Getting your bearings

Bearings have been used for thousands of years — styled from wood, metal, and now plastic. These rolling components are critical in motion control applications, assisting in sliding, turning, and rolling motion of machine components. Using bearings correctly enhances the speed and efficiency of machinery; for example, smaller bearings usually translate to faster accelerations. However, it is important to remember several other factors that impact bearing speed — lubrication, materials, loads, retainers, and more.

What bearing attributes are most closely linked to speed, and how?

Geoff/GGB: Because dry bearings aren't lubricated, the bearing material's mechanical, thermal, and tribological properties have more influence on sliding speed. Specifically, maximum sliding speed for a dry bearing is determined by temperature rise due to frictional heating, and its consequential effects on mechanical strength and wear rate. Frictional heat generated in a dry bearing is a function of sliding speed, applied specific load, and bearing material friction coefficient. Dry bearings have a limiting sliding speed of about 2.5 m/s.

In bearings operating under marginal or boundary lubrication with oil or grease, the same material properties determine limiting sliding speed. Oil lubrication reduces friction, providing limiting sliding speed higher than 2.5 m/s. With grease lubrication, maximum sliding speed is limited by the grease's tendency to separate at high shear rates.

Dan/SKF: Bearing speed limits are generally associated with the lubricant's temperature range and the mechanical stability of bearing materials. In bearings, friction or resistance to rotation or sliding is made up of rolling and sliding friction in the contact areas of raceways, the contact areas between the rolling elements and cage, the friction in the lubricant, and the sliding friction of contact seals. At extremely high speeds, the stability and strength of the bearing cage or retainer, centrifugal and gyratory forces, and lubrication of the contacting surfaces also come into play.

Nancy/PKB: Speed is affected by many factors, including bearing geometry. Bearing rings must be ground to tight tolerances and polished to a smooth and shiny finish. Bearing steel must be of high quality with small, evenly spaced carbides. Large carbides and inclusions reduce the quality of the steel and create “tear outs” and “comet tails” in the raceway; this slows the bearing and shortens its operating life. Finally, retainers must be piloted so they do not drag on any of the bearing components. Retainer drag slows the bearing and results in premature assembly failure.

What are some of the limiting factors associated with bearings when it comes to speed, and how do you overcome them?

Brad/New Hampshire Ball Bearings: Traditional stamped-steel retainers are limited in their operating speed. On the other hand, machined retainers made from phenolic are porous, which allows the retainer to be vacuum-impregnated with base oil for faster operation. Other polymer retainers contain a slight graphite fill, which gives them their lubricity. Oil or grease plating typically outperforms traditional grease fills. Finally: the viscosity also affects running torque and heat generation.

Nancy/PKB: The upper-level assembly into which bearings are placed can limit speed. Contamination, ambient or fluctuating temperatures, and moisture as well as vacuum conditions, salt baths, and other environmental factors should be considered for optimum performance. Special coatings, various ball and retainer materials, and a myriad of bearing lubricants satisfy specific environmental conditions, speed, temperature, and other factors.

What special precautions should designers consider in regard to bearings or any other component?

Dan/SKF: Designers need to determine the operating conditions to which bearings will be exposed beforehand — and have a basic understanding of the recommended operating conditions and limits for the specific bearings they select for a job. Bearing geometries and materials can be customized for given sets of conditions, but one size does not fit all. Too many times a bearing performs satisfactorily — until the same design is used in another application and the bearing fails.

Brad/New Hampshire Ball Bearings: An especially sensitive issue is fitups, such as the mating of the shaft to the bearing bore. It is critical that the mating components be ground to precision tolerances and sized to minimize interference and burrs. An excessive interference fit reduces the radial play in the bearing and potentially limit its speed capability and life.

Will there be intermittent or constant loads? For high-speed operation, load becomes a critical factor when calculating the bearing theoretical life. A few tips: Keep the loads in check based on the bearings dynamic load capacity, and determine how much fatigue life is expected. Also consider a preload mechanism, such as a light spring load, to minimize skidding and control the bearing's end and radial play.

External sealing mechanisms protect the bearings. Contacting seals integral to the bearing design are detrimental at high speeds and can cause excessive heat generation. As a result, non-contacting shields are recommended.

Geoff/GGB: Plain journal bearings should be assembled with the manufacturer's recommended interference in the housing and clearance on the shaft. The shaft should be finished to recommendations; a ground finish is preferred, although sometimes a super-fine finish may be required. These recommendations take into account differential thermal expansion between the bearing and its assembly. Under high-speed conditions, good thermal transfer between the bearing and housing is important.

Ray/Schneeberger: One of the most frequently overlooked items is environment. Will the bearing need to operate in a machine tool where there will be chips, coolants, or grinding swarf? If this is overlooked, it can be costly to the machinery once in production.