Case study: Condition assessment of non-inspectable bearings

Non-inspectable bearings

Sometimes some bearings in wind turbine gearboxes are not reachable with endoscope probe. This is a common issue for specific gearbox models, where several bearings are partially or completely non-inspectable. That is caused by either the tight construction of the bearing itself or shaft elements -like oil deflectors- which impede the access.

We have two examples of non-inspectable ball bearings in Picture 1. In this case the bearings are physically not accessible.

Non-inspectable ball bearings

We can see an example of a partially inspectable ball bearing in Picture 2.

Partially inspectable ball bearing

In this case only the generator side half inner race is visible. This picture seems to represent the bearing in good condition. Time was spent for this bearing inspection, but it was not possible to detect any significant defect on the visible portion of the inner race. The outer race was not reachable with endoscope probe due to the tight bearing construction.

How to assess the condition of non-inspectable bearings?

How to retrieve information about non-inspectable bearings? Vibration analysis is always a challenge when performed on wind gear units, but if performed by specialists, it can supply excellent information about bearings’ condition.

The analyses must have three main goals:

  • bearing defect detection;
  • defect severity assessment;
  • corrective action recommendation.

The demodulation techniques allow even non-advanced analyses to detect bearing defects, especially on high speed bearings. Vibration analysis can supply good information about defect severity too. The severity assessment is usually the most complicate task in the analyses, and a specialist is needed for that. Only the specialist can clearly define the severity of the fault condition and suggest a proper timing for the corrective action.

A vibration analysis performed on the gearbox, where the bearing in the Picture 2 was inspected, revealed a ball bearing defect with critical severity (Picture 3).

Signal from defective ball bearing

Vibration analysis on wind turbine drive train with portable analyzer

Contextually to the endoscopic inspection -with suitable wind conditions- it is possible to perform vibration measuring with a portable analyzer, although better with a multi-channel one in order to reduce the turbine run time. Several sensors with magnetic support are moved point to point to collect data on key measurement locations of all the drive train components.

Data collection with a portable analyzer

This way it is possible to collect additional information from the rotating equipment, but with some disadvantages:

  • personnel must stay in the nacelle during 30 min turbine run;
  • we can obtain a quick view of the vibrational behavior of the entire drive train, with no chance to analyze trends or data collected in different operating conditions;
  • data analysis is performed later in the office, with no possibility to implement the acquisition with further data in case it is needed.

What to do if we detect a defect on a bearing at an early stage and we are using a portable device? Obviously, our recommendation should be monitoring the defect progression. It means that we should climb the turbine periodically to collect additional data on the drive train, on the base of the recommended frequency.

Vibration analysis on wind turbine drive train with online systems

This is the recommended method for analysis on wind turbines. Only permanent installations ensure the best reliability on identifying symptoms and preventing potential future failures. The main advantages of this method are:

  • personnel do not need to stay in the nacelle during data collection;
  • we can obtain different data sets in different operating conditions, and we can compare a lot of data sets collected in the same operating condition at different times;
  • data analysis is performed continuously, with the chance to detect the defect as soon as it can be revealed by vibration data, with possibility to implement the acquisitions with additional measurement tasks in case they are needed;
  • possibility to set alarm thresholds on data trends, to simplify the analysis of huge amount of data and keep under control hundreds of machine trains weekly.

From the condition monitoring point of view, one of the main advantages of the CMS is the possibility to follow up the development of the defect since its first detection (Picture 5).

Increase amplitude at the HSS GS-RS bearing BPFI

This allows drive train operation using the residual life of the damaged component, until the severity of the condition becomes dangerous (Picture 6). From the maintenance management point of view, this allows the maintenance work to be planned well in advance and at the most appropriate time to minimize production loss.

Acceleration spectrum collected before bearing replacement and inner race damage


The presence of non-inspectable bearings in gearboxes for wind applications must always be considered when establishing a proper monitoring strategy of these components. Vibration analysis is a valid tool for determining the condition of bearings that cannot be directly inspected, and for monitoring them over time. Online systems are the best tool for continuous monitoring of the drive train.

Moventas is a specialist in condition monitoring

Moventas provides a top level service in condition monitoring and diagnosis of the main components of wind turbines. Our condition monitoring engineers in the Condition Monitoring Centers in Finland, USA and Italy, provide customers high level support and training.

For more information please contact our sales or Lead Service Engineer Danilo Scamonatti at

Download this case study here: Case study: Condition assessment of non-inspectable bearings

Contact us at
For clients in North America:

Contact for more information

Please fill out the contact form, send and we will be in touch with you shortly.

  • This field is for validation purposes and should be left unchanged.