Improved component design and advances in the properties of sealing materials have increased the potential for engineers to apply highly effective sealing solutions. Mike Holland, general manager of ERIKS Sealing Technology, explains
Engineering equipment in today’s manufacturing and process industries is expected to last longer and to function under ever tougher environmental conditions; and reliability of machinery is essential to ensure that production lines or operations are kept running at the maximum rate. To maintain smooth running, good seals are essential as a means of preventing leaks of oils and greases, and defending machinery against the ingress of common contaminants such as dust and dirt.
Oil seals are probably the most common type of seal used in industry, yet this commodity product contains a wealth of technical innovation, especially in terms of materials used. A fresh look at the anatomy of an oil seal reveals how the latest products have been developed to optimise operating life and minimise the potential for failure or poor performance.
To ensure optimal selection when specifying a seal, engineers must consider application, thermo-chemical and mechanical criteria. Material selection is governed by thermo-chemical criteria, such as temperature and media.
Elastomers used as seal materials are typically compounds containing both the base polymer and many other ingredients that affect material properties in either use or manufacture. Additives in high performance modern lubricants and oils may result in chemical degradation of traditional sealing materials, for example leaching out plasticisers. Material compounds are continually developed to ensure reliable operation, in accordance with advances in other technologies.
Each seal material has its own optimum range and therefore temperature is an important parameter to consider. An elastomer can harden and crack as a result of thermal stress when operating outside of its optimum range; in nitrile rubber, heat ageing is a more common cause of failure than wear. However, if the mechanical criteria allow, upgrading to a PTFE (polytetrafluoroethylene) seal can extend the thermal limit of a seal to meet such demands. PTFE is a thermoplastic with an operating temperature that typically ranges between -70°C to 200°C (and in some applications can reach 260°C). The high temperature resistance, wide compatibility with chemicals and low friction profile of PTFE has enabled the use of this material as a highly successful sealing material in many applications, including food and beverage, chemical, pharmaceutical, machinery and automotive.
The right seal
The use of correct seal geometry may mitigate the constraints of mechanical application criteria such as dynamic motion, pressures, surface finish, abrasion and geometry. For example, in rotary seals, where higher shaft speeds are exhibited, the permissible pressure differential across the seal becomes smaller. Increased shaft speed produces greater frictional heat generation, but so too does pressure applied to the seal as more of the lip surface is forced against the shaft. Since too much friction can result in faster wear and shorter seal and shaft life, design variations that help offset these negative effects of higher shaft speeds need to be built into the seal. These design enhancements can include reducing the radial load of the seal lip, upgrading the sealing material to one that can handle higher temperatures, or optimising the shaft sealing surface.
Once the thermo-chemical and mechanical criteria have been satisfied, engineers must still ensure regulatory compliance, such as FDA compliance in the food and pharmaceutical industries, demanding up-to-date knowledge of elastomer and polymer specific legislation.
With a wide range of specialised seals now available, each with their own set of features, understanding the function of a seal in any given application is essential if engineers are to increase the performance and extend the service life of industrial machinery, while also reducing maintenance costs in the process.
It is true to say that even the best components will deliver a shorter operating life than the components they protect, especially in harsh operating environments where temperatures and contamination run at high levels. However, continued innovation over recent years has delivered enhanced component design and material properties that are worthy of closer examination.
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