Reliability Theory Applied to Wind Turbine Gearbox Bearings

Transcription

1 Reliability Theory Applied to Wind Turbine Gearbox Bearings Rob Budny President, RBB Engineering GRC 2015 All Members Meeting

2 Overview A comment on models Gearbox component failure distribution GL 2010 requirements for gear and bearing fatigue Life Two typical high speed gearbox topologies Effect of part count on system reliability Effect of increased bearing reliability on system reliability Conclusions

3 All Models Are Wrong. Some are Useful. Since all models are wrong the scientist cannot obtain a "correct" one by excessive elaboration. On the contrary following William of Occam he should seek an economical description of natural phenomena. Just as the ability to devise simple but evocative models is the signature of the great scientist so overelaboration and over parameterization is often the mark of mediocrity -George E.P. Box George Box was a great statistician, who made important contributions to Quality Control, Engineering, and DOE His insight into the inherent limitations of models has been important to me throughout my career I decided to create a simple reliability model of two wind turbine gearbox bearing systems, and see what I could learn from them 3

4 Gearbox Failure Breakdown by Component Type 8% Failure data submitted in 2013 and different 25% OEM s 577 incidents 67% Bearings Gears Other *Data Source: NREL GRC Failure Database Best opportunity to improve gearbox reliability is to improve bearing reliability

5 GL 2010 Gear and Bearing Fatigue Requirements GL Requirements for Gears Failure Mode GL Requirement Required Confidence Methodology Pitting 1.2 Safety Factor 99% ISO Bending Fatigue 1.5 Safety Factor 99% ISO GL Requirements for Bearings Failure Mode GL Requirements Required Confidence Methodology Pitting (RCF) 130,000 hrs 90% ISO (Basic) Pitting (RCF) 130,000 hrs 90% ISO (Modified) Pitting (RCF) 175,000 hrs 90% ISO/TS 16281:2008 Each gear requires safety factor applied to loads, 99% confidence of survival No safety factor on bearing loads,90% confidence of survival No requirement to consider effect of number of gears and bearings on system life

6 Low Part Count Gearbox Layout Annulus 17 bearings 9 gears Planet Pinion HSS-A Gear HSS-B HSS-C 1PL-A 1PL-B Pinion PLC-A Sun PLC-B ISS-A Gear ISS-B ISS-C LSS-A LSS-B LSS-C

7 High Part Count Gearbox Layout 1 Annulus 32 bearings 13 gears Planet 2 Annulus 1PL-A 1PL-B 1PL-C 1PL-D Planet Pinion 1PLC-A Sun 1PLC-B 2PL-A 2PL-B HSS-A Gear HSS-B HSS-C 2PLC-A Sun 2PLC-B LSS-A LSS-B LSS-C

8 Bearing System Reliability Model Assumptions Failure of any bearing results in gearbox failure Reliability calculations assume one failure mode Pitting fatigue (RCF) of bearings All bearings assumed to just meet minimum GL requirements (175,000 hrs) Many other failure modes affect wind turbine bearings Micropitting Skidding or smearing WEC Debris or fretting damage Bearings assumed free of manufacturing defects Mounting Grind temper Inclusions Design inputs assumed correct Applied loads Misalignment Load share

9 Bearing Unreliability Calculations Bearing System Unreliability vs Time Unreliability Operating Time (years) Hi-Part Count L10 Low Part Count L10 Hi Part Count L01 Low Part Count L01

10 Prevention of Non-Classical Failure Modes Failures other than that caused by classical rolling-element fatigue are considered avoidable if the component is properly designed, handled, installed, and not overloaded -Irv Zaretsky As described on the assumptions slide, the reliability calculations assume that the only failure mode is RCF Proper design practices can greatly reduce the number of nonclassical bearing failures Skidding, micropitting: Use of TRB s can prevent these failures WEC, debris: Case carburized bearings, with optimal retained austenite can prevent WEC failures Fretting: Proper turbine operation, bearing coatings can prevent failures Non-Uniform load share: Use of statically determinate bearing arrangements 10

11 Conclusions L10 life requirement for wind turbine gearbox bearings probably not optimal for lowest cost of energy Gearbox failures are extremely costly Approximately two thirds of wind turbine gearbox failures are the result of bearing failures More reliable bearings show big improvements in gearbox system reliability Will come at a cost, and effect gearbox design Very likely that increased reliability will justify any cost increase, but should be confirmed Effect of gearbox part count not considered in current GL requirements Part count becomes less important as component reliability increases System reliability models, like those being proposed by the AGMA Wind Turbine gearing committee can provide powerful information to wind turbine buyers Buyers can better evaluate expected gearbox reliability Models must use consistent reliability parameters across manufacturers Assumptions used in the reliability calculations must be validated

12 Acknowledgements My thanks to the following individuals, who helped inspire and inform this presentation: Bob Errichello, GEARTECH Ryan O Connor, EDF Irv Zaretsky, NASA Glenn Research Center (ret) 12

13 Online Resource: Gearboxfailure.com

14 Thank you for your attention! Questions?