IMPROVING PITCH
BEARING RELIABILITY
2/9/16
AWEA O&M Seminar
Rob Budny, President RBB Engineering
Function, Architecture, Challenges
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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
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Architecture
• Inner ring, outer ring, two rows of
balls, cage or spacers, seals
• 8 point contact bearing
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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
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COMMON FAILURE MODES
Ellipse Spill
Normal Contact
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Contact with 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
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Pitch Bearing Damage from Ellipse Spill
Bearing Damage from Ellipse Spill
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True Brinelling
True Brinelling Stress Contour
(Source: GEARTECH)
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True Brinell Dent
(Source: GEARTECH)
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
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False Brinelling
Damage from False Brinelling
False Brinelling Mechanism
(Source: GEARTECH)
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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
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Fretting Corrosion
Fretting Corrosion Mechanism
Fretting Corrosion Damage to Pitch Gear
(Source: GEARTECH)
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(Source: GEARTECH)
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
© Copyright 2016 RBB LLC
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Macropitting
Macropits on Pitch Bearing
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Macropits can have several root causes
• Subsurface shear stress causes cracks which eventually reach surface and coalesce
• Presence of nonmetallic inclusions in critical subsurface locations
• Surface defects (PSO or GSC), which serve as crack initiation sites
Each root cause affects pitch bearings, but surface defect most common
Breakdown of the roller path surface results in failure of the bearing
© Copyright 2016 RBB LLC
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Ring Cracks
Crack In Bearing Outer Ring
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Crack Through Threaded Hole in Bearing Ring
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
© Copyright 2016 RBB LLC
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FAILURE DETECTION
Grease Analysis
Grease Sample “Normal Bearing Condition”
(Source: Monitek, a division of Frontier Pro Services)
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Grease Sample “Abnormal Bearing Condition”
(Source: Monitek, a division of Frontier Pro Services)
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
© Copyright 2016 RBB LLC
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FAILURE PREVENTION
Design/Procurement Specification
Pitch Bearing and Hub Finite Element Model
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Threaded Hole Finite Element Model
Require fully flexible analysis,
include effects of blade and hub
stiffness
Require manufacturing
tolerance sensitivity study
Require analysis of stress
concentrations
Limit contact stress
Limit ellipse spill
Seal requirements
Cage requirements
H-Seal
(Source: Kaydon)
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Manufacturing Quality
Damage to Single Race
High Accuracy Required for Load Share
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Accuracy of Component
Geometry
Preload setting
Heat treatment
Assembly cleanliness
Macropitting Near Fill Hole
© Copyright 2016 RBB LLC
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Grease Selection
Riffel Grease Test Rig
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Important grease parameters
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Riffel Test Specimens
(Source: Exxon Mobil)
(Source: Exxon Mobil)
Base oil viscosity
Oil separation
Pumpability
Low temperature viscosity
Mechanical stability
Fretting and false brinelling resistance
Fretting resistance tests
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Riffel test
ASTM D4170 test
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ASTM D4170 Test Rig
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Autolube Systems
Schematic of Autolube System
Comparison of Manual Grease Fill vs Autolube
(Source: SKF)
(Source: SKF)
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FUTURE DESIGN TRENDS
Alternative Bearing Architectures
Two Row Angular Contact Ball Bearing
Two Row Angular Contact Roller Bearing
(Source: Kaydon)
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(Source: Kaydon)
Much less sensitive to manufacturing tolerances
Much less susceptible to ellipse spill
Roller bearings have higher load capacity than ball bearings
© Copyright 2016 RBB LLC
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RESOURCES FOR MORE
INFORMATION
Recommended Resources
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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
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