Experimental Analysis of Chemically Degraded Lubricant’s Impact on Spur Gear Wear
Round 1
Reviewer 1 Report
The manuscript studies the effect of the added acid (HCl) to the lubricant on wear. The topic is interesting, and the experimental data collected using multiple facilities are presented. However, since the presented results were obtained at the specific HCl concentration (36.46% v/v) and under limited testing conditions, they may not be valid if the HCL concentration is lower and/or the operating conditions are different to the studied ones. The reported results may be too specific and of limited applicability. This concern needs to be addressed.
Some specific comments are listed below.
Title: It seems it is a bit too broad, in particular, the term “effect of lubricant degradation”, which can be caused by various reasons. It would be good to narrow it down to reflect the focus of this work, that is, effect of HCl induced degradation on …
Abstract: The statement that “the pH meter has become a viable option for detecting oil degradation and offering a useful way to keep track of lubricant quality” is based on the data obtained under the conditions studied and presented in the manuscript. It is likely that changes in the pH value are not significantly enough under other conditions, which are not included in this study. Thus, further work seems required to make it as a generalised statement, that is, it is also true under other conditions. If not, this statement needs to be revised by adding some specific condition when it holds.
Section 1 Introduction: Please clarify specific knowledge gap(s) on the topic – “information on the effect of oil deterioration on gear wear is scarcer” (lines 96-97, page 2 of 22). The sentence “In conclusion, this research sheds light on the value of measuring pH for lubricant degradation forecast and deepens our comprehension of the role that lubricant degradation plays in gear wear” (lines 106-108, page 3) should be presented in the conclusion section. As mentioned before, this reviewer wonders how this conclusion was drawn from fundamental understanding or through case studies. If it is the latter case (I believe so), the results may be of limited applicability and the value of this study has certain limitation.
Section 2 Experimental test setup and methodology: The wrong unit (in m) is used in “… to measure the ferrous (ranges from 40m to 400m) and non-ferrous (ranges from 135m to 450m) particles” (lines 132-133, page 4). Check the English of the sentence “…, surface roughnesses of varying sizes are analysed” (line 172, page 4). Figure 2 (page 5): the connection of the 3 boxes needs to be clarified. If the tested samples and/or their wear particles in boxes 1 (macro scale) and 2 (micro scale) were analysed using nano-scaled surface characterisation, this step should be linked to both boxes 1 and 2 directly. My suggestion is to place the 3rd box below the first 2 if my understanding is correct. The information on the lubricity tester should be included in section 2.2.
Section 3 Results and discussions: Please check and revise the statement “This observation is consistent with the well-established principle that a rise in temperature increases metal-to-metal contact, …” (lines 218-219, page 6). Normally it is the other way around, that is, increasing metal-to-metal contact results in an increase in temperature because friction and wear caused by direct face to face contact generates heat in the process. Some important statements need to be supported by evidence. For example, “The failure of the lubricant oil to create a thick enough film on the contacting surfaces is likely to blame for this phenomenon” (lines 230-233, page 6). Justify the use of HCl and its v/v% and the speed, torque and duration of the test in section 3.2. Table 1: provide information on how the error was defined and calculated. Also, were these chemical properties obtained before or after the test? Please clarify when the data were collected in the caption. Section 3.2.1.2: Some statements of load-carrying capacity cause some confusion. It stated earlier that the load-carrying capacity of the HCl added lubricants reduced (lines 301-302, page 9). Also Figure 6(b) shows that the weld loads of oil 1 with HCl and oil 2 with HCl are lower than those without HCl, showing that the load-carrying capacity of the oil samples with HCL reduced by about 20%. The latter statement “In other words, HCl forms a corrosive layer that increases wear while decreasing friction and possibly improving surface load-bearing capacity” (lines 309-310, page 10), “So, it can be concluded that the with mixing of the aqueous HCl the load carrying capacity increases” (lines 330-331, page 10) and the information about Figure 7 cause some confusion. To my understanding, the effects of adding HCl to the lubricant are complex. On one hand, the function of extreme pressure additives and the formation of an oxide layer, which potentially reduce wear, needs to be considered. On the other hand, a corrosive layer formed by added HCl often results in a high wear rate. The resultant wear rate depends on a number of factors including the basic lubricant, HCl concentration, material properties, operating conditions and environmental conditions (humidity and temperature). Due to this challenge, the results generated at a specific condition may not be applicable to other conditions. Having said that, if the study is about a typical case where gear wear often experiences, it has a high practical value. A convincing justification of this case is needed.
Section 4 Conclusions: Once again, those results were obtained under the specific conditions in terms of the material, HCl content and operating conditions. They may not be applicable when the conditions and material are different.
Author Response
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Reviewer 2 Report
This paper investigated impact of lubricant degradation on spur gear wear through an experimental study. The experiment setup, instrumentation, along with results under two case studies were discussed in details. HCl was added to the oil as a way to simulate degraded oil. Various indicators for both oil degradation and gear wear progression were monitored. The results have shown pH value may be a more accurate measure for oil degradation. Overall, it is an interesting study. There are a few major issues the reviewer thinks that are necessary to be addressed:
1) How valid is it to simulate oil degradation by adding HCl? Pardon me, if this was discussed in the manuscript already but I missed it.
2) How much impact HCI has on the investigated metal materials (e.g., wear mass increased in Case 2)?
3) Using Case 2 as an example, can the mass removed be completely attributed to oil degradation, whose level is simulated by adding HCl in this study? If not, by how much percentage can it be attributed to oil degradation? This appears necessary to have a quantitative study (assumed this is what authors tried to accomplish) on the effect of lubricant degradation on spur gear wear.
The reviewer would like to see these issues clearly addressed in a revised version, which can be routed for another round of review.
Author Response
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Reviewer 3 Report
Quite a comprehensive study was conducted by authors to explore the effect of lubricant degradation on spur gear wear.
Here are some comments:
It is pretty inconsistent usage or description of tolerances and errors in the results, e.g., in Table 1 and Table 2 there are indicated errors, while wear mass is measured accurately in micrograms.
Typeset should be corrected in some cases, e.g., the caption of Table 2 on one page, the table itself on the other, and numerical values and measurement units separated at the end of the row (in the caption of Table 1).
There are several occasions of wrong measurement units, e.g., 138400 g (which is 138,4 kg) is wear mass, in 2.2.a, particles in the range of 40 m to 400 m!
In the section Experimental Setup, there is no detailed description of how the system actually works.
The data about the gearbox is insufficient for the experiment to be recreated by others. The gearbox must be completely described dimensionally (axis distance, gears with or without profile shift, number of teeth, even the addendum tip radius).
In section 2.1, how is the (catalogue) data, line 122, full-scale torque, important for the experiment or the results?
Author Response
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Reviewer 4 Report
A purely experimental paper that reports on lubricant degradation effects. The paper adequately presents its case and claim to novelty. The experimental data seems compelling and supports the presented hypotheses.
Author Response
The authors would like to convey their heartfelt appreciation for the reviewer's time and attention to this review. Your feedback is invaluable to us. Thank you very much.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The revised manuscript looks much stronger. Good work!
I don't have further comments for improvement.
Author Response
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Reviewer 2 Report
Thanks to the authors for detailed explanations in response to the reviewer's comments and updated manuscript, which can be recommended for publication in the journal of lubricants.
Author Response
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Reviewer 3 Report
In the new version of the manuscript, there are some grammatical errors, e.g., in line 99, shouldn’t there be a comma after the second square bracket? Some professional proofreading is advisable.
There is an issue in the description of the experiments in the manuscript. In line 133, there is a mention of Test 1 and Test 2. Then, there is Case 1 in lines 241 and further, and Case 2 in lines 280 and further. In Case 2, two separate, unnamed tests are announced but some additional tests were performed. In lines 475-477, test-1 and test-2 are mentioned, but those are not the same tests as those from the beginning because the latter address difference in oils state rather than working parameters. Finally, in the Conclusion section, cases are mentioned again. It is quite confusing and the reader has to take notes and create a structure to get the overall view of the tests performed and the items they are performed on. For such a comprehensive study it wouldn't be a bad idea to give a table showing all the tests and cases performed and to equalize designations. In that way, a reader would be able to easily distinguish between the tests and the results and this would, in the opinion of this reviewer, give additional quality to the manuscript.
Author Response
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