Testing Grease Consistency

Round 1

Reviewer 1 Report

• For some readers, a brief description of basic grease structure (thickener, which holds the oil and gives the grase stability) might be helpful. An REM picture of a thickener structure would also help. But as readers should be aquainted with grease, it is not necessarily needed.
• Fig. 3 would benefit from some more data on the used greases an forces for the curves. Can a correlation be seen? Maybe only give the data for 2-3 curves for the reader to get some idea of the used grease and forces.
• Fig. 12: is there a picture missing? Or is the intention that the grease in the cup and the grease on the fingers is to be compared - then this should be noted.

Author Response

“For some readers, a brief description of basic grease structure (thickener, which holds the oil and gives the grase stability) might be helpful. An REM picture of a thickener structure would also help. But as readers should be aquainted with grease, it is not necessarily needed.”

Thank you for your comments. The paper is mainly aimed at grease or other lubrication industry professionals who likely know the fundamentals of grease. Additional information on measurements procedure are added and figures are improved by adding color to the measured data.

“Fig. 3 would benefit from some more data on the used greases an forces for the curves. Can a correlation be seen? Maybe only give the data for 2-3 curves for the reader to get some idea of the used grease and forces.”

Colors have been added along with a label for each plot. Please see Fig. 4 in revised manuscript.

“Fig. 12: is there a picture missing? Or is the intention that the grease in the cup and the grease on the fingers is to be compared - then this should be noted.”

The intent is for the reader to compare the grease in the cup to the grease on the fingers. This has been clarified in the caption.

Reviewer 2 Report

The manuscript aims at finding correlations between grease consistency and the general rheological grease properties. The limitations of the different possible techniques used to measure grease consistency are analyzed. The role of the penetration test, largely considered as the most important test to assess the properties of greases and to select commercial products for specific applications, is questioned. The limitations of the NLGI scale grades and of the penetration test alone, such as its dependence on the specific operator and on the chosen geometry, are evidenced and thoroughly discussed in the manuscript.

The analysis of the possible correlations between grease quantities is of great interest for the scientific and technological communities and can contribute to raising awareness on the limitation of the NLGI scale alone. The article is original and well written, the literature is well synthesized and the conclusions are interesting.

As a general comment, I would suggest better develop the Sections of the manuscript in which the Methods and Results/Discussion are described. The used procedure and the data interpretation are not always clear to the reader. For example, the information regarding the specific instruments used to perform all the tests, the specific procedures followed, the number of samples per measured point and per material, are lacking in the manuscript. In most of the graphs, a specific reference to the tested greases is lacking as well. The standard used to perform tests and previous literature are largely and correctly referenced in these Sections. However, to improve the readability of the manuscript, I would suggest briefly recall definitions and methodologies in the manuscript itself. It is clear that the Authors performed extensive experimental tests on several grease samples. However, the collected data and graphs should be better organized and discussed.

Specific comments:

1. Lines 134-136: ‘Two parameters will be investigated here…’ The definition of the investigated parameters (yield point and flow point), as defined by Cyriac et al., should be briefly recalled here, to improve Section readability.
2. Section 2.2. A picture of the instrument used for the ‘rheometer oscillatory test’ could help.
3. Section 3 (Details of Rheometer Penetration Test). The whole section lacks clarity and it is difficult to follow. I suggest considering re-structuring it. Specific comments in the following points (4,5,6,7,8).
4. The quantities used to define Figure 2 are not clear to me. Could you please briefly define what you mean with ‘5% Trim’, ‘100% Trim’, ‘Pre-shear’?
5. PU2 is mentioned only in Section 4.1. The description of the selected Materials should be anticipated and placed before the experimental results.
6. Line 224: ‘This figure includes a wide variety of forces and grease types. For some greases, a low normal force…’. Could the Authors be more specific on this? What do you mean by ‘wide variety’? Which parameter defines the variety? What is the value associated with a ‘low’ normal force?
7. Figure 3. The legend is missing in the Figure, so it is not possible to correlate a specific curve to the corresponding material. Is it possible to associate different behaviors to different grease compositions or consistency? In the graph, the lines are continuous. Are they the sum of several sampled points or they are the result of a trend/fit? Could you please comment on the experimental errors?
8. Figure 4. The chosen markers are quite big. Is it possible to reduce them? Are the errors hidden behind the points? Could you please refer in the graph to specific materials as defined in Table 2?
9. lines 276-279: ‘Because a wide range of consistencies…’. Could you be more precise? Which greases showed variations and which didn’t?
10. Line 279: ‘In order to obtain an even wider array…’. Scientifically speaking, I am not completely convinced that a mix of grease is, in general, still a grease. This is possible, probably if the greases can be mixed. Have you previously checked the mixability of the different products? Please provide further details on this.
11. Lines 319-320: ‘For simplicity, only one set of data will be presented; however, this set is particularly representative of all data collected.’ Could the Authors please expand this? How did you decide that the chosen set is representative of all the others?
12. Figure 7: the different greases cannot be distinguished in the graph.
13. Line 346-347: ‘As expected, based on another study [8], there is a general positive correlation between these two measurements’. The main goal of the manuscript is to identify these correlations. Is it possible to further investigate this correlation, modeling the trend? The whole discussion should be based on more quantitative parameters, if possible.
14. Is Figure 11 a duplicate of Figure 10? If yes, I would suggest keeping a single picture, in which both the legend and the trend are present.

Author Response

“The manuscript aims at finding correlations between grease consistency and the general rheological grease properties. The limitations of the different possible techniques used to measure grease consistency are analyzed. The role of the penetration test, largely considered as the most important test to assess the properties of greases and to select commercial products for specific applications, is questioned. The limitations of the NLGI scale grades and of the penetration test alone, such as its dependence on the specific operator and on the chosen geometry, are evidenced and thoroughly discussed in the manuscript.

The analysis of the possible correlations between grease quantities is of great interest for the scientific and technological communities and can contribute to raising awareness on the limitation of the NLGI scale alone. The article is original and well written, the literature is well synthesized and the conclusions are interesting.

As a general comment, I would suggest better develop the Sections of the manuscript in which the Methods and Results/Discussion are described. The used procedure and the data interpretation are not always clear to the reader. For example, the information regarding the specific instruments used to perform all the tests, the specific procedures followed, the number of samples per measured point and per material, are lacking in the manuscript. In most of the graphs, a specific reference to the tested greases is lacking as well. The standard used to perform tests and previous literature are largely and correctly referenced in these Sections. However, to improve the readability of the manuscript, I would suggest briefly recall definitions and methodologies in the manuscript itself. It is clear that the Authors performed extensive experimental tests on several grease samples. However, the collected data and graphs should be better organized and discussed.”

Thank you very much for your comments and interest in our research. These sections have been edited in response to the specific comments below. In addition, more specific information has been added about procedures and instruments used in Section 3 and Section 4.1.

“Specific comments:

Lines 134-136: ‘Two parameters will be investigated here…’ The definition of the investigated parameters (yield point and flow point), as defined by Cyriac et al., should be briefly recalled here, to improve Section readability.”

The text has been revised to include a brief description of the referenced yield stress definition.

“Section 2.2. A picture of the instrument used for the ‘rheometer oscillatory test’ could help.”

A picture of the rheometer used has been added as Figure 2.

“Section 3 (Details of Rheometer Penetration Test). The whole section lacks clarity and it is difficult to follow. I suggest considering re-structuring it. Specific comments in the following points (4,5,6,7,8).

The quantities used to define Figure 2 are not clear to me. Could you please briefly define what you mean with ‘5% Trim’, ‘100% Trim’, ‘Pre-shear’?”

A thorough description of each procedure has been provided to reduce ambiguity of this set of experiments.

PU2 is mentioned only in Section 4.1. The description of the selected Materials should be anticipated and placed before the experimental results.

Table 2 has been moved into Section 3.

Line 224: ‘This figure includes a wide variety of forces and grease types. For some greases, a low normal force…’. Could the Authors be more specific on this? What do you mean by ‘wide variety’? Which parameter defines the variety? What is the value associated with a ‘low’ normal force?

The descriptions of the range of grease types and forces have been more specifically defined in the text. The range of forces is between 2 and 8 Newtons and the range of greases is between grade 00 and grade 2. Using this information, one can make the assumption that a “low” normal force may possibly be 2 N if testing a grade 2 grease.

Figure 3. The legend is missing in the Figure, so it is not possible to correlate a specific curve to the corresponding material. Is it possible to associate different behaviors to different grease compositions or consistency? In the graph, the lines are continuous. Are they the sum of several sampled points or they are the result of a trend/fit? Could you please comment on the experimental errors?

A legend has been added to this figure linking a grease/force combination to each line. It is unlikely that this plot could be used to observe grease behavior since, for example, the grade 2 grease tested with 8 N shows similar behavior to the grade 0 grease tested with 2 N. The data collected was a large number of points and are plotted as lines for simplicity. The main focus of this plot is to observe the general trend of penetration over time so as to suggest the minimum amount of time for tests to be conducted quickly. Because the results for each grease/force combination showed that it reaches steady state before 20 seconds, positional error over time was not a main focus of this set of measurements.

Figure 4. The chosen markers are quite big. Is it possible to reduce them? Are the errors hidden behind the points? Could you please refer in the graph to specific materials as defined in Table 2?

Here, samples were measured three times and the average value is presented. The data points were large to enhance visibility but have been reduced slightly. Even with the size reduction, error bars are negligibly larger than the data points and have been omitted. In addition, the data labels have been changed to indicate the corresponding grease from Table 2.

“lines 276-279: ‘Because a wide range of consistencies…’. Could you be more precise? Which greases showed variations and which didn’t?”

It is advantageous to have as many data points as possible. A remark has been added about how the PU2 and AlC2.1 greases changed consistency significantly with shear while the LiC greases did not.

“Line 279: ‘In order to obtain an even wider array…’. Scientifically speaking, I am not completely convinced that a mix of grease is, in general, still a grease. This is possible, probably if the greases can be mixed. Have you previously checked the mixability of the different products? Please provide further details on this.”

The only greases mixed are greases in the same “family” by the same manufacturer that have very similar formulations where the main variation is thickener concentration (as well as additive amounts). These greases were not found to show any meaningfully deviant behavior from expected trends.

However, even if this were not the case it could arguably be a good thing that a mix of greases not be compatible to observe whether the consistency tests studied still end up yielding the same results.

“Lines 319-320: ‘For simplicity, only one set of data will be presented; however, this set is particularly representative of all data collected.’ Could the Authors please expand this? How did you decide that the chosen set is representative of all the others?”

The set of data given is representative of others because each grease individually shows a general positive correlation between rheometer penetration and cone penetration but all of them plotted together do not show a cohesive trend.

The main point of showing these figures is to indicate that there is at least one case where there is not a strong overall correlation between these tests. Because there is at least one case where this approach should not be used, it appears to be an unreliable method (if seeking to estimate cone penetration) and should be avoided unless seeking a slightly quicker and simpler method to roughly compare the consistencies of two samples.

“Figure 7: the different greases cannot be distinguished in the graph.”

These plots have been changed to differentiate the greases from each other.

“Line 346-347: ‘As expected, based on another study [8], there is a general positive correlation between these two measurements’. The main goal of the manuscript is to identify these correlations. Is it possible to further investigate this correlation, modeling the trend? The whole discussion should be based on more quantitative parameters, if possible.”

The main point in displaying this figure is to show that there is at least one grease that significantly deviates from the expected trend. Because this indicates that there is some notable discrepancy between these two methods, a correlation is not given because it is not recommended to correlate the two for practical use.

As Figure 9 (now Figure 10 after adding a picture of the rheometer) also indicates, the yield stress appears to be the cause of this discrepancy. In fact, a close examination of the paper that sets forth this yield stress definition even indicates that it does not correlate well with cone penetration.

“Is Figure 11 a duplicate of Figure 10? If yes, I would suggest keeping a single picture, in which both the legend and the trend are present.”

Yes, this figure is a duplicate. This was done to avoid cluttering the image and to give two different points of focus. The first time the figure is presented is intended to serve as an overall indication of a tight correlation relative to the other methods investigated. The second time the figure is presented is intended to quantify that correlation.