Dr. Stephen Ekwaro-Osire Professor Mechanical ...

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Dr. Andy Swift. Professor ... A. Swift, J. Chapman. Wind Science and ... development initiative to improve gearbox reliability in wind turbines. This initiative ...
Dr. Stephen Ekwaro-Osire Professor Mechanical Engineering Dr. Andy Swift Professor Wind Science & Engineering Research Center Dr. Jamie Chapman Adjunct Professor Wind Science & Engineering Research Center Gear with Asymmetric Teeth for Use in Wind Turbines There is a concern on the reliability of wind turbines, directed to the fact that gearbox failure has been a major problem in the wind industry. The objective of this research was to design and construct a test bed for testing the performance of asymmetric gears. The tests to be performed on the test bed include gear dynamics, tip relief modification, high-contact-ratio and wear.

Proceedings of the SEM Annual Conference June 7-10, 2010 Indianapolis, Indiana USA ©2010 Society for Experimental Mechanics Inc.

Gear with Asymmetric Teeth for use in Wind Turbines S. Ekwaro-Osire 1, T.-H. Jang, A. Stroud, I. Durukan, F.M. Alemayehu Mechanical Engineering Department Texas Tech University Lubbock, TX 79409-1021 A. Swift, J. Chapman Wind Science and Engineering Research Center Texas Tech University Lubbock, TX 79409-1021 ABSTRACT In the US, wind energy is one of the electrical energy sources that are growing fastest. The growth has been linear at a rate of about 20% to 30% per year over the last decade. For the next two decades, the wind industry has set its goal to more than 20%. However, there still remains concern on the reliability of wind turbines. This concern is often directed to the fact that gearbox failure has been a major problem in the wind industry. Compared to the other wind turbine components, gearbox failures result in the second highest down time per failure. Currently, there are several initiatives underway to improve gearbox reliability in wind turbines. Additionally, due to the increasing performance requirements, there has also been need of new gear designs. Recently, theoretical analyses have shown that asymmetric gears may offer a potential to reduce the costs associated with the gear failures, while at the same time maintaining the fatigue life. Also, for wind turbine gearboxes, the gears experience only uni-directional loading. In these instances, the geometry of the drive side does not have to be symmetric to the coast side. This allows for the designing of gears with asymmetric teeth. The objective of this research was to design and construct a testbed for testing the performance of asymmetric gears. The tests to be performed on the testbed include gear dynamics, tip relief modification, high-contact-ratio, and wear. BACKGROUND In the US, wind energy is one of the fastest growing electrical energy sources [1]. Its growth has been linear at a rate of about 20% to 30% per year over the last decade [2]. For the next two decades, the wind industry has set its goal to more than 20%. Despite the projected increase in this sector, concern still remains about the reliability of wind turbines. Since gearbox replacement is expensive and gearbox reliability is a key issue, research in gearbox technology has increased in importance. With the recognition of the impact of gearbox failures, there is wide spread agreement in the wind sector community that in the next several years, the turbine drive-train technology will be evolved significantly to improve reliability and reduce cost and weight [3]. Recently, due to the increased interest in accounting for the uncertainty involved in wind turbines, there has been an interest in utilizing probabilistic techniques in the design of wind turbines. Veldkamp [4] presented a probabilistic approach for the design of wind turbines. In his study, he noted that the important stochastic parameters influencing fatigue loads include wind characteristics (e.g., average wind speed, cut out wind speed, turbulence intensity, and wind field shear), material properties (e.g., fatigue strength), geometry (e.g., dimensions), and aerodynamics (e.g., edge moment blade). Musial et al. [5] discussed a multi-year research and development initiative to improve gearbox reliability in wind turbines. This initiative comprised both testing and analysis efforts. This initiative seeks to address the problem within the design process by developing solutions in order to speed up improvements in wind turbine gearbox. The approach proposed involves drive-train analysis, dynamometer testing, and field testing. Another comprehensive initiative is the “UpWind” project underway in Europe [6]. This is the largest long-term wind energy research project that the European Union has ever funded. 1

Corresponding author: [email protected]

T. Proulx (ed.), Experimental Mechanics on Emerging Energy Systems and Materials, Volume 5, Conference Proceedings of the Society for Experimental Mechanics Series 16, DOI 10.1007/978-1-4419-9798-2_9, © The Society for Experimental Mechanics, Inc. 2011

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66 In this initiative, a multi flexible body dynamics-based wind turbine model with a detailed model of the gearbox was created. The gears inside the gearbox and their tooth contact were modeled with high detail. Experimental data was used to verify the model. Lately, there has been a lot of research activity on spur gears with asymmetric teeth. New gear designs are needed because of the increasing performance requirements placed on wind turbines, such as high load capacity, high endurance, low cost, long life, and high speed. In wind turbine gearboxes, the gears experience only uni-directional loading. In this instance, the geometry of the drive side does not have to be symmetric to the coast side. This allows for the design of gears with asymmetric teeth. The theoretical work conducted thus far [7-15] have shown that asymmetric gears may offer a potential to reduce the costs associated with the gear failures, while at the same time maintaining fatigue life. Other than the theoretical studies of asymmetric gears, there have not been experimental studies published. Hence, the overall objectives of the research was to (1) design and construct a gear testbed, (2) conduct performance tests on asymmetric gears, and (3) conduct reliability analysis and dynamic finite element analysis on asymmetric gears. This paper focuses only on the design and construction of a gear test bed. The tests the testbed can perform include gear dynamics, tip relief modification, high-contact-ratio, and wear. ASSYMETRIC GEARS In previous studies, related to bending stress and load capacity, high performance has been achieved for gears with asymmetric teeth. These gears provide flexibility to designers due to their non-standard design. If they are correctly designed, they can make important contributions to the improvement of designs of gears in wind turbine industry [9, 12, 13, 16]. An asymmetric tooth is shaped using different pressure angles on the drive side and coast F F side of the tooth (see Figure 1). In the design of asymmetric teeth, the choice of pressure angles on drive and coast side Coast Coast Drive Drive is very important to obtain satisfactory performances.Most side side side side of the recent research on the benefits of involute spur gears with asymmetric teeth has focused on geometrical design and stress analysis [16-20]. It has been shown that as the (a) (b) pressure angle increases, the root fillet stress and contact Figure 1: Asymmetric gear teeth, (a) pressure stress decrease significantly. In few studies [21], the effects angle larger on drive side than on coast side, (b) of various parameters, such as pressure angle and tooth pressure angle smaller on drive side than on height on the dynamic load and the static transmission coast side. errors of spur gears with asymmetric teeth, were investigated. On comparing the spur gears with asymmetric and symmetric teeth, it was shown that for asymmetric teeth, increasing the addendum leads to a significant decrease in the dynamic factor, static transmission error, and root fillet stress. Karpat and Ekwaro-Osire [7] studied the wear of involute spur gears with asymmetric teeth under dynamic loading. They observed an interaction between wear and dynamic loads for spur gears with asymmetric teeth. It was shown that, as the pressure angle on the drive side increases, wear depth decreases considerably. GEAR TESTING TESTBEDS A mechanical system exhibits combined parametric excitation and clearance type non-linearity. For the validation of the analytical solution, Blankenship and Kahraman [22] developed a gear dynamics test rig in order to demonstrate non-linear behaviors in mechanical oscillator and compared with numerical integration results and experimental measurements. Petry-Johnson et al. [23] developed a spur gear efficiency test machine for the experimental investigation of high-speed spur gear efficiency for both jet-lubricated, dry sump conditions and diplubricated conditions. The experimental results showed that the influence of rotational speed, oil viscosity, oil bath level, and rotational direction on load independent power loss was quantified. Based on their experiments, the authors proposed an efficiency model to predict the instantaneous mechanical efficiency of a gear pair under typical operating, surface, and lubrication conditions. The predictions from the model were shown to be within 0.1% of the measured values [24]. Seetharaman and Kahraman [25] performed experiments over a wide range of operating speed, temperature, oil levels, and key gear design parameters. Among others, their studies included the spin power losses of spur gear pairs operating under dip-lubricated conditions. Their measurements indicated that the static oil level, rotational speed, and face width of gears have a significant impact on spin power losses. Kahraman and Blankenship [26] designed a gear test rig to investigate the influence of involute contact ratio and linear involute tip relief on the torsional vibration behavior of a spur gear pair. The influence of involute contact ratio on dynamic transmission error is quantified and a set of generalized, experimentally validated design