Wind turbines face harsh environmental conditions that are even more challenging for offshore units. Proper selection of specialty lubricants that allow wind turbines to run with maximum availability by avoiding non-scheduled shutdowns is one of the key contributors to achieving fast return on investment.
On the other hand, operating conditions and long maintenance intervals can stress most lubricants to the point that they may interfere with reliability before they can be replaced. With regard to the various rotating parts of wind turbines (e.g., gearbox, bearings, brakes and shrink discs), advanced technology lubricants have been demonstrated to contribute to increased component reliability due to their enhanced ability to perform at temperature extremes, resist extreme loads, and protect against corrosion and mechanical wear.
An understanding of the key features and benefits of modern lubricants – and how to apply them to best realize those benefits – can help to extend lubrication intervals, improve equipment reliability and maximize total cost of ownership.
The best selection and application of a lubricant is ensured by understanding application details, including the component to be lubricated, the type(s) of surfaces in contact (metal-to-metal, metal-toplastic, etc.) and the component’s relative motion (sliding vs. rolling friction). Environmental conditions and one’s long- and short-term reliability goals – such as extended lubrication intervals, sealing compatibility, cold/hot temperature operation or elimination of corrosion – also should affect choice of lubricant.
Knowledge of modern lubrication technology enables the prioritization of goals and will allow the best chance to achieve one’s objectives. Application details can be summarized by the acronym LETS (load, environment, temperature, speed).
Load is an estimation of the amount of force or stress under which the component to be lubricated is operating. Bearings are sometimes referred to as operating under light, moderate or heavy loads. A common rule is that bearings operating at or above 20 percent of their rated load-carrying capacity are considered heavily loaded. Shock or vibration and frequent start-stop cycles are clues that a bearing application is under stressed load conditions.
Lubricants chosen for high or stressed load applications usually contain solid lubricants and/or high base oil viscosity. Environment asks the end user to consider the effects of the general climate and surroundings. Among the variables are seasonal and daily temperature fluctuations; atmospheric contamination, including dust, dirt or fly ash; wind; salt spray; and humidity.
Substations in warm areas will face different environmental challenges than those in Canada or north China. Offshore environments are more corrosive than onshore areas. Many lubricants used on wind turbine bearings are selected due to their good low-temperature performance; however, in many climates, that benefit is of limited usefulness. Lubricants intended for cold temperature service often are formulated with low viscosity base oil to counter the effect of temperature on the viscosity of oil and the consistency of grease. In some cases, what works well in cold environments may not work well in hot environments.
Temperature considers the actual temperatures to which the component and lubricant will be exposed, from start-up to normal operating temperature; this is not the same as ambient temperature. An example of temperature would be the difference between ambient air temperature (environmental temperature) at a substation and the actual temperature inside a gearbox at that substation. There may be a significant difference between these two temperatures.
Components To Be Lubricated
In a perfect world, it would be easy to lubricate an entire device with one lubricant. In reality, different bearings, gearboxes, slides, threaded connections, orings and other components that make up a device may not all be best served with one lubricant.
Different types of bearings and gears have been designed to have various strengths for different kinds of service, and they often must be lubricated with different lubricants to help ensure they can deliver their performance benefits. Spindle bearings are lubricated with different lubricants than are tapered roller bearings, and worm gears are lubricated with different lubricants than are spur or helical gears.
Keep LETS in mind as it relates to each component in the device. Certain components are more sensitive to some LETS details than others. Each material of construction may have different compatibility with various lubricants. Many elastomeric materials may shrink or swell to different degrees, depending on the base oil of the lubricant with which they are in contact. Components made with copper, brass or silver may react with or be corroded by lubricants with active sulfur chemistry. Sulfur-based additives often are used in industrial gear oils and some greases.
The act of lubrication is no small detail. There are many types of lubricants that offer higher chances of success in equipment being installed in wind turbines. The equipment manufacturer and end user begin to affect reliability of their equipment as soon as they select lubricants with which to service it. These limitations are not evident or often thought of upon initial lubrication or servicing of the device; rather, they present themselves over time as the lubricant is stressed by environmental and static conditions. Better reliability will come from the use of lubricants that do not change (degrade) over time and that are properly applied to the components they are intended to protect from wear and corrosion. Knowledge of various lubricants and their respective strengths and weaknesses will serve design engineers and maintenance professionals well. Better reliability means higher profitability for the end user and speaks to the quality of equipment sold by various equipment manufacturers.