Bearings Resist Corrosive Acids, Alkalis and Gases

Bearing manufacturers have a graduated programme of high performance materials, surface finishes and lubricants, to help manufacturers optimise the life costs of machines across all industry sectors. The complexity of modern manufacturing processes mean that rolling bearings are increasingly called upon to operate in environments that expose them to corrosive mediums such as water, acids, alkalis and gases. Moreover, although bearing manufacturers have developed advanced sealing systems to combat corrosion, these do not provide an effective solution where aggressive mediums are concerned. As a result, manufacturers such as NSK have developed a response based upon a graduated programme of high performance materials, surface finishes and specially developed lubricants, including solid types. The programme is helping manufacturers to optimise the life costs of their machines across all industry sectors: food processing, semi-conductor manufacturing, metal plating, chemical processing, agriculture, iron and steel manufacturing, fisheries, transportation - and many more. The fundamental role of rolling bearings in all types of machinery and equipment highlights the importance of combating corrosion, as a means of ensuring, continuous reliable operation. What must be borne in mind is that even standard industrial bearings are precision-engineered components with very small internal operating clearances, a factor that makes them particularly sensitive to contamination. This is particularly significant, as, in many cases, it is the service life of rolling bearings that dictates the required maintenance effort and, hence, the availability for service of a machine or appliance. Probably the most common bearing contaminant is water, or moisture. In addition to being forcibly injected by means such as high-pressure hose downs, moisture can be drawn into sealed ball bearings by the vacuum created as bearing internal air pockets cool after machines shut down. With bearings at standstill, moisture suspended within the grease lubricant, gravitates to points of contact between balls and raceways therefore exposing these areas to corrosion. This is evidenced by internal inspection of moisture-contaminated bearings, which will often reveal patches of corrosion on the raceway spaced at the same pitch as the ball complement. At best, the effect of this on bearing performance is substantially increased noise and running roughness, leading to the development of flaking on the corrosion-pitted raceway and premature bearing failure. At worst, the result is bearing seizure. NSK's response to this problem is the Spacea and Silver-Lube ranges of bearings. These employ a combination of materials, coatings and lubrication technologies that provide reliable performance and endure a multitude of industrial environments. The Spacea corrosion resistant series is divided into a number of specific types, offering graduated levels of performance. For operation in water-based environments, stainless steel, hybrid, and fluoride low temperature chrome plated bearings are available. The stainless steel bearings are the base standard of the range; they are designed for high humidity environments - as found in food processing, fisheries, chemical processing and semi-conductor equipment cleaning. The martensitic stainless steel used in the manufacture of these bearings is also employed for the bearing inner and outer rings of the hybrid series bearings. However, these bearings differ in that silicon-nitride balls, fluororesin cages and waterproof grease or solid lubricants enable them to provide up to five times the life of stainless steel types in applications subject to watersplash or immersion. Also designed to cope with the rigours of food processing environments is the NSK's Silver-Lube series. These are fully assembled, corrosion resistant, mounted ball bearing units: they help users achieve hygiene improvements while reducing downtime and maintenance costs. In addition to corrosion resistance, the major benefit of Silver-Lube bearings is that they are pre-assembled; each insert is inclusive of stainless steel raceways, cage, balls and grub screws, and protected against contamination by a highly-effective stainless steel flinger and seal arrangement. For more specialised applications in food processing, semi-conductor cleaning equipment and transportation, NSK manufactures fluoride low-temperature chrome-coated bearings. These have stainless steel inner and outer rings that are coated with a fluoride low temperature chrome. In addition, for applications where grease lubrication is not viable a fluororesin cage is available. The major benefits offered by these bearings is that they provide higher corrosion resistance and durability than stainless steel bearings and bearings that are hard chrome or electroless nickel plated. Where the problems are related to alkali and weak acid environments, the Spacea corrosion resistant range provides a graduated package comprising three bearing types: precipitation hardened stainless steel bearings, nickel alloy-coated bearings and ceramic bearings. Both precipitation-hardened and nickel alloy-coated bearings offer higher corrosion resistance than martensitic stainless steel types. The former is a hybrid design that includes ceramic balls and a self-lubricating fluororesin cage. The performance of nickel alloy types also exceeds that of bearings employing hard chromium plating, by virtue of greater hardness. Typical areas of application for these bearings include liquid crystal and semi-conductor manufacturing, hard disc manufacture, metal plating and food processing. Even higher levels of corrosion resistance are provided by the first of the Spacea range's all-ceramic bearing solutions. These are oxide or silicon-nitride based and designed to offer longer life than stainless steel and hybrid bearings in alkali and weak acid environments. Oxide-based ceramics are the least expensive of the options while silicon-nitride bearings are the choice for corrosive applications where high temperatures and heavy-load conditions are encountered. The second ceramic bearing option offered is intended for use in environments where strong acids, strong alkalis or corrosive gases are encountered. Employing carbide-based material, these bearings offer the highest corrosion resistance of any ceramics, enabling them to provide long life in these environments. Such conditions are routinely encountered in film cleaning systems, chemical processing, metal plating and liquid crystal and semi-conductor manufacturing equipment. A lower cost option for light load applications in aggressive environments is also available. These are fluororesin bearings marketed under the name of 'Aqua-Bearings'. Providing corrosion resistance equivalent to ceramics, and ten times the durability of stainless steel bearings under light loads, they are designed for use in strong acid, strong alkali and corrosive gas environments. Typically these are encountered in various cleaning and disinfecting processes, chemical processing, plating, liquid crystal and semi-conductor manufacturing.