Wind Turbine Gearbox: Introduction

The blades, the hub, and the main shaft of a wind turbine harness the kinetic energy of the wind and convert it into rotational mechanical energy.

The speed of this rotation is too slow to drive the generator efficiently, so a gearbox converts the high-torque and low-speed mechanical energy of the main shaft into the low-torque and high-speed mechanical energy that drives the shaft of the generator.

The fundamental properties of solids and liquids are involved in the functioning of a gearbox.

A solid is a material that does not change its shape or its volume. Any significant change of the shape and/or volume means that it will not function as designed. The housing and moving parts of a gearbox are solids.

A liquid is defined as matter that does not change its volume but will take the shape of its container. This important physical property of a liquid helps protect the solid parts of a gearbox from wear and failure. The gearbox lubricant is a liquid.

Solid and liquid materials work together to help the gearbox function properly.

Gears and bearings fail because of the changes of the shape and/or volume of the solid materials of which they are made.

When any two solid moving parts of gears or bearings are in direct contact with each other, the surface roughness of the solid materials causes contact stress that is “normal” or perpendicular to the contact surfaces. Localized micro deformation of the parts occurs due to contact stress.

The direct contact between the two solid surfaces also generates the shear stress that is tangential to the contact surfaces. This stress shears away small amounts of the solid materials.

Both the contact stress and the shear stress are major causes for wearing and failure of the solid moving parts in a gearbox, and can be minimized with the use of a lubricant.

The liquid properties of the lubricant help reduce wear of the moving parts in a gearbox.

First, the liquid lubricant trapped in between two solid surfaces helps even out the contact stress between the two because the liquid lubricant will take the shape of its container, and fill in small voids in the rough surface.

Also, the liquid lubricant evenly transfers the normal forces between the two solid surfaces because it is incompressible; it does not change the volume. These effects provide a much smoother interface between the contact surfaces and thus reduce the contact stress significantly.

Second, because the solid surfaces are not in direct contact, shearing between the solid surfaces is replaced with shearing between thin layers of the lubricant. This helps eliminate the shear stress in the solids, and prevents wearing, heating, or even welding of the surfaces. The resistance of the liquid to such shearing is related to its viscosity.

Viscosity measures the resistance to flow in a liquid. A high viscosity fluid, such as gear oil, flows slowly. Low viscosity fluids like water flow easily. Viscosity of most liquids decreases as temperature rises, and increases at low temperature.

High temperatures will cause gear oil viscosity to de-crease. The resulting breakdown of the thin film of oil between the parts causes accelerated wear.

On the other hand, low temperatures will cause gear oil viscosity to increase, making it difficult for the oil to flow to the places where it is needed.

For these reasons, gear oil temperatures are carefully monitored and controlled.

Proper inspection and maintenance of gearbox lubricants and lubricating systems are vital.

Filter servicing, periodic oil sampling and analysis, and correct oil levels are important parts of the gearbox maintenance routine, which is one of the most important tasks performed by a wind turbine technician.

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