A GENERAL GUIDE TO THE PRINCIPLES, O P E R ATION AND TROUBLESHOOTING OF

H Y D R O D Y N A M I CB E A R I N G S

The Best Support In The Bu s i n e s s .

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Preface

IntroductionHistoryTypical 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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During eve ry second of eve ry 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 pre s s u re is placed

on those invo l ved to correct the problem(s). It is the intention of this presentation to assist the reader in pro b l e m

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

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

Rolling contact – load supported by balls or ro l l e r s .Hyd ro s t a t i c – load supported by high pre s s u re fluid.Hyd ro d y n a m i c – load supported by a lubricant film.Ma g n e t i c – 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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HISTORYIn the late 1880s, experiments we re beingconducted on the lubrication of bearing sur-faces. The idea of “f l o a t i n g” a load on a filmof oil grew from the experiments ofBeauchamp Tower and the theoretical work ofOsborne Re y n o l d s .

PIVOTED SHOE THRUST BEARINGS (Fig. 1)Prior to the development of the pivoted shoet h rust bearing, marine propulsion relied on a“ h o r s e s h o e” bearing which consisted of seve r a lequally 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 we re on the ord e rof 40 psi. Comparison tests against a pivo t e dshoe thrust bearing of equal capacity showe dthat the pivoted shoe thrust bearing, at only1/4 the size, had 1/7 the area but operated suc-cessfully with only 1/10 the frictional drag ofthe horseshoe bearing.

In 1896, inspired by the work of OsborneReynolds, Albert Kingsbury conceived and test-ed a pivoted shoe thrust bearing. According toDr. Kingsbury, the test bearings ran well. Smallloads were applied first, on the order of 50 psi(which was typical of ship propeller shaft unitloads at the time). The loads were graduallyincreased, finally reaching 4000 psi, the speedbeing about 285 rpm.

FIRST APPLICATION In 1912, Albert Kingsbury was contracted bythe Pennsylvania Water and Power Company toapply his design in their hydroelectric plant atHoltwood, PA. The existing roller bearingswere causing extensive down times (several out-ages a year) for inspections, repair and replace-ment. The first hydrodynamic pivoted shoethrust bearing was installed in Unit 5 on June22, 1912. At start-up of the 12,000 kW unit,the bearing wiped. In resolving the reason forfailure, much was learned on tolerances and

Figure 1 Hydrodynamic, Equalizing Pivoted Shoe Thrust Bearing

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finishes re q u i red for the hyd rodynamic bear-ings to operate. After properly finishing therunner and fitting the bearing, the unit ranwith continued good operation. This bearing,owing to its merit of running 75 years withnegligible wear under a load of 220 tons, wasdesignated by ASME as the 23rd In t e r n a t i o n a lHistoric Mechanical Engineering Landmarkon June 27, 1987.

JOURNAL BEARINGS

The cylindrical hydrodynamic journal bearing isthe most basic hydrodynamic bearing. It has acylindrical bore, typically with two axial groovesfor lubrication. This bearing has a high loadcapacity, and the simple design is compact, bi-rotational, and easy to manufacture. However,

as the design speeds of machines increased, itwas found this bearing had limitations due to oilwhirl. Oil whirl is very undesirable because ofhigh vibration amplitudes, forces, and cyclicstresses that are imposed on the shaft, bearingsand 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 approx i m a t e l ys e ve n t y - f i ve years ago. Ex t e n s i ve tests and appli-cations have proved the pivoted shoe journal bear-ing to be most effective in eliminating oil whirl.

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Figure 2 Hydrodynamic Pivoted Shoe Journal Bearing

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TYPICAL 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 provid-ed additional benefits in regard to shockabsorbing capability, and allowed damping asa design parameter to control vibration.These considerable benefits allowed the d e s i g nto be used in a wide variety of applications.Indeed, the invention made it possible to buildthe high-tech machines and ships 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 para m e t e r sand operation of the hyd rodynamic pivoted shoe bearing..

HYDRODYNAMIC BEARINGSBearings transmit the rotating shaft’s loads to thefoundation or machine support. Hydrodynamicbearings transmit (float) the load on a self-renewing film of lubricant. Thrust bearings sup-port the axial loads. Radial loads are supportedby journal bearings. The machine and bearingcan be classified as horizontal or vertical depend-ing on the orientation of the shaft. The bearingsmay be solid for assembly over the end of theshaft, or split for assembly around the shaft..

HYDRODYNAMIC PRINCIPLE JOURNAL BEARINGS Based on his theoretical investigation of cylin-drical journal bearings, Professor Os b o r n eReynolds showed that oil, because of its adhe-sion to the journal and its resistance to flow (vis-cosity), is dragged by the rotation of the journalso as to form a wedge-shaped film between thejournal and journal bearing (Fig. 4). This actionsets up the pressure in the oil film which there-by supports the load (Fig. 5).

This wedge-shaped film was shown by Reynoldsto be the absolutely essential feature of effectivejournal lubrication. Reynolds also showed that

“if an extensive flat surface is rubbed over aslightly inclined surface, oil being present, therewould be a pressure distribution with a maxi-mum somewhere beyond the center in the direc-tion 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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Figure 4 Hydrodynamic Principle

Figure 5 A figure from Reynold’s Paper “On the Theory of Lubrication”showing oil film pressure distribution.

Figure 6 Illustration from a page from AlbertK i n g s bu ry ’s Paper “ D evelopment of the Kingsbu ry Thrust Bearing”

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about the theoretical center of pressure, the oilp re s s u res would automatically take the theore t i-cal form, with a resulting small bearing frictionand absence of wear of the metal parts. In thisway a thrust bearing could be made with seve r a lsuch blocks set around in a circle and with pro p-er arrangements for lubrication.” The same con-cept applies to the pivoted shoe journal bearing.

As with the plain cylindrical bearing, the pivot-ed shoe thrust and journal bearings rely onadhesion of the lubricant to provide the filmwith a self-renewing supply of oil.

BASIC PIVOTED SHOE THRUST (& JOURNAL) PARTS (See Fig. 7)This section discusses the associated thru s tbearing parts, with corresponding journal bear-ing 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 or shrinkfit, or it may be an integral part of the shaft.The collar is called a runner in ve rt i c a lmachines. (In the radial direction, the shaftjournal transmits the radial loads to the journalshoes through the lubricant film.) In hydrody-namic bearings, the fluid film is on the order of.025 mm (.001”) thick. With this and theinformation from HYDRODYNAMIC PRIN-CIPLE, two points can be realized:

1. The stack-up of tolerances and misalignmentin hydrodynamic bearings has to be conserva-tively less than .025 mm (.001”), or somemeans 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 lubri-cant to the surface.

THRUST SHOE (JOURNAL SHOE) ASSEMBLYThe shoe (also called a pad, segment, or block)is loosely constrained so it is free to pivot. Theshoe has three basic features - the babbitt, body,and pivot, and so is usually referred to as anassembly.

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 supporting struc-ture which holds the babbitt and allows free-dom to pivot. The material is typically steel.Bronze is sometimes used (with or without bab-bitt) depending on the application. Chromecopper is used to reduce babbitt temperature.

PIVOT - The pivot allows the shoe to rotate andform a wedge. It may be integral with the shoeb o d y, or be a separate insert. The pivot surface isspherical to allow 360˚ rolling freedom.

BASE RING (ALIGNING RING) The base ring loosely holds and constrains theshoes against rotating so as to allow freedom top i vot. It may have passages for the supply of lubri-cant, and contain features to adapt for misalign-ment and tolerance in the parts. The base ring(aligning ring) is keyed or doweled to the housingto pre vent rotation of the bearing assembly.

LEVELING PLATES The leveling plates (not applicable to journalbearings) are a series of levers designed to com-pensate for manufacturing tolerances by distrib-uting the load more evenly between thrustshoes. The leveling plates also compensate forminor housing deflections or misalignment

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Figure 7 Thrust and Journal Bearing Part Schematic.

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between the collar and the housing’s supportingwall. A description is given later under the sec-tion on misalignment.

LUBRICANT The lubricant is another important “element”of the bearing (See Fig 8-1). The loads are trans-mitted from the shaft to the bearing throughthe lubricant which separates the parts and pre-vents metal to metal contact. The lubricantalso serves to carry heat caused by friction outof the bearing.

RELATED PARAMETERS TOLERANCE, ALIGNMENT AND EQUALIZATION In a machine, alignment and load distributionare not perfect because of manufacturing toler-ances in the housing, shaft and bearing ele-ments. 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 housingb o res 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 considered sta-tic misalignment and can be adjusted at assemblyif proper design features are incorporated. Ot h e rs o u rces of misalignment termed dynamic mis-

alignment are due to operating or changing condi-tions such as: thermal housing distortion, shaftdeflection from imposed loads, movement causedby thermal expansion, movement caused by set-tling of foundations, pipe strain, etc.

In pivoted shoe thrust bearings, static misalign-ment and manufacturing tolerances in the shoeheight are accommodated by the leveling plates.

Referring to Figure 8-2, the load transmitted bythe rotating collar to any thrust shoe forces theshoe against the upper leveling plate behind it.If one shoe were slightly thicker than the others,the resulting higher film force bears the shoedown against the upper leveling plate. Eachupper leveling plate is supported on one radialedge of each of two adjacent lower levelingplates. The lower leveling plates rock veryslightly and raise the shoes on either side and soon around the ring. This feature also compen-sates for minor housing deflections or misalign-ment between the housing’s supporting wall andthe 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, shoe move-ment, and thermal expansion of the parts. If settoo tight, power is wasted. If too loose, theunloaded side shoes are too far from the shaft todevelop a film pressure and can flutter causingdamage to the unloaded shoes. Filler plates and

Figure 8-1 Pivoting Shoe Hydrodynamic Film Formation.

Figure 8-2 Thrust Bearing Load Equalization and Alignment Features.

shim packs provide a means for setting end playand axial positioning of the rotating elements.Adjusting screws are also used to accomplishthis function.

PRELOAD Preload pertains mostly to journal bearings andis a measure of the curvature of the shoe to theclearance in the bearing. The shoe curvature isanother parameter which effects the hydrody-namic film, allowing design variations in bear-ing 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 theheat generated from oil shear, before re - e n t e r i n gthe bearing. It must also be warm enough to

f l ow fre e l y, and filtered so that the average part i-cle size is less than the minimum film thickness.

Various methods are applied to provide lubri-cant to the bearing surfaces. The bearing cavi-ties can be flooded with oil such as vertical bear-ings which sit in an oil bath. The bearings canalso be provided with pressurized oil from anexternal lubricating system. The flow path of ahorizontal, flooded pivoted shoe thrust bearingis shown in Figure 10.

For high speed bearings, the frictional lossesfrom oil shear and other parasitic losses begin toi n c rease exponentially as the surface speedenters a turbulent regime. The amount oflubricant re q u i red increases pro p o rt i o n a t e l y.In d u s t ry trends for faster, larger machinesnecessitated the design of lower loss bearings.This has been incorporated by the introductionof other methods of lubrication.

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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 (journal)surface between the shoes. Rather than floodingthe bearings, sufficient oil is applied to the mov-ing surface al l owing the bearing to run part i a l l ye vacuated. Such a method of lubrication re d u c e sparasitic churning losses around the collar andb e t ween the shoes.

In 1984 Kingsbury introduced its Leading EdgeGroove (LEG) Thrust Bearings (Fig. 11), anoth-er technology developed for high tech machines.In addition to reducing oil fl ow and power loss,LEG lubrication greatly reduces the metal temper-a t u re of the shoe surface. This effectively re d u c e soil flow re q u i rements and power loss whilei m p roving the load capacity, safety and re l i a b i l i t yof the equipment. Rather than flooding the bear-ing or wetting the surface, the LEG design intro-duces cool oil directly into the oil film (Fig. 12),insulating the shoe surface from hot oil thata d h e res to the shaft. The same technology isapplied 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 re q u i red to re m ove the heatgenerated by friction in the oil. The housing maysimply be air cooled if heat is low. Ve rtical bearingstypically sit in an oil bath with cooling coils (Fi g .14), but the oil can also be cooled by an externalcooling system as typical in horizontal applications.The heat is re m oved by a suitable heat exc h a n g e r.

OPERATION AND MONITORINGUnder operation, the capacity of hyd ro d y n a m i cbearings is restricted by minimum oil film thicknessand babbitt temperature. The critical limit for low -speed operation is minimum oil film thickness. Inhigh-speed operation, babbitt temperature is usual-ly the limiting criteria.

Te m p e r a t u re, load, axial position, and vibrationmonitoring equipment are used to evaluate the oper-ation of the machine so that problems may be iden-tified 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 moni-tored through the use of temperature detectors.The temperature of the bearing varies signifi-cantly with operating conditions and also variesacross and through the shoe. Therefore, for themeasurement to be meaningful, the location ofthe 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 in thedirection of rotation and 75% of the radial widthof the shoe measured from the ID to the OD. Ina journal bearing, sensor location should be 75% ofthe arc length on the center line of the shoes. T h i sposition re p resents the most critical area because itis the point where peak film pre s s u res, minimumfilm thickness, and hot temperatures co-exist.

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Figure 14 Vertical Thrust Bearing Oil Sump and Cooling Coils.

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INTRODUCTIONThis section presents an overview of a structuredbearing troubleshooting approach. T h eapproach is developed based on an understand-ing of bearing operation and the potential effectsof related parameters. Of particular interest arethe rotating journal, collar, or runner, the bab-bitted shoe surface, all contact points within thebearing assembly, and the lubricating oil.Machine specific operational and performancedata must also be considered.

The following approach can be used with all typesof fluid-film bearings, but the discussion is cen-t e red on equalizing thrust bearings. These bearingscontain the most moving parts and are widelyused. The re m a rks made herein are readily adapt-able to other fluid-film bearing types (i.e., non-equalizing thrust bearings, pivoted shoe journalbearings, journal shells).

When evaluating bearing distress, the babbittedshoe surface is commonly the only area that isexamined. Although a great deal of informationcan be extracted from the babbitt appearance,additional information exists elsewhere. These“secondary sources” of diagnostic informationoften prove to be very valuable, since the babbit-ted surfaces are usually destroyed in a cata-strophic bearing failure. Even a bearing wipe,which is the most common appearance of dis-tress, may hide valuable information.

The “t e x t b o o k” cases of distress modes are espe-cially useful in diagnosing problems prior to thedamage that occurs when a bearing can no longers u p p o rt an oil film. T h rough the prudent use oft e m p e r a t u re and vibration monitoring equipment,routine oil analyses, lubrication system eva l u a t i o n sand machine operational performance re v i ew s ,bearing distress may be identified and eva l u a t e db e f o re catastrophic failure occurs.

Bearing health is commonly monitored throughthe use of temperature measurements. Be awarethat temperature sensors may be mounted in awide variety of locations, with a correspondingvariation in temperature. The specific locationand type of sensor must be known in order for themeasured temperature data to have any real value.

DISCUSSION (Refer to Fig 7) To begin an evaluation, the bearing assemblyshould be completely disassembled. In this man-ner all of the bearing components may be eva l u a t-ed. Do not clean the bearing, since valuable infor-mation may be lost.

BASE RINGExamine the base ring. During routine operation,the lower leveling plates may form indentations inthe base ring, on either side of the dowels thatlocate them. The indentations should be identicaland barely noticeable. De e p, wide indentations arean indication of a high load. The rocking strip onthe bottom of the lower leveling plates contacts thebase ring, and its condition presents another indi-cation of bearing load.

The cleanliness of the bearing and oil can also bedetermined, since deposits are often trapped inthe base ring. Evidence of water contamination,particularly in vertical machines, may go unno-ticed unless the base ring is examined.

LEVELING PLATESThe spherical pivot in the rear of each thrust shoerests in the center of a flat area on the hard e n e dupper leveling plate. This flattened area is sus-ceptible to indentation due to the point contactof the pivot. The indentation is easily identifiedby a bright contact area. This area indicatesw h e re the shoe operates on the upper leve l i n gplate, and its depth gives an indication of load.Close examination of the upper leveling platenear the contact area may also produce evidenceof electrical pitting.

As noted previously in SECTION I, the upperl e veling plates interact with the lower leve l i n gplates on radiused “wings.” The upper leve l i n gplates are typically hardened; the lowers are not.When new, the leveling plates have line contact.T h e re is little friction between the wings, andthe bearing can react quickly to load changes.Depending on the nature and magnitude of thet h rust load, the wing contact areas will incre a s ein time. The contact region of the wings, againnoted by bright areas, will normally appear larg-er on the lower leveling plates. If the ro t a t i n gcollar is not perpendicular to the shaft axis, thel e veling plates will continuously equalize, caus-ing rapid we a r.

SHOE SUPPORTThe shoe support is the hardened spherical plugin the rear face of each thrust shoe. Based on themagnitude and nature of the thrust loads, thespherical surface will flatten where it contactsthe upper leveling plate. The contact area willappear as a bright spot on the plug. If evidenceof hard contact exists (a large contact spot), restthe shoes (pivot down) on a flat surface. If theshoes do not rock freely in all directions theyshould be replaced.

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The pivot can also appear to have random con-tact areas, indicating exc e s s i ve end play, or it maybe discolored, indicating lack of lubrication.

SHOE BODYThe shoe body should be periodically examinedfor displaced metal or pitting. Indentationsroutinely occur where the shoe contacts thebase ring shoe pocket in the direction of rota-tion. Displaced metal exhibiting a coarse grainmay indicate erosion damage; bright or peenedspots may indicate unwanted contact.Depending upon the shape of the individualpits, pitting may indicate corrosion or undesir-able stray shaft currents.

SHOE SURFACEWhen evaluating the shoe surface, the first stepshould be to determine the direction of ro t a-tion. This may be accomplished by eva l u a t i n g :

• Abrasion scratches• Discoloration (75-75 location)• Babbitt flow• Babbitt ove r l a y• T h rust shoe/base ring contact

Use caution when evaluating babbitt ove r l a y(babbitt “rolled ove r” 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, withno babbitt voids or overlays. The dull grey finishof a brand new shoe may remain unchanged aftermany hours of operation, or it may appear glossyin spots or in its entire t y. Routine thermalc ycling of the bearing may cause the emergenceof a mild “s t a r b u r s t” or mottled pattern in thebabbitt. This is harmless, providing the shoe isflat and cracks do not exist.

SCRATCHESAbrasionA bearing surface exhibiting circ u m f e re n t i a lscratches is the result of abrasion damage ( Fig. 15). Abrasion is caused by hard debris,which is larger than the film thickness, passingt h rough the oil film. The debris may embed itselfin the soft babbitt, exhibiting a short arc on theshoe surface, ending at the point the debrisbecomes embedded. Depending on the debriss i ze, the scratch may continue across the entireshoe surface.

Abrasion damage becomes worse as time pro-gresses. Surface scratches allow an escape forlubricating oil in the oil wedge, decreasing thefilm thickness. This will eventually lead to abearing wipe.

Figure 15 Thrust Shoe Surface Abrasion.

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Another source of abrasion damage is a roughjournal, collar or runner surface. Roughnessmay be due to previous abrasion damage. It mayalso be from rust formed after extended periodsof down time. New bearings should not beinstalled when the rotor is visibly 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 (axialclearance). Random scratches may also indicatecareless handling at installation or disassembly.

In order to eliminate abrasion damage, thelubricating oil must be filtered. If the oil can-not be filtered or has degraded, it should bereplaced. It is important to evaluate the filter-ing system, since the problem may be an incor-rectly sized filter. The filter should only passdebris smaller in size than the predicted bearingminimum film thickness.

In addition to filtering/replacing the oil, thee n t i re bearing assembly, oil re s e rvoir and pipingshould be flushed and cleaned. The originalbearing finish should also be re s t o red. Jo u r n a lshoes typically must be replaced, but if the cor-rection leaves the bearing within design toler-ance, the bearing may be re u s e d .

Although the babbitted surface is usually dam-aged more severely, the rotating collar or jour-nal surface must also be evaluated. Debris par-tially lodged in the babbitt may score the steelsurfaces. These surfaces must be restored by lap-ping or hand stoning.

DISCOLORATIONTin Oxide Damage This is one of several electrochemical reactionswhich eliminate the “embedability” propertiesof a fluid-film bearing. Tin oxide damage isrecognizable by the hard, dark brown or blackfilm that forms on the babbitt (Fig. 16).

Tin oxide forms in the presence of tin-basedbabbitt, oil and salt water, beginning in areas ofhigh temperature and pressure. Once it hasformed, it cannot be dissolved, and its hardnesswill prevent foreign particles from embeddingin the babbitt lining.

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

In addition to replacing the oil, the entire bear-ing assembly, oil reservoir and piping should be

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Figure 16 Tin Oxide Damage

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flushed and cleaned with mineral spirits. Thebearing shoes should be replaced. The condi-tion of the rotating journal, collar or runner sur-faces must also be evaluated. They must berestored to original condition, either by lapping,hand stoning or replacement.

OverheatingOverheating damage may represent itself inmany ways, such as babbitt discoloration, crack-ing, wiping or deformation. Repeated cycles ofheating may p roduce thermal ratcheting, a typeof surface deformation that occurs in anisotro p i cmaterials (Fig. 17). These materials possess dif-ferent thermal expansion coefficients in eachcrystal axis.

Oil additive packages may “plate out” at rela-tively high bearing temperatures. The platingtypically begins in the area of highest tempera-ture, the 75-75 location (Fig. 18).

O verheating may be caused by numero u ssources, many of which concern the quantityand quality of the lubricant supply. Among the

possible causes are:• Improper lubricant selection• Inadequate lubricant supply• Interrupted fluid film• Boundary lubrication

The following conditions may also cause ove r h e a t i n g :

• Improper bearing selection• HP lift system failure• Poor collar, runner or journal

surface finish• Insufficient bearing clearance• Excessive load • Overspeed• Harsh operating environment

Verify that the quantity and quality of oil flow i n gto the bearing is sufficient. These values shouldbe available from the bearing manufacture r.

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 the

Figure 17 Thermal Ratcheting

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shoes. 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 lap-ping, hand stoning or replacement.

VOIDSElectrical Pitting Electrical pitting appears as rounded pits in thebearing lining. The pits may appear frosted (Fi g19), 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 pittingusually occurring where the oil film is thinnest.

As pitting progresses, the individual pits losetheir characteristic appearance as they begin tooverlap. Pits located near the boundary shouldstill be intact. The debris that enters the oil

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Figure 18 Overheating, Oil Additives Plated Out

Figure 19 Stray Shaft Currents/ ElectricalPitting (Frosting).

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begins abrasion damage. Once the bearing sur-face becomes incapable of supporting an oilfilm, the bearing will wipe. The bearing mayrecover an oil film and continue to operate, andpitting will begin again. This process mayoccur several times before the inevitable cata-strophic bearing failure.

Electrical pitting damage is caused by intermit-tent arcing between the stationary and ro t a t i n gmachine components. Because of the small filmthicknesses re l a t i ve to other machine clearances,the arcing commonly occurs through the bear-ings. Although the rotating and other stationarymembers can also be affected, the most seve re pit-ting occurs in the soft babbitt.

Electrical pitting can be electrostatic or electro-magnetic in origin. Although both sourcesresult in pitting damage, they differ in originand destructive capabilities.

El e c t rostatic shaft current (direct current) is themilder of the two. Damage pro g resses slow l y,and it always occurs at the location with the low-est resistance to ground. It can be attributed tocharged lubricant, charged drive belts, orimpinging part i c l e s .

This type of shaft current can be eliminatedwith grounding brushes or straps. Bearing iso-lation is also recommended.

Electromagnetic shaft current (alternating cur-rent) is stronger and more severe than electrosta-tic current. It is produced by the magnetizationof rotating and/or stationary components.

This type of current will not always occur at thelocation of lowest resistance. Because the curre n tis stro n g e r, bearing damage is often accompaniedby journal, collar or runner damage.

Electromagnetic currents are best eliminated bydemagnetizing the affected component.

Figure 20 Edge Load Pivoted Shoe showing Babbitt Mechanical Fatigue

Figure 21 Edge Load Journal Shell with Babbitt Mechanical Fatigue

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Grounding brushes or straps may or may not behelpful. The bearings should also be isolated.

The lubricating oil must be filtered or replaced.Pitting damage often blackens the oil and fills itwith debris. In addition to filtering/replacing theoil, the entire bearing assembly, oil reservoir andpiping should be flushed and cleaned. The origi-nal bearing finish should also be re s t o re d .Journal shoes typically must be replaced, but ifthe correction leaves the bearing within designtolerance, the bearing may be reused. The con-dition of the rotating journal, collar or runnersurfaces must also be evaluated. It must berestored to original condition, either by lapping,hand stoning or replacement.

FatigueFatigue damage may re p resent itself as intergranularor hairline cracks in the babbitt. The cracks mayappear to open in the direction of rotation. Pi e c e s

of babbitt may spall out or appear to be pulled awayin the direction of rotation. The cracks extendt ow a rd the babbitt bond line, and may re veal theshoe backing (Figs. 20-22).

A combination of causes contribute to fatiguedamage, but concentrated cyclic loading is usuallyi n vo l ved. The fatigue mechanism invo l ves re p e a t-ed bending 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• Bent shaft• Thermal cycling• Vibration.

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Figure 22 Babbitt Fatigue in a Thin Thrust Plate

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Performance data should be reviewed to deter-mine if a vibration increase occurred. The level-ing plate wings should be examined for signs ofexcessive wear, indicating the rotating collar orrunner is not perpendicular to the shaft axis.

High bearing temperature may also be consideredas a contributing factor to fatigue damage. Astemperatures increase, the fatigue strength ofbearing materials decreases.

The lubricating oil must be filtered or replaced. In addition to filtering/replacing the oil, theentire bearing assembly, oil reservoir and pipingshould be flushed and cleaned. Depending onthe extent of damage, voids in the babbitt can bep u d d l e - re p a i red. The original bearing finishmust be restored. Journal shoes may also be pud-dle-repaired and refinished. If this cannot bedone, the shoes must be replaced.

Although the babbitted surface is usually dam-aged more severely, the rotating collar or journalsurface must also be evaluated. This surface mustalso be restored to original condition, either bylapping or hand stoning.

Ca v i t a t i o nCavitation damage appears as discreet irre g u l a r-ly-shaped babbitt voids which may or may notextend to the bond line. It may also appear asl o c a l i zed babbitt erosion. The location of thedamage is important in determining the tro u b l es o u rce (Figs. 23-25).

Often called cavitation erosion, cavitation dam-age is caused by the formation and implosion ofvapor bubbles in areas of rapid pressure change.Damage often occurs at the outside diameter ofthrust bearings due to the existence of highervelocities. This type of damage can also affect sta-tionary machine components in close proximityto the rotor. Based on its source, cavitation can beeliminated in a number of ways:

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

Figure 23 Thrust Shoe Cavitation Damage In Babbitt Face

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The lubricating oil must be filtered or replaced.In addition to filtering/replacing the oil, theentire bearing assembly, oil reservoir and pipingshould be flushed and cleaned.

Depending on the extent of damage, voids inthe babbitt can be puddle-repaired. The origi-nal bearing finish must be restored. Journalshoes may also be puddle-repaired and refin-ished. If this cannot be done, the shoes must bereplaced.

Although the babbitted surface is usually dam-aged more severely, the rotating collar, runneror journal surface must also be evaluated. Thissurface must also be restored to original condi-tion, either by lapping or hand stoning.

ErosionErosion damage may appear as localized babbittvoids with smooth edges, particularly in the

direction of rotation. Damage is more likely tooccur 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, bab-bitt erosion may occur.

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

CorrosionCorrosion damage is characterized by the wide-spread removal of the bearing lining by chemi-cal attack. This attack produces a latticeworkappearance. The damage may be uniform with

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Figure 24 Thrust Shoe Cavitation Towards Outside Diameter

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the affected elements being “washed away,”leaving the corrosion resistant elements behind.Corrosion may also affect the rotating collar,runner or journal, appearing as random, wide-spread rust or pitting. The pits are easily dis-tinguished from electrical pitting, since they arenot as uniform or smooth-bottomed.

Corrosive materials may appear in the lubricat-ing oil through:

• Decomposition of oil additive s• Acidic oxidation products formed in serv i c e• Water or coolant in lube oil• Direct corrosive contamination.

Bearing housing seals, oil additive packages,and oil reservoir operating temperatures shouldbe evaluated as an initial step in eliminatingcorrosion. The integrity of cooling coils shouldalso be examined.

The cause of corrosion is best detected byknowledge of the babbitt composition and anoil analysis. Corrosion can be eliminated byreplacing the lubricating oil. In addition, theentire bearing assembly, oil reservoir and pipingshould be flushed and cleaned. If the originalbearing finish cannot be restored, the bearingmust be replaced.

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

COLLAR/RUNNER/JOURNAL SURFACEThe most commonly overlooked bearing com-ponent is the collar. It is the single most impor-tant part of the bearing. Collar rotation drawsoil into the region between the collar and shoesurfaces. Oil adheres to the collar and is pulledinto pressurized oil wedges. This occurs due tothe collar surface finish. If the collar finish istoo smooth (better than 12 RMS), it will notmove an adequate supply of oil; too rough, andthe bearing shoes will be damaged. Ideally thefinish should be between 12 - 16 RMS.

Each time a bearing is inspected, the collarshould be inspected and worked as necessary.Glossy areas on the collar can easily be removedby hand scrubbing with a soft 600 grit oilstone.Collars with significant operating time may

h a ve lost their original surface flatness. This flatness, as well as the surface finish,should be restored.

If a split runner is used, it should be separatedinto halves and evaluated. Relative motionbetween the halves will result in fretting damageto the runner, as well as potential cavitation-likedamage to the bearing surfaces.

It is very important that the collar faces be par-allel, and perpendicular to the centerline of theshaft. If the collar is not within tolerance, theresultant “wobble” will force the shoes and lev-eling plates to constantly equalize, causing rapidleveling plate wear (Fig. 26).

OILA quick visual examination of the oil or oil fil-ter may be all that is required to determine thata problem exists and that further investigationis necessary. Cloudy or discolored oil indicatesthat a problem exists.

Figure 25 Cavitation Damage on OutsideDiameter of Collar

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 information yourequest. As a minimum, the following should besupplied:

• Particulate density• Particulate breakdown• Viscosity• Water contamination• Chemical breakdown

The amount of particulate, as well as its content,can identify potential trouble spots. Oil viscosi-ty will decrease in time, and whether or not dis-tress is suspected, it should be periodically evalu-ated. Water contamination is extremely unwant-ed, since it can cause rust and oil foaming, and ifit is drawn into the oil film, bearing failure. Achemical breakdown of the oil will help to deter-mine the integrity of additive packages and thepresence of unwanted contaminants.

OPERATIONAL DATAAnother important source of diagnostic informa-tion is unit operational data. Identifying periodsof load or speed changes, recent maintenance, orthe performance of related machinery may alsohelp determine the root cause of distress.

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

Figure 26 Leveling Plate Wear due to Collar Wobble

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References1.) So h re, J.S., and Nippes, P. I . ,“ El e c t romagnetic Shaft Cu r rents andDemagnetization on Rotors of Turbines andC o m p ressors,” Proceedings of the Se ve n t hTurbomachinery 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

“T h e o ry 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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