Wind Turbine Gearbox Force and Stress Analyses

Three different types of forces act on gears and bearings in a gearbox. They are gravitational forces, frictional forces, and contact forces. The most significant are contact forces, due to the magnitude of the contact stresses that these forces are generating on the surfaces of the moving parts during operation.

Contact force occurs when the tooth of a gear is engaged by the tooth of another gear. There may be more than one contact force acting on a gear if the gear engages multiple gears in the assembly.

The maximum contact stress is positively proportional to the contact force and inversely proportional to the contact area. For a given amount of contact force, the smaller the contact area, the greater the contact stress will be.

Determined by the shape of the gear teeth, the contact area between the two engaging teeth is always a very small, narrow rectangular area. The contact stress on the engaging teeth, therefore, is always very significant by magnitude. This stress is measured in units of 1000 pounds per square inch or kilopounds per square inch, which is abbreviated ksi. Contact stress can easily reach levels of 100 ksi or more, causing the wear and possible failure of gear teeth.

A misaligned gear shaft will make the already small contact area between two engaging teeth even smaller, resulting in a sharp increase of the contact stress, accelerated wear, and premature gear failure. Careful alignment to tolerances is a critical part of gearbox maintenance.

Alignment also has a significant impact on the contact stress acting on the surfaces of the rollers and rings of the bearings.

A misaligned gear shaft will cause the small contact area between the roller and the ring of the bearing to become even smaller, resulting in a sharp increase of the contact stress. This causes accelerated wear, and eventual bearing failure.

Before failure, the wearing of the gears and the bearings will disturb the designed mass distributions of the moving parts, causing the mass center of the rotating parts to deviate from the center of rotation, which in turn will cause the gearbox to vibrate.

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