A gearbox is typically used in a wind turbine to increase rotational speed from a low-speed rotor to a higher speed electrical generator. A common ratio is about 90:1, with a rate 16.7 rpm input from the rotor to 1,500 rpm output for the generator. Some multimegawatt wind turbines have dispensed with a gearbox. In these so-called direct-drive machines, the generator rotor turns at the same speed as the turbine rotor. This requires a large and expensive generator. Other wind turbines on the market sit in-between, with gearbox ratios of about 30:1, dispensing with the highest speed stage in a typical gearbox. There is a trade-off between the reliability of gearboxes and gear stages and the cost of slower, higher torque generators.
The design of a wind turbine gearbox is challenging due to the loading and environmental conditions in which the gearbox must operate. Torque from the rotor generates power, but the turbine rotor also applies large moments and forces to the wind-turbine drivetrain. It is important to ensure that the drivetrain effectively isolates the gearbox, or to ensure that the gearbox is designed to support these loads, otherwise internal gearbox components can become severely misaligned. This can lead to stress concentrations and failures.
Wind-turbine drivetrains undergo severe transient loading during start-ups, shut-downs, emergency stops, and during grid connections. Load cases that result in torque reversals may be particularly damaging to bearings, as rollers may be skidding during the sudden relocation of the loaded zone. Seals and lubrication systems must work reliably over a wide temperature variation to prevent the ingress of dirt and moisture, and perform effectively at all rotational speeds in the gearbox.
Gear and bearing fatigue standards by AGMA and ISO are used for design, these only capture a subset of the potential failure modes of the components. For instance, the ISO 6336 gear standard provides an established method for calculating resistance to subsurface contact failure and for tooth root breakage. The standards are doing their job, but these are not the most common failure modes observed in windturbine gearboxes. More common causes of failure are manufacturing errors such as grind temper or material inclusions, surface related problems, such as scuffing or micropitting, and fretting problems from small vibratory motions, such as may occur when a machine is parked. Scuffing is adhesive wear and subsequent detachment and transfer of particles from one or both of the meshing teeth (ref ISO13989-1). It can happen quickly and is generally considered to be associated with an absence or breakdown of the lubricant film under high loads (ISO 13989-2). Micropitting is a surface fatigue resulting from generation or numerous surface cracks, and is associated with insufficient film thickness (ISO 15144-1). Film thickness is affected by sliding speed, load, temperature, surface roughness, and chemical composition of the lubricant.
Many wind-turbine gearboxes have also suffered from fundamental design issues such as ineffective interference fits that result in unintended motion and wear, ineffectiveness of internal lubrication paths and problems with sealing. Improving the resistance of future gearbox designs to all these issues is a key for the future cost of energy generated by wind turbines.
Filed Under: Gearboxes