The following article was provided by RBB Engineering, and is specific to the issues related to the oils utilized in gearboxes:
It is well known that gearbox failures represent one of the largest maintenance costs for wind turbine owners. Many of these failures are due to factors that are beyond the influence of the turbine owner, such as inherent design issues, manufacturing defects, and high load events. There is a significant contributor to wind turbine gearbox failures that is within the owners influence however, and that is the condition of the gearbox oil. A properly designed and implemented lubrication system maintenance program can result in significantly higher gearbox reliability and lower gearbox replacement costs. Lack of attention to this area however can result in lower gearbox reliability, and increased replacement costs. At its simplest, a lubrication maintenance program has four goals:
- To keep the oil within upper and lower temperature limits during operation
- To keep the oil particle count below specified limits
- To keep the water content below specified limits
- To keep the additive content and viscosity within specification
In order to understand why these goals are important, it is necessary to understand the functions that oil performs in a gearbox, and how it performs them. Functions of oil include the removal of heat, the prevention of corrosion, and debris transport, but the primary function is to provide a film that separates two mechanical bodies, such as gears or bearing components, which would otherwise be in direct contact. There are three distinct lubrication regimes, which are shown in Figure 1 in what is known as the Stribeck curve.
The figure shows the three regimes of lubrication, and how the coefficient of friction varies with the lubrication regime. The most desirable regime from a reliability perspective is known as hydrodynamic lubrication, and in gear and bearing contacts there is a specialized type of this known as elastohydrodynamic lubrication (EHL). When operating in this regime, the two surfaces are completely separated, and the entire contact load is supported by the oil film. The second regime is known as mixed film lubrication. In this regime, the surfaces are not completely separated, and some of the load is carried by local high spots on the surfaces known as asperities, while the balance of the load is carried by the oil film. Low speed stage gears and bearings in wind turbine gearboxes often operate in this regime, and rely on special oil additives to create chemical films on the contacting surfaces, and prevent excessive wear. We will discuss the role of additives in more detail later in this article. The third regime is known as boundary lubrication. In this regime, none of the load is carried by the oil film. There are several factors that determine the thickness of the oil film, including the applied load, speed, and geometry of the contacting surfaces, but the most important factor is the viscosity of the oil. Viscosity is a measure of the resistance of a fluid to flow. The higher the viscosity of the oil, the thicker the film that separates the contacting surfaces. The viscosity of oil is not constant, it varies with temperature. The higher the temperature, the lower viscosity. It is for this reason that one of the goals of a lubrication maintenance program is to keep the oil temperature well within the gearbox manufacturer’s maximum temperature specification. Operating at higher temperatures results in lower viscosity oil, and lower film thickness. If the reduction in viscosity is large enough to change the lubrication regime in any contact in the gearbox from hydrodynamic to mixed-film, or from mixed-film to boundary, the rate of wear in the contact will increase, resulting in a decrease in gearbox reliability. An additional consequence of high oil temperature is an accelerated rate of oxidation of the oil, and a reduction in its useful life. The temperature of the gearbox oil should be carefully monitored. Indications of abnormal oil temperature should be investigated and the root cause identified and corrected, even if the gearbox is operating at oil temperatures below the manufacturers fault threshold. Keeping the oil above a minimum temperature is also important to ensure that lubricant is flowing as needed whenever the gearbox is rotating. If the oil temperature is too low, the viscosity of the oil can increase to the point that lubricant will not flow to some areas of the gearbox. To prevent this from occurring, the oil heating system must be kept in good operating condition, and turbine cold startup guidelines established by the manufacturer must be followed.
The second element of the lubrication maintenance program involves maintaining the cleanliness of the oil. There are a number of sources of contaminants of oil, including dust and sand from the atmosphere, and internally generated particles, primarily wear debris from gears and bearings. Particles can also be introduced into the gearbox during oil top off, if the oil is not sufficiently filtered before it is introduced into the gearbox.
Figure 2 shows photomicrographs of oil samples taken from a barrel, a tanker, and a mini-container, and compares them to the requirement for oil introduced into the gearbox. The presence of particles, particularly wear debris from gears, has a very harmful effect on bearing lives. The thickness of the oil film developed in the EHL regime is very thin, on the order of 0.25 to 1.25 µm. Particles larger than this that pass through the contact zone can result in dents in bearing surfaces. The dents have raised shoulders that protrude beyond the lubrication film thickness, and contact asperities on the mating surface. These contact events cause the protruding material to fail, and create the beginnings of a point surface origin (PSO) macropit, leading to premature failure. The presence of metallic particles in the oil can also result in accelerated chemical deterioration of the oil by acting as a catalyst for certain harmful chemical reactions. In order to prevent failures of this type, it is imperative that the gearbox lubrication system have a well operating filtration system, including both inline filtration and offline, or kidney loop filtration. Inline filtration systems typically have two stages, a fine filtration filter that removes particles 10 µm and larger, and a coarse filtration filter that removes particles 50 µm and larger. Offline filtration systems are particularly important if the lubricant pump is mechanically powered. If the lubricant pump is electrically powered, the lubricant flow rate is independent of gearbox rotational speed, resulting in increased effectiveness of the inline filtration system.
Figure 3 shows a cutaway view of a typical two stage gearbox filter. The filtration circuit is usually designed with a bypass around the fine filter, to allow for low temperature startup conditions, and to prevent a dirty filter element from restricting oil flow to the gearbox. When the filter is in bypass, a switch is made, and the turbine operator is notified of the condition. If the indication is made due to a dirty filter, it is important that the filter be replaced with a clean one as soon as possible. Best practice dictates that the filters be changed at every service interval, or upon a bypass indication, whichever comes first. The coarse filter is not capable of keeping the oil clean enough to prevent damage to the bearings. The offline filter is typically designed to remove even smaller particles, usually 3 µm and larger, or water, or both. Offline filters operate whenever the gearbox oil temperature is above a threshold value.
The mention of the ability of some offline filters to remove water from the oil brings us to the next element of a lubrication maintenance program, keeping the water content of the oil as low as practicable. This may well be the most important of the four elements. The presence of excessive amounts of water in gearbox oil has a number of consequences, all of them bad for gearbox reliability. The most damaging consequence of excessive water content in oil is premature failure of bearings. The exact mechanism of damage is unknown, but one theory is that the water provides a source of hydrogen which results in embrittlement of the bearing steel. According to the Timken Products Catalog, a bearing that operates in oil containing 1,000 parts per million (PPM) of water has a reduction in life of nearly 70% compared to a bearing that operates in oil containing 100 PPM of water. Reliably keeping water content at that level can be very difficult, and may not be attainable with some oils or in some geographic locations, but the fact remains that the lower the water the content in the oil, the longer the expected life of the gearbox bearings. Large amounts of water in the oil can bind with some of the additives used in the oil, causing them to fall out of solution. The reduced level of additive in the oil reduces the intended effect of the additives, and can clog lubrication orifices and filters. Free and emulsified water in the oil can also result in damaging corrosion on gears and bearings. Water can enter the gearbox oil either from exposure to rain or some other source of water, or from condensation of the water content in the air. Adequately protecting the gearbox from the environment and maintenance of seals is the key to preventing the ingress of water from the first means. Preventing water from humid air from entering the gearbox oil can be accomplished by using a filtered breather equipped with a desiccant, and regularly changing the desiccant when it can no longer absorb water, as indicated by a change of color in the desiccant.
A regular oil monitoring program can provide the operator with valuable information on the condition of the gearbox oil, including water content, cleanliness, viscosity, and additive levels. Additives have been used in oil since the 1920’s. Some additives result in new properties of the lubricant, others enhance existing properties, and still others reduce the rate of undesirable changes to the oil as it ages. Additives are used to prevent scuffing of gears, reduce the rate of wear of bearings, prevent micropitting, provide corrosion protection, prevent foaming, and reduce the change in oil viscosity with temperature. These properties are all very important to the reliable operation of a wind turbine gearbox, and appropriate amounts of each additive must be present in the gearbox oil in order to achieve the performance benefits they can provide. Each oil manufacturer uses a different combination and concentration of additives, and provides guidance on acceptable ranges of each additive for its oil. Regular monitoring of the additive content of the oil can provide a good understanding of how well the oil is performing its intended functions. If the additive content is not within the manufacturer’s specification, the performance of the oil will be reduced, and the reliability of the gearbox may be affected. When selecting an oil analysis company, it is important to choose one that has extensive experience in the wind industry, as the requirements of a wind turbine gearbox differ from those in other applications. Also, since establishing trends in oil analysis results is often more important than measuring the absolute value, it is a good practice to use the same facility for all of the testing, to reduce process and equipment induced variability.
Effort spent ensuring that gearbox oil temperature stays within normal operating limits, is clean, has the lowest practicable water level, and meets all of the manufacturer requirements for additive content, viscosity, and other parameters will result in increased gearbox reliability. The ROI on this effort is large, and represents an opportunity for most wind farm operators to improve the performance of their projects.
The proceeding article was provided by RBB Engineering