Select bearings with advanced features
Bearing selection normally begins with calculating the L10 life, which reflects the estimated life that 90 percent of a given group of bearings will achieve prior to fatigue failure. This simple estimate is based upon the actual loads and speeds applied to the bearing, as well as the bearing’s dynamic capacity. However, the L10 estimate doesn’t factor product quality, the product’s life-extending features, maintenance, or the operating environment. Today, bearing manufacturers provide a variety of feature-rich products that are specifically designed to prolong the life, particularly in environments detrimental to bearings. As a result, it’s important to consider the product design as a valuable contributor toward facility uptime and reliability.
The main threat to bearing life is the harsh environments that expose bearings to contamination from both moisture and solid particles. Moisture contamination may include caustic and non-reactive chemicals during wash-down procedures, application overspray, condensation, or the general nature and environment of the application. Exposure can also include dirt, debris, dust and shavings that can accumulate around the bearing. If wind or aggressive airflow exists near the bearings, then the threat from contamination is elevated further.
Bearing manufacturers offer many different, often premium, products that help combat these threats. The features can include design and material upgrades to the primary and supplementary product components. The number one advanced feature designed to prolong bearing life is a severe-duty seal. The seal should have multiple contact extensions that isolate the bearing cavity from the external environment, even during operation. The contacting lips are elastomeric and provide positive engagement on a rotating surface. The downside of some of these seals is that the lips can increase drag and limit the speed capability of the bearing.
Flingers (a sealing feature) rotate with the shaft or inner ring. The purpose of the flinger is to essentially "fling" any particles away from the bearing. The flinger acts like a radial centrifuge, leveraging centrifugal acceleration to slingshot any approaching moisture or debris away from the seal contact surfaces and the bearing. In an advanced design, they’re usually made from corrosion-resistant materials, such as stainless steel or nickel-plated steel. Some are offered with larger diameters that create a higher rim speed to more effectively relocate the foreign material.
Cage design also plays a major factor in achieving the L10 bearing life. Cages designed with pockets symmetrically placed throughout the cage, with compartmentalized construction, allow the grease to accumulate closer to the critical areas of the bearing. These types of cages will also guide fresh grease into those areas more efficiently than standard cages.
The bearing housing design and structure also plays an important role. Pockets or areas where moisture and material accumulate will increase the potential of bearing contamination, as well as corrode the housing surface. Designs that help direct the flow of liquids or contaminants away from the bearing are beneficial. These housing features include gradual surface draft away from the shaft, drip rails, drainage ports, solid structure and generally smooth periphery absent of depressions.
End closures are excellent accessories to prolong bearing life when shaft lengths stop at the bearing. Closures that effectively seal off contamination offer another layer of protection to the seals and are an excellent addition for applications with routine wash-down, or where debris can build around the bearing. When an application’s shaft length doesn’t stop at the bearing, end closures are provided with a knockout hole to allow the shaft to extend through them, yet still deflect contaminants. The best closures are corrosion resistant and attach mechanically to the bearing housing with a positive interlock that seals the two components together.
Although most bearing housings are offered in painted or plated cast iron, many manufactures currently offer housings made from 300-series stainless steels and polymers with integral anti-microbial agents. Additionally, the balls/rollers, inner and outer rings are commonly offered in 300-series stainless steels or a variety of corrosion-resistant platings. These upgraded features will mean these bearings will last longer, with no chipping, flaking, or rusting.
Advanced bearing designs will also offer bearing housings with re-lubrication ports that allow grease to enter the bearing cavity, while purging excessive or contaminated grease out and away from the bearing. The ability to re-grease anti-friction bearings is important because if the bearing doesn’t receive fresh grease, the life of the bearing is limited to the life of the grease within the bearing. Many of the corrosion-resistant series bearings are offered with grease fittings made from stainless steel.
It’s well understood in the industry that bearing reliability is important to maintain operational efficiency and uptime. Selecting bearings with advanced features that prevent life-limiting environmental influences will enable facilities to save both time and money.
Galen Burdeshaw is Baldor customer order engineering manager, Dodge bearings and PT components. For more information, visit www.baldor.com.