Handbook of Lubrication Episode 2 Part 6 pps

In some cases the debris becomes completely embedded in the bearing material and results in only super-ficial wear damage with no significant effect on bearing performance.. diameter tin

FIGURE 2 Section through babbitt thickness showing advanced stage of fatigue cracking in babbitt with cracks extending to bond and intersecting (Magnification × 75.) (From Burgess, P B., Lubr Eng., 9(6), 309, 1953 With permission.)

478 CRC Handbook of Lubrication

FIGURE 1 The effect of lining thickness on babbitt fatigue strength.

(From Szeri, A Z., Ed., Tribology: Friction, Lubrication, and Wear,

Hemisphere Publishing, Washington, D.C., 1980 With permission.)

machinery in which the bearings are used and is in the form of weld spatter, grinding wheel abrasive, and foundry sand Other debris can enter the bearing from the environment.3 Regardless of the source, the debris can cause scoring and tracking of the bearing surface and embedding of the debris in the surface Journal scoring can also occur, but its severity

is dependent upon the relative hardness of the debris and journal materials In some cases the debris becomes completely embedded in the bearing material and results in only super-ficial wear damage with no significant effect on bearing performance In other cases, severe wear damages both the bearing and journal Increased wear and ultimate failure of the bearing occurs Figure 5 shows circumferential scoring, tracking, and embedded debris in

a 76-mm (3-in.) diameter tin-base babbitt bearing with an embedded debris particle at the end of a score mark

Self-propagating mechanical wear by debris can also occur One such type of wear has

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FIGURE 3 Sketch showing babbitt fatigue initating wiping damage.

FIGURE 4 Babbitt fatigue in 178-mm (7-in.) diameter turbine bearing with thick babbitt layer 1,2

been termed wire-wooling or machining-type wear because the debris generated resembles wire wool or metal turnings by a machine tool.4,5In this type of wear the journal is usually more severely worn than the bearing Severity of the damage is dependent upon the chromium content of the journal steel High chromium, e.g., 12% chromium, is particularly susceptible

to this type of wear damage as seen in Figure 6 Figure 7 is the mating bearing which was used The damage is initiated by debris embedded in the bearing material which generates additional journal steel debris and forms a hard steel scab in the babbitt surface This scab acts as a tool which further propagates the wear and often results in catastrophic damage Simple scoring damage on a journal can sometimes be incorrectly identified as wire-wooling

or machining-type wear, but in many cases is not self-propagating

Another type of mechanical wear results from self-loading or radial binding of a bearing and its journal Jamming of debris in the bearing clearance, too tight a radial fit, or dimen-sional interference from differential thermal expansion6can cause high wear of both bearing and journal Figures 8 and 9 show a graphite bearing and its journal which were worn by self-loading Both axial and circumferential cracks occurred in the graphite bearing

Electrical Damage

Wear is sometimes experienced in rotating machinery as a result of the passage of current between the bearing and its mating surface, journal, or thrust runner.7,8 Sparking between the surfaces causes pitting damage to both surfaces Pits on the harder journal or runner surfaces are usually considerably smaller than those on the bearing Multiple pits, closely spaced, produce a frosted appearance of the surfaces; and the removal of fused metal particles cause the surfaces to be rough The rough surfaces produce further wear by mechanical abrasion An additional consequence of sparking is deterioration of the lubricant and possible contamination of the lubricant and the lubricating system by spark debris In extreme cases, the passage of current can cause an increase in the temperature of the parts which may damage the bearing or the lubricant

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FIGURE 7 Damaged babbitt bearing used with journal of Figure 6 1,2

Regardless of their size, electrical pits have a characteristic appearance The bottoms are rounded and have a smooth, shiny, melted appearance (Figure 10) Usually the periphery

of the pit at the bearing surface has a ridge of melted metal In some cases this ridge is worn away by contact with the journal For cases in which wiping damage has been su-perimposed, electrical pitting can be identified as the cause by examining the harder journal surface which operated against the bearing

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FIGURE 10 Electrical pitting in tin-base babbitt produced by high current (Magnification × 15.) (From Boyd, J and Kaufman.

H N., Lubr Eng., 15(1), 28, 1959 With permission.)

FIGURE 11 Electrical pitting in tin-base babbitt produced by low current (Magnification × 15.) (From Boyd, J and Kaufman,

H N., Lubr Eng., 5(1), 28, 1959 With permission.)

Figure 12 shows schematically some sources of bearing currents in rotating machinery.

Table 1 reviews these sources and methods of eliminating or reducing the bearing currents Because bearing currents may be produced by a variety of conditions, no single method of measuring potentials is suitable in all cases Table 1 indicates the best location for taking measurements In the case of the dissymmetry effect, measuring the potential between the shaft and one bearing may be unreliable This is because both bearings are in series with the generated emf and a temporary large resistance in the one bearing may make the potential across the other bearing negligible The most reliable method is to measure the potential between the extremities of the shaft Using this method on ordinary electrical apparatus with

484 CRC Handbook of Lubrication

FIGURE 12 Principal sources of bearing current in rotating machinery (From Boyd, J and Kaufman, H N.,

Lubr Eng., 15(1), 28, 1959 With permission.)

Table 1 (continued) PRINCIPAL SOURCES OF BEARING CURRENT AND METHODS OF CURRENT CONTROL

a See Figure 12.

journal bearings, it was found that potentials less than 300 mV cause no significant electrical damage The corresponding value for machines with ball and roller bearings is 100 mV Since electrostatic effects are greatly influenced by humidity and by surface conditions, the measurement of electrostatic potential is apt to be extremely erratic Absence of elec-trostatic potential during a set of measurements does not necessarily mean that such potentials are not present under other conditions Generally speaking, electrostatic potentials ordinarily

do not produce sustained currents of large magnitude The intermittent charging and dis-charging, however, can eventually produce enough bearing damage to cause failure

The erratic nature of electrostatic potentials makes it difficult to set practical limits for satisfactory operation It is known that peak voltages of 20 V or more can produce bearing damage Reducing the voltage to the order of 1 V by some form of grounding device ordinarily eliminates the trouble

The main methods of eliminating or reducing damage due to bearing currents include: (1) eliminating the source, (2) insulating the machine parts, (3) grounding the shaft, and (4) modifying the machine design

Damage from Thermal Effects

The physical properties of some materials, such as the tin-base babbitts can differ along different axes of the grains making up their structure Such anisotropic properties, coupled with differences in orientation of grain axes, can result in grain distortion when thermal cycling is imposed This effect is shown schematically in Figure 13 If this grain distortion occurs in a babbitt bearing, the journal can contact the distorted or raised grains and result

in slight wear or burnishing This produces a mottled appearance of the bearing surface as shown in Figure 14 Mottling is usually not detrimental to bearing performance However,

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FIGURE 13 Sketch showing effect of thermal cycling on tin-base babbitt grains 1,2

FIGURE 14 Surface mottling on 178-mm (7-in.) diameter bearing due to anisotropy of tin-base

babbitt 1,2

in some cases of severe grain distortion, cracks can occur in the babbitt surface along the grain boundaries as fatigue from thermal cycling

Another thermal phenomenon results from the reduced strength properties of babbitt with increasing temperature At elevated temperatures babbitt will undergo creep with rippling

of the surface and subsequent wiping.9

WIPING Wiping is the smearing or removal of bearing material from one point and the redeposition

at another point on two surfaces in sliding contact Superficial wiping in which bearing performance is not significantly affected can occur from either a temporary overload or temporary loss of lubricant If either the overload or loss of lubricant is of long duration, severe damage frequently results Bearing misalignment often results in wiping damage Wiping damage is shown in Figure 15 on a 178-mm (7-in.) diameter bearing Babbitt smeared by wiping is shown in the developed view of Figure 16

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FIGURE 15 Wiping damage on a 178-mm (7-in.) diameter bearing.

FIGURE 16 Developed view of babbitt smeared by wiping 1,2

Wiping sometimes is the indirect result of blistering at the interface of babbitt metal bonded to steel This is a rare occurrence caused by hydrogen gas, trapped in the steel during manufacture, later diffusing to the interface where sufficient pressure is developed to cause the babbitt to blister and to be wiped by the journal or runner Figure 17 is a section through babbitted steel which shows the inclusions in steel through which the gas can migrate to cause a blister Figure 18 is a section through a blister showing the separation at the bond

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FIGURE 17 Section through babbitted steel showing babbitt blister formation at locations above inclusions in steel.

shows leaded bronze corrosion combined with fatigue on a 229-mm (9-in.) diameter railroad diesel bearing Figure 21 shows a metallographic section through a copper-lead bearing in which lead corrosion occurred The black voids at the bearing surface were pockets of lead removed by the corrosive attack Figure 22 is a metallographic section showing copper corrosion in a copper-lead bearing The white copper grains at the bearing surface have been chemically attacked

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FIGURE 20 Corrosion and fatigue of a leaded-bronze railroad diesel bearing 1,2

FIGURE 21 Section through a copper-lead bearing showing lead corrosion (Magnification × 150.) (From

Burgess, P B., Lubr Eng., 9(6), 309, 1953 With permission.)

EROSION Erosion is the removal of material from the bearing surface by fluid action which results

in the formation of voids or pits in the surface It can be produced by changes in the direction

of flow of high-velocity fluid streams or by the abrasive action of debris in the fluid stream

as it impinges on the bearing material

Cavitation erosion is a type of erosion in which the formation and collapse of gas bubbles

in the lubricant produces high localized pressures which result in fatigue pitting of the bearing surface Figure 25 shows erosion damage on a 44-mm (1.15 in.) diameter aluminum bearing from a high-speed gas compressor Figure 26 is an enlargement of the damage

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FIGURE 24 Enlargement of fretting corrosion of bronze bushing

of Figure 23 (Magnification × 15.) 1,2

FIGURE 25 Cavitation erosion of a 44-mm (1.75-in.) diameter aluminum bearing 1,2

ROLLING ELEMENT BEARINGS

W J Derner and E E Pfaffenberger

ROLLING BEARING TYPES Rolling contact bearings are generally categorized by the type of rolling element and the manner in which it is used The most obvious divisions are between ball and roller bearings and between radial and thrust bearings, while some angular contact bearings are utilized for both radial and thrust loads

Individual sections in this chapter will cover (1) types of rolling element bearings and their selection criteria, (2) dimensional standards, (3) characteristics of materials employed, (4) rolling bearing theory, (5) load, speed, and related application limits, (6) lubrication, and (7) failure analysis Load rating for individual bearings, their application ranges, and many other details are available in catalogs and related literature from bearing suppliers

Ball Bearings (Figure 1)

Radial and Angular Contact

The most common design of radial ball bearing is the Conrad type where, in general, five to nine balls are inserted between the inner and outer rings Where a greater radial capacity is desired, filler type rings utilize notches on one shoulder so that more balls can

be inserted An optimum capacity is achieved through the use of a split inner or outer ring wherein a maximum number of balls can be inserted with the retainers This latter design, however, requires external means for holding the ring halves together so that load can be divided between contacts on both halves

Self-aligning ball bearings are available in double row varieties in which the outer ring raceway is of a larger radius than the ball and the inner has two raceways ground in it, one for each row of balls

Thrust Ball Bearings

These are available in designs for single direction as well as double direction thrust and normally are found with 90° contact angles

Roller Bearings

A variety of rollers have developed for use in bearings including the early spring-wound cylindrical, solid and hollow cylindrical, tapered, and spherical rollers Retainers (cages) used to space the rolling elements may be land riding, roller riding, or supported on a raceway Retainers may be (1) machined out of solid, (2) stamped and formed, (3) fabricated and fastened by cold heading or riveting, or (4) molded of one or two elements Since retainer contact with the roller or ring involves some sliding, a lubricant should be chosen which is compatible with the nature of the contact as well as the material of the retainer

Radial and Angular Contact ( Figure 2 )

Tapered roller bearings — Supported between two cones of different angles, the tapered

roller centers itself between them and recognizes a certain axial force which maintains its contact with a lip or rib generally on the inner ring Its contact with that rib involves sliding which must be lubricated to prevent wear and to dissipate the heat generated

Cylindrical roller bearings — This type generally runs cooler than other roller bearings

because of the narrow and uniform shape of the Hertzian contact with no more roller end contact with the ribs or flanges than is required to provide guidance or location In

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cations similar to those in screw-down bearings in rolling mills Since many of these run at low speeds or are even subjected to only momentary, limited rotation (and/or oscillation), lubrication requirements are highly specialized and require careful evaluation

Cylindrical roller bearings — Conventional roller thrust bearings with flat plate surfaces

are utilized in low-speed applications where high thrust capacity is essential Tandem cy-lindrical roller thrust bearings are used in many applications wherein single-helical gears are run at low speeds Because of the relatively high slip in these applications, as well as the generally severe contact conditions in all slow speed and/or oscillating operation, special attention to cooling and extreme pressure (EP) additives in the lubricant are required

Spherical roller bearings —- Spherical thrust bearings require the same attention as do

most tapers, but absorb misalignments which would not be acceptable to cylindrical and tapered varieties Spherical thrust bearings are provided with both symmetrical and asym-metrical roller designs The latter are somewhat limited in having a moderately loaded rib

or locating flange as a sliding contact which must be lubricated as with the tapered roller bearings In lower-speed applications where hydrodynamic films are not readily generated

at the rib roller end contact, wear becomes a significant factor limiting the life or function

of these bearings In all cases of angular contact, spherical or tapered roller thrust bearings, the relation of thrust to radial load must be carefully controlled to ensure that the bearing does not come apart Manufacturers’ recommendations must be carefully adhered to

Mounted-Bearing Units

A significant proportion of rolling element bearings are supplied in integral housings with seals which offer advantages in that they do not require a large, continuous machined housing While a great majority are grease lubricated, for adverse environmental conditions some are provided with complete lubricating systems to include cooling and filtration For severe contamination, flushable seals (Figure 4) have lube fittings separated from the main lubri-cation system of the bearing Some applilubri-cations are so severe that frequent and heavy relubrication is relied on to purge the system of contaminants and to exclude water vapors due to “breathing” where intermittent operation is encountered

ROLLING BEARING STANDARDS

Boundary Dimensions

The great majority of the world rolling element bearings are in compliance with boundary dimension plans adopted by the International Standards Organization (ISO) Domestic stand-ards originated with the Anti-Friction Bearing Manufacturer’s Association (AFBMA) have been taken over by American National Standards Institute (ANSI) which, as a participating body in ISO work, offers the most complete and authoritative set of standards for use in this country

A great majority of the so-called inch series of bearings have been superceded by metric series which fit a number of well-established boundary plans.1 Of particular note, a new series of tapered roller bearings has achieved a reduction in the multiplicity and complication

of sizes with worldwide acceptance in an ISO standard.2

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FIGURE 3 Roller thrust bearings.