Understanding the Life of Power Transmission Elements of Wind Turbine Systems

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1 Understanding the Life of Power Transmission Elements of Wind Turbine Systems Jian Cao and Q. Jane Wang Northwestern University March 2010

2 Northwestern University 2/22

3 Wind Resource Assessment 3/22

4 Google 4/22

5 5/22

Windmill gearboxes: Power up to 5-10 MW, going to 15 MW unbalanced/unstable

6 Great Challenges Advanced product design requirements impose great challenges: high power density high efficiency high reliability extreme conditions e.g. Windmill gearboxes: Power up to 5-10 MW, going to 15 MW unbalanced/unstable loading very long operating duration extreme weather conditions minimal maintenance. 6/22

7 Counterformal Contact in Gears From A. Erdemir Typical line contact during a meshing cycle in helical/ spur/worm/straight bevel gears 7/22

8 Gear as an Example Shaved R a =698.8 nm, R q =886.4 nm, R t =18430 nm Honed R a =270.5 nm, R q =346.3 nm, R t =7200 nm Ground R a =512.6 nm, R q =657.0 nm, R t =10720 nm Polished R a =189.2 nm, R q =280.2 nm, R t =5430 nm A 3-dimensional mixed EHL model capable of handing real machined roughness is needed even for line contact problems Surface roughness is usually of the same order of magnitude as, or greater than, the possible EHL (elastohydrodynamic lubrication) film thickness Surface topography is 3-dimensional, although macro contact geometry may be simplified to 2-dimensional. 8/22

3-D Line Contact Mixed EHL Model The Reynolds Equation: ρ 3 p ρ 3 p ( ρh) ( ρh) h + h = U + x 12η x y 12η y x t Film Thickness Equation: 2 x h = h ( t) + + v( x, y, t) + δ ( x, y, t) + δ ( x, y, t) 0

9 3-D Line Contact Mixed EHL Model The Reynolds Equation: ρ 3 p ρ 3 p ( ρh) ( ρh) h + h = U + x 12η x y 12η y x t Film Thickness Equation: 2 x h = h ( t) + + v( x, y, t) + δ ( x, y, t) + δ ( x, y, t) R x Surface Deformation: 2 p( ξ, ς, t) vxy (,, t) = dξ dς π E ' 2 2 ( x ξ) + ( y ς) Load Equation: W () t p( x, y,) t dxdy p = Ω Lubricant Viscosity Model: Ω η = η 0e α p Ren et al., J. Tribology, 2009 Chen, W. W., Wang, Q., Wang, F., Keer, L. M., and Cao, J. J. Applied Mechanics (2008) Based on the mixed EHL model by Zhu & Hu ( ) for point contacts, and contact model with mixed FFT approach by W. Chen & Q. Wang (2007) 9/22

Line Contact Mixed EHL Solution Film Thickness or Gap

each other at a rolling speed of 500 mm/s and a

883 GPa Both hydrodynamic lubrication and surface asperity

10 Line Contact Mixed EHL Solution Film Thickness or Gap Pressure Two 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 GPa Both hydrodynamic lubrication and surface asperity contacts are simulated with a unified equation system and numerical approach. 10/22

11 Typical Mixed EHL Solutions For a spur gear set under LPSTC conditions, P H =2.919 GPa, SR=114.3% From Zhu et al., J. Tribology, /22

12 Friction in Gears Tooth contact friction is often the single largest source of power loss in a gearbox Total friction is the sum of hydrodynamic friction and asperity contact friction Friction can be predicted based on the mixed lubrication analysis In hydrodynamic areas: using Bair & Winer s non-newtonian elasticviscous fluid model:.. τ τ = L τ γ ln 1 Gx η τ L In contact areas: using an experimentally estimated boundary lubrication coefficient of friction (Typically ~ ). Contact Area Hydrodynamically Lubricated Area Pressure Lubricant Film Thickness Subsurface Stress Field Contact friction is usually dominant /22

13 Friction Reduction Strategy Friction coefficient Boundary lubrication 1 Rougher Surfaces Actual Operating Point Mixed lubrication By low friction materials, coatings, and lubricant additives By optimization of design, operating conditions, and surface finish By improved lubrication techniques, lubricants, and surface textures Full-film (Elastohydrodynamic or hydrodynamic) Lubrication 2 Smoother Surfaces Current Status 3 Improved Friction Curve Lubricant Film Thickness (λ) Ratio (1) Reduce contact friction coefficient (2) Reduce asperity contact (3) Reduce hydrodynamic friction 13/22

14 Effect of Surface Finish Surface roughness and its orientation greatly affect the lubrication performance and friction. In most cases the smoother the better 14/22

15 Possible Friction Reduction in Gears From A. Martini, D. Zhu, and Q. Wang, /22

50%) 8 N - number of stress cycles until initiation 8 V - stressed volume 8 τ e - effective stress

16 Stress Based Fatigue Life Models Ioannides-Harris Model (1985): Zaretsky life model (1987): 1 e ec ln ~ N τ e dv S V ln 1 S ~ N e V ( τ e τ u) z c dv 8 S - probability of survival (e.g. 50%) 8 N - number of stress cycles until initiation 8 V - stressed volume 8 τ e - effective stress (based on calculated 3D stress) 8 z - depth below the surface 8 τ u, e, c, etc - materials-related constants 16/22

17 Comparison between Predicted Pitting Life and Test Results From Zhu, Ren and Wang, /22

18 Surface Finish Effect on Pitting Life From Zhu, Ren and Wang, /22

19 Opportunities and Challenges Opportunities: Recent advancement in contact and lubrication research has provided powerful tools for friction/efficiency and life/durability analyses Challenges: 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 19/22

Technology Challenges Gearbox performance Operating expenses to high Capital expenses still exceed DOE performance goals Rotor stretching strategy Wind plants under-performing 10% Why: Bearing

Unscheduled maintenance, low reliability, lack O&M automation Fatigue load & deflection control required Tower clearance limit, materials, aeroacoustics limiting tip speed, dynamic stability?

20 Technology Challenges 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? Offshore COE Cost Breakdown LRC & Lease Cost 6% Electrical Infrastructure 12% Eng/Permits 4% O&M (After Tax) 13% Support Structure 14% LRC & Lease Cost 10% Electrical Infrastructure 7% National Renewable Energy Laboratory M. Robinson Misc BOS 13% Turbine 32% Offshore Warranty 6% Onshore COE Cost Breakdown O&M (After Tax) 9% Foundation 3% Misc BOS 11% Turbine 60% Existing design codes & tools should achieve 20 year life & reliable power performance predictions; What are we missing? 20/22

21 Future Trends Simulation-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. 21/22

THANK YOU jcao@northwestern.edu www.mech.

22 THANK YOU google Jian Cao

23 23/22

24 MW Installed Growth of Wind Energy Capacity Worldwide Actual Pacific Rest of the World Asia North America Europe Projected Pacific Rest of the World Asia North America Europe Jan 2009 Cumulative MW = 115,016 Rest of World = 23,711 North America = 27,416 MW U.S 25,170 Canada 2,246 Europe = 63,889 MW 120, , ,000 90,000 80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 0 '00 '01 '02 '03 '04 '05 '06 '07 '08 '09 ' EU US Asia Rest of the World Pacific Sources: BTM World Market Update 2007; AWEA, January 2009; Windpower Monthly, January 2009 National Renewable Energy Laboratory 24/22

The Ohio State University, Columbus, OH PLEASE SCROLL DOWN FOR ARTICLE

The Ohio State University, Columbus, OH PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [Li, Sheng] On: 24 June 2010 Access details: Access Details: [subscription number 923369461] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered