Gearbox oil leaks present a special challenge in an outdoor green-power application. Unfortunately, they have been so prevalent, leaks are defined by their degree. Drips, leaks, seeps, and weeps all contribute to the costs of maintenance and to the environment in which we work, both in nacelle and out.
The most common cause of gearbox oil leaks: excessive oil
Modern wind-turbine gear designs provide a necessary power-to-weight ratio. But relatively small housings alone lack the capacity to reject the heat generated in the process of converting low speed high-torque rotational energy from the rotor into energy for the generator. Summer heat, high humidity, and high altitude all reduce a system’s ability to cool.
These gearboxes have internal plumbing with orifices to control pressure and nozzles to deliver cool, clean oil to bearings and gear meshes. An integrated oil pump moves oil from the sump on the high-speed side, through a filter and a cooler as directed by the manifold block. The oil level in the gearbox is a delicate balance between enough oil to fill the entire lubrication circuit during operation and the lowest oil-sump level.
Many conventional seals that wear over time due to rotating-shaft contact can be expensive to replace. A benefit of a low sump oil level system is the choice to use labyrinth seals. These are non-contact seals that have no wearable parts. Once installed they rarely need replacing. An improper installation will be detected during testing.
Labyrinth seals work best during operation but are susceptible to leaking when flooded with oil, especially at rest. As the oil flows between the rotating and stationary staggered rows, it loses pressure to turbulence and drops back into the gearbox. When the oil level is too high, the excess leaks out.
In this case, first consult your owner’s manual. It has been my experience that the OEM recommended oil level is exactly halfway between the minimum and maximum (usually not more than two inches between), and should be viewed on the manual sight glass after 30 minutes at rest. Generally, no oil can be viewed in a sight glass during operation, even though this might be several inches below the minimum line. Critical measurements are oil temperatures at various sensors, and oil pressure before and after the filters.
Leak cause #2: choked gearbox
As the turbine begins operation, the gearbox warms and the air inside it expands and escapes through an air breather located on the top. As it cools after turbine operation, a vacuum pulls in air from the nacelle. Some air-breather filters made of paper elements are intended to catch particles in the air while others with a dry desiccant remove moisture.
If the filter becomes clogged and loses the ability to pass air, the gearbox will breathe through the labyrinth seals and expel oil along the way. One common cause of a clogged air filter is oil saturation from splashing gearbox oil. When this occurs, both have to be replaced. As a possible alternative, a small plug (not dissimilar to the labyrinth) can be inserted which lets air pass, but not oil.
One issue specific to dry-desiccant air breathers is the number of ventilation holes opened during activation of the filter. Opening a few of the ventilation holes (letting air flow through the filter) activates these filters. Not opening all ventilation holes may extend filter life. The unfortunate consequence can be a leaking gearbox, which may need more than a few small holes to breathe.
It has been my experience that opening all of the holes on these types of filters will help minimize gearbox leaks at ideal oil levels. Evaluate your situation and compare the costs of potential reduced filter life against cleaning oil leaks. WPE
Story By: Paul Baker, director of business development, Frontier Pro Services, www.frontierpro.com
Filed Under: Gearboxes, O&M