Types of failure in wind turbines gearbox stages

Bearings are mechanical parts designed to reduce the friction between a shaft and the pieces connected to it, which support it and ease its movement, so they are items of critical wear of the machinery in wind turbines.

In the multiplier, the bearings are exposed to radial forces from the weight of the shafts and axial forces from the rotor load. For them to function at full capacity it is vital for them to be well lubricated and protected against corrosion and entry of contaminants.

The main shaft of the wind turbines comes from the blades. In some models it passes through one or two principal bearings, and ends in the slow stage of the multiplier, the first of three phases that are normally found in this mechanical part that multiplies the input revolutions to transmit them to a shaft that culminates in the power generator.

Each of the three phases of the multiplier, called low speed, planetary, and output or high speed stage, can suffer failures linked to their gear mechanism and bearings.

Each of the three phases of the multiplier, called low speed, planetary, and output or high speed stage, can suffer failures linked to their gear mechanism and bearings.

Low speed stage

In the low speed stage failures arise fairly frequently and are hard to detect, because the rotation speed is very slow. Visual inspection and measurement by means of other technologies have proved ineffective for the early detection of the most commonly found problems that affect this stage such as micropitting, microscopic cracks in the gears that can, with time and pressure, cause them to break; or problems of axial force that damage the input bearing.

Planetary stage

The planetary stage is mechanically complex. It consists of a large number of gears with different speeds that share the load dynamically throughout the geometry. In this phase, the problems of micropitting and wear from rubbing and axial load are the most frequent.

Often, the operators do not know the geometry of the planetary stage and, as a result, it is very difficult to identify a problem. This lack of knowledge of the planetary geometry comes about because frequently the manufacturers fail to supply this information in order to keep their control over maintenance.

As in the low speed stage, the preventive maintenance techniques are unable to make a reliable prediction of the existence of a problem in this phase. Borescopy, one of the most commonly used techniques, consists in the visual inspection of the gears through an optical fibre, but the structural complexity that characterises the planetary stage means that it cannot always reach the points that present a potential risk of failure.

Analysis through vibrations also has its weaknesses due to a lack of knowledge of the make-up of the unit. The diagnosis becomes complex because without a knowledge of the geometry it is not possible to relate the wave shapes with a problem or to know whether they represent a pattern of normal operation.

Fast Stage

Of the three stages that normally make up a multiplier, the fast stage is the most simple to observe because the bearings are nearest to the casing. The speed is another point that favours the detection because, being faster, it is possible to repeat a large number of cycles with an unvarying point of contact.

The kind of failure that tends to happen in this stage is wear by rubbing, something that occurs fundamentally when the machine is stopped. When a wind turbine is not operating, there is not a total stoppage; there is always an angle of rotation remaining in the blades. The force of this movement is increased in the multiplier, which transmits it to the fast shaft and, in the absence of opposition of the dynamic load of the generator, it makes the bearings begin to slip, which generates wear and breakage in the bearing races.

In order to minimise this effect, the bearing manufacturers have come up with a compound called black oxide, which covers them and, for a time, prevents this slippage from wearing or breaking the races. This is not a definitive solution, but it delays the appearance of problems caused by rubbing.

One of the problems that is arousing the greatest interest just now in the wind sector, and in bearings in particular, is WEC (White Etching Cracks). While there is no precise knowledge of the evidence of generation and propagation of this effect, the presence of hydrogen and compounds such as sulfur or manganese of the oil additives generates surface corrosion that is noticeable through a whitening of the surface, which gives its name to this problem.