RECENT PERFORMANCE MODELING of wind-turbine-gearbox bearings with field data suggests that taming torque reversals could lengthen a gearbox-bearing life up to two fold. The field data was collected by engineers at AeroTorque and supplied to bearing simulation and prediction firm Sentient Science. The later company has collected data on millions of military aircraft bearings and developed the algorithms, called DigitalClone, for predicting the conditions that promote the longest bearing life.
The two plot shows that lowering peak stresses lengthens the life of a bearing. Furthermore, lowering loads so that WEA does not form lengthens life further. Generally, bearings found with WEA at failure did so earlier than those without evidence of WEA. A conclusion is that WEA significantly reduces bearing rolling contact fatigue life at various stress levels as applied on Sentient Science DigitalClone models in computational testing. Source: Sentient Science
Bearings seem the weak link in wind turbine gearboxes. Once an axial crack forms in a bearing race, the bearing begins to shed debris into the oil and eventually, gear-teeth-surfaces wear and if the bearing is not replaced, the entire gearbox can need replacing, sometimes a $350,000 job.
All wind turbines are affected by transient drivetrain loads from extreme wind and severe stops triggered by various faults codes. To make matters worse, transient load events have increased as turbines are built larger to produce more power.
Recent thinking has it that torque reversals from sudden stops and wind events, such as gusts and storms, produce rapid torque swings from positive to negative, and in a manner that essentially hammers bearings as load zones shift from one side of a bearing race to the other. The pounding on hard subsurface occlusions in the bearing material produces the axial crack. “Research and monitoring operating turbines have shown that the torque reversals and impact loads are a leading cause of axial-crack damage and White Etch Area,” said Doug Herr, General Manager at AeroTorque. Axial cracks are often initiated by the phenomena of White Etch Areas.
One solution to the problem may be an Asymmetric Torque Control (ATC), an AeroTorque device intended to reduce the magnitude and speed of impact, and reversal loads. It has shown to reduce peak stresses in bearings during reversals by up to 14%. The company has collected significant amounts of field data on torsion reversals during turbine stopping and other events, and say the ATC provides a considerable reduction in drivetrain loading, up to 54% in forward and 74% in reverse, even on turbines with active controls. The question AeroTorque engineers’ asked was: How do you convert that finding to a life factor and better calculate value and ROI?
To calculate the bearing’s life, the two companies decided to evaluate the life of a 1.5 MW, high-speed bearing under a Class-1 representative duty cycle that included severe real-world conditions. These bearings have experienced high rates of white-etch areas in the field.
AeroTorque provided its data on hard-stop amplitudes and frequencies while Sentient Science studied failed bearing samples and modeled the white-etch like inclusions to generate a representative microstructure model. The company’s modeling approach was chosen because it can accurately calculate bearing life by accounting for wind loading events, material quality and inclusions, material microstructure, surface finish, and lubricant.
The company ran the simulations through its DigitalClone life-prediction tool to assess the effect on high-speed-bearing performance with new duty cycles and with inclusions added.
Results showed that a white etched damaged area reduced bearing L50 life based on rolling contact fatigue by up to 45%. If WEA can be prevented using ATC, bearing L50 life can be extended by up to a factor of two, from 2.89 to 5.39 years. The teams are continuing to further examine the effects of dynamic reverse loads and impact loads, which were not considered in this program. Sentient says its customers control over 40% of the U.S. wind fleet. 
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