Sizing and Applying Rolling Element Bearings

General Sizing Procedures

Rolling element bearings are mechanically complex compared to plain bearings, but are indispensable in a large variety of functions.

Standard ratings such as those provided by ABMA/ANSI give an accurate comparison on various bearings. The two standard ratings for rolling element bearings are the static load rating Co, for bearings that rotate only occasionally or not at all, and the basic load rating C, for rotating bearings. Bearing manufacturers usually provide extensive formulas and tables to help determine those ratings, as the guidelines and calculations can get complex and vary widely among possible applications.

Static or non-rotating loads must be analyzed for their effect on bearing structural stability, reliability, system noise, and vibration levels. Other factors that must be considered include the nature of the load, the frequency of occurrence, and the characteristics of the bearing support structure.

Using the basic load rating, C, standard formulas have been developed to predict the statistical life of a bearing under a defined set of conditions. These formulas are based on an exponential relationship of load to bearing life.

"Rating life" describes the fatigue life that may be expected from 90 percent of a given group of approximately identical bearings under equal conditions of load and speed. It's commonly referred to as L10 life, B-10 life, or minimum life, and can be calculated if C and the equivalent radial load (to be explained) are known.

Reliability and rating life concepts apply to statistical fatigue life, which can differ appreciably from the actual operating life because of external or unexpected factors. It's also possible to establish a statistical life for a group of bearings at reliability levels higher or lower than the normal 90 percent base. The bearing manufacturer should review applications requiring higher degrees of reliability.

Relationships derived from standard formulas typically show that doubling the speed of either a ball or roller bearing cuts the life in half, while halving the speed doubles the life. Doubling the load on a ball bearing reduces its life to one-eighth, while halving the load increases the life eight times. For a roller bearing, doubling the load reduces the life to one-tenth, while halving the load increases life ten times.

Applications and Load Evaluation

Rolling bearings run the gamut of practical applications, and many types of loads, whether radial, thrust (axial), or a combination of radial and thrust affect bearing performance and life.

Loading analysis converts all bearing loads to an equivalent load Pe, and permits a direct comparison with published bearing ratings. Various operating factors are applied in a formula to yield a constant stationary radial load. This representative load should result in the same bearing fatigue life attained under actual loading.

Where several bearings are being evaluated as alternatives, it may be helpful to construct a spreadsheet that accounts for the factors and calculations and establishes each bearing's equivalent radial Pe and L10 life.

Varieties of loading patterns can affect bearing selection and life. Some of these are variable loads and speeds, which usually change in a known manner; shock loads, which can cause damage if too severe; or repetitive and oscillating loads, which vary stress cycling and make bearing selection difficult. Others include unbalanced loads, which usually move in phase with a rotating shaft, and inertia loads, caused by changes in velocity or direction.

Radial bearings support shafts that transmit torque through belts, chains, gears, wheels, cranks and similar devices. The major bearing forces produced by belt and chain drives are radial, a product of the power transmitted, sprocket pitch diameter or sheave diameter, and rotational speeds. In addition, calculations for belt drives require the center distance between shafts, number, size, and type of belts, component spacing along the shaft, and direction of driving force. Tension factors available in manufacturers' tables can also be used for approximate calculations.

Gear drives may exert combined radial and thrust forces on bearings. The direction and magnitude of forces from helical, worm, bevel, and spiral bevel gears depend on the nature of the tooth contact. Because forces interacting on a bearing can require complex, multiple calculations, the bearing manufacturer is sometimes an essential consultant in the process.

How They Work

A rolling element bearing consists of an inner ring, an outer ring, balls or rollers, and a retainer or separator. The first three components support the load, while the retainer positions and guides the rolling elements. Various protective housings and sealing mechanisms may be integrated into or assembled around the bearing as well.

While the ball bearing is probably the most common and familiar, rolling element bearings also include needle, taper, cylindrical, and spherical roller bearings (which are actually barrel-shaped). There are variations of these basic types that fill special needs, such as double-row spherical roller bearings to handle heavy-duty loads.

The dynamics of a bearing include: torque, or the frictional resistance to rotation; speed, which may be limited by thermal, lubricant, or design factors; vibration, which exists in all bearings but may be amplified by other components; shaft control, with possible radial, axial or angular deviations for the intended path of movement; and thermal characteristics, especially the need to offset heat generation with heat dissipation.

Bearing components, although usually made of hardened steel, react as elastic bodies under load. A continuous flexing and wave motion of the roller material occurs as it deforms in front of and behind the moving contact zone, and the cyclic strain can eventually lead to general material fatigue. Other major factors to consider are the elastic deflection, contact stress (a complex component of surface fatigue failure), and rolling and sliding friction.

The proper raceway curvature design, established by the inner and outer rings, will help control elastic deformation, contact stress (and contact area), and rolling and sliding friction. Contact stresses are also influenced by the amount of clearance in a bearing, since a tighter fit keeps more rollers in contact with the races and distributes the load more, although a really tight fit amounts to a preloaded state (and that's a different situation). The retainer provides positive separation and guiding of adjacent rolling elements. It also limits and absorbs various dynamic forces between the rollers.

Steel used to manufacture bearing elements must possess high strength, endurance, toughness, wear resistance, dimensional stability, and freedom from internal defects. Heat treatment during manufacture lends considerably to these qualities.

Most bearing accessories help a bearing adapt to the machinery. Housings, seals, tapered adapter sleeves, locknuts, lock washers, flingers and clamp plates are among the add-ons. To meet diverse application needs, other special bearing features are available, such as self-alignment, permanent lubrication, temperature, vibration or speed sensors, high temperature operation, and custom sealing arrangements.

Failure Modes

The most common failure modes of rolling element bearings are contact fatigue, wear, plastic flow, and fracture.

These classes are further divided into very specific types of failure that can be traced back to their root cause, which may be improper installation, misalignment, contamination, inadequate lubrication, or over-loading. Or, perhaps the bearing exceeded its expected life at the rated load, although for safety many operations forego this eventuality with scheduled replacement - it is the premature failure that is cause for investigation.

Contact fatigue can occur at or beneath the surface. Wear encompasses a wide variety of damage, such as fretting, abrasion, scoring, corrosion, and electrical pitting. Plastic flow includes brinelling (indentation under excessive load or impact), cold working, and hot working. Fracture, the actual cracking or breakage of a component, is usually caused by overload, improper fitting, bending fatigue, or a component defect.

Installation, Maintenance, and Troubleshooting

The same factors that make a successful installation are also the key areas in effective maintenance - and likely suspects when troubleshooting. Some of the main considerations are mounting tightness, alignment, lubrication, overloads, and vibration.

Initial installation weighs heavily on the success of a bearing's service life. Closely following the procedures and recommendations in the service instructions provided with each bearing will ensure correct shaft seat integrity, mounting tightness, unit alignment, and maintenance.

Sealing to retain lubricant and exclude contaminants is critical to bearing life. Because a seal can be a complex assembly of flingers, lips, labyrinths, springs, and shields, the installation directions should be taken seriously - don't force or otherwise damage the seal. Cleanliness and proper alignment are both essential to bearing life and sealing effectiveness.

In a properly lubricated rolling element bearing, a thin film of lubricant separates the rolling elements from the raceways and prevents contact with the asperities, or high points of the metal surfaces. Lubricant also protects against corrosion, dissipates heat, excludes contaminants, and flushes away wear products. Lubrication for rolling element bearings must withstand higher rates of shear and mechanical working than in other mechanical components, being subject to both rolling and sliding contact and extremely high contact pressures.

Petroleum-based or synthetic lubricants are used, either in grease or oil form. Grease characteristics vary considerably, depending on application requirements. As a lubricant, oil is more versatile than grease, although it is more difficult to seal and retain in bearings and housings. It can be pumped, circulated, filtered, cleaned, heated, cooled, and atomized. Oil is better suited for severe applications that include extreme speeds and high temperatures. Both an adequate oil supply and proper viscosity are critical to bearing performance.

Synthetic lubricants are becoming more widely used, especially in military, aerospace, and other demanding fields. While they permit a broader operating temperature range and provide good lubrication beyond the limits of petroleum lubricants, they are also considerably more expensive and may have a shorter life.

Some special applications use dry lubricants, although these are too limiting and expensive for normal rolling element bearing use.

One area of particular concern is to avoid mixing incompatible lubricants that can cause premature bearing failure when combined. This is especially important when re-lubricating previously installed bearings.

Pumps and Systems, February 2007