Exploring limiting factors of wear in pitch bearings of wind turbines with real-scale tests

Abstract

Abstract. Oscillating movements under load can cause wear in rolling bearings. Blade bearings of wind turbines are subject to both. To know how to avoid wear in these bearings is important since they ensure the operational safety of the turbine. Oscillations of blade bearings vary in load, speed, and amplitude. The objective of this work is to find limits of these operating parameters with regards to wear occurrence. To this end several tests with real-size bearings were carried out. The test parameters are based on typical operating conditions of a reference turbine. The size of the bearings and the test parameters differ from other published tests for oscillating bearings, where often scaled bearings are used results. The test results show that wear occurs for every tested combination of load, speed, and amplitude of a steady oscillating movement. Even if the wear characteristics differ between tests, each of them resulted in wear. Hence, no wear limits can be defined with the tested grease and within typical operating conditions of a wind turbine below which wear does not occur. Tests with a discontinuity in the steady oscillation movement, however, did not result in wear. Such discontinuities can be longer movements embedded in steady oscillations. They are characteristic to wind turbine operation, where longer movements are a reaction to wind gusts.

Wear Development in Oscillating Rolling Element Bearings

“…BEHNKE ET AL. also showed results of the wear tests of real-size blade bearings with the same grease as the lubricant [5]. For the smaller amplitude ratios, the wear already occurred for only 1000 cycles, which matches the fast wear initiation obtained in the tests with the type 7220 bearings in this paper.…”

“…At a more advanced stage, after 80,000 cycles, the damage pattern shown in Figure 5c appears. Some parts of the wear mark still appear reddish-brown and on the left side at middle height a bigger material spall can be observed (5). It is also noticeable that large parts of the damage mark have been smoothed and have a mirror-like appearance (4).…”

“…and WANDEL ET AL. found that very small oscillation movements are most likely not the most critical ones when in terms of wear as a failure mode and the middle range of amplitude ratios has a greater impact [5,6] . However, the stochastic nature of the wind and operating conditions of the wind turbine as well as the influence of the specific order of occurrence of cyclic movements of the blade bearings makes rating the risk of wear damage a very difficult topic.…”

“…This method is usually called individual pitch control (IPC). Recent investigations have shown that, in contrast to CPC, IPC can decrease the number of very small oscillation movements and lead to generally larger oscillation amplitudes [4,5]. These oscillation angles depend on the individual controller and turbine design and do not allow for a general statement.…”

The effect of site-specific wind conditions and individual pitch control on wear of blade bearings

“…Tests on small-scale bearings showed that interrupting oscillating and wear-critical movements with lubrication runs effectively prevents wear. This was even tested and verified for real-scale blade bearings with about 5 m diameter Behnke and Schleich, 2023). In general, two different types of lubrication runs can be differentiated.…”