1. Challenges of Large MMW Gearboxes in Wind Turbine Applications
Michael Sirak, PE, GE Transportation
Tony Giammarise, GE Transportation
Cornell University
12-13 June 2009
GE Transportation
Industry Trends
DriveTrain Description
Challenges Moving Forward
Design
Manufacturing
Validation

2. GE … a tradition of innovation
Founded in 1878 as the Edison Electric Co.
130 years of successful operation
320,000 employees worldwide
Strong technology focus … Research centers in Germany, India, China and the U.S.

7. Land Based Technology Status
Today’s Land Base Utility Class Turbines
1.5 – 3.0 MW Upwind Configuration
80 – 100 Meter Tower Height
Conventional 3 stage gearbox
Distributed component drivetrain
Full Span Pitch Control
Tapered Cylinder Steel Towers
~65 kWhr/kg tower top mass
200 MW + Windfarms
Clipper
Performance
98% Availability
40+% Capacity Factor at IEC-II 8.5 m/s
CF% +10 pts in last 5 yrs

8. Offshore Technology Status
Development and demo stage
19 Projects, 900 MW Installed, shallow water
3 – 4 MW Upwind Configuration
80 m towers
Monopile & gravity foundations < 15m
Steel Tapered Towers
Infrastructure not yet available in US
Performance
Average 45+% Capacity Factor
Availability & Cost Not Well Established
11 c/kwhr UK Thames Estuary site
GE 3.6 MW – Arklow Banks

9. US 20% Wind Outlook Wind Resource Technology Issues Grid Integration
US has outstanding wind resource
Broadly distributed thru mid-west & west
Offshore in NE
US Has the Wind Resource
Technology Issues
Need for technology improvements to compete with cheap coal
Offshore wind not mature
Turbines 5-10 MW req’d
20% Wind Distributes per Demand & Resource
Grid Integration
Integration of wind forecasting tools
Consolidation of load balance zones
System Balkanization a barrier
Transmission
Corridors an Essential Part of the Solution

10. WinDS Analysis to Achieve 20%
Analysis Summary
Existing Technology Achieves 6-8% Penetration by 2030
20% Requires Technology Improvement
Improvements In Cost, Capacity Factor, and O&M needed
One cent reduction in COE billion dollar economic savings per year
Land Technology Improvements
Turbine size increasing from 1.5 to 5 MW
20% Reduction In Capital Cost per kW
15% Increase In Capacity Factor
30% Reduction in O&M Costs
GW Scale Windfarms
Offshore Technology Improvements
Turbine size increasing from 3 to 10 MW
20% Reduction In Capital Cost
15% Increase in Capacity Factor
50% Reduction in O&M Costs

11. 7GA87 Gearbox

12. 7GA87E – 1.5MW 1:78 ratio Compound Planetary
3 stage CP design with helical gearing
Straddle mounted planet bearings
Flexible, splined ring gear
No reversed bending loads on planet teeth
One high speed stage
Over 1300 produced
Oldest fleet at 36 months

13. Current GET Gearboxes Differential Planetary + Parallel Shaft
2.0 – – 18 RPM
1400 – 1900 KNm Input Torque
60:1 – 97:1 Ratio Possibilities
Approximate Weight: 39,000 Lbs
Horizontal and vertical output shaft 550mm CD
2350 / 2150mm Overall Length
2 Simple Planetaries + Parallel Shaft
2.3 – – 16 RPM Input Speed
1500 – 1920 KNm Input Torque
78:1 – 136:1
Approximate Weight: 46,500 Lbs
Horizontal Output 550mm CD
2550mm Overall Length
Compound Planetary + Parallel Shaft
1.4 – – 20 RPM
700 – 970 KNm Input Torque
59:1 – 86:1 Ratio Possibilities
Approx Weight: 33,000 Lbs
Horizontal Output 550mm CD
.Working on Vertical Output Shaft for 2009.
2200mm Overall Length

14. Common Gearbox Components 2 Simple Planetary + Parallel Shaft Arrangement
Torque Arms – Transfers torque and wind load to turbine structure
Low Speed Annulus Gear – Non-Rotating, fixed to Input Housing.
High Speed Pinion Bearings– supports gear and braking loads.
High Speed Annulus Gear – Non-Rotating, fixed to Input Housing.
Input Housing – Supports gearbox torque and blade loads.
High Speed Planet Gears – Typically 3.
High Speed Pinion – driven by high speed gear. Drives the generator.
Input Carrier – Connects Gearbox to Blades
Low Speed Planet Gears – Typically 3, 4, or 5 gears. Meshes with Annulus and Sun Gear.
High Speed Gear – fixed to sun gear, drives high speed pinion.
Pitch Tube – Conduit for electric or hydraulic supply to blade control system – rotates at rotor speed.
Low Speed Sun Gear – Output of first speed increasing Stage. Inputs power to High Speed Planetary Stage
High Speed Gear Bearings – Supports Sun Gear & High Speed Gar loads.
High Speed Sun Gear – Output of second speed increasing Stage. Inputs power to High Speed Parallel Shaft Stage
Input Carrier Bearings – Supports blade loads
High Speed Carrier Bearings – Supports Gear loads

15. Constraints – Design Perspective
Design Complexity & challenges
Dynamic Loads – Extreme coherent gust winds, Turbulent shear winds
Low “Lamda” – Design challenge to maintain Lamda ratios >1
High Torque Density
Loads prediction & understanding
Torque Arm Design – Currently used for a MW turbine Gearbox, but as the MW rating increases, this design becomes complex
Adequate Cooling & heating – Challenge due to extreme environmental conditions
Noise control – Huge factor due to noise regulations
Vibrations
Seal designs – Current Gearbox industry is challenged is oil leaks either on input side / output side
Reliability – Design life 20 years
Weight – Shipping and uptower

16. Growing wind turbine scale necessitates development of new drive train technology
3.6 MW HS Pinion Bearing
Roller Speed: 92 ft/s
Rotation
1,500 RPM
Film Thickness (mm)
Problem
Conventional 3-stage gearbox-based drive trains are sub-optimal as wind turbine rated power grows from 1 MW to 5 MW
Cause is increasing size of problematic HS Pinion Bearings which require decreasing oil viscosities
Forces a dangerous compromise … select low viscosity oil to support higher speed bearings OR higher viscosity oil to protect the low speed gear stages
Solution
Develop technology that eliminates the need for the 3rd (high speed) stage of the gearbox
Above Line:
Film Thickness > Gear Surface Finish
1.5 MW HS Pinion Bearing
Roller Speed: 65 ft/s
Impact of change from ISO VG 320 to ISO VG 460 oil on Low Speed Planetary gear film thickness
Rotation
1,500 RPM

17. Relative Sizes & Conversion of mils & microns
~ Diameter of human hair, thickness of copy paper
~ 0.1mm = 100 microns = .004 inch
1 mil = .001 inch = 25.4 microns
1 micron = 1 m = 1e-6 meters = .001 mm
Film thickness on low speed wind bearings ~ 0.15 microns!

18. Gear Design Process

19. Constraints – Manufacturing Perspective
IH process
Size of Gearing
Size of gears used causes a challenge in manufacturing a quality gear, current industry supported by few suppliers in market
Heat Treatment Process
Several heat treatment process available but process is generally difficult to monitor and maintain long term process capability
Steel Quality
Casting & Machining
Cleanliness
Plays a major role in the overall reliability of the gear box from performance & life stand point
Grinding
Process Quality
Very critical to this business as cost of producing a non-quality part accumulates into a higher maintenance costs
Gear Mfg defect- not acceptable

21. Constraints – Validation Perspective
Comprehensive commercial testing
Component testing
Sub-system testing
HALT - Every new production or significant change
Field testing - ~ typically 6 months field test
Cost – testing costs are expensive but critical
Larger gearboxes = larger test stands

22. High Speed Gear Bearing
Component testing … demonstrated 99% reliability at 95% confidence
Input Housing
Carrier
High Speed Gear Bearing

23. Systems level testing Cold Chamber Lube System Flow Rate
Strain Gauge Testing

25. Validates deflection calculations in structures, shafts and bearings
Gear Mesh Pattern
All gear contact patterns were good and passed typical acceptance criteria.
Gear contact patterns & condition are best indicators of system validation.
Validates deflection calculations in structures, shafts and bearings
Validates adequate lube flow

26. Wind farms in Ocean- greater challenge for maintenance
Constraints – Maintenance Perspective
Large Cranes Used for installation, repair & accessibility
Filtration – Lube oil used needs regular filtration & maintenance
Contamination of Oil w/metal particles
Load Management
Monitor noise
Periodic oil sampling
Shipping & Logistics – Weight & shape of the Gearboxes causing a challenge for shipping & logistics
Condition based monitoring system – Proactively monitors, detects impending drive train issues, enabling increased availability & decreased maintenance costs
Uptower Inspection Technology – Barkhausen Noise, Eddy Current
Can only climb two or three towers per day!
Wind farms in Ocean- greater challenge for maintenance

27. Barkhausen Noise – Uptower Inspections1 st
tooth
Leveraging uptower inspection and repair .
Surface defect Caused by grind temper
No other teeth had comparative BH values as
the damaged teeth. GB was repaired and
placed back in service. 1 st
tooth 2 nd
tooth
2nd tooth

28. Gear Case Depth Eddy Current Inspection
Description
GE-conceived technology that enables non-destructive examination of Gear manufacturing quality
Benefits
Advances state of the art in gear manufacturing and wind turbine gearbox quality
Eliminates need for sacrificial parts during gear manufacturing
Reduces gear inspection process cycle time … manufacturing productivity improvement
Enables examination of gears after installation in the gearbox
Eddy Current System
Gear Case Depth Profile
Gear Case
Case

29. 3.5 MW IntegraDrive in the space of a 3-stage 2.3 MW gearbox
IntegraDrive geared generator
Location of 2 MW generator coupling
PM Generator
Compound Planetary Gearbox
3.5 MW IntegraDrive in the space of a 3-stage 2.3 MW gearbox
Generates 6% higher AEP versus conventional DFIG drive train
Conventional Gbx & Generator
IntegraDrive
No. of Bearings 26 11
No. of Gear Meshes 15 6
Efficiency Range
96.5% %
99.3%
System Efficiency
90%
93%
Modular flywheel coupling system … common interface for multiple suppliers

30. June 12-13 2009, Cornell University
Challenges of Design, Manufacturing Validation of large Gearboxes used in wind Turbine Applications
June , Cornell University
Mike Sirak
Tony Giammarise

31. X section of a typical 1.5 MW Wind Turbine
Gear Drive converts ~ 18 rpm input to 1440 rpm output; hence a speed increasing drive system
GE Energy has the entire system. We at Power Drives are building only the Gear drive portion of the system
This is a talk to show you some of the hardware, and the manufacturing methods/ materials used

32. Stats for a typical 1.5 MW system Cut in wind speed 3.5m/s
Near Future 2.0 to 2.5 MW near term, land based (already here in Europe)
Stats for a typical 1.5 MW system
Cut in wind speed 3.5m/s
Cut out Wind speed 25m/s
Rotor diameter 77m
Rotor speed to 20.4 rpm
Swept Area m2
Tower height 80 to 100 m
Stats for a mating gearbox
Input rpm 18
Output rpm 1440
Ratio
Input power 1660Kw
Input rated Torque Nm

33. The practical issues with Big wind machines and the scale of things :
Installation
logistics and access
Scale reflected in cost and available suppliers; their capacity and location, logistics
Unscheduled maintaince
sky hook
~240 ft, (80 m)

34. Typical Design deliverables include
Gearbox Design is based on a design process where requirements are flowed to Drive Systems from:
Turbine manufacturers
public standards
insuring bodies
Internally imposed guidelines
Typical Design deliverables include
Geometrical/Dimensional envelope, details in aux equipment placement
Power capabilities, input rpm’s output rpm’s
Weight limit
Environmental operational conditions
Temperature extremes
Wind class
Emergency stops/overload conditions
Noise limits
Reliability guarantees
Certification from insuring body

35. Gear box materials flow from design requirements, customer requirements and insurance bodies
Structural Components
Moderately large Machined Ductile Iron Castings with
Soundness requirements
Mechanical and Microstructural requirements
Environmental Requirements (Cold weather, -40C)
Gearing
Forged raw materials
Clean Steel Technology required for gear steels
Carburized and hardened
Extensive high tolerance machining AGMA class 12 and higher
Extensive Secondary heat treatments/ finishing operations/ inspections for final acceptance

36. Bearings
Extensive use of rolling element bearings
~ less than 10% mass of a 1.5 MW gear box are the bearings
Commercially produced by the leading global bearing suppliers
Clean steel technology required
Both thru hardened and case carburized utilized
Bearing issues:
design loads and their complete measurement
Global supply issues of capacity timely deliveries
Earlier generation of gear boxes had life limited bearing failures .

37. 20000 lbs of 32000 lbs total of the gearbox mass for a 1
20000 lbs of lbs total of the gearbox mass for a 1.5 MW Box is structural castings. Half of that is in one casting.

38. Heavy walled Ductile Iron Issues
First stage planetary gears carrier cast ductile iron
Heavy walled Ductile Iron Issues
Designing for optimized weight to strength ratios
Soundness of castings and surface quality
Consistency of Mechanical properties and microstructure throughout
Cold weather requirement and fracture toughness measurement
Global supply chains also represent additional manufacturing/ quality risks

39. Ductile (spheroidal) Iron metallurgy
What it it?
Why Ductile?
Standard structural material, Effectively easy to cast, Lower costs as compared to cast steel, better yields and comparable mechanical properties, machines readily, and insuring bodies allow it!
Properties
100X Microstructure ferritic ductile Iron
Common grade used by many Wind turbine producers

40. Other structural Castings found in wind turbine system are similar to those within the gear box.
Hub Casting
Bedplate Casting

41. Bearing locations in a 1.5MW compound planetary gear arrangement
Bearings shown in various colors

42. Examples of Roller bearing types and usage within a 1.5 MW gear box

43. Overview of Wind Gearing
Forged raw materials using
Clean Steel Technology & high Hardenability Steels
Carburized and hardened helical gearing
Extensive high tolerance machining AGMA class 12 and higher for reduction of noise, better load sharing
Extensive heat treatments/ finishing operations/ inspections for final acceptance

44. Clean Steel Technology a Definition
Decreasing steel quality
Single melted standard air
Steel making methods utilized for Clean Steel
Bottom taping EAF
Ladle refined
Deoxidized
Vacuum degassed
Bottom poured ingot or con-cast
Protected from re-oxidation during teeming/casting
Careful limits on Hydrogen and Oxygen
All of the above need to be done to assure high quality steel
Double melting
Gear & Bearing quality
Increasing cost X 2x

45. Manufacturing sequence for typical precision Wind gears
Steel making/ ingot production
Grades 18CrNiMo7-6, 43B17, 4820 and similar
Clean steel technology required/ No naughty bits in the steel/ inspect it
Forging operations to rough shape ingot into forged blank
heat it/ beat it / heat treat it and then inspect it
Gear making operations
rough it, hob it ( First outline of the tooth form )
prep it for carburizing
pump carbon into the surface (Extensive diffusion based high temperature controlled cycles changing the near surface to a different alloy!)
quench it to make it hard
temper it to make is a little less hard but improve it’s toughness
grind it to very accurate finish dimensions
inspect it, measure it, making sure it meets print and is free from manufacturing defects
protect it during storage/shipment

46. Typical Case Carburized tooth form and the scale of things
Why Case harden gears
Resist bending Fatigue loads
Resist Surface contact Fatigue
Add compressive residual stress to aid in the fatigue loading capabilities
Core structure remains ductile
Highest allowable design loads per all international standards
Large capacity worldwide for this type of hardening process
Surface Hardness
Effective case depth
Distance in mm from ground surface
Case hardening Terms
Surface Hardness
Effective Case depth to HRC 50
Pitch line Case depth
Root hardness
Core hardness
Core
region
A tooth form showing the surface hardening; scale is in mm

47. Typical Carburized Gear Process
Steel Supplier
Steel Run
(Heat)
Ingots
29” square
Billets
14.5” round corner
240k lbs
30 ingots/run
1 billet/ingot
Gear Supplier
Forging Supplier (Q =13)
Rough Machining
Forgings
Mults
1 gear/forging
1 forging/mult
6-8 mults/billet
GE Transportation
Assembly into Gearbox
Heat Treat
Carb/Quench
Hard turn/ Grinding
2 gears/heat treat
1 gear/grind
1 gear/gearbox

48. Typical Gear Quality Checks

49. Modern Gear Grinders required to meet the tolerance needs for wind gearing. High quality tolerances needed to reduce noise

50. The requirements of Wind Gearing also require large capital investments.
Current grinding technology both measures, distributes stock and self corrects dimensions, before and during grinding

51. Gear Tolerances and the scale of things
Typical AGMA Class 12 gearing found on a 40 inch diameter high speed gear
Small numbers!
These magnitudes require temperature controlled grinding processes, temperature controlled measurement processes to produce accurate gears. Accurate tooth form assures uniform loading

52. Issues in gear manufacture for wind gear systems
Special problems of large forgings
Distortion during heat treatment
Distortion during quenching
Achieving adequate case and or core properties in large parts
Damaging hard surfaces during final grind operations
Capacity worldwide for large high tolerance gear manufacture

53. Design success means:
Understand the requirements
Know and measure loads
Utilize knowledge in Public Standards
Understand the operating environment
Detail and define requirements and flow them from System, to sub-System, to each component
Inspect Components using the right methods and technologies
Manufacture carefully Utilizing the right advanced and traditional tools
Test to failure
The outcome is, the ability to achieve the level of reliability desired
Thank you

54. Scale of things: 1.5 MW units 1660 Kw = 2226 HP= 1.6 MW
GE locomotive=4400 HP,so from a power view, half a locomotive up a pole.
The entire structure, including the pole is ~ 220 Tons excluding the foundation=weight of locomotive
The foundation weights ~500 tons

55. Operating Environment
-40C to +40 C
Lake Superior Late March w/ machines as cold as the photo looks
Above the Columbia River, mid summer