Wind turbine gearbox operates under a wide array of highly fluctuating and dynamic load conditions caused by the stochastic nature of wind and operational wind turbine controls. Micropitting damage is one of failure modes commonly observed in wind turbine gearboxes. This article investigates gear micropitting of high-speed stage gears of a wind turbine gearbox operating under nominal and varying load and speed conditions. Based on the ISO standard of gear micropitting (ISO/TR 15144-1:2010) and considering the operating load and speed conditions, a theoretical study is carried out to assess the risk of gear micropitting by determining the contact stress, sliding parameter, local contact temperature and lubricant film thickness along the line of action of gear tooth contact. The non-uniform distributions of temperature and lubricant film thickness over the tooth flank are observed due to the conditions of torque and rotational speed variations and sliding contact along the gear tooth flanks. The lubricant film thickness varies along the tooth flank and is at the lowest when the tip of the driving gear engages with the root of the driven gear. The lubricant film thickness increases with the increase of rotational speed and decreases as torque and sliding increase. It can be concluded that micropitting is most likely to initiate at the addendum of driving gear and the dedendum of driven gear. The lowest film thickness occurs when the torque is high and the rotational speed is at the lowest which may cause direct tooth surface contact. At the low-torque condition, the varying rotational speed condition may cause a considerable variation of lubricant film thickness thus interrupting the lubrication which may result in micropitting.

Micropitting damage is one of the failure modes commonly observed in gears leading to destructive failures, which in turn results in unplanned shutdown and expensive replacement, such as those observed in wind turbine gearboxes. This study investigates gear micropitting initiation and propagation when subjected to varying torque loads under a constant rotational speed. The study employs both experimental gear testing and analytical evaluation based on the ISO Technical Report of Gear Micropitting, ISO/TR 15144-1:2010 and the recently revised ISO/TR 15144-1:2014. Initiation and propagation of micropitting are assessed in testing by quantifying the development of micropits and their progressive rate after specific numbers of running cycles at step-up torque levels. The analytical study is conducted to validate the prediction of micropitting using the ISO/TR recommended procedures by comparing the results with the occurrence of micropits in the tested gears.The gear test results show that micropitting initiates at the pinion dedendum but escalates at the addendum, because of the greater severity of progressive micropitting at the dedendum of the mating wheel where the tip relief area first comes into mesh. The analytical results, based on varying surface roughness measurements obtained from the tested gears, confirm that the maximum contact stresses and minimum specific lubricant film thicknesses occur in these regions. The specific lubricant film thickness varies considerably because of changes of surface roughness after gears are subjected to various running cycles under varying torque levels. It has found that the excessive loading, gear tooth micro-geometry, surface roughness and lubricant film thickness are the main factors affecting micropitting.

This study investigates the probabilistic risk of gear tooth flank micro-pitting in wind turbine (WT) gearboxes and shows how relatively slow rate of supervisory control and data acquisition (SCADA) data, recorded during operation, can be used to analyse the onset of gear surface damage. Field measured time series of SCADA signals, including wind speed, generator power and rotational speed, were used to obtain the statistical variation of gear shaft torque and rotational speed. From the SCADA data obtained over a 2.2 year period random number datasets of smaller sizes were selected. Based on these random number datasets the effect of gear shaft torque and rotational speed variations on the probabilistic risk of gear micro-pitting was investigated. Determinations of the gear tooth flank contact stress and lubricant film thickness were based on the technical report of gear micro-pitting, ISO/TR 15144-1 (2010). The study has shown that the considered pinion gear is subjected to high load conditions resulting in high contact stresses. The variation of rotational speed causes greater sliding between the gear teeth. The results of specific lubricant film thicknesses have shown that there is considerable risk of gear micro-pitting under the operational conditions recorded from the SCADA field data.