The bearing necessities – stress-testing blade bearings

With wind turbines in the 7-10MW range, the wind drives blades up to 80m in length but, in the future, that may easily increase to 100m. As blades become longer, the stress on every component increases, which poses a serious problem for blade bearings in particular. Professor Jan Wenske from the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) Fraunhofer is part of a new highly accelerated pitch-bearing test (HAPT) project that will investigate the causes of bearing failure, which are poorly understood at present.

“The statistical database for downtime and failure is small even though wind power has been around for over 30 years,” Wenske says. “But we know from our data and from the work of other research institutes that the pitch system as a whole is the number one cause of downtime. This includes not only bearings, but also electrical systems and electronics. Nevertheless, bearings are a significant factor, especially in turbines of 5MW or more. With the next generation of turbines in the range of 10MW, there is an exponential increase in forces and bend moments, so the challenge is even greater.

Reality check

“Bearings are the weak part of the design. They are the bottleneck with systems of this size. We are working on a way to design and test blade bearings under realistic conditions to improve the reliable operation of future turbines.”

IWES Fraunhofer is working with slewing ring manufacturer IMO Group and the Institute of Machine Elements, Engineering Design and Tribology (IMKT) on the HAPT project, which will develop a test bench for blade bearings and a method for calculating their service life. The goal of the project, which has received €10.7m in funding from the German Federal Ministry for Economic Affairs and Energy, is to simulate 20 years of operation in just six months of testing.

“We started investigating this topic four years ago, and we contacted IMO. Now, more suppliers are becoming interested in the project because they want to learn more about what we are doing. At the moment, turbines of 7-8MW are the most common, and the cost can be up to €36,000 a day if one is down. For offshore turbines, the downtime could be as much as three weeks; so, the potential for lost revenue is very high. As the load increases, faults in the blade bearings also increase exponentially and the rate of damage rises,” Wenske says.

When the HAPT project is complete, it will yield a new and larger test rig to accurately model the performance of blade and pitch bearings in larger wind turbines, and it will provide the industry with the prerequisites for the computational design of blade bearings with more precise dimensions. The three partners involved in the project bring different skills and experience.

Fraunhofer has essential turbine competence, and it can test and model advanced turbines and control systems. It is accustomed to the process of accelerated testing for many components, which Wenske says involves more than simply increasing parameters of stress and frequency.

“We need a new testing rig for the size of bearings that we are talking about,” Wenske says. “We need to be able not only to increase oscillation but also to examine the action of lubrication films. We can increase loading but the stress in the material must not be so excessive that it causes failures that would not be seen in the real operation of turbines. We need a rig in which we can use frequency, loading, sequencing and other elements in the right combination. It is clear that we are not looking for the effects that are already well understood, but for something new.”

IMKT, which already has a test rig for bearings up to nearly 1m in diameter, has a great deal of experience in testing, as well as detailed knowledge of how lubrication films perform. IMO understands the manufacturing process, and the current limitations on design and stress loading, so it will keep its ideas for optimisation realistic.

The next milestone in the project is the planning stage for the test rig. There is a lot of pressure to complete that within a strict timescale, given that HAPT is a publicly funded project, but Wenske is confident that the deadline will be met as the partners are fully engaged.

“There is a lot of knowledge about the common way to load and operate bearings, but there is a lack of knowledge about the special conditions that affect pitch bearings,” Wenske says. “They do not move continuously, and there is a lot of variation and oscillation. They are different from blade bearings, which are coupled to the weak, low-stiffness component of the blade, so there is big deformation; however, on the other side, it sits on a hub made of steel, so this is not a typical application for standard calculations. Pitch bearings have been used for over 20 years, but the loads were not as high back then as they are now.”

Reduce the load

When HAPT improves the industry’s understanding of blade-bearing design, there is hope that the use of individual pitch control (IPC) systems for load reduction, which is a primary goal for manufacturers, will become a reality. IPC balances out the loads across individual blades and reduces the overall load, but there is little reliable information available concerning the suitability of blade bearings for IPC.

“IPC means continuous movement, because the pitch of the blade changes all the time to cope with wind share, gusts and tower shadow effects,” Wenske says.

So, some roller bearings work with very small movements, and that can lead to new mechanical stresses and failures. The wind energy industry is innovative but it can be slow to risk new technologies.

“Companies do not want to make dramatic changes, but with turbines of 8MW and more the goal is to bring down the cost of using bigger blades, so you have to overcome the fact that bigger rotors lead to exponentially larger forces. We have to find a smart way to bring loads down, which means IPC or using better, newer or less material.

Cost-effective solution

“The industry must be sure that IPC works. IPC is essential to build a cost-efficient turbine, but so far, manufacturers have not cleared their bearings for use in IPC. So, we have a bottleneck. IPC reduces loads in a theoretical environment but the components are not there, so we can’t look at bearings in operation and the industry is, therefore, looking for a solution.”

HAPT aims to provide a test bed that could simulate the load to which bearings would be exposed were they to have the weak structure of a rotor blade attached, which Wenske believes would go beyond what manufacturers have done in testing. An accurate assessment of stresses is essential to open the door to IPC and to develop turbines with longer blades.

Once the bearings bottleneck has been resolved, Wenske says that the industry will tackle the problem of extending rotor blades beyond 80m, which will require the development of new technology. For now, though, he is focused on the task at hand, which will pave the way for the next phase of turbine innovation.  ?