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. However, one significant contributor to failures that an owner can control is the condition of the gearbox oil.
A properly designed and implemented lubrication system main-
tenance program can result in sign-
ificantly higher gearbox reliability and lower gearbox replacement costs. Lack of attention to this area, though, can lead to failures and unnecessary expenses. 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; and
- To keep the additive content and viscosity within specification.
In order to understand why these goals are important, it is necessary to know the many functions that oil performs in a gearbox. Oil’s roles include the removal of heat and prevention of corrosion, but the primary function of oil in a gearbox is to provide a film that separates two mechanical bodies, such as gears or bearing components, that 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). 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. In the third regime, known as boundary lubrication, 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 the 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 manufacturer’s fault threshold. Keeping the oil above a minimum temperature is also important to ensure that the 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 the 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 oil contaminants, including dust and sand from the atmosphere, as well as 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 these 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 edges that also 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 macro-pit, 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 and offline filtration 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.
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, in order to allow for low-temperature startup conditions and 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 there is a bypass indication, 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 50 µm 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 work continuously and can achieve a fine level of filtration due to their very low flow rates, typically around 1 gallon per minute.
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 below a threshold value. This may well be the most important of the four elements. The presence of excess water in gearbox oil has a number of consequences, and all of them are very 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 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, but the fact remains that the lower the water 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 the solution. The reduced level of additives in the oil reduces the intended effect of the additives and can clog lubrication orifices and filters. Free water in the oil can also result in damaging corrosion of 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 condensation from entering the gearbox oil can be accomplished by using a 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.
We have discussed the effect of water content, cleanliness and oil viscosity on the performance of the gearbox, but we have not yet discussed the types of additives typically used in gearbox oils and how they function.
Additives have been used in oil since the 1920s. 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 for the proper performance of a wind turbine gearbox, and appropriate amounts of each additive must be present in the gearbox oil in order to receive 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.
While there is little that the wind turbine owner can do to control the rate at which additives are depleted from the oil (other than keeping water levels low), regular monitoring of the additive content of the oil can provide a good understanding of how well the oil is performing its intended functions.
Effort spent ensuring that gearbox oil stays within normal operating limits for temperature, is clean, has low water content and meets all of the manufacturer requirements for additive content and viscosity will result in increased gearbox reliability. The return on investment for this effort is large and represents an opportunity for most wind farm operators to improve the performance of their projects. w
Marketplace: Lubrication & Filtration
Good Lubrication Practices Can Reduce Gearbox Failures
By Rob Budney
While there are many issues beyond the control of owners and operators, gearbox filtration should not be one of them.
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