Pitch Bearing Testing at IWES

We carry out realistic, accelerated tests on roller and ball-bearings under various operating conditions – including Individual Pitch Control (IPC) – to develop and validate new design and calculation methods, and to scrutinize root causes of faults and fault mechanisms.

In operation

© Foto Martina Buchholz
© Foto Martina Buchholz
© Foto Martina Buchholz

In cooperation with Senvion GmbH, Fraunhofer has developed a blade bearing test stand for the entire hub/blade bearing/ rotor blade group of turbines up to 3 MW. The test rig – without OEM specific additional parts – is available for public use.
 

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.

 

Targets 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 time-accelerated endurance testing. 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/compression of the testing period to times which are acceptable within the framework of wind turbine development. Access for other customers 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.

 

 

 

 

Technical data
 

• 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

Download data sheet

Starting operation in 2018

Testing concept emulating operating conditions

© Foto IMO

Quality check on a mounted bearing

© Foto IMO

Assembly of a bearing for a turbine with Individual Pitch Control

Designed for pitch bearings with a diameter up to 5 meters, this test bench will cover the current and next generation of offshore turbines up to 10 MW. The mechanical emulation of interface stiffnesses during the “Highly Accelerated Pitch Bearing Test” (HAPT) allows for a compact layout and more cost-effective testing.


Standard practice for designing wind turbine blade bearings is dimensioning, which is largely based on experience. The operating conditions in this rotating union present a particular challenge for the rolling bearings used. Their design works as long as no major changes are introduced to the bearing structure or type of bearing loads. Common standards for determining bearing life (ISO 281) and certification guidelines fall short in this respect. For changes in the operating conditions, such as increased load, new control methods, e.g. Individual Pitch Control (IPC), and increased size, this situation poses a risk for reliable operation.

 

Aims

In the research project HAPT (Highly Accelerated Pitch Bearing Test) the pitch, yaw and main bearing manufacturer IMO, the Institute for Machine Design and Tribology (IMKT) of Leibniz Universität Hannover and Fraunhofer IWES are developing the basis for a computational design, which cleverly combines theory, design and validation. A key aspect is to define the system limits for observing the working condition broadly since the components surrounding the bearing as well as pitch system control have a significant influence on the mechanisms of damage and thus on the life of a wind turbine.


Testing strategy

For validating theories of life-limiting states and accelerated test methods, IWES is setting up a large-scale test bench for current and future generations of turbines up to 10 MW. The test bench concept focuses on the stiffnesses of the attachment parts hub and blade, load application in the bearing as well as new control methods such as IPC. The condition of the bearing which is not visible from outside is determined by measuring the thickness of the lubrication film during operation. In addition to the large-format bearings, smaller-scale tests validate the theoretical findings. Commissioning of the test infrastructure is scheduled for 2018. After that, several endurance tests on actual-size pitch bearings are planned for a period of two years.
 

By simulating the real installation situation, different load scenarios and reproducible conditions can be emulated and blade bearings tested in various operating modes: dynamic application of load in five degrees of freedom, pitch, interim inspections. The effects of control concepts on bearing life can be checked and repeated any number of times. The reliability and availability of wind turbines can be increased, and costs reduced, as a result.


Technical Data

• Bending moments up to 50 MNm
• Bearing diameter up to 5 m
• Pitch activity
• Simultaneous test of two bearings
• Accelerated test procedure
• Emulation of the stiffnesses of hub and blade

Download data sheet

Full-scale test stand for complete nacelles in the power output range from 2 to 8 MW