Dr Felix Guerzoni, Shell Lubricants' product application specialist, explains the benefits for wind turbine engineers of the company's new high-performance, synthetic wind turbine blade bearing grease, Shell Gadus S5 V110KP 1.5.

Wind turbines represent a challenging application for lubrication, not only because of the remoteness of the wind farms and the sheer height of the turbines themselves, but also because of the ambient conditions in which they operate and the loads and vibrations to which components are exposed. 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 bearings' design and typical failure modes, lowest ambient operating temperatures, servicing frequency, method of application and compatibility of factory-fill with service-fill greases, among other considerations.

There are currently two approaches to the development of greases for main bearing, blade pitching, yaw bearings and generator bearings: developing a multipurpose grease suitable for the lubrication of all components, or developing a number of greases designed for each given component. 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 considered carefully.

The selection of greases for wind-turbine-bearing lubrication requires an understanding of the bearings' design, typical failure modes and servicing frequency, among other considerations. Adjustment of the blade angles of a wind turbine is an important practice not only for the safe operation of the turbine (especially in the presence of high winds), but also to control its power output. 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. While vibration and minor oscillations can 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 specialised greases designed to overcome multiple lubrication challenges.

Outstanding Performance in Variable Conditions

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 <-40?C to >50?C are not uncommon, often with wide seasonal and daily swings.

It is typical for today's wind turbine designs to be fitted with centralised greasing systems of progressive or single-line type, with narrow-diameter feeder lines that distribute the grease to the lubrication points, which necessitates that greases have excellent low-temperature pumpability. In the Kesternich flow-pressure test (DIN 51805), the Shell Gadus S5 V110KP demonstrates its advanced fluidity even at lower ambient temperatures, a critical requirement in blade-bearing grease systems incorporating narrow-diameter lines from divider blocks to bearings. In this test, the pressure required to force a strand of the grease in a continuous stream out of a test nozzle is determined. Ideally, pressures should remain low even at sub-ambient temperatures, ideally <1,400 mbar.

Wet, Corrosive Environments

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 saltwater 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 of 1-5, with 0 being no corrosion and 5 representing heavy corrosion with corroded areas covering more than 10% of the running track surface Shell Gadus S5 V110KP 1.5 blade-bearing grease has been run through a number of other industry standard tests to demonstrate its advanced performance in the area of resistance to wear in oscillating and fretting wear-type conditions.

Long Grease Life

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 that wind farm operators are looking for when selecting a grease and to ensure optimal reliability for their assets. The Shell Gadus S5 V110KP 1.5 grease uses synthetic base oils instead of mineral oils and so 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.

Outstanding Protection Against Wear

The blade-bearing application involves 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 that 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.

Shell Gadus S5 V110KP 1.5 blade-bearing grease has been designed to help provide increased resistance against false brinelling and to help prevent wear at the rolling contacts, even under high and variable load conditions. One of the most critical requirements is the ability of the lubricant to function over a wide operating temperature range.

Rothe Erde, one of the world's leading manufacturers of blade bearings for wind turbines, specifies that greases for use in its blade bearings must pass the Rothe Erde/IME Riffel corrosion test (FE 61001).

This test was developed specifically by the Institute for Machine Elements and Machine Design (IME) at RWTH 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 then goes through one million cycles of alternating 70kN axial load. At the end of the testing cycles, the bearings are removed and the depth of wear on the bearing raceway measured using a profilometer.

A visual assessment of the degree of corrosion is conducted with a rating on a scale of 1-5, with 1 being the least corrosion and 5 the worst and highest level of corrosion. The Shell Gadus S5 V110KP 1.5 blade-bearing grease gives a corrosion rating of 1, riffle-wear-scar depth maximum of 2?m and mean riffle-wear-scar depth of <1?m, showing advanced protection against this wear mechanism.

The Riffel Test

The Riffel test can also be run under modified test conditions, with the FAG four-point contact ball bearing axially loaded under dynamic conditions +/-23.3kN, using a hydropulser across only five rolling elements, and subjected to a swing of 7Hz. This results in an approximate 3,000MPa maximum contact pressure at the inner ring. The NaCl solution is injected into the bearings at eight locations and the load cycles increased to 1.1 million cycles. In the modified Riffel test, Shell Gadus S5 V110KP 1.5 showed significantly lower frictional torque and element surface damage depth in both contaminated and contamination-free environments relative to existing in-service blade-bearing greases. A common failure mode for blade bearings is that of fretting wear or false brinelling, a wear mechanism that can be addressed through careful grease design.

When developing greases for demanding applications such as that of blade bearings in wind turbine applications, 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, on and offshore, with extremes of temperature, exposure to salt spray and infrequent maintenance, bearing lubrication is a challenge. Shell's new Gadus S5 V110KP 1.5 blade-bearing grease has been formulated with these challenges in mind, and through judicious selection of synthetic base fluid, thickener and specific additive combinations, a grease with advanced corrosion against fretting wear, high load-carrying performance and exceptional low-temperature pumpability can be obtained. This product builds on successful experiences in the wind and aviation industries to help satisfy the demands of today's active pitching systems for multi-megawatt wind turbine-operating in both on and offshore environments.