Using data to extend main bearing life through predictive maintenance
Recognizing the issue that main bearings, unlike the gearbox, do not have great attention paid to the lubrication, with no filter for removal of metallic particulates, no dryers or desiccant breathers labour and no lubricant change at 5 year intervals (typical), Romax InSight developed a process for extending bearing life.
Now flushing around 50 bearings annually, Romax continues to optimize the predictive maintenance methodology. Thorough flushing of the bearing to remove all the grease and repacking with fresh grease resets the lubrication conditions. Data gathered from flushed bearings has consistently shown significant reduction in metallic debris, water content, temperature and vibration; extending bearing life significantly.
Understanding the failure to catch it earlier
Spherical roller bearings are selected primarily due to cost and their ability to handle a large amount of radial load and misalignment. However, they have been found to have limited life in many turbine designs. The failure mode is complex, but a key factor is that the thrust load is supported principally by the downwind bearing; the downwind rollers often skew during rotation as well as sliding. This raises surface stresses and leads to surface initiated micro pitting. The debris generated causes bearing and cage wear, debris denting and early life failure as the bearing spalls out.
There are a number of commonly used industry practices for managing main bearing health, which include alarms that are triggered when certain factors arise. Once an alarm is sounded, by vibration or temperature, the bearing is inspected to confirm the damage highlighted in the data.Historically the options for life extension at this stage have been limited to a time when a crane can be arranged. If the bearing is in poor condition, it is common to pump in additional grease to buy some time (purging). Combined with derating, the bearing can be limped along without tripping the turbine on temperature as often, whilst repairs are arranged. When the bearing is overheating, typically with cracking on the raceway shoulder at bot- tom dead centre, it is prudent to shut down. Both examples, inefficient purging and waiting time for repairs on inactive turbines, can cost production revenue significantly.
Early detection triggers preventative maintenance
Preventative maintenance by grease flushing extends the bearing life. To get the maximum benefit,the early detection of poor lubrication conditions or early stage wear and spalling is required. A periodic grease sampling and analysis pro- gram provides insight into the lubrication quality. For vibration data, Romax has developed special algorithms that provide 12-18 months on the initial spalling on rollers and raceways – this is a critical point where the damage accumulation in the bearing accelerates. Figure 1 provides a case study with one year’s warning on bearing failure, when the first debris dents where appearing on the inner race.
The same Cloud-based software shows that temperature warnings are very late stage, even using an advanced algorithm that corrects for environmental fluctuations and tracks deviations from the fleet mean. The software allows users to pull all forms of useful data into decision making: SCADA, vibration, particle counters, lubricant analysis, inspection and maintenance data. Point B is a good time to use this data to schedule grease flushing.
Combining technologies for best practice Combining technology for early detection and bearing life extension allows for best practice management of main bearing health and reduces the cost of failure. A predictive maintenance program incorporates assessing principally the vibration and grease data to develop a priority list of bearings that require flushing. Then batches of bearings can be flushed during convenient service periods to extend life. To perform the flushing, Romax developed a special flushing machine that is installed and allows spraying and slow rotation of the bearing to remove all the grease. For severely damaged bearings, larger pieces of raceway can be removed manually beforehand to prevent ingress further into the bearing, typically these can be found at the rear of the bearing near bottom dead centre. The process allows for >95% removal of the old, contaminated grease. An endoscopic inspection at this stage confirms the removal and allows complete access to all positions on both the upwind and downwind side to fully quantify the damage to the bearing.
Results from flushing
Introducing a new maintenance practice to the wind industry requires significant trial and evaluation by owners. During 2014-2016, Romax conducted trials with over 10 owner/operators, including gathering and examining data to investigate the effect on lubrication conditions, temperature and vibration. Figure 2 provides some typical examples from bearings that had started down the road to failure, but were in earlier stages. If the grease is laden with iron and water, replacing it resets it to fresh grease. After 104 days running the iron concentration was still low. The temperature data is for a bearing that was degraded enough to be overheating, here the flushing extended life more than one year, allowing repairs to be delayed and downtime minimized.
With this process proving successful, Romax is looking forward to using flushing to allow in-field trials of different greases and sees this as the next area where life extension (e.g. use the best per- forming grease) could be achieved.