How to Avoid Trouble • Knowledge of Load, Fits, Mounting conditions •
Lubrication
• surrounding vibration • • Water • Does one need such detailed observation???
Failure by Abrasive Particles
outer race of a spherical roller bearing
Scratching
Indentation
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Failure due to Inadequate Lubrication If the lubricant film between raceways and rolling elements becomes too thin, the peaks of the surface asperities will momentarily come into contact with each other. Polishing of two surfaces (as in lapping) occurs in initial phase. Small cracks then form in the surfaces and this is known as SURFACE DISTRESS. Consequently material is heated to such temperatures that rehardening takes place.
outer ring of a spherical roller bearing-Mirror finish
This produces localized stress concentrations that may cause cracking or flaking or transfer of material (commonly know as SMEARING) from one surface to the other.
100 x m
agnific ation
Surface distress in the form of a band encircling a roller.
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Smearing on the surface of a roller from a spherical roller bearing - 100 x magnification.
A cylindrical roller with end smearing
Smeared face of a cylindrical roller bearing inner ring.
Smeared outside surface of a spherical roller bearing outer ring
Failure by Vibration inner and outer ring of a cylindrical roller bearing
outer ring of a self aligning ball bearing damaged by vibration
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I.I.T. Bombay
Rust and Corrosion Condition: Rusting at spacing equal to the distances between the rolling elements Causes: Ingress of water or corrosive material (such as acid) Condensation of moisture contained in the air.
Deep seated rust in the outer ring of a deep groove ball bearing
Poor packaging and storing conditions, and handling with bare hands.
Solutions
Improvement in sealing effect Measures for preventing rusting when not operating for a long period of time.
I.I.T. Bombay
Fretting and Fretting Corrosion Meaning:
!
Condition: $
! ! %
Causes: &
!
! Poor lubrication (no lubrication) Fluctuating load Vibration during transportation
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I.I.T. Bombay
Fretting and Fretting Corrosion Condition: ' ( Cause: Relative movement between bearing & housing. Insufficient radial interference. (dirt?).
Fretting on Steel Backing
Solutions: Review bearing interference and back contact conditions between bearing back & housing )
I.I.T. Bombay
Electrical Pitting Condition: *
+ !
! , Causes: ! Solutions: )
! !
"
$ -
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Flaking
I.I.T. Bombay
Meaning= “Peeling off” Results: *
.
Flaking…….
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
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Flaked inner ring of a spherical roller bearing. Very high axial load compared to the radial load.
Flaking in the outer ring of spherical roller bearing that has been mounted in a housing with oval bore
I.I.T. Bombay
Cracking Meaning= breaking, splitting, or snapping apart Results: " Causes:
Solutions Improve dissipation of heat from bearing Prevention of misalignment improvement in clearance Selection of suitable lubricant and determination of optimum lubricant feeding rate
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 mm2 (0.01 in2). – 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 loss Monitoring cost < Overall maintenance cost
13
I.I.T. Bombay
Criteria to Monitor Bearing Temperature o Thermocouple ???? o ? Lubrication breakdown ? o Predicts only malfunctioning. o Fatigue damage???? o POOR-INDICATOR????
Vibration & Noise Wear
Table: Variation of annual average bearing temperature, °C
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I.I.T. Bombay
Vibration measurements o o o o o
Detection of cracks (incipient fatigue damage) Damage spots generate structure borne sound Recoding by acceleration pick-ups 1 < fdefect < 100 kHz Relative measurement. Trend analysis. Comparison with identical bearing locations o Early diagnosis of localized damage Vibration pick-ups must be firmly attached. Measuring locations in load zone Sound expected by damaged bearing > Overall sound of machine
Bearing Defect Frequencies FTF = 0.5 * f r * 1 − BSF = 0.5 *
BPFO =
BPFI =
BD PD
PD BD xf r 1 − BD PD
I.I.T. Bombay
Natural frequencies = [20kHz, 1MHz] 2
x cos 2 β
n BD * fr 1− x cos β 2 PD n BD * fr 1 + x cos β 2 PD
BD = ball or Roller Diameter PD = Pitch Diameter = Contact angle n = Number of balls or rollers f r = Re lative speed between the inner and outer race
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I.I.T. Bombay
Measuring Instruments
I.I.T. Bombay
Shock pulse meter Measure compression waves, or shock pulse originated by impact. Micro shocks (high frequency) Magnitude of shock wave increases when bearing surfaces are stressed or damaged. Carpet value, Max value, and Index Number
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Measuring instruments……..
: High frequency vibration time signal of a bearing (a) Without fault (b) with a cavity on rolling surface
