Wind turbines typically achieve 140,000 – 150,000 operating hours during their 20- to 25-year lifetime, so demands on quality and reliability of lubricants are high. Additionally, given the challenges of up-tower oil change, upkeep requirements must be low.
Compare this with the demands on a passenger car. During a typical 10- to 15-year lifetime, a modern car is generally limited to between 2,500 and 5,000 operating hours, with annual services that include an oil change.
Functions of oil
Most wind turbines fitted with a gearbox operate under difficult environmental conditions, with weight and space limitations requiring a compact design. So, with high power loads passing through small gearboxes, substantial internal heat is generated. A key function of oil lubrication in a turbine gearbox is to dissipate this internal heat. Additionally, the continuous circulation of the oil in the system allows for oil filtration, which can enhance both oil and gearbox operational life. A third function is that an oil-cooler can be integrated into the system to control operating temperatures.
Oil companies claim that synthetic gearbox oils offer superior protection against gear and bearing wear such as micropitting and scuffing — material transfer between sliding tooth surfaces. Oxidation resistance, low-temperature fluidity, filterability and keep-clean properties all extend exchange intervals by several years.
Synthetic gear oils are also important in compact yaw and pitch drive planetary gearboxes. The application is markedly different than in the main gearbox, as these yaw actions are not continuous but stop and start, so require an oil with good wear protection. In most rotating equipment, the gap between exposed stationary and rotating bearing parts is closed with a seal, which keeps the lubricant in, and dust and dirt out.
Large single rotor bearings used in several types of wind turbines can be lubricated by grease or oil, explains Alexander Peter, application engineer of German bearing supplier IMO. "Up to about 2MW power ratings, grease lubrication is common. But with larger capacities a switch is usually made to oil, mainly because, within larger bearing units more internal heat is generated, which can be dissipated through oil circulation, and the oil can be filtered as a lifetime-enhancing benefit."
Hydraulic systems, like those for the pitch drives used by some major turbine suppliers, require a different synthetic oil type, which can deal with, among other things, high pressure in the system.
In conventional fast-speed geared wind turbines with a separate main shaft, grease lubrication is common for the main bearing(s). Because the bearings are fairly small, friction-free seals can be used and no bearing pre-heating is required during cold weather.
The same grease lubrication can be used for the rotor blade pitch bearings and yaw bearing, where operation is discontinuous. A challenge for pitch bearing lubrication is that movement is restricted to a section of a full circle. And, because no circulation of the lubricant is needed, the bearings of high-speed generators normally use a synthetic grease developed for electric machines.
Open gear systems — exposed structures like the yaw gear with yaw motors — provide another challenge, resolved by using grease that typically contains extreme pressure additives and finely dispersed graphite to withstand heavy loads.