With the ever-increasing pressure to reduce LCOE in the wind industry, asset reliability is becoming a primary focus for wind farm owners and operators. While many solutions exist to mitigate drivetrain component failures, pitch bearing failures are still poorly understood and often result in increased and unexpected maintenance costs.
Increasing rotor diameters and harsher operating conditions increase the stress on pitch bearings. More sophisticated pitch control strategies, designed to reduce drivetrain loads, often result in an increased duty cycle on pitch bearing. This increased duty cycle further contributes to premature failures. Premature pitch bearing failures affect a wide variety of turbine and bearing OEMs and grease suppliers across many operating locations.
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Unlike other drivetrain components (e.g. gearbox, main bearing and generator) the operation of pitch bearings is still relatively poorly understood by asset owners. Typically, owners simply rely on the OEMs to just replace the failed part. However, it is essential to conduct a Root Cause Analysis investigation following each failure in order to develop effective solutions to mitigate the risk of future failures and reduce maintenance costs.
ONYX recommends the following process for failure investigation:
- Review all relevant documentation related to the pitch bearing failures and seal leaks/failures and develop a preliminary fishbone diagram
- Conduct in situ inspections of any potential seal leaks/failures, bolted connections, presence of cracks, etc.
- Carry out a workshop disassembly and detailed inspection
- Run follow on metallurgical investigation to eliminate potential material and manufacturing defects
- Complete FEA modelling to assess the effectiveness of available solutions (stiffening plates, compression straps, bearing replacement with larger bearing, etc)
Figure 1: Bearing disassembly
Figure 2: Pitch bearing FEA modelling
ONYX has performed Root Cause Analysis on a number of pitch bearings across several OEMs, with the following failure modes observed most frequently:
- Ellipse truncation
- Cage wear and surface fatigue
- Outer race cracks originating as stress concentrations such as lifting features, bolt holes and ball fill plugs
- Quality issues from poor induction hardening
Less commonly observed are seal failures, soft zone damage, fretting corrosion and lubrication failures due to hydraulic oil contamination.
Figure 3: Soft spot damage
Figure 4: Pitch bearing crack
The condition of pitch bearings is difficult to assess using off the shelf condition monitoring systems, due to the non-periodic oscillating nature of their operation. Difficult access for inspections can also make manual routine checks impractical or ineffective. At the same time, an undetected pitch bearing failure could have catastrophic consequences, such as blade loss or hub cracks.
SCADA data analysis can be effective for pitch bearing fault detection, however this often depends on having the appropriate SCADA tags available. Grease sample analysis is used by many operators but need to be accompanied by a robust sampling process to ensure consistency of results. Other monitoring techniques are available such as displacement monitoring and Acoustic Emission measurements. For the failures ONYX has experienced, the effectiveness of these solutions will depend on the failure mode.
As is often the case in the wind industry, there is no one size fits all solution. A good understanding of turbine specific failure modes is essential for the development of an effective risk mitigation strategy.