Drive train components

Position of blade bearings of a wind turbine

The rotor blade bearings of wind turbines are subject to unfavorable operating conditions. They have to accommodate high bending moments while standing still or rotating at very low speeds. The surrounding parts, especially the rotor blade, provide limited stiffness. It is not possible to apply typical standards for calculating the service life of bearings, such as ISO 281, and the experience-based construction of bearings is reaching its limits due to new bearing designs and operating modes such as Individual Pitch Control (IPC). The damage mechanisms in the bearing depend on a range of factors, which, in turn, have differing effects depending on the actual bearing and lubricant.


In such situations, the safe use of blade bearings in the turbine can only be ensured through highly realistic testing. To this end, Fraunhofer IWES, in cooperation with Senvion GmbH, has developed a blade bearing test stand which enables testing of the entire hub/blade bearing/ rotor blade group. In this set-up, all the significant interfaces are modelled realistically. A maximum bending moment of 15 MNm can be created on the rotor blade; this moment can be divided into flapwise and edgewise bending. Additional load arms on the free blade bearing flanges ensure realistic deformation of the hub flange on the blade bearing. Pitch movements
with amplitudes of up to 5° can be realized when subject to loading. The test stand has 400 measurement channels and can also accommodate high-frequency systems for monitoring vibrations and lubricant film thickness.


Goals of testing

The Fraunhofer IWES’ testing strategy is essentially divided into functional and fatigue testing. During functional testing, the dominant damage mechanisms in the bearing are determined. These damage mechanisms define the program for subsequent timeaccelerated endurance testing. Fraunhofer IWES also handles the set-up of the test program, taking the time series of the load simulation as the basis. A comprehensive data analysis permits the acceleration/com-pression of the testing period to times which are acceptable within the framework of wind turbine development. The test stand, without the additional parts from Senvion GmbH, is also available for public use. The test stand can be used directly to test the hub/blade bearing/rotor blade assemblies; Fraunhofer IWES can also arrange the production of the corresponding components for testing individual bearings.


• Max. bending moment 15 MNm
• Pitch movement when subject to loading with +/- 5°
• Running of generic programs as well as modified time series
• 400 measurement channels
• Measurement of thickness of lubrication film

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© Photo Martina Buchholz
© Photo Martina Buchholz

The test bench for wind turbine main shafts is designed for accelerated fatigue testing of main shafts of 2 to 5 MW wind turbines. The test bench has a modular design, so that different main shaft designs can be tested under a realistic load situation. Due to its modular design nature, it is also possible to adapt to different component test setup as, e.g. main bearings, pitch bearings or main frames.

The aim of the fatigue tests is not only to damage the specimen, rather it is important to reproduce a realistic loading and thus wear and tear as occur in operational wind turbine main shafts. For this reason we have conducted a detailed analysis of simulated and measured load time series for the main shaft. After processing the data using the „Rain-Flow- Counting“ method, lifetime damage is calculated based on Miner‘s rule.

An important output of those analyses is the information on which degrees of freedom are relevant for fatigue damage mechanisms and thus helping us to design an adequate test bench.

- Max. bending moment: 15 MNm
- Max. radial force: 3MN- 300 kW
- Max. rotational speed: 60 rpm Drive power: 300 kW
- Heavy-duty foundation
- Flexible test arrangements possible

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The development of a suitable Condition Monitoring System (CMS) for the damage detection in gearboxes is especially important for offshore applications. Gearboxes are among the components with the longest down times in wind turbines (WT). Service and maintenance are - especially in the offshore sector - associated with high costs. For this reason, more and more gearboxes are being equipped with oil sensors which, as part of an on-line CMS, monitor the condition of gearboxes and give timely fault reports to operators. In this way, the risks of damages could be reduced and gearbox breakdowns could be avoided. However, oil sensor technology reliability at present does not meet the high demands of WT operators as in the field false reports often occur resulting in unnecessary and costly maintenance operations.

Oil Quality as Condition Indicator

The sensors can detect faults or defects based on the oil quality. Various studies on Oil-Condition-Monitoring have detected the relationship between the gearbox condition and the oil quality. However, it is still unclear, whether sensors can ascertain the gearbox oil condition reliably. Suitable testing facilities are required for ensuring oil sensor reliability and at the same time increase WT availability. In order to achieve this objective, oil sensors must undergo rigorous testing before they are installed in WT gearboxes.

DegradO – New Test Methods for Oil Sensors

The main focus of the BMU project „DegradO“ at the Fraunhofer IWES lies in the area of WT degradation under the influence of offshore environmental conditions. At present, in the growing wind industry there is a lack of procedures for the simulation of load and environmental conditions. The aim of DegradO is to develop new testing methods for investigations into the degradation processes in WT and their components. By using suitable test facilities, the reproducibly simulation of the environmental conditions under realistic conditions will be possible.

One of these test facilities is an oil sensor test stand, whereby Fraunhofer IWES reproducibly simulates the operating conditions of a WT gearbox oil circuit. In order to test the performance of different oil sensors, two oil ageing stages are simulated. The sensors are tested under changing operating conditions and Contaminating pollutants such as wear debris, water, dust and air. In this way, the capability of the oil sensors to provide reliable values can be assessed. The main objective is the development and subsequent provision of standardised oil sensor tests for the industry.


In agreement with customers, the sensors can be tested for different gearbox oil operating and ageing conditions. By employing various oil contaminants and operating conditions, real-life scenarios can be recreated. The focus of the tests are reliability, accuracy and oil sensor operational capability under the effects of influences such as particles, water or air in the oil. If required, additional vibration and temperature tests can be carried out under realistic conditions in a HALT (Highly Accelerated Life Test) / HASS (Highly Accelerated Stress Screen) test chamber.

Target groups

• Wind farm operators
• Wind turbine manufacturers
• Gearbox manufacturers
• Oil sensor manufacturers
• Oil producers

Technical data

• Oil types: mineral oil, PAO, PAG. Tests with Fresh and aged oil possible
• Temperature: 30 ° C -100 ° C
• Flow rate: up to 15l / min
• Oil pressure: 10 bar
• Insertion of contaminants: water, Ferromagnetic and non-Ferromagnetic particles. Analysis of air-oil-dispersion possible by Flender gears
• Installation options: 6 sensors in the horizontal pipe, 4 sensors in the vertical pipe. Possible variation depending on the size of the sensors

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