Bearing Failures-An Overview

8/13/2019 Bearing Failures-An Overview

1/17

Failure of Rolling Bearings:

An Overview

!" ##"

$ %&

$ '()'*+ *,-,

Rolling Bearing Damage

Bearing Life depends on:

Total number of stress cycles Loads incurred by rolling

elements and raceways.

Survival of dynamically stressed

Bearings is based on material fatigue

(pitting). Stage II

III


2/17


3/17

Indentations caused by faulty mounting

Dented roller

surfaceInner ring

raceway

Double row cylindrical roller

bearing suffered improper handling

Fatigue

Limited endurance limit

I.I.T. Bombay


4/17

Fatigue

Condition:

||

Causes:

Solutions: ! "

###

I.I.T. Bombay

Damage???? Unusual operational behavior!!

Uneven running

Uncommon running noise

Resistance to smooth running

Flaked surfaces

High Friction Detrimental preload

Poor lubrication Damaged surfaces

High Temperature

High VibrationMonitoring

I.I.T. Bombay


5/17

Knowledge of Load, Fits, Mounting conditions

Lubrication

surrounding vibration

Water

Does one need such detailed observation???

How to Avoid Trouble

I.I.T. Bombay

Failure by Abrasive Particles

outer race ofa spherical

rollerbearing

Scratching Indentation


6/17

Failure due to Inadequate Lubrication

outer ring of a sphericalroller bearing-Mirror finish

If the lubricant film between raceways

and rolling elements becomes too

thin, the peaks of the surfaceasperities will momentarily come intocontact with each other. Polishing oftwo surfaces (as in lapping) occurs in

initial phase.

Small cracks then form in the surfacesand this is known as SURFACE

DISTRESS.

Consequently material is heated tosuch temperatures that rehardening

takes place.

This produces localized stress concentrations that may causecracking or flaking or transfer of material (commonly know asSMEARING) from one surface to the other.

Surface distress in the

form of a bandencircling a roller.

100xmagnification


7/17

Smearing on the surface of aroller from a spherical roller

bearing - 100 x magnification.

A cylindrical roller

with end smearing

Smeared

face of acylindricalroller bearinginner ring.

Smeared outsidesurface of aspherical roller

bearing outer ring

Failure by Vibration

outer ring of a selfaligning ball bearing

damaged by vibration

inner and outer ring of acylindrical roller bearing


8/17

Rust and Corrosion

Condition: Rusting at spacingequal to the distances betweenthe rolling elements

Causes: Ingress of water orcorrosive material (such as acid)

Condensation of moisturecontained in the air.

Poor packaging and storing

conditions, and handling with barehands.

Solutions Improvement in sealing effect

Measures for preventing rusting when not operating for a long period of time.

I.I.T. Bombay

Deep seated rust in the outerring of a deep groove ball

bearing

Fretting and Fretting Corrosion Meaning: ! !

Condition: $ !

%

Causes: & ! !

Poor lubrication (no lubrication)

Fluctuating load

Vibration during transportation

I.I.T. Bombay


9/17

Fretting and Fretting Corrosion

Condition: '

(

Cause: Relative movementbetween bearing & housing.

Insufficient radial interference.

(dirt?).

Solutions: Review bearing interference and back contactconditions between bearing back & housing )

Fretting on

Steel Backing

I.I.T. Bombay

Electrical Pitting

Condition: * +

!

!

,

Causes:

!

Solutions:) !

!

" $-

I.I.T. Bombay


10/17

FlakingMeaning= Peeling off

Results: * .

I.I.T. Bombay

Causes:

Installation error, misalignment, improper handling, etc.

ingress of foreign objects, debris, rusting, etc.

Solutions

Eliminate causes.

Adapt bearing with larger capacity

Increase viscosity of oil/grease

Flaking.


11/17

Flaking in the outer ring ofspherical roller bearing that hasbeen mounted in a housing withoval bore

Flaked inner ring of a

spherical roller bearing. Veryhigh axial load compared to

the radial load.

Cracking

Meaning= breaking, splitting,or snapping apart

Results: "

Causes: !

Loose fit, excessive impacts

Solutions

Correction of fits

Vibration isolation

I.I.T. Bombay


12/17

SeizureMeaning=

Results: -

!

!

I.I.T. Bombay

Causes: . Poor lubrication or improper lubricant.

Smaller Clearance.

Excessive load (or pre-load).

Installation error.

SolutionsImprove dissipation of heat from bearing

Prevention of misalignment

improvement in clearance

Selection of suitable lubricant and

determination of optimum lubricant feedingrate


13/17

Bearing Maintenance

1. Statistical Method (bearing life estimation)

Probabilistic rolling contact fatigue.

Few bearings breakdown earlier than their life.

Unnecessary replacement of few perfectly good

condition bearings.

2. Bearing Condition Monitoring

Detect crack size.

Warns if crack area > 6.45 mm

2

(0.01 in

2

). Needs crack propagation theory (-ies)

I.I.T. Bombay

Monitoring Bearing

Low failure (damage) rate Subjective

supervision Reactive maintenance

Damage hazardous to human life

Unexpected damage leads long production

lossMonitoring cost < Overall maintenance

cost

I.I.T. Bombay


14/17

Criteria to Monitor

Bearing Temperature

o Thermocouple ????

o ? Lubrication breakdown ?

o Predicts only malfunctioning.

o Fatigue damage????

o POOR-INDICATOR????

Vibration & Noise

Wear

I.I.T. Bombay

Table: Variation of annual average bearing temperature, C


15/17

Vibration measurementso Detection of cracks (incipient fatigue damage)

o Damage spots generate structure borne sound

o Recoding by acceleration pick-ups

o 1 Overall soundof machine

I.I.T. Bombay

Bearing Defect Frequencies

=

PD

BDfFTF r 1**5.0

= 2

2

cos1*5.0 xPD

BDxf

BD

PDBSF r

= cos1*

2x

PD

BDf

nBPFO r

+= cos1*

2x

PD

BDf

nBPFI r

r racer and oute inne

theed betweenlative spef

ollersballs or rNumber ofn

gleContact an

meterPitch DiaPD

terller Diameball or RoBD

r Re=

=

=

=

=

I.I.T. Bombay

Natural frequencies = [20kHz, 1MHz]


16/17

I.I.T. Bombay

Measuring Instruments Shock pulse meterMeasure compression waves, or shock pulse

originated by impact.

Micro shocks (high frequency)

Magnitude of shock wave increases whenbearing surfaces are stressed or damaged.

Carpet value, Max value, and Index Number

I.I.T. Bombay


17/17

: High frequency vibration time signal of a bearing

(a) Without fault (b) with a cavity on rolling surface

Measuring instruments..

Measuring instruments..

Enveloping Method

(a) No defect. (b) Wear (c) High friction

I.I.T. Bombay

Documents

Bearing Failures-An Overview