Full Life Wind Turbine Gearbox Lubricating Fluids

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Industrial gear box lubricants typically are hydrocarbon based mineral oils with considerable amounts of additives to overcome the lack of base fluid properties like wear protection, oxidation stability, load carrying capacity, low temperature solidification and drop of viscosity at higher temperatures. For today's wind turbine gearboxes, the requirements are more severe and synthetic hydrocarbon oils are used to improve on this, but all such hydrocarbon based lubricants require significant amounts of Extreme Pressure (EP) additives to meet performance requirements. Perfluoropolyether (PFPE) fluids provide load carrying capacity as an inherent property. During the course of the project with the main tasks ... continued below

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Lutz, Glenn A.; Jungk, Manfred; Bryant, Jonathan J.; Lauer, Rebecca S.; Chobot, Anthony; Mayer, Tyler et al. February 28, 2012.

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Description

Industrial gear box lubricants typically are hydrocarbon based mineral oils with considerable amounts of additives to overcome the lack of base fluid properties like wear protection, oxidation stability, load carrying capacity, low temperature solidification and drop of viscosity at higher temperatures. For today's wind turbine gearboxes, the requirements are more severe and synthetic hydrocarbon oils are used to improve on this, but all such hydrocarbon based lubricants require significant amounts of Extreme Pressure (EP) additives to meet performance requirements. Perfluoropolyether (PFPE) fluids provide load carrying capacity as an inherent property. During the course of the project with the main tasks of 'Establish a Benchmark', 'Lubricant Evaluation', 'Full Scale Gearbox Trial' and 'Economic Evaluation', the PAO Reference oil exhibited significant changes after laboratory gear testing, in service operation in the field and full scale gearbox trial. Four hydrocarbon base oils were selected for comparison in the benchmarking exercise and showed variation with respect to meeting the requirements for the laboratory micro-pitting tests, while the PFPE fluid exceeded the requirements even with the material taken after the full scale gear box trial. This is remarkable for a lubricant without EP additives. Laboratory bearing tests performed on the PFPE fluids before and after the full scale gear box trial showed the results met requirements for the industry standard. The PFPE fluid successfully completed the full scale gear box test program which included baseline and progressive staged load testing. The evaluation of gears showed no micro-pitting or objectionable wear. By the final stage, lubricant film thickness had been reduced to just 21% of its original value, this was by design and resulted in a lambda ratio of well below 1. This test design scenario of a low lambda ratio is a very undesirable lubrication condition for real world but creates the ability to test the lubricating fluids performance under the most extreme conditions. The PAO Reference oil also passed its testing without any noticeable deterioration of the gear surface. However the PAO Reference oil was replaced midway through the progressive loading, as the lubricant was burned in an attempt to raise the sump temperature to the same levels as for the PFPE. Both materials experienced a decrease of viscosity during their respective run times. The viscosity index decreased for the PAO there while there was a slight increase for the PFPE. FZG laboratory gear tests and measurements of the drive motor's current during the full scale gear box trial were made to characterize the relative efficiency between the PFPE fluid and the PAO Reference oil. In the FZG laboratory efficiency test, the PFPE fluids show much higher churning losses due to their higher viscosity and density. The analysis seems to show that the efficiency correlates better to dynamic viscosity than any other of the measured metrics such as film thickness. In load stages where the load, speed and temperature are similar, the PFPE fluid has a greater film thickness and theoretical gear protection, but requires a larger current for the drive motor than the PAO. However in load stages where the film thickness is the same, the PFPE fluid's reduced dynamic viscosity gives it a slight efficiency advantage relative to the PAO reference oil. Ultimately, many factors such as temperature, rotational speed, and fluid viscosity combine in a complex fashion to influence the results. However, the PFPE's much lower change of viscosity with respect to temperature, allows variations in designing an optimum viscosity to balance efficiency versus gear protection. Economic analysis was done using Cost of Energy calculations. The results vary from 5.3% for a 'Likely Case' to 16.8% for a 'Best Case' scenario as potential cost improvement by using PFPE as the gearbox lubricating fluid. It is important to note the largest portion of savings comes in Levelized Replacement Cost, which is dictated by the assumption on gearbox reliability. Thus, verifying and quantifying the potential of PFPE fluid to effect gearbox reliability is the key assumption that would need to be further validated. In summary the proof of concept to use PFPE fluid as wind turbine gear box lubricant was validated with this project. The increase in life time was qualitatively demonstrated and this supports the need for future activity of field trials and laboratory aging studies to quantify the predicted 20 year life. With micro-pitting being the major failure mechanism in the last years, recent publications show that white etch cracking of bearings seem to have the highest impact on wind turbine reliability. With its higher film thicknesses compared to PAO reference oils, PFPE fluids have the potential to reduce this failure occurrence as well.

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  • Report No.: DOE/EE0001364-1
  • Grant Number: EE0001364
  • DOI: 10.2172/1041556 | External Link
  • Office of Scientific & Technical Information Report Number: 1041556
  • Archival Resource Key: ark:/67531/metadc830754

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  • February 28, 2012

Added to The UNT Digital Library

  • May 19, 2016, 3:16 p.m.

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  • July 22, 2016, 7:25 p.m.

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Lutz, Glenn A.; Jungk, Manfred; Bryant, Jonathan J.; Lauer, Rebecca S.; Chobot, Anthony; Mayer, Tyler et al. Full Life Wind Turbine Gearbox Lubricating Fluids, report, February 28, 2012; United States. (digital.library.unt.edu/ark:/67531/metadc830754/: accessed September 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.