Wind turbine gearbox lubrication requires special consideration

Industrial gearboxes are highly engineered machines. Many components in gear drives have tight tolerance limits and an optimized gear geometry. It stands to reason that 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 improper lubrication. If the wrong type of oil or incorrect viscosity is used, gear failures are sure to happen. But this is preventable. Proper oil viscosity provides a film between meshing gear teeth. It is very thin and keeps the gear teeth from contacting metal-to-metal. Oil suppliers provide product data sheets with useful information, but the true test of a 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 these present challenges that are not found in other equipment.

Gearboxes are the weakest link and the most expensive component in a wind turbine; they are expensive to repair and replace. They are relatively small but very complex with their low- and high-speed geometry. A gearbox in a wind turbine connects a low-speed shaft turned by the rotor blade with a high-speed shaft that drives the turbine. The low-speed shaft is typically supported by two large bearings. Lubricants in the gearbox and bearings play a vital role in ensuring that a wind turbine operates effectively.

Aside from suitable viscosity, wind turbine gearbox oils must have an extra-high film strength, low pour point and resistance to corrosion, good water tolerance, good cleansing and dispersing ability, they must be resistant to foaming and facilitate filtering, provide protection against micro-pitting, and they must be non-toxic.

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 to properties.

At a recent technical dinner meeting of the Toronto Section of the STLE (Society of Tribologists and Lubrication Engineers), Michelle Graf, the product manager of hydraulic and industrial gear oils for the Lubrizol Corp. in Wickliffe, OH, presented an overview of wind turbine gearbox lubricants and performance parameters, followed by a discussion of tests needed to adequately evaluate lubricant performance. Graf is also co-chair of the American Wind Energy Association’s Operation and Maintenance Working Group.

Graf spelled out the specific requirements for wind turbine gearbox lubricants, saying that they should be 90-99% base oil and 1-10% additives. The base oil can be either mineral oil or PAO. The base stocks need to be synthetic products because of the extended drain requirements of wind turbine gearboxes.

The potential benefits of synthetic base stock are that they can operate in a wide range of temperatures and that they 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, wind turbine gearbox oils must also inhibit foaming. Formulation balance is critical because so many components of a formulation are surface-active. Thus, proper formulating is a real balancing act.

There are various key tests available to assure appropriate and effective performance of wind turbine gearbox oils. They include general industrial gear bench tests, mechanical and field tests, and specific tests that OEMs can perform. The governing standard is DIN 51517-3. Wind turbine suppliers, gearbox manufacturers and lubricant vendors are useful sources of information.

Proper selection, application and condition monitoring of wind turbine gearbox lubricants is essential for achieving maximum service. Because oils and additives break down over time, it is essential that gearbox oil is replaced regularly. An oil sampling program is effective in monitoring gearbox health.

Checking oil levels regularly should be part of a preventive maintenance program. Too little lubrication can damage gearing; to much may cause churning and higher operating temperatures. Either result in decreased efficiency and reduced oil and seal life. It is important to check for leaks at shaft seal areas and to make sure that filters and strainers remove contaminants when the gearbox is equipped with a pressurized lubrication system. Replace them when changing the oil.

The choice of wind turbine gear 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 turbine performance and increase reliability. Among them are new products related to automatic lubrication systems.

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

Above all else, the main issue is to keep the gearbox lubricant as clean and contaminant-free as possible. This minimizes downtime, reduces maintenance and repair costs, and increases machine availability.

MRO

Steve Gahbauer is an engineer and a Toronto-based freelance writer.