Wind power’s weakest link

How to maximize the performance of turbine gearboxes.

March 30, 2012   by Steve Gahbauer, Contributing Editor

Many components in highly engineered industrial gear drives have tight tolerance limits and an optimized geometry. It stands to reason such equipment needs special care. The successful operation and longevity of a gearbox is directly related to regular maintenance and proper lubrication.

Gear oil has two main purposes: it reduces wear and dissipates heat. Most gearbox failures can be attributed to the wrong type of oil or incorrect viscosity. Proper oil viscosity provides a film between meshing gear teeth. It’s very thin and prevents the gear teeth from metal-to-metal contact. Although oil suppliers provide product data sheets to their customers, the true test of gear oil is how it works in the system.

This is true for any type of gearbox, but proper lubrication and lube cleanliness are absolutely crucial to wind turbine gearboxes because they present challenges that are not found in other equipment. Gearboxes are the weakest link and the most expensive component in a wind turbine. Although relatively small, they’re very complex because of their low- and high-speed geometries.

The gearbox connects a low-speed shaft turned by the rotor blade with a high-speed shaft that provides the drive. A low-speed shaft is typically supported by two large bearings. Lubricants in the gearbox and bearings ensure the turbines operate effectively.

Aside from suitable viscosity, oils must be non-toxic and have: an extra high film strength; low pour point and resistance to corrosion; water tolerance; good cleansing and dispersing ability; resistance to foaming and they facilitate filtering; and provide protection against micro-pitting. Above all, they must be reliable and long lasting because access to wind turbine gearboxes is difficult and expensive. Maintenance and oil changes require a tall crane and, in many cases, easement rights on properties.

At a recent technical session of the Toronto Section of the Society of Tribologists and Lubrication Engineers (STLE), Michelle Graf, product manager of hydraulic and industrial gear oils for Lubrizol Corp. in Wickliffe, Ohio, presented an overview of wind turbine gearbox lubricants and performance parameters, starting with their composition.

Graf, also co-chair of the American Wind Energy Association’s Operation and Maintenance Working Group, said they should be 90% to 99% base oil and 1% to 10% additives. The base oil can be either mineral oil or polyalphaolefin (PAO). Base stocks need to be synthetic products because of the extended drain requirements of wind turbine gearboxes.
Synthetic base stocks operate within a wide range of temperatures and have very versatile viscosity characteristics. Additives must be formulated for extra-high pressure, anti-wear and rust inhibiting properties. Aside from reducing friction and dissipating heat, oils must also inhibit foaming. Balance is critical because so many components of a formulation are surface-active, making proper formulation a real balancing act.

Testing performance
There are various tests available to assure lubrucants perform effectively, including general industrial gear bench tests, mechanical and field tests, and specific tests performed by OEMs. The governing standard is DIN 51517-3. If you need more guidance, wind turbine suppliers, gearbox manufacturers and lubricant vendors are useful sources of information.

Proper selection, application and condition monitoring of lubricants maximizes service capability. Because oils and additives break down over time, lubricants need to be replaced regularly. Make regular oil-sampling part of a preventive maintenance program. Too little lubrication damages gearing; too much may cause churning and higher operating temperatures. Either results in decreased efficiency and reduced oil and seal life.

Check for leaks at shaft seal areas and make sure filters and strainers remove contaminants when the gearbox is equipped with a pressurized lubrication system. Replace them when changing the oil.

The choice of lubricant depends on turbine size, turning speed, transmission design, operating range and filtration, and on the type of lubricant delivery system. There are many things to consider, but solutions are available. For instance, SKF offers a wide range of items for wind turbine gears and bearings that are designed to boost performance and reliability. Among them are new products related to automatic lubrication systems.

Graf says other points to consider are anticipated oil life, compatibility with current lubes and warranty issues. It’s a good idea to discuss specific needs with suppliers and to look at the total cost versus only the cost of oil.

Keep the lubricant as clean and contaminant-free as possible to minimize downtime, reduce maintenance and repair costs; and increase machine availability.

Steve Gahbauer, an engineer and Toronto-based freelance writer, is the former engineering editor of PLANT. E-mail

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