Lubricants answer wind power performance demands

Lubricant are now a major element in the tribological system to be taken into account in the design of wind power stations.

The European Union has decreed that about 21% of the European energy demand has to be covered by renewable energy sources by the year 2010, and wind energy is bound to play a major role in this development.

The performance of today's wind power stations ranges from several megawatts up to 10MW for prototypes already under development.

Due to this increase in performance the lubricant assumes an ever-greater importance, meaning that it is a major element in the tribological system to be taken into account in the design of wind power stations.

Only special lubricants are able to cope with the operating conditions of highly loaded components like gears and enhance their reliability.

In wind power stations both fluid (lubricating oils) and consistent (lubricating greases) lubricants are used.

The main task of the lubricant is to ensure reliable operation of the machine elements.

Important lubrication points in wind turbines are in the main gear drive, the yaw system gear, the main and generator bearing, the pitch adjustment unit and the nacelle slewing ring.

High demands are made on service life, load-carrying capacity and thermal resistance.

All these characteristics should be maintained over an extended period of time.

Therefore, plant operators require prolonged oil service life and grease relubrication intervals.

While the engine oil of a car is changed after 15,000 to 30,000km, which corresponds to an oil service life of 300 to 600h at an average speed of 50km/h, the gear oil in a wind turbine is changed after as many as 25,000 to 50,000h service.

Commodity lubricants as defined by various standards cannot meet the expectations of today's wind industry.

The ever increasing performance requirements made on wind turbine gears have led to higher loads and temperatures in a smaller space.

Therefore, nowadays the main gearboxes of wind turbines are generally lubricated with synthetic oils.

In this context, it needs to be observed that different base oils (polyalphaolefin, polyglycol or rapidly biodegradable ester) are used to formulate these gear oils.

Experience gained with usual synthetic lubricants has shown, however, that even these products cannot always meet the high requirements made in terms of wear protection of rolling bearings, micro-pitting resistance, foam and residue formation.

The global lubricant specialist Kluber Lubrication has made it its job to develop a new series of gear oils which fulfil the operators' demands for synthetic products with different base oils and eliminate the weak points of existing lubricants.

After having gone through numerous development stages and extensive performance tests, three synthetic high-performance gear oil series are now available: Klubersynth GEM 4 N, a polyalphaolefin; Klubersynth GH 6, a polyglycol; and Klubersynth GEM 2, a rapidly biodegradable ester.

These synthetic lubricants offer higher ageing resistance, improved load-carrying capacity and lower friction than mineral oils, and thus enable extended service life and very economical operation.

In Germany, industrial gear oils are classified in accordance with DIN51517.

Part 3 of this standard defines the requirements gear oils exposed to high loads need to fulfil.

In addition to the usual tests on viscosity, pour point, foaming characteristics, steel and copper corrosion, the scuffing load characteristics of the oils are determined in the FZG (Gear Research Centre) scuffing load test.

DIN51517, Part 3, stipulates a scuffing load stage greater then 12 for gear oils.

The task of a gear oil is not just limited to gear lubrication.

The oil should also be suitable for the lubrication of the rolling bearings incorporated in the gearbox.

Therefore, the revised standard DIN51517, 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 antiwear 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.

With the high-performance gear oils of Kluber Lubrication wear is just one third of the maximum permissible value.

Klubersynth GEM 4-230 N, Klubersynth GH 6-320 and Klubersynth GEM 2-320 comply with the performance parameters stipulated in the standard without problems.

These results, however, are insufficient for assessing the gear oil performance for wind turbines.

Further tests should be performed.

In addition to an improved scuffing load resistance at higher circumferential speed, the Kluber special oils achieve the load stage greater than 10 in the micro-pitting test, classified as "high".

This high micro-pitting resistance is not only achieved at 90C but also at an injection temperature of 60C (common in wind power stations).

The Kluber lubricants were subjected to adequate tests at both temperatures.

A trouble-free operation of a wind-power station is also determined by the antiwear properties of the lubricant at low gear speeds, as the planetary gear stage is run at the lowest speed.

Here a test method developed by FAG provides useful information.

The Kluber gear oils pass the test run with wear being below 40mg and therefore fall under the "low" wear category, which is the best possible classification.

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 determined on the FZG test rig.

The new gear oils reduce temperatures by up to 20C and power losses by up to 18% compared with standard gear oils.

These advantages contribute considerably to increasing the efficiency and energy output of wind turbine - worth up to several thousand Euro over the operating time of the plant.

More than 1000 wind power stations are already lubricated with the high-performance gear oils of Kluber Lubrication and many others are to follow.

Operators are convinced of the good wear protection for both the gear and the rolling bearings, the improved micro-pitting resistance as well as the high purity throughout the service life of the gear oils.

Also, economic aspects like high operational reliability, long oil change intervals and low friction values lead to cost reductions.

The service package consisting of extensive consulting by the Kluber sales team as well as oil analyses ensure that operators will always be provided with the lubricant tailored to suit the individual requirements of wind power stations.

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