Vibration Institute © SKF Group Slide 0 2011/SKF - Vibration Institute... · Vibration Institute ©SKF Group Slide 4 Purpose of a bearing and friction • To provide low friction

Vibration Institute © SKF Group Slide 0

Vibration Institute Piedmont Chapter Symposium 2011

Presented by Tom McDermott

SKF Sr. Application Engineer

Friday May 13, 2011

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The contents of this publication are the copyright of the publisher and may not be reproduced (even extracts) unless prior written permission is granted. Every care has been taken to ensure the accuracy of the information contained in this publication but no liability can be accepted for any loss or damage whether direct, indirect or consequential arising out of useof the information contained herein.


Discussion topics

• Bearing basics

• Bearing life expectancy

• Bearing failure statistics

• Pre-operational damage mode causes

• Operational damage mode causes

• Identifying loading patterns

• ISO Standard 15243

• Bearing damage analysis

• Securing evidence

• Conducting analysis


SKF bearing basics

• Purpose and functions of a bearing

• Bearing components and materials

• Types of bearing loads

• Rolling elements – ball vs. roller

• Contact angle

• Precision class

• Radial and axial clearance


Purpose of a bearing and friction

• To provide low friction rotation of machine parts.

• To support and locate rotating equipment.

Resistance to motion which occurs when one object slides or rubs against another object.

If not controlled, friction will result in:

• Heat generation • Increased wear

• Increased noise • Loss of power


Roles of a bearing

• Reduce friction

• Transmit loads

• Support the shaft

• Locate the shaft


Bearing components

Outer ring

Cage / retainer

Inner ring

Rolling elements (balls)

Outer ring raceway

Bore surface

Inner ring raceway

OD surface


Functions of the cage

• Minimize friction and heat generation.

• Prevent contact between adjacent rolling elements.

• Guide the rolling elements.

• Provide a surface for the lubricant to adhere to.

• Retain the rolling elements when bearings of a separable design are mounted or dismounted.


Types of bearing loads

Radial load

Axial load

Combined load


Point and line contact


Types of rolling elements

Spherical roller(asymmetrical)

Taper roller

Spherical roller(symmetrical)

Needle roller

Cylindrical roller

Ball


Types of ball bearings

Self-aligningAngular contactDeep groove


Types of roller bearings

Cylindrical SphericalTaper CARB


Load carrying capacity relative to bearing type

Load carrying capacityis expressed as the “basic dynamic load rating”or “C” in catalogs


Contact angle

• The lower the contact angle, the higher the radial load capacity

• The higher the contact angle, the higher the thrust load capacity


Bearings and contact angles

As contact angle increases, radial load capacity decreases; and axial load (i.e. thrust) capacity increases.


Precision classes


Note: Radial clearances are not the same as precision classes

Radial clearance

Axialclearance

Bearing internal clearances

C1 < C2 < CN < C3 < C4 < C5


How does temperature affect internal clearance?

Reducedradialclearance

Expansion

Compression


Bearing life expectancy

Based upon five assumptions :

1. The bearing is defect free.

2. The correct bearing type and size is selected for the application.

3. Dimensions of the bearing mating parts are correct.

4. The bearing will be mounted without damage.

5. Good lubrication in the correct quantity will always be available to the bearing.


Circle of bearing life


Why bearings fail

Four predominant causes of premature bearing failure

• 90% of bearings outlive their machinery

• 9.5% of bearings will be removed for preventative reasons

• 0.5% of bearings fail in their application (and this is generally preventable)

16% Poor Installation

36% Poor Lubrication

14% Contamination

34% Fatigue


Pre-operational damage mode causes

• Damage during transportation, handling and storage.

• Incorrect shaft and housing fits.

• Defective bearing seats on shafts and in housings.

• Faulty mounting practices.

• Static misalignment.

• Passage of electric current through the bearing.


Operational damage mode causes

• Static vibration

• Operational misalignment

• Ineffective sealing

• Ineffective or inadequate lubrication

• Passage of electric current through the bearing

• Excessive loading


Identifying loading patterns: inner ring rotation

LoadZone

LoadZone

Load


Identifying loading patterns: outer ring rotation

LoadZone

LoadZone

Load


Load zone when thrust loads are excessive


Thrust load + radial load = combined loads


Internal preload & out of round housing


Out of round housing visible in outer ring


Affects of misalignment in a bearing


Misalignment in a ball and roller bearing


Bearing damage

analysis


Classifications: ISO system

• The ISO classification system is divided in six main areas and then further divided into sub-areas.

• Going through the table, 15 categories in total can be observed in which the damage can be classified.

• These categories will be covered, one by one, indicating the features. A number of typical examples are shown.

• There are some other reasons for bearing damage, such as design problems, etc. These are not classified in the ISO standard.


Bearing damage classifications: ISO 15243


Fatigue: subsurface fatigue

• Repeated stress changes

• Material structural changes

• Micro-cracks under the surface

• Crack propagation

• Flaking, spalling and peeling

1. Fatigue1.1. Subsurface fatigue

1.2. Surface initiated fatigue


Fatigue: subsurface fatigue




Fatigue: surface initiated fatigue

• Surface distress

• Reduced lubrication regime

• Sliding motion

• Burnishing, glazing

• Asperity micro-cracks

• Asperity micro-spalls





Hair strand

(cross section)

50 microns

Dirt particle

1 micron

oil film = 0.2 micron

10 microns










Hydrogen Embrittlement


Wear: abrasive wear

• Progressive removal of material

• Ingress of dirt particles

• Accelerating process

• Dull surfaces

2. Wear2.1. Abrasive wear

2.2. Adhesive wear


Wear: abrasive wear


2.2. Adhesive wear


Wear: abrasive wear


Wear: adhesive wear

• Low loads

• Accelerations

• Smearing / skidding / galling

• Material transfer / friction heat

• Tempering / re-hardening

• With stress concentrations and cracking or flaking


2.2. Adhesive wear


Wear: adhesive wear


2.2. Adhesive wear


Wear: adhesive wear


Corrosion: moisture corrosion

3. Corrosion3.2.1. Fretting corrosion

3.1. Moisture corrosion

3.2. Frictional corrosion

3.2.2. False brinelling

• Oxidation / rust

• Chemical reaction

• Corrosion pits / flaking

• Etching (water/oil mixture)
















Corrosion: frictional corrosion: fretting





• Micro-movement between mating surfaces

• Oxidation of asperities

• Powdery rust / loss of material

• Occurs in fit interfaces transmitting loads









5/13/2011 © SKF Group Slide 55SKF Field Training Series


Corrosion: frictional corrosion: false brinelling





• Rolling element / raceway

• Micro movements / elastic deformations

• Vibrations

• Corrosion / wear / shiny /

red depressions

• Stationary: rolling element pitch

• Rotating: parallel flutes










Electrical erosion: excessive voltage

• High current / sparking

• Localized heating in very short Interval / melting / welding

• Craters up to 100 µm

4. Electricalerosion

4.1. Excessive voltage

4.2. Current leakage









Electrical erosion: current leakage

• Low current intensity

• Shallow craters closely positioned

• Development of flutes on raceways & rollers, parallel to rolling axis

• Dark gray discoloration












Plastic deformation: overload

• Static or shock loads

• Plastic deformations

• Depressions in rolling element distance

• Handling damages

5. Plasticdeformation

5.1. Overload

5.2. Indentation from debris

5.3. Indentation by handling




5.1. Overload






Plastic deformation: indentation from debris

• Localized overloading

• Over-rolling of particles ð dents

• Soft / hardened steel / hard mineral


5.1. Overload






5.1. Overload






Plastic deformation: indentation from handling


5.1. Overload




Indentation from handling: localized overloading


Fracture: forced fracture

• Stress concentration > tensile strength

• Impact / overstressing

6. Fracture

6.1. Forced fracture

6.2. Fatigue fracture

6.3. Thermal cracking



6. Fracture







Fracture: fatigue fracture

• Rings and cages - Crack initiation / propagation

• Exceeding fatigue strength under bending

• Finally forced fracture

6. Fracture





Fracture: fatigue fracture


Fracture: thermal cracking

• High sliding and / or insufficient lubrication

• High friction heat

• Cracks at right angle to sliding direction

6. Fracture





Fracture: thermal cracking

6. Fracture





Classifications: securing evidence

• Collect operating data, monitoring data

• Collect lubricant samples

• Check bearing environment

• Assess bearing in mounted condition

• Mark mounting position

• Remove, mark and bag bearing and parts

• Check bearing seats

• Lubricant condition (color, presence of water, viscosity, consistency, distribution in the bearing, etc.)


Classifications: conducting the analysis

• Examine bearing and parts

• Record visual observations

• Record pictures of bearing and pertinent parts

• Use the failure modes to eliminate improbable causes and determine the original cause of the failure

• Use external resources such as SKF Bearing Inspector at @ptitudeXchange.com or SKF Bearing Installation and Maintenance Guide #140-710

• Contact external resources for assistance, if needed

• Initiate corrective action, if desired.

• Consider SKF analysis services ($)


Available training courses

• WE201: Bearing Maintenance and Technology

• WE202: Bearing in Rotating Machinery Applications

• WE203: Lubrication in Rolling Element Bearings

•WE204: Root Cause Bearing Damage Analysis


Thank you!


Documents

Vibration Institute © SKF Group Slide 0 2011/SKF - Vibration Institute... · Vibration Institute ©SKF Group Slide 4 Purpose of a bearing and friction • To provide low friction