Experimental Study of the Thermal and Wear Characteristics of a Foil Bearing Lubricated with a Low-Boiling Liquid

Round 1

Reviewer 1 Report

1.               Abstract need a lot of improvement. It's a generalized abstract. The results are not included nor the impact of research is created for readers

2.               Introduction does not cover or discuss research on foil bearing with the liquid cooling medium.

3.               In the introduction status quo on the topic is also not discussed.

4.               In the introduction gaps are not identified. Accordingly, novelty is not defined considering it.

5.                Results on lines 326 to 330 in the conclusion are generalized and could be concluded with simulation or calculation.

6.               Knowledge value of research or novelty is below par.

 

 

1.     Line 41 to 43 and 83 to 89, correct English syntax 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The article is dedicated to foil bearings that are of great interest to scientist and engineers at the present period, the trend to oil-free technologies in turbo machinery is obvious. The authors study experimentally the tribological behavior of the first generation foil bearings of different sizes and from different materials, under lubrication with low-boiling liquid. Unfortunately, the design of the study is elaborated not well enough to provide useful reliable results. The design, the methodology and the presentation quality must be improved significantly. The paper in the present form must be rejected, but resubmission after the improvements is welcomed because the issue is interesting and can potentially provide scientific and practical value.

The main reason for such a decision is that the authors missed the control over the tested samples of bearing. They do not provide any measurements of the bearings after assembly and before testing them. The authors state themselves in Line 228 that the technology used to manufacture bump foils is characterized by poor repeatability. The abnormal result in Fig. 9 when the geometrically less tight bearing demonstrates more heat generation is the additional and expressive evidence that the actual geometry is out of control. Thus, the obtained results on heating and wear cannot be consider reliable and representative, and are not for any practical use.  I can recommend the authors to read some studies where the problem of measuring the actual parameters of the foil bearing assemblies before tests is successfully solved, so that there are not any reasons to doubt in what was tested:

-          https://doi.org/10.3390/app13074555

-          https://doi.org/10.1115/1.4031496

-                  -     https://doi.org/10.1115/1.3159379

Another weak side of the study is that the results of analysis and conclusion are not in good agreement with the objectives and ideas of the study set by the title, abstract and the first part of the work. The only reliable conclusion in the study that is actually supported by the experimental results and the methodology is the positive role of the offered cooling system for the initial purposes. It provides good effect and can be used for the further improved studies. Other results on heating, wear and torque are not reliable enough due to the mentioned reasons.

Additionally, the authors also state some other problems with the design of the experimental study and test rig, e.g. possible axial displacements during reassembly (Line 300), insufficiency of 200 cycle tests for the purposes of the study (Line 314). In aggregate, this leads to heterogeneity of the results obtained, which is noted by the authors themselves.

Since I’d like to learn an improved version of this study in future, there are some questions and issues below also aimed to increase its quality in different aspects:

1. The authors in the introduction did not mentioned any work related to foil bearing lubricated with low-boiling liquids, and even state in Line 89 that previous works were “mostly for air lubrication”. Actually, there is a number of experimental and theoretical works on the issue by NASA and some other researchers worth to be read and mentioned, e.g.

- https://doi.org/10.2514/6.1991-2108

- https://doi.org/10.2514/6.1994-868

- Walton, Heshmat , Compliant Foil Bearings For Use In Cryogenic Turbopumps, 1994

- https://doi.org/10.2514/6.1997-3100

- DOI 10.1088/1757-899X/147/1/012043

- R.L. Fuller, Principal Engineer Barber-Nichols Inc., Liquid Hydrogen Supported Foil Bearings for Launch Vehicle Propellant Densification

2. Introduction in the present form sometimes looks li a textbook, e.g. Lines 51-59. The readers of such papers are often are well versed in the subject area, so it’s better to focus on the detailed analysis of scientific background, e.g. like I mentioned in the previous item, than providing basic general information.

3. I believe, the title should better reflect the subject of the study, like thermal and wear behavior. Also I find studying different bearing designs in the work instead of “various operating conditions” like written in the title.

4. Please clarify how the top and bump foil are fixed in the bearing housing.

5. The use of electromagnetic servomotor (its ram is shown in Fig.4) is not explained, it is not clear how it can affect the results.

6. It would be useful to specify the main properties of the used lubricant, such as density, dynamic viscosity, boiling temperature, etc.

7. There is a lack of analysis and discussion of the results in Section 3.1. In the present form it’s more like justification of choosing the study methodology, namely using the cooler, but the reader would like to learn more about the bearings performance. Of course, after controlling the parameters of the samples.

8. The results from the torque sensor in Fig. 14 are almost not representative as they do not provide valuable information of the bearings performance. We tried to use similar torque sensors in our practice to measure the torque in fluid film bearings, and this is a quite challenging task. The whole system must perform smoothly, including the drive, the couplings, the seals, the system must be well aligned, etc. In particular, I recommend using contactless seals for the purpose, because in your case the impact of the face seal and rolling bearings makes it difficult to analyze the bearing itself.  

 

Summarizing the above, the work requires sufficient improvement in most of aspects, including design, methodology, analysis of background and the results, more meaningful and reasonable conclusions. I wish all the best to the authors and will be interested in reading the improved work in future.

Author Response

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Reviewer 3 Report

Although it seems too long, the title precisely defines the topic of the article. The summary covered the purpose and main conclusion of the research. All information in the introduction leads to the purpose of the research. The applied method is valid for this type of research. The research results are adequately and clearly presented in plots. The discussion of the results follows logically from the data presented in the plots. The conclusions derived from this research are concisely and precisely defined. In general, the entire paper is well organized and represents a contribution to foil bearing research.

Author Response

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Round 2

Reviewer 1 Report

All the queries have been addressed 

Author Response

We thank the reviewer for reviewing our manuscript and for all comments that helped us improve the quality of our publication.

Reviewer 2 Report

The authors have performed and described additional experimental tests, which significantly improved the article and made the obtained results logical. Only a few minor revisions are still required to avoid further readers’ questions.

1. According to the new data, the measured difference between the clearances of bearings with bores of 25.9 and 26.0 mm was about 8 µm in one case, and about 0.5 µm in the other. These data also require additional explanation why, with the same difference in roughness, the clearance values are so different (almost 16 times).

2. The conclusions made regarding the effect of roughness on the final clearance and preload are presented in an overly imperative and categorical manner in the last paragraph of section 3.2 and in the Conclusion, lines 528-530. Although such a relationship may be realistic and roughness possibly may be the main factor in this matter, but before further testing, it remains only a hypothesis. The authors should present this just as a hypothesis, or conduct an additional numerical verification using some kind of engineering software.

3. Although the authors have presented the information about the actual bearing clearances, reading section 3.1 for the first time still cause confusion about the obtained results, because the corresponding explanation is shown only in the next in section 3.2. Even after reading section 3.2 the brief analysis of figures 9-12 is complicated due to the mismatch in the bearings' parameters. It is advisable to either include the data on the actual clearance values in the figures 9-12, or place the data from Section 3.2 before the main experimental results.

Author Response

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