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Successful sealing? It's a material matter

01 October 2008

Les Hunt finds out what’s new in seals, seal services and sealing technologies

As with most sealing problems, the basic trick is to maximise sealing compressive force without over-stressing the sealing material and risking catastrophic failure. Fortunately, advances in seal materials and, to a lesser extent, energising techniques have made life a lot easier for the fluid sealing specialist, and there are no shortages of ideas out there to help you with your sealing problems.

SKF, probably best known to us for its linear and rotary bearing systems, has been very acquisitive on the seals front in recent years. About two years ago, it acquired the Austrian machined polymer seal firm, Economos, whose products will be familiar to DPA readers. SKF has invested substantially in the business and continues to develop the special ‘Ecopur’ polymer grades that made the Economos brand name famous.

The latest Ecopur materials are claimed to cover the requirements of just about every industrial application. Combining resistance to abrasion, compression and tearing, and with a choice of application-specific properties, these materials can be machined to produce a variety of formats, including U-rings, lip seals, wipers, chevron packings and dampers. Ecopur is particularly tolerant of mineral oil and biodegradable hydraulic oils up to 60ºC and water up to 40ºC. Depending upon the design of the sealing element and its housing, Ecopur seals can also be used at pressures of up to 400bar – even higher where anti-extrusion rings are used.

There are a number of variants in the range, including H-Ecopur, which is FDA approved and highly resistant to hydrolysis; G-Ecopur, offering similar properties to H-Ecopur, but which can be used in the manufacture of seals up to 4,000mm diameter; T-Ecopur, which is modified for low temperature applications down to –50ºC, and S-Ecopur, a new polyurethane brand with optimised friction and wear characteristics for severe duties such as water hydraulics and oil-free pneumatics.

For more extreme applications, the harder grade, X-Ecopur offers higher extrusion resistance than standard materials and therefore higher pressure capabilities for the same seal geometry. Seals of 2,000mm diameter are also possible with this material. Generally, Ecopur seals are CNC machined, but SKF Economos offers moulded production for higher quantities, which does reduce the unit price.

If there’s one industry that does tend to demand the very best from a seal manufacturer, it’s motorsport. The huge engineering investment by this sector has produced a lot of innovation, which readily serves the needs of other industries, as motorsport bespoke seal manufacturer, Race-Tec NAK has discovered.

Race-Tec’s sealing technology is based on three key materials: PTFE (polytetrafluoroethylene) for the lip material, a perfluoroeleastomer (FFKM) substrate and metal housings made from high performance alloys. PTFE is very widely used in the manufacture of seals. It offers unrivalled chemical resistance and has an extraordinary working temperature range of –250ºC to +250ºC or even higher. And, of course, its low friction characteristics are legendary.

Perfluoroelastomers offer many of the properties of PTFE but, additionally, the flexibility of an elastomer. Temperature range is not so dramatic as PTFE, but a perfluoroelastomer seal will operate at temperatures in excess of 300ºC, easily surpassing the temperature performance of more conventional sealing elastomers.

The housings and outer rings of high-performance seals are also subject to chemical attack and extreme thermal cycling, and Race-Tec’s choice of alloy reflects this. The company uses high corrosion resistant alloys and will also use exotic metals such as titanium when weight and strength are deemed important properties of the seal.

Race-Tek manufactures and tests its seals in house, and uses tools such as finite element analysis during the design process. It boasts a range of capabilities that enable it to mould the elastomeric sealing elements and bond them to a variety of exotic metal substrates.

Seals with a purpose
Whereas Race-Tec will produce a tailor-made precision seal to meet the needs of a particularly demanding application, Trelleborg’s take on application-specific seal manufacturing is rather different, if one of the company’s recent projects is anything to go by.

Essentially, Trelleborg recognised that the current method of sealing car shock absorbers left a lot of room for improvement. An automotive shock absorber includes a hydraulic piston, which moves within the absorber housing, against which it must be sealed. Traditionally, a band of PTFE based sealing material is manually wrapped around the piston - a time-consuming procedure that doesn’t exactly lend itself to the volume needs of the automotive industry.

Trelleborg’s design team has come up with a solution to this problem that involves pulling a thin, flat disc of its Turcon PTFE sealing material over the piston. In order to speed the process, the company has developed a special automated assembly machine that removes the need for manual intervention. As well as banding the piston, the process is engineered to incorporate a lip, which enhances the friction characteristics, improving wear resistance and overall shock absorber performance.

The bronze-filled Turcon material can operate over a temperature range of -40ºC to +120ºC and withstand peak pressures of 8MPa with a linear movement of up to 3m/s, ensuring long seal life. Users of Trelleborg’s automatic shock absorber banding machines are claiming to meet carmakers’ targets of up to 100,000km before replacement is needed.

While on the subject of automotive sealing, ThreeBond has just launched a single-component, liquid-silicone gasket material that can be dispensed by hand during installation or when undertaking maintenance. The new TB1215 material seals flanges against water or oil, not just in automotive components, but also in general engineering, industrial power transmission, agricultural or marine applications. Available in 200g pressurised canisters, 310ml cartridges and tubs of up to 250kg weight, this solvent-free, low-odour and non-acidic RTV-silicone forms a rubber-like, highly elastic gasket that offers good shear resistance and excellent adhesion - even to slightly contaminated surfaces.

At your service
If a sealing problem is giving you a headache, then why not consult the experts. Ceetak offers a design service based on a long-term partnership with its key distributor, Parker Pradifa Seals. Between them, these companies can offer years of materials development and seals application engineering, often in demanding niche sectors such as life sciences. Aggressive media, wide temperature variations and the typical accreditation and compliance regimes demanded by the sector (FDA/USP Class VI) can almost stall a project unless you have the necessary in-house experience and knowledge.

For the life scinces industry alone, Ceetak can provide 19 specially formulated compound recipes, including different grades of EPDM, FKM, NBR, FFKM and silicone. Each of these compounds has FDA or USP accreditation, and offers long-term sealing performance in processes involving steam, high purity or aggressive media.

In one application, the company was called upon to provide O-rings for steam valves installed in sterilising pipework. The size and geometry of the O-ring was standard, but providing a suitable material was more challenging. The application involved not only high working temperatures and pressures, but also potential contact with lactic acid at high temperatures; USP Class VI approval was also required.

Ceetak recommended its newly developed V1274-75 FKM compound, which was able to tolerate superheated steam. The company submitted the material for USP type approval against Class VI, which requires compounds to be made from known bio-compatible ingredients as well as meeting tighter requirements for low extractables. It passed with a matter of weeks and the valve manufacturer subsequently changed the specification for other valve product lines to include the new O-rings. Technical director, Andrew Kownacki puts it in a nutshell:

“Our customers tend to come to us with a difficult sealing application, which we then dissect, exploring every aspect in order to provide better seal performance. This could mean designing a new seal, recommending a particular material or even developing a new application-specific compound. Whatever the outcome, we strive to provide a high-quality engineering design service.”

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