Boilermakers Guide to: Bearing Failure Diagnostics

By addressing the root cause, the conditions promoting ultimate failure are either eliminated or

reduced to minimise the onset or accelerated progression towards failure and hence replacement. The

result is that the system enjoys an extended life and the maintenance costs are thus reduced. Any

activity aimed at addressing the root cause is seen as being proactive. To this end, oil analysis,

balancing and alignment of rotating equipment is a prime example of Proactive Maintenance. By

setting these parameters correctly, the consequential failure owing to vibration damage is eliminated

or at least postponed significantly.

M K Williamson Pr.Eng. B.Sc.

This document is and remains the intellectual property of John C. Ellis. The document is confidential and is issued with the understanding that no original or copy shall be made available for use by third

parties.

Issue: 1 2 3 4 5

Date: 07/06/10 16/11/11

Comment: 1st Issue Formatted

1 Table of Contents

1 Table of Contents ................................................................................................................... 2

2 Purpose .................................................................................................................................. 4

2.1 Economic Aspects: ............................................................................................................................ 5

2.2 General Aspects: ............................................................................................................................... 5

2.3 Safety and Environmental aspects: .................................................................................................. 5

3 Scope ..................................................................................................................................... 5

4 References ............................................................................................................................. 5

5 Abbreviations ......................................................................................................................... 6

6 Preparation ............................................................................................................................ 7

7 Procedure Guidelines .............................................................................................................. 8

8 Preliminary Steps .................................................................................................................... 8

8.1 Required Information ....................................................................................................................... 8

8.2 General Inspection ............................................................................................................................ 9

9 Disassembly ........................................................................................................................... 9

9.1 Angular Contact Ball Bearings .......................................................................................................... 9

9.2 Deep Groove Bearings .................................................................................................................... 10

9.3 Spherical Roller Bearings ................................................................................................................ 10

10 Bearing Terminology ............................................................................................................. 11

11 Recording Visible Damage ..................................................................................................... 11

12 Bearing Troubleshooting ....................................................................................................... 12

13 Bearing Removal ................................................................................................................... 13

14 Bearing Analysis - External Surfaces ...................................................................................... 14

14.1 Corrosion ........................................................................................................................................ 14

14.1.1 Simple Corrosion ........................................................................................................................................ 14

14.2 Fretting Wear (Or “Fretting Erosion”) ............................................................................................ 16

14.3 Fretting Corrosion ........................................................................................................................... 19

14.4 Fretting Fatigue............................................................................................................................... 20

14.5 Color ............................................................................................................................................... 20

15 Internal Surfaces ................................................................................................................... 20

15.1 Scoring ............................................................................................................................................ 20

15.2 Brinelling ......................................................................................................................................... 21

15.3 False Brinelling ................................................................................................................................ 22

15.4 Cage ................................................................................................................................................ 24

15.5 Raceways ........................................................................................................................................ 24

15.6 Smearing ......................................................................................................................................... 25

15.6.1 Run-in Procedure (When speed cannot be varied): .................................................................................. 29

16 Macroscopic Evaluation ........................................................................................................ 29

17 Basic Load-Path Patterns and Interpretation – Courtesy SKF .................................................. 29

2 Purpose

This guide outlines the required conventions, procedures and methodologies for Bearing Failure

Diagnostics and Exception Reporting within a Proactive precision Maintenance (PpM) realm. It also

defines the roles and responsibilities for carrying out such work.

This is not a paper on “Failed Bearing Evaluation”. The aim of the paper is to encourage evaluation of

the machine train, including the failed bearings to arrive at the root cause of the “Bearing Failure”.

Another aim of this guide is to promote the practice of detection and correction, fixing the problem,

not just detecting the failures. Too often we hear of or see success stories on totally wrecked bearings

presented alongside the spectral and vibration data that detected the failure. Many analysts and

engineers consider this success, yet the root cause is never identified. The bearings are replaced and

the unit is put back into service only to suffer the same fate. To this end, there must be a multi-stage

approach: the vibration monitoring program must detect the problem at the earliest opportunity, the

maintenance department must act. Before the bearings are re-installed a “Bearing Failure Diagnostic”

must be carried out. This does not just involve a study of the failed bearings, but also requires an in-

depth understanding of the bearing arrangement, the operating conditions and the actual load

distribution verses the expected load conditions.

The key then is when to intervene. It is essential that the bearing be changed at the right time. Change

it too early; end users consider the PpM programme as ineffective. Change it too late and it may

damage other components along with the evidence indicative of the root cause of the failure.

Remember the aim of a PpM maintenance philosophy is to be proactive and precise, not reactive.

As part of the authors’ Proactive precision Maintenance (PpM) strategy, the booklet serves:

To provide a procedure that enables a thorough analysis of a failed bearing in order to identify

the root cause.

To provide guidance on the frontline analysis of the bearing that is required before the bearing

is removed from site.

To provide basic guidance on how to dis-assemble a rolling element bearing and conduct an

expert review of not only the path patterns, but the fretting patterns and other evidence as

presented by the failed bearings.

To provide clear and concise guidance on how to interpret some of the less discussed failure

modes that affect machine reliability and/or performance.

To provide guidance on the report layout and recommendation/s that must be entered into an

exception report so the condition is rectified before a machine is re-conditioned or re-

furbished.

To provide guidance so assets realise in as short a time as possible, the industry and business

value elements of an effective PpM Program.

It is important to understand that this paper does not attempt to address all bearing failure modes,

just those that the author feels are not adequately covered by publications provided by international

standards and bearing companies. The paper is a compilation of the authors own experience and it is

highly recommended it be read along with ISO 15243.

2.1 Economic Aspects:

cost-effective design to optimize CAPEX,

cost-effective operation to optimize OPEX,

improved profitability (by reducing revenue loss),

LCC/whole-life management,

reduced cost of insurance;

2.2 General Aspects:

Retention of operator license (being able to operate),

Life extension of capital equipment,

Improved product quality,

Better equipment purchase decisions,

Better resource planning;

2.3 Safety and Environmental aspects:

Improved personnel safety,

Reduced catastrophic failures,

Reduced environmental impact,

Improvement of safety procedures and regulations

Compliance with authority requirements

3 Scope

This process is applicable to all sectors.

4 References

ISO 15243:2004

ISO 5593:1997

ISO 281:1990

ISO 281:1990/AMD 2:2000

ISO 6601:2002

ISO 8785:1998

Why Bearings Fail - SKF

5 Abbreviations

CAPEX capital expenditure

CDF cumulative distribution function

CM condition monitoring

CMMIS computerized maintenance management information system

DHSV downhole safety value

ESD emergency shutdown

FTA fault-tree analysis

FMECA failure mode, effect and criticality analysis

HIPPS high-integrity process-protection system

KPI key performance indicators

LCC life cycle cost

LEL lower explosion limit

MEG mono ethylene glycol

MI maintainable item

MTBF mean time between failures

MTTF mean time to failure

MTTR mean time to repair

MTTM mean time to maintain

MUT mean up time

MDT mean down time

NDT non-destructive testing

OPEX operational expenditure

PM preventive maintenance

POSM production operation and start-up manual

P&ID process and instrument diagram

PSD process shutdown

PSV process safety valve

QRA quantitative risk assessment

RA reliability and availability

RAM(S) reliability, availability, maintainability (and safety)

RBI risk-based inspection

RCM reliability-centred maintenance

RM reliability and maintenance

SIL safety integrity level

SSIV subsea isolation valve

TEG triethylene glycol

TTF time to failure

TTR time to repair

WO work order

SAP CMMS system

6 Preparation

Any personnel working on or around heavy machinery must attend the appropriate training course

and have been judged competent.

If required, engineering consultant, mechanical technician or Operations technician to apply

for permit(s).

Obtain work permit(s).

All personnel involved must hold/attend a toolbox talk.

Before intervention, discuss isolation/spading of machine with operations staff and the Central

Control Room (CCR). This is to ensure that where operationally possible, all the required

machinery is off line and safe. It’s often much simpler to get machines changed over during

night shift than a busy day shift.

Ensure all safety inhibitions/isolations are in place before entry into restricted modules. i.e.

CO2 inhibition is required before entry into most turbine generators. Opening the door to a

module may trigger an alarm or worse shutdown the machine; to inhibit safety systems special

permission is required from the installation/site manager or OIM so plan in advance.

Before removal ensure failed bearings are correctly marked so their location and orientation is

known. Once removed they must again be labelled and placed in individual ziplock bags

including a sample of the lubricant. Do not remove any grease or throw away any debris.

PpM BP Always locate and speak to the system owner prior to working in his/her area. Besides

being good manners, a discussion with the system owner about the performance of or recent

maintenance intervention carried out on the unit, is invaluable as a diagnostic tool. The

information must be noted and verified if possible before conducting the diagnosis. Recent oil

changes must be noted. If any re-greasing has taken place note the lubricant type and

quantity. Verify the type of grease used by asking for the grease gun and taking a sample of

the grease found within. For engineers writing a comprehensive report, all information must

be factual and traceable.

PpM BP Do more than what’s expected of you; leave the site cleaner and safer than the state

you found it in. Clean up any accidental spillage, and double check you have correctly closed

up and tightened up. While moving around the unit, use your senses, note anything that may

be of concern. Be sure, check again, and then report to the system owner, supervisor or the

control room as required.

PpM BP If involved in the re-installation of the bearing, wherever possible, best practice is to

always install a bearing with the designation facing outboard of the unit.

7 Procedure Guidelines

The accuracy of this PpM technique as with most techniques is limited by the quality and repeatability

of the process; hence it is vital that the guidance in this booklet is closely followed if this tool is to

provide effective results.

8 Preliminary Steps

Following bearing removal, the bearing should be engraved with a unique identifying serial number on

the same side of both the inner and outer ring; the bearings should be marked on the outboard side

(Side of bearing that faced outwards). This is very important; if a bearing shows signs of an excessive

axial load, knowing which way the bearing is installed will assist with determining whether the load

was imposed by “Cross Location” (No space for axial thermal expansion) or external sources such as an

incorrectly spaced coupling. If the unit is vertical indicate “Top” and “Bottom”.

8.1 Required Information

The unit/skids nameplate details and child nameplate details including serial number should be

recorded, along with the following information:

Time and date of bearing removal

Reason for removal i.e., vibration, temperature, noise, seized, other maintenance, etc

Equipment removed from, including position

Bearing manufacturer

Designation

Date of installation

Vibration level at time of removal and if possible, vibration signatures

8.2 General Inspection

An inspection of the general condition of the bearing should be performed and noted on a

bearing evaluation sheet. This general evaluation should consist of the following:

Ability to rotate the bearing by hand

Condition of external surfaces such as rings, seals, shields, etc

Condition of cage and lubricant, if possible, without disassembly

If seals or shields are present, they can be removed by using a screwdriver of the proper size to

pry them away from the inner ring and then removed

Using a clean tongue depressor remove a grease sample from as close to the bearing races as

possible and place in a small plastic bag

The grease sample should be worked to a thin film and held up to a bright light and the

presence of dirt, water, or other contaminates noted and recorded

The bearing can now be cleaned using a solvent or diesel fuel and a parts cleaning brush NOTE:

Observe applicable safety precautions when handling solvents

Following cleaning, the bearing should rotate freely by hand. Any discrepancies should be

noted

Spray the bearing with WD40, CRC or similar substance to prevent corrosion from occurring if

the bearing is not to be analyzed straight away

9 Disassembly

Disassembly of bearings cannot be accomplished without minor damage to the bearing components.

However, this damage, nicks in the balls or fresh smears in the race, are readily discernible from the

damage which occurred during operation and should be noted in the evaluation.

9.1 Angular Contact Ball Bearings

To disassemble angular contact ball bearings perform the following:

Place the bearing face (thin side) down inside a clean box or other suitable container in order

to prevent the balls/rollers from rolling away.

Use a two small pieces of wood as a spacer to support the outer ring of the bearing at

sufficient height to allow the cage and balls to clear the outer ring. Then strike the inner ring

with a mallet at the high point; the bearing should separate into individual components.

9.2 Deep Groove Bearings

In order to disassemble a deep groove ball bearing the cage must be first removed, the difficulty of this

task is dependent on the type of cage present. A snap in phenolic cage can be removed by using a

screw driver or punch and hammer, and a two piece phenolic cage can be disassembled using the

appropriate size drift to drive out the pins which hold the cage together. However, disassembly of the

two piece riveted metal cage can be difficult and is made much easier if a drill press is utilized during

the following procedure:

Place an indentation in the centre of each rivet using a centre punch and hammer.

Clamp the bearing to the drill press (firmly, any movement, the drill will keep breaking) and

drill the rivets with a 10mm end mill, if not available use a twist drill bit of the same diameter

as the rivet. Only the head of each rivet needs to be removed and not the entire rivet.

Remove the cage halves and clean the bearing and cage assembly with a brush and solvent to

remove metal slivers (do not rotate).

Slide all of the balls to one side of the bearing and clamp bearing in a vice on the side opposite

from the balls.

Note: A face shield should be utilized during the next step as there is a chance the bearing may

fracture.

Slowly tighten vice until the rings are distorted enough to allow the inner ring to move past the

last ball. This will require a few mils of deflection and be accompanied by a snapping sound.

Release the tension on the vice and place the bearing components in a clean container.

9.3 Spherical Roller Bearings

Spherical Roller Bearings are simple to dismantle by simply using a flat screw driver. After rotating the

outer ring perpendicular to the inner ring, align a rolling element with the filling slot or cut-out in the

inner ring then simply pry the rolling elements loose one by one. Once you have released about a third

of the rolling elements the cage and remaining roller assembly should simply drop out. If not continue

to remove rolling elements until it does.

10 Bearing Terminology

Cag

e

Outer Ring Shoulder

Ball (Rolling Element)

Outer Ring Side Face

Inner Ring Side Face

Outer Ring Raceway

Inner Ring

Inner Ring Raceway

Inner Ring Shoulder

Outer Ring

Outside

Diameter Bore

Figure 1 - Bearing Terminology for Ball Bearings (Courtesy SKF)

Outer Ring

Cage Inner Ring

Side Faces

Lubrication

Groove and

Holes (W33)

Spherical Roller

(Rolling Element) Guide Ring

Inner Ring

Raceways

Outer Ring

Raceways

Figure 2 - Bearing Terminology for Roller Bearings (Courtesy SKF)

11 Recording Visible Damage

All of the surfaces must be carefully examined and exceptions recorded if an accurate assessment of

the cause of failure is to be determined. In most cases, a 10x magnifying glass, a good light, a

lampshade for diffusing the light so reflective surfaces can be viewed or photographed, will suffice.

To make a “Light Tent” which diffuses bright light so reflective surfaces can be photographed. Place a

white piece of paper on the desk, place the bearing component on the paper and cover with the lamp

shade, cover the top of the lampshade with another piece of white paper after cutting out a hole in

which the camera or microscope lens can fit, place desk lamps around the outside of the lampshade,

two should do, one on either side. This will diffuse the light and get rid of all the problems normally

associated with studying or photographing a reflective surface.

12 Bearing Troubleshooting

The bearing troubleshooting process follows the same general steps for any machinery problem or

failure, and begins with history. If failure rates seem excessive and a failure pattern emerges,

vibration, oil or phase analysis of the unit as a whole may be warranted. Important questions for

consideration include:

Frequency of failure – What has been the failure rate for bearings in a particular machine or

group of machines? Has the failure rate remained constant, or has it increased or decreased?

Was there a time when MTBF was extended? If so, what changes have occurred and when did

the failures begin?

What is the timing between detection to machine failure, or shut-down?

What bearings were originally specified for the machine?

What bearings are currently installed? If different than originally specified, why were they

changed?

Have any changes been recorded in the operation, type or replenishment of the lubrication

system.

What changes, if any have been made to the lubricant delivery system?

When was the last scheduled greasing or oil change? How much grease oil was used?

What changes have been made to the lubricant? What was originally specified versus what is

being used today? Why was the change made?

What is the condition of the lubricant in the machine?

Is there any correlation between bearing failure rates and any lubrication changes?

Have there been any changes in installation and maintenance practices? Have specifications

or standards been altered for better or worse?

Are the bearings suitable for the duty required? (Unless you are experienced and familiar with

bearing life calculations, it is recommended you consult a bearing application engineer)

If in a pump and the bearings in question are angular contacts (72 or 73 series), are they a

matched pair, are they the right way round (most common arrangement is back to back).

What other design changes have been made to the machine, including piping, couplings, etc.?

Has there been a change in process, including product, throughput, and speed?

Check the alignment reports if available.

Have there been any changes to seals, such as type of seal or seal material?

Do maintenance records indicate any unresolved machine deficiencies from the previous

repair, such as shaft runout, or shaft or housings out of specification?

If dealing with housings, take note of where the locating rings were installed, does the

arrangement allow for thermal expansion?

If dealing with bearings mounted on adapter or withdrawal sleeves, find out the tightening

angle, amount of drive up or mounting method employed.

This list will grow depending on the machine type, and the process. It is important to gather this

information before, or at a minimum, during the repair, to ensure all possible failure sources are

considered. Cross sectional drawings are a must as most failure analysis involves understanding the

bearing arrangement. Most incorrect analysis of bearing failures arises from not fully understanding

the design of the bearings and the arrangement that they’re in.

13 Bearing Removal

Bearing removal is very important. It is essential to preserve all evidence of a bearing failure or of a

removal / replacement before a bearing has reached full life expectancy. Best practice calls for

inspection of every bearing removed from a machine, even when the removal is performed on a

preventive basis, allowing underlying fault sources to be detected and remedied. Although possibly

repeated, the recommended steps include:

Gather a sample of the lubricant, whether oil or grease, in a clean container. If the grease

condition varies with proximity to the bearing, it may be wise to gather several samples. New

grease pumped in as a last minute attempt to avert a failure does not provide an accurate

description of the environment inside the bearing. Look to the opposite side of the housing

for older grease.

Mark the location of the fixed or stationary ring with respect to its anchor, such as housing.

When cut open, the fretting patterns on the bearing can be compared to the likely load zone.

Where practical, use tools that will remove the bearings without loading the raceways or

rolling elements. Although damage created when pulling a bearing off a shaft by its outer ring

is evident when performing root cause failure analysis, it may mask other symptoms. If

removing the bearing from a shaft using a puller, rotate the puller and keep the screw fixed as

this will distribute the load evenly and not indent the bearing.

Place the bearing in a plastic bag or other packaging before setting it down on a surface that

could add secondary contamination.

Label the bearing or package with the machine identification and the bearing position.

Basic failure analysis and troubleshooting should be conducted before a new bearing is installed in the

machine. Obvious failures associated with the shaft and housing, and the lubricant, can be addressed

before reassembly. In this way, a repeat of the failure can be avoided.

In cases of catastrophic failure, high cost, or high production impact, it may be beneficial to enlist the

services of an expert in bearing failure analysis to thoroughly inspect the bearing and lubricant, to

develop a more accurate failure cause. Additional tests may include the use of microscopes to look at

metal structure and the progression of fatigues and cracks, and more complete lubricant analysis. It

has been said the “bearing does not lie.” If the bearing was not run to the point of melt-down, the

root cause is typically evident, down to the location where a torch was used to heat the inner ring, or

the spot where it was struck by a hammer to drive it on that last 1/8 inch (3 mm) on the shaft.

Careful observation of the failed parts and associated components will eliminate other causes and will

help identify the true root cause of the bearing failure.

14 Bearing Analysis - External Surfaces

This section on external surfaces describes the checks on the physical condition of the bearing and

machine interfaces where the root cause of the bearing failure is often determined. The information

obtained from the examination of the points of contact between the bearing and the machine should

be correlated with conditions found on the rollers and raceways to obtain the most probable cause of

bearing failure.

A review of bearings removed, performed by the U.S. Navy, indicated that over one-half of bearing

failures were the result of contamination and improper installation. Contamination problems are

normally manifested on the internal surfaces while indicators of installation problems are evident on

internal and external surfaces.

Degradations and wear on external surfaces can be evaluated by observing the type and patterns of

Simple Corrosion, Fretting Corrosion, Fretting Wear, scoring marks, and colour of components.

14.1 Corrosion

Corrosion of bearing surfaces is very common and consists of “Simple Corrosion” (Rust) and “Fretting

Corrosion” (Combination of the mechanical process “Fretting”, and “Corrosion” which is the oxidation

of the “Fretted” particles in the presence of water, these oxides are often harder than the bearing

surfaces and these abrasives rapidly abrade the surfaces, so wear accelerates).

14.1.1 Simple Corrosion

Simple corrosion is usually the result of water or moisture coming into contact with an unprotected

surface of the bearing and simply rusting the steel. This type of corrosion is usually brown in colour

and takes place on the faces and shoulders where there is no contact between the balls, races, shaft,

or housing.

A thin protective oxide film forms on clean steel surfaces exposed to air. However this film is not

impenetrable and if water or corrosive elements make contact with the steel surfaces, patches of

etching will form; this development soon leads to deep seated rust. The salts that are present in fresh

water constitute, together with the water, an electrolyte which causes galvanic corrosion, also known

as “Water Etching”. Salt Water, is therefore highly corrosive to bearing surfaces. In some cases, water

or other contaminates will interact with the lubricant and form acids which result in dark brown

and/or black stains on the balls and races. This type of corrosion can take the form of very fine crack

like lines in equipment which has not been in operation for some time. In advanced states, severe

pitting can take place. “Simple Corrosion” will form points for surface initiated spalling to occur if

formed in the raceways, the spalling will often be roller spaced, as the water or electrolyte will tend to

concentrate in areas where there is the greatest amount of surface tension, this is often between the

rolling elements and raceway exposed to air. As a result, the spalling will often only be found on the

inner ring as 1) The lubricant will tend to gather at the lowest point, the loaded zone, which protects

the bottom rollers and outer ring in the loaded zone, but runs off the tops of the rollers and therefore

exposes the contact point between the top of the roller and inner ring. 2) At the top of the bearing the

rolling elements are supported by the inner ring, with a clearance at the top of the rollers, as there is

only a thin protective film between inner ring and roller, etching or pitting is accelerated in this area.

Figure 3 - Simple Corrosion (Rust)

Figure 4 - Image of a segment of an inner ring representative of spalling due to water etching (black stains).

Figure 5 - Image of an outer ring from a spherical roller bearing showing water etching and score marks at roller spaced

intervals. Figure 5b. Close-up showing the water marks and the fracture. Figure 5c shows the ring split slightly. Figure 5d is a

macroscopic image of the fracture.

14.2 Fretting Wear (Or “Fretting Erosion”)

The “American Society for Metals” (ASM) Handbook on Fatigue and Fracture defines fretting as: "A

special wear process that occurs at the contact area between two materials under load and subject to

minute relative motion by vibration or some other force."

Fretting Wear is the result of small amounts of relative motion between the external surfaces of the

bearing, the shaft, housing, and/or bearing outer caps. This relative motion causes adhesive wear to

take place due to metal-to-metal contact at high points between the two surfaces.

Severe fretting can create weak spots in the bearing which will result in cracking and eventual spalling

of the bearing. In addition, heavy fretting on the guide or floating bearing outer ring may cause the

bearing to lock-up in the housing which will cause the bearing to become overloaded and result in

failure.

Unlike simple corrosion, fretting wear will often have a pattern associated with it which can reveal

misalignment, improper clearances, and/or excessive loading.

Therefore, careful mapping of the fretting patterns will greatly assist in accurately diagnosing bearing

failures. The amount of fretting wear, is a function of fit and load, and thus is an excellent indicator to

the cause of bearing failure.

In vertical applications where no parasitic or radial forces exist, the fretting wear will be uniformly

distributed near the centre, usually offset by the contact angle of the bearing in the same plane as the

wear track created by the ball on the outer race.

Since the rings are flexible and the bearing generates pressures in excess of 300,000 PSI the ring flexes

in response to the resulting pressure wave creating the relative motion between the bearing and the

housing which causes the adhesive wear. Studies by the U. S. Navy have indicated that this type of

fretting can occur after as little as 100 hours of operation.

A radially loaded bearing, as found in a horizontal machine, will show fretting, slightly offset from the

centre, but will be heavy only in the load zone and tapper off to little or no fretting in the upper half of

the bearing on the outer ring. In the case of a preloaded bearing the fretting wear will be evident

around the full circumference of the outer ring.

In both cases mentioned above the inner ring of the bearing will show signs of fretting the entire 360,

but will be displaced either side of centre as a function of the load.

Excessive fretting can occur when any of the following conditions exist:

Heavy loads and severe vibration.

Improper shaft geometry.

Housing imperfections.

Should a bearing be subjected to heavy axial loading (Thrust), the fretting will occur at opposite sides

of the inner and outer rings.

Poor housing and bearing fits, where the bearing is not fully supported by the housing, results in

excessive flexing of the outer ring in localized areas, this has the effect of increasing the amount of

stress on the bearing, and shortening bearing life.

Bearing misalignment is a leading cause of premature bearing failure and can easily be avoided by

properly installing the bearing. A skewed fretting wear pattern on the outer ring but normal light

fretting in the centre of the inner ring bore would indicate misalignment. In order to avoid this type of

problem, squareness of the shaft shoulder should be confirmed, as well as ensuring the bearing is

positioned snugly against the shoulder during installation.

Tapered housings, fluted housings and loose fits produce fretting wear on the portion of the bearing in

contact with the housing and not in the area of no contact.

A fluted housing where the outer ring is supported on the high points of the housing, does not provide

sufficient resistance to rotation to prevent the outer ring from chattering in the housing, the bearing is

noisy in service.

Figure 6 - Image of the inner ring bore of a angular contact ball bearing showing fretting wear to the bottom of the image and

the fretting corrosion; patchy spread from the centre of the ring to the bottom. To the left is a macroscopic image of the

fretting wear following incorrect setting of the preload in an X arrangement.

Figure 7 - Image of an outer ring from an angular contact ball bearing showing fretting corrosion in the loaded zone; the dark

patch to the bottom of the image is the face of the bearing and this is indicative of the load distribution. The uneven spread of

the fretting corrosion is indicative of a loose fit in the housing. The dark areas show where there was contact with the housing,

the reflective or light patches being the areas where there was no contact with the housing.

14.3 Fretting Corrosion

“Fretting” or “Fretting Wear” as earlier mentioned is the result of small amounts of relative motion

between the external surfaces of the bearing, the shaft, housing, and/or bearing outer caps. This

relative motion causes adhesive wear to take place due to metal-to-metal contact at high points

between the two surfaces. “Fretting Corrosion” occurs when these small particles removed by fretting

are oxidized. The oxides are usually harder than the underlying metal, so wear accelerates as the

harder particles abrade the metal surfaces further. Unprotected bearings on larger units can suffer

serious degradation, especially when exposed to seawater.

Figure 8 – a) Inner ring bore showing fretting corrosion (dark patches) the lighter areas are indicative of fretting wear. Figure

8b is a macroscopic representation of the fretting corrosion.

14.4 Fretting Fatigue

When cracks in either surface are created due to excessive fretting, this is known as “Fretting Fatigue”.

It is the more serious of the two phenomena because it can lead to catastrophic failure of the bearing.

14.5 Color

The colour of the bearing, overall and in specific locations, provides information on the operating

environment especially temperature. As the operating temperature increases the bearing components

will change from a bright shiny or steel gray colour to a straw, reddish brown, blue and finally black

with increase in operating temperature. As a result of operating at excessive temperature the steel

hardness is reduced and operating life significantly shortened.

A determination should be made as to the cause of overheated bearing components. For example, the

bearing could be the cause of the excessive heat generation due to a reduction of internal clearance by

a tight fit, the use of a bearing with insufficient radial internal clearance for the application, or by

failure of the lubricant. In addition, an external source of heat could be conducted to the bearing

which would cause premature failure. Thus, the bearing and surrounding components should be

examined for signs of thermal distress.

Look for lack of lubrication that can eventually cause the bearing to seize:

You will see" mirror like" surfaces on the metal parts that look like the piece was "lapped".

The metal will become discoloured and soften as it anneals. Annealing can occur any time the

temperature exceeds 300°F (150°C):

Straw yellow 600° F. 315° C.

Brown 700° F. 370° C.

Blue 800° F. 425° C.

Black 900° F. 480° C.

15 Internal Surfaces

The internal surfaces consist of the rolling elements, raceways, internal shoulders or lands, and the

cage. In normal operation all of the load should be carried by the rolling elements and the raceways

and no signs of excessive abrasive wear should be apparent on the cage or the shoulders. For phenolic

or other non-metal cages, a minor amount of rubbing on the shoulder is acceptable since the cage

uses one of the shoulders as a guide. Steel cages ride on the rolling elements and should never come

into contact with the inner or outer ring.

15.1 Scoring

There are two types of scoring, axial and circumferential, both of which are usually indicative of either

poor bearing mounting procedures, or fits. While axial scoring can be created during bearing

installation or removal a determination of the relative age of the marks can normally be made, and a

conclusion reached as to whether or not the cause was mounting or dismounting. Whether there was

excessive interference between the bearing and either the shaft or the housing can also be

determined by examination of the scoring. Furthermore, insufficient heating of the bearing, hard

particle contamination or a lobed shaft can cause scoring of the inner ring and damage to the shaft.

Excessive interference will expand the inner ring, taking up all the internal clearance and preloading

the bearing.

Circumferential scoring is caused by the shaft being undersized or the housing being oversized and is

indicative of the bearing turning on the shaft or in the housing. In some cases excessive load may

cause the bearing to spin or creep relative to the machine mounting surface and/or fastening devices.

In addition, damage to the bearing face can result due to rubbing (Smearing) of the bearing and the

shaft shoulder and lock nut/washer. In both cases this may result in sufficient damage to cause

transverse cracks on the ring affected, which rapidly develops into spalls, in the worst case scenarios

the ring may actually fracture right through, resulting in catastrophic failure.

Excessive scoring of either type will result in premature bearing failure and can be avoided by carefully

measuring the shafts and the housings dimensions carefully, in 4 planes, and installing the bearing in

accordance with the vendors’ recommendations.

Figure 9 – Image of an outer ring showing circumferential scoring indicative of a loose fit.

15.2 Brinelling

Refers to a material surface failure caused by contact stress that exceeds the material limit. This failure

is caused by just one application of a load great enough to exceed the material limit. The result is a

permanent dent or "brinell" mark. It is a common cause of roller bearing failures, and loss of preload in

bolted joints when a hardened washer is not used. Engineers can use the Brinell hardness of materials

in their calculations to avoid this mode of failure. A rolling element bearing's static load rating is

defined to avoid this failure type.

Figure 10 – Brinelled inner ring of angular contact ball bearing. This was done while the coupling half was being fitted with a

hammer.

15.3 False Brinelling

When a bearing is not running, there is no lubricant film between the rolling elements and the

raceways. The absence of lubricant film gives rise to metal to metal contact and the vibrations produce

small relative movements of rolling elements and rings. As a result of these movements, small particles

break away from the surfaces and this leads to the formation of depressions in the raceways. This

damage is known as false brinelling, sometimes also referred to as wash boarding.

Balls produce sphered cavities while rollers produce fluting. In many cases, it is possible to discern a

reddish brown deposit or red rust at the bottom of the depressions. This is caused by oxidation of the

detached particles, which have a large area in relation to their volume, as a result of their exposure to

air. There is very rarely any visible damage to the rolling elements. The higher the energy of the

vibration the more severe the damage is. The period of time the unit is exposed to parasitic vibrations

and the magnitude of the bearing internal clearances also influence development, whereas the

frequency of the vibration does not appear to have any significant effect. Roller bearings have proved

to be more susceptible to this type of damage than ball bearings. This is considered to be because the

balls can roll in every direction. Rollers, on the other hand, only roll in one direction; movement in the

remaining directions takes the form of sliding. Cylindrical roller bearings are the most susceptible. The

fluting resulting from vibrations sometimes closely resembles the fluting produced by the passage of

electric current. However, in the latter case the bottom of the depression is dark in colour, not bright

or corroded. The damage caused by electric current is also distinguishable by the fact that the rolling

elements are marked as well as the raceways.

To clarify; False Brinelling is a commercial term, used to describe the “Fretting” that takes place,

between rollers and raceways in the absence of a lubricant while the unit is at rest, but still subjected

to vibration normally from other units running nearby, commonly described as parasitic vibration.

So, False Brinelling is damage caused by fretting without corrosion, this causes imprints that look

similar to Brinelling, but are caused by a different mechanism.

It is important to note that in the presence of a lubricant false brinelling does not occur. In normal

operation, a rolling-element bearing has the rollers and races separated by a thin layer of lubricant

such as grease or oil; hydrodynamic or elasto-hydrodynamic lubrication. If a bearing is stationary but

subject to these parasitic vibrations under load, the lubricant is pushed out of the loaded area. Since

the bearing is rolling only small distances (less than roller spacing), and there is no increase in

temperature, there is no action or movement that causes the soap to release mineral oil thereby

replacing the displaced lubricant. While bearings and races appear smooth, they are microscopically

rough. Thus, the high points of each surface come into contact, but the "valleys" do not. The contact

zone is thus reduced, increasing the contact stress. This causes pieces of each surface to break off or to

become pressure-welded then break off. The addition of phosphorous, which is an extreme pressure

additive, will alleviate the condition to a certain degree, but again this EP additive is not being replaced

in the contact zone. So the relief is only temporary.

The broken-off pieces are called wear debris. Wear debris is highly destructive. As mentioned, it has a

large surface area in relation to its volume thus larger than the surrounding surface finish and can

oxidize (rust). This produces a compound that is harder than the steel itself which is more abrasive,

therefore wear is accelerated. Since there are only small relative motions between the surfaces in

contact, the damage is on-going, creating more wear debris.

False Brinelling is another failure mode that is not taken seriously, yet the prevention is so simple.

Common solutions include:

Keeping the spare/standby unit at a distance from the duty unit, which is on and vibrating

Manually rotating the shafts of the spare/standby units on a regular basis

Regularly switching between the units so that both are in regular operation

Isolating each unit from parasitic vibration

Figure 11 – A presentation of brinelling at the spalling stage, showing its progressive development from an indentation to

spalling.

15.4 Cage

The colour and presence of any signs of abrasive wear should be noted. The colour of the cage, steel or

phenolic, will darken with increases in temperature. As previously mentioned, with the exception of a

phenolic cage which uses a land as a guide there should be no signs of abrasive wear or cracks in the

cage.

The cage pockets will often show signs of scoring due to the fact sliding and not rolling takes place

between the cage and the rolling elements. In the ideal situation, the condition of each cage pocket

will show uniform wear patterns and any pocket to pocket deviation should be noted and the cause

determined. The most common cause of excessive scoring of the pockets is hard particle

contamination.

When a cage is exposed to vibration, the forces of inertia may be so great as to cause fatigue cracks to

form in the cage material, sooner or later these cracks lead to cage fracture.

Excessive speed, also subjects the cage to heavy forces of inertia that may lead to fractures.

Where the cage is concerned, sliding cannot be eliminated in the contacts regions of the other

components of the bearing. This explains why, when a bearing is starved of lubricant, the cage is the

first component to be affected. In cases where the cage is centred on the rolling elements, as in a deep

groove ball bearing, the resultant forces arising from cage wear may lead to cage failure within a very

short space of time.

If the rings of a deep groove ball bearing are fitted out of alignment, the path of the balls has an oval

configuration. As the cage is centred on the balls, it has to change shape for every revolution it

performs. Fatigue cracks then form, and sooner rather than later, these cracks lead to fractures.

15.5 Raceways

The raceways are the portions of the inner and outer rings between the lands where the rolling

elements support the load. The area of contact is referred to as the ball path or path pattern and is the

portion of the bearing which sees the cyclic stress which limits bearing life.

This path pattern varies in appearance according to the rotational and loading conditions, by

examining the path pattern in a dismantled bearing, it is possible to gain a better understanding of the

conditions under which the bearing was operated. By learning to distinguish between normal and

abnormal path patterns, it is possible to assess whether the bearing has run under the proper

conditions pertaining to the application. In the majority of cases the damage to the bearing originates

within the confines of the raceway, once the significance of the path pattern has been learned, the

appearance and location of the patterns prove to be very useful aids in diagnosing the cause of the

failure or damage.

As previously mentioned, pressures in this region can exceed 300,000 PSI. This high pressure can cause

bearing failure through the following two mechanisms:

1. Surface initiated spalling

2. Fatigue spalling

Surface initiated spalls are caused by hard or soft particle denting, water etching, false brinelling,

incorrect mounting procedures scoring the ball path, and/or electrical pitting of the raceways. These

indentations create stress rises which cause the metal to fracture and spall out. Soft particle dents can

be caused by thread, tobacco, cage material, and lubricant impurities. Hard particle denting can be

caused by the metal oxide particles formed due to simple and fretting corrosion. Electrical pits are

often the result of improperly grounding arc welders as well as the presence of stray currents in the

rotating assembly of electrical motors.

Fatigue spalls occur when the bearing has work hardened the raceway, resulting in the embrittlement

of the material, and finally loss of material.

The observable difference between the two types of spalls are 1) the surface initiated spall has an

arrowhead shape often with a point at the tip of the arrow and 2) a fatigue or subsurface initiated spall

is usually oval and has steeper sides than a surface initiated spall.

15.6 Smearing

Smearing as described in this instance is a condition that arises as a result of the rolling elements

sliding rather than rolling in the loaded zone. In vertical applications load is distributed among all the

rolling elements (as opposed to four or five in a similar sized bearing in a horizontal application).

Therefore the minimum load required to ensure the rolling elements do not slide is greater.

Inadequate loading therefore causes the elements to break through the lubricant film as a result of the

sliding which results in metal to metal contact.

This type of wear is known as smearing, (the transfer of material from one metal surface to another.)

The process requires surface-parallel motion (sliding) in the contact, and is often accompanied with

high friction heat. The rapid increase in temperature of the inner ring causes the inner ring to expand

faster than the shaft to a point where the interference fit is compromised and the shaft will begin to

rotate in the inner ring causing a dangerous catastrophic failure. The expansion of the inner ring also

causes a preload condition; the increased drag not only enhances inner ring rotation but also increases

the temperature to increase further to a point where the lubricant is no longer able to sustain the load

and catastrophic failure results.

Figure 12 – Images of a failure caused by smearing. Note the deep indentations left in the inner ring by the rolling elements

and also the bent shaft. The inner ring and shaft have reached alarmingly high temperatures due to the loss of interference as

a result of the heat spike causing the inner ring to expand rapidly and rotate on the shaft. Notice again the cage, still intact,

evidence beyond doubt that there was indeed lubricant present.

Figure 13 – Inner ring bore showing the extent of the inner ring rotation.

Figure 14 - This image shows an inner ring with the same indentations as Figure 12, but in this case the grease/sealing

arrangement kept oxygen out, hence the ring does not show any discolouration due to oxidation. Again notice the

indentations formed as the unit came to a stop and the steel was still malleable. This smearing failure was caught early as the

unit was screeching. Notice how the inner ring had only just broken away, evidenced by the circumferential scoring.

An additional cause of smearing can be inadequate grease duct arrangement, excessive quantity of

grease and/ or incorrect grease used. This is especially true in vertical units and occurs when the base

oil viscosity or consistency of the grease is too high. The soap in grease is not a lubricant, it’s simply a

carrier; it is the mineral oil that lubricates the bearing. While replenishing the grease in a unit or after

refurbishment it is important that the soap is pushed out of the bearing as soon as possible. The soaps

consistency increases drag and causes the rolling elements to slide, hence the need to only replenish

units while they are running.

The grease used in the majority of offshore installations had a NGLI classification of 3. Most offshore

units are also designed with their overall mass in mind, and due to the fact that most motors are direct

start, their rotors are very light. In addition, due to the wide temperature variations (-20 to 25oC)

which cause the lubricants consistency/viscosity to fluctuate considerably. It became the authors

prime focus to have all pumps and motors greased with an NGLI classification of 2. This reduced the

number of failures; especially of vertical units in the winter. As the reduced consistency is more

suitable at lower temperatures during start of the units, and did not have any adverse effect during the

summer months.

So, as discussed this condition often arises when the base oil viscosity or consistency of the grease is

too high, or, as is common with some motors, the bearing is subjected to a light load condition thereby

reducing the amount of friction required to keep the elements rolling. The weight of the rotor is often

insufficient, in some cases the DE bearing is preloaded by means of spring washers at the NDE. In the

case of vertical motors the load is distributed amongst all the rolling elements hence the minimum

load requirement is higher than that of a horizontal unit. The horizontal motor has its load distributed

amongst only those elements that are in the load zone, normally 3 - 5.

The minimum requisite load is the term used to describe this minimum load a bearing must be

subjected to dependent on running speed and viscosity of the lubricant. The author has witnessed

many instances whereby a “D Frame” 50kw induction motor has been replaced by a seemingly

identical “D Frame” 50kw induction motor. However the motors are not identical, by understanding

bearing designations, the trained eye would be able to identify straight away, by looking at the

nameplate, that the DE of one is fitted with an “N” series (cylindrical roller bearing) and the other a “6”

series (deep groove ball bearing). If the motor with the “N” series at the DE, designed with a belt pull

factor in mind, is fitted to a centrifugal pump where the residual imbalance of the shaft and the weight

of the rotor is the only load, the motor will fail rapidly. The cylindrical roller has a far higher load

carrying capacity; therefore it requires a higher minimum requisite load.

Preloading of arrangements fitted with ball bearings, by means of springs at the NDE, or above/ below

the non-located bearings outer ring, is often the only force acting on the bearing. Should there be no

preloading, the rolling elements will slide as the rolling elements are very often in transition between

the raceway i.e. not contacting either side of the raceway and very lightly loaded radially also resulting

in smearing and excessive bearing noise. In extreme cases the bearing makes a screeching noise on

start up as well as sounding very rough.

The initial heat spike initiated upon start-up with this failure mode will more often than not damage or

cause the bearing to fail. This is one of the main reasons the author does not recommend or condone

and will not participate in no-load runs on electric motors. Firstly, because of smearing in motors with

rolling element bearings and secondly, there is no point. If a bearing is damaged the defects are hardly

ever discernible under no-load conditions. From a vibration collection standpoint, we do NOT collect

vibration data unless there is a 70% load on the unit, otherwise the defect frequencies just do not

show up, be they electrical or mechanical. Also remember that some failure modes will propagate

from below but parallel (orthogonal) to the bearings surface, when you start an electric motor with no

load, the first few microns 20 – 25um will be under a tremendous amount of cyclic shear stress. When

the bearing is smearing the magnitude of the shear stress causes microscopic cracking, the bearing

may not appear to be damaged, but it’s not long before the cracks migrate to the surface of the

bearing which then causes flaking and spalling.

In order to avoid damage to rolling elements bearings after a refurbishment or grease replenishment

while at standstill it is advised that a proper run-in procedure be followed. A proper run-in procedure

will provide the following results:

Expel the excess grease found in the bearing and system (the soap of the grease causes

smearing)

Orient the lubricating film on each contact surface

Establish a low equilibrium operating temperature

Achieve a sealed-for-life lubrication condition

Where manufacturers’ guidelines are incomplete or ambiguous, a Kluber specific recommended run-in

procedure where the speed cannot be varied is described below in detail. Although there are

alternatives for applications where speed or temperature can be varied or monitored.

15.6.1 Run-in Procedure (When speed cannot be varied):

Run-in at constant speed is possible. In this operation, the bearing should run at full speed for about

12 seconds. After stopping, the heat in the bearing dissipates. In this way a dangerous temperature

rise is prevented. The non-running time depends on the various design factors, but it should be at least

5 times greater than the running time. This process is repeated until the bearing temperature becomes

constant.

Figure 15 - Run-In Procedure when speed cannot be varied.

16 Macroscopic Evaluation

A macroscopic examination of failed beatings can be used to determine the true or root cause of

bearing failures, identify problems with lubrication, bearing design, installation and operation, and

evaluate the accuracy of the vibration analysis program.

17 Basic Load-Path Patterns and Interpretation – Courtesy SKF

There are many ways bearings can be damaged before and during mounting, and in service. The

pattern or load zone produced by the action of the applied load and the rolling elements on the

internal surfaces of the bearing is a clue to the cause of failure.

To benefit from a study of load zones, you must be able to differentiate between normal and abnormal

patterns. The figures illustrate how an applied load of constant direction is distributed among the

rolling elements of a bearing. The large arrow indicates the applied load. The series of small arrows

show the share of this load supported by each ball or roller in the bearing.

Figure 16 - Rotating Inner Ring Load

The figure illustrates the load zone resulting if the outer ring rotates relative to a load of constant

direction, or where the inner ring rotates and the load also rotates in phase with the shaft.

Figure 17 - Combined Axial and Radial Loading

Combined axial and radial load will produce a pattern shown in the figure above. With combined load,

the loaded area of the inner ring is slightly off-center, and the length in the outer ring is greater than

that produced by radial load, but not necessarily 360 degrees. In a double-row bearing, a combined

load will produce load zones of unequal length. The thrust carrying row will have a longer stationary

load zone. If the thrust is of sufficient magnitude, one row of rolling elements can be completely

unloaded.

Figure 18 - Axial Load

The load path shows uniform wear on both the inner and outer ring. Pure thrust (axial) load is rare. If

axial load is present, it is usually accompanied by radial load.