Bearings get tough on corrosion

June 1, 2000
Bearings are getting a leg up on harsh environments. Find out how new materials are helping

Harsh environments associated with food and beverage, paper processing, steel, and chemical plants have always been tough on bearings. Small wonder. Rusting of cast-iron housings, peeling of nickelplated housings, and premature failure of rotating steel components can all lead to a quick demise.

The answer to these problems lies in several protection methods used by a variety of bearing manufacturers. Polymer materials, for example, protect or replace cast iron and steel housings. Stainless steel and ceramic materials, on the other hand, let bearings operate in severe applications without the worry of corrosion. Also, hard coatings on inner and outer races improve corrosion resistance, reduce wear, and extend bearing fatigue life. And other components, such as seals, end covers, and grease, are used to stop corrosive contaminants from entering a bearing.

Rust-free housings

Several approaches are used to protect bearing housings from corrosion, ranging from epoxy-painted cast iron and nickel-plated steel to polymer coatings, or housings made entirely of polymers. These methods protect the outer surface of the bearing housing from rust, but not the functional components inside. Polymers offer two other advantages -- a clean appearance, which is desirable in food processing applications, and light weight.

Manufacturers such as NTN Bearing Corp., Mt. Prospect, Ill., combine housing protection with stainless steel inserts to obtain better corrosion protection than standard cast iron units. In such a bearing, stainless steel components are packed with high-temperature polymer grease containing rust and oxidation inhibitors. The grease is USDA approved for use in food processing plants where there is a possibility of incidental contact with food.

NTN surrounds the bearing insert with a white fiberglass-reinforced polyester housing. Although the polyester housing is suitable only for light loads, about 1/3 that of a stainless steel unit, it has more resistance to corrosion from water and acid, and it reduces bearing weight 30 to 60%.

Besides food-processing, these bearings are suitable for plants in chemical, pulp and paper mill, and marine industries.

Survivor PT bearings from the Fafnir Bearing Div. of The Torrington Co., Torrington, Conn, take a somewhat different approach. The corrosion- resistant bearing features a thermoplastic polymer housing that is more durable than nylon or coatings, and doesn't scrape or flake off during operation. For corrosive applications, you can switch from a cast iron housing to this type of polymer housing without reducing load capacity. It resists caustic chemical washdowns as well as steam and high temperatures to 250 F (brief exposures to 320 F).

The bearing rings are protected by Fafnir's proprietary thin dense chrome (TDC) coating that resists cracking and peeling. Other components are made of stainless steel or polymer materials to further enhance corrosion resistance and service life.

The Great Bend Packing Co., Great Bend, Kans., switched to these corrosion- resistant bearings on their meatpacking plant conveyors, which face harsh detergents during washdowns to ensure bacterial control. The new bearings are still operating after two years whereas other anti-corrosion bearings were being changed out in 60 to 90 days.

Survivor PT bearings also replaced standard and even nickel-plated bearings at a major garlic processing plant. The original bearings were unable to withstand harsh operating conditions on conveyors due to a combination of daily steam washdowns and caustic sulfuric fumes emitted by the garlic cloves.

Meat, poultry, and other food processing plants tend to go with white housings, which can be easily inspected for cleanliness. Klean-Gard ball bearings from Rexnord Corp., Link-Belt Div., Indianapolis, combine cleanability and corrosion protection in a white polypropylene housing that meets USDA and FDA requirements.

Each bearing is factory lubricated with food-grade grease. An auxiliary shaft seal ring with spring-loaded double- lip seal on the front and rear of the housing guards the bearing from contaminants while retaining lubricant.

A look inside

There are two basic ways to prevent corrosion in the working components of a rolling-element bearing: by using stainless steel or ceramic components, and by applying protective coatings. Each method has tradeoffs in higher cost and lower load capacity, says Jeff Koziol of The Torrington Co.

Stainless steel. This type of bearing contains stainless steel balls, rings or races, and locking collars. Cages may be stainless or composite material. Such a bearing costs 21/2 to 3 times as much as a conventional one.

Most of these bearings use grade 400 stainless steel, which has moderate corrosion resistance. Its load capacity is comparable to that of conventional bearing steel (AISI 52100).

Grade 300 stainless steel bearings, available from Torrington, offer better corrosion resistance, but at the expense of reduced load capacity and speed. Such bearings are a good choice for food-processing applications where corrosion resistance is more important than load capacity.

Ceramic balls. Hybrid bearings contain ceramic balls with steel races. The ceramic material is chemically inert, which makes it corrosion-resistant. But ceramic components are mostly limited to the balls in precision ball bearings, and such bearings cost 11/2 to 5 times as much as comparable bearings with steel balls. Development and application of other ceramic components, such as rollers and races, have been limited by cost and strength constraints.

In addition to corrosion resistance, hybrid bearings with ceramic balls also run faster (20 to 40% more than conventional bearings), reduce vibration (for more accurate machining), and operate at high temperatures with marginally low lubrication.

Coatings. In some bearings, the inner and outer rings wear a protective coating. The most common types are thin dense chrome and zinc chromate. The other bearing parts are made of less expensive stainless steel.

Coated bearings cost 11/4 to 21/2 times more than standard bearings. The degree of corrosion protection depends on the type of coating and the application. Some coatings also increase bearing wear life because they harden the raceways.

For example, AquaSpexx bearings from The Timken Co., Canton, Ohio, specialize in combating corrosion damage caused by water. They feature a proprietary zinc-alloy coating that is applied to inner and outer races and other functional components as needed for the application.

The company first applied this coating to small tapered roller bearings for boat trailer wheels. Larger sizes, up to 24.5-in. OD, are being introduced for more demanding applications.

The coated bearings are suitable for difficult environments such as steel mills, paper mills and off-highway vehicles. A rolling mill environment is especially severe. Here, roll neck bearings must battle extremes of heat and intrusions from scale and water. In such cases, custom seals should be added to exclude contaminants.

These bearings have undergone tests in both the laboratory and in demanding service applications. In a standard salt spray test (ASTM B117), bearing races with the AquaSpexx zincalloy coating outlasted 440 C stainless and TDC coated races by at least 3.5 times. In a seaming head application, they last five times as long as standard bearings.

Linear too

Linear bearings, both plain and rolling element, are also engaged in the corrosion battle says Steve Feketa of Thomson Industries Inc., Port Washington, N.Y. For example, the company supplies plain bearings with either corrosion-resistant replaceable polymer inserts or permanent liners.

First is the Nyliner bearing, which consists of an aluminum bushing and a replaceable polymer insert. The most commonly used polymer is self-lubricating, has low water absorption, and is reinforced for load capacity. Other polymers can be substituted to provide resistance to different chemicals.

Second, a FluoroNyliner Bushing bearing offers a fluoropolymer lining on the ID. The fluoropolymer is inert and therefore impervious to any corrosive elements. As the fluoropolymer material wears in, it deposits a layer of PTFE lubricant on the mating shaft to reduce friction and wear.

These bearings run on 303 or 316 stainless steel shafts. The polymer materials in both bearings have very good corrosion resistance, but the FluoroNyliner material has better wear resistance.

In the rolling-element arena, the company supplies Ball Bushing bearings with 440 stainless steel outer races, and both ceramic and 440 stainless steel balls. The stainless steel has moderate corrosion resistance, however, and ceramic materials may be too costly for the average application.

Thomson can apply the following corrosion-resistant coatings to inner and outer races of Ball Bushing bearings:

• Nickel plating has high corrosion resistance, but poor adhesion to the parent metal. Its load capacity is about 10% of that of conventional bearing steel.
• Zinc cobalt dichromate has good corrosion resistance, but its soft surface leads to adhesive wear. Its load rating is 10 to 15%.
• Thin dense chrome, which has much less corrosion resistance, and its load capacity is about 50% of that of conventional bearing steel.

The company is introducing a new bearing for washdown applications. It has a polymer outer race, austenitic stainless steel rolling elements, and an inner race (shaft) that either has a corrosion-resistant coating or is made of austenitic stainless steel. This bearingshaft combination offers high corro - sion resistance, but with 50% of the normal load rating of a conventional Ball Bushing bearing.

In the wings

Researchers are looking at new ways to make steel bearing components more corrosion resistant. For example, FAG Kugelfischer Georg Schafer AG in Germany is working on a new process that injects nitrogen under pressure into steel to improve its corrosion resistance as well as toughness and strength. Called Cronidur 30, the new steel was developed jointly by the Ruhr University Bochum, steel manufacturer VSG Essen, and FAG Kugelfischer.

It is said to be more corrosion resistant than the stainless steel used in many bearings. It also has good wear resistance and fatigue life. Engineers anticipate applications in aircraft and aerospace engines.

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