Sources of time‐varying contact stress and misalignments in wind turbine planetary sets

Abstract

Recent data shows that 90% of large wind turbines include a gearbox, and industry forecasts expect this fi gure to remain relatively stable. With global annual volumes (2009) of around 18,600 units, the quality, cost and performance of gearboxes is of paramount importance to the wind sector. The industry has been focusing some attention on gearbox reliability, as demonstrated by a growth in the number of specifi c seminars and collaborative programs on this topic. One aspect that needs to be brought to an industry-wide forum is the understanding of the complexity of bearing design in the gearbox and the careful attention that needs to be paid to ensure a reliable gearbox design. This paper seeks to address this issue by clear demonstration of design issues using a model of the gearbox from the National Renewable Energy Lab's Gearbox Reliability Collaborative. Detailed models are presented with focus on determining the quality of the function of the planetary gear stages. Key design drivers are discussed such as the quality of alignment at the gears and bearings and the loads and stresses seen on these components. Under a design load case with a signifi cant rotor off-axis moment the stresses in the planet gears and bearings are investigated. It is shown how the misalignment of the planet pins varies with the rotation of the planetary set and how subsequently time-varying contact stresses and load distributions occur in the planet gears and bearings. These factors strongly infl uence the fatigue life of the gearbox components as well as the level of vibration. Design tools are then used to demonstrate how small variations in the clearances of the planet carrier bearings can have a big effect on the quality of the design. Numerical studies show where optimal clearance settings lie and how the misalignment of the planetary set can be improved. Furthermore, a demonstration is made of how redesign of the bearing arrangement and subsequent optimization of the planet tooth geometry further improves the misalignment and results in signifi cantly reduced time-varying contact stresses, better load distribution and reduced vibration. It is illustrated that small clearances, such as in the carrier bearings, can have a large effect on the performance of the design and a study shows how to identify and reduce time-varying misalignment and contact stresses resulting in lower vibration, lower fatigue and a more reliable product.

On long-term fatigue damage and reliability analysis of gears under wind loads in offshore wind turbine drivetrains

“…Alongside the dominant high ratio gearboxes, medium ratio gear trains, direct drives, hydraulic drives, differential and variable speed gearboxes are available commercially in the wind turbine market. Yet, the majority (90%) of drivetrains in currently 2 installed turbines are based on gear technology [1]. The wind turbine gearbox failure investigations carried out by Musial et al [2] indicate that the design process may have the biggest contribution to the premature failures of wind turbine gearboxes.…”

Improving wind turbine drivetrain designs to minimize the impacts of non-torque loads

“…Rolling element bearings operate in a finite life region above the fatigue load limit. In the scenario of 189% rated torque, the planet bearing forces exceeded their fatigue limits (defined by Crowther et al ), and thus, they rapidly decrease the bearing life. Non‐torque loads and manufacturing tolerances cause cyclic planet bearing forces about rotor rotation frequency and its higher harmonics.…”

“…Gear tooth edge loading was observed on the upwind side of the ring gear teeth during the GRC experiments as predicted by the analytical and computational models . The edge loading was caused by unequal planet bearing loads between upwind and downwind rows and by misalignment induced by non‐torque loads.…”

Comparison of Planetary Bearing Load-Sharing Characteristics in Wind Turbine Gearboxes

“…Gravity is an important factor as it introduces fundamental excitations in the rotating carrier frame of planetary gear sets. The effect of carrier bearing clearance on planetary load sharing subject to rotor moments has also been studied (Crowther et al, 2011, LaCava et al, 2013, Guo et al, 2014. Rotor moments impact planet load sharing, gear and bearing alignment, and bearing contact conditions and stress (Park et al, 2013, Gould and Burris, 2016, Dabrowski and Natarajan, 2017.…”