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Time-dependent analyses of wear in oscillating bearing applications

Authors:
Fabian Schwack at Vestas Wind Systems A/S
Fabian Schwack
Artjom Byckov at Leibniz Universität Hannover
Artjom Byckov
Norbert Bader at University of Twente
Norbert Bader
Gerhard Poll at Leibniz Universität Hannover
Gerhard Poll
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Abstract and Figures

The most common wear damage modes in oscillating bearings are false brinelling and fretting corrosion. Under the aspect of big enough oscillating amplitude to contact area ratio, false brinelling can be described as the incubation process of fretting corrosion. Incubation process means, that at the beginning of the oscillating motion a lubricant film is present. If so, mild wear will occur, called false brinelling. The lubricant will squeezed out with time. Thus, the oxid film will be destroyed. The oxide particles lead to abrasive wear. This paper will show experimental results of bearings which operate oscillating. The analyses are focused on the time-dependent spreading of wear and the occurrence of different wear modes. Furthermore, fatigue spalls are analysed. The experiments are accomplished on several bearings with a constant amplitude, frequency and load. The used lubricant is mineral oil. The experiments were conducted using different number of cycles between 100 and 5.000.000.
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TIME-DEPENDENT ANALYSES OF WEAR IN OSCILLATING BEARING
APPLICATIONS
TRACK OR CATEGORY
WEAR
AUTHORS AND INSTITUTIONS
Fabian Schwack, Artjom Byckov, Norbert Bader, Gerhard Poll
Institute of Machine Design and Tribology (IMKT), Leibniz Universitaet Hanover,
INTRODUCTION
Oscillating bearings can be found in several industrial applications. The minute motion is
intentional in some applications like blade bearings for wind turbines [1]. Other bearings in technical
systems are unintentionally affected by vibrations due to operation or shipping.
The dominating and therefore most focused upon damage modes in oscillating bearings are false
brinelling and fretting corrosion [2]. False brinelling was first mentioned by ALMEN in 1937 [3].
Figure 1 shows a false brinelling damage and the typically occurring areas due to slip. False brinelling
can be found in lubricated bearings operating under small pivoting angles. The greyish color occurs due
to the formation of magnetite, i.e. .
First effects of fretting wear were mentioned in 1927 by TOMLINSION [4]. The term fretting is
used for damages where two contacting surfaces are subjected to relative slip [5]. The main difference
between false brinelling and fretting corrosion is, that false brinelling occurs under mixed lubrication
conditions, while fretting corrosion occurs under dry contact conditions. In unlubricated contacts the
oxide film will be time-dependently destroyed due to the metal-metal contact. The oxide particles in the
contact spot will then lead to abrasive wear. Figure 1 shows a fretting corrosion damage of a point-
contact. The reddish color occurs due to the formation of hematite, i.e. alpha -  [6].
Figure 1: Different areas of false brinelling damage [7] and typical fretting corrosion damage
Under the aspect of a certain ratio of displacement to Hertz’ian contact area () [8], see Figure 1,
false brinelling can be described as the incubation process of fretting corrosion [9] [10]. In this case,
incubation process means, that at the beginning of the oscillating motion a lubricant film is present. If
so, mild wear occurs (false brinelling). Beneath a certain  -ratio the lubricant can be squeezed out
of the contact. The metal-metal contact leads to fretting corrosion [11]. In this paper effects of the
incubation process are analyzed for an angular contact ball bearing which is lubricated with mineral oil.
The operating parameters (pivoting angle, oscillating frequency, load and lubrication) are constant for
all experiments. The experiments were run for  up to    cycles to show the progress of wear
with increasing cycles. For the analyses of the wear marks, different wear characteristics were examined
according to literature.
TEST CONDITIONS
The experiments were conducted on a test rig equipped with a servo-motor. The motor allows the
required oscillation motion. The bearings were loaded under pure axial load. For the experiments
angular contact ball bearings of the size 7208 were used. Table 1 shows the experimental data.
Bearing
Bearing size
7208
Inner diameter
40 mm
Outer diameter
80 mm
Number of rolling elements
14
Lubricant
Type
Mineral oil
Kinematic viscosity (40 °C)
100 mm/s²
Experimental data
Pivoting angle
1,2°
Oscillating frequency
5 1/s
Axial load
8500 N
No. of cycles

Table 1: Experimental data
RESULTS
The test bearings were analyzed with a laser-scanning microscope (Keyence vk-x200). Each
contact spot between raceway (inner and outer ring) and rolling element was analyzed. Thus, roughly
300 laser-scans were carried out. Figure 2 shows the occurring wear in seven contact spots between
raceway and rolling element. Also four contact spots under dry conditions are visualized. All contact
spots were subjected to a different number of cycles.
Figure 2: Results with different number of oscillating cycles under lubricated and dry conditions
Figure 3 shows some results of the experiments. Figure 3.1 shows the heavily damaged area vs. cycles
on the inner and outer ring. The damaged area increases with the number of cycles. For    cycles
the area on the outer race slightly decreases, which could be affected by statistical deviations. Figure 3.2
shows the distribution of undamaged, mildly damaged and heavily damaged area. This graph shows that
the heavily damaged area increases with rising number of cycles. Finally Figure 3.3 and 3.4 show the
hematite and magnetite portions for the inner and outer raceway. The hematite portion increases with the
rising amount of cycles. Also the magnetite portion rises. In Figure 3.3 some statistical deviations can be
seen at    cycles. With    cycles the portion of magnetite decreases while the portion of
hematite rises due to covering effects and/or tribochemical reactions.
The results show, how different oscillating wear phenomena occur with increasing amount of cycles.
Figure 3: Overview of results
KEYWORDS
Fretting wear, false brinelling, minutely vibrating
REFERENCES
[1] Schwack, F., Stammler, M., Poll, G., and Reuter, A. 2016. Comparison of Life Calculations for Oscillating Bearings Considering Individual Pitch Control in Wind Turbines. Journal of
Physics: Conference Series 753 (11) 112013.
[2] Errichello, R. 2004. Another Perspective: False Brinelling and Fretting Corrosion. Tribology & Lubrication Technology 60 (4) 34–36.
[3] Almen, J. O. 1937. Lubricants and False Brinelling of Ball and Roller Bearings. Mechanical Engineering 59 (6) 415–422.
[4] Tomlinsion, G. A. 1927. The Rusting of Steel Surface in Contact. Proceeding of the Royal Society of London. Series A. 115 (771) 472–483.
[5] Uhlig, H. H., Ming-Feng, I., Tierney, W. D., and McClellan, A. 1953. Fundamental investigation of fretting corrosion. NACA Technical Note 3029.
[6] Godfrey, D. 1999. Iron oxides and rust (hydrated iron oxides) in tribology. Lubrication Engineering 55 (2) 33–37.
[7] Schwack, F. and Poll, G. 2016. Service Life of Blade Bearings. Problems Faced in Service Life Estimation of Blade Bearings. Windtech International 2016, Nov/Dec, 19–22.
[8] Maruyama, T., Saitoh, T., and Yokouchi, A. 2016. Differences in Mechanisms for Fretting Wear Reduction between Oil and Grease Lubrication. Tribology Transactions, 1–9.
[9] Godfrey, D. 1956. A Study of Fretting wear in mineral oil. Lubrication Engineering 12 (1) 37–42.
[10] Ming-Feng, I. and Rightmire, B. G. 1956. An experimental study of fretting. Proceedings of the Institution of Mechanical Engineers 170 (1) 1055–1064.
[11] Godfrey, D. 2003. Fretting Corrosion or False Brinelling? Tribology & Lubrication Technology 59 (12) 28–30.
... controllers 2,3 often leads to more cycles, starved lubrication and increased risk of wear and more pronounced wear. 11 Nevertheless, some individual pitch controllers may positively affect the bearings since the pitch sequences are more favourable. 12 17 Stammler et al. used the pitch drive of a 3-MW-call pitch bearing with 2.3-m diameter to compare friction torque models with measurements. ...
... Any constant oscillation causes wear in a bearing if the number of cycles is high enough. 11 Stammler et al. showed that longer movements that interrupt sequences of short oscillations can prevent wear. 12 These movements are called protection runs. ...
... These results are in line with the incubation hypothesis. 11 The tests of the T-Solid bearing resulted in less pronounced wear. For x/2b = 2.67 the 69% of the exposed areas show severe wear. ...
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... For example, in wind turbines, pitch bearings are simultaneously subjected to intentional oscillations caused by pitch control operations and unwanted oscillations caused by uneven airflow [7,8]. Oscillations can cause micro movements between rollers and raceways [9,10], and false brinelling and fretting corrosion will become typical major failure pattern after a certain number of oscillation cycles [11,12]. Similar examples can be found in antennapointing mechanisms in space applications [13] and heavy machinery such as cranes [14]. ...
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... When wear conditions are met, surface alterations can appear after less than 1000 cycles, and brownish-red corrosion products are clearly visible after at least 5000 cycles (Schwack et al., 2017;Wandel et al., 2023). Effective protection runs are one means to prevent wear on raceways. ...
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View
... When wear conditions are met, surface alterations can appear after less than 1000 cycles, and brownish-red corrosion products are clearly visible after at least 5000 cycles (Schwack et al., 2017;Wandel et al., 2023). Effective protection runs are one means to prevent wear on raceways. ...
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View
... The whole area covered by the rolling element shows severe wear. Parts of the mark show red-brown and black deposits which indicate the iron oxides hematite and magnetite [24,25,26]. ...
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View
... The whole area covered by the rolling element shows severe wear. Parts of the mark show red-brown and black deposits which indicate the iron oxides hematite and magnetite [24][25][26]. ...
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... False Brinelling and Fretting Corrosion can occur under similar conditions if the oscillation amplitude is so small that slip occurs in a fixed contact region [18]. If the lubricant was squeezed out of the contact zone [19,20] or the lubricant was not able to separate the contacting surfaces [21,22] or if no lubricant was used, Fretting Corrosion likely occurs. While False Brinelling triggers mild adhesive wear, the surfaces are heavily stressed by Fretting Corrosion. ...
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... As explained in Sect. 1, the oscillation wear in the form of false brinelling and/or fretting corrosion is also considered one of the critical failure modes of the pitch bearing. Recent research has shown that the occurrence of the oscillation wear damage, its severity and speed of damage propagation are influenced by many parameters such as oscillation amplitude and frequency, contact loads and lubricant and so on [7,14]. It is impossible to make a final conclusive assessment on whether or not such wear damage occurs based on the current state of industry knowledge, however there appears to be a consensus that the more successive oscillation cycles with small amplitude the bearing experiences, the higher the risk of such wear damage. ...
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