1

Wind Turbine Gearbox Failures

Robert Errichello GEARTECH

2

WIND TURBINE TYPICAL GEAR FAILURES

BENDING FATIGUE

HERTZIAN FATIGUE

WEAR

SCUFFING

CRACKING

3

BENDING FATIGUE OCCURS IN 3 STAGES

Load

Crack Initiation Ratchet Marks

Crack Propagation Beach Marks

Fracture Ductile / Brittle

4 Bending Fatigue/High-Cycle/Root Fillet Cracks

Classic bending fatigue with crack initiation at critical point of maximum bending stress. Curved trajectory of crack growth is typical as shown by adjacent fracture. Failure was caused by incomplete flame hardening.

Copyright 2000 by GEARTECH, Townsend, MT 59644 All rights are reserved.

5 Bending Fatigue/High-Cycle/Tooth End Cracks

Bending fatigue cracks initiated in root fillets at left ends of teeth due to incomplete hardening on tooth ends.

Copyright 2000 by GEARTECH, Townsend, MT 59644 All rights are reserved.

Scuffing in slide direction

Crack origins

Beach marks

6

FATIGUE SURFACE MORPHOLOGY RATCHET MARK

ORIGIN

BEACH MARK FRETTING CORROSION

ORIGIN

FRACTURE

7

DEPTH BELOW SURFACE

TENSILE STRESS NEAR CASE/CORE BOUNDARY

CASE/CORE BOUNDARY IS CRITICAL

8

Case History of Bending Fatigue Failure of INT Pinion

Courtesy of Rainer Eckert, NW Labs

9

A

B C

D D

E

F

Bending Fatigue on INT Pinion

B

MnS inclusions

10

A

B C

D D

E

F

Inclusion Origin on Tooth Fragment

B

MnS inclusions

11

A

B C

D D

E

F

Close-up of Origin

B

MnS inclusions

12

A

B C

D D

E

F

SEM Close-up of Inclusion 100X

B

MnS inclusions

13

A

B C

D D

E

F

SEM Close-up of Inclusion 550X

B

MnS inclusions

14

A

B C

D D

E

F

SEM Close-up of Inclusion 35X

B

MnS inclusions

15

A

B C

D D

E

F

SEM/BSE Close-up of Inclusion 50X

B

MnS inclusions

16

A

B C

D D

E

F

SEM/BSE Close-up of Inclusion 100X

B

MnS inclusions

17

A

B C

D D

E

F

EDS Analysis (fingerprint) of Inclusion (SEM microscope)

B

MnS inclusions

18

A

B C

D D

E

F

EDS Analysis (fingerprint) of Matrix (SEM microscope)

B

MnS inclusions

19

A

B C

D D

E

F

Conclusions

B

MnS inclusions

1. Bending fatigue is primary failure mode 2. No surface defects were found at tooth surface 3. No material defects due to heat treatment were found 4. Fatigue crack initiated subsurface 5. Alumina inclusion was found at the crack origin 6. Root cause of bending fatigue is inadequate material cleanliness

20

HOW TO PREVENT BENDING FATIGUE

USE STEEL THAT IS: CLEAN, HARDENABLE CARBURIZED SHOT PEENED FINE GRAINED REDUCE: BAINITE, PEARLITE NETWORK CARBIDES MICROCRACKS FLAWS BENDING STRESS

21

HERTZIAN FATIGUE

22

HERTZIAN FATIGUE - 3 GENERAL MODES

MACROPITTING

MICROPITTING

SUBCASE FATIGUE

23 Hertzian Fatigue/Macropitting

Macropitting on a wind turbine pinion caused by misalignment due to excessive bearing clearance. Micropitting surrounding macropits is secondary failure. Note similar damage on each tooth including wear step at end of contact.

Copyright 2000 by GEARTECH, Townsend, MT 59644 All rights are reserved.

Macropitting

Micropitting

24

HOW TO PREVENT MACROPITTING

Reduce contact stress

Use clean steel

Use carburized steel

Grind / hone gear teeth

Keep lube cool, clean, dry

Use high viscosity lube

25

MICROPITTING

26 Hertzian Fatigue/Micropitting

Micropitted gear teeth appear dull, etched, or stained with patches of gray. It attacks high points on gear tooth surfaces such as crests of undulations, peaks of cutter scallops, and ridges of grinding lay.

Copyright 2000 by GEARTECH, Townsend, MT 59644. All rights are reserved.

27

HOW TO PREVENT MICROPITTING

Grind / hone / polish gear teeth Avoid shot-peened flanks Make hardest gear as smooth as possible Make pinion 2 HRC points harder than gear Use oil with high micropitting resistance Keep lube cool, clean, dry Use high viscosity lube Use high speeds Coat teeth with phosphate, Cu, or Ag Run-in with special lube and controlled loads

28

SUBCASE FATIGUE

29 Hertzian Fatigue/Subcase Fatigue

Subcase fatigue may be caused by inadequate case depth, inclusions near case/core boundary, or grind temper. In the helical gear shown, it was caused by grind temper. Fine, longitudinal cracks and sharp edged craters are typical.

Copyright 2000 by GEARTECH, Townsend, MT 59644. All rights are reserved.

30

HOW TO PREVENT SUBCASE FATIGUE

Reduce contact stress

Use hardenable steel

Use optimum case / core properties

Avoid overheating & grind temper

Keep subsurface stress < subsurface strength

31

WEAR

32

33 Wear/Adhesion

Adhesion is due to microwelding and tearing of asperities. Typically, mild adhesion occurs during running-in and subsides after it removes local asperities. The photo shows some local scuffs (severe adhesion).

Copyright 2000 by GEARTECH, Townsend, MT 59644. All rights are reserved.

Scuffing

34

HOW TO PREVENT ADHESIVE WEAR

Use smooth tooth surfaces

Run-in new gearsets at reduced loads

Keep lube cool, clean, dry

Use high speeds

If low speed, use nitrided gears & high-viscosity lube

If very low speed, avoid S-P additives

35

FRETTING CORROSION

36 Wear/Fretting Corrosion

Fretting corrosion on teeth of a wind turbine pinion. It occurred because the turbine was parked for three months and the drive train vibrated when wind buffeted the rotor.

Copyright 2000 by GEARTECH, Townsend, MT 59644. All rights are reserved.

37

SCUFFING

38 Scuffing (severe adhesion)

Scuffing on teeth of a wind turbine pinion. It occurred because the oil supply was interrupted. Copyright 2000 by GEARTECH, Townsend, MT 59644. All rights are reserved.

39

SPECIFIC FILM THICKNESS DEFINES LUBE REGIME

hmin 1

2 = (12 + 22)1/2 = hmin /

Lube Regime< 1 Boundary

1 2 Mixed Film> 2 Full EHD

40

GEAR LIFE DEPENDS ON

Gear Life

Micropitting Adhesion Scuffing

Macropitting

41

HOW TO PREVENT SCUFFING

OPTIMIZE GEAR GEOMETRY / ACCURACY

USE NITRIDED STEEL

GRIND / HONE GEAR TEETH

COAT GEAR TEETH w/ PHOSPHATE, Cu, Ag

USE HIGH-VISC, ANTISCUFF LUBE

COOL GEAR TEETH WITH LUBE

RUN-IN NEW GEARSETS AT REDUCED LOADS

42

GRINDING CRACKS

43 Cracking/Grinding Cracks

Grinding cracks occur on ground flanks usually perpendicular to the grind direction. They are relatively shallow and parallel or in a crazed pattern. Nital etch may show grind temper as in the above photo.

Copyright 2000 by GEARTECH, Townsend, MT 59644. All rights are reserved.

44

HOW TO PREVENT GRINDING CRACKS

Use proper wheel, feed, speed, & coolant

Keep retained austenite < 20%

Keep case free of carbide networks

Keep surface hardness < 60 HRC

Inspect for temper w/ acid etch

Inspect for cracks w/ magnetic particle

45

CASE / CORE SEPARATION

46 Cracking/Case-Core Separation

Case-Core separation is brittle fracture that usually occurs because case depth is too deep. Chevron marks on the fracture surface point to the crack origin.

Copyright 2000 by GEARTECH, Townsend, MT 59644. All rights are reserved.

Chevron marks

47

HOW TO PREVENT CASE / CORE SEPARATION

Control case depth at tooth tips

Avoid narrow toplands

Mask toplands to restrict carbon penetration

Chamfer all edges of teeth

Use clean steel with high fracture toughness

Do not refrigerate

Temper immediately after quenching

Do not shot peen flanks

Wind Turbine Gearbox FailuresWIND TURBINE TYPICAL GEAR FAILURESBENDING FATIGUE OCCURS IN 3 STAGESBending Fatigue/High-Cycle/Root Fillet CracksBending Fatigue/High-Cycle/Tooth End CracksFATIGUE SURFACE MORPHOLOGYCASE/CORE BOUNDARY IS CRITICALCase History of Bending Fatigue Failure of INT PinionBending Fatigue on INT PinionInclusion Origin on Tooth FragmentClose-up of OriginSEM Close-up of Inclusion 100XSEM Close-up of Inclusion 550XSEM Close-up of Inclusion 35XSEM/BSE Close-up of Inclusion 50XSEM/BSE Close-up of Inclusion 100XEDS Analysis (fingerprint) of Inclusion (SEM microscope)EDS Analysis (fingerprint) of Matrix (SEM microscope)Slide Number 19HOW TO PREVENT BENDING FATIGUEHERTZIAN FATIGUEHERTZIAN FATIGUE - 3 GENERAL MODESHertzian Fatigue/MacropittingHOW TO PREVENT MACROPITTINGMICROPITTINGHertzian Fatigue/MicropittingHOW TO PREVENT MICROPITTINGSUBCASE FATIGUEHertzian Fatigue/Subcase FatigueHOW TO PREVENT SUBCASE FATIGUEWEARSlide Number 32Wear/AdhesionHOW TO PREVENT ADHESIVE WEARFRETTING CORROSIONWear/Fretting CorrosionSCUFFINGScuffing (severe adhesion)SPECIFIC FILM THICKNESS DEFINES LUBE REGIMEGEAR LIFE DEPENDS ON HOW TO PREVENT SCUFFINGGRINDING CRACKSCracking/Grinding CracksHOW TO PREVENT GRINDING CRACKSCASE / CORE SEPARATIONCracking/Case-Core SeparationHOW TO PREVENT CASE / CORE SEPARATION