A Grease with Low Base Oil Viscosity Improves Main Bearing Lubrication
The lubrication of main bearings in wind turbines is a difficult business. The lubricant has to cope with false brinelling, with challenging operating aspects such as high loads and varying temperatures, and with the need to protect components against wear. Last, but not least, lubricants have to be useable in centralised 
lubricating systems. The classical approach in wind turbine main bearing lubrication has been to use greases with a high base oil viscosity, which were regarded as the optimum. Meanwhile, bench tests and field experience have shown that greases with a low base oil viscosity (i.e. in the range of 130–150mm/s2) can protect the bearing better than other lubricants. The key to success is the combination of the low-viscosity base oil with suitable extreme pressure and antiwear additives.
By Thomas Jørgensen, Klüber Lubrication, Germany
Increased Challenges for Wind Turbines
Wear protection under high loads and vibrations, good grease distribution, efficient oil separation and reduction of component temperatures are some of the issues which need to be considered, especially for main rotor bearings in both geared and direct-drive turbines. There are further challenges for manufacturers of lubricants from developments in turbine technology. One development has been the tendency for turbines to have higher power generation capacities, which also means that they have bigger and heavier rotor bearings. A second trend has been the requirement of longer lifetime (i.e. a minimum of 20 to 25 years). These conflicting requirements make the main bearing an extremely challenging application. Mechanical challenges which have been known from the early days of wind turbine design are becoming even more obvious with increasing turbine dimensions. The fact that the bearings operate at low speed and, at the same time, are subjected to alternating loads and torques with constantly varying directions of loads and movements, are problems that the industry is still trying to solve using a variety of techniques.
