General Bearing Design

Bearing Design & Lubrication

Introduction The components of industrial machinery requiring grease lubrication include bearings, couplings, open gears and a variety of other moving parts. The widest use of grease is in lubricating bearings, which are critical elements in equipment used in steel mills, mining, construction and transportation the industries largely determining the nations economic stability.

A bearing is a housing or support for a rotating part (a shaft rotating within a bearing) or one that moves linearly (moves axially within the bearing). A bearing may also restrain motion in some manner. Bearings are of two basic types: plain and rolling-element. Plain bearings are based on sliding motion between a stationary and a moving member. Rolling-element bearings have either balls or rollers that accommodate motion between the stationary and moving parts. In either case, a film of lubricant separating moving surfaces is essential for long service life.

Plain bearings that support loads perpendicular to their axis of rotation are called journal bearings; rolling-element bearings carrying similar loads are called radial bearings. Bearings of either type that support loads parallel to their axis of rotation are called thrust bearings.

Operating Ranges for Different Types of Bearings

There are many different types of anti-friction bearings but when all the factors relating to load spectrum and capacity, size, performance, costs, bearing life and reliability are analyzed, designers in most industries rarely deny the logic and value-analysis benefits of the tapered roller bearing. Of course, other bearing types have their accepted fields of use as discussed in the following review, but the tapered roller bearing is the most versatile of all bearing types.

Fluid Film Bearings The most widely used plain bearing. They rely on lubricant viscosity to separate the bearing surfaces. The rotating shaft drags the lubricant around forming a supporting wedge. These bearings usually need a lubrication system. The continuous lubrication acts to cool the bearing allowing high shaft speeds and heavy loads.

Rolling Element Bearings

Rolling element bearings generally consist of two rings (races or raceways) with a set of rolling elements running in their tracks. The rolling elements take the form of balls or various types of rollers. The outer race is located in a housing and the inner race on the shaft. Often one of the races is not fixed axially but is free to move to allow for shaft movement. Due to the inertia forces on the bearing shaft speeds are limited but they have an advantage over fluid film bearings as a lubrication system is generally not needed.

Porous Metal Bearings These bearings are made from a porous metal, typically sintered bronze, impregnated with lubricant. They are cheap and simple and quite sufficient for many applications. Applications include shafts for light power transmission, typically electric motors and small engines.

Dry rubbing bearings These are usually plain plastic bushes which have to be able to run with marginal or no lubrication. The main requirement of dry bearings is that the bearing surfaces have a low coefficient of friction. These bearings are used in undemanding applications such as low speed moving parts in domestic appliances, instrument and electro-mechanical devices. Bearings may be self-lubricated or externally lubricated with oil or grease. Grease is generally preferred under high loads or when good adhesion to bearing surfaces or good sealing properties are necessary. Fluid Film

Plain Bearings

A plain bearing is the most basic type and contains no moving parts . In their simplest form, these bearings consist of load-carrying cylindrical inserts, made of a material or alloy that is softer than the part that slides or moves against it. Consequently, the bearing assumes most of the wear. This is an important economic advantage because bearings are more conveniently replaced or adjusted than the relatively inaccessible moving components . Plain bearings are used mainly in applications where the loads are relatively light and the motion is relatively continuous. The prime example is crankshafts and connecting rods in internal combustion engines. To work efficiently, there must be some clearance between the stationary part and the rotating part. Extreme care needs to be taken to ensure that particle contaminants do not collect in this clearance in order that rapid wear is avoided. Regular maintenance is therefore very important with most plain bearing applications. In view of the need for frequent maintenance anti-friction bearings with rolling elements have largely replaced plain bearings in modern applications. Plain bearings can be described by their configuration, by their motion, or by the type of loading they accommodate. Thus, the major categories of plain bearings are journal, guide and thrust bearings.

Journal Bearings A journal or sleeve bearing consists of a cylindrical housing supporting a rotating shaft. The term journal refers to the portion of a shaft contained within a bearing; sleeve refers to the bearing configuration. The terms are commonly used interchangeably. If the bearing is a full-cylindrical, 360 design it is called a bushing. A shaft that is loaded in a single direction can be supported by a journal bearing in the form of a partial cylinder. Such a bearing supports the shaft in the load zone only. For example, cranes, earth-moving equipment and railroad journals use partial-cylinder bearings to support loads directed against the top portion of an axle.

Configurations of plain journal bearings Journal bearings frequently contain two or more parts to facilitate removal or replacement. Automotive engine main bearings, for instance, contain two half-round sleeves which contain the crankshaft journals. A single-piece bearing may be difficult to remove because of flanges on the shaft, which may be too large to be pulled through the bearing housing, or perhaps the shaft is simply too long or too heavy to be easily freed from the bearing. Guide Bearings

Guide bearings support reciprocating rather than rotating machine parts; loading is generally less than in a journal bearing. Interior surfaces may be grooved to help distribute lubricant and relieve pressure. Equipment using guide bearings include crossheads in steam engines and some air compressors.

Plain guide bearing accommodates linear motion

Thrust Bearings

Thrust bearings accommodate the axial movement of a rotating shaft. They are usually used in conjunction with journal bearings and are lubricated by grease, which leaks from the ends of the journal housings.

Rolling-Element Bearings The main function of any rolling Bearing type is the decreasing of friction which could be caused from any elements related rotation, in order to reach the maximum efficiency of the power, which could have been converted from torque power into heat power, so the main function for the bearings is decreasing the "converted to heat" power. A great variety of rolling bearing types and designs are known, the manifolds of the bearings is justified by their various purpose of application, each type of bearing has a certain characteristic technical properties, rolling bearings have the following advantages compared with the plain bearings:

Low starting moment. Low friction at all speeds. Low energy consumption. High reliability. Small width. Low consumption of lubricant. Long relubrication intervals. Easy to mount and dismount. Standardized dimensions.

A rolling-element bearing has balls or rollers situated between a stationary housing and a moving journal. Rollers can be cylindrical, spherical or tapered. Roller bearings with relatively long, small-diameter rollers are called needle bearings.

Rolling element bearings generally consist of two rings (races or raceways) with a set of rolling elements running in their tracks. The rolling elements take the form of balls or various types of rollers. The outer race is located in a housing and the inner race on the shaft. Often one of the races is not fixed axially but is free to move to allow for shaft movement. Due to the inertia forces on the bearing shaft speeds are limited but they have an advantage over fluid film bearings as a lubrication system is generally not needed. Typically, the outer race is stationary and the inner race is affixed to a rotating shaft. Unlike plain bearings, rolling-element bearings are made of hard steel alloys because the small rolling elements must carry a wide range of loadings and unit stresses at the contact surfaces can be very high. Most rolling-element bearings used in industry are grease lubricated.

Rolling-element bearings are frequently called antifriction bearings although the frictional torque of a full fluid-film plain bearing can be as low as that of a rolling-element bearing. However, starting friction in a plain bearing is usually higher than that of a rolling-element bearing.

The following are some of the very common types of bearings, a certain bearing type is chosen in order to make it particularly suitable for the desired applications, however it is not possible to apply general application rules for the selection of any type of bearings, as several factors must be considered and assessed in relation to each other they are all produced by most of the bearings manufacturers:

Deep Groove ball bearing Cylindrical Roller Bearing Taper Roller Bearings

Spherical roller and Needles bearings

Basic types of rolling-element bearings Ball Bearings Ball bearings are, perhaps, the most familiar type of rolling-element bearing. Radial ball bearings contain the rotating motion of a shaft and are functionally similar to plain journal bearings. Ball thrust bearings are the functional equivalent of plain thrust bearings.

All types of ball bearings have a point contact between the balls and the races. Because of this point contact for a given load capacity it is generally necessary to specify a larger ball bearing than a tapered roller bearing which distributes the load over the length of the roller (line contact). The most popular type is the deep groove ball bearing which is suitable for light radial loads only. Axial or shock load conditions should be avoided since this can lead to rapid failure. It is also important to ensure that the lubricant is clean since contaminants tend to get trapped in the race groove, which ultimately limit performance and bearing life.

A ball-bearing assembly includes balls, a retainer, races, the rotating shaft and the supporting housing. The balls are made of hardened steel, ground to a true sphere, and polished to a fine finish. The balls are held in position by a retainer, or spacers, and roll between races which must also be ground and polished to a fine finish. This type of bearings divided into two categories

Deep groove ball bearings are supplied as an assembly with inseparable races; they necessarily have a radial clearance which can only be adjusted by the fits of the inner and outer races. They are mostly used where size, load capacity and radial clearance are not important, but where ease of assembly and low cost are.

1- Single row deep groove ball bearings

Single row deep groove ball bearings are:

simple design;

carry considerable axial loads in either direction, even at high speeds; and

little attention in service.

Shielded deep groove bearings are intended to be used in any applications where the inner ring is to rotate. The steel shield forms a small gap with the inner ring shoulder, oil and rubber wear-resistant synthetic rubber, seal against a recess in the inner ring.

Single row deep groove ball bearings have limited ability to accept any misalignment.

Single row deep groove ball bearings are the most popular of all the ball bearings in use today, because they are various types and are cost effective relative to performance.

Radial ball bearings Radial ball bearings use a versatile design that permits relatively high-speed operation under a range of load conditions. Bearings consist of an inner and outer ring with a cage containing a complement of precision balls.

The standard Conrad-type bearing has a deep-groove construction capable of handling radial and axial loads from either direction. The maximum-capacity type supports primarily radial loading.

An extremely wide variety of sizes is available in extra-light to heavy series. Various shield and seal configurations help protect internal bearing components and retain lubricants.

Angular contact ball bearings The angular contact ball bearing is designed to overcome some of the limitations of the deep groove ball bearing, by improving its ability to cope with the combined thrust loads as well as radial loads. Although load capacity is increased compared to the deep groove ball bearing, it is significantly less than the equivalent dimensioned tapered roller bearing (see Fig.). Single row angular contact ball bearings

are of non-separable design, usually arranged so that they can be adjusted against a second bearing. They are not very tolerant of any misalignment between shaft and housing and this can have a serious effect on bearing life. Single-row bearings have high thrust capacity in one direction. Some single-row bearings are specifically designed for duplex mounting in sets for maximum performance. Double-row Conrad bearings can accommodate thrust in both directions.

The special geometry of angular contact bearing raceways and shoulders creates ball contact angles that support higher axial loads. Different designs offer contact angles ranging from 20 to 40. Higher angles provide more axial load capacity for longer service life operating under axial and radial loads.

2- Double Row Angular Contact Ball Bearings

Double Row Angular Contact Ball Bearings correspond in function to two single row bearings arranged back-to-back. These bearings can accommodate axial loads in both direction, as well as, tilting moments.

Bearings with one-piece inner ring have a filling slot at one side. If directional axial loads must be accommodated, the bearings should be arranged so that the axial load acting on the shaft is not directed towards the filling slot.

Angular misalignment can only be accommodated between the ball bearing and the raceways by force, however, this produces increased ball bearing loads and may lead to a reduction of bearing life

Roller Bearings A roller-bearing assembly consists of rollers, a retainer, races, a shaft and a bearing housing and seals. As with ball bearings, the contact surfaces of roller bearings must have a fine surface finish that perform with maximum efficiency.

Cylindrical roller bearings

The most common use of cylindrical roller bearings is at non-locating positions where it is necessary to accommodate thermal expansion effects by allowing axial displacement (floating) of the shaft relative to the housing. Inner and outer races are separable (which facilitates mounting and dismounting), and the cylindrical rollers have line contact with the races so they can carry more radial load than the point contact of ball bearings. The cage must align and retain the rollers which necessitates a heavier cage and fewer rollers when compared to a tapered roller bearing of similar size, and hence a lower load capacity. The rollers are not true cylinders but are usually crowned or end relieved to reduce stress concentrations at the ends of the roller-race contact. Designs are also available to carry a limited axial load by integrating additional flanges on the inner or outer race. However, the full axial thrust must be taken between the flanges and the roller ends which can lead to high stress concentrations . Cylindrical roller bearings can carry heavy radial loads and operate at high speeds.

Tapered roller bearings

Because of their geometry and design features, tapered roller bearings provide several important and unique performance characteristics enabling them to meet a wide range of application requirements. Tapered roller bearings consist of four basic components. These are the inner race (cone), the outer race (cup), tapered rollers and a cage (roller retainer) (Fig. 1). Under normal operating conditions the inner race, outer race and rollers carry the load while the cage spaces and retains the rollers. The inner race, roller and cage is referred to as the 'inner race assembly' and this is usually separable from the outer race, facilitating equipment build.

Fig 1 Tapered Roller Bearing , the tapered roller bearing can handle both radial and thrust loads

The tapered roller bearing combines the benefits of all the other bearing types as well as offering additional advantages :

Because of its tapered roller/race geometry, a tapered roller bearing can carry both heavy radial and thrust loads or heavy combined loads than angular contact ball bearings(Fig. 1)

Excellent load carrying capacity/cross section ratios provide economic bearing arrangements. A longer relative life for a given bearing size Reduced bearing size for a given load capacity. high reliability and low operating temperatures Adjustability : for optimum performance either endplay or preload values can be specified according

to the design requirements. Tapered roller bearing bearings are less sensitive to misalignment and offer long life Less sensitive to contaminated environments due to the natural pumping action which forces any

particle contaminants out of roller/race contact area. Low friction coefficient and high speed capabilities due to true rolling motion, when compared to

other roller bearing types. Simple mounting and dismounting with separable inner and outer races. A choice of mounting arrangements to suit the loading aspects and design constraints of an

application. Lower price for a given calculated fatigue life This adds up to optimum bearing system performance for virtually any application, together with cost

effective design and manufacture of equipment, as well as reduced maintenance and longer life.

Axial loads are only supported in one direction. Account must be taken of the fact that due to the angle of the rollers radial loads will generate axial forces. To counteract this bearings are usually employed in pairs, either face to face or back to back. The bearings can then be adjusted against each other to provide preload which determines the internal clearance. This versatile bearing is especially popular in the automotive industry.

Spherical roller bearings

The self aligning feature of spherical roller bearings allows minor angular displacements between shaft and housing to be accommodated. They have a high radial load carrying capacity, but under heavy load the stress is not evenly distributed and true rolling motion only occurs at two contact points on each roller. This naturally induces skidding along the roller length and therefore the spherical roller bearing has a higher coefficient of friction and lower speed capabilities than other types of roller bearings. This adds up to optimum bearing system performance for virtually any application, together with cost effective design and manufacture of equipment, as well as reduced maintenance and longer life.

The cage must be of an extremely robust construction to counteract the roller skewing effect which increases the cage moment of inertia and limits the number of rollers. It is best suited to applications where there is a risk of misalignment at assembly, and where speed and deflection criteria are not exacting. Mounted on adapter or withdrawal sleeves and housed in plummer blocks they present economic bearing arrangements. Also available with seals for maintenance-free operation. . However, they are somewhat speed limited. Needle roller bearings Needle roller bearings are similar to cylindrical roller bearings but with long, thin rollers, giving them a very compact cross-section. Needle bearings, containing cylindrical rollers with a high length-to-diameter ratio, provide the highest load capacity for a given radial space of any rolling-element bearing. They are very adaptable and have a high radial load capacity in relation to their sectional height, but can cope only with very light axial loads. This type frequently has no inner race and can accommodate oscillating motion. Needle bearings without a retainer, or cage, and a full-complement of rollers provide high load capacity, but are speed limited. Needle bearings with a retainer contain fewer rollers and, therefore, have a lower load capacity, but can operate at higher speeds. Wide variety of designs including combined bearings for radial and axial loads providing simple, compact and economic bearing arrangements. Typical applications are in the synchromesh mechanisms of automotive gearboxes, and as planetary gear bearings in light duty epicyclic hub-reduction units.

Lubrication of Bearings

Proper lubrication is essential to successful performance of any bearing and necessarily includes the selection of an adequate type of lubricant, the right amount of lubricant and the correct application of the lubricant on the bearing.

The fundamental functions of a lubricant are as follows :

To prevent metallic contact between the rolling elements, raceways, and cage To prevent the bearing from corrosion and wear To separate mating surfaces and reduce friction. To transfer heat (with oil lubrication).

To reduce rolling and sliding friction.

To protect the highly finished surfaces of the rolling elements and races against corrosion and pitting. This is extremely critical to service life.

Sealing and cooling of the bearing.

To act as a sealant.

These functions include consideration of the lubrication and generated film thickness on the raceway (simulated according to elastohydrodynamic effects) and on rib/roller end contact.

Bearings may be lubricated with grease or oil-in special cases with a solid lubricant-the choice of which depends primarily on the temperature range, operating speeds, and loading conditions of the bearings concerned. All lubricants must be changed from time to time because their properties deteriorate as a result of aging and contamination of the lubricants. The limited speeds for both grease and oil lubrication are usually given in the bearing tables.

Grease Lubrication

Grease lubrication is generally used for bearings operating under normal conditions. Grease has certain advantages over oil because it's easily retained in housing as well as its sealing effect against the entry of moisture and outside impurities. In general, the bearing should only be 1/3 to 1/2 partly filled with grease because over filling will cause rapid temperature rise, particularly if speeds are high.

Oil Lubrication

Oil lubrication is generally used when high speeds or operating temperatures are beyond the effective range of greases, or when it is so designed that heat developed in the bearing must be transferred away through a lubricating oil circulation system. Viscosity of the lubricating oil is the main factor to be considered to suit the diverse applications with respect to speed, temperature, and loading conditions. Oil lubrication is effective; however, its oil feeding and sealing devices must be provided. Some of them are recommended as follows:

Lubrication of Plain Bearings

The mode of lubrication of a plain bearing depends on the conditions that affect the bearings ability to develop a load-carrying fluid film to separate the journal and bearing surfaces. If such a film is not produced (or before it is produced), the lubrication mode is termed boundary or mixed-film lubrication where the surfaces are not completely separated and some metal-to-metal contact occurs. If a lubricating film is formed with sufficient pressure to separate the journal and bearing surfaces, the lubrication mode is termed hydrodynamic or full fluid-film lubrication.

Hydrodynamic Lubrication

Keeping a liquid film intact between surfaces moving with respect to each other is generally done mechanically, as by pumping. In a cylindrical journal and bearing, the rotary shaft acts as a pump to maintain the lubricant film. The journal floats on a film of oil with an equilibrium thickness established between oil input and oil leakage (mostly at the bearing ends).

The equilibrium thickness of the oil film can be altered by:

Increasing load, which squeezes out oil. Increasing temperature, causing more oil leakage. Changing to a lower viscosity oil, which also causes more oil leakage. Reducing journal speed, which generates a thinner oil film.

Lubrication of a journal rotating in a cylindrical bearing offers the classic example of the hydrodynamic theory of bearing friction, as described by Osborne Reynolds in 1886. The theory assumes that under these conditions, friction occurs only within the fluid film, and is a function of fluid viscosity.

As speed increases, the wedging action of the lubricant moves in the direction of rotation, and pressure within the film becomes greater so the journal is now riding on a full fluid-film and hydrodynamic lubrication is reached. If loading on the bearings is increased sufficiently, the hydrodynamic film may collapse and the bearing will revert to the boundary-lubrication mode.

Elastohydrodynamic Lubrication

As pressure or load increases, viscosity of the oil also increases. As the lubricant is carried into the convergent zone approaching the contact area, the two surfaces deform elastically due to lubricant pressure. In the contact zone, the hydrodynamic pressure developed in the lubricant causes a further increases in viscosity that is sufficient to separate the surfaces at the leading edge of the contact area. Because of this high viscosity and the short time required to carry the lubricant through the contact area, the lubricant cannot escape, and the surfaces will remain separated.

Load has little effect on film thickness because at the pressures involved, the oil film is actually more rigid than the metal surfaces. Therefore, the main effect of a load increase is to deform the metal surfaces and increase the contact area, rather than decrease the film thickness.

The principles governing the lubrication of rolling-element bearings differ from those of plain bearings. In a full fluid-film plain bearing, the journal load is supported by a continuous hydrodynamic lubricant film which keeps the two contact surfaces separated.

In a rolling-element bearing, unit pressures are extremely high between the relatively small rolling elements and their raceways. Lubricants subjected to the high pressure within the contact zone of a rolling-element bearing undergo a dramatic increase in viscosity. This enables the lubricant film to withstand the high contact stresses while preventing contact between the rolling surfaces. Pressures of this magnitude do not exist in a full fluid-film plain bearing and lubricant viscosity is unaffected.

The high contact pressures in a rolling-element bearing also elastically deform the rolling surfaces to enlarge the contact area which supports the load. The combination of surface deformation and hydrodynamic lubricating action produces a thin, elastohydrodynamic (EHD) lubricant film which provides for lubrication within the contact zone of rolling-element bearings.

The two major considerations in EHD lubrication are :

The elastic deformation of the contacting bodies under load.

The hydrodynamic effects forcing the lubricant film to separate the contacting surfaces while the pressure is deforming them.

Elastohydrodynamic (EHD) lubrication

Film thickness on the raceway

The importance of the elastohydrodynamic lubrication mechanism lies in the fact that the lubricant film thickness between the two contacts can be related to the bearing performance. The thickness of the generated film depends on the operating conditions such as :

Velocity Loads Lubricant viscosity Pressure/viscosity relationship.

Analytical relationships for calculating the minimum and the average film thickness have been developed : Minimum film thickness (based on Dowson Equation) :

hmin = KD (oV)0.7a0.54W-0.13R0.43

where : hmin = minimum lubricant film thickness KD = constant containing moduli of elasticity o = lubricant viscosity at atmospheric pressure V = relative surface velocity a = lubricant pressure viscosity coefficient W = load per unit length

R = equivalent radius Average film thickness (based on Grubin Equation) :

h = 0.039 (Va)0.728 (P/ l)-0.091 (S 1/ R)0.364

where : h = lubricant film thickness (mm) = viscosity of lubricant V = surface velocity a = lubricant pressure viscosity coefficient P = load between inner race and rollers l = effective length contact between rollers and inner race S 1/ R = sum of inverses of contact radii

The major factors influencing the lubricant film thickness are viscosity and speed whereas load has less importance. These thin EHD films are often not much larger than the surface roughness height.

The fatigue life of a bearing is related in a complex way to speed, load, lubrication, temperature, setting and alignment. The lubricants role in this interaction is determined primarily by speed, viscosity and temperature; and the effects of these factors on bearing fatigue life can be dramatic. For example, in a test program, table 5-A, two bearing test groups were subjected to conditions of constant speed and load. Differing film thicknesses were achieved by varying operating temperature and oil grade, and thereby, oil operating viscosity. Life was dramatically reduced at higher temperatures, with lower viscosity, and thinner resultant films

Film thickness at rib/roller end contact The contact between the large end of the roller and the inner race rib is described as an elastohydrodynamic contact or a hydrodynamic contact (elastic deformations are negligible). As the roller/rib loads are much lower than the roller/race loads, the film thickness at the rib/roller end contact is usually larger than the film thickness on the roller/race contact (approximately 2 times more). Nevertheless, in severe conditions, scoring and/or welding of the rib/roller asperities can occur. This may be related to speed, oil viscosity, load or inadequate oil supply to the rib/roller end contact. In these conditions, the use of Extreme Pressure (EP) lubricant additives may help prevent scoring damage.

Grease lubrication: Greases provide a lubricating film on the surfaces of rolling elements, separators, and raceways. The lubricant is actually a thin film of oil which is released as the three-dimensional fibrous network within the grease is ruptured under shear. Only that portion of the grease in intimate contact with moving surfaces breaks down; the balance remains intact and functions as a sealant.

When a freshly charged bearing begins to rotate, grease is thrown from the rolling elements and is rapidly circulated through the housing. After a short time, grease from the rotating outer race is then thrown back onto the rolling elements where shearing takes place. This turbulent environment at the outset of rotation creates frictional heat which attains a maximum and then gradually diminishes as the continual shearing action releases a lubricating oil film. As lubrication takes effect, the temperature of a properly charged bearing will drop and assume equilibrium.

WARNING: Bearing lubrication is critical. Failure to maintain proper lubrication can result in equipment failure, creating a risk of serious bodily harm.

Boundary Lubrication

The simple assumptions made in discussing fluid film lubrication are hardly ever valid in practice. Under certain conditions -- such as shock loading, steady heavy load, high temperature, slow speed, and critically low viscosity -- the lubricant system no longer remains in the hydrodynamic regime. A situation arises wherein there is intermittent contact between the surfaces, resulting in a significant rise in temperature and subsequent destruction of the contacting surfaces. Under these circumstances, the fluid film is no longer capable of adequately protecting the surfaces, and other approaches must be employed such as adding film-forming additives. Because of the generation of relatively high levels of friction and heat, and the resulting high rate of surface wear, boundary lubrication is not the most desirable mode of operation. However, at times, it is completely unavoidable.

Grease should be introduced to the bearing where fluid pressure is least at the point of maximum clearance within the bearing. Grooves are often incorporated in the interior surface of a journal bearing to relieve pressure and to store reserve lubricant. When loading is in one direction, axial grooves running lengthwise on the bearing surface and located in areas of low pressure will not disturb the lubricating film and can relieve pressure.

When the direction of loading is variable, the location of pressure extremes within the bearing is also variable. Under these conditions, well spaced annular or circumferential grooves will relieve pressure without substantially interrupting lubricating films. Axial grooves should be beveled so lubricating grease is more easily swept from the groove by the rotating shaft.

Lubrication of Rolling-Element Bearings

The lubricant in a rolling-element bearing has three functions:

1. To reduce rolling and sliding friction.

2. To protect the highly finished surfaces of the rolling elements and races against corrosion and pitting. This is extremely critical to service life.

3. To act as a sealant.

Generally , rolling bearings are lubricated with grease or oil. Grease is generally preferred as it is easier to retain in the housing, provides a better barrier to contaminants and is less likely to drain away from the bearing surfaces, therefore providing more reliable lubrication. Oil, on the other hand, is a better lubricant and is essential for high speed and high temperature operation (over 100 C). Bearings are greased by completely filling the bearing (not the housing). They are then occasionally re-lubricated to maintain maximum efficiency. The simplest form of oil lubrication is an oil bath. The oil level should almost reach the centre of the lowest rolling element. This method is acceptable provided that the operating temperature is not excessive. Another common and simple method is oil splash. This method is frequently used in applications such as gearboxes. If the operating temperature excludes the use of an oil bath/splash then an oil circulation system or oil injection system must be used. This aids in the cooling of the bearing.

Sources of friction Rolling action is the predominant source of friction in a rolling-element bearing. Rolling friction arises from metal deformation when a ball or roller under load moves along the surface of a race. A buildup of deformed material preceding the rolling element offers resistance to motion which, in turn, creates frictional heat. Other

less prominent sources of frictional heat in rolling-element bearing include sliding, slippage, and abrasive action.

A limited amount of sliding friction results when the spacers between rolling elements contact the raceway. Friction also arises from slippage of rolling elements. This occurs in the unloaded region of a bearing where clearance between rolling elements and raceway is a maximum. Slippage also increases with decreasing speed because the reduction of centrifugal force on the rolling elements results in greater clearance. Friction can also result from rusting or corrosion of metal surfaces which produces abrasive oxide particles.

Oil Bath Lubrication Oil bath lubrication is widely used method in the case of low or medium speeds. The oil is picked up by the rotating bearing elements. The oil should be at the centre of the lowest rolling element. It is desirable to provide a sight gauge so the proper oil level may be maintained.

Dripping Oil Lubrication

This method is suitable for the application where a small quantity of lubricating oil is constantly fed into the bearing without interruption. The excessive amount of oil dripped may cause a rise in the temperature of the bearing.

Oil Jet Lubrication

Oil jet provides a very effective lubricating method for high speed applications. It is important to ensure that sufficient amount of oil will reach the bearing components and will be able to dissipate the heat generated by friction. The velocity of the oil jet, usually 15 M/S, must enable some of the oil to penetrate through the turbulent air membrane surrounding the bearing. The position of the oil jet should be placed between the inner race and cage of the bearing.

Oil Mist Lubrication

This method is often used for high speed applications, such as grinding spindles. The oil mist is produced in an atomizer. Dry compressed air, after filtered, is used in the oil lubricator. The oil is

then introduced into the bearings. The air current will also serve to cool the bearing, and its slightly higher pressure in the housing will also prevent impurities from entering. The small quantity of oil can be regulated so that the lubricant friction is practically negligible.

Solid Lubrication

Sometimes it is found that the addition of a small amount of solid lubricant, such as MoS2, into the grease will greatly improve the lubricating properties. In some special cases where very high temperature or high vacuum prevails, solid lubrication will be the solution. Oil Circulation System Circulating lubrication is commonly used for high speed operation requiring bearing cooling and for bearings used at high temperatures. Oil is supplied by the pipe at the top, it travels through the bearing, and drains out through the pipe an the left. After being cooled in a reservoir, It returns to the bearing through a pump and a filter. The oil discharge pipe should be larger than the supply pipe so that an excessive amount of oil will not back up in the housing . This method of lubrication is the common practice of spindle design for high speed precision machine tools.

Oil Injection System Jet lubrication is often used for ultra high speed lubrication such as bearings in jet engines with a dm.n value (dm: pitch diameter of rolling element set in mm; n: rotational speed in rpm) exceeding one million. Lubricating oil is sprayed under pressure from one or more nozzles directly into the bearing. In the diagram the lubricating oil is sprayed on the inner ring and cage guide face. In the case of high speed operation, the air surrounding the bearing rotates with it causing the jet to be deflected. The jetting speed of the oil from the nozzle should be more that 20% of the circumferential speed of the inner ring outer surface

Oil Splash Lubrication With this lubrication method, oil is splashed onto the bearings by gears or a simple rotating disc installed near bearings without submerging the bearings in oil. It is commonly used in automobile transmission and final drive gears. The figure shows this lubrication method used on a reduction gear

The high contact pressures in a rolling-element bearing also elastically deform the rolling surfaces to enlarge the contact area which supports the load. The combination of surface deformation and hydrodynamic lubricating action produces a thin, elastohydrodynamic (EHD) lubricant film which provides for lubrication within the contact zone of rolling-element bearings.

Grease lubrication: Greases provide a lubricating film on the surfaces of rolling elements, separators, and raceways. The lubricant is actually a thin film of oil which is released as the three-dimensional fibrous network within the grease is ruptured under shear. Only that portion of the grease in intimate contact with moving surfaces breaks down; the balance remains intact and functions as a sealant.

When a freshly charged bearing begins to rotate, grease is thrown from the rolling elements and is rapidly circulated through the housing. After a short time, grease from the rotating outer race is then thrown back onto the rolling elements where shearing takes place. This turbulent environment at the outset of rotation creates frictional heat which attains a maximum and then gradually diminishes as the continual shearing action releases a lubricating oil film. As lubrication takes effect, the temperature of a properly charged bearing will drop and assume equilibrium.

Choice of lubricant

The choice between oil or grease lubrication depends on the relationship of journal speed to viscosity. Slower journal speeds have higher viscosity requirements while high speeds call for a light-bodied oil. Bearings designed for low speed operation usually have a relatively large clearance between shaft and housing, while high-speed bearings usually have a much smaller clearance.

Rolling Element Bearing Applications

Bevel Gear Pinion

Requirements = Pinion must be accurately located axially and have a high radial capacity at the gear end

LH bearing = X arranged taper roller bearings locate the shaft and carry the high axial load. Spacers accurately control the axial clearance in the bearing pair. X arrangement is less sensitive to misalignment allowing the roller bearing to locate the shaft radially.

RH bearing = NUP roller bearing has a high radial load capacity. The NUP design requires no locating shoulders that would make assembly difficult. The sliding fit is in the housing

Car Wheel

Requirements = Wheel must be accurately located with small clearance in the bearings

LH bearing = The O arrangement allows the bearing clearance to be adjusted through the inner race. Since the load direction is stationary relative to the shaft the inner race can be a sliding fit.

RH bearing = O arranged taper roller bearings resist side forces on the wheel better than X arranged bearings. This is important due to the short shaft.

Electric Motor

Requirements = Simple and quiet. Axial forces are low but shaft speed is high

LH bearing = Deep groove ball bearing axially locates the shaft

RH bearing = This is the output end of the shaft and the bearing floats in the housing

Bearing housings differ in design depending on the application and serve to support the bearing and contain the lubricant. Suitable seals are usually provided to exclude water, dust, dirt or other external contaminants from the bearing components and to prevent leakage of the lubricant from the housing.

Rolling Element Bearings Selection The table below demonstrates the features of each bearing type enabling a suitable bearing to be chosen for a particular application.

Having chosen the bearing type the bearing size can be calculated. A bearing is selecting based on its quoted value of static load carrying capacity or dynamic load carrying capacity. If the bearing is stationary for long periods or rotates slowly and is subject to shock loads then the bearing selection procedure is based on the Static Load Carrying Capacity. For more continuous operation the bearing is selected on the Dynamic Load Carrying Capacity.

Static load Carrying Capacity The Basic Static Load Rating given for each bearing in bearing catalogues is based on the stationary axial and radial forces acting on the bearing.

When bearings are subject to both radial and axial loadings the equivalent static load must be found thus:

Po = XoFr + YoFa

If only radial forces act then

Po=Fr

Where Po = The equivalent static bearing load (N) Fr = static radial load on the bearing (N) Fa = static axial load on the bearing (N) Xo = static radial factor Yo = static axial factor

N.B. The values of Xo & Yo are given in the Bearing Data

The Basic Static Load Rating Co can be calculated from

Co = SoPo

where Co = basic load rating (N) , o = static safety factor and Po = equivalent static bearing load (N)

Values of So depend on the type of bearing and the requirements regarding quiet running.

Guidelines on Static Safety Factor

If the bearing is stationary for long periods or rotates slowly and is subject to shock loads then the bearing selection procedure is based on this basic load rating. Values of Co for each bearing are quoted in the bearing catalogues. Choose a bearing whose quoted value for Co is equal to the required value of Co calculated above.

Dynamic Load Carrying Capacity The dynamic load carrying capacity of a bearing is dependent on the dynamic forces acting on the bearing as well as the basic static forces. The first step is therefore to calculate the Static Load Rating before continuing with the following procedure.

If the bearing is subject to radial and axial forces then an equivalent dynamic bearing load must be calculated.

P = XFr + YFa

P = The equivalent dynamic bearing load (N) Fr = static radial load on the bearing (N) Fa = static axial load on the bearing (N) X = radial factor Y= axial factor

When Fa = 0 or Fa is relatively small up to a limiting case of Fa/Fr = e (where e is a certain limiting value) then

P = Fr

N.B. The values of X, Y & e are given in the Bearing Data

Once a value for the equivalent dynamic bearing load is obtained it can be used to calculate the dynamic load rating of the bearing. This value is used to select the bearing. Each bearing in the bearing catalogue has a quoted value for dynamic load rating and so a bearing should be chosen that has a higher rating than the one calculated.

The dynamic load rating that is quoted in the catalogues for each bearing is dependent on the required life of the bearing and the equivalent dynamic bearing load (P).

The ISO equation for basic rating life is

Where L = basic rated life, Millions of revolutions C = basic dynamic load rating P = equivalent dynamic bearing load p = exponent in the life equation p = 3 for all ball bearings p = 10/3 for all other roller bearings

Basic Rated Life of Bearings

The basic rated life (defined as the number of revolutions that 90% of a group of identical bearings would be expected to achieve) is determined from the length of life that is required of the bearing. Typical life expectancies of required of various machines are given below.

Machine Usage Hours intermittent- domestic machines 300-3000 short periods- hand tools, construction machines 3000-8000 high reliability for short periods- lifts, cranes 8000-12000 8 h/day partial use gears, motors 10000-25000 8 h/day full use machine tools, fans 20000-30000 continuous use 40000-50000

N.B. The Following figure shows that the same calculated fatigue life as for ball or cylindrical roller bearings can be achieved under the same combined axial / radial load by a tapered roller bearing of a much smaller bore ; alternatively, a tapered roller bearing of the same bore can achieve much greater fatigue life.

Fig. 2 The calculated fatigue life for ball , cylindrical roller bearings and a tapered roller bearing of a much smaller bore

True rolling motion

The extensions of the raceways and rollers of a tapered rollerbearing are designed to converge at a common point on the axis of rotation (fig. 2). This results in true rolling motion of the rollers on the raceways, at every point along the roller body.

Fig. 2 On-apex design results on true rolling motion at all points along the roller body.

Radial & thrust load capability

The angled raceways allow the tapered roller bearing to carry combinations of radial and thrust loads. The greater the angle between the outer race and bearing center line the greater is the ratio of thrust to radial load capacity (fig. 3). Long line roller/race contact gives the tapered roller bearing a high load carrying capacity. This and the capability to carry radial loads, thrust loads, or any combination of the two, makes tapered roller bearings the ideal choice for many applications. For a given bore it is possible to select a specially light or heavy section to meet application load/duty requirements (fig. 4)

Fig. 3 Designs to support radial and thrust loads in Fig. 4 Designs to suit the space available

Combination

Bearing damage Analysis

Recognize and preventing Damage

Recognizing and preventing damage can dramatically increase bearing life and decrease the potential for improper handling, installation and adjustment. Additionally, it can highlight other conditions associated with the system design and/or operation. Our service engineering team can help customers apply proper practices in their plants, helping to decrease downtime

Damage to bearings while handling before and during installation and damage caused by improper installation, setting and operating conditions are, by far, the causes of the largest percentage of premature trouble.

In the following, examples are shown of the most common types of damage and some of the causes of this damage.

In many cases the damage is easily identified by the appearance of the bearing, but it is not easy and sometimes it is impossible, to determine the exact cause of that damage. As an example, a bearing with scored and heat discolored roller ends and rib is easily identified as a burned up bearing and damaged beyond further use. The cause of the burning or damage, however, might be traced to any one of a number of things such as insufficient or improper lubricant. It may be the wrong type of lubricant or the wrong system for supplying lubricant. Perhaps a lighter or a heavier lubricant is needed or an extreme pressure type of lubricant rather than a straight mineral oil and a circulating oil system needed rather than an oil level or splash system. This type of damage could be caused by excessively tight bearing setting or a combination of too tight setting and inadequate lubrication.

From this it can be seen that simple examination of a bearing will not reveal the cause of the trouble. It can reveal if the bearing is good for further service, but often it is necessary to make a thorough and complete investigation of the mounting, installation and parts affecting the bearing operation to determine the cause of the damage. Unless the true cause of the damage is found and corrected, the replacement bearing will be damaged in the same manner and again there will be premature trouble. This information is not an attempt to make "trouble shooters" or "bearing experts" of all who read it. It is intended to caution users about possible causes of damage and alert them to take preventive action. With proper precautions during the handling, assembly and operation of bearings, almost all damage can be prevented. It is much easier, and a great deal less expensive, to prevent damage than to determine and correct the cause of damage after the machine or equipment is in operation.

Typical modes of failures

Mode of contact fatigue

Geometric stress concentration Geometric stress concentration fatigue results from locally increased stress at the ends of roller/race contact.

Point Surface Origin (PSO) PSO is fatigue damage that has its origin associated with surface asperities, which act as local stress concentrations.

Peeling This type of fatigue is characterized by a shallow < 2.5 m m (0.1 m in) deep, spalling which sometimes occurs locally around bruises, grooves, or ends of roller/race contacts where the EHD film is lost by leakage.

Transverse cracking fatigue

a) Non-propagating spall

Inclusion origin spall b) Spall propagated by hydraulic pressure Damage by mechanisms other than contact fatigue

Abrasive Wear

Spalling Wear from foreign material. Debris bruises on all contact surfaces due to hard particles in the lubricant

Brinelling Brinelling is the plastic deformation of bearing element surfaces due to extreme or repeated shock loads.

False brinelling False brinelling is recognisable by the grooves worn into the raceways by axial movement of the rollers

during transportation. Cage damage

Cage breakage

Results of good practices

In the preceeding comments, the results of bad handling, improper assemblies, adjustments and operating conditions have been stressed and the resulting damage shown. The following image shows what happens when there is good lubrication, good assembly and maintenance and the proper fitting practice for the bearing application has been followed. This bearing shows that, with reasonable care in machining the parts and in the assembly and maintenance, it is not difficult to get excellent life. This bearing operated for over 400,000 km (250,000 miles) in a bus and is still in excellent condition and probably would run for many more kilometres.

How to determine probable causes

PLAIN BEARING FAILURES PLAIN BEARING DAMAGE

The identification of the cause of damage or failure in plain bearings is crucial before remedial action can be taken to prevent further recurrence.

The first step is a visual examination of the damaged component. The following photographs can then be used to find a match. "A picture is worth a thousand words." However, do not stop at this stage. More than one mechanism of failure can lead to superficially similar bearing damage. Moreover, once failure has been initiated, the original mechanism may lead to other failure mechanisms and it is essential to identify the original cause.

We are primarily concerned with bearings from industrial machines. The vast majority of these are designed to operate with a hydrodynamic lubricating film and are based on white metals (Babbitts) - alloys of tin and lead. Most of the photographs are of damaged white metal bearings, but some examples of other materials are included.

White metals are chosen because they are of low hardness and melting point, so that, in the event of failure, damage should be limited to the bearing, a comparatively cheap and readily replaced component, leaving the expensive rotor unharmed. These desirable characteristics, however, place limits on their operation (see below). The effect of temperature on the hardness of white metals, and hence their ability to carry load, is critical:

Suggested Temperature Limits for Whitemetal Bearings

OPERATING LIMITS

The Figure below gives a working envelope for steadily-loaded journal bearings in terms of the main operating parameters: load, speed, lubricant viscosity and bearing geometry.

Load Capacity Diagram for Steadily-Loaded Plain Journal Bearings

(EDSU 66023 )

The Figure below gives a working envelope for steadily-loaded thrust bearings in terms of load capacity and operating speed.

Limits of Safe Operation

Note. Both these figures are for steadily-loaded operation. Under alternating load conditions, such as occur in the bearings in reciprocating machines or in the bearings of rotating machines subject to vibration (rotor out-of-balance, rotor instability), a separate limitation is imposed by the fatigue strength of the bearing material. In the case of reciprocating machines, higher strength bearing materials can be used, but with the risk of damage to the bearing journal.

Index of Failure and Damage Mechanisms

1. Lubrication Breakdown

2. Wear

3. Fatigue

4. Cavitation Erosion

5. Fretting Damage

6. Chemical Effects

7. Thermal Ratchetting of Tin-Rich White Metal

8. Electrical Damage

9. Incompatible Materials

10. Manufacturing and Assembly Errors

1. Lubrication breakdown

1. Start-up wiping damage

2. Excessive load at start-up

3. Loss of clearance in tilting-pad journal bearing at start-up

4. Typical wiping failure

5. Lubricant breakdown with lead-bronze bearing

6. Summary

1.1 Start-up wiping damage

Minor rub in bottom of bearing subject to a vertical load

Main Characteristics

Minor wiping - melting and resolidification of white metal at right hand side of damage patch (the black axial mark is a photographic artefact and should be ignored) - in centre of bottom half of bearing subjected to a vertically downward load.

Cause

Damage is caused by rubbing of journal before hydrodynamic lubricating film has developed.

Note

When hydrodynamic film is present the load-line, and the minimum film thickness, is displaced in the direction of rotation. Hence any damage due to small dirt particles, transient lubrication film breakdown is displaced from the centre of the loaded half of the bearing.

Possible Confusing Damage

Fretting damage caused by vibration from an external source when machine is not operating with the journal resting in the bearing also occurs on the bottom centre line.

Comments

This type of damage is limited to heavily loaded bearings, particularly in machines subject to frequent starts. The main effect is optical with the damage no more than a witness mark; it has no effect on the performance of the bearing and is only found when the bearing is examined at overhaul.

Minor wiping or plastic deformation can be a desirable feature, assisting run-in, the bearing generating its own profile to suit the operating conditions.

Similar witness marks may also be found at the edges of a bearing where there is misalignment or manufacturing error.

1.2 Wiping damage caused by excessive load at start-up

Wiping of white metal thrust pad caused by successive starts under escessive load


Obvious melting of bearing metal that has been carried over the trailing edge of the pad and solidified in layers where subjected by cool oil flowing between the pads.

Cause

Excessive thrust load at starting has meant that the bearing metal has been subjected to rubbing and frictional overheating before the development of hydrodynamic lubricating film. The molten white metal has resolidified on coming into contact with the cool oil flowing between the pads. The photograph shows successive layers of resolidified white metal following a number of starts.

Comment

If load at start up cannot be reduced, the only cure is to use a larger bearing.

1.3 Loss of clearance in tilting-pad journal bearing at start-up

Loss of clearance in five pad, tilting-pad journal bearing

at high-speed start-up causing wipe at the centres of all the pads


Small patch of wiping in centre of all of the pads.

Cause

Establishment of thermal equilibrium in a tilting-pad journal bearing takes some time because of the restricted thermal path through the pad pivots. With rapid start-up, sufficient radial thermal

expansion can occur in the pads to take up the radial clearance, preventing the formation of a hydrodynamic lubricating film and causing wiping at the centres of the pads.

Comment

This type of failure can be avoided by extending the run up time to full speed or by increasing the radial clearance in the bearing if this is acceptable for the performance of the bearing for the normal operating conditions.

1.4 Typical wiping failure

Wiping of journal bearing showing resolidified white

metal in the central circumferential oil groove


The photograph shows a typical wiping failure with the molten white metal from the bearing surface solidifying in a cooler part of the bearing, in this case the central oil supply groove.

Cause

Wiping is caused by loss of the hydrodynamic lubricating oil film during running. There are a number of reasons why this can occur. These are discussed in the summary sheet at the end of this Section.

Possible Confusion Damage

Wiping can be a secondary event initiated by fatigue damage or electrical erosion reducing the effective load carrying area of the bearing. The signatures of these failure mechanisms are described in the appropriate Sections and should be looked for, particularly in the nominally unloaded parts of the bearing.

Comment

Do not change to a bearing material with higher load-carrying capacity unless it can be established that failure has been the result of inadequate strength in the white metal. Treating the symptoms is not the way to provide a cure. The cause of failure must be identified

1.5 Lubricant breakdown with lead-bronze bearing

Crack in crankpin of reciprocating compressor caused by

lubrication breakdown failure of lead-bronze big-end bearing


The photograph shows a section through one 10 inch diameter crankpin from a 6-throw reciprocating compressor after a lubrication failure. The lead-bronze bearing had wiped, but, in contrast to the case with a white metal bearing that limits the temperature rise to about 240C, much higher temperatures have been reached and this has resulted in quench cracking of the steel crankpin and total loss of a large crankshaft.

Cause

This is a lubrication breakdown failure (see summary sheet at end of section for possible reasons).

Note

When wiping occurs with a lead containing copper alloy (lead bronze, copper lead), only a small proportion of the bearing metal melts (the lead phase); high enough temperatures are generated by rubbing against the solid material leading to cracking of the journal by repeated quenching as it leaves the load zone.

Comments

White metal bearings should be the first choice where loading conditions permit their use. Lead-bronze can be used for little-end bearings where loads tend to be high, but the lower speed oscillating conditions do not tend to give rise to quench cracking. Note that hardened steel journals have to be used with these harder bearing alloys. As a rule of thumb journal hardness should be at least three times that of the bearing metal.

Lubrication Breakdown: Summary

Characteristics

Melting of white metal, often resolidifying in cooler parts of the bearing (e.g. in the oil grooves, or in the upper half of the bearing, though in the case of minor wipes it will be confined to the trailing edge of the damaged region.

Re-deposited bearing material does not form a metallurgical bond with the underlying material and can often be easily peeled off (particularly if whitemetal).

Circumferential scoring of wiped bearing usually occurs.

Possible Causes

Wiping is caused by a failure to form a hydrodynamic oil film at start up or reduction in lubricant film thickness (sometimes complete breakdown of the lubricating film), resulting in overheating and melting of the bearing material (partial melting in the case of the higher strength lead-bronzes and copper-leads). It should be noted that in high-speed machines wiping can occur without complete film breakdown if the temperature in the oil film rises above the melting point of the bearing material (about 240C in the case of whitemetals).

A number of possible causes are listed below. It is unlikely that failure occurs as a consequence of poor bearing manufacturing quality.

1. Inadequate supply of lubricant. This can occur at start up in low temperature conditions if the lubricant viscosity in the reservoir is so high that flow rate is too low. (Viscosity at start up should not exceed 2000 cSt.)

2. Use of lubricant of too low viscosity grade for the application

3. Lubricant supply is too hot leading to thin lubricant film

4. Misalignment leading to edge loading.

5. Excessive load causing high temperature and thin lubricant film. (Excessive start-up load, preventing or delaying the generation of a hydrodynamic lubricating film can be a problem with hydrodynamic bearings.)

6. Excessive shaft orbiting due to bearing instability (e.g. sub-synchronous whirl), but this can also give rise to fatigue failure.

7. Shaft operating load line close to an oil groove causing breakdown of the oil film or oil starvation. The particular case of gear bearings, where the load line is a combination of gravity and gear thrust, should be noted.

8. Loss of bearing area by some other failure mechanism, e.g. fatigue, electrical erosion damage.

Actions

The first check is whether the failure has occurred for the first time or whether there is a history of similar failures on the particular machine, or other similar machines. The operating conditions of the equipment leading up to failure are very important in understanding the cause. For example: has the failure occurred at start-up; was there an increase in vibration levels; has there been a problem with the lubricant pump or was the oil at the correct operating level in a self-contained system; was the filter element blocked?

The profile of the wiping patch may indicate the occurrence of misalignment. Measuring the shell thickness can in some instances indicate the exact position of the shaft when the failure occurred.

Check that the position of wiping corresponds to the predicted minimum film thickness in the bearing (this can be calculated using the techniques listed below). If the actual and predicted positions do not correspond then look for conditions that may result in the applied load being in a different direction to that expected. Note again that changes in gear loading will alter the direction of the resultant load.

Check damage to the mating shaft. Surface scoring and/or thermal cracking can occur; this risk is greater with harder bearing materials (e.g. copper and aluminium alloys).

Useful Analysis Techniques

Calculation of bearing operating conditions (lubricant flow rate, temperature, lubricant film thickness, attitude angle) can provide good supporting evidence. See, for example, ESDU Data Items 84031, 90027 (journal bearings), 82029, 83004 (thrust bearings).

2. Wear Bearing materials are chosen to be soft, not only does this allow them to conform to the journal or thrust collar to accommodate slight misalignment (conformability), but also to allow hard contaminant particles to embed so that they do not score the counterface (embeddability). There is, however, a limit to the latter and with severely contaminated lubricants particles small enough to enter the oil film at the supply groove, but larger than the minimum oil-film thickness can remain proud in the soft bearing metal and score the shaft or thrust collar - scoring.

Scoring results in an increase in clearance that eventually requires the bearing to be replaced. More seriously, however, wear in non-cylindrical profile bearings (e.g. lemon, multi-lobe), used to give enhanced stability, tends to produce a more cylindrical bore that can result in the bearing developing instability. Hydrodynamically-lubricated bearings can operate satisfactorily with a certain amount of scoring; however, a stage is eventually reached when it is no longer possible to generate an oil film at start-up and wiping failure occurs (see Lubrication breakdown).

Particles in high concentration, but small enough to pass through the oil film can erode the soft bearing material in the direction of oil flow in the bearing - erosion. Larger particles trapped in the oil grooves can also cause erosion damage.

Other wear mechanisms such as fatigue, cavitation erosion, fretting, chemical attack and electric erosion are dealt with separately.

2.1 Scoring damage of white metal thrust pad by dirt

Scoring of thrust pad by dirt in oil


Score marks follow the direction of motion in the bearing, like the grooves in a gramophone record. The score marks start where the dirt particle bridges the film and form a continuous mark up to the end of the bearing surface, the trailing edge on the right of the pad in the Photograph, or stop with an embedded particle (see following example). Scoring is an example of abrasive wear; adhesive wear gives discontinuous tears rather than clear uninterupted scores.

Cause

Contamination of the lubricant with hard particles small enough to enter the oil film at the point of maximum film thickness, but too large to pass right through without bridging the film. See also in the photograph how the soft white metal has been dragged over the trailing edge of the pad

Comments

Scoring damage indicates inadequate filtration of the oil or failure to change the oil filter when necessary.

2.2 Scoring of white metal journal bearing

a. Dirt particles embedded in b. Iron print from bearing surface

white metal bearing


Photograph a shows typical score marks in the white metal surface ending in an embedded foreign particle.

Cause

Contamination of the lubricant with hard particles small enough to enter the oil film at the point of maximum film thickness, but too large to pass right through without bridging the film.

Note

The embedded particles may be analysed to identify the source of the contamination. For example, iron or steel particles can be identified by iron printing using absorbent paper soaked in potassium ferrocyanide solution. Photograph b shows an iron print from a bearing surface.

Comment

Scoring damage indicates inadequate filtration of the oil or failure to change the oil filter when necessary.

2.3 Scoring of lead-bronze journal bearing

Gross scoring damage to lead-bronze journal bearing


Deep circumferential scoring of bearing surface in the direction of rotation.

Cause

Gross contamination of lubricating oil.

Note

Note the continuous score marks and the dragging over of the bearing metal at the downstream end of the half bearing (right hand side in Photograph).

Comment

This was a main bearing of a three-throw reciprocating pump. The damage occurred at commissioning because the crankcase was not cleaned out before starting.

2.4 Erosion damage of white metal journal bearing

Erosion of surface of bearing caused by the presence of

large amounts of small size dirt particles in the oil


The arrow in the photograph shows the direction of rotation. The erosion damage is clearly caused by dirt particles entering via the oil feed holes in the central circumferential oil groove. Erosive wear is shown by roughening and dulling of the white metal surface.

Cause

Contamination of the lubricant by fine particles.

Possible Confusion with Other Types of Damage

Cavitation of the oil film gives similar looking damage (see cavitation erosion), but in the present case the damage is in the centre of a bearing land away from the oil feed.

2.5 Erosion damage to axial oil groove in journal bearing

Erosion damage to inlet oil groove caused by

particles in oil too large to enter oil film


Edges of oil groove have been eroded away by particles trapped in oil groove as they were too large to enter bearing oil film.

Cause

Oil contamination.

Note

Axial oil grooves should have dirt escape gutters cut at 45 at ends of groove. These gutters should be deep enough to discharge the dirt particles, but not so deep that they cause excessive sideways drainage of oil from the groove.

Comment

This damage indicates inadequate filtration of the oil.

2.6 Summary

Characteristics

Wear by hard particles larger than the minimum oil film thickness in the bearing give characteristic continuous scores extending to a discontinuity in the bearing surface (oil groove, trailing edge of thrust pad) or ending in an embedded foreign particle.

Erosive wear is a fatigue process causing small scale removal of material that results in a characteristic roughening of the surface. Superficially this can resemble cavitation damage (see cavitation erosion), and electric erosion/spark erosion, but the former occurs in the centre of bearing lands away from oil feed holes and the latter results in small rounded shiny melt pits that are quite different in appearance.

Possible Causes

Two causes of wear damage are discussed in this section: abrasive wear caused by the cutting action of hard particles that bridge the bearing oil film; erosive wear, the small scale removal of material by fatigue resulting from the mechanical action of small particles repeatedly striking the surface.

Actions

Where damage is caused by solid particles in the lubricant, the only remedy is improved filtration of the oil. This should be no problem where there are duplicate filters, but single filters with over-pressure relief can release particles into the lubricant when the filter becomes blocked and there is a risk of particle release when filter changes are made.

Magnetic plugs should be fitted to reduce damage by ferrous wear particles.

Analytical Methods

Analysis of filter debris may be useful in identifying the source of solid contaminants, with iron printing using absorbent paper soaked in potassium ferrocyanide a useful technique for the identification of ferrous particles embedded in soft bearing materials.

3. Fatigue White metal bearing alloys have low strength and readily suffer fatigue damage when subject to reversing loads, such as occur in reciprocating machines (engines, reciprocating compressors, ram pumps), and also when subject to vibration caused by out-of-balance or dynamic instabilities.

White metals are always used as a lining on a backing of a harder material, usually mild steel, but more rarely bronze. The fatigue strength of the white metal can be increased by reducing the thickness of the lining, but with the penalty of reducing the conformability and embeddability of the bearing material.

White metal thicknesses of 1 to 3 mm (thick wall bearings) are most commonly used in industrial machines; this not only gives good conformability and embeddability, but may also provide a molten layer that allows a machine to be run down safely in the event of failure without damage to the rotor. Thick wall bearings can be repaired by re-metalling. Thin wall bearings with white metal linings in the range 0.08 to 0.12 mm are only used in reciprocating applications, in particular high-speed engines, to give enhanced load-carrying capacity; these bearings cannot be repaired.

See Section 7 for a different fatigue mechanism, thermal fatigue. Mechanical erosion (Section 2) and cavitation erosion (Section 4), which are other forms of fatigue damage, are also treated separately.

3.1 Fatigue of White Metal Journal Bearings

Initial stages of fatigue showing bruising of the white metal surface


Both photographs show the characteristic crazy cracking of the white metal surface. Where loose pieces are formed these may remain in the bearing (photograph b) or come away and get trapped so that they melt and wipe, but this is a secondary effect, not the primary damage. The loose pieces tend to have an aspect ratio (length / thickness) of about 5:1, so that on thick wall bearings the fatigued areas are large

Cause Excessive alternating loading. Note Photograph b, where the fatigue damage is on one side of the bearing

suggests that there may have been slight misalignment.

3.2 Fatigue of White Metal Journal Bearing Caused by Vibration

White metal journal bearing showing fatigue in both bearing halves


Fatigue of journal bearing removed from a high-speed centrifugal compressor (16,000 rev/min) following a 12-hour vibration incident.

Cause

Synchronous vibration causing excessive loading on the white metal. Note that the fatigue patches are diametrically opposed, showing that the vibration mode was elliptical.

Note The circumferential bands of fatigue correspond to the dovetail grooves that were machined in the bearing shell.

Comment

This bearing remained in service for six weeks after the vibration had been suppressed. This shows the remarkable load-carrying capacity of the oil film formed by hydrodynamic action. The loss of bearing material would of course have prevented the bearing from starting up satisfactorily. This type of machine was prone to a peculiar type of intermittent synchronous vibration instability. In one machine, with a rotational speed of 26,000 rev/min, the manufacturer supplied lead-bronze bearings with a lead overlay in an attempt to prevent fatigue failure, though this was treating the symptoms rather than addressing the real problem. Bearing fatigue still occurred,

leading to lubricant film breakdown and cracking of the expensive rotor by thermal shock (see Section 1: Lubrication Breakdown with Lead-Bronze Bearing) that incorporated an integral gear. The problem was successfully solved by changing to a modified design of bearing (elliptical bore) that inhibited the instability.

3.3 Premature Fatigue of White Metal Journal Bearing

Premature fatigue caused by poor bonding of white metal to bearing shell

Exposed dovetails in machined oil groove showing lack of bonding between the edges of the dovetail and the white metal (the black lines at the edges of the dovetails)


This has the appearance of typical fatigue cracking. The tell-tale bubbles in the central oil groove that have been caused by vaporisation of oil trapped in the cast iron bearing shell point to poor bonding. This suggests that the fatigue damage has also been the result of poor bonding.

Cause Premature fatigue from inadequate bonding of the white metal to the bearing shell resulting in decreased fatigue strength of the white metal lining.

Note

te the presence of the dovetails in the bearing ell. This has been used in the mistaken pression that the strength of the white metal is reased by giving it a mechanical bond. The rmful effect of dovetails in preventing the mation of a metallic bond because of the different rmal conductivities and rates of expansion in el and white metal is shown in photograph b ere the machined oil groove has exposed the vetails revealing lack of bonding between the ite metal and the sides of the dovetails.

Possible Confusion with Other Types of Damage

The appearance of premature fatigue is very similar to that or normal fatigue. It can be confirmed by removing loose pieces and examining the bearing shell. In the case of normal fatigue the tin lining on the shell will still be present.

Comment

Cast iron should not be used for bearing shells. Cast iron is porous and absorbs lubricating oil in service. This oil is almost impossible to remove and prevents satisfactory re-metalling of the shell. The bonding should be checked by ultrasonic testing before re-metalled bearings are put into service.

3.4 Fatigue Failures: Summary

Section through fatigued white metal showing crack penetrating to the backing layer, but not to the backing metal

Fatigued white metal showing presence of tin layer on steel backing

Characteristics

Fatigue damage is readily distinguished from other failure mechanisms. It manifests itself by crazy cracking on the surface; the cracks propagate through the white metal to the tin lining on the bearing shell where they continue until joining up with another crack and form a loose piece that detaches from the backing. These loose pieces have an aspect ration of about 5:1 (Fig. a). The full fatigue strength of the white metal lining depends on a sound metallurgical bond between the backing material and the white metal that is obtained by tinning the backing before pouring on the molten white metal. If a sound metallurgical bond is not created, the fatigue strength is markedly reduced, making the bearing susceptible to premature fatigue failure. Normal and premature fatigue can be distinguished by examining the bottom of the fatigue pit; this should show a coating of tin on the backing, not steel or a rusty surface (Fig. b).

Fatigue failure is normally confined to journal bearings and rarely occurs with thrust bearings. The loose pieces of white metal can remain in place, held in by the closing fitting journal, or disappear by getting trapped so that they rub and wipe.

Possible Causes

Fatigue in journal bearings can result from inadequate design, but is usually the result of excessive alternating loading.

Actions

Where fatigue is the result of excessive load, the only remedy is to change to a more fatigue resistant bearing material. Where it is a consequence of vibration, the vibration level has to be reduced.

4. Cavitation Erosion This is a particular form of fatigue caused by rapid fluctuation of pressure in the bearing oil film. When the pressure is low, bubbles of vapour or dissolved gas are formed and then collapse as they go into a high pressure region.

Vaporous cavitation, where the bubble collapse is much more violent, results in shock waves in the lubricant film that cause fatigue failure in the white metal surface. This differs from normal fatigue in that small pits are formed rather than loose pieces. Cavitation damage occurs where there are reciprocating loads, either as part of the normal loading cycle or because of high-frequency vibration. It can also occur in bearings where there are sharp discontinuities in the thickness of the bearing oil film

Gaseous cavitation, in which the bubbles are of gas from solution in the lubricant, is much less energetic than vaporous cavitation as bubble dispersion depends on re-absorption of the gas by diffusion rather than instantaneous collapse. Gaseous cavitation can still, however, cause damage to soft white metal bearings.

4.1 Cavitation Erosion Damage in Reciprocating Engine Bearing

Cavitation erosion damage to engine bearing caused by rapid approach of shaft to bearing, collapsing vapour bubbles in the oil film. (Photograph: Glacier Metal Co.)

Main Characteristics Cavitation damage results in roughening of the bearing lands. It takes the form of arrows with the arrow head pointing against the direction of motion.

Cause

Rapid fluctuations of pressure in the oil film in reciprocating machine causing the formation of bubbles as the shaft moves away from the bearing followed by rapid collapse of the bubbles as the load is applied and the shaft approaches the bearing.

Note The damage is in the middle of the bearing lands, away from the oil

groove and the oil feed holes. Possible Confusion with Other Types of Damage

Cavitation damage is very similar to the damage resulting from dirt erosion (see Section 2.4: Erosion damage of white metal journal bearing). In the latter case, however, the arrow head starts at the oil feed hole

Comment

This is a case of vaporous cavitation. For an example where the less violent gaseous cavitation was involved, see Section 4.3: Cavitation Damage Caused by Bearing Instability below and Section 6 "Copper deposit on thrust bearing".

4.2 Cavitation Erosion of White Metal Thrust Collar

Photograph a

Cavitation erosion of white metal thrust face

Photograph b

Steel thrust collar with lubricant feed grooves on end of gear shaft

Main Characteristics Photograph a shows typical roughening of white metal caused by erosion.

Cause Photograph b shows the thrust collar associated with the cavitation damage shown in Photograph a. This is an unusual arrangement with the oil grooves machined in the hard steel thrust collar, the reverse of the more normal arrangement with the grooves in the soft white metal.

Comment The problem was caused by the sharp fall in pressure at the beginning of the grooves in the steel collar, followed by bubble collapse as the oil re-entered the high pressure region downstream of the groove. The problem was solved by changing to the more normal arrangement with the grooves in the white metal face.

4.3 Cavitation Damage Caused by Bearing Instability

Photograph 'a'

Cavitation erosion damage to white metal journal bearing in high-speed refrigeration compressor subject to instability

Photograph b

Part of Photograph 'a' at higher magnification


This is another aspect of cavitation erosion damage, in this case the bearing of a high-speed (25,000 rev/min) refrigeration compressor that was subject to a synchronous vibration. The higher magnification Photograph b shows erosion damage resembling the effect of wave action on a sandy shore.

Cause The refrigerant was Refrigerant R12 that was in contact with the lubricating oil. Because of the vibration, the refrigerant came out of solution in the oil as the shaft moved away from the bearing, only to go back into solution as the pressure increased as the shaft approached the bearing.

Note This was a case of gaseous cavitation, giving a less energetic bubble collapse, but sufficiently energetic to cause cavitation damage to the bearings.

Comment For another example where gaseous cavitation was involved, see Section 6 "Copper Deposit on Thrust bearing".

CAVITATION DAMAGE: SUMMARY

Characteristics

Cavitation erosion gives a fine scale roughened texture to the damaged surface. This could possible be confused with wear by fine particles (see Section 2.4: Erosion damage of white metal journal bearing) or even electrical erosion damage (see Section 8: Electrical damage), but can readily be distinguished by closer examination (electrical damage, for example, shows rounded shiny pits from which molten metal has been removed) and the circumstances in which the damage occurred.

Possible Causes

Cavitation damage is associated with high-speed reciprocating machines, or machines in which the bearings are subject to high frequency vibration. Gaseous cavitation should be suspected in machines handling lubricating oils with gas in solution, most particularly refrigeration compressors in which the refrigerant is in contact with the lubricating oil.

Actions

In many cases a modification of the bearing design can prevent bubble formation in a sensitive part of the bearing. Where cavitation is associated with vibration, it is a secondary effect and the cure is to remove the cause of the vibration.

5. Fretting Damage Fretting is a form of adhesive wear; it occurs as the result of small scale oscillatory movement. There are three situations in which this can result in damage in plain bearings.

Fretting can occur at the contact of a journal in the bearing in a machine that is subject to external vibration. This causes a change in the bearing profile that can affect its performance. Secondly, fretting can also occur on the backs of bearing shells that are not given an adequate interference fit in the housing (see Section 10: Manufacturing and assembly errors). The third situation is at the contact between the pivots of tilting pads and the carrier, where the pads are subject to fluttering through loading by a vibrating shaft

5.1 Fretting in a Journal Bearin

Documents

General Bearing Design