1 Introduction Electrical energy from wind turbines (WT) already plays a crucial role in the energy supply of Germany. Energy supply from renewable sources like wind is planned to increase in the nearer future for Germany and the European union [1-3]. A key aspect for energy providers is the Levelized Cost of Electricity (LCOE). A significant share of the overall cost of WTs stems from maintenance and servicing [4, 5]. The main bearing is one key component of a WT. Up to 30 % of WT main bearings experience failure during their planned service life [6-8]. Science and Research 14 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0003 Influence of wear and manufacturing inaccuracies on the performance of a conical plain bearing main bearings for wind turbines Jan Euler, Georg Jacobs, Timm Jakobs, Thomas Decker, Noah Smeets, Julian Röder* submitted: 19.09.2024 accepted: 6.03.2025 (peer review) Presented at GfT Conference 2024 * Jan Euler, M. Sc. Orcid-ID: https: / / orcid.org/ 0000-0001-9293-3219 Prof. Dr.-Ing. Georg Jacobs Orcid-ID: https: / / orcid.org/ 0009-0009-7683-6350 Timm Jakobs, M. Sc. Thomas Decker, M. Sc. Orcid-ID: https: / / orcid.org/ 0000-0002-3296-7166 Noah Smeets, B. Sc. Julian Röder, M. Sc. Orcid-ID: https: / / orcid.org/ 0000-0002-8701-6384 Chair for Wind Power Drives der RWTH Aachen University. Electrical energy harvested by wind turbines already constitutes the largest portion of the current electricity mix in Germany. With increasing penetration of wind energy in the energy sector, wind turbine availability and reliability become more important during turbine design. One component which is prone to failure is the wind turbine’s main bearing. A damaged main bearing results in long downtimes and costly repairs as the drivetrain needs to be dismantled using large cranes or specialised crane vessels for offshore turbines. One possible remedy is the use of plain bearings as main bearings instead of the commonly used rolling element bearings. Plain bearing main bearings can be designed with segments and thus potentially repaired up-tower without the costly dismantling of the drivetrain. Therefore, they can drastically reduce repair costs in case of bearing failure. To this end the novel FlexPad bearing design was developed at the CWD. The general applicability of the FlexPad design was shown in previous publications. During wind turbine operation, which is influenced by the intermittent weather and wind conditions and includes start-stop events, wear is a crucial aspect that needs to be accounted for during the design stage of plain bearings. Until now, there is no knowledge about the influence of wear on the profiling and performance during production loads of the FlexPad. Kurzfassung This paper presents a method to quantify the influence of wear on the contours profiling. The wear’s effect on the segment profiling is assessed by means of surface measurements of the FlexPad bearing. The complex conical geometry necessitates a novel measurement and post processing approach. Using this new approach, the FlexPad contour is evaluated pre and post testing. Based on the measurements the influence of wear and thus the changed contours on the bearing´s performance is evaluated by means of multi body elastohydrodynamic simulations. Moreover, the measured contours profiles are compared to an ideal profile, evaluating the influence of possible manufacturing inaccuracies. The investigated profiles of factory-new segments showed good conformity with the nominal profile, but due to their waviness there is a less favourable pressure distribution. Run-in segments show slight wear and smoothing. Both have a positive effect on the pressure distribution in simulative tests. Schlüsselwörter main bearing, drivetrain, FlexPad bearing, surface measurement, elastohydrodynamic simulations, simulative tests Currently all commercially available WT use rolling bearings as main bearings [9]. The replacement of a faulty rolling element main bearing is especially costly as the rotor needs to be dismounted and the WT drivetrain to be disassembled. One possible approach to address this challenge is the use of plain bearings as WT main bearings. Plain bearings can be designed with segmented sliding surfaces. In case of failure sliding segments can potentially be exchanged individually. This would result in reduced maintenance costs, as the segment exchange could happen up-tower without the use for expensive cranes and the overall downtime would be reduced. Plain bearings as WT main bearings are currently subject to research and development but are not used commercially to date. One such plain bearing design is the FlexPad main bearing which was designed and tested experimentally on different scales at the CWD [10-16]. The Flex- Pad is a double conical plain bearing design with a segmented outer sliding surface which is connected to the housing by a flexible arm structure (see Figure 1). The flexibility allows for the sliding segments to follow the deflection of the shaft which reduces edge wear and improves load distribution. In addition to the macro-geometry design (see Figure 1) a converging gap between shaft and bearing is essential for the hydrodynamic operation of a plain bearing. The converging gap leads to pressure build-up when oil is forced through by the rotation of the shaft. Radial journal bearings achieve this converging gap with an eccentricity between shaft and journal. Segmented bearings can also utilize a surface profile, e.g. a wedge-shaped inlet surface machined on the segment (see Figure 2 right grey areas). The shape of this surface profiling influences the hydrodynamic performance of a plain bearing and improves to the load carrying capacity as well as reducing the frictional losses during the bearing’s operation. The shape was addressed in various studies starting with the simple Rayleigh step from 1918 to 3D topologies in recent studies [18-23]. Naturally, abrasive wear influences the bearings profiling and thus has influence on its hydrodynamic performance. As with most common segmented plain bearings the FlexPad also utilizes profiling on its sliding surfaces to improve performance [24]. The design for the initial profiling was done by hand. To the authors knowledge there are no studies focussing on the effect of profiling for conical plain bearings except the works of Jakobs et al. [24]. A deeper understanding of the influence of profiling on the performance of conical plain bearings is therefore necessary. The FlexPad sliding surface contour was designed with a simple profile. The profile of the contour is shown in Figure 2. Figure 2 left shows the depth of removed material from the surface of the ideal circular surface of a slice of the segment. Denoted is the depth of removed material correspon- Science and Research 15 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0003 Pad area parallel to shaft Areas of retraction forming the profile of the pad Outlet wedge Inlet wedge Direction of shaft rotation and fluid transport over the segment Profile function -15 15 Figure 2: Target profile of the investigated FlexPad design in 2D (left), target profile in 3D (right) Figure 1: Original FlexPad design (1); Schematic depiction of the FlexPad’s flexibility (2) [17] (1) (2) The contour follows a circular progression which exceeds the depth of the segment profile significantly in scale. Furthermore, the measured section does not follow a circular path but cuts straight across the segment (see Figure 3, d). The measured contour also follows a curvature (see Figure 3 e) with only the outer chamfers of the profile shown in Figure 2 being discernible. To determine the machined profile of the segment the underlying curvature must be removed through post processing. As the FlexPad has a conical shape one could assume the measured contour follows a circle. This, however, is not the case. The macro geometry of the measurement is analogous to a conic section. As the cutting plane is roughly parallel to the opposing generating line of the cone, the measured macro geometry can be approximated with a parabola. A parabola with its apex and the coordinate centre can mathematically be described as: 1 For a conic section the parameter a can be defined as 2 The length AF −− refers to the distance between the apex and the focal point of the parabola. The focal point of the parabola stemming from a conic section is at the intersection between the cutting plane and the cone’s centre line. Through the breadth of the measured contour, the axial position of the measurement along the segment and thus its total position can be determined. The Flex- Pad segments are measured with a 90° angle respective to the backside of the segment. The cone angle of the FlexPad V2 is 46.7° [12]. Using this knowledge, the focal point and its distance to the apex can be determined, as shown in Figure 4. With the function of the parabola known for each measurement, its value can be deducted from the measured contour. This results in a profile measurement similar to the target profile shown in Figure 2. 2.2 Measurement Results To establish a baseline for the observed wear and damage, the profile of a segment which has not been used in test bench trials is measured. The results are shown in Figure 5. Depicted is the following: 1. The target profile. 2. The contour as measured and adjusted by the parabola function, which was determined through the measurement position. The measured contour shows good agreement with the target contour. Between ten and fifteen degrees of centre ( ) = = 1 4 Science and Research 16 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0003 ding to the angular position on the sliding surface. The profiling is symmetrical to allow for usage of segments for both rotor side (RS) and generator side (GS). The profile is most pronounced at the edges of the sliding segment, where the most material is removed to further reduce the possibility of edge wear. Towards the centre the profile follows a slight ramp to induce the formation of a convergent lubrication gap. In the centre of the segment the profile forms a plateau and no material is removed. This allows for large load bearing surface. The sliding surface of the FlexPad’s sliding segments consist of a thermally sprayed coating of cobalt-chromium alloy with integrated hexagonal bohrnitrite. The surface material was especially chosen for its wear resistance and low friction coefficient [16, 25]. Nevertheless, wear can still be expected to occur during the bearing’s lifetime, due to the special load conditions experienced by a WT’s main bearing (e.g. start-stop operation under the influence of high radial load) where mixed friction operation cannot be avoided completely. In this study the wear driven profile change between factory new and tested segments of the FlexPad bearing is investigated. To this end a simple measurement approach is presented to determine the contour and its profile for conical sliding surfaces. The profile of the measured contours of worn and unworn segments are compared to the target profiling of the contour. The influence of wear and manufacturing inaccuracies on the FlexPad profile and on its performance is assessed by means of a multi-body elasto-hydrodynamic (MB-EHD) simulation. 2 Contour Measurements To assess the effect of wear and manufacturing inaccuracies on the bearing’s performance, the profile of tested segments needs to be determined. Currently, there is no standardised method for the contour and profile measurement of conical plain bearings. The conical shape results in the contour dominating the measurement. An assessment of the profile is not possible without postprocessing. Therefore, a novel approach is proposed in this study. 2.1 Measurement Method To measure the contour of the FlexPad segments a Hommel T8000 (see Figure 3, c) is used. The measurement system has a resolution of 0.1 μm. The contour is determined by a tactile measurement along the Xand Ycoordinates of the segment surface. The individual segments are measured in 10 distinct radial measurement positions as shown in Figure 3 b. The measurement positions are identical for all segments and are determined by individual positioning slots in the measurement bracket (see Figure 3, a and b). For common radial plain bearings a measurement like this results in a directly visible profile of the measured plain bearing [26]. The approach however is not suitable for the FlexPad geometry. the desired chamfers are distinctly visible. In the centre (± 5 °) no material is removed from the original circular sliding surface for the factory new segments. Between the chamfers and the centre inlet and outlet wedges are visible, starting at around ± 6.5 ° of centre. Unlike the target contour, the measured contour features a slight waviness. The influence of the minimal manufacturing inaccuracy on the bearing’s performance is further analysed in chapter 3. Some segments on the rotor side show clear indications for three body abrasion (see Figure 7, right). The abrasion groove follows a circular pattern. Therefore, it was most likely caused by metallic debris dragged around for one or more revolutions causing abrasive wear. The abrasion is present in almost all RS segments and becomes more pronounced towards the centre of the loaded zone for production loads and start/ stop procedures. The circular abrasion can also be documented with the contour measurements (see Figure 8). The parabola sub- Science and Research 17 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0003 a) b) c) d) e) measurment positions direction of measurment μ Figure 3: Bracket for segment measurement (a and b), contour measurement system (c), linear measurement progression (d), Measurement without post processing (e) 46.7 ° Measurement position Focal point 90 ° Figure 4: Geometric determination of parabola focal point centre two distinct collapses are visible. This is caused by the measurement progression path cutting through the groove two times, as the circular arc of the groove is Science and Research 18 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0003 traction derived from the measurement position yields expected results. The chamfers and inlet/ outlet wedges are relatively unfazed by the abrasion. In the segment start/ stop wear groovelike abrasion Figure 6: Lower RS bearing halve top view (left), segment in three o’clock position with circular groovelike abrasion (right) Rotation Figure 5: Contour for an untested segment in comparison with the target contour Rotation Figure 7: Contour measurements for a rotor side segment in abrasion grove height traversed. For these contours no following MB-EHD simulation is performed as the abrasion is only localized in a small segment area. The FlexPad prototype was subject to intense testing of startand stop procedures. The amount of start-ups corresponds to the amount a WT main bearing would see during its predicted service live [10]. The segments in the lower halve of the RS cone show signs of wear and some groovelike abrasion in accordance with the remaining segments. In Figure 8 the measured contour of a tested RS segment is shown in comparison to the target contour. The tested segment was mounted in the bottom position of the RS as shown in Figure 6. Again, the profile determined via parabola subtraction is shown. The measurement position is in the segment centre where significant wear is visible. The measured profile is visibly less wavy than the contour of the untested segment, see Figure 5. The contour seemingly follows the same progression as for the untested segment. However, the inlet wedge progresses to the segment centre instead of stopping at ± 6.5 °. As the contour measurement is aligned to 0 mm at its highest point, the total depth of material loss cannot be determined using this method. Overall the contour shows little signs of wear and material removal. The visible smoothening of the segment however, warrants further investigation in future studies. 3 Simulation Results and Discussion Contours and their profiling have a large influence on the bearing’s performance. To assess the influence of the worn profile for the FlexPad bearing, MB-EHD simulations were performed using the determined contours. The simulation setup is identical to previous studies [10, 12, 24, 27, 28]. The contour profile was integrated with a grid size of 4 mm. Investigated was the bearing’s performance under stationary production load conditions. For all simulations one profile was applied to all segments and over the whole segment length. For the target profile this is clearly suitable as all segments were designed with this profiling in mind. For the measured contour of factory new segments this already constitutes a compromise as not all measured contours were 100 % identical. However, all showed the same general profile progression. Therefore, a representative profile was chosen from a centre measurement position (see, Figure 5). Contours that documented the groovelike abrasion were therefore unsuitable for simulation as the area could not sensibly be extended over the whole segment. For worn contours from segments of the bottom RS position a representative profile was chosen and applied for all segments (see, Figure 9). The bottom centre on the RS would typically experience the most amount of load during production load conditions and start/ stop procedures. This is due to the weight of the Rotor. The compromise, to use this contour a representative for the worn FlexPad, was deemed to be acceptable as a worst-case investigation. Future studies should further investigate the influence of 3D contour profiling for individual segments and non-uniform contour profiling for all segments on the bearing’s performance. The results of the simulative investigation on the bearings performance for different profiles are listed in Table 1. Listed are the values for the maximum hydrodynamic pressure, the maximum average segment pressure, the total amount of friction loss, the percentage of pressured area and the amount of solid body contact force relative to the sum of all forces. The target profile achieves the lowest maximum pressure of 17.9 MPa. The factory new profile results in a 65 % increased maximum pressure relative to the target contour. The worn contour has a significantly lower maximum pressure relative to the target profile (11 % increase). The increased maximum pressures for the factory new profile are a result of the overall wavi- Science and Research 19 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0003 Rotation Figure 8: Contour of tested segment from the bottom RS position The worn profile shows further reduced friction loss, due to the slightly reduced pressured area and lack of solid body friction. Bearings featuring the factory new profile demonstrate the lowest amount of hydrodynamically pressured area and are the only sample for which solid body contact would occur. In Figure 9 the difference in hydrodynamic pressure distribution corresponding to the difference in oil film height for worn and factory new contours is shown. For the most highly pressured segment (see Figure 9, highlighted segments) it can be seen, that the waviness of factory new contours results in the same waviness for the lubrication gap. The sudden fluctuations in the height of the lubrication gap result in a smaller loaded zone for the induvial segments and thus higher maximum pressu- Science and Research 20 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0003 ness and slight manufacturing inaccuracies of the nonidealized profile. The improved performance of the worn profile is due to the overall less wavy surface which features an elongated inlet wedge. An elongated inlet wedge was already demonstrated to yield positive effects on pressure distribution for the FlexPad [24]. Both waviness reduction and the elongated inlet wedge are a result of the running in process, which is common for plain bearings. The maximum average segment pressure is comparable for all profiles, as the overall load distribution in the FlexPad bearing is unaffected by the profile. The worn profile shows a slightly reduced maximum average segment pressure. This is caused by a slightly improved force distribution among segments. Simulated bearings with the factory new profiles demonstrate the highest friction loss, caused by the solid body contact. Target contour Factory new contour Worn contour Maximum hydrodynamic pressure [MPa] 17.9 29.5 19.8 Maximum average segment pressure [MPa] 2.08 2.12 2.01 Friction loss [W] 407 427 387 Pressured area*) [%] 47.8 44.4 46.7 Solid body contact**) [%] 0 0.2 0 *) area of the bearing surface with hydrodynamic pressure above 1 Pa **) amount of force carried through solid body contact relative to the sum of all forces Table 1: Simulation result for different contours under production load conditions worn contour worn contour factory new factory new Oil film height [μm] Figure 9: Oil film height (left) and hydrodynamic pressure (right) for both a FlexPad bearing with factory new and worn contours (shown is the RS in the front), segment with highest pressure highlighted in black res. The area of maximum pressure is also the area for which solid body contact occurs. For the investigated load conditions the target profile performed best. Factory new profiles displayed waviness which was disadvantageous regarding pressure distribution and thus maximum pressures. Maximum pressures are significantly increased for the measured contour, caused by the waviness of the profile. The increased maximum pressures could lead to damage in the running in process and should be kept in mind during bearing design. The worn contour profile showed clear improvement compared to factory new contour profiles due to running in. Worn profiles displayed no waviness and featured advantageously elongated inlet wedges. For the worn profiles the bearing achieves a performance comparable to the idealised target profile. The wear experienced by the FlexPad during test procedure is therefore not critical. The FlexPad segments contour profiling can be expected to remain functional during the service life of its wind turbine, should no extraordinary events occur. 4 Conclusion Plain bearings play an ever-greater role in drivetrains of WTs. Current industry and academic research explore the feasibility of plain bearings as WT main bearings. Modern plain bearings feature sophisticated surface profiles to improve performance and guaranty safe operation. In this study the influence of wear on the profile of the novel conical plain bearing main bearing concept FlexPad was determined, using 2D contour measurements with post processing. Investigated were factory new segments and worn segments from a highly loaded position within the bearing. The determined profiles were compared to the target profiles in their progression. Further their effect on the bearing’s performance during production load conditions was investigated by means of simulations. The progression of the factory new profiles showed a very good agreement with the target profiles Profiles of worn segments showed expected signs of running in i.e. smoothing and elongated inlet wedges. During simulations of production load conditions, the bearings with factory new profiles demonstrated significantly higher maximum hydrodynamic pressures than the idealised target profile. The worsened behaviour is most likely caused by the profile’s waviness and slight manufacturing inaccuracies. For the worn profiles the bearing’s performance was comparable to results for the idealised target profiles with slight improvements in friction loss and maximum hydrodynamic pressures. The measured contour showed little wear, which improved the bearings performance rather than worsening it. The FlexPad profile therefore demonstrated good wear resistance for the approximated service live of a wind turbine, as more than 10.000 start stop procedures were performed. Future studies will further combine the presented method with the 3D profile methods investigated by Jakobs et al. [24] to achieve an improved understanding of the effect of homogeneously worn contour profiles. The obtained measurements will also be used to assess the influence of surface roughness on breakaway torque and overall performance, i.e. maximum pressures, friction loss and load distribution. References [1] Climate Action: 2050 long-term strategy. 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