September 2020

44 n SERVICE AND REPAIRS September 2020 www.drivesncontrols.com Gearboxes get a new life after catastrophic failures O perations and maintenance costs represent about a quarter of the whole-life costs of wind power plants, so keeping gearboxes functioning properly is crucial to running wind power projects successfully and profitably. Badly damaged gearboxes that some might regard as unsalvageable can sometimes be repaired by experts with the right skills and experience. Dana SAC UK, part of Brevini Motion systems, has recently refurbished four similar wind turbine gearboxes that were extensively damaged, yet still worth repairing. The repairs all involved the same type of helical gearbox with partly monoblock casings that made both the strip-down and installation of gear wheels challenging. There was a risk that the gear cases would crack when pressing the wheels off. “Because we were able to inspect four units in the same period, we found them at varying stages of the same failure mode, enabling us to identify the point of initiation,”explains Andrew Baggaley, aftermarket manager with Dana’s service team.“The bearing pairs on the third line were being overloaded. The premature bearing wear had allowed misalignment with the preceding and following lines. Subject to how long the unit was continuing to run, the case bores and – ultimately – the second line wheel and fourth line pinion shaft, were also destroyed.” Further analysis revealed the main cause of the failures to be thrust overload, which could be seen from the asymmetrical bearing inner race wear which was always in the same direction. This had also worn the bores. “The gear case was set up and clocked on the horizontal borer,”Baggaley continues.“This confirmed that the general accuracy of the gear case was good – parts of both the bore pair were unworn, giving us the original centrelines. Accordingly, the line could be re- bored oversize with a counter bore added for a top-hat flange.” The low-speed shaft interface was also damaged internally where it grips the input shaft. Unusually, the pick-up had been caused during the original installation when the parts were mated.“While many high-value components had been essentially written-off, we were keen to find out whether refurbishment was still practically and commercially viable,”Baggaley recalls. An in-depth investigation resulted in a detailed report with costed recommendations. The rebuilding and refurbishment of the gearboxes was achieved using a combination of new and existing parts. All of the re-used parts underwent NDT (magnetic particle inspection) testing. Bore liners were fitted before installing bearings, and axial floats were set, checked and recorded. The gearboxes were spin-tested and their bearing temperatures monitored to ensure they plateaued at an acceptable level. The lubrication systemwas also refurbished. A large-area, high-efficiency filter and a remotely monitored particulate sensor were added. The refurbished gearboxes were returned ready-to-fit, demonstrating that refurbishment can be possible, even with extensive damage, and at less than half the cost of buying a new gearbox. Using reverse- engineered parts can cut costs and save time compared to using OEM components. “We have seen a number of these types of failure coming through to our servicing team and are confident in our approach which offers significant benefits to the customer,” Baggaley concludes.“For this project, as with all work undertaken by our servicing team, we focused on minimising disruption and delays. Some of the issues uncovered at the investigation stage were potentially very expensive for the customer, so our approach of re-using parts where commercially viable and practical, meant that we both kept costs to a minimum, as well as avoiding unnecessary delays with the manufacture or delivery leadtime for new parts.” n Four wind turbine gearboxes have recently been restored by UK experts following catastrophic in- service failures. The repairs, using a combination of new and existing parts, saved time and money compared to replacing the boxes entirely or relying on OEM parts. This photo shows the scale of the damage to the gearboxes before they were repaired Damage to the gearbox’s thrust bearings (shown on the right) was extensive