How to reduce WEC in Wind turbines - Midpoint BearingHow to reduce WEC in wind turbines Field track...

White Etching Cracks (WEC)How to reduce WEC in Wind turbines

Summary: WEC and cause for WEC

What is White Etching Cracks (WEC)?

WEC are material changes that can occur in rolling bearingsteels and have significantly higher hardness than the originalstructure. Under mechanical load, cracks may develop betweenthe white, hard areas and the matrix that, under strain,propagate to the surface and lead to failure. These cracks occurin both through hardened and case hardened rolling bearings.

How do WEC occur?

Schaeffler has comprehensive testing facilities for assessing theinfluences and developing solution concepts for reducing the riskof WEC.

According to our current level of knowledge, WEC occur only asa result of unspecified additional loads (such as electricity,dynamics, mixed friction) that supplement the usual bearingloads and not to a root cause in the bearing itself or in thebearing design.

Influences, which could have an effect on component fatigue,such as contact pressure, an insufficient level of steel purity oroverloading, do not cause the occurrence of WEC.

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How to reduce WEC in wind turbines

Summary: Schaeffler solutions

Through hardened bearings with Durotect® B

Schaeffler recommends Durotect® B (black oxide coating)

Ø Good protection against WEC

Ø Longest and best field experience: failure rate 0,02%482.000 Bearings, < 100 Failures in the past 9 years

Ø Economical solution

Ø Durotect® B also minimizes the risk of damage caused byslippage, improves running-in behavior and providesprotection against corrosion

The WEC risk can be limited by the design of the bearingand the selection of lubricants or materials.

Lubrication advise

Ø Oil changes: keep the oil history clean and document the oilchanges

Ø Avoid water in lubricant as far as possible.

Further Schaeffler solutions

Special carbonitriding steel Mancrodur® with Durotect® BHigh chromium special steel Cronidur® 305



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White BandsRelated to classic fatigue

White etching layer WEL ButterflyWhite flanks around non metallic inclusions

White Etching Cracks (WEC)Crack networks including white etching flanksA type of White Etching Area WEAà Nital etching: phase that do not corrode, appear white

Related designations:White structure flakingBrittle crack networkAxial Cracks (referring to damage appearance atmartensitic steels)

Other White Etching Areas (WEA)


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Affected Applications

WEC occurs in many sectors of Industry includingAutomotive

WEC occurs in all bearing components, except cages

Øindependent from line or point contact

Øindependent from bearing type

Øindependent from material (except Cronidur)

Øindependent from heat treatment

WEC failures are not unique to wind turbines. There aremany other applications that have experienced problemsrelated to WEC.

Martensitic

Bainitic

Case hardened


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Affected Applications in wind turbines

Rotor shaftSpherical roller bearingsTapered Roller bearings

GearboxCylindrical roller bearingsTapered Roller bearingsin Planet Gears

Intermediate speed shaftHigh speed shaft

How do White Etching Cracks occur?How to reduce WEC in wind turbines

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Electricity Mixed Friction Dynamics

Reduceelectrostaticsand current

flows

Reducemixed friction

as far aspossible

Reducevibrations

Reducetractionslippage

Preventunfavourable

bearingkinematics

WEC

A d d i t i o n a l L o a d s

P r e v e n t i v e M e a s u r e s

lead to

reduce the risk of

Lubricationin combinationwith auxiliary

loadsinfluences

How do White Etching Cracks occur?How to reduce WEC in wind turbines

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Preventive MeasuresThe risk of WEC can be reduced if variouspoints are observed in design of the bearingarrangement and the selection of lubricantsand materials.

Reduceelectrostatics andcurrent flows

Optimise groundingconcept (couplingdesign, insulatingcoatings, ...),quality/concept of drive(windings) andcontrol/inverter

Identify electrostaticsources: belt drives,rotor blades, paper rollcontact, flowing liquids.Take appropriatemeasures

Reduce mixedfriction as far aspossible

Pay attention tolubrication conditions

Prevent mixed friction(κ < 1)

Ensure low roughnessof element at risk offailure (normally innerring)

Avoid excessively highviscosities (κ > 10) athigh n · dM values

Ensure sufficient oilrelease from greases

Reduce vibrations

Avoid high axial androtationalaccelerations

Reduce highfrequency vibrations(> 100 Hz)

Reduce tractionslippage

Achieve preload/lowoperating clearance asfar as possible

Specific load (Fr/C0)not too low ordifference fromminimum load not toolarge

Keep splash losseslow (avoid high oilflows, large outletholes)

Use design withoptimised tractionslippage

Preventunfavourablebearing kinematics

Select optimum bearingtype for frictionkinematics: low pvvalue

Keep differential andspin slippage low

Pay attention toskewing(tapered roller bearings,cylindrical rollerbearings)

reduce the risk of

Recommended Schaeffler Solution: Through hardened + Durotect® B

Through hardened bearings with Durotect® BSchaeffler recommends Durotect® B (black oxide coating)

Ø Good protection against WEC

Ø Longest and best field experience: failure rate 0,02%482.000 Bearings, < 100 Failures in the past 9 years

Ø Economical solution

Ø Durotect® B also minimizes the risk of damage caused by slippage, improves running-inbehavior and provides protection against corrosion

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What is Durotect ® B?The coating system Durotect® B has been consistentlydeveloped from conventional black oxide finishes to giveincreased performance.

AdvantagesDurotect® B…Ø minimizes the risk of damage caused by slippageØ improves running-in behaviorØ provides protection against corrosionØ increases the level of robustness to WEC independent

from base materialSchaeffler therefore always recommends its customersto use gearbox bearing supports in Durotect® B.

FeaturesØ Coating comprising mixed iron oxidesØ Colour: dark brown to deep black

Schaeffler's Surface Technology Competence Center hasdeveloped an ideal coating system to reduce/eliminateWEC damage.


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Field track recordIt has been proven statistically that black oxide finishing of outer and inner rings as well asrollers significantly reduces the formation of white etching cracks.

Longest and best field experience: failure rate 0,02%482.000 Bearings, < 100 Failures in the past 9 years

13,448

24,092

35,818

43,60550,978

66,75570,530

54,115

89,520

33,288

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

Ist 2005 Ist 2006 Ist 2007 Ist 2008 Ist 2009 Ist 2010 Ist 2011 Ist 2012 Ist 2013 Ist kum.2014

Piec

es


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Field track recordLongest and best field experience: failure rate 0,02%482.000 Bearings, < 100 Failures in the past 9 years

Statistics in relation to 482.000 bearings (delivered 2005 – 2014)Bearing locationin wind gearboxes

Planet bearings 45 %High speed shaft bearings 35 %Intermediate speed shaft bearings 20 %

Heat treatment(all bearings are through hardened)

Martensitic 55 %Bainitic 45 %

Bearing type Cylindrical roller bearings 80 %Tapered roller bearings 20 %

Lubricants(In general the 482.000 bearings areoperated in all common lubricants used inwind gearboxes. This is our experience interms of prevalence of wind oils.)

Mobil SHC XMP 320 70 %Castrol Synthetic X 320 20 %Castrol Synthetic A 320 5 %Texaco Meropa 320 5 %

Failure free running time maximum time: 9 years, average time: 4 years

How to reduce WEC in Wind turbines - Midpoint BearingHow to reduce WEC in wind turbines Field track...

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