What Is The Gear System Used In Wind Turbines?

Wind turbines convert the kinetic energy in wind into electrical energy. The key components of a wind turbine include the rotor blades, gearbox, generator, nacelle, and tower. The rotor blades capture the wind energy and rotate the main shaft connected to them. This shaft rotates at a low speed of about 30-60 rpm. The gearbox is a critical component that increases this low rotational speed to a high speed of 1000-1800 rpm suitable for electricity generation. Essentially, the gearbox serves as a transmission system between the low-speed shaft turned by the rotor blades and the high-speed shaft connected to the generator. By stepping up the rotational speeds, the gearbox enables the generator to produce electricity at grid-level AC frequencies.

Function of the Gearbox

The primary function of the gearbox in a wind turbine is to step up the slow rotational speed of the turbine rotor to a much faster speed that is required by the generator to produce electricity effectively. The rotor blades spin slowly, even at their tips, while the generator needs to spin very fast – around 1000-1800 rpm. Therefore, a gearbox is used to bridge this large speed difference.

Most large modern wind turbines use a planetary gearbox design. The gearbox steps up the low input speed from the main rotor shaft to a high output speed for the generator. A gear ratio of approximately 1:100 is common. For example, if the rotor spins at 20 rpm, the gearbox output to the generator will be around 1500-1800 rpm.

Without the gearbox, the rotor would need to spin dangerously fast to directly produce the required generator speeds. Or large heavy generators would be needed, reducing efficiency. So the gearbox plays an essential role in the drive train by allowing much higher generator speeds than the low rotor speeds.

This speed-increasing function enables electricity generation at optimum speeds from the slow rotational motion of the turbine blades. In summary, the gearbox provides a vital speed conversion between the rotor and generator (International falls obituaries daily journal).

Gearbox Types

There are several main types of gearboxes used in wind turbines including planetary, parallel-shaft, and helical gearboxes. These different types vary in their design and operation:

Planetary gearboxes are one of the most common types used in wind turbines. As explained on the Felsics website, “Planetary gearboxes have high gear ratios and are very compact in design. Multiple planets or pinions spin on their own bearings around a sun gear.” This allows them to reach the high speeds and torque needed for generating electricity.

Parallel-shaft gearboxes are another option, consisting of separate input and output shafts connected via gears. They tend to be larger in size compared to planetary gearboxes. According to JS KeWei’s website, parallel-shaft gearboxes can have up to “three stages” and utilize helical gears for smooth power transmission.

Helical gears, with their angled teeth, help minimize noise and vibration in the turbine drivetrain. Helical gearing can be implemented in both planetary and parallel-shaft wind turbine gearboxes. The angle of the teeth allows for gradual contact and a quieter operation.

In summary, planetary, parallel-shaft, and helical gears each serve important roles in wind turbine drivetrains, with tradeoffs in size, gear ratios, and noise levels.

Main Shaft

The main shaft in a wind turbine connects the rotor hub to the gearbox (Source). It rotates at the same speed as the rotor, which in modern wind turbines is typically around 10-20 rpm. The main purpose of the main shaft is to transmit the rotational power from the rotor to the gearbox, where the rotational speed will be increased.

The main shaft is usually made of forged steel and designed to withstand extreme torque. The torque is highest when the wind turbine rotor is starting up at low speeds. The diameter of the main shaft can be over 1 meter on large multi-megawatt turbines (Source). The main shaft is connected to the rotor hub through a bolted flange connection and to the gearbox input shaft, also via a flange.

Proper design and maintenance of the main shaft is crucial to prevent failures that could require expensive repairs. The main shaft must be precisely aligned and balanced. Main shaft bearings also require regular lubrication and inspection.

High Speed Shaft

The high-speed shaft connects the gearbox to the generator in a wind turbine. As the name suggests, this shaft rotates at a high speed in order to drive the generator. According to the Department of Energy, the high-speed shaft rotates at about 1,200-1,800 rpm in most wind turbine designs (https://www.energy.gov/eere/wind/how-wind-turbine-works-text-version).

The high-speed shaft experiences high loads and fatigue which can lead to bearing failures if not properly maintained. Research has focused on developing prognostic systems to monitor the health of the high-speed shaft bearings and detect potential issues early on (https://www.sciencedirect.com/science/article/pii/S0003682X17300130). Proper lubrication, alignment, and balancing are also critical to ensuring a long service life for the high-speed shaft.

As wind turbines continue to increase in size and power generation capacity, demands on the high-speed shaft also increase. Manufacturers are focused on design improvements such as larger bearings and shafts made of higher strength materials to ensure reliability and uptime of the high-speed shaft component.

Gearbox Maintenance

The gearbox is one of the most critical components of a wind turbine which requires regular monitoring and maintenance to maximize reliability and performance. Proper lubrication maintenance is essential to reduce friction and wear. Synthetic oils are commonly used as they offer superior lubrication over long operating durations at high loads compared to mineral oils. Oil filtration systems should be installed to remove contaminants and water accumulation. The oil quality should be periodically analyzed to detect early signs of abnormal gear wear.

Vibration monitoring is another key aspect of gearbox maintenance. Accelerometers are mounted on the gearbox to detect any abnormal vibration signals that may indicate early fault development. Advanced signal processing techniques allow earlier detection of potential faults compared to traditional vibration analysis methods. Vibration monitoring combined with oil analysis provides effective condition monitoring to minimize unplanned downtime.

Various gearbox failure modes need close monitoring such as gear wear, bearing failures, and shaft misalignment. Both offline and online condition monitoring systems are utilized to detect these failure modes early on. This enables timely maintenance actions before catastrophic failures occur, maximizing gearbox reliability and availability.^[1]

Gearbox Failure Modes

Some common gearbox failure modes in wind turbines include bearing wear, gear tooth damage, and issues with lubrication oil (https://www.nrel.gov/docs/fy12osti/53084.pdf). Bearing wear is one of the most prevalent failure modes, often caused by improper installation, lack of maintenance, or fatigue from normal use over time (https://www.pall.com/en/oil-gas/blog/wind-turbine-gearbox-failures.html). This can lead to excessive clearances, vibration, noise, overheating, and eventual failure. Gear tooth damage like pitting, scuffing, or breakage is another frequent issue, resulting from improper tooth design, manufacturing defects, overload, or lack of lubrication. Finally, oil breakdown and contamination can reduce lubrication effectiveness which accelerates wear and increases friction.

Condition Monitoring

Condition monitoring is critical for detecting issues and preventing failures in wind turbine gearboxes. There are several techniques used:

Oil analysis involves taking oil samples and analyzing them for particles, water content, and viscosity. Higher levels can indicate wear and contamination in the gearbox. Oil analysis helps identify problems early so maintenance can be scheduled before failure occurs (source).

Vibration analysis uses sensors to detect abnormal vibration patterns that may indicate developing faults in bearings, gears, or other components. Vibration monitoring is one of the most effective ways to identify gear or bearing damage (source).

Thermography uses infrared cameras to measure temperature variations on gearbox components. Localized hot spots can indicate excessive friction and impending failure. Thermal imaging helps pinpoint exactly where faults are occurring (source).

Gearbox Design Improvements

Gearbox manufacturers are constantly working to improve the reliability and performance of wind turbine gearboxes. Some key areas of innovation include:

Materials

New steel alloys and coatings allow gears and bearings to withstand higher loads and resist wear and corrosion. Manufacturers like ZF are using case-hardened steel, nitriding steel, and chrome-molybdenum steel for gearbox components.1

Lubrication

Synthetic lubricants with improved viscosities can operate over a wider temperature range. Condition monitoring systems help ensure oil cleanliness. ZF uses its Variacon lubrication system to provide optimal lubrication.2

Filtration

Advanced filtration systems remove microscopic particles from the oil to prevent gear wear. Some systems use electrostatic filtration or have built-in sensors to monitor oil cleanliness.

Cooling

New cooling jackets, pipes, and pumps allow better heat dissipation from the gearbox. This prevents overheating of the lubrication oil and bearings.

Future Outlook

The future of wind turbine gearbox technology looks promising as innovations continue to improve reliability and efficiency. Some key areas researchers are exploring include:

Lightweight materials like composites and alloys are being used more frequently in gearbox designs. These materials can handle high torque loads while reducing overall weight. According to an article on Energy5, “Replacing traditional materials with lightweight composites and alloys could potentially reduce gearbox mass by up to 70%.” This weight reduction will lead to lower maintenance costs and longer gearbox lifetimes.

Another article on Energy5 discusses advancements in magnetorheological fluids. These “smart fluids” can change viscosity in response to magnetic fields. Integrating them into gearbox lubrication systems enables active control of lubricant properties. This optimizes lubrication, reduces friction and wear, and improves heat dissipation.

In general, gearbox reliability is predicted to improve through condition monitoring systems and smarter control algorithms. Sensors and software will enable predictive maintenance before failures occur. New designs will also continue mitigating problematic issues like bearing wear and lubrication deficiencies seen in past gearboxes.