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Understanding the Life of Power Transmission Elements of Wind

Turbine Systems

Understanding the Life of Power Transmission Elements of Wind

Turbine Systems

Jian Cao and Q. Jane Wang

Northwestern University

March 2010

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Northwestern UniversityNorthwestern University

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Wind Resource Assessment

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Google

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Great ChallengesGreat ChallengesAdvanced product design requirements impose great challenges:

high power density high efficiency high reliabilityextreme conditions

e.g. Windmill gearboxes:Power up to 5-10 MW, going to 15 MWunbalanced/unstable loadingvery long operating durationextreme weather conditionsminimal maintenance.

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Counterformal Contact in GearsCounterformal Contact in Gears

Typical line contact during a meshing cycle in helical/ spur/worm/straight bevel gears

From A. Erdemir

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Ra=698.8 nm, Rq=886.4 nm, Rt=18430 nm Ra=512.6 nm, Rq=657.0 nm, Rt=10720 nm

Ra=270.5 nm, Rq=346.3 nm, Rt=7200 nm Ra=189.2 nm, Rq=280.2 nm, Rt=5430 nm

Shaved Ground

Honed Polished

Surface roughness is usually of the same order of magnitude as, or greater than, the possible EHL (elasto-hydrodynamic lubrication) film thicknessSurface topography is 3-dimensional, although macro contact geometry may be simplified to 2-dimensional.

A 3-dimensional mixed EHL model capable of handing real machined roughness is needed even for line contact problems

Gear as an ExampleGear as an Example

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3-D Line Contact Mixed EHL Model3-D Line Contact Mixed EHL ModelThe Reynolds Equation:

Film Thickness Equation:

Surface Deformation:

Load Equation:

Lubricant Viscosity Model:

( ) ( , , )pW t p x y t dxdyΩ

= ∫∫

0peαη η=

2

0 1 2( ) ( , , ) ( , , ) ( , , )2 x

xh h t v x y t x y t x y tR

δ δ= + + + +

3 3 ( ) ( )12 12

p p h hh h Ux x y y x t∂ ρ ∂ ∂ ρ ∂ ∂ ρ ∂ ρ∂ η ∂ ∂ η ∂ ∂ ∂

⎛ ⎞⎛ ⎞+ = +⎜ ⎟⎜ ⎟

⎝ ⎠ ⎝ ⎠

2 2

2 ( , , )( , , )' ( ) ( )

p tv x y t d dE x y

ξ ς ξ ςπ ξ ςΩ

=− + −

∫∫

Based on the mixed EHL model by Zhu & Hu (1999-2000) for point contacts, and contact model with mixed FFT approach by W. Chen & Q. Wang (2007)

Ren et al., J. Tribology, 2009Chen, W. W., Wang, Q., Wang, F., Keer, L. M., and Cao, J. J. Applied Mechanics (2008)

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Line Contact Mixed EHL SolutionLine Contact Mixed EHL Solution

Film Thickness or Gap PressureTwo shaved cylindrical surfaces running against each other at a rolling speed of 500 mm/s and a slide-to-roll ratio of 25%. Max. Hertzian pressure 1.883 GPa

Both hydrodynamic lubrication and surface asperity contacts are simulated with a unified equation system and numerical approach.

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Typical Mixed EHL SolutionsTypical Mixed EHL SolutionsFor a spur gear set under LPSTC conditions, PH =2.919 GPa, SR=114.3%

From Zhu et al., J. Tribology, 2009

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Friction in GearsFriction in GearsTooth contact friction is often the single largest source of power loss in a gearboxTotal friction is the sum of hydrodynamic friction and asperity contact frictionFriction can be predicted based on the mixed lubrication analysis

In hydrodynamic areas:using Bair & Winer’snon-Newtonian elastic-viscous fluid model:

⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

L

L

xG ττ

ηττγ 1ln

..

In contact areas: using an experimentally estimated boundary lubrication coefficient of friction(Typically 0.08 ~ 0.12). -1.5 -1.0 -0.5 0.0 0.5 1.0

Contact Area

HydrodynamicallyLubricated Area

Pressure

Lubricant FilmThickness

SubsurfaceStress Field

0.07 ~ 0.15 ).Contact friction is usually dominant

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Friction Reduction StrategyFriction Reduction Strategy

(1) Reduce contact friction coefficient (2) Reduce asperity contact (3) Reduce hydrodynamic friction

Mixedlubrication

Boundarylubrication

Full-film (Elastohydrodynamic orhydrodynamic) Lubrication

By low friction materials, coatings,and lubricant additives

Fric

tion

coef

ficie

nt

Lubricant Film Thickness (λ) Ratio

Actual Operating Point

1

2

3

1

23 By improved lubrication techniques,

lubricants, and surface textures

By optimization of design, operatingconditions, and surface finish

Improved Friction Curve

Rougher SurfacesSmoother Surfaces

1

2Current Status

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Effect of Surface FinishEffect of Surface FinishSurface roughness and its orientation greatly affect the lubrication performance and friction. In most cases the smoother the better

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Possible Friction Reduction in GearsPossible Friction Reduction in Gears

From A. Martini, D. Zhu, and Q. Wang, 2007.

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Stress Based Fatigue Life ModelsStress Based Fatigue Life ModelsIoannides-Harris Model (1985):

Zaretsky life model (1987):

8 S - probability of survival (e.g. 50%)8 N - number of stress cycles until initiation8 V - stressed volume8 τe - effective stress (based on

calculated 3D stress)8 z - depth below the surface8 τu, e, c, etc - materials-related constants

∫∫∫V

ece

e dVNS

τ~1ln

∫∫∫−

V

cuee dV

zN

S)(~1ln ττ

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Comparison between Predicted Pitting Life and Test ResultsComparison between Predicted Pitting Life and Test Results

From Zhu, Ren and Wang, 2009

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Surface Finish Effect on Pitting LifeSurface Finish Effect on Pitting Life

From Zhu, Ren and Wang, 2009

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Opportunities and ChallengesOpportunities and Challenges

Opportunities: Recent advancement in contact and lubrication research has provided powerful tools for friction/efficiency and life/durability analysesChallenges:

Market and technology development constantly imposes new challenges (higher power, higher efficiency, better reliability,more compact sizes, lower costs, etc.)There is still a gap between fundamental research and industrial applications

Simulation-based analyses integrated in design for surface strength and friction/efficiency is far behind that for structure strength with FEA

20/22National Renewable Energy Laboratory – M. Robinson

Offshore COE Cost Breakdown

LRC & Lease Cost6%

Electrical Infrastructure

12%

Eng/Permits 4%

Support Structure14%

Misc BOS13%

Offshore Warranty

6%

Turbine32%

O&M (After Tax)13%• Gearbox performance

• Operating expenses to high• Capital expenses still exceed DOE

performance goals• Rotor stretching strategy• Wind plants under-performing 10%

Why:• Bearing failures; inaccurate

internal loads?• Unscheduled maintenance, low

reliability, lack O&M automation• Fatigue load & deflection control

required• Tower clearance limit, materials,

aeroacoustics limiting tip speed, dynamic stability?

Technology Challenges

Onshore COE Cost BreakdownO&M (After Tax)

9%LRC & Lease

Cost10%

Electrical Infrastructure

7%Foundation

3% Misc BOS11%

Turbine60%

Existing design codes & tools should achieve 20 year life & reliable power performance predictions;

What are we missing?

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Future TrendsFuture TrendsSimulation-based tribological analyses, efficiency and surface failure predictions will be integrated in design packages. More precise machining, better surface finish and other surface enhancement techniques (such as coatings) will be widely used especially for critical heavy-duty gears.Advanced lubricant/additive/coating interfacial system design will be developed, significantly improving performance, efficiency and life. New material development is needed.

THANK YOUjcao@northwestern.edu

www.mech.northwestern.edu/fac/caogoogle “Jian Cao”

THANK YOUTHANK YOUjcao@northwestern.edujcao@northwestern.edu

www.mech.northwestern.edu/fac/caowww.mech.northwestern.edu/fac/caogoogle google ““Jian CaoJian Cao””

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Growth of Wind Energy Capacity WorldwideGrowth of Wind Energy Capacity Worldwide

EUUS

AsiaRest of the World

Pacific

National Renewable Energy Laboratory

010,00020,00030,00040,00050,00060,00070,00080,00090,000

100,000110,000120,000

'00 '01 '02 '03 '04 '05 '06 '07 '08 '09 '10 11 12

MW

Inst

alle

d

Sources: BTM World Market Update 2007; AWEA, January 2009; Windpower Monthly, January 2009

Pacific

Actual Projected

Pacific

Rest of the World Rest of the World

Asia Asia

North America North America

Europe Europe

Jan 2009 Cumulative MW = 115,016

Rest of World = 23,711

North America = 27,416 MWU.S 25,170

Canada 2,246

Europe = 63,889 MW