While reliability challenges continue to face the installed wind turbine fleet, newer multi-megawatt wind turbine designs with larger blade diameters and tower heights, increased pitch control and greater focus on offshore siting, continue to impose new challenges to bearing and lubricant manufacturers alike.

The selection of greases for wind turbine bearing lubrication requires an understanding of the design and typical failure modes for the bearings, lowest ambient operating temperatures, servicing frequency, method of application and compatibility of factory fill with service fill greases, to name a few considerations. There are currently two approaches to the development of greases for main bearing, blade/pitching and yaw bearings and generator bearings, namely development of a multi-purpose grease suitable for lubrication of all applications, or greases designed specifically for the given application.

Each approach has its pros and cons, and the balance between ultimate bearing reliability and ease of use and ordering for service technicians has to be weighed carefully.

Adjustment of the blade angles of a wind turbine is an important practice, not only for safe operation of the wind turbine (especially in the presence of high winds), but also to control the power output of the turbine. Adjustment of the blades is via blade (or pitch) bearings which are slewing rings – typically double raced four-point contact ball bearings bolted to the blade hub. Such bearings are exposed to very high dynamic loads from the blades. The blade bearing application not only involves significant vibration, but also minimal rotation for the rolling elements in the bearing. While vibration and minor oscillations result in a major failure mode (that of false brinelling), newer wind turbine designs involve more active pitching. This lubrication system and the environment in which it operates require specialized greases designed to overcome multiple lubrication challenges.

Irrespective of the lubrication application in a wind turbine (whether it be main gearbox oil, hydraulic fluid or grease), one of the most critical requirements is the ability of the lubricant to function over a wide operating temperature range, from extreme climate (Arctic) conditions to high ambient temperature. Temperatures from <−40ºC to >50ºC are not uncommon, often with wide seasonal and daily swings. It is typical for today's wind turbine designs to be fitted with centralized greasing systems of progressive or single-line type with narrow diameter feeder lines which distribute the grease to the lubrication points. (This requires that greases have excellent low temperature pumpability.)

Note: In the Kesternich Flow Pressure Test (DIN 51805), the test grease demonstrated excellent fluidity even at lower ambient temperatures, a critical requirement in blade bearing grease system incorporating narrow diameter lines from divider blocks to the bearing gives. In this test, the pressure required to force a strand of the grease in a continuous stream out of a test nozzle is determined. Pressures should remain low even at sub-ambient temperatures, ideally <1400 mbar. The test blade bearing grease developed for this application gave values of only 375 mbar even at -45ºC, with values of <1400 mbar as low as −60ºC. Performance in the Lincoln ventmeter test further validates the excellent low temperature pumpability.

Performance characteristics

Wind farms are commonly located in coastal areas and increasingly offshore, which results in an increased potential for (salt) water ingress. As a result, greases for wind turbine lubrication must be able to demonstrate excellent rust and corrosion resistance, even in a salt water corrosion environment.

The SKF Emcor test (ASTM D6138) Corrosion Preventive Properties of Lubricating Greases Under Dynamic Wet Conditions is an industry standard test used to determine the anti-corrosion properties of greases when exposed to waters of varying quality in contact with double row self-aligning ball bearings. At the end of the test, the bearing raceways are examined and degree of corrosion rated against a defined rating scale from 1 to 5, with 0 being no corrosion and 5 representing heavy corrosion with corroded areas covering more than 10% of the running track surface. When run with distilled water the test blade bearing grease will give a rating of 0, and 1 or less than 1 with synthetic sea water, indicating excellent corrosion-preventive properties.

The blade bearing application involves not only significant vibration, but also minimal rotation for the bearing members. A common failure mode for blade bearings is that of fretting wear or false brinelling, a wear mechanism which can be addressed through careful grease design. This type of wear is a result of the very slow and infrequent oscillations in a grease lubricated pitch bearing. The oscillatory motion is insufficient to enable or promote formation of an effective lubricating film between the rolling members and raceway. The test blade bearing grease has been designed to provide increased resistance against false brinelling and to prevent wear at the rolling contacts even under high and variable load conditions.

Standard test protocols

One of the world's leading manufacturers of blade bearings for wind turbines, Rothe Erde, specifies that greases for use in their blade bearings must pass the Rothe Erde/IME Ripple & Corrosion Test (FE 61001).

This test was developed specifically by the Institut Fuer Maschinenelemente und Maschinengestaltung, Aachen University, Germany, to evaluate the performance of blade bearing greases and their tendency to prevent false brinelling due to oscillation and wear. The test grease is packed into a FAG four point contact roller bearing through which 1% NaCl solution is passed, and the bearing is then cycled through 1 million cycles of alternating 70 kN axial load. At the end of the testing cycles, the bearings are removed and the ripple depth on the bearing raceway measured. Furthermore, a visual assessment of the degree of corrosion is conducted with a rating on a scale of 1 to 5, with 1 being the best and last corrosion and 5 the worst and highest level of corrosion. The test blade bearing grease gives a corrosion rating of 1 and ripple depth maximum of 2 m and mean ripple depth of <1 m, respectively showing excellent protection against this wear mechanism.

The test blade bearing grease has been run through a number of other industry standard tests to demonstrate its excellent performance in the area of resistance to wear in oscillating and fretting wear type conditions. These include the SNR FEB 2, or false brinelling rolling bearing grease tester, and the Fafnir Fretting test (ASTM D4170). Again, the test lubrication performed well in these trials. Performance in the anti-wear and load carrying (extreme pressure, EP) Four Ball Method wear prevention (ASTM D2266) or the extreme pressure (ASTM D2596) tests, and FAG (Schaeffler) FE8 DIN 51819 Roller bearing test further demonstrate the excellent performance of this grease.

Longevity requirements

While the temperatures to which greases are exposed in a wind turbine are far from the highest in the industry, long grease life is certainly a feature wind farm operators are looking for when selecting a grease and to ensure optimal reliability for their assets. The Shell test grease utilises synthetic base oils instead of mineral oils and as such provides a number of performance benefits, including extended oil life (oxidation stability), improved low temperature pumpability and improved wear protection at higher operating temperatures thanks to lower friction coefficients.

When developing greases for demanding applications – such as that of blade bearings in wind turbine application – a deep understanding of the bearing design, operating conditions (load, vibration, operating temperature) and failure mechanisms are key to providing long service life and reliability to wind farm operators. With the demanding environments in which wind turbines operate, both on- and off-shore with extremes of temperature, exposure to salt spray and infrequent maintenance, bearing lubrication is a challenge. It is only through judicious selection of synthetic base fluid, thickener and specific additive combinations that optimal performance characteristics be achieved. The end goal is the development of lubricants with excellent corrosion against fretting wear, high load carrying performance and exceptional low temperature pumpability.

In other words, the aim is to develop and utilise greases that satisfy the demands of today's active pitching systems for multi-mega watt wind turbine operating in both on- and off-shore environments. ♦
 

This article was published in the January/February 2014 issue of Renewable Energy Focus magazine.

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