GB2421549A

GB2421549A - Spring-energised seal

Info

Publication number: GB2421549A
Application number: GB0525551A
Authority: GB
Inventors: Alistair Wright
Original assignee: Trelleborg Wills Polymers
Current assignee: Trelleborg Wills Polymers
Priority date: 2004-12-15
Filing date: 2005-12-15
Publication date: 2006-06-28
Also published as: WO2006064255A1; GB0525551D0; GB0427488D0

Abstract

A seal comprises an enclosed annular cavity and a spring disposed within the cavity. The seal is made by providing a separate jacket 302 having an open annular cavity, locating the spring 310 in the cavity and bonding an annular plug 324 to close the cavity. The spring may be a U-shaped or cantilever spring 310, helical ribbon spring or coil spring. The jacket 302 and plug 324 may be made from a high-performance polymer such as PTFE, polyurethanes, PEEKs, ETCFE, PFA or FEP eg by CNC operations or by injection moulding. The jacket 302 may have an external annular recess 326 between two sealing lips 328. High pressure fluid can enter the recess 326 to exert a biasing force on the sealing lips 328. The annular recess (426, fig.4) may be less pronounced. The plug may be wrapped or coated in a melt-processable (thermoplastic) material before insertion into the spring cavity, the assembly then being heated in an oven. Alternatively, an adhesive may be applied to the jacket and/or the plug or the plug may be formed by pouring molten thermoplastic material into the cavity after insertion of the spring. In a modified seal, fig.5, a coil spring 510 is used and there is no recess in the active face of the seal. A rigid reinforcing member 532 is located within the coil spring 510. A reinforcing member may be used with the cantilever spring of figure 4.

Description

* 2421549

I

SPRING-ENERGISED SEAL

The present invention relates to spring-energised seals, and in particular to filled seals, and to a method of manufacturing such seals.

Spring-energised seals are used in a manner similar to conventional elastomer seals, e.g. an 0-ring. Elastomer seals often fail due to conditions present in their operating environment, such as chemical attack, extreme heat or cold, extrusion, friction, or compression set (a permanent deformation, or set, resulting from compressive stress). However, spring- energised seals have a different structure and do not behave like a conventional elastomer and therefore avoid such causes of seal failure.

The basic spring-energised seal design has two components; a generally Ushaped jacket made of a high performance polymeric seal material and a metal spring-loading device.

The U-shaped jacket is pressure actuated so that fluid pressure of the fluid, or fluidised solid, being contained by the seal energises the sealing lips, forcing the seal surface of the lips against the mating hardware. As the fluid pressure increases, so does the load on the sealing lips to enhance resulting sealing contact. The jackets are typically machined rather than injection-moulded, so that the exterior contour of the seal may be easily enhanced for a specific surface contour. The use of polymeric materials and the machining of the seal jackets makes spring-energised seals adaptable to a wide range of applications.

The spring of a spring-energised seal provides a biasing force against the sealing lips such that when the fluid pressure is insufficient to bias the lips, the spring supplies all of the load required for biasing the sealing lips into the surrounding hardware. The spring also typically compensates for variations in tolerances and normal wear of the seal. As a seal loading device, a metal spring is more accurate than other devices, such as 0-rings, for the control of friction. Three types of metal springs are typically used: a cantilever-beam spring, a round wire in a slanted coil, and a helical-ribbon spring. Load ratings of the spring required for a particular application may be tailored to the applications linearfriction or torque requirements.

A common application for spring-energised seals is in equipment used within the food and/or drug industries. In these applications it is necessary to encapsulate the spring to prevent food or drug stuffs from being trapped and becoming prone to bacterial entrapment that can contaminate the food and/or drug stuffs. A known method of encapsulating the spring is to fill the jacket cavity, thus embedding the energising spring, with a silicon potting compound that meets the various drug and food agency standards. However, such silicon filled seals exhibit certain disadvantages. The presence of the silicon potting compound within the seal jacket cavity tends to restrict the movement of the seal lips and thus impair the functionality of the seal. Additionally, the silicon potting compound tends to be susceptible to damage by cleaning agents and solvents used around the seal and is also susceptible to damage from high pressure cleaning operations typically used in cleaning food andlor drug preparation equipment.

An alternative to filling the jacket cavity with silicon potting compound is shown in United States Patent No. US 5,799,953. This document discloses a spring energised seal in which the jacket includes an annular cavity having two annular lips, the spring being disposed within the annular cavity. A cap is disposed over the annular lips and encapsulates the cavity and thus the spring. Cooperating projections and channels lock together the cap and jacket. In an alternative embodiment, the cap and jacket are bonded together. However, by encapsulating the spring by sealing the annular cavity the ability to energise the seal by virtue of fluid pressure is lost or significantly impaired. Furthermore, fluid pressure on the cap can cause the cap to flex inwards towards the cavity, thus exerting a flexing force on the region of the joint between the cap and jacket lips that can cause the cap to become at least partially disconnected from the jacket, thus re-exposing the spring.

United States Patent No. US 5,163,692 discloses a spring energised seal in which a U- shaped spring is integrally bonded to an elastomer seal body. Since the elastomer body is moulded around the spring the spring must be supported in a mould, thus meaning that the spring is not entirely encapsulated by the elastomer body.

According to a first aspect of the present invention there is provided an annular seal comprising an annular one piece body having an enclosed annular cavity formed therein and a spring disposed within the annular cavity formed therein and a spring disposed within the annular cavity such that the spring is encapsulated, wherein the annular body includes a pair of opposed seal lips and an annular recess formed between the seal lips arranged in use to receive a pressurised fluid, the seal lips being arranged in use to be urged apart by the pressurised fluid. Preferably, the spring comprises a cantilever spring having a pair of spaced apart opposing legs. The annular recess may therefore extend between the opposing legs.

According to a second aspect of the present invention there is provided an annular seal comprising an annular one piece body having an enclosed annular cavity formed therein and a spring disposed within the annular cavity such that the spring is encapsulated, wherein annular reinforcing member is also disposed within the annular cavity and is arranged to prevent the cavity from collapsing under excess pressure.

Preferably, the spring includes an annular cavity in which the reinforcing member may be coaxially located. The spring may be a helical or coil spring.

According to a third aspect of the present invention there is provided a method of manufacturing a seal comprises providing an annular seal jacket having a sealing face and having an open annular cavity formed in the seal jacket, the annular cavity opening in a face other than the sealing face; inserting a spring into the open annular cavity; and providing an annular plug in the annular cavity, the annular plug being bonded to the annular seal jacket within the annular cavity, such that the spring is encapsulated.

Additionally, the method may further comprise inserting an annular reinforcing member into the open annular cavity in addition to the spring. Advantageously, the reinforcing member may be coaxially disposed inside the spring.

Additionally, the step of providing the annular plug may comprise introducing molten thermoplastic material into the annular cavity in the seal jacket and allowing the molten thermoplastic material to cool.

Alternatively, the step of providing the annular plug may comprise providing a preformed annular plug, inserting the annular plug into the annular cavity and bonding the annular plug to the seal jacket.

Additionally or alternatively, the bonding step may comprise providing a layer of thermoplastic material on a portion of at least one of the annular plug and annular cavity and inserting the plug into the open annular cavity and heating the assembled seal jacket, spring and plug to a sufficiently elevated temperature to at least partially melt the layer of thermoplastic material, whereby the seal jacket and plug become bonded to one another.

Additionally, the step of providing a layer of thermoplastic material may comprise either wrapping a film of thermoplastic material around at least a portion of the annular plug, wrapping a film of thermoplastic material around at least a portion of the annular cavity or coating at least a portion of at least one of the annular plug and annular cavity with a thermoplastic material in a molten or semi-molten state.

Alternatively, the bonding step may comprise coating a portion of at least one of the annular plug and annular cavity with an adhesive and inserting the annular plug into the annular cavity.

Additionally or alternatively, the method may further comprise removing any excess thermoplastic material from the assembled seal jacket, spring and plug prior to heating the assembly.

Preferably, the thermoplastic material may comprise any one of FEP (fluorinated-ethylene propylene), PFA (perfluoroalkoxy), PTFE (polytetrafluoroethylene), ETFE (ethylenetetrafluoroethylene) and PE (polyethylene).

Additionally or alternatively, the method may further comprise performing one or more finishing processes on the assembled seal jacket, spring and plug after the heating step.

The seal jacket and plug preferably comprise the same material and preferably comprise a polymeric material.

The spring preferably comprises one of a cantilevered spring, a helical ribbon spring or a slanted coil spring.

Embodiments of the present invention will now be described, by way of illustrative example only, with reference to the accompanying figures, of which: Figure 1 illustrates in partial cross-section an energised-seal according to the prior art; Figure 2 illustrates the prior art seal of Figure 1 in use; Figure 3 illustrates a seal according to a first embodiment of the present invention prior to assembly; Figure 4 illustrates a seal according to a second embodiment of the present invention prior to assembly; and Figure 5 illustrates a seal according to a third embodiment of the present invention.

A spring-energised seal according to the prior art is illustrated in Figure 1. The seal 1 comprises a jacket 2 made from a conventional high-performance polymer, such as a low- friction PTFE resin such as (Turcon ), a fluoropolymer e.g. ETFE, thermoplastic- elastomer compounds, ultra-high molecular-weight polyethylene compounds or any other suitable seal material. The jacket 2 is machined into the desired shape using conventional CNC operations such that the seal has an inner lip 4 and an outer lip 6 with a substantially U-shaped cavity 8 formed there between. Located within the cavity 8 is an energising- spring 10, which in the example shown in Figure 1 is in the form of a cantilever spring.

The spring 10 is held within the cavity 8 by means of inner and outer flanges 12, 14 formed on the seal jacket 2 around the respective inner and outer periphery of the cavity 8. The seal shown in Figure 1 also has a silicon filler 16 provided in the jacket cavity 8. In Figure 2 the prior art seal 1 illustrated in Figure 1 is shown housed within an annular recess 18 so as to provide a seal between opposing surfaces 20 and 22. A typical application in which such an arrangement may be found is in a piston in which the recess 18 is formed within the piston itself and the seal I is provided to provide a seal between the piston and the internal surface of the bore in which the piston is housed. The piston may, for example, be a plunger for dispensing fixed volumes of a food stuff.

The component parts of a spring-energised seal according to an embodiment of the present invention are shown in Figure 3. As with the prior art, a jacket 302 is provided in which an open cavity 308 is formed. An energising-spring 310 is located within the cavity 308. In Figure 3, the spring 310 comprises a U-shaped or cantilever spring. Additionally, a plug 324 is provided that is manufactured so as to provide a close fit within the cavity 308 of the seal jacket 302. Both the jacket 302 and the plug 324 may be manufactured through normal CNC operations or by injection moulding from conventional high-performance polymers such as PTFE, polyurethanes, PEEKs (polyetheretherketones) ETCFE, PFA (perfluoroalkoxy resins), FEP or other fluorinated polymers. As shown in Figure 3, the jacket 302 is formed with an external annular groove or recess 326 between the two sealing lips 328. The annular groove 326 is preferably dimensioned such that it extends between the opposing legs of the cantilever spring when assembled. However, in other embodiments the legs of the spring may not extend on either side of the annular groove, or the spring may be a helical or coil spring, in which case the seal jacket either side of the annular groove is preferably of solid material. In use, the seal is located such that high pressure fluid can enter the annular groove 326, the fluid pressure exerting a biasing force on the sealing lips 328. In other words, even though the spring 310 is completely encapsulated by the seal jacket 302 and plug 324, the seal can still be pressure energised by the fluid that the seal is preventing from leaking. Consequently, this embodiment of the present invention provides a seal having the advantages of a filled spring energised seal, in terms of encapsulation of the spring, whilst retaining the performance of a non- filled pressure energised seal.

An alternative embodiment of a seal according to the present invention is illustrated in Figure 4. In an analogous manner to Figure 3, the seal shown in Figure 4 is shown in the dis-assembled state and also comprises a seal jacket 402, cantilever spring 410 and plug 424. The seal jacket has an internal cavity into which the spring is located. In an analogous manner to the seal shown in Figure 3, the seal jacket 402 shown in Figure 4 comprises a pair of opposing seal lips 428 with an external annular groove, or recess, 426 formed there between. Whilst the annular recess 426 of the seal shown in Figure 4 is not as pronounced as that of the seal shown in Figure 3, it provides the same function of allowing the seal to be pressure energised in use.

To assemble the seal in accordance with embodiments of the present invention, the spring 310 is inserted into the spring cavity of the jacket as is normal with conventional spring- energised seals. The plug is then preferably wrapped or coated in a melt- processable (thermoplastic) material, such as FEP (fluorinated-ethylene propylene), PFA (perfluoroalkoxy), PTFE (polytetrafluoroethylene), ETFE (ethylenetetrafluoroethylene) or PE (polyethylene) and inserted into the spring cavity. The choice of melt-processable material is dependent upon the intended end-application of the energised-seal and will be appreciated that other thermoplastic materials that are compatible with the jacket material may be used. Any excess melt-processable material may be trimmed from the back of the seal. The assembly is then preferably restrained in a polymer or metal groove so as to provide support to the jacket and plug during the subsequent processing step. The assembly is then heated in an oven to a temperature suitable to melt, or partially melt, the melt-processable material so as to weld the plug and jacket together. The assembly is heated for a duration long enough to cause the weld to be completed. After cooling, the seal is removed from the groove and may be finished through some clean-up operations, such as supplementary machining, as required.

In alternative embodiments rather than wrapping the plug in the meltprocessable material, the periphery of the spring cavity in the jacket may be lined with the melt-processable material. The unwrapped plug is then introduced into the spring cavity and remaining process steps completed as with the previously described embodiments. Alternatively either or both of the plug and jacket may be coated in a layer of molten, or semi-molten, thermoplastic material, for example by dipping them into a container of molten material.

In further alternative embodiments a suitable adhesive may be applied to one or both of the plug and jacket in place of the melt-processable material, in which case the further steps of heating the assembled seal may be omitted, unless the adhesive is heat activated. The adhesive bonds the plug and jacket together.

In a further alternative embodiment the annular plug may be formed by pouring molten thermoplastic material into the spring cavity in the seal jacket after the spring has been inserted and simply allowing the molten material to set. This eliminates the need for heating the assembly to bond the plug to the jacket and also avoids the need for forming the annular plug and seal jacket cavity to close tolerances, since any variations in the dimensions of the jacket cavity will be automatically accommodated by the molten thermoplastic material.

An advantage of the construction and method of assembly of the seals in accordance with embodiments of the present invention is that the plug and corresponding internal cavity in the seal jacket are located on a nonactive face of the seal, by which it is meant that they are located on a face of the seal on which no pressurised sealing fluid bears. As a consequence, the weld/bond formed between the plug and seal jacket is not exposed to any significant forces in use and is thus highly resistant to failure.

The assembled seal is analogous to the silicon filled seal I shown in Figures 1 and 2 in that the spring is fully encapsulated so as to prevent food or drug stuffs from being trapped or becoming prone to bacteria entrapment that can contaminate the food or drug stuffs. The energised seals according to embodiments of the present invention are therefore fully compliant with the various food and drug agency standards.

A further embodiment of the present invention is shown in Figure 5. As with the previously illustrated embodiments, the seal comprises a seal jacket 502, a spring 510 and a jacket plug 524. In the example shown in Figure 5 the seal is fully assembled. In the embodiment illustrated the spring 510 comprises a helical or coil spring, which has a substantially circular cross-section. Although there is no annular groove or recess in the active face of the seal, pressure energisation may still occur by the system fluid pressure acting on the face 530 of the seal. This area is pressed in by the system pressure, which in turn pushes the sealing lips 528 against the hardware. The spring counteracts the fluid pressure, by virtue of its resistance to deformation, to prevent the front of the seal from collapsing in on itself. To further guard against collapse of the seal a substantially rigid reinforcing member 532 is located within the helical or coil spring. The reinforcing member may be either a hollow or solid wire formed into a circle with the ends being optionally welded together. The reinforcing member is fitted inside the helical or slant coil spring prior to the spring being inserted into the seal jacket. The reinforcing member prevents the spring from being over compressed in high pressure applications where the system pressure is acting on the front face of the seal. If this pressure is too great it can crush the seal by collapsing the spring cavity and the spring inside it. With the reinforcing member present the front face of the seal can be pushed in by the system pressure only as far as the to the reinforcing member, which in turn is supported by the spring and body of the jacket the spring. The cross section of the reinforcing member is preferably the same as the internal diameter of the cross section of the spring, less the amount the spring is compressed by when installed in its hardware. The reinforcing member may, in addition to being used with the helical and slant coil springs as mentioned, also be used with the cantilever design as shown in Figure 4. The reinforcing member in this instance would be fitted between the front of the seal and the spring to prevent the seal collapsing as before.

There would be no need to fit a reinforcing ring to the seal type shown in Figure 3 as the seal jacket follows the contours of the spring and all pressure energisation would act on the sealing lips as required.

Seals according to embodiments of the present invention exhibit a number of significant advantages over conventional silicon filled seals known from the prior art. For example, the seal has increased stability that substantially prevents the seal from distorting in dynamic applications, which increases the serviceable life of the seal. Increased stability is provided by the enhanced physical attributes of the polymer material that the plug is manufactured from and also from the fact that the plug and jacket are welded together during manufacture, thus effectively providing a one piece seal. The increased stability of the seal permits it to be used at higher pressures than is currently possible with conventional silicon filled seals, which are very prone to distortion under high pressure, thus causing premature seal failure. The seal's functionality is also increased due to the absence of silicon filler that can restrict the movement of the seal lips. In addition, since the plug is manufactured from the same high-performance material as the seal jacket, the use of cleaning agents and solvents around the seal during use will not effect the seal's functionality or life, the seal is less susceptible to damage from high pressure cleaning operations from the front and rear of the seal and the seal is less susceptible to damage when subjected to high back pressure when is use, which extends the seal's life.

Additional benefits of seals in accordance with embodiments of the present invention over the silicon filled seals and elastomers of the known prior art are a greater operational temperature range, no gas absorption and much reduced susceptibility to explosive decompression (ED) .

Claims

I. An annular seal comprising an annular one piece body having an enclosed annular cavity formed therein and a spring disposed within the annular cavity formed therein and a spring disposed within the annular cavity such that the spring is encapsulated, wherein the annular body includes a pair of opposed seal lips and an annular recess formed between the seal lips arranged in use to receive a pressurised fluid, the seal lips being arranged in use to be urged apart by the pressurised fluid.
2. An annular seal according to claim I, wherein the spring comprises a cantilevered spring having a pair of spaced apart Opposing legs.
3. An annular seal according to claim 2, wherein the annular recess extends between the opposing legs of the cantilever spring.
4. An annular seal comprising an annular one piece body having an enclosed annular cavity formed therein and a spring disposed within the annular cavity such that the spring is encapsulated, wherein annular reinforcing member is also disposed within the annular cavity and is arranged to prevent the cavity from collapsing under excess pressure.
5. An annular seal according to claim 4, wherein the spring includes an annular cavity in which the annular reinforcing member is coaxially located.
6. An annular seal according to claim 4 or 5 wherein the spring comprises a helical or coil spring.
7. An annular seal according to any preceding claim wherein the one piece body comprises a polymeric material.
8. A method of manufacturing a seal comprises: providing an annular seal jacket having a sealing face and having an open annular cavity formed in the seal jacket, the annular cavity opening in a face other than the sealing face; inserting a spring into the open annular cavity; and providing an annular plug in the annular cavity, the annular plug being bonded to the annular seal jacket within the annular cavity, such that the spring is encapsulated.
9. A method according to claim 8 further comprising inserting an annular reinforcing member into the open annular cavity in addition to the spring.
10. A method according to claim 9, wherein the annular reinforcing member is coaxially disposed inside the spring.
11. A method according to any one of claims 8 to 10, wherein the step of providing the annular plug comprises introducing molten thermoplastic material into the annular cavity in the seal jacket and allowing the molten thermoplastic material to cool.
12. A method according to any one of claims 8 to 10, wherein the step of providing the annular plug comprises providing a preformed annular plug, inserting the annular plug into the annular cavity and bonding the annular plug to the seal jacket.
13. A method according to claim 12, wherein the bonding step comprises: providing a layer of thermoplastic material on a portion of at least one of the annular plug and annular cavity and inserting the plug into the open annular cavity; and heating the assembled seal jacket, spring and plug to a sufficiently elevated temperature to at least partially melt the layer of thermoplastic material, whereby the seal jacket and plug become bonded to one another.
14. A method according to claim 13, wherein the step of providing a layer of thermoplastic material comprises wrapping a film of thermoplastic material around at least a portion of the annular plug.
15. A method according to claim 13, wherein the step of providing a layer of thermoplastic material comprises wrapping a film of thermoplastic material around at least a portion of the annular cavity.
16. A method according to claim 13, wherein the step of providing a layer of thermoplastic material comprises coating at least a portion of at least one of the annular plug and annular cavity with a thermoplastic material in a molten or semi- molten state.
17. A method according to claim 12, wherein the bonding step comprises coating a portion of at least one of the annular plug and annular cavity with an adhesive and inserting the annular plug into the annular cavity.
18. A method according to any one of claims 13 to 16, wherein the method further comprises removing any excess thermoplastic material from the assembled seal jacket, spring and plug prior to heating said assembly.
19. A method according to any one of claims 13 to 18, wherein the thermoplastic material comprises one of FEP (fluorinated-ethylene propylene), PFA (perfluoroalkoxy), PTFE (polytetrafluoroethylene) ETFE (ethylenetetrafluoroethylene) and PE (polyethylene).
20. A method according to any one of claims 8 to 19, wherein the method further comprises performing one or more finishing processes on the assembled seal jacket, spring and plug after the heating step.
21. A method according to any one of claims 8 to 20, wherein the seal jacket and plug comprise the same material.
22. A method according to any one of claims 8 to 21, wherein the seal jacket and the plug comprise a polymeric material.
23. A method according to any one of claims 8 to 22, wherein the spring comprises a cantilevered spring, a helical ribbon spring or a slanted coil spring.