Low temperatures are, however, often not considered in advance. The consequences can be an immense maintenance effort and high costs.
Many problems can be avoided by using a lubricant that is suited to the particular friction point and its operating conditions. Only lubricants that are especially designed for operation at low temperatures will function reliably in these conditions. Such speciality lubricants also ensure that wind turbines work reliably at high ambient temperatures and smoothly across a wide temperature range - as even in Nordic countries, summers can be warm.
Safe cold start of gears
While industrial gearboxes are mostly subject to relatively constant ambient temperatures, wind turbines are in the open and subject to the seasons. Such conditions call for high-performance gear oils based on carefully selected raw materials and additives suited to the requirements of the specific material combinations and loads.
Modern additive technologies in gear oils ensure that gears are reliably protected against tooth and bearing damage, even at low oil temperatures.
Gear oils from Klüber Lubrication, the tribology expert, follow an holistic approach, which not only considers the gear and the lubricant components, but also focuses on competent advice and suitable test methods.
As temperature changes, oil viscosity changes in a non-linear way. The oil becomes thinner with increasing temperature, while it becomes more viscous as temperatures decrease.
The viscosity index (VI) describes an oil's viscosity-temperature behaviour. The higher the viscosity index, the smaller the viscosity change of an oil at fluctuating temperatures.
Operators of wind power stations should use synthetic gear oils with a high viscosity index (VI) exclusively to ensure the load-carrying capacity of the oil is sufficient during cold starts and to cover a wide temperature range. The viscosity index depends, among other factors, on the base oil used; viscosity index improvers are often used to increase a base oil's viscosity index.
However, VI improvers in gear oils also have their disadvantages. The polymers often used in these oils become sheared over time, leading to viscosity loss and reduced load-carrying capacity. There are products on the market that cover a wide temperature range without VI improvers, e.g. Klübersynth GEM 4-320 N, a gear oil based on polyalphaolefin.
During start-up of wind turbines at low temperatures gear oils are often heated to overcome high viscosity and ensure safe lubrication, avoiding damage and to obtain the maximum permissible viscosity for the components to be lubricated.
After heating, the gear oils must show a continuously high performance until the required service viscosity is reached.
Overheating and resulting oil damage is to be avoided. The heating elements should be applied over a wider surface area rather than in just a few spots and have a maximum heat rate of 0.8-1.5 W/cm². Additional circulation can prevent gear oil damage. Gear oils must not show irreversible chemical damage after long storage and at low temperatures, e.g. in unheated warehouses or during transport.
An important requirement in the development of Klübersynth GEM 4-320 N was low residue formation and good filterability. Traditional metal additives like zinc, molybdenum and magnesium were not used for its formulation and it has a high shear stability to ensure the above requirements are met. Comprehensive tests performed together with the oil filter manufacturers used showed that filtering installations are not damaged, even at low temperatures and with high oil viscosities.
Lower service temperature of bearing greases
The characteristics of a lubricating grease, e.g. consistency, shear viscosity or base oil viscosity change depending on the mechano-dynamic loads and temperatures. These changes may affect the function of the wind turbine bearings. They are often operated at very low operating temperatures that may even be below the base oil's pour point.
While generator and main bearings heat up internally during operation as heat is generated in the nacelle, the exposed pitch bearings are subject to the lowest temperatures, almost the same as the ambient temperature. The quoted service temperatures of lubricating greases are guide values that depend on the lubricant's composition and the application method. So what happens if temperature sinks below the lower service temperature of a grease? The grease stiffens, however, it should still be soft and adhesive, ensuring that the grease is not displaced and maintains a good lubricating effect in the tribo-contact area.
The lower service temperature is the lowest temperature at which the product passed the flow pressure test or the low-temperature torque (IP 186) test. It does not necessarily mean that the product's lubricating effect at low temperatures is good. The flow pressure test and the low-temperature torque test only provide indications of the grease's low-temperature characteristics. With suitable low-temperature component tests (e.g. SNR-FEB 2), further information on a grease's low-temperature characteristics can be obtained.
Results obtained in SNR-FEB 2 rolling bearing grease test
The antiwear behaviour of lubricating greases in rolling bearings subject to small oscillating rolling and sliding motion is determined on the SNR FEB 2 rolling bearing grease tester. Since the wear pattern in this test resembles the indentation caused in the Brinell hardness test, the SNR FEB 2 test is also referred to as "false Brinell test". An axial load of 8000 N is applied, corresponding to a Hertzian pressure of 2100 N/mm2, with a frequency of 24 Hz and oscillating ± 3° of angle with a test duration of 5 or 50 hours. The temperature of the lower shaft washer is -20 °C or ambient temperature.
Klüberplex BEM 41-141 was tested at ambient temperature, resulting in less than 5 mg of wear, and at -20 °C, resulting in less than 20 mg – both are excellent values, which many other lubricants on the market do not attain.
Function of the additives
Lubricating greases for rolling bearings in wind turbines should be equipped with an additive package suited to the special operating conditions. High ambient temperatures are not essential to provide the chemical energy for additives to react. Hertzian pressure causes the lubricant temperature in the lubrication gap to increase quickly until the temperature required for additive reaction is reached. The function of the antiwear and extreme-pressure additives is ensured for reliable wear protection even at very low ambient temperatures. The decisive criterion for successful development of a low-temperature grease is the lubricating effect of the grease as a whole rather than just the low-temperature characteristics of the individual grease constituents. Klüberplex BEM 41-141 is a special lubricating grease suited to the operating conditions of rolling bearings in wind turbines.
Pumping lubricants at low temperatures - lubricating systems
Lubricating systems are expected to pump the grease reliably to the lubrication points within the temperature range. To ensure the grease is pumped even at the lower service temperature of the grease, modifying the lubricating system can be helpful; e.g. increasing the tubing diameter or heating the system have proven effective in practice. The total back-pressure in the lubricating system caused, e.g. by distributor blocks or pipe friction should definitely not exceed the maximum permissible delivery pressure of the system.
Central lubrication systems
Most gear rim/pinion drives on pitch and yaw bearings are still lubricated by hand. To keep downtime to a minimum, maintenance has to be reduced. Central lubrication systems are increasingly used for the relubrication of these open gears. The necessarily adhesive lubricants, which are very viscous, have to be pumpable even at very low operating temperatures.
Reduce stocks to a minimum
Turbine operators traditionally have considerable logistics and stock-keeping measures in place to meet the requirements of the many different friction points and their various operating conditions. The risk of confusing different lubricant types was also very high.
Klüber Lubrication’s tribology experts have developed Klüberplex BEM 41-141 for all relevant rolling bearings and Klübersynth AG 14-61 for open gears and for all temperatures. Combined with the high-performance gear oil Klübersynth GEM 4-320 N, operators can improve the operational reliability of their wind turbine gearboxes in all climates, reduce downtime and increase their plant's effectiveness, while drastically reducing stock costs by requiring only three lubricants to cover the main applications.