Modern gearboxes provide greater reliability to the wind-energy industry thanks to design and operations improvements, such as bearing selection, design process, manufacturing quality, transportation, lubrication systems, and end-of-line testing. Reliability issues will never completely go away, but keeping on top of issues should reduce maintenance costs at aging wind parks, with problematic models, or gearbox subcomponents and batches.
Currently the common failure modes are parallel-stage bearing axial cracking, planet-bearing fatigue (wear out), and gear-tooth cracks or liberation. Most often failures are caused by micropitting, heat treat type or quality, harsh loading, and poor lubrication. The root cause is often not just one source. Many parallel stage issues can be repaired uptower, where planet stage repairs typically take place downtower.
Gearbox life can be extended with good maintenance practices. For example, a supply of dry, clean lubricant affects bearing life significantly. Ensure filters are changed regularly and an oil sampling and analysis program is in place. Condition-based maintenance can be applied to the oil – flush and change in accordance with quality requirements. Watch out for water in oil: breathers, sealing, oil, and climate conditions are all influencing factors.
Good filtration can extend both the life of the oil and the gearbox. Offline filters, typically 3 micron, can be retrofit if necessary. These have a lower flow rate, running in a kidney loop, and effectively reduce wear from debris. Other options are available to ensure dryer oil.
A routine maintenance schedule tends to reduce reliability issues. If a large wind farm is having two to four issues per year with main bearings and gearboxes, scheduling repairs concurrently saves on crane out. Knowledge that the failure is impending can also reduce downtime. Cliché alert: knowledge equals money, however, when dealing with reliability issues you need to be losing money first and then save money through good management.
The condition of your assets is the first place to start. Temperature and oil analysis should be a routine practice. Both will inform partially about the condition of the gearbox. Temperatures will rise when bearings are significantly damaged, improperly set, or the cooler is not functioning well. Be careful though, often it tells you nothing as far as bearing cracks. Make sure to check how much debris is being generated by recording the filter condition – is it full of metal? Record a picture of the swarf magnet as well. This low-cost activity allows a short list for borescope inspections – also not expensive when attributed to a short list – 10 to 20% population.
Lastly, determine what other approaches are useful. Some damage is hard to find using these basic approaches, such as parallel-stage bearing cracks and planet-bearing damage. Portable and permanent vibration monitoring costs a bit more and is a large capital cost retrofit but can effectively identify damage well ahead of a failure.
By: Ashley Crowther, Global VP – InSight, Romax Technology, Inc.
Filed Under: Components, Featured, Gearboxes