As the wind industry continues to develop into a key provider of electricity around the world, operators and manufacturers are demanding more from their machines. This continual evolution of wind has led to larger turbines and larger wind farms. As output increases, so do the acting forces and importance lubricants play in the reliability of these powerful machines. Lubricants ensure the efficient operation of these facilities and are an integral part of the system. As a result, lubricants must be accounted for in the design and maintenance in this demanding field.
Wind turbines have a unique list of requirements from lubricants. The main task of the lubricant is to ensure reliable operation of the machine elements. The lubricant should also meet demands for service life, load carrying capacity and thermal resistance over an extended period of time. Meeting these demands with taller and bigger turbines produces an increasingly challenging environment. Important lubrication points in wind turbines include the main gear drive, yaw system gear, main and generator bearing, pitch adjustment unit, and nacelle slewing ring.
Changes to the design of wind turbines create additional demands on lubricants that are used in the wind power industry. As interest in the industry grows, engineers work to improve the efficiency and the output of wind turbines. Gearbox designs contain more equipment designed to produce more work, and more work leads to the generation of more heat in the gearbox. As a result, lubricants must function at higher operating loads while helping to reduce temperatures in the gearbox.
Mineral oils cannot meet these demands, and operators of wind power plants have turned to synthetic oils to satisfy these demands (improved thermal resistance, better viscosity characteristics, product longevity and longer machine component life). Different base oils (polyalphaolefin, polyglycol or rapidly biodegradable ester) are used to formulate these gear oils. Today, however, many basic synthetic oils cannot meet the new requirements created by changes in the industry. As a result, operators are turning with increasing frequency to new, higher-performance synthetic oils.
The lubricant industry is responding with specialty products that meet and even exceed the standards set before them. These new lubricants offer high thermal resistance, resistance to oxidation, a more consistent viscosity at rising or falling temperatures, lower friction coefficients, high wear protection for bearings and gears, good load-carrying capacity in bearings and gears, and low residue formation. They also offer extended service intervals, resulting in more economical operations. Developing these lubricants requires knowledge of additives – their chemistry, which additives to use, what combinations to use them in – and of base oils. The purer the molecular structure of the base oil, the better the lubricant.
Wind lubricants are designed for very specific applications, such as the generator bearing in a wind turbine gearbox. The service technician must get to the top of a 200- to 300-foot tower while the product is raised to the top of the tower in a pail.
New tests and standards
Like the traditional synthetic oils that preceded them, new, high-performance synthetic oils are subject to the tests of original equipment manufacturers (OEMs) and must meet a number of universal standards. For example, industrial gear oils are classified in accordance with Deutsches Institute for Normung (DIN) 51 517. Part 3 of this standard defines the requirements for gear oils that are exposed to high loads. In addition to the usual tests on viscosity, pour point, foaming characteristics, and steel and copper corrosion, the scuffing load characteristics of the oils are determined in the Gear Research Center (FZG) scuffing load test. DIN 51 517, Part 3, stipulates a scuffing load stage greater than 12 for gear oils.
Because gear oils should also be suitable for lubricating the rolling bearings in the gearbox, the standard DIN 51 517, Part 3, also contains the FE 8 rolling bearing test rig developed by the rolling bearing manufacturer FAG. The FAG FE 8 test rig can be used to assess the anti-wear properties of an oil and its effect on the rolling bearing service life. In this test, the wear of the rolling elements should not exceed 30 mg.
The assessment of gear oil performance for wind turbines also includes tests that measure scuffing load resistance and micro-pitting resistance. A test developed by FZG measures anti-wear properties of the lubricant at low gear speeds as the planetary gear stage is run at the lowest speed. In this test, better-performing lubricants fall within the low-wear category.
Gear efficiency is determined to a large extent by the friction characteristics of the lubricating oil. The friction coefficients of different base oils can be seen in the result of the FZG test rig. Today’s gear oils can reduce temperatures by as much as 68°F (20°C) and power losses by as much as 18% when compared to standard gear oils. Cutting power loss leads to more power being delivered to the rotor, as well as increased profits simply by increased performance of the lubricant.
As the industry continues to develop by finding more efficiencies, lubricants are also contributing to cost-savings through better power transfer and component reliability. As a result, the value placed upon high-performance lubricants continues to increase. What was once a commodity selected on the basis of price is now considered by many as a machine element, carefully specified in much the same way gears and other components are specified.
Jestin Hulegaard is wind account manager at Kluber Lubrication. He can be reached at firstname.lastname@example.org.