the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jan 2024
Load case selection for finite element simulations of wind turbine pitch bearings and hubs
Abstract. Finite element simulations of large rolling bearings and structural parts are an indispensable tool in the design of wind turbines. Unlike simpler structures or smaller bearings in rigid environments where analytical formulas suffice, wind turbine components require a more comprehensive approach. This is because analytical formulas often fall short in predicting load distributions and stresses, leading to inadequate designs. However, due to the size of the finite element models and operational loads involved, it’s necessary to strike a balance between achieving realistic results and keeping computational times manageable. This study focuses on the selection of load cases for simulations of pitch bearings and hubs of wind turbines. The models for these contain the hub, the pitch bearings, the inner parts of three blades, and any necessary interfaces parts. The simulation results allow the calculation of static and fatigue strength. Given the complexity of the problem, with each rotor blade having six degrees of freedom, five types of loads, and the pitch angle, the potential combinations of loads would result in an unmanageably high number of required simulations. The present work exploits relationships between load components and the rotor position to reduce the number of load cases needed for fatigue calculations. The IWT 7.5-164 reference turbine and three commercial turbines serve as the basis for case studies which include bin counts and exemplary finite element simulations. The blade’s azimuth angle and bending moments of one blade allow determining the loads at all three blade roots with a reasonable degree of confidence.
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RC1: 'Comment on wes-2023-163', Jonathan Keller, 31 Jan 2024
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In this paper, the authors present a methodology for selection (or really, downselection) of load cases related to finite element analyses of a pitch bearing design. In general, the paper is well written and structured and presents a nice contribution on the topic of pitch bearing design.
Lines 4-5: I believe this sentence could be written a bit better, as I’m not sure the “size” of the model or the (magnitude) of the operational loads matters. I think the sentiment being expressed is something like “However, due to the number of contacting bodies and operational load cases involved, it’s necessary to…” Then again, in the case of a structure such as a pitch bearing, it’s more like the small size and large number of contact patches compared to the large overall size of the bearing itself that is what we mean by “size” of the model. But, I think the provided sentence gives the proper gist of things.
I will admit the large number of very short paragraphs typically of 1-3 sentences in Sections 1 through 2.1 make things a bit hard to read. I recommend a little effort to group like paragraphs together. For example, the paragraphs in lines 16-43 and then separately 44-56 can all be combined into a single paragraph. Sections 2.2 through 4 are much better.
Figures 1 and 2: If possible, I believe it would be helpful for the reader to label each box that corresponds to the descriptions in the text. For example, in Figure 1 it might be “Load time-series”, “Rainflow counts”, “Rotor model”, “Finite element analysis”, and “Stress calculations”.
Lines 72-74: I think the second version of the sentence is written more clearly than the first, honestly, as I don’t think we’re talking about “signals”. It is most easily cleared up by changing the first sentence to “…to which the loads on a pitch bearing are affected by not only the loads on the blade to which it is connected, but also the loads from the other blades as transmitted through the hub.” I’m not sure if the second sentence is then needed, or maybe the first can be deleted and just the second kept.
Although nice to have, I believe Figures 3-5 could be placed side-by-side or otherwise condensed a bit more, as 3 independent figures here seem a bit overkill. I suppose that’s just a preference thing though. I would label the primary and secondary blades in Figure 5 (or some version of it) though.
Lines 103-106: I am curious as to the message intended here. Everything is pretty cut-and-dry to this point, but these sentences beg as may questions as they answer, or maybe they don’t answer anything at all? It almost feels like it’s a description of future work that might be included in a conclusions section. I don’t have any more specific comment here, but rather I’m just generally a bit puzzled as to what to make of these sentences.
Section 2.3: I’m also a bit confused by the title of this section and the description of “missing” pitch angles. I believe what lines 111-115 is saying is that the pitch angles of the secondary blades can simply be assumed the same as the primary blade. In that respect, it’s not missing, so it feels like this statement is better suited in the previous section. Similarly with the hub moments, as these are best derived from the blade root moments. I guess I also don’t understand how the blade azimuth angle would be “missing” from a simulation. Overall, I believe some additional context would be helpful here, or my other suggestion would be to just combine this with Section 2.2. I really must be missing something, as later on I also don’t seem to understand the value of Figure 12.
Table 1: I will admit I only “sorta” get what is being described here, but also sorta not. I was also puzzled by the statement “The results are arbitrarily chosen”. It feels like a better use of some text would be to maybe show an example real result for the IWT7.5 in an Appendix, if such a result would be too long in the main body.
Figure 8: It’s a bit hard to interpret as is. I see in the second row that the moments are thick lines and the forces thin ones. But I don’t see any thick lines in the first row? It also looks like there are two sets of thin lines in the first row, one solid and one dashed, but the text says only one force? It also feels like 30 seconds is more than needed, something like 20 seconds (or 3 revolutions) is probably sufficient. I might also suggest expanding the plot vertically, or plotting on a 2x2 grid rather than a 4x1 grid to make the plots more visible.
Figure 9: What do the vertical yellow lines indicate?
Figure 11: Is currently quite small and should be increased in size, especially the font size.
Minor typos:
Line 7: I think it should be “…interface parts” or “…interfacing parts”.
Line 34: I believe the citation style here should look like “Chen and Wen (2012) simulated…”
Line 313: I believe the volume, issue, page numbers and doi are missing from Chen and Wen. 134 (4): 041105, https://doi.org/10.1115/1.4007349.
Line 134: Missing space between sentences.
Line 186: I think what is being described here is the “bolt holes”, instead of “bore holes”. Similar in line 187. Maybe this is just me though, and “bore hole” is common in the pitch bearing community.
Line 191: I’m not sure what the “As axial location” here indicates.
Line 222: I believe it should be “Both moments had positive effects…”
Line 292: I believe the citation style here should look like “…based on in Becker and Jorgensen (2023).”
Citation: https://doi.org/10.5194/wes-2023-163-RC1
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