable for higher operating temperatures are polyureas and acrylic monomer reinforced hydrocarbon elastomers.

Claims

1. A sealing ring comprising .a sealing member having a contact surface for engagement with a surface to be sealed and extending over at least 50 per cent of the axial extent of the sealing ring, and an elastomeric energising member for radially pressing the contact surface into engagement with the surface to be sealed, characterised in that the sealing member (5) comprises a polymeric material possessing good wear resistance and a higher modulus of elasticity than the energising member (6) at the operating temperature of the sealing ring (4) , and in that the sealing member (5) is bonded to the energising member (6) over the area of contact therebetween (7).

2. A sealing ring as claimed in claim 1 in which the sealing member (5) is bonded to the energising member (6) over the whole of the contact area therebetween (7) .

3. A sealing ring as claimed in claim 1 or 2 in which the sealing member (5) is substantially rectangular in cross-section.

4. A sealing ring as claimed in claim 3 which is substantially rectangular in cross-section.

5. A sealing ring as claimed in any one of the preceding claims in which a recess (15) is formed in an axial end face of the ring (4) to receive an anti-extrusion ring (16) adjacent one end of the contact surface.

6. A sealing ring as claimed in any one of the preceding claims in which the sealing member (5) has a radial thickness between 10 and 70 per cent of the radial thickness of the energising member (6).

7. A sealing ring as claimed in claim 6 in which the sealing member (5) has a radial thickness between 20 and 35 per cent of the radial thickness of the energising member (6).

8. A sealing ring as claimed in any one of the preceding claims in which the composition of the sealing member (5) is such that a test at the operating temperature of applied load (L) against strain (H) based on the decrease in height of a standard cylindrical specimen 11mm high and 16mm diameter subject to a strain at the rate of lmm/minute produces a result above a line (X - Y) between a point of 25 kgf load and 5 per cent deformation and a point of 120 kgf and 30 per cent deformation.

9. A sealing ring as claimed in any one of the preceding claims in which the composition of the energising member (6) is such that a test at the operating temperature of applied load (L) against strain (H) based on the decrease in height of a standard cylindrical specimen 11mm high and 16mm diameter subject to a strain at the rate of lmm/minute produces a result below a line (X - Y) between a point of 25 kgf load and 5 per cent deformation and a point of 120 kgf and 30 per cent deformation.

10. A sealing ring as claimed in any one of the preceding claims in which the polymeric composition of the sealing member is a polyurethane.

11. A sealing ring as claimed in any one of the preceding claims in which the elastomeric energising member is composed of nitrile rubber.

12. A sealing ring substantially as herein described with reference to Figures 1, 4 and 5; Figure 2 or Figure 3 of the accompanying drawings.