1. AWEA O&M Seminar
Rob Budny, President RBB Engineering
IMPROVING PITCH
BEARING RELIABILITY
2/9/16

2. © Copyright 2016 RBB LLC
2
Function, Architecture, Challenges
2
• Function
• Allow variable pitch position of
blade
• Transmit blade loads into hub
• Loads include axial, radial,
and moment components
that constantly change
direction and magnitude
• Architecture
• Inner ring, outer ring, two rows of
balls, cage or spacers, seals
• 8 point contact bearing
• Design Challenges
• Long time spent in fixed position
• Very small amplitude oscillating
motion, not rotation
• High loads
• Flexible mounting structures
(blade and hub)
• Exposure to environment
• Cost constraints
Typical 8 Point Contact Bearing
Bearing Cross Section

3. COMMON FAILURE MODES

4. ©Copyright 2016 RBB LLC 4
Ellipse Spill
• Design intent is for contact ellipse to be completely contained in raceway
• Under some load conditions, contact ellipse can spill over the end of the raceway
• Results in very high stresses on the end of the raceway
Contact with Ellipse SpillNormal Contact

5. ©Copyright 2016 RBB LLC 5
Pitch Bearing Damage from EllipseSpill
Bearing Damage from Ellipse Spill

6. ©Copyright 2016 RBB LLC 6
True Brinelling
• Named for James Brinell, inventor of Brinell hardness test
• True Brinelling an overload phenomenon, resulting in dent due to subsurface
yielding
• True Brinell dents have raised shoulders and original machining marks in crater
True Brinell Dent
(Source: GEARTECH)
True Brinelling Stress Contour
(Source: GEARTECH)

7. © Copyright 2016 RBB LLC 7
False Brinelling
• Name from appearance of false brinelling wear scar, looks like indentation from true
brinelling
• Mechanism for false brinelling is very different than true brinelling
• False brinelling a type of adhesive wear, occurs under boundary lubrication
• Generates form of iron oxide known as magnetite
• Creates wear scar without raised shoulders. Machining marks in wear scar worn away
• Wear scar harmful, as is wear debris
• Can progress to macropitting or fretting corrosion
FalseBrinelling Mechanism
(Source: GEARTECH)
Damage from FalseBrinelling

8. © Copyright 2016 RBB LLC 8
Fretting Corrosion
• Name derives from appearance of fretting corrosion wear scar; it looks like rust
• Fretting corrosion is severe adhesive wear, occurs under unlubricated conditions
• Fretting corrosion has very high rate of wear, extremely damaging
• Generates a form of iron oxide known as hematite
• Wear scar can serve as failure initiation point, wear debris is extremely abrasive
Fretting Corrosion Mechanism
(Source: GEARTECH)
Fretting Corrosion Damage to Pitch Gear
(Source: GEARTECH)

9. © Copyright 2016 RBB LLC 9
Macropitting
• Macropits can have several root causes
• Subsurface shearstress causes cracks which eventuallyreachsurface and coalesce
• Presence of nonmetallicinclusionsincritical subsurface locations
• Surface defects (PSO or GSC), which serve as crack initiationsites
• Each root cause affects pitchbearings, but surface defect most common
• Breakdownof the rollerpathsurface resultsin failure of the bearing
Macropits on Pitch Bearing

10. © Copyright 2016 RBB LLC 10
Ring Cracks
• Holes for grease fittings or handling features in bearing rings act as stress risers
• These features can initiate cracks which result in failure of bearing
Crack In Bearing Outer Ring Crack Through Threaded Hole in Bearing Ring

11. FAILURE DETECTION

12. © Copyright 2016 RBB LLC 12
Grease Analysis
• Grease analysis can provide advanced warning of pitch bearing deterioration
• Size, shape, and amount of wear particles can be interpreted to determine severity
and type of wear in bearing
Grease Sample “Normal Bearing Condition”
(Source: Monitek,a division of Frontier ProServices)
Grease Sample “Abnormal Bearing Condition”
(Source: Monitek,a division of Frontier ProServices)

13. FAILURE PREVENTION

14. © Copyright 2016 RBB LLC 14
Design/Procurement Specification
• Require fully flexible analysis,
include effects of blade and hub
stiffness
• Require manufacturing
tolerance sensitivitystudy
• Require analysisof stress
concentrations
• Limit contact stress
• Limit ellipse spill
• Seal requirements
• Cage requirements
H-Seal
(Source: Kaydon)
PitchBearing and Hub Finite Element Model
Threaded Hole Finite Element Model

15. © Copyright 2016 RBB LLC 15
Manufacturing Quality
• Accuracy of Component
Geometry
• Preload setting
• Heat treatment
• Assembly cleanliness
High Accuracy Required for Load Share
Damage to Single Race
Macropitting Near Fill Hole

16. © Copyright 2016 RBB LLC 16
Grease Selection
• Important grease parameters
• Base oil viscosity
• Oil separation
• Pumpability
• Low temperature viscosity
• Mechanical stability
• Fretting and false brinelling resistance
• Frettingresistance tests
• Riffel test
• ASTM D4170 test
RiffelGrease Test Rig
(Source: ExxonMobil) RiffelTest Specimens
(Source: ExxonMobil)
ASTM D4170 Test Rig

17. © Copyright 2016 RBB LLC 17
Autolube Systems
Schematic of Autolube System
(Source: SKF)
Comparison of Manual GreaseFill vs Autolube
(Source: SKF)

18. FUTURE DESIGN TRENDS

19. © Copyright 2016 RBB LLC 19
Alternative Bearing Architectures
Two Row Angular Contact Ball Bearing
(Source: Kaydon)
TwoRow Angular Contact RollerBearing
(Source: Kaydon)
• Much less sensitive to manufacturing tolerances
• Much less susceptible to ellipse spill
• Roller bearings have higher load capacity than ball bearings

20. RESOURCES FOR MORE
INFORMATION

21. © Copyright 2016 RBB LLC 21
Recommended Resources
• RBB Engineering and GEARTECH online resource for gear and bearing failure mode
identification and failure prevention information, GearboxFailure.com
• AWEA Recommended O&M Practices, RP 814 ‘Wind Turbine Pitch Bearing Grease
Sampling and Procedures”
• ASTM D7690, Standard for Grease and Oil Particle Analysis
• NREL Wind Turbine Design Guideline DG03: Yaw and Pitch Rolling Bearing Life