Root Cause 0502.pdf

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Copyright 2002, Electrical Apparatus Service Association, Inc. (Version 502CI-502)

Root Cause Failure Analysis Introduction

Root Cause Failure AnalysisThis book was developed to help electric motor techni-

cians and engineers prevent repeated failures because theroot cause of failure was never determined. There arenumerous reasons for not pursuing the actual cause offailure including:

A lack of time.

Failure to understand the total cost.

A lack of experience.

A lack of useful facts needed to determine the rootcause.

The purpose of this book is to address the lack ofexperience in identifying the root cause of motor failures. Byusing a proven methodology combined with extensive listsof known causes of failures, one can identify the actualcause of failure without being an industry expert. In fact,

Electrical Apparatus Service Association, Inc.

1331 Baur Boulevard St. Louis, Missouri 63132 USA

314-993-2220 Engineering Fax 314-993-2998 www.easa.com

The information in this book was carefully prepared and is believed to be correct,

but EASA makes no warranties respecting it and disclaims any responsibility or liability of any kind

for any loss or damage as a consequence of anyones use of or reliance upon such information.

Many of the pictures in this book are of failures thathave occurred where the actual cause was identified.However, in some cases the exact cause was neververified, nonetheless they are included along with theauthors opinion of the most likely cause. In othercases, the pictures are of parts that have not failed, butthe pictures are useful in illustrating how and where thepart could fail.

It is difficult to segregate each type of failure into nicedistinct categories and to do so would require jumpingback and forth from section to section which wouldcause some amount of discontinuity. Hence, there is acertain amount of overlap and duplication of photos toclarify specific points.

There is no attempt to single out a particular motormanufacturer or to suggest that one product has moredefects or failures than another. For this reason, wehave not identified the manufacturer of the parts ormotors. In some cases, the failed part is not even anoriginal equipment part. Also, we have made no effortto identify whom may have repaired a particular motor.The intent of this book is not to place blame but to assistin a correct diagnostic procedure that will preventrepetitive failures.

The authors would like to express our appreciation toall those who have donated pictures for this edition andhope that we will continue to receive more pictures ofunique types of failures to fill the gaps.

EDITORS NOTE

when properly used, this material, will polish ones diagnos-tic skills that would qualify one as an industry expert.

The book is divided into the various components of anelectric motor. In addition to a brief explanation of thefunction of each component and the stresses that act upon

them, numerous examples of the most common causes offailure are also presented.

Since it is not always possible to pinpoint the exact causeof failure, some examples are used more than once. Due toa lack of all the necessary facts associated with the applica-tion and history of a given machine, it is only possible toassign the root cause to the most probable scenario.

A reference section is included at the back of this book forthose wanting to further research root cause failure analy-sis.


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Copyright 2002, Electrical Apparatus Service Association, Inc. (Version 502CI-502)

Root Cause Failure Analysis Table of Contents

Table of ContentsSection

Root Cause Methodology................................................................................................................................... 1

Bearing Failures ................................................................................................................................................. 2Winding Failures ................................................................................................................................................ 3

Shaft Failures ..................................................................................................................................................... 4

Rotor Failures..................................................................................................................................................... 5

Mechanical Failures ........................................................................................................................................... 6

DC Motor Failures .............................................................................................................................................. 7

Accessory Failures ............................................................................................................................................. 8

Case Studies ...................................................................................................................................................... 9

Reference Materials ......................................................................................................................................... 10


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Copyright 2002, Electrical Apparatus Service Association, Inc. (Version 502CI-502) 1 - 1

Root Cause Failure Analysis Root Cause Methodology Section 1

1Root Cause MethodologySection Outline Page

Introduction to failure surveys ......................................................................................................................... 1-2

Root cause methodology ................................................................................................................................1-2Summary of motor stresses ............................................................................................................................ 1-3

Analysis of the motor and system ...................................................................................................................1-4

Arriving at the correct conclusion ....................................................................................................................1-5

Basic AC motor nomenclature and common alternatives ............................................................................... 1-6

Basic DC motor nomenclature and common alternatives ............................................................................... 1-7

Methodology forms

Appearance of motor and system............................................................................................................. 1-8

Application considerations ........................................................................................................................ 1-9

Maintenance history ...............................................................................................................................1-10

Motor system and environment checklist ............................................................................................... 1-11

Stator coil layout for location and identification of fault ........................................................................... 1-12

Inspection reports ................................................................................................................................... 1-13


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1 - 2 Copyright 2002, Electrical Apparatus Service Association, Inc. (Version 502CI-502)

Root Cause Failure AnalysisSection 1 Root Cause Methodology

INTRODUCTION TO FAILURE SURVEYSMost failure survey data for electric motors is influenced

by the particular industry, the geographic location and thecombination of the motors in use. Therefore, specific num-bers may not always be relevant.

Most failure surveys focus on the component that actuallyfailed but do not address the root cause of that failure. As an

example, a bearing failure is not the root cause, it is simplythe component that failed. The root cause may be contami-nation, vibration, lack of lubrication, etc.

The data provided by the Institute of Electrical and Elec-tronics Engineers (IEEE) study shown in Figure 1 is helpfulin that it points to the most likely cause of motor failure byvirtue of which component has failed. It then becomes theresponsibility of those analyzing the failure to search for theroot cause that led to the failure of that particular compo-nent. These percentages may vary for a specific industry orlocation.

The real challenge lies in reducing the large category ofunknown failures. It is these unknown failures that makeanalyzing the entire motor system so critical.

Each section of this book provides a detailed list ofpossible root causes of failure for a particular motor compo-nent. And in most cases, an example of that type of failureis also provided.

ROOT CAUSE METHODOLOGYRoot cause methodology is a step-by-step method for

examining a failed motor and its system. It focuses on thestresses that acted upon the failed component. By betterunderstanding the stresses that acted upon a failed part, the

service center is more likely to uncover the root cause of thefailure.

The five key steps in root cause methodology are:

Failure mode:The manifestation, form or arrange-ment of the failure (e.g., turn-to-turn, phase-to-phase,etc.).

Failure pattern:How the failure is configured (e.g.,

symmetrical or nonsymmetrical). Appearance:A visual examination of the failed part,

the entire motor and the system in which it operates.Care must be taken to inspect all motor parts fordamage, contamination, moisture, cracks or other signsof stress.

Application: A close examination of the work per-formed by the motor and the characteristics of thosetypes of loads.

Maintenance history: An examination of the workperformed to keep the motor and system in properoperating condition.

In an ideal world, all relevant information pertaining to theapplication, appearance and maintenance history is avail-

able prior to the actual inspection of the motor or failedcomponent. However, in real life, the methodology usuallyunfolds by first inspecting the failed part, then the motor andfinally acquiring information about the application, appear-ance of the system and the systems maintenance history.This sequence is usually driven by the urgency to return themotor to service as well as the availability of application andhistorical data.

The good news is, in some cases, the root cause of failureis obvious. Such examples could be:

A balancing weight comes loose and strikes the winding.

The winding is saturated with water.

The bearing lubricant is contaminated.

However, in a case where the root cause must be known,it is imperative that none of the steps of the methodology beskipped.

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lamrehT X X X X X

/cirtcelEcirtceleid X X X

lacinahceM X X X X X

cimanyD X X X X X

raehS X X X

/noitarbiVkcohs X X X X X

laudiseR X X X

citengamortcelE X X X X

latnemnorivnE X X X X X

TABLE 1: MOTOR COMPONENTS/STRESSES

Bearing 51%*

Stator winding 16%*(May have been voltage,

water, overload, etc.)

External 16%*(Environment, voltageand load will likely

occur again)

Shaft/coupling 2%Rotor bar 5%

Unknown 10%*(No root cause failureanalysis performed)

* For each component shown, appropriate measures

to either prevent or predict the failure could greatly

reduce three-quarters of motor failures.

FIGURE 1: DISTRIBUTION OF FAILED

COMPONENTS

A Survey of Faults ..., IEEE Petro-Chemical Paper

No. PCIC-94-01, Olav Vaag Thorsen and Magnus Dalva.


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Motor

Power supply

Power source Utility Co-gen

Mechanical system

Mechanical device Pump Fan Compressor Mechanical Transmission drive Machine tool Conveyor belt

Meter

Process

Process requirement Flow Mixing Grinding Handling Conveyance Machining

Power transmission

Belting Direct connect Clutch Gears

Motor controls

Variable-frequency drive Soft start Wye-Delta Across-the-line Sensors Metering

Mounting base

Plate Rails

C-face P-Base

Electricity

Ambient

Moisture, wind snow, rain Chemical Temperature

Air flow Vibration Noise

FIGURE 2: THE TYPICAL MOTOR AND SYSTEM

stresses exceeds the design capacity, then the life of thesystem may be drastically reduced and catastrophic failure

could occur.These stresses can be broken down into the following

groups or classifications:

Bearing stresses: Thermal, dynamic and static load-ing, vibration and shock, environmental, mechanical,electrical.

Stator stresses:Thermal, electrical, mechanical andenvironmental.

Rotor stresses: Thermal, dynamic, mechanical, envi-ronmental, magnetic, residual, miscellaneous.

Shaft stresses:Dynamic, mechanical, environmen-tal, thermal, residual, electromagnetic.

For a more detailed summary of these stresses, seeTable 2.

ANALYSIS OF THE MOTOR ANDSYSTEM

Surrounding the motor is a system that consists of thepower supply, mounting, coupling and driven equipment.The environment, including the ambient, acts as an um-brella covering all of the elements of the system. Even the


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end product or process can be considered part of thissystem. (See Figure 2.)

Many factors affecting the system will also affect themotor and may contribute to the motor failure and vice-versa. Failure to consider each of these elements of thecomplete motor system could lead to an incorrect diagnosisof the root cause of failure. An effective tool for a systemsapproach is to conduct a failure mode effect analysis

(FMEA) of the complete system. The idea is to determinewhat the possible failure modes are for a component andthen determine how that failure can impact the systemwhere the component resides. This will offer at least someof the possible scenarios that can lead to a motor failure.

It is important to note that a number of failure mechanismscan lead to the same failed part with a common mode andpattern of failure. As examples, improper voltage, too muchload, blocked ventilation, excessive cycling and excessiveambient can all produce the same type of winding failure. Itis not always possible to correctly identify the problemwithout considering the entire system.

In many cases, arriving at the correct conclusion is aprocess of elimination driven by the collection of accuratedata and facts associated with the system. At the risk ofstating the obvious, failure to eliminate the root cause willusually assure expensive downtime and repeated motorfailures. A classic example is the repeated replacement offailed bearings without ever trying to assess the root causeof failure.

ARRIVING AT THE CORRECTCONCLUSION

When analyzing a motor failure, it is important not toassume facts that may fill in the gaps in information suppliedby the customer.

The service center often does not know much about themotor application, much less the power supply and/ormaintenance history. The customer dealing with the servicecenter is probably not the person who removed the motorfrom service, and may not be the operator who is familiarwith the motor or its application.

Incorrect, incomplete or even misleading information isthe common. It may be impossible to draw the correctconclusion from the evidence provided. Never assume apiece of evidence exists just to force the conclusion to fitthe facts.

When a conclusion is built around erroneous informationmingled with facts, the root cause of failure is seldomcorrect. The result is additional failures or assigning blameto the wrong parties.

Example:A winding has failed, after a very short run time,with a turn-to-turn failure. The customer might believe thatthe motors short life indicates poor workmanship, whetherthe motor is new or rewound.

The customer failed to advise (or the service center failedto ask) that the motor was operating on a pulse modulatedwidth (PWM) drive with a 100 (30.5 m) cable run. This wouldhave been a valuable piece of information for the servicecenter and, at the same time, it would have accuratelydescribed the motors power supply.

Without the knowledge of the PWM drive, the servicecenter forces the conclusion that the motor manufacturermust have damaged the winding, even though there was nosuch evidence. The manufacturer must have damaged it insome not so obvious way.

The wrong party is assigned responsibility for, and thecost of, repairing the failed motor. More importantly, the

problem is not fixed and will likely occur again.The location of the failure is critical evidence that may

explain the real reason for the winding failure. If the turn-to-turn failure is in a coil connected to a line lead, then atransient voltage could be the culprit. The location of thisfailure should alert the service center to find out more aboutthe power supply.

When a motor is operating from a PWM drive, especiallywith a long cable run [more than 50 (15.25 m)], a turn-to-turn failure in the lead coil is classic indication of high voltagespikes produced by that PWM drive and the long cable run.

The difference in knowledge will:

Assign the responsibility and cost of the repair to thecorrect party.

Give credibility to the service center.

And most importantly, make sure the root cause of thefailure is identified and corrected.

RESOURCE MATERIALS

The following pages provide some useful resources tohelp correctly identify motor failures including basic nomen-clature for horizontal and vertical motors, charts for thecollection of data, and lists of questions useful in analyzinga motor failure.


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Bearing carrier

Bearing holder

Bearing quill

Top hatRunner

Rain bonnet

Drip cover

Coupling

Stand tube

Oil dam

Stand pipe

Air baffle

Shroud

Air deflector

Clearance fit

Flame path

Shaft opening

End bracket

End bell

Terminal box

Outlet boxConduit box

Junction box

Keyway

Shaft

Foot

Coils

Coils

Windings

Rotor skew

Rabbet fit

Spigot fit

Fan cover

Fan shroud

Bearing cap

Bearing retainer

Back cap

Grease line

Stator laminations

Stacked statorCore iron

Core plate

Punchings

Stator laminations

Stacked stator

Core iron

Rotor laminations

Rotor core

Rotor laminations

Rotor core

Fill pipe

Drain pipe

Rotor fan blades

Rotor fins

End turns

Coil extensions

End ring

Eye bolt

Lifting eye

Stator shroud

Belly band

External cooling fan

Anti-rotation device

Anti-backlash assembly

Non-reverse ratchet

Other key nomenclature items:

Thrust washer

Spring washer

Pre-load washer

Wave washer

Oil ring

Oil slinger

Sleeve bearing

Babbitt bearing

Plain bearing

Bearing shell

Shaft

Boldtextindicates

terminology used

in this book.

BASIC AC MOTOR NOMENCLATURE AND COMMON ALTERNATIVES


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End bracket

Brush posts

Brush stud

Brush arm

Brush box

Brush holder

Commutator

Interpole

Commutator coil

Brush holder yoke

Brush holder insulator

Brush holder ring

Banding

Glass banding

Frame Field coil

Shunt

Pole iron

Armature

Key

Louvered ventilation covers

Shaft

Boldtextindicates

terminology used

in this book.

BASIC DC MOTOR NOMENCLATURE AND COMMON ALTERNATIVES


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METHODOLOGY FORMSAPPEARANCE OF MOTOR AND SYSTEM

METI SKRAMER

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lladroceR?debburtfahsrosnoitanimalrotorehtevaH.tcatnocrotatsdnarotorfosnoitacol

?esoolsgnicarbliocroslioc,skcitspotehterA

gniggolcforaelcdnaeerfsegassapgniloocrotorehterA?sirbed

notisI?eruliafgnidniwehtfonoitacollacisyhpehtsitahW

ehtfI?dnenoitcennocetisopporodnenoitcennocehthtiweruliafehtsierehw,yllatnozirohdetnuomsirotom

?deliafsesahproesahphcihW?kcolcehtottcepserronruttsrifehtnieruliafehtsI?deliafsliocfopuorghcihW

?lioctsrif

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tfahsdnarotorneewtebtnemevomfosngisynaerehterA?snoitanimaldnasrabro

neeberehtsahrodednetnisametsysnoitacirbulehtsI

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).knil


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METHODOLOGY FORMS

APPLICATION CONSIDERATIONS

METI SKRAMER

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sihtnodeliaftnempiuqerosrotomrehtoynaevaH

?noitacilppa

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?ecivresnineebrotomehtsahgnolwoH

?gnitarepoelihwrognitratsnoliafrotomehtdiD

rolaunamasihtsI?detratsrotomehtsinetfowoH,atled-eyw,gnidniw-trapatisI?noitarepocitamotua

dohtemenil-eht-ssorcaro)DFV(evirdycneuqerfelbairav?gnitratsfo

?dedivorpsinoitcetorpfoepyttahW

ehtdeppirtrodevomertahW rotom ?enilehtmorf

ehtsierehW rotom lamronehteratahwdnadetacolsawtahW?setarepotihcihwnisnoitidnoclatnemnorivne

ehtnehwekiltnemnorivneeht rotom ?deliaf

ehtdnuoraerutarepmettneibmaehtsawtahW rotom ta?riafonoitalucricerynaerehtsaW?eruliafehtfoemiteht

?etauqedariagniloocfoegnahcxeehtsI

evirdycneuqerfelbairavaybdeilppusrewopehtsIehtneewtebnurelbacehtfoecnatsidehtsitahW?)DFV(

?rotomehtdnaDFV

gnitnuomdnagnilpuocehtebircseduoydluowwoH?daolnevirdehtrofdohtem


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MOTOR SYSTEM AND ENVIRONMENT CHECKLIST

SINEWAVE POWER

Across-the-line starting Controlled start

Voltage RMS current Reduced voltage

Vab _______ IA ______________

Part-windingV

ac_______ I

B______________ Wye-delta

Vbc

_______ IC

______________ Soft start

_______

% unbalanced

NON-SINEWAVE POWER

Type of drive Known transients

Pulse width modulated Lightning

Other _______________ Switching

Other __________

Cable run length ________

Recorded incidents OtherTrips Power factor correction

Failed starts Surge capacitors

Known harmonics Lightning arrestors

Other ______________ Reactors

Voltage variations

POWER SUPPLY INFORMATION

V

Time

VMax

= _______

d

d

v

t

= ________

V

Time

High

Low

ENCLOSURE AND ENVIRONMENT

Location of motor

Outdoors Confined space

Indoors Other ___________________

Examine ambient Recent events

High temperature Rain

Low temperature Flood

Moisture conditions Spills

Altitude, coastal or mining Lightning

Other _______________ Other __________

List possible contaminants

__________________________________________

__________________________________________

Abovegrade

Belowgrade

At grade

Ground level

MOUNTING AND COUPLING

Equipment type Description

Pump Centrifugal

Reciprocating

Submersible

Blower/Fan _________________________

_________________________Compressor Reciprocating

Rotary screw type

Material handling _________________________

(Conveyor, _________________________

crusher, etc.) _________________________

Starting requirements

Inertia Torque

Low Constant

Medium Variable

High Constant horsepower

Other______________

Starting cycle

Acceleration ____________________

Frequency/on-off time ____________________

Loading conditions

Light High

Medium Overload

APPLICATION INFORMATION

Direct coupled

Integral cantilever

Solid shaft, coupled lower endHollow shaft, coupled top end

Wall mountedCeiling mountedOther

Integral but foot mounted

Common shaft

Overhung load

BeltsSprocketOther

Risetime


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12:00

6:00

3:009:00

12:00

6:00

3:009:00

Drive end Opposite drive end

The leads are F1 mounted

F2 mounted

The connection is on the drive end (DE)opposite drive end (ODE)

How many leads are there? _____

How are the leads marked? _____

Mark the location of failure(s) above. Identify on which end of the motor

the failure occured and it's position on the clock.

Identify which coil in the group failed, and relationship to the lead coil.

Is there core damage?YesNo

How many slots are affected? __________

Length of damaged area? _____________

If so, how extensive is the damage?

Is there grease, water or dirt on the windings?

Number of: Poles _____

Slots _____

Coils per group _____

Circuits _____

Connection Wye Delta

STATOR COIL LAYOUT FOR LOCATION AND IDENTIFICATION OF FAULT


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Root Cause Methodology Sect

1 Copyright 2002, Electrical Apparatus Service Association, Inc. (Version 502CI-502)

Root Cause Failure Analysis

AS FOUND REPORT

OPPOSITE

DRIVE END

(ODE) MAG. CTR.

DRIVE END

(DE)3:00

12:00

9:00

6:00

3:00

12:00

9:00

6:00

Disassembled By:Mechanical Readings:

Electrical Readings:Reassembled By:

Tested By:

CUST:P.O. #:JOB #:DATE:

OVERALL LENGTH

NORMAL THICKNESS

INSIDE DIAMETER

OUTSIDE DIAMETER

WINDING FLAPS

FRAME CLEARANCE

WINDING PITCH

NUMBER OF LEAD WIRES

SIZE OF LEAD WIRE

SIZE OF MAGNET WIRE

TYPE OF MAGNET WIRE

NUMBER OF TURNS PER COIL

NUMBER OF COILS PER POLE

NUMBER OF SLOTS

CONDITION OF CONNECTION LEADS

INITIAL 12:00

3:00

6:00

9:00

FINAL 12:00

3:00

6:00

9:00

IB OB

AIR GAP(AS VIEWED FROM COUPLING END)

DRIVE END

OPP. DRIVE END

MEASUREMENTS

BRG. HOUSING I.D.

BRG. O.D.

CLEARANCE

BRG I.D.

JOURNAL O.D.

CLEARANCE

MFG.

IB OB

BEARINGS

SHAFT

TIR INITIAL

FINAL

STRAIGHTENING METHOD

NOT APPLICABLE

PEENING

HEATING

BENDING

LOCATION OF BEND

(DISTANCE FROM IB END)

JOURNAL RECONDITION

NOT RECONDITIONED

CHROME PLATE DEPOSITION THICKNESS

DRIVE END

OPPOSITE DRIVE END

END FLOAT

ENTER

VALUES

MECHANICAL REPAIR

COND. BEFORE REPAIR

DISASSEMBLY

MANUFACTURER

SERIAL NO.

HORSE POWER

VOLTAGE

PHASES

NO LOAD CURRENT 1ST PHASE

2ND PHASE

3RD PHASE

SERVICE FACTOR

NEMA TYPE

RPM

BEARING TYPE

FULL LOAD AMPS

MOTOR INFORMATION

TEST

YES NO COUPLING OR PULLEY ATTATCHED TO MOTOR ON ARRIVAL?

DELTA COIL CONNECTION WYE COIL CONNECTION

YES NO CONNECTION BOX ATTATCHED TO MOTOR ON ARRIVAL?

1009080

70

60

50

40

30

20

1098

7

6

5

4

3

2

1

0 1 2 3 4 5 6 7 8 9 10

1 MIN 30 SEC1 MIN 45 SEC

2 MIN 0 SEC

3 MIN4 MIN

5 MIN

0 SEC15 SEC

30 SEC

45 SEC1 MIN 0 SEC

1 MIN 15 SEC

6 MIN7 MIN

8 MIN

9 MIN10 MIN

NO LOAD SPEED (RPM)

INSULATION RESISTANCE

TEST VOLTS

MINUTES INTO TEST

RESISTANCE : VALUE @ 10 MIN:


RESISTANCE(MEGAHOMS)

INSULATION RESISTANCE DURING TEST (IF REQUIRED)

POLORIZATION INDEX =

MAJOR REPAIR ITEMS

INSPECT/CLEAN

REWIND

RESTACK ROTOR

BEARING REPLACEMENT

JOURNAL SURFACE RECONDITION

SHAFT STRAIGHTENED

SHAFT REPLACED

HOUSING LINE BORED

ROTOR BALANCED

PAINTED

OTHER

1. BEARING DE ODE

GOOD CONDITION

CHATTER

FROZEN TO SHAFT

SCORED/WIPED

OTHER

2. LUBRICATION

INBOARD NORMAL VARNISHED

OUTBOARD NORMAL VARNISHED

3. ROTOR

GOOD CONDITION ARCED/FUSED AREAS

DISCOLORED/HOT SPOTS ROTOR FAN CRACKED

CRACKED ROTOR BARS RUBBED TO STATOR

OTHER

4. SHAFT

GOOD CONDITION DIAM @ SLEEVE BEARING

BENT RUN OUT / TIR

JOURNAL SURFACE DAMAGED

OTHER

5. STATOR


DISCOLORED/HOT SPOTS BENT LAMINATIONS

RUBBED/WARPED/WORN LOOSENESS

OTHER

6. FLAME PATH BUSHING

GOOD CONDITION I.D.

WEAR OUT OF ROUND/TIR

SCORING JOURNAL CLEARANCE

7. SUSPECTED CAUSE OF FAILURE, COMMENTS, SUGG.

SLEEVE

ANTIFRICTION


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Section 1 Root Cause Methodology

1 - 14 Copyright 2002, Electrical Apparatus Service Association, Inc. (Version 502CI-502

Root Cause Failure Analysi

AS RELEASED REPORT

OPPOSITE

DRIVE END

(ODE) MAG. CTR.

DRIVE END

(DE)3:00

12:00

9:00

6:00

3:00

12:00

9:00

6:00

Disassembled By:Mechanical Readings:

Electrical Readings:Reassembled By:

Tested By:

CUST:P.O. #:JOB #:DATE:

OVERALL LENGTH

NORMAL THICKNESS

INSIDE DIAMETER

OUTSIDE DIAMETER

WINDING FLAPS

FRAME CLEARANCE

WINDING PITCH

NUMBER OF LEAD WIRES

SIZE OF LEAD WIRE

SIZE OF MAGNET WIRE

TYPE OF MAGNET WIRE

NUMBER OF TURNS PER COIL

NUMBER OF COILS PER POLE

NUMBER OF SLOTS

CONDITION OF CONNECTION LEADS

INITIAL 12:00

3:00

6:00

9:00

FINAL 12:00

3:00

6:00

9:00

IB OB

AIR GAP(AS VIEWED FROM COUPLING END)

DRIVE END

OPP. DRIVE END

MEASUREMENTS

BRG. HOUSING I.D.

BRG. O.D.

CLEARANCE

BRG I.D.

JOURNAL O.D.

CLEARANCE

MFG.

IB OB

BEARINGS

SHAFT

TIR INITIAL

FINAL

STRAIGHTENING METHOD

NOT APPLICABLE

PEENING

HEATING

BENDING

LOCATION OF BEND

(DISTANCE FROM IB END)

JOURNAL RECONDITION

NOT RECONDITIONED

CHROME PLATE DEPOSITION THICKNESS

DRIVE END

OPPOSITE DRIVE END

END FLOAT

ENTER

VALUES

MECHANICAL REPAIR

COND. AFTER REPAIR

ASSEMBLY

MANUFACTURER

SERIAL NO.

HORSE POWER

VOLTAGE

PHASES

NO LOAD CURRENT 1ST PHASE

2ND PHASE

3RD PHASE

SERVICE FACTOR

NEMA TYPE

RPM

BEARING TYPE

FULL LOAD AMPS

MOTOR INFORMATION

TEST

YES NO COUPLING OR PULLEY ATTATCHED TO MOTOR ON ARRIVAL?

DELTA COIL CONNECTION WYE COIL CONNECTION

YES NO CONNECTION BOX ATTATCHED TO MOTOR ON ARRIVAL?

1009080

70

60

50

40

30

20

1098

7

6

5

4

3

2

1

0 1 2 3 4 5 6 7 8 9 10

1 MIN 30 SEC1 MIN 45 SEC

2 MIN 0 SEC

3 MIN4 MIN

5 MIN

0 SEC15 SEC

30 SEC

45 SEC1 MIN 0 SEC

1 MIN 15 SEC

6 MIN7 MIN

8 MIN

9 MIN10 MIN

NO LOAD SPEED (RPM)

INSULATION RESISTANCE

TEST VOLTS

MINUTES INTO TEST



RESISTANCE(MEGAHOMS)

INSULATION RESISTANCE DURING TEST (IF REQUIRED)

POLORIZATION INDEX =

MAJOR REPAIR ITEMS

INSPECT/CLEAN

REWIND

RESTACK ROTOR

BEARING REPLACEMENT

JOURNAL SURFACE RECONDITION

SHAFT STRAIGHTENED

SHAFT REPLACED

HOUSING LINE BORED

ROTOR BALANCED

PAINTED

OTHER

1. BEARING DE ODE

GOOD CONDITION

CHATTER

FROZEN TO SHAFT

SCORED/WIPED

OTHER

2. LUBRICATION

INBOARD NORMAL VARNISHED

OUTBOARD NORMAL VARNISHED

3. ROTOR


DISCOLORED/HOT SPOTS ROTOR FAN CRACKED

CRACKED ROTOR BARS RUBBED TO STATOR

OTHER

4. SHAFT

GOOD CONDITION DIAM @ SLEEVE BEARING

BENT RUN OUT / TIR

JOURNAL SURFACE DAMAGED

OTHER

5. STATOR


DISCOLORED/HOT SPOTS BENT LAMINATIONS

RUBBED/WARPED/WORN LOOSENESS

OTHER

6. FLAME PATH BUSHING

GOOD CONDITION I.D.

WEAR OUT OF ROUND/TIR

SCORING JOURNAL CLEARANCE

7. SUSPECTED CAUSE OF FAILURE, COMMENTS, SUGG.

SLEEVE

ANTIFRICTION


18/249


Root Cause Failure Analysis Bearing Failures Section 2

2Bearing FailuresSection Outline Page

Determining bearing life .................................................................................................................................. 2-2

The fatigue process and stresses that act upon rolling element bearings ...................................................... 2-2Methodology for analyzing rolling element bearing failures ...................................................................... 2-4

Tips for interpreting bearing failures .........................................................................................................2-4

Lubrication ................................................................................................................................................ 2-5

Thermal stress ........................................................................................................................................2-10

Dynamic and static loading stress .......................................................................................................... 2-13

Vibration and shock stress ..................................................................................................................... 2-15

Environmental stress ..............................................................................................................................2-17

Mechanical stress ................................................................................................................................... 2-19

Electrical stress ...................................................................................................................................... 2-21

Vertical motor bearing systems: Special cases ...................................................................................... 2-24

Introduction to sleeve bearing failures .......................................................................................................... 2-29

Methodology for analyzing sleeve bearing failures .................................................................................2-30

Thermal stress ........................................................................................................................................2-31

Babbitt grade .................................................................................................................................... 2-32

Some common causes of failure ......................................................................................................2-32

Dynamic and static loading stress .......................................................................................................... 2-35

Environmental stress ..............................................................................................................................2-37

Mechanical stress ................................................................................................................................... 2-39

Vibration and shock stress ..................................................................................................................... 2-41Electrical stress ...................................................................................................................................... 2-42


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Root Cause Failure AnalysisSection 2 Bearing Failures

DETERMINING BEARING LIFEBearing life is a function of rotational speed, dynamic

load, lubricant quality, impact loading and bearing size. Theprediction of rating fatigue life, commonly referred to as L

10

life is based on the assumption that the ultimate cause offailure is material fatigue. Excessive heat, lack of lubricant,or excessive loads simply accelerate the fatigue process.The L

10

life is the estimated time for 10% of a large popula-tion to fail. If L

10is one year, then L

50(the point at which half

the bearings will have failed) is 5 times that or 5 years. Thismeans that for an application with a L

10life of 1 year, 10%

of the bearings may fail within that first year, and that one-half the bearings may fail after 5 years.

The life for ball bearings is approximately inversely pro-portional to the load cubed and inversely proportional to thespeed. These relationships are only valid within certainconstraints relating to the bearing size, design, lubrication,temperature, load and speed. Bearings are subject to speedlimitations that are affected by the size and material of thebearing, as well as the lubricant. Oil lubrication increasesbearing speed limits by at least 10 to 15%.

L10

= (C/P)p

When rpm is constant, L10h

can be derived:

L10h

= 1,000,000/60n (C/P)p

Where: L10

is basic rating life, millions of revolutions

p = 3 for ball bearings

p = 10/3 for roller bearings

C = Bearing dynamic load rating

P = Equivalent bearing load

n = Rotational speed, rpm

The bearing industry has long used this formula to predictbearing life. The L

10bearing life gives satisfactory assur-

ance of bearing life for the purpose of selecting theappropriate bearing for each application.

In the real world, manufacturers try to reduce costs byusing the smallest bearing that will give satisfactory perfor-mance. Sometimes motors are built with smaller bearingsthan are prudent. End users apply motors for applications(and in environments) for which they were not intended. Inaddition, maintenance personnel do not always lubricatebearings on schedule. The repair industry has to contendwith each of these realities.

Rolling element bearings

Thermal stress.................................................... 2-10

Dynamic and static loading stress ...................... 2-14

Vibration and shock stress ................................ 2-15

Environmental stress .......................................... 2-17

Mechanical stress............................................... 2-19

Electrical stress .................................................. 2-23

PHOTOGRAPHS OF BEARING FAILURES

Sleeve bearings

Thermal stress.................................................... 2-33

Dynamic and static loading stress ...................... 2-36

Environmental stress .......................................... 2-38

Mechanical stress............................................... 2-40

Vibration and shock stress ................................. 2-41

Electrical stress .................................................. 2-42

The mode of bearing failure is fatigue, which may begreatly accelerated by the factors listed later in this section.

THE FATIGUE PROCESS ANDSTRESSES THAT ACT UPON ROLLINGELEMENT BEARINGS

Microscopic subsurface fractures of metal due to cyclicloading stress, producing thin layers of surface separa-tion, which flake off (spalling).

Some increase in noise and vibration will occur.

A change in critical dimension occurs.

Noise, vibration, friction, heat and wear accompaniedby more advanced spalling. It is no longer safe orprudent to operate the machine.

The final step is advanced spalling, usually followed bycatastrophic failure. (See Figure 1.)

The above 5 steps outline the failure process; the rate atwhich that process occurs depends on the variables in theL

10formula, but can be further influenced by several external

factors. The majority of bearing failures can be attributed to

a variety of stresses that can be grouped as follows:Thermal stress

Friction.

Lubricant.

Ambient.

Dynamic and static loading stress

Radial.

Axial.

Preload.

Vibration and shock stress

Rotor.

Driven equipment.

System.Environmental stress

Condensation.

Foreign material.

Excessive ambient.

Poor ventilation.

Mechanical stress

Loss of clearance.

Misalignment.

Shaft fit out of tolerance.

Housing fit out of tolerance.


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Electrical currents

Rotor dissymmetry.

Electrostatic coupling.

Static charges.

Variable frequency drives.

Since more than half of electric motor failures start asbearing failures (Figure 2), it is important to correctly analyze

the failure to determine the root cause to prevent futurefailures. Because severe thermal failures also destroy thelubricant, evaluation of the bearing independent of thesystem is difficult (Figure 3).

FIGURE 1: THE FATIGUE PROCESS

Bearing 51%*

Stator winding 16%*(May have been voltage,

water, overload, etc.)

External 16%*(Environment, voltage

and load will likelyoccur again)

Shaft/coupling 2%Rotor bar 5%

Unknown 10%*(No root cause failureanalysis performed)

* For each component shown, appropriate measures

to either prevent or predict the failure could greatly

reduce three-quarters of motor failures.

FIGURE 2: DISTRIBUTION OF FAILEDCOMPONENTS

A Survey of Faults ..., IEEE Petro-Chemical Paper No.

PCIC-94-01, Olav Vaag Thorsen and Magnus Dalva.

FIGURE 3: LUBRICANT DESTROYEDBY THERMAL STRESS

These photographs show the progression of a fatiguefailure, from microscopic fractures, through spalling, tocatastrophic failure. How quickly this happens de-pends on speed, time, temperature, load, vibration andlubricant.


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METHODOLOGY FOR ANALYZING

ROLLING ELEMENT BEARINGFAILURESThere are five key areas which should be considered and

related to one another in order to accurately diagnose theroot cause of rolling element bearing failures. They are:

Failure mode.

Failure pattern.

Appearance.

Application.

Maintenance history.

FAILURE MODES

Failure modes can be grouped into twelve categories,

which are usually the result of combined stresses acting onthe bearing to the point of damage or failure. This isarbitrarily referred to as the failure mode.

Fatigue.

Fretting.

Smearing.

Skidding.

Scoring.

Abrasive or abnormal wear.

Corrosion.

Lubrication failure.

True or false brinelling.

Electric pitting or fluting.

Cracks.

Seizures.

These modes do not represent the cause of the bearingproblem; instead they are the result or way that the problemis manifested.

FAILURE PATTERNS

Closely associated with the failure mode, yet different, isthe failure pattern. Each bearing failure has associated withit a certain pattern which can be grouped into some combi-nation of the following categories.

Temperature levels (discoloration).

Noise levels.

Vibration levels.

Lubrication quality.

Condition of mounting fits.

Internal clearances.

Contamination.

Mechanical or electrical damage.

Load paths and patterns (alignment).

TIPS FORINTERPRETING BEARINGFAILURES

In order to correctly interpret a bearing failure, it ishelpful to mark the position of each bearing as it isremoved. When axial thrust is a factor, the direction ofthrust may point to a coupling problem or an internalpreload condition. A practical method is to use a diegrinder or engraver to identify which side of each bearingis toward the rotor before the bearings are removed.Dissect the bearing using a die grinder rather than atorch, which heats and destroys evidence.

The wear pattern on the raceways offers importantevidence. Axial displacement (thrusting) is indicated bya ball path that is offset to opposite sides of the inner andouter races. Misalignment is indicated by a ball path thatangles from one side of the outer race to the other. Adisplaced (cocked) inner race is indicated by a widerpath on the inner race. Internal misalignment (indicatedby the angled ball path) often results from a cockedbearing bracket, or a bearing housing that has beenbored and sleeved improperly.

In the case of thrust bearings, indications of internalmisalignment are important because misalignment willdrastically shorten bearing load capacity and life.

Some bearing failures, especially sleeve bearing fail-ures, can only be interpreted in conjunction with thelubricant. When possible, preserve a sample of thelubricant for analysis. In the case of rolling elementbearings, the appearance of the lubricant can be criti-cally important.

If a bearing is to be sent out for outside expertise, donot clean the bearing first! Sandwich bags are great forpackaging a bearing with its lubricant before shipping.

Above, the side of the bearing toward the rotor has beenmarked prior to dissection. Below, a dissected ball bear-ing.


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APPEARANCE CONSIDERATIONS

When coupled with the mode and pattern of failure, themotor, bearing and load appearance usually give a clue asto the possible cause of failure. The following checklist willbe useful in the evaluation.

Are there signs of contamination in the area of thebearings? Any recent welding?

Are there signs of excessive temperature anywhere inthe motor or driven equipment?

What is the quality of the bearing lubricant?

Are there signs of moisture or rust?

What is the condition of the coupling device used toconnect the motor and the load?

What levels of noise or vibration were present prior tofailure?

Are there any missing parts on the rotating member?

What is the condition of the bearing bore, shaft journal,seals, shaft extension and bearing cap?

What was the direction of rotation? Was there anoverhung load or any axial thrust? Are they supported

by the bearing wear patterns? Does the outer or inner face show signs of fretting?

Is the motor mounted, aligned and coupled correctly?

Do not destroy the failed bearing until it has been properlyinspected. It is also important to save a sample of thebearing lubricant.

APPLICATION CONSIDERATIONS

Usually it is difficult to reconstruct the actual operatingconditions at the time of failure. However, a knowledge ofthe general operating conditions will be helpful. The follow-ing items should be considered:

What are the load characteristics of the driven equip-

ment and the loading at time of failure? Does the load cycle or pulsate?

How many other units are successfully operating?

How often is the unit started?

What type of bearing protection is provided?

Where is the unit located and what are the normalenvironmental conditions?

Is the motor enclosure adequate for the application?

What were the environmental conditions at time offailure?

Is the mounting base correct for proper support to themotor?

Is the belting or method of connection to the load

correct for the application?

MAINTENANCE HISTORY

An understanding of the past performance of the motorcan give a good indication as to the cause of the problem.Again a checklist may be helpful.

How long has the motor been in service?

Have any other motor failures been recorded and whatwas the nature of the failures?

What failures of the driven equipment have occurred?Was any welding done in the area?

When was the last time any service or maintenancewas performed?

What operating levels (temperature, vibration, noise,etc.) were observed prior to the failure? What trippedthe motor off the line?

What comments were received from the equipmentoperator regarding the failure or past failures?

How long was the unit in storage or sitting idle prior tostarting?

What were the storage conditions?

How often is the unit started? Were there shutdowns?

Were the lubrication procedures correct?

Have any changes been made to surrounding equip-ment?

What procedures were used in adjusting belt tensions?

Are the pulleys positioned on the shaft correctly and asclose to the motor bearing as possible?

LUBRICATIONBecause lubrication is inseparable from many bearing

failures, there is lubrication information distributed through-out the bearing failure section. This portion of the sectionfocuses specifically on lubrication issues, to facilitate its useas a reference.

The role of lubricant is to reduce friction between therolling or sliding parts, dissipate heat generated by thebearings and protect the surface finish of the bearing partsfrom corrosion. To a lesser extent, lubrication excludesforeign contamination by displacement.

Lubrication normally means either grease or oil, each ofwhich can be delivered by several different methods.

GREASE LUBRICATION

Grease is oil suspended in a base so that the oil isavailable to lubricate the bearing as needed. Grease lubri-cation is almost exclusively for ball and roller bearings.

When a bearing housing is designed for grease lubrica-tion, a cavity is provided within the bracket to hold a quantityof grease. Some designs incorporate metering plates andsimilar methods to regulate the flow of grease to the bearing.An inner bearing retainer (or cap) is often provided to retainthe grease and exclude contamination. In many cases, theretainer also is used to establish endplay.

Lubrication can be affected by temperature, environmen-tal conditions, dynamic bearing load, and speed. Lubrication

selection can affect vibration levels, bearing temperatureand longevity.

Grease selection should consider the above variables.The best grease for an open pit copper mine in the desert[130oF (54C) ambient] is probably not the best grease touse in the arctic [-50oF (-45C) ambient]. The same is truefor dry climates (5% humidity) versus coastal regions (98%humidity). Additional concerns include contamination of thelubricant. Contamination, high temperature and friction re-duce the effectiveness of lubricants.


23/249



Present research is making it possible to predict bearinglife more accurately. The use of Elasto-Hydrodynamic Lu-brication theory (EHL), introduced in the 1960s, for calculatingfilm thickness and pressure profiles, has been the key tomany investigations and the base for understanding failuremodes. Since the early 1970s, lubrication and film thicknesshave been recognized as significant factors in the lifeequation. The ABMA Standard 9/ANSI B3.15, and ISO 281

standards were modified in 1972 and 1977 respectively, toinclude this effect by the addition of the a2 (material) and a3(operating conditions) life adjustment factors.

Typical factors used are shown in Figure 4. The latestefforts have been in the area of particle contamination andlubricant cleanliness. These new studies are tending toreshape the life prediction equations. According to onebearing manufacturer, the true nature of the failure modemechanism was hidden and not understood until recently

for the following reasons: The high loads used to accelerate testing resulted in

insufficient time for wear to manifest itself.

Surface initiated cracks, from particle indentation,

FIGURE 4: LIFE ADJUSTMENT FACTOR

VS. VISCOSITY RATIO

FIGURE 6: GREASE TEMPERATUREPROPERTIES

TEMPERATURE VS. OXIDATION LIFE

FIGURE 5: LIFE ADJUSTMENT FACTORVS. CONTAMINATION-LOAD

which penetrated into deeper areas of high stressand culminated in flaking, could not be distinguishedfrom flaking caused by cracks formed below thesurface.

Based on these latest studies, bearing life theory hasbeen further refined to use a family of curves to establish anadjustment factor to the unmodified life. Of primary impor-tance is the !factor used to correct for contamination. An

accurate assessment of the!factor requires an analysis ona computer with accurate knowledge of the application.Figure 5 is typical of the curves used to determine the lifeadjustment factor for contamination. These refinements,along with similar actions taken by other manufacturers, canonly lead to a more precise determination of bearing life. Inaddition to new life prediction theories, new lubricants andlubrication methods are being devised which will extend theoperating life. Synthetic greases are capable of extendinggrease life significantly as indicated by the oxidation char-acteristics shown in Figure 6. Although grease life is afunction of more than just oxidation life, it is a good indicatorof the type of gain that can be made using synthetic grease.

Synthetic greases can be formulated with a lower sensi-tivity to temperature variations, and therefore, have a largeruseful temperature range and the potential for lower losses.

The question frequently asked about greases, deals withthe compatibility of them if mixed during the relubricationprocess. Table 1 is a guideline to assist in this process. If indoubt, do not mix without checking with the lubricant manu-facturer.

Lubrication arrangements for grease-lubricated bear-ings, shown in Figure 7, vary among manufacturers and

designs. Grease viscosity, motor mounting position, andbearing enclosure impact the effectiveness of the lubrica-tion porting. For example, the grease-through design shownin Example C does not work well with a double-shieldedbearing.

While some margin exists, a good rule of thumb forbearing temperature is 80-90-100, where 80o C is theoperating temperature, 90 o C is the alarm setting, and100o C is the shutdown limit. For higher temperatures,synthetic lubricants (oil or grease) are available. In general,


24/249


25/249



tion. When a high ambient condition exists, or when it isdesirable to lower bearing temperatures, a forced lubrica-tion system is used.

Most sleeve bearings require 2 to 3 gallons per minute(1.5 to 2 liters per minute) for adequate lubrication. Tocontrol the volume of oil through a forced lubrication system,the inlet is pressurized and oil forced through a small orifice(or metering plate). System pressure is 10 to 15 psi, and

orifice sizes are typically around 0.030 (0.8 mm) to providethe desired flow rate. To test the flow rate, use a bucket tomeasure the oil exiting the bearing for one timed minute.One common cause of apparent oil leaks is a missingorifice. This occurs because the orifice is installed in themotor piping, and can get lost when the motor plumbing isdisconnected.

Table 3 provides some clues based on the appearance ofthe oil.

LUBRICATION PRECAUTIONS All motor housings, shafts, seals and relubrication paths

must be kept thoroughly clean throughout the motor's life.

Avoid any dirt, moisture, chips or foreign matter contami-nating the grease.

Identify the temperature range for the application andselect a grease that will perform satisfactorily.

Over greasing may cause elevated bearing and/or wind-ing temperatures which can lead to premature failures.Be sure to properly purge excess grease.

When regreasing, be sure that the new grease is compat-ible with the existing grease and that it has the desiredperformance characteristics.

Synthetic grease may not be as suitable as petroleumgreases for high-speed applications. Some applicationsmay require an extreme pressure (EP) grease.

Some common greases are not suitable for motorapplications. If they are too soft, whipping can occur. Iftoo stiff; noise and poor bleeding characteristics canoccur.

Do not try to lubricate sealed bearings.

preventing corrosion during long idle periods. Bearingslubricated by oil mist should have seals or bearing isolatorsto contain the oil. Recovery methods vary from drip cups topassing the exiting vapor through a reclassifier. A reclassifierreconsolidates the oil droplets.

Oil mist has drawbacks, each of which is difficult to detectuntil the motor has been dismantled. First, oil mist is a vaporthat can exit the bearing chamber and cause other prob-

lems. Environmental contamination may result when thevapor recovery system fails. Oil chemically attacks someinsulation materialsespecially lead wire insulation.

Oil selection is affected by the application, temperature,environment and bearing design. Aside from the obviousfactors already listed, oil viscosity can affect vibration levelsof sleeve bearing machines by altering the stiffness of theshaft-bearing interface. As a rule-of-thumb, the closer theratio of bearing length to bearing diameter is to 1, the moreimportant oil viscosity is likely to be.

FORCED LUBRICATION

Forced lubrication systems are added to reduce bearing

temperature (Figure 8). In effect, the forced lubricationsystem simply increases the size of the oil reservoir. Therole of the oil reservoir is to ensure a steady supply of oil tolubricate the bearings, but also to cool the oil by recircula-

FIGURE 8: SLEEVE BEARING MOTOREQUIPPED WITH A FORCED LUBRICATION

SYSTEM

The piping is part of a forced lubrication system used toreduce bearing temperatures.

ecnaraeppA

evahyamtahW

deneppah

.ttibbabdetlem,lionaelCretfadeddasawliO

.deliafgniraeb

.ecnaraeppaykliM .lioehtniretaW

.yddumsraeppaliO .lioninoitanimatnoC

.sgnidniwdekaos-liO

laeshtnirybalevissecxEootlevellio,ecnaraelc

gniraebdezirusserp,hgih-decrofrorebmahc

ootemulovnoitacirbul.hgih

TABLE 3: APPEARANCE OF OIL


26/249



OVERLUBRICATION

Thin grease used in this roller bearing migrated past theinner bearing cap. A lip seal would prevent this byretaining the grease.

Grease quiets a noisy bearing. This bearing was noisy forquite some time.

Overgreasing a noisy bearing treats the symptom ratherthan the cause. The result may cause other problems,most notably an increase in winding temperature.

The upper bearing carrier, bolted inside the end bracket,has too much clearance to the shaft. Gravity, grease anddirt are not a good combination.


27/249



noitidnocgnirotinoM erutarepmeT

lamroN )C08(F071

mralA )C09(F091

nwodtuhS )C001(F012

TABLE 4: BEARING MONITORING

TEMPERATURES

Add 30C when synthetic lubricants are used, however,synthetic grease is often not suitable for high-speedapplications.

THERMAL STRESSA rolling element bearing should operate at temperatures

not in excess of 100C. The rule of thumb, 80-90-100,refers to an operating temperature of 80C (170F), analarm temperature of 90 C (190 F) and a shutdowntemperature of 100C (210F). (Note: 30C may be addedfor synthetic lubricants, however, synthetic grease is oftennot suitable for high-speed applications.) (See Table 4.) Forsealed bearings, the rpm rating is significantly lower than foropen bearings.

At temperatures above 100 C (130 C for syntheticlubricants), thermal expansion of the component parts mayreduce the internal clearance, resulting in premature failureof the bearing. In addition, lubricant breakdown will result inhigher bearing temperatures and bearing failure.

Bearing temperature is affected by the temperature of thesurroundings (air, windings, rotor), as well as by the lubri-cant (type, quantity, viscosity and condition), the bearingitself (internal clearance, open/shielded/sealed,) and load

(dynamic load, direction of load, speed and impact cycling).The bearing should be sized appropriately for all of theseconditions, but in the real world not all equipment is createdequal. Understanding the root cause may lead to sugges-tions to modify a unit to make it more suitable, or evenreplace it.

THERMAL STRESS

Symptoms of overheating are discoloration of the races,balls and cages from straw to blue. Temperatures inexcess of 400F (205C) can anneal the race and ballmaterials. The resulting loss in hardness reduces thebearing capacity, causing early failure.

Courtesy of The Barden Corporation

Heat discoloration indicates the inner race reached700F (370C). Possible causes include loss of fit to theshaft, lubricant failure or improper installation. Localizeddiscoloration may indicate that a torch was used to heatthe inner race.


28/249



THERMAL STRESS

Lubricant failure will lead to excessive wear, overheatingand subsequent bearing failure.


This motor failure started as a failed bearing. The burntpaint shows the extreme heat created by this failure. Thebearing failure resulted in damage to the rotor as well asthe stator.

Diligence and protection mean the difference betweenminor damage and this type of failure.

The temperature of this bearing exceeded the droppingpoint of the grease. This is the temperature at which oilseparatesor drops outfrom the grease base. Thisbearing failure led to the bent shaft.


29/249



THERMAL STRESS

By the time this bearing failed, the shaft temperatureexceeded 900F (480C).

This bearing shows signs of heat discoloration. It wasoverheated prior to installation.

In this case, the heat source was a stalled rotor. Heatmigrated from the rotor through the shaft to the bearing.Different greases have different dropping points (thetemperature at which oil separates from the greasebase).

The additional air shroud on the drive end deflects airacross the drive end bearing housing. If removed by anend user or previous repairer, the drive end bearingtemperature will increase.

Loss of lubrication damaged this spherical roller bearing.


30/249



DYNAMIC AND STATIC LOADINGSTRESS

Load characteristics of the bearing in its unique applica-tion include radial load and/or thrust load. Radial load mayresult from a belted application (Figures 9 and 10), misalign-ment or other factors not immediately apparent. Thrustloads may be external or internal in source. A vertical

application supporting a pump may require a thrust bearingcapable of handling substantial thrust loads. An identicalmotor might be designed with different bearings for high-,medium- or low-thrust applications. Thrust load may alsoresult from internal preloading of the motor. Prior machinework such as a shaft replacement, missing gaskets orswapped bearing caps (when both bearings are the samesize) can also cause this condition.

For vertically-mounted machines, axial thrust load of thenon-thrust bearing may result from improper assembly.Thermal expansion of the shaft during service may movethe axial load from the thrust bearing to the non-thrustbearing. This is also true of axially-loaded horizontal ma-chines. It is worth noting that there are end users who install

horizontal motors in nonstandard positions (Figure 11),reducing the effectiveness of the lubrication paths. If some-thing about the evidence doesnt seem to fit, it may indicatean unusual mounting condition. Look for an answer that fitsALL the evidence.

Axial loading may also result from improper alignment ifthe coupling preloads the locating bearing; for examplewhen a rigid coupling is used, if the installer pries thecoupling halves apart (or draws them together using thecoupling bolts) after the motor base is secured.

FIGURE 9: RESULTS OF EXTREMEOVERHUNG LOAD ON A BELTED

APPLICATION

FIGURE 10: BELTED APPLICATION

Pulley diameter and number of belts can affect radialloads. See alignment material in Section 6.

FIGURE 11: HORIZONTAL MOTOR MOUNTED

VERTICALLY

An end user may save money by purchasing a hori-zontal C-face motor instead of a vertical. However,this may reduce the effectiveness of lubrication paths.


31/249



DYNAMIC AND STATIC LOADING STRESS

These shafts show signs of a classic case of excess radialload on a ball bearing.

This application requires a roller bearing for the drive end.The pulley should be installed as close to the bracket aspossible. Worn belt grooves increase belt slip and maycause the operator to overtighten the belts and overloadthe bearing.

This bearing stopped rotating, but the undersized shaftdid not. A heavy radial load caused this unique failurepattern.

Severe spalling caused by excessive load. This spallingis part of the natural failure process as a bearing reachesthe end of its life.


Heavy shock loads can cause unusual fractures of theouter race and/or balls.


32/249


33/249



VIBRATION AND SHOCK STRESS

A split outer race (circumferentially) is caused by highshock impact. This unusual failure is more common inapplications such as a crusher or hammer mill.

The bearing of this motor was damaged by vibrationcaused by a damaged cooling fan.

Rotational shock load caused this coupling to fracture.Most service centers do not receive the coupling with themotor, so evaluating a bearing failure without all theevidence can be tough. The coupling can provide valu-able information.

Heavy axial loading or axial impact can chip the outer raceshoulder of a roller bearing.

Courtesy of Koyo

Excessive load may cause bearing cage failure.


34/249



ENVIRONMENTAL STRESSLubricants are produced with varying degrees of mois-

ture resistance. It is up to the end user to select the mostappropriate lubricant for the application. Condensation maycause rust on the surface of the bearing and internal motorparts. Corrosion on the bearing raceways or rolling ele-ments will work quickly to further damage the bearingsurface.

Foreign material may include liquid or vapor that attacksthe bearing surface or the lubricant. Examples include nitricor hydrochloric acid, which can flash-rust a bearing whenthe vapor is present, even in small quantities. Foreignmaterial also includes grease incompatibility. SeeTable 1 (Page 2-7) for specifics, but the results of mixingincompatible greases vary. Some combinations result in asoupy liquid while others harden into a solid mass thatresembles plastic.

Excessive ambient temperature is not restricted to the air

surrounding the motor. An exposed steam line near one endof a motor may elevate temperatures on that end only.Radiant heat sources may be a considerable distance from

the machine and still raise bracket temperaturewithout affect-ing air temperature. A motor operating within a confinedspace (e.g., compressor) may be subject to recirculation asthe temperature of the cooling air is raised each time it

passes through the motor. The smaller the "T, the lesseffective the cooling medium becomes. ("T is the tempera-ture difference, in this case between the air in and air out.)

In the case of restricted ventilation, the temperature of thewindings and rotor increases. The shaft functions partly as a

heat sink to conduct heat away from the rotor. That, in turn,increases the bearing and lubricant temperature. Buildup ofcontamination (dirt, pulp, product) on the exterior of the motor

insulates the bearing, trapping heat.

ENVIRONMENTAL STRESS

Grease was flushed from this bearing. Water, steam orsolvents are often the cause of this type of damage.

The lubricant was washed out of the bearing. Rust isevident.

The irregular striped discoloration was caused whencontamination was pressed in the roller path.

Corrosion caused the initial damage to this roller bearing.


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ENVIRONMENTAL STRESS

Dust and other fine dry contaminants absorb oil andthicken the grease base.

Grease compatibility problems may result from mixingincompatible greases, or from ingress of other contami-nants. Dry powders may absorb the oil causing thegrease to thicken.

Restricted ventilation may increase winding and/or rotortemperatures. Heat transfers to the bearing housing andelevates the bearing temperature.

Dirt in the roller path imbeds in the raceway, decreasingbearing life.


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MECHANICAL STRESSBearing failures can also result from a variety of mechani-

cal causes, either internal or external in origin. Contaminationand/or corrosion may reduce the clearance between theshaft and end bracket resulting in heat-generating friction.

Misalignment of the motor and driven equipment in-creases the dynamic load on the bearing. Improper

manufacturing or repair procedures may result in a loss ofinternal bearing clearance. A shaft fit that is too large, or abearing housing that is too small, results in a tighter fit andreduces internal clearance of the bearing. Too loose a fitmay permit the bearing to slip on the shaft (or in thehousing), generating more heat.

In specific cases, use of the wrong bearing for theapplication can lead to the same failures. Vibrator (shaker-screen) motors are designed with loose shaft fits and tighthousing fits. They require the use of C4 internal clearancebearings. Some dragline motors utilize higher interferencefits between the shaft and bearing (m6 rather than k5), butmay also adjust the bearing housing fit to preserve thebearings internal clearance.

Crushers are often fitted with spherical roller bearings ontapered journals. The distance the bearing is advanced ontothe tapered journal controls the internal clearance of thebearing. Once the bearing is removed, it is too late to checkthe internal clearance.

MECHANICAL STRESS

Discoloration and scoring is the result of the outer raceslipping in the bearing housing.


Heavy ball path wear indicates a tight fit.


This drive end bearing was forced over the bearing lockwasher after the inner race spun and got hot enough toforge. A heavy axial preload from the load caused thefailure.


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MECHANICAL STRESS

Loss of fit damaged this bearing. The inner race spun on the shaft, generating heat. Thermal breakdown of the lubricantfollowed causing the rolling elements to seize and forge to the inner race, which expanded it further.

Loss of fit (left) may follow a bearing failure or it may result from corrosion, product contaminants or insufficient clearance.A motor in a corrosive atmosphere, operating infrequently, is susceptible to this mode of failure. The combination of analuminum bracket and steel shaft can be vulnerable. Resulting friction could cause the shaft to seize, or friction-generatedheat could weaken the shaft (right).

Severe vibration literally forged the inner race of this bearing once the race temperature reached 1200F (650C).


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FIGURE 12: SHORTED LAMINATIONS

This severe damage resulted from a bearing failure thatprogressed.

ELECTRICAL STRESSCurrent discharge from voltage passing through the bear-

ings can damage them. These shaft voltages have longbeen associated with medium and large electric machines;however, the increased used of variable frequency drives(VFDs) has since resulted in shaft voltages in much smallermotors.

In standard machines, any break from uniformity in therotor or stator can cause shaft voltages. Shorted lamina-tions (Figure 12), gaps in the stator laminations (as occurwith large machines built with segmented laminations),variations in air gap or spacing for f ields or interpoles in a DCmachine; all can result in shaft voltages in rotating equip-ment. Shaft voltages may also result from static electricdischarge from the driven equipment or process. Oneexample is a large continuous paper roll, where staticelectricity can build up and discharge through the bearings.

Indications of shaft voltages are fluting when rpm issteady, or frosting when speed varies continuously. In somecases, the appearance of the balls offers the best clue.Instead of a highly-polished finish, the rolling elements may

have a dull appearance.The rule of thumb for voltage limits is 100 mV for ball

bearings and 200 mV for sleeve bearings. Variable fre-quency drives can result in shaft voltages as high as 20 to25 volts. Because of capacitive coupling between the rotorand stator, both bearings must be electrically isolated. Thestandard method of insulating only one bearing will notprotect bearings in a machine operated from a VFD.

FIGURE 13: METHODS OF PROTECTING AGAINST SHAFT CURRENTS

METHODS OF PROTECTION

Before pulse width modulated (PWM) inverters, shaft-riding brushes were used or the opposite drive end bearingwas insulated. Insulating the bearing was the preferredmethod. This breaks the circuit and interrupts the flow ofvoltage. (See Figure 13.) A good analogy is a light switch:When the switch is turned off, the light goes off because theswitch breaks the circuit.

The grounding brush provides a parallel path to the


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When insulating a bearing housing, the repairer must

also insulate the face of the bearing cap. The bearing

cap could come into contact with the face of the

bearing, bypassing any insulation on the bearing

housing.

FIGURE 15: PRECAUTION WHEN INSULATINGBEARING CAPS

FIGURE 14: INSULATING WITH CERAMIC

SPRAY

Ceramic or aluminum oxide spray is one method ofinsulating. Above, an opposite drive end bearing jour-nal, and below, vertical motor bearing carriers, all ofwhich have been insulated with ceramic spray.

bearing, diverting some of the current from the bearing tothe brush. Voltage follows the path of least resistance, so ifthe brush is highly conductive and has good contact with theshaft, most of the voltage will flow through the brush. But asthe shaft oxidizes or as dirt builds up on the shaft, theresistance through the brush/shaft connection increases.The bearing becomes the path of least resistance and moreof the voltage flows through the bearing.

A partial list of better preventive measures includes:

Install ground brushes on both ends.

Insulate both bearing housings.

Insulate both shaft journals.

Use ceramic (insulated) bearings.

Use bearings with ceramic balls.

Install in-line filters between the motor and VFD toreduce the problem.

Improve grounding of the motor and drive.

If a motor is critical, a short-term corrective action is todecrease the switching frequency of the drive to less than5 kHz. That may permit the motor to operate until anotheroption can be implemented.

Grounding brushes still have all the problems mentionedpreviously, but are utilized by some manufacturers. For verylarge machines, a copper toothbrush style brush is avail-able. In most cases, the brush is constructed like a

conventional carbon brush, but with a high silver content toincrease conductivity.

Grounding brushes should be located as close as practi-cal to the bearing. The longer the supporting bracket, thehigher the resistance of the bracket/brush/shaft path.

Ceramic spray can applied to the shaft journal, and mustbe precision-ground to size (Figure 14). Ceramic chipseasily, so handling requires care. Because the layer ofceramic is relatively thin, care should be taken when balanc-ing a shaft with ceramic-coated journals. The rotor weightshould not be placed on the journals, for balancing orinspection, because the point-loading is likely to break theceramic loose from the shaft. The damage often does notshow up until the motor is in service, at which time theceramic fractures, leaving the bearing with a loose shaft fit.

A thermal spray aluminum oxide may be used for sleevebearing exteriors. Aluminum oxide is the same materialused for emery cloth and abrasive grinding wheels. With thealuminum oxide bearing shell, vibration can eventuallycause the bearing housing to wear due to the abrasiveaction. The higher the vibration, the more likely this is tooccur. Aluminum oxide coatings can also be compromisedby moisture and corrosion.

Insulating the bearing housings requires that other parts(like bearing caps) not bypass the insulation. When abearing exhibits evidence of shaft currents, and the housingis insulated, verify the integrity of the insulation with that endof the motor assembled. (See Figure 15.)

Space-age epoxy putties (Devcon, Belzona) also can beused, but caution should be exercised to avoid exceedingthe load capacity of these materials.


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ELECTRICAL STRESS

Fluting due to shaft currents on both a roller bearing,above, and a ball bearing, below.

When rotational speed varies, the shaft currents maycause a dull, frosted appearance instead of fluting.

The spacing of the fluting marks depends on rpm, diam-eter, radial load and magnitude of the shaft voltage.

Fluting only occurred on the non-loaded roller path be-cause the arcing occurred only at the gap between the

rollers and the race. A good analogy is the points in anolder automobile ignition system.

The arcing on this ball was caused by welding done nearthe motor.


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VERTICAL MOTOR BEARINGSYSTEMS: SPECIAL CASES

There are several features unique to vertical motors.Those features are grouped together here for the conve-nience of those inspecting vertical machines. Because avertical motor is often coupled to a pump, the motor may berequired to support the weight and thrust load of the pump

as well as the weight of the rotor. With pump designsranging from low- to medium- to high-thrust, the upperbearing arrangement of seemingly identical vertical motorscan vary tremendously according to the bearing size, quan-tity and direction of thrust. (See Figure 16.)

When an end user changes the pump without matchingthe thrust load requirement to the thrust load capacity of themotor, or when an aftermarket spare motor is purchased,there is potential for misapplication.

The service center may not be aware of all the circum-stances surrounding a bearing failure. A key considerationis the length of time in service with the same pump and thrustload for the motor. Recent installation, pump work or otherchanges are cause for further investigation.

The following checklist will help focus the inspection onprobable causes:

Has the pump been recently replaced or serviced?

Was any base or foundation work done?

Has the motor been coupled to the same pump? Has itbeen moved recently?

Has there been a change in the material being pumped?

Are there records of vibration levels and/or current?

Is there on-line monitoring equipment for vibration/current? Are the records available?

Has there been any recent maintenance to the motor orpump?

Is the pump or motor part of a redundant system? If so,

are some units run continuously or is the startingsequence alternated?

Have maintenance personnel recently checked thealignment or vibration?

Following is a list of possible misapplications for verticalmachines:

Mismatch of thrust needs.

- High-thrust bearings coupled to a low-thrust pump.

- Low-thrust bearings coupled to a high-thrust pump.

- Lack of upthrust capability on a pump with occa-sional upthrust.

Bearing arrangement has been changed for occa-sional upthrust, but no clamping ring/thrust shoulder isprovided. The bearing orientations are correct, but theupthrust bearing cannot function because there isnothing to thrust against. (See the top illustration inFigure 16.)

Bearing thrust capacity has been changed by adding orremoving a bearing without changing lubrication provi-sions. If a thrust bearing is removed from a2-thrust-bearing arrangement, the lower bearing shouldbe removed, with a spacer (Figure 17) used beneaththe remaining bearing. A clue is to compare the oil level

COUPLING

ADJUSTING NUT

LOCK WASHER

BEARING HOLDER

RATCHET CAP

SHAFT

THRUST BEARING

TOP BEARINGLOCK WASHER

TOP BEARINGLOCK NUT

TOP BRACKET

TOPBEARING CAP

O - RING

TOP BEARINGCAP BOLT

BEARINGHOLDER

OIL DAM

BEARING HO

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