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Thrust Bearing

LEVELING PLATESET SCREW

BASE RING

LOWER LEVELING PLATE

UPPER LEVELING PLATE

THRUST SHOE ASSEMBLY

BASE RING KEY

COLLAR

BASE RING KEY SCREW

BASE RING DOWEL

JOURNAL SHOE

JRL SHOE STOP PIN

RETAINING PLATE SCREW

RETAINING PLATE

ALIGNING RING

RETAINING PLATE

RETAINING PLATE SCREW

RETAINING PLATE

JOINT SCREWRETAINING PLATE

JOINT DOWEL

ALIGNING RING

ANTI-ROTATION PIN

Journal Bearing

A GENERAL GUIDE TO THE PRINCIPLES, OPERATION AND TROUBLESHOOTING OF

HYDRODYNAMICBEARINGS

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ts PrefaceIntroductionHistoryTypical Applications

Section I

Hydrodynamic Bearings

Hydrodynamic Principle

Basic Pivoted Shoe Thrust (& Journal) Parts

Related Parameters

Operation And Monitoring

Section II

Introduction

Discussion

Base Ring

Leveling Plates

Shoe Support

Shoe Body

Shoe Surface

Collar/Runner/Journal Surface

Oil

Operational Data

Recommended Reading and References

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KPreface

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During every second of every day, machines all over the world are working to provide the products we demand.

These machines rely on the successful support of bearings. If a machine goes off line, extreme pressure is placed

on those involved to correct the problem(s). It is the intention of this presentation to assist the reader in problem

solving by providing background information on hydro-dynamic bearings and distress modes.

INTRODUCTIONBearings which support rotating shafts can be classified into four basic categories:

Rolling contact load supported by balls or rollers.Hydrostatic load supported by high pressure fluid.Hydrodynamic load supported by a lubricant film.Magnetic load supported by magnetic fields.

This guide contains information on hydrodynamic, pivoted shoe bearings using oil as a lubricant.However, much of the information can be applied to hydrodynamic bearings in general.

Section I describes the principles, parts, related parameters and operation of the bearing in order to provide a base for a better understanding of Section II.

Section II provides an overview of a structured troubleshooting approach, with information on distress modes and recommended repair.

Holtwood Generating Station (See page 4)

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ceHISTORYIn the late 1880s, experiments were beingconducted on the lubrication of bearing surfaces. The idea of floating a load on afilm of oil grew from the experiments ofBeauchamp Tower and the theoretical work of Osborne Reynolds.

PIVOTED SHOE THRUST BEARINGS (Fig. 1)

Prior to the development of the pivoted shoethrust bearing, marine propulsion relied on ahorseshoe bearing which consisted of severalequally spaced collars to share the load, eachon a sector of a thrust plate. The parallel sur-faces rubbed, wore, and produced considerablefriction. Design unit loads were on the orderof 40 psi. Comparison tests against a pivotedshoe thrust bearing of equal capacity showedthat the pivoted shoe thrust bearing, at only1/4 the size, had 1/7 the area but operatedsuccessfully with only 1/10 the frictional dragof the horseshoe bearing.

In 1896, inspired by the work of OsborneReynolds, Albert Kingsbury conceived andtested a pivoted shoe thrust bearing. Accordingto Dr. Kingsbury, the test bearings ran well.Small loads were applied first, on the order of50 psi (which was typical of ship propellershaft unit loads at the time). The loads weregradually increased, finally reaching 4000 psi,the speed being about 285 rpm.

FIRST APPLICATION

In 1912, Albert Kingsbury was contracted bythe Pennsylvania Water and Power Companyto apply his design in their hydroelectric plantat Holtwood, PA. The existing roller bearingswere causing extensive down times (severaloutages a year) for inspections, repair andreplacement. The first hydrodynamic pivotedshoe thrust bearing was installed in Unit 5 onJune 22, 1912. At start-up of the 12,000 kWunit, the bearing wiped. In resolving the reason for failure, much was learned about

Figure 1 Hydrodynamic, Equalizing Pivoted Shoe Thrust Bearing

KPreface

tolerances and finishes required for the hydro-dynamic bearings to operate. After properlyfinishing the runner and fitting the bearing,the unit ran with continued good operation.This bearing, owing to its merit of running75 years with negligible wear under a load of220 tons, was designated by ASME as the23rd International Historic MechanicalEngineering Landmark on June 27, 1987.

JOURNAL BEARINGS

The cylindrical hydrodynamic journal bearingis the most basic hydrodynamic bearing. It hasa cylindrical bore, typically with two axialgrooves for lubrication. This bearing has a highload capacity, and the simple design is compact,bi-rotational, and easy to manufacture.

However, as the design speeds of machinesincreased, it was found that this bearing hadlimitations due to oil whirl. Oil whirl is veryundesirable because of high vibration amplitudes, forces, and cyclic stresses that areimposed on the shaft, bearings and machine.

Efforts to suppress and eliminate oil whirl haveresulted in a variety of fixed geometry bearingswhich are modifications to the profile of thebearing bore. Variations are the lemon bore,pressure dam, lobed, and other fixed profilebearings. The pivoted shoe concept (Fig. 2) wasfirst applied to journal bearings approximatelyseventy-five years ago. Extensive tests and applications have proved the pivoted shoe jour-nal bearing to be most effective in eliminatingoil whirl.

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Figure 2 Hydrodynamic Pivoted Shoe Journal Bearing

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ceTYPICAL APPLICATIONSEarly history had proven that hydrodynamicpivoted shoe bearings provided considerablebenefits. They were smaller, less expensive,required less maintenance, lasted longer, andwere more efficient. The oil film also provided additional benefits in regard toshock absorbing capability, and alloweddamping as a design parameter to controlvibration. These considerable benefits allowedthe design to be used in a wide variety ofapplications. Indeed, the invention made itpossible to build the high-tech machines andships of today.

Figure 3 Hydrodynamic Pivoted ShoeThrust and Journal Bearing

INDUSTRIAL APPLICATIONSHydroelectric Generators Hydraulic TurbinesSteam Turbines Gas TurbinesDredge Pumps Boiler Feed PumpsHigh Speed Blowers Centrifugal CompressorsElectric Motors Deep Well PumpsOil Pumps Cooling PumpsPulp Refiners TurbochargersAir Preheaters Rock CrushersExtruders

SHIPBOARD APPLICATIONSMain Propeller Journals Propeller Line Shaft Turbine-Generator Sets Main Gear BoxClutch PumpsBlowers Auxiliary Machinery

This section describes the principles, parts, related parametersand operation of the hydrodynamic pivoted shoe bearing.

HYDRODYNAMIC BEARINGSBearings transmit the rotating shafts loads to the foundation or machine support.Hydrodynamic bearings transmit (float) theload on a self-renewing film of lubricant.Thrust bearings support the axial loads. Radialloads are supported by journal bearings. Themachine and bearing can be classified as hori-zontal or vertical depending on the orientationof the shaft. The bearings may be solid forassembly over the end of the shaft, or split forassembly around the shaft.

HYDRODYNAMIC PRINCIPLE JOURNAL BEARINGS

Based on his theoretical investigation of cylindrical journal bearings, Professor OsborneReynolds showed that oil, because of its adhesion to the journal and its resistance toflow (viscosity), is dragged by the rotation ofthe journal so as to form a wedge-shaped filmbetween the journal and journal bearing (Fig.4). This action sets up the pressure in the oilfilm which thereby supports the load (Fig. 5).

This wedge-shaped film was shown byReynolds to be the absolutely essential featureof effective journal lubrication. Reynolds also

showed that if an extensive flat surface isrubbed over a slightly inclined surface, oil beingpresent, there would be a pressure distributionwith a maximum somewhere beyond the centerin the direction of motion.

PIVOTED SHOE

Applied to hydrodynamic pivoted shoe thrustbearings (Fig. 6) Albert Kingsbury stated: If ablock were supported from below on a pivot, at

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KSection I

JOURNAL BEARING

OIL WEDGE

ADHESION

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Figure 4 Hydrodynamic Principle

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Figure 5 A figure from Reynolds Paper On the Theory of Lubrication showing oil film pressure distribution

Figure 6 Illustration from a page from AlbertKingsburys Paper Development of the Kingsbury Thrust Bearing

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friction and absence of wear of the metal parts.In this way a thrust bearing could be madewith several such blocks set around in a circleand with proper arrangements for lubrication.The same concept applies to the pivoted shoejournal bearing.

As with the plain cylindrical bearing, the pivoted shoe thrust and journal bearings relyon adhesion of the lubricant to provide thefilm with a self-renewing supply of oil.

BASIC PIVOTED SHOE THRUST (& JOURNAL) PARTS (See Fig. 7)This section discusses the associated thrustbearing parts, with corresponding journalbearing information in parenthesis.

ROTATING COLLAR (JOURNAL)The collar transmits the thrust load from therotating shaft to the thrust shoes through thelubricant film. It can be a separate part andattached to the shaft by a key and nut orshrink fit, or it may be an integral part of theshaft. The collar is called a runner in verticalmachines. (In the radial direction, the shaftjournal transmits the radial loads to the journal shoes through the lubricant film.) Inhydrodynamic bearings, the fluid film is onthe order of .025 mm (.001) thick. With thisand the information from HYDRODYNAMICPRINCIPLE, two points can be realized:

1. The stack-up of tolerances and misalign-ment in hydrodynamic bearings has to be lessthan .025 mm (.001), or some means of adjustment has to be incorporated.

2. The collar surfaces must be flat and smooth(and journal surface cylindrical and smooth) incomparison to the film thickness, but not sosmooth as to inhibit the adhesion of the lubricant to the surface.

THRUST SHOE (JOURNAL SHOE) ASSEMBLY

The shoe (also called a pad, segment, or block)is loosely constrained so it is free to pivot. The shoe has three basic features - the babbitt,body, and pivot, and so is usually referred to as an assembly.

BABBITT The babbitt is a high-tin material,metallurgically bonded to the body. As withthe collar, the babbitt surface must be smoothand flat in comparison to the film thickness.

The babbitt is a soft material (compared to theshaft) which serves two functions: It traps andimbeds contaminants so that these particles donot heavily score or damage the shaft. It alsoprotects the shaft from extensive damage shouldexternal conditions result in interruption of thefilm and the parts come in contact.

BODY The shoe body is the supportingstructure which holds the babbitt and allowsfreedom to pivot. The material is typicallysteel. Bronze is sometimes used (with or without babbitt) depending on the application.Chrome copper is used to reduce babbitt tem-perature.

PIVOT The pivot allows the shoe to rotateand form a wedge. It may be integral with theshoe body, or be a separate insert. The pivotsurface is spherical to allow 360 rolling freedom.

BASE RING (ALIGNING RING)

The base ring loosely holds and constrains theshoes against rotating so as to allow freedom topivot. It may have passages for the supply oflubricant, and contain features to adapt for misalignment and tolerance in the parts. Thebase ring (aligning ring) is keyed or doweled tothe housing to prevent rotation of the bearingassembly.LEVELING PLATES

The leveling plates (not applicable to journalbearings) are a series of levers designed to

KSection I

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Figure 7 Thrust and Journal Bearing Part Schematic

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compensate for manufacturing tolerances bydistributing the load more evenly betweenthrust shoes. The leveling plates compensatefor minor housing deflections or misalignmentbetween the collar and the housings support-ing wall. A description is given later under thesection on misalignment.

LUBRICANT

The lubricant is another important elementof the bearing (See Fig 8-1). The loads aretransmitted from the shaft to the bearingthrough the lubricant which separates the partsand prevents metal-to-metal contact. Thelubricant also serves to carry heat caused byfriction out of the bearing.

RELATED PARAMETERS TOLERANCE, ALIGNMENT AND EQUALIZATION In a machine, alignment and load distributionare not perfect because of manufacturing tolerances in the housing, shaft and bearingelements. There are three areas of concern:

1. The squareness of the collar (and parallelismof the journal) to the axis of the shaft which isassembled to, or machined on the shaft.

2. The alignment of shaft with the bearing andhousing, which is a manufacturing tolerancestack-up of the bearing parts and the housingbores and faces.

3. The alignment of shafts between machineswhich are aligned and coupled together on site.

Misalignment of the shaft to the bearing andhousing, and between machines is consideredstatic misalignment and can be adjusted at

assembly if proper design features are incorpo-rated. Other sources of misalignment termeddynamic misalignment are due to operating orchanging conditions such as: thermal housingdistortion, shaft deflection from imposed loads,movement caused by thermal expansion, move-ment caused by settling of foundations, pipestrain, etc.

In pivoted shoe thrust bearings, static mis-alignment and manufacturing tolerances in theshoe height are accommodated by the levelingplates.

Referring to Figure 8-2, the load transmittedby the rotating collar to any thrust shoe forcesthe shoe against the upper leveling platebehind it. If one shoe were slightly thickerthan the others, the resulting higher film forcebears the shoe down against the upper levelingplate. Each upper leveling plate is supportedon one radial edge of each of two adjacentlower leveling plates. The lower leveling platesrock very slightly and raise the shoes on eitherside and so on around the ring. This featurealso compensates for minor housing deflec-tions or misalignment between the housingssupporting wall and the collar face.

END PLAY AND RADIAL CLEARANCE

End play is the axial thrust bearing clearancewhich is the distance the shaft can movebetween opposing thrust bearings. For journalbearings, the radial clearance is half the differ-ence of the journal bearing bore and journaldiameter. End play and radial clearance arerequired to allow for misalignment, shoemovement, and thermal expansion of theparts. If set too tight, power is wasted. If too

Figure 8-1 Pivoting Shoe Hydrodynamic Film Formation

Figure 8-2 Thrust Bearing Load Equalization and Alignment Features

loose, the unloaded side shoes are too far fromthe shaft to develop a film pressure and canflutter causing damage to the unloaded shoes.Filler plates and shim packs provide a meansfor setting end play and axial positioning ofthe rotating elements. Adjusting screws arealso used to accomplish this function.

PRELOAD

Preload pertains mostly to journal bearingsand is a measure of the curvature of the shoeto the clearance in the bearing. The shoe cur-vature is another parameter which effects thehydrodynamic film, allowing design variationsin bearing stiffness and damping to control thedynamics of the machine. The geometry anddefinition are given in Figure 9.

LUBRICATION

For hydrodynamic bearings to operate safelyand efficiently, a suitable lubricant must alwaysbe present at the collar and journal surfaces.The lubricant needs to be cooled to remove

the heat generated from oil shear, before re-entering the bearing. It must also be warmenough to flow freely, and filtered so that theaverage particle size is less than the minimumfilm thickness.

Various methods are applied to provide lubri-cant to the bearing surfaces. The bearing cavities can be flooded with oil such as verticalbearings which sit in an oil bath. The bearingscan also be provided with pressurized oil froman external lubricating system. The flow pathof a horizontal, flooded pivoted shoe thrustbearing is shown in Figure10.

For high speed bearings, the frictional lossesfrom oil shear and other parasitic losses beginto increase exponentially as the surface speedenters a turbulent regime. The amount oflubricant required increases proportionately.Industry trends for faster, larger machinesnecessitated the design of lower loss bearings.This has been incorporated by the introduc-tion of other methods of lubrication.

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KSection I

Figure 9 Pivoted Shoe Journal Bearing Preload

1 Oil enters annulus in base ring.

2 Oil passes through radial slots in back face of base ring.

3 Oil flows through clearance between base ring bore and shaft.

4 Oil flows to inner diameter of rotating thrust collar.

5 Oil flows between shoes and into the films.

6 At the collar rim, oil is thrown off into space around the collar.

7 Oil exits tangentially through the discharge opening.

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Directed lubrication directs a spray of oil froma hole or nozzle directly onto the collar (jour-nal) surface between the shoes. Rather thanflooding the bearings, sufficient oil is appliedto the moving surface allowing the bearing torun partially evacuated. Such a method oflubrication reduces parasitic churning lossesaround the collar and between the shoes.

In 1984 Kingsbury introduced its LeadingEdge Groove (LEG) Thrust Bearings (Fig. 11),another technology developed for high techmachines. In addition to reducing oil flow andpower loss, LEG lubrication greatly reduces themetal temperature of the shoe surface. Thiseffectively reduces oil flow requirements andpower loss while improving the load capacity,safety and reliability of the equipment. Ratherthan flooding the bearing or wetting the surface,the LEG design introduces cool oil directly intothe oil film (Fig. 12), insulating the shoe surfacefrom hot oil that adheres to the shaft. Thesame technology is applied to pivoted shoe journal bearings (Fig. 13).

Figure 10 Thrust Bearing Typical Oil Flow Path

Figure 11 Leading Edge Groove (LEG) Thrust Bearing

Oil enters sump and is pumped through a filter and cooler.

Oil passes through inlet orifice which controls flow rate.

COOLING SYSTEM A cooling system is required to remove the heatgenerated by friction in the oil. The housing maysimply be air cooled if heat is low. Vertical bear-ings typically sit in an oil bath with cooling coils(Fig. 14), but the oil can also be cooled by anexternal cooling system as typical in horizontalapplications. The heat is removed by a suitableheat exchanger.

OPERATION AND MONITORINGUnder operation, the capacity of hydrodynamicbearings is restricted by minimum oil film thick-ness and babbitt temperature. The critical limit for low-speed operation is minimum oil filmthickness. In high-speed operation, babbitt temperature is usually the limiting criteria.

Temperature, load, axial position, and vibrationmonitoring equipment are used to evaluate theoperation of the machine so that problems may beidentified and corrected before catastrophic failure.

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Figure 12 Leading Edge Groove Lubricant Flow Path.

Figure 13 Leading Edge Groove (LEG) Journal Bearing

Of these, bearing health are commonly monitored through the use of temperaturedetectors. The temperature of the bearing variessignificantly with operating conditions and alsovaries across and through the shoe. Therefore,for the measurement to be meaningful, thelocation of the detector must be known.

The recommended location for a detector istermed the 75/75 location on a thrust shoeface, i.e. 75% of the arc length of the shoe inthe direction of rotation and 75% of the radialwidth of the shoe measured from the ID to theOD. In a journal bearing, sensor location shouldbe 75% of the arc length on the center line ofthe shoes. This position represents the most critical area because it is the point where peakfilm pressures, minimum film thickness, and hottemperatures co-exist.

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Figure 14 Vertical Thrust Bearing Oil Sump and Cooling Coils

KSection II

INTRODUCTIONThis section of the paper presents an overviewof a structured bearing troubleshootingapproach. The approach is developed based onan understanding of bearing operation and thepotential effects of related parameters. Of par-ticular interest are the rotating journal, collar,or runner, the babbitted shoe surface, all con-tact points within the bearing assembly, andthe lubricating oil. Machine specific opera-tional and performance data must also be considered.

The following approach can be used with alltypes of fluid-film bearings, but the discussionis centered on equalizing thrust bearings.These bearings contain the most moving partsand are widely used. The remarks made hereinare readily adaptable to other fluid-film bear-ing types (i.e., non-equalizing thrust bearings, pivoted shoe journal bearings, journal shells).

When evaluating bearing distress, the babbittedshoe surface is commonly the only area that isexamined. Although a great deal of informa-tion can be extracted from the babbitt appear-ance, additional information exists elsewhere.These "secondary sources" of diagnostic infor-mation often prove to be very valuable, sincethe babbitted surfaces are usually destroyed ina catastrophic bearing failure. Even a bearingwipe, which is the most common appearanceof distress, hides valuable information.

The "textbook" cases of failure modes are especially useful prior to the damage thatoccurs when a bearing can no longer supportan oil film. Through the prudent use of tem-perature and vibration monitoring equipment,routine oil analyses, lubrication system evalua-tions and machine operational performancereviews, bearing distress may be identified andevaluated before catastrophic failure occurs.

Bearing health is commonly monitored throughthe use of temperature measurements. Beaware that temperature sensors are mounted ina wide variety of locations, with a correspond-ing variation in significance. The specific location and type of sensor must be known in order for the measured temperature data tohave any real value.

DISCUSSION To begin an evaluation, the bearing assemblyshould be completely disassembled. In this manner, all of the bearing components may be evaluated. Do not clean the bearing, since valuable information may be lost.

BASE RINGExamine the base ring. During routine opera-tion, the lower leveling plates (LLP) will formindentations in the base ring, on either side ofthe dowels that locate them. The indentationsshould be identical and barely noticeable. Deep,wide indentations are an indication of a highload. The rocking strip on the bottom of theLLP contacts the base ring, and its conditionpresents another indication of bearing load.

The cleanliness of the bearing and oil can alsobe determined, since deposits are often trappedin the base ring. Evidence of water contamina-tion, particularly in vertical machines, may gounnoticed unless the base ring is examined.

LEVELING PLATESThe spherical pivot in the rear of each thrustshoe rests in the center of a flat area on thehardened upper leveling plate (ULP). This flat-tened area is susceptible to indentation due tothe point contact of the pivot. The indentationis easily identified by a bright contact area. Thisarea indicates where the shoe operates on theULP, and its depth gives an indication of load.Close examination of the upper leveling platenear the contact area may also produce evidence of electrical pitting.

As noted previously in SECTION I, the upperleveling plates interact with the lower levelingplates on radiused "wings." The upper levelingplates are typically hardened; the lower levelingplates are sometimes not. When new, the leveling plates have line contact. There is littlefriction between the wings, and the bearing can react quickly to load changes.

Depending on the nature and magnitude of thethrust load, the wing contact area will increasein time. The contact region of the wings, againnoted by bright areas, will normally appearlarger on the lower leveling plates. If the rotat-ing collar is not perpendicular to the shaft axis,the leveling plates will continuously equalize,causing rapid wear.

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SHOE SUPPORTThe shoe pivot is the hardened spherical plugin the rear face of each thrust shoe. Based onthe magnitude and nature of the thrust loads,the spherical surface will flatten where it con-tacts the upper leveling plate. The contact areawill appear as bright spot on the plug. If evi-dence of hard contact exists (a large contactspot), rest the shoes (pivot down) on a flat surface. If the shoes do not rock freely in alldirections they should be replaced.

The pivot can also appear to have randomcontact areas, indicating excessive end play, or it may be discolored, indicating lack oflubrication.

SHOE BODYThe shoe body should be periodically examined for displaced metal or pitting.Indentations routinely occur where the shoecontacts the base ring shoe pocket in the direction of rotation. Displaced metal exhibit-ing a coarse grain may indicate erosion damage; bright or peened spots may indicateunwanted contact. Depending upon the shapeof the individual pits, pitting may indicate corrosion or undesirable stray shaft currents.

SHOE SURFACEWhen evaluating the shoe surface, the first

step is to determine the direction of rotation.This may be accomplished by evaluating:

Abrasion scratches Discoloration (75-75 location) Babbitt flow Babbitt overlay Thrust shoe/base ring contact

Use caution when evaluating babbitt overlay(babbitt "rolled over" the edges of the shoes),since it may appear on both the leading andtrailing shoe edges.

NORMAL

A healthy shoe will exhibit a smooth finish,with no babbitt voids or overlays. The dullgrey finish of a brand new shoe may remainunchanged after many hours of operation, orit may appear glossy in spots or in its entirety.Routine thermal cycling of the bearing maycause the emergence of a mild "starburst" ormottled pattern in the babbitt. This is harmless, providing the shoe is flat and cracks do not exist.

SCRATCHES

AbrasionA bearing surface exhibiting circumferentialscratches is the result of abrasion damage (Fig.15). Abrasion is caused by hard debris, which islarger than the film thickness, passing through

Figure 15 Thrust Shoe Surface Abrasion

KSection II

the oil film. The debris may embed itself in thesoft babbitt, exhibiting a short arc on the shoesurface, ending at the point where the debrisbecomes embedded. Depending on the debrissize, the scratch may continue across the entireshoe surface.

Abrasion damage becomes worse with time.Surface scratches allow an escape for lubricatingoil in the oil wedge, decreasing the film thick-ness. This will eventually lead to bearing wipe.

Another source of abrasion damage is a roughjournal, collar or runner surface. Roughnessmay be due to previous abrasion damage. Itmay also be from rust formed after extendedperiods of down time. New bearings should notbe installed when the rotating component isvisibly damaged.

Random scratches, which may run a staggeredpath both circumferentially and radially, aremore likely to appear in the unloaded bearingor unloaded portion of the bearing. In a thrustbearing, it may indicate excessive end play(axial clearance). Random scratches may alsoindicate careless handling at installation or disassembly.

In order to eliminate abrasion damage, thelubricating oil must be filtered. If the oil cannot be filtered or has degraded, it should bereplaced. It is important to evaluate the filtering

system, since the problem may be an incorrectlysized filter. The filter should only pass debrissmaller in size than the predicted bearing minimum film thickness.

In addition to filtering/replacing the oil, theentire bearing assembly, oil reservoir and pipingshould be flushed and cleaned. The originalbearing finish should also be restored. Journalshoes typically must be replaced, but if the correction leaves the bearing within design tolerance, the bearing may be reused.

Although the babbitted surface is usually damaged more severely, the rotating collar orjournal surface must also be evaluated. Debrispartially lodged in the babbitt may score thesteel surfaces.These surfaces must be restored by lapping or hand stoning.

DISCOLORED

Tin Oxide Damage This is one of several electrochemical reactionswhich eliminate the embedability properties of a fluid-film bearing. Tin oxide damage (Fig.16) is recognizable by the hard, dark brown or black film that forms on the babbitt.

Tin oxide forms in the presence of tin-basedbabbitt, oil and salt water, beginning in areasof high temperature and pressure. Once it has formed, it cannot be dissolved, and its

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Figure 16 Tin Oxide Damage

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hardness will prevent foreign particles fromembedding in the bearing lining. This allowsabrasion damage to occur. Pieces of tin oxidemay break off during operation and score thejournal, collar, or runner. The formation of tinoxide will also eliminate bearing clearance.

This damage may be stopped by eliminatingsome or all of the contributing elements. Thelubricating oil must be replaced. A reduction in oil temperature may also discourage theformation of tin oxide.

In addition to replacing the oil, the entire bearing assembly, oil reservoir and pipingshould be flushed and cleaned with mineralspirits. The bearing shoes should be replaced.The condition of the rotating journal, collar orrunner surfaces must also be evaluated. Theymust be restored to original condition, either by lapping, hand stoning or replacement.

OverheatingOverheating damage may represent itself inmany ways, such as babbitt discoloration (Fig.18), cracking, wiping or deformation. Repeatedcycles of heating may produce thermal ratcheting(Fig. 17), a type of surface deformation thatoccurs in anisotropic materials. These materials possess different thermal expansion coefficientsin each crystal axis.

Overheating may be caused by numeroussources, many of which concern the quantityand quality of the lubricant supply. Among thepossible causes are:

Improper lubricant selection Inadequate lubricant supply Interrupted fluid film Boundary lubrication

The following conditions may also cause overheating:

Improper bearing selection HP lift system failure Poor collar, runner or journal surface finish Insufficient bearing clearance Excessive load Overspeed Harsh operating environment

Figure 17 Thermal Ratcheting

Figure 18 Overheating, Oil Additives Plated Out

KSection II

Verify that the quantity and quality of oil flowing to the bearing is sufficient. These values should be available from the bearing manufacturer.

If thermal ratcheting has occurred, examine theshoes for the existence and depth of cracks.Remove the cracks and restore the original shoesurface. If this cannot be done, replace theshoes. Journal shoes typically must be replaced,but if the correction leaves the bearing withindesign tolerance, the bearing may be reused.

The condition of the rotating journal, collar orrunner surfaces must also be evaluated. It mustbe restored to original condition, either by lapping, hand stoning or replacement.

Oil StarvationIt is often possible to distinguish oil starvation(i.e. the total absence of lubricant) from a lessthan adequate oil flow by closely examining thebabbitted surface of the bearing.

In Fig. 19, the babbitt has been completelyremoved from the shoe surface. If there hadbeen any oil flow to the bearing, it would havesufficiently cooled the region to allow the moltenbabbitt to at least solidify and accumulatebetween the shoes. With no oil coming in atthe leading edge of the shoe, this area typically shuts down, resulting in the babbittbeing eliminated in the corner near the outerdiameter in Fig. 20.

In combination assemblies, such as KingsburysCH unit, the cut-off of oil supply will also leadto destruction of the journal shell, where themolten babbitt can pool in the lower half. Theextreme temperatures in such cases can lead tothe fusion of the oil circulator to the collar (see Fig. 21), which has no quench marks inthis instance, prime evidence that no lubricantever reached the parts.

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Figure 19 Pivoted Shoe Showing Complete Babbitt Removal

Figure 20 Outer Edge Babbitt Erosion

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VOIDS

Hydrogen BlistersIn certain occasions, hydrogen gas becomestrapped during the casting or forging of partsmade of steel. If the steel base is babbitted andthe gas eventually migrates to the surface, blisters can form in the area, significantly weakening the babbitt bond (see Fig. 22).

Thermal annealing has been proven to preventblisters from forming by diffusing out hydro-gen before the part is babbitted. In Figure 23,the annealing process was accidentally missedand large hydrogen blisters emerged during themachining process.

Electrical Pitting Electrical pitting (Fig. 24) appears as roundedpits in the bearing lining. The pits may appearfrosted, or they may be blackened due to oildeposits. It is not unusual for them to be verysmall and difficult to observe with the unaidedeye. A clearly defined boundary exists betweenthe pitted and unpitted regions, with the pit-ting usually occurring where the oil film isthinnest. As pitting progresses, the individualpits lose their characteristic appearance as they begin to overlap. Pits located near the bound-ary should still be intact. The debris that entersthe oil begins abrasion damage. Once the bear-ing surface becomes incapable of supportingan oil film, the bearing will wipe. The bearingmay recover an oil film and continue to oper-ate, and pitting will begin again. This processmay occur several times before the inevitablecatastrophic bearing failure.

Electrical pitting damage is caused by intermit-tent arcing between the stationary and rotatingmachine components. Because of the smallfilm thicknesses relative to other machineclearances, the arcing commonly occursthrough the bearings. Although the rotatingand other stationary members can also beaffected, the most severe pitting occurs in thesoft babbitt.

Electrical pitting can be electrostatic or electromagnetic in origin. Although both

Figure 22 Hydrogen Blisters In The Babbitt Figure 23 Large Hydrogen Blisters

Figure 21 Oil Circulator Fused To Collar

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sources result in pitting damage, they differ inorigin and destructive capabilities.

Electrostatic shaft current (direct current) is themilder of the two. Damage progresses slowly,and it always occurs at the location with thelowest resistance to ground. It can be attributedto charged lubricant, charged drive belts, orimpinging particles.

This type of shaft current can be eliminatedwith a grounding brushes or straps. Bearing isolation is also recommended.

Electromagnetic shaft current (alternating current) is stronger and more severe than electrostatic current. It is produced by the magnetization of rotating and/or stationarycomponents. This type of current will notalways occur at the location of lowest resistance.Because the current is stronger, bearing damageis often accompanied by journal, collar or runner damage.

Electromagnetic currents are best eliminated bydemagnetizing the affected component.Grounding brushes or straps may or may not be helpful. The bearings should be isolated.

The lubricating oil must be filtered or replaced.Pitting damage often blackens the oil and fills it

with debris. In addition to filtering/replacingthe oil, the entire bearing assembly, oil reservoirand piping should be flushed and cleaned.

The original bearing finish should also berestored. Journal shoes typically must bereplaced, but if the correction leaves the bearingwithin design tolerance, the bearing may bereused. The condition of the rotating journal,collar or runner surfaces must also be evaluated.It must be restored to original condition, eitherby lapping, hand stoning or replacement.

FatigueFatigue damage (Fig. 25, 26, 27) may representitself as intergranular or hairline cracks in the babbitt. The cracks may appear to open in thedirection of rotation. Pieces of babbitt may spallout or appear to be pulled away in the direction ofrotation. The cracks extend toward the babbittbond line, and may reveal the shoe backing.

A combination of causes contribute to fatiguedamage, but concentrated cyclic loading is usuallyinvolved. The fatigue mechanism involves repeatedbending or flexing of the bearing, and damageoccurs more rapidly with poor bonding.

It is important to note that fatigue damage willoccur without poor bonding. Fatigue can occurwhen conditions produce concentrated cyclicloads, such as:

Misalignment Journal eccentricity Imbalance

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Figure 24 Stray Shaft Currents/Electrical Pitting (Frosting)

Figure 25 Edge Load Pivoted Shoe Showing Babbitt Mechanical Fatigue

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Bent shaft Thermal cycling Vibration

Performance data should be reviewed to determineif a vibration increase occurred. The leveling platewings should be examined for signs of excessivewear, indicating the rotating collar or runner is notperpendicular to the shaft axis.

High bearing temperature may also be consideredas a contributing factor to fatigue damage. As temperatures increase, the fatigue strength of bearing materials decrease.

The lubricating oil must be filtered or replaced. In addition to filtering/replacing the oil, the entirebearing assembly, oil reservoir and piping shouldbe flushed and cleaned.

Depending on the extent of damage, voids in thebabbitt can be puddle-repaired. The original bearing finish must be restored. Journal shoes mayalso be puddle-repaired and refinished. If this cannot be done, the shoes must be replaced.

Although the babbitted surface is usually damagedmore severely, the rotating collar or journal surfacemust also be evaluated. This surface must also berestored to original condition, either by lapping orhand stoning.

Fretting CorrosionFretting, like fatigue, is due to a cyclical vibration, but it is normally due to minute oscillations between two non-rotating surfaces.

Prime examples are the contact areas betweenshoe supports and leveling plates, journal bearing shells and the housing (see Fig. 28), andparts with an interference fit, such as the collarand shaft. As can be noted in the photograph,fretting corrosion is characterized by a red-orangeiron oxide, which forms on the freshly machinedsteel surfaces. To reduce the risk of recurrence, thereplacement parts should be adjusted to obtain abetter fit that eliminates the oscillations.

This journal shell had too much clearancebetween its outer diameter and the housing,which resulted in the typical orange-brown oxidation of the exterior surface of the bearing.

SpraggingIn a tilting-pad journal bearing, the upper padstypically receive no load. These pads tend to floatback and forth between the pivot point and the

Figure 26 Edge Load Journal Shell With Babbitt Mechanical Fatigue

Figure 27 Fatigue

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shaft. The frequency of motion is usually abouthalf that of the rotating speed of the journal. This self-excited motion is frequentlydetrimental to the pad, causing a cyclical loadon the leading edge that erodes away the bab-bitt (Fig. 29 and 30). Pads with a smaller arc-length have been found to be much less proneto spragging, so a 5-pad bearing will be lesslikely to experience this phenomenon than a 4-pad bearing. When a bearing must have lessthan five pads and there is a relatively light load,the axial length of the bearing can be reducedto exert more force on the pads. In addition,when there is limited preload on the originalpads, the bearing manufacturer should considerintroducing a relief into the leading edge of thepads in order to eliminate self-excited vibration.

CavitationCavitation damage appears as discreet irregularly-shaped babbitt voids which may ormay not extend to the bond line. It may alsoappear as localized babbitt erosion. The location of the damage is important in determining the trouble source.

Often called cavitation erosion, cavitationdamage (Fig. 32 and 33) is caused by the formation and implosion of vapor bubbles in

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Figure 28 Fretting Corrosion of Shell And Housing

Figure 30 Spragging Effects On Pad

Figure 29 Eroded Babbitt From Spragging

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I areas of rapid pressure change. Damage oftenoccurs at the outside diameter of thrust bear-ings due to the existence of higher velocities.This type of damage can also affect stationarymachine components in close proximity tothe rotor.

Based on its source, cavitation can be eliminated in a number of ways:

Radius/chamfer sharp steps Modify bearing grooves Reduce bearing clearance Reduce bearing arc Eliminate flow restrictions (downstream) Increase lubricant flow Increase oil viscosity Lower the bearing temperature Change oil feed pressure Use harder bearing materials

The lubricating oil must be filtered orreplaced. In addition to filtering/replacing theoil, the entire bearing assembly, oil reservoirand piping should be flushed and cleaned.

Depending on the extent of damage, voids inthe babbitt can be puddle-repaired. The original bearing finish must be restored.Journal shoes may also be puddle-repaired and refinished. If this cannot be done, theshoes must be replaced.

Although the babbitted surface is usually dam-aged more severely, the rotating collar, runneror journal surface must also be evaluated. This

surface must also be restored to original condition, either by lapping or hand stoning.

ErosionErosion damage may appear as localized bab-bitt voids with smooth edges, particularly inthe direction of rotation. Damage is more likely to occur in stationary members.

As a rule of thumb, if the babbitt has beenaffected, the cause was cavitation damage, noterosion. Since erosion is caused by suddenobstructions in oil flow, it is more likely tooccur in other areas, since the babbitt is underhigh pressure. Once damaged, however, babbitt erosion may occur.

Corrective action is similar to that employedin eliminating cavitation damage, with theemphasis on streamlining oil flow through thebearing.

Figure 33 Cavitation Damage on OutsideDiameter of Collar

Figure 32 Thrust Shoe Cavitation Towards Outside Diameter

CorrosionCorrosion damage (Fig. 34) is characterized by the widespread removal of the bearing lining bychemical attack. This attack produces a latticeworkappearance. The damage may be uniform with theaffected elements being "washed away," leaving thecorrosion resistant elements behind.

Corrosion may also affect the rotating collar, run-ner or journal, appearing as random, widespreadrust or pitting. The pits are easily distinguishedfrom electrical pitting, since they are not as uniform or smooth-bottomed.

Corrosive materials may appear in the lubricatingoil through:

Decomposition of oil additives Acidic oxidation products formed in service Water or coolant in lube oil Direct corrosive contamination

Bearing housing seals, oil additive packages, andoil reservoir operating temperatures should be evaluated as an initial step in eliminating corrosion. The integrity of cooling coils shouldalso be examined.

The cause of corrosion is best detected by knowledge of the babbitt composition and an oilanalysis. Corrosion can be eliminated by replacingthe lubricating oil. In addition, the entire bearingassembly, oil reservoir and piping should beflushed and cleaned. If the original bearing finishcannot be restored, the bearing must be replaced.

The rotating collar, runner or journal surface must also be evaluated and restored to originalcondition, either by lapping or hand stoning.

COLLAR/RUNNER/JOURNAL SURFACEThe most commonly overlooked bearing component is the collar. It is the single mostimportant part of the bearing.

Collar rotation draws oil into the region betweenthe collar and shoe surfaces. Oil adheres to the collar and is pulled into pressurized oil wedges.This occurs due to the collar surface finish. If thecollar finish is too smooth (better than 12 RMS),it will not move an adequate supply of oil; toorough, and the bearing shoes will be damaged.Ideally the finish should be between 12 - 16 RMS.

Each time a bearing is inspected, the collar shouldbe inspected and worked as necessary. Glossy areason the collar can easily be removed by hand scrubbing with a soft 600 grit oilstone. Collarswith significant operating time may have lost theiroriginal surface flatness. This flatness, as well as the surface finish, should be restored.

If a split runner is used, it should be separated intohalves and evaluated. Relative motion between thehalves will result in fretting damage to the runner,as well as potential cavitation-like damage to thebearing surfaces.

It is very important that the collar faces be parallel,and perpendicular to the centerline of the shaft.

Figure 34 Example of Bearing Corrosion

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If the collar is not within tolerance, the resul-tant "wobble" (Fig. 36) will force the shoesand leveling plates to constantly equalize, caus-ing rapid leveling plate wear.

OILA quick visual examination of the oil or oil filter may be all that is required to determinethat a problem exists and that further investi-gation is necessary. Cloudy or discolored oilindicates that a problem exists.

A thorough oil analysis can provide very usefuldata to assist in diagnosing bearing or machinedistress. Be aware that the usefulness of theanalysis is directly related to the informationyou request. As a minimum, the followingshould be supplied:

Particulate density Particulate breakdown Viscosity Water contamination Chemical breakdown

The amount of particulate, as well as its content, can identify potential trouble spots.Oil viscosity will decrease in time, andwhether or not distress is suspected, it shouldbe periodically evaluated. Water contaminationis extremely unwanted, since it can cause rustand oil foaming, and if it is drawn into the oilfilm, bearing failure. A chemical breakdown ofthe oil will help to determine the integrity ofadditive packages and the presence of unwantedcontaminants.

OPERATIONAL DATAPerhaps the most important source of diagnostic information is unit operationaldata. Identifying periods of load or speedchanges, recent maintenance, pad temperatureand vibration level trends, or the performanceof related machinery may also help determinethe root cause of distress.

Vibration data or an analysis may help discoverexisting problems, as well as examining theremaining bearings in a troubled unit.

AFTERMARKET SUPPORTKingsbury operates aftermarket support facili-ties capable of inspecting and evaluating allmakes, models, and types of fluid-film thrustand journal bearing designs. In most cases,Kingsbury is able to provide an evaluation and recommended repair, replacement, or performance upgrade at no charge and no

Figure 36 Leveling Plate Wear Due ToCollar Wobble

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eferencescommitment to the customer. In addition,Kingsbury Field Service Technicians andEngineers are available to assist either in person or by phone and other means of communication. Please visit our websitewww.kingsbury.com for further details.

References1. Sohre, J.S., and Nippes, P.I.,"Electromagnetic Shaft Currents andDemagnetization on Rotors of Turbines andCompressors", Proceedings of the SeventhTurbomachinery Symposium (1978)

Recommended Reading andReferences"Practical Machinery Management for ProcessPlants", Bloch & Geitner, Volume 2, GulfPublishing Company

"Thermal Aspects of Fluid Film Tribology",Oscar Pinkus, ASME Press

"Theory and Practice of Lubrication forEngineers", by Dudley D. Fuller, Wiley &Sons, Inc.

EPRI GS-7352 Project 1648-10 Final Report"Manual of Bearing Failures and Repair inPower Plant Rotating Equipment", MTI

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