Know More About Seals

Whatever the nature of the equipment, it is a safe assumption that its reliable operation depends, among other components, on seals, ranging from tiny O-rings to custom engineered large diameter roll-line seals. Wherever there are aggressive operating conditions, seals form the first line of defense. If a seal fails to function effectively, the consequences can be expensive, including increased consumption of lubricant, clearing of hazardous waste and equipment failure. The bottom line: Unplanned shutdowns and lost productivity. SKF offer a wide range of standard and custom built seals, including all-rubber, metal cased, fabric reinforced and metal clamp seals, with spring loaded and springless sealing lips. For example, in demanding roll line applications SKF large diameter seals with an elastomeric outside diameter provide a robust sealing solution. These seals effectively retain oil or grease and prevent the ingress of contaminants, improving bearing performance and reducing lubrication costs. The elastomeric outer also protects the housing bore and minimises the risk of damage during installation or removal, while the use of a springless lip seal, designed to pump contaminants away from the lip, enhances seal life and preserves the condition of lubricant and bearing. Large diameter seals, between 203mm and upwards of several meters in diameter, offer a unique combination of properties, as they have to function under exceptionally high levels of stress for long periods of time without degradation, yet have to be easy to fit and replace. It is therefore crucial to select large diameter seals that have been developed specifically for each application, as the use of incorrect designs or materials will rapidly lead to failure. For example, when SKF were working on their Bearing Life Theory (adopted in ISO 281) their research indicated that up to 14% of all premature bearing failures could be directly attributed to contaminants entering bearings either via poorly designed or specified seals, or because the materials used were unsuitable for each application. SKF's latest large diameter seals, are therefore manufactured from advanced engineering plastics, including PTFE (polytetrafluoroethylene) and HNBR (hydrogenated acrylonitrile butadiene rubber). PTFE is a thermoplastic polymer with a chemical resistance that is far superior to that of any other material used in conventional seals. The operating temperature of PTFE extends from -70°C to +200°C and in some applications can reach +260°C. Furthermore, the polymer has a smooth, dirt resistant surface and an extremely low coefficient of friction, with the ability to tolerate dry running and also provide insulation against the passage of electric current. The wide range of PTFE materials available with filler variants includes glass fibre, carbon, graphite, molybdenum disulphide, metal oxides, polymeric fillers, and combinations of these, each possessing specific properties for different applications and working conditions. Seal materials are chosen for their resistance to swelling, elasticity, chemical or thermal resistance, or gas permeability, with all filled PTFE materials offering greater resistance to wear and deformation than conventional seal materials. By comparison, unfilled PTFE usually provides lower friction than filled versions, although its resistance to wear and deformation under load is limited. HNBR, on the other hand, is particularly suitable for applications where aliphatic hydrocarbons, such as propane or butane and mineral oils and greases, or sulfonated crude oils are present. It is also ideal for use with many diluted acids, bases, salt solutions and even in high temperatures and glycol water mixtures. Another recent seal material offering benefits in heavy industry is fluoro-rubber, or FKM, which combines outstanding levels of resistance towards heat, chemicals, ageing and ozone, with very slight gas permeability. The material has exceptionally good wear characteristics, even under harsh environmental conditions, and can withstand operating temperatures of up to +200°C. Fluoro-rubbers are also resistant to oils and hydraulic fluids, fuels and lubricants, mineral acids, and aliphatic as well as aromatic hydrocarbons, which can cause seals made from other materials to fail. Note that although FKM is an excellent material from which to produce seals used in harsh environments, it must be handled carefully. FKM is stable and harmless in most operating conditions but if exposed to extreme temperatures above +300°C, it gives off hazardous fumes that can be harmful and must be handled carefully. For heavy industrial applications where downtime is critical and shaft removal is impractical, allrubber split seals are available. All-rubber seals are simply placed around the shaft and pushed into the seal bore, then held firmly by a cover plate, which compresses the split joint together. These perform most effectively with grease or heavy lubricants and can also be used with the latest generation of lubricating oils. In addition to a full range of seals in standard sizes, we also provide a nationwide service, through a network of ten dedicated service centres, for the design and manufacture of customised seals. Custom seals are produced using specially developed SKF machine systems. These standalone systems incorporate CNC lathes with the latest digital controls, automatic swarf suction and advanced, simple to use software, for the production of seals from 3 to 600 mm in diameter. With seals no longer being commodity products, but rather essential components in the evolution of production systems, the need for these products to be tailored to individual applications is becoming ever greater. New seal materials such as PTFE, HNBR and FKM are helping engineers throughout industry to achieve optimum performance from a wide range of machinery, while reducing downtime, maintenance, and ultimately costs. Source:bearing.com.cn