IMPROVING PITCH BEARING RELIABILITY

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

2 Function, Architecture, Challenges 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 Typical 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 Bearing Cross Section 2

3 COMMON FAILURE MODES

4 Ellipse Spill Normal Contact 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 Copyright 2016 RBB LLC 4

5 Pitch Bearing Damage from Ellipse Spill Bearing Damage from Ellipse Spill Copyright 2016 RBB LLC 5

6 True Brinelling True Brinelling Stress Contour (Source: GEARTECH) 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 Copyright 2016 RBB LLC 6

7 False Brinelling Damage from False Brinelling False Brinelling Mechanism (Source: GEARTECH) 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 7

8 Fretting Corrosion Fretting Corrosion Mechanism (Source: GEARTECH) Fretting Corrosion Damage to Pitch Gear (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 8

9 Macropitting Macropits can have several root causes Macropits on Pitch Bearing 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 9

10 Ring Cracks Crack In Bearing Outer Ring 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 10

11 FAILURE DETECTION

12 Grease Analysis Grease Sample Normal Bearing Condition (Source: Monitek, a division of Frontier Pro Services) 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 12

13 FAILURE PREVENTION

14 Design/Procurement Specification Pitch Bearing and Hub 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 Threaded Hole Finite Element Model H-Seal (Source: Kaydon) 14

15 Manufacturing Quality Damage to Single Race High Accuracy Required for Load Share Accuracy of Component Geometry Preload setting Heat treatment Assembly cleanliness Macropitting Near Fill Hole 15

16 Grease Selection Riffel Grease Test Rig (Source: Exxon Mobil) Important grease parameters Base oil viscosity Oil separation Pumpability Low temperature viscosity Mechanical stability Fretting and false brinelling resistance Riffel Test Specimens (Source: Exxon Mobil) Fretting resistance tests Riffel test ASTM D4170 test ASTM D4170 Test Rig 16

17 Autolube Systems Schematic of Autolube System (Source: SKF) Comparison of Manual Grease Fill vs Autolube (Source: SKF) 17

18 FUTURE DESIGN TRENDS

19 Alternative Bearing Architectures Two Row Angular Contact Ball Bearing (Source: Kaydon) Two Row Angular Contact Roller Bearing (Source: Kaydon) Much less sensitive to manufacturing tolerances Much less susceptible to ellipse spill Roller bearings have higher load capacity than ball bearings 19

20 RESOURCES FOR MORE INFORMATION

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 21

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