GB2033497A - Seals - Google Patents

Abstract

An O-ring seal, washer or gasket is formed with an inner core 10 of elastomeric material and an outer layer 11 of softer elastomeric material, to accommodate, for a given contact stress, greater surface asperities and/or greater relative movement of surfaces to be sealed than would an O-ring, washer or gasket of uniform material. In another embodiment an elastomeric washer is provided on each main face with a washer of softer elastomeric material. <IMAGE>

Description

SPECIFICATION Improvements in or relating to elastomeric O-rings, washers and gaskets and to seals embodying such O-rings, washers and gaskets This invention relates to elastomeric O-rings, washers and gaskets for seals and to seals embodying such O-rings, washers and gaskets. The expression O-rings, washers and gaskets is used to include also sealing elements which are not complete annular elements, e.g. U-rings.

In an O-ring seal, the elastomeric material has to provide a contact stress, that is to say the deformation of the O-ring material must provide interfacial pressure between the O-ring material and the surface against which it seals. Also the O-ring must deform to correspond with irregularities in that surface. The choice of the material for the O-ring is a compromise between these two requirements; simple theory and experimental evidence however indicate that the use of a harder or softer material does not affect the seal performance directly. Similar considerations apply to other elastomeric sealing elements such as washers and gaskets.

According to one aspect of the present invention, an elastomeric O-ring, washer or gasket for a seal has surface regions of elastomeric material which is softer than the elastomeric material underlying the surface regions.

The invention includes within its scope and elastomeric washer or gasket for a seal comprising a planar elastomeric element having surface portions over both its major flat surfaces which are of softer material than the elastomeric material between said surface portions.

According to another aspect of the present invention, an elastomeric O-ring, washer or gasket for a seal is formed with an elastomeric core of harder material than the surface of the O-ring.

The hardness of the material may decrease monotonically, e.g. in one or more steps through the thickness of the material to the outer surface. In the simplest form an O-ring may comprise an annular core of relatively hard elastomeric material with a surface consisting of a softer elastomeric material.

The modulus of elasticity of the core may typically be at least twice as great as the modulus of elasticity of the surface material.

The O-ring, washer or gasket of the present invention separates the two sealing functions, i.e.

the provision of a contact stress and the deformation into surface features, and hence enables the seal design to be improved. The hard core provides the main contact stress and the softer surface more readily deforms to the surface irregularities. It thus enables lower leakage rates to be achieved at an equivalent level of seal compression compared with a conventional O-ring seal, washer or gasket of uniform material. Conversely, a required level of leakage can be achieved with relaxed manufacturing tolerances and surface roughness requirements.

The invention furthermore includes within its scope a seal comprising an elastomeric O-ring or gasket as described above arranged between surfaces of substantially rigid material, wherein the softer elastomeric material around said core has a thickness at least twice the height of surface asperities on said surface. With a conventional seal of uniform material, a substantial seal compression may be required to achieve the necessary level of contact stress, particularly with a surface which is not smooth. If relative movement between the surfaces occurs, for example due to vibration or thermal change, effective seal compression may be reduced and unacceptable contact stresses may result. With the O-ring or gasket of the present invention, adequate contact stress may be maintained with larger surface defects or larger relative movements.Considered in another way, for a given initial seal compression, a higher level of seal integrity is obtainable. A lower seal compression is particularly advantageous if an O-ring or gasket has to be retained in a groove, since the groove width has to be increased for high levels of seal compression.

A further advantage of the reduced compression is the associated reduction in clamp load, which can be of importance in reducing weight. With the seal construction described above, it may be possible to change production techniques to take advantage of the greater freedom, for a given leakage requirement, to change tolerances on compression and surface roughness.

The invention still further includes within its scope a seal comprising an elastomeric element in the form of an O-ring, washer or gasket engaging rigid surfaces against which the element is to effect a seal wherein the element has surface regions, at least in the areas of its surface engaging said rigid surfaces, of elastomeric material which is softer than the elastomeric material underlying said surface regions.

These and other features of the invention will be more fully explained in the following description wherein reference is made to the accompanying drawings in which: Figure 1 is a cross-section through an elastomeric O-ring for a seal; Figure 2 is a diagram, to a much larger scale than Fig. 1, showing deformation of part of an O-ring into surface asperities of a rigid element; and Figure 3 is a section through an annular washer constituting another embodiment of the invention.

Referring to Fig. 1, there is shown an O-ring for a seal comprising an annular core 10 of a relatively hard elastomeric material. Such an O-ring might be produced for example a standard O-ring of 80 I.R.H.D. (International Rubber Hardness Degrees) and coating it with a sheet of softer elastomer of say, 60 I.R.H.D, The Youngs Modulus (Modulus of Elasticity) of the core is thus between 2 and 3 times as great as that of the outer surface coating. In use, such an O-ring is compressed between two surfaces of rigid elements which are to be sealed; for example the seal may be in an annular gap between an inner element and an outer element, being retained by location in a groove in one of the elements. The thickness of the soft layer 11 is preferably made at least twice the height of surface asperities on the surfaces in contact with the O-ring.

The coating 11 may be of the same elastomeric material as the core or of a different material.

In practice a single coating has been found adequate although it would be possible to have more than one layer over the core.

Fig. 2 illustrates the deformation, at 15, of part of an elastomeric O-ring 16, into a depression 17 in a rigid element 18. It has been shown (Leahy, 1971, 5th Int Conf. on Fluid Sealing, B.H.R.A. Paper C6) that the extent of the deformation of an elastomer of modulus E into a defect of depth A and width 2B as determined by the contact stress f is given by the expression: ?T a f=E-. - (1) 3 B The peak contact f, stress for laterally unconstrained O-rings in compression has been given by the empirical expression (Lindley 1976 J, IRI. July/Aug pp 209-213):

where x is the compressed seal height and d is the cross-sectional diameter of the O-ring.

These two expressions may be combined to give the approximate relationship between the surface parameter A B and seal compression, as follows: A x = 1.4(-)* (3) B d Equation (3) implies that the depth of penetration of an elastomer into a given surface defect is, to a first approximation, only dependent on the seal compression and is not dependent on the seal hardness. This is confirmed by experimental evidence.

In considering the much more complex seal surface as a whole, experimental results have confirmed that there is an approximate correlation of seal compression with a surface parameter, which is equivalent to the A ratio in equation (3) above.

B In practice, the tolerances on O-rings and on the surfaces which they engage are such that often broad ranges in seal compression may occur. To ensure that a given minimum seal contact stress or seal compression is achieved, considerable means and very high maximum seal compressions may be required with a conventional O-ring of uniform material.

The minimum initial compression must be adequate to provide the necessary level of leak tightness. High initial seal compression, to guard against a reduction in effecting seal compression in service, may lead to unacceptable stresses within the bulk of the O-ring and to problems in retaining the O-ring within a groove, the width of which has to be increased to cope with high levels of seal compression.

With the O-ring of Fig. 1, if the modulus of the surface material is E, and that of the core is E2, it may be shown from equations (1) and (2) above that

As an example of the effect of a composite O-ring, it may be shown from equation 4 that, fora ratio of E2 - of 2, to infill a constituent of a surface characterized by an E, a - value of 0.3, the required compression is reduced to 5% compared with 13% for a B conventional O-ring of uniform material (i.e. when E2 - is unity).

E, The choice of the ratio E1 - may be made to suit the particular seal application.

E2 The O-ring described above is particularly advantageous in achieving high integrity sealing at low levels of contact stress. If the surfaces to be sealed are relatively movable, the bulk deformation of the seal can accommodate changes in separation of the sealing surfaces with only small changes in leakage.

Although, in this description of one embodiment, reference has been made more particularly to an O-ring, it will be readily apparent that the concepts disclosed can be applied to seal shapes other than O-rings, for example, U-rings, and gaskets, e.g. flat elastomeric gaskets.

Fig. 3 is a section through an annular washer. In a washer which is to be sandwiched between two flat parallel surfaces and which has to seal against these surfaces, it is only necessary to have the softer elastomeric material on these surfaces; there is no need for it to surround completely a core of harder elastomeric material. The washer of Fig. 3 comprises a flat annular disc 20 of elastomeric material sandwiched between two layers 21, 22 of softer elastomeric material. Such a washer may be fabricated from materials of the two different degrees of hardness with the layers bonded together or alternatively the softer surface layers may be formed by putting coatings on the inner layer 20, e.g. by a printing process, or by chemically softening the surface portions of a washer formed initially of the harder material.

Surface layers formed by coating or by surface treatment of the inner layer may be quite thin compared with the inner layer, e.g. having a thickness of 0.1 mm for a washer of 5 mm thickness. The thickness required for the softer portion however will depend on the size of the surface asperities of the rigid surfaces between which the washer is compressed. Preferably the softer surface regions have a thickness at least twice the height of such surface asperities.

Claims (12)

1. An elastomeric O-ring, washer or gasket for a seal having surface regions of ealstomeric material which is softer than the elastomeric material underlying the surface regions.

2. An elastomeric washer or gasket for a seal comprising a planar elastomeric element having surface portions over both its major flat surfaces which are of softer material than the elastomeric material between said surface portions.

3. An elastomeric O-ring, washer or gasket for a seal formed with an elastomeric core of harder material than the surface of the O-ring or gasket.

4. An elastomeric O-ring as claimed in claim 1 wherein the hardness of the material decreases monotonically through the thickness of the material from the centre to the outer surface.

5. An elastomeric O-ring, washer or gasket for a seal comprising an annular core of relatively had elastomeric material with a surface coating of a softer elastomeric material.

6. An elastomeric O-ring, washer or gasket for a seal as claimed in any of the preceding claims 3 to 5 wherein the modulus of elasticity of the core is at least twice as great as the modulus of elasticity of the surface material.

7. A seal comprising an elastomeric O-ring, washer or gasket as claimed in any of the preceding claims 3 to 6 arranged between surfaces of substantially rigid material, wherein the softer elastomeric material around said core has a thickness at least twice the height of surface asperities on said surfaces.

8. A seal as claimed in claim 7 and wherein there is relative movement between said surfaces, the elasticity of the O-ring or gasket being such that the O-ring or gasket being such that the O-ring or gasket remains in contact with both surfaces throughout said relative movement.

9. A seal comprising an elastomeric element in the form of an O-ring, washer or gasket engaging rigid surfaces against which the element is to effect a seal wherein the element has surface regions, at least in the areas of its surface engaging said rigid surfaces, of elastomeric material which is softer than the elastomeric material underlying said surface regions.

10. A seal as claimed in claim 9 wherein the softer surface regions have a thickness at least twice the height of the surface asperities of said rigid surfaces.

11. An elastomeric O-ring or gasket for a seal substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

12. An elastomeric washer for a seal substantially as described with reference to Fig. 3 of the accompanying drawings.

1 3. A seal having an elastomeric O-ring or gasket substantially as hereinbefore described with reference to the accompanying drawings.