Numerical and Experimental Investigation of Different Oil Levels and Operation Conditions on the Individual Hydraulic Losses of Spherical Rolling Bearings

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript focuses on the component-level hydraulic loss mechanism of spherical roller bearings (Type 22320) under different oil levels and operating conditions. Adopting a combined approach of "CFD two-phase flow simulation + experimental validation", it systematically analyzes the effects of oil level, inner ring speed, and lubricant temperature on hydraulic losses, and clarifies the dominant role of rolling element churning losses. The results provide direct engineering reference value for the optimal design of bearing energy efficiency. The manuscript is logically structured overall with rigorous methodology, and the simulation data show good agreement with the experimental results.

Revision suggestions：

1. In the model construction method of the simulation setup section, the full-bearing model approach proposed by FELDERMANN is adopted, and the lubrication gap volumetric flow is neglected. However, how is the width parameter of the selected gap determined? Relevant basis should be supplemented. Meanwhile, does this width parameter affect the analysis results?
2. The guidance method of the selected bearing is not mentioned in the manuscript. The friction torque caused by factors such as friction between the cage and the guiding surface, as well as differential sliding, is not considered. It is recommended to supplement relevant explanations in the manuscript.
3. In Section 2.1.2, it is stated that "specific bearing parameters can be found in Table??". Is "Table??" a formatting error? Additionally, the manuscript lacks the table presenting specific bearing parameters, and relevant content should be supplemented.
4. In Section 2.2.3, should the definition and duration of the "rotation cycle" be explained? It is recommended to add supplementary descriptions.
5. Compared with Reference [1], what are the innovations of the research method in this manuscript? Is it merely the difference in the selected bearing type?

 

Author Response

Comment 1:  In the model construction method of the simulation setup section, the full-bearing model approach proposed by FELDERMANN is adopted, and the lubrication gap volumetric flow is neglected. However, how is the width parameter of the selected gap determined? Relevant basis should be supplemented. Meanwhile, does this width parameter affect the analysis results?

Answer 1: We agree with this comment and added more information at line 148: 
The width of the gap is not a fixed value, but is determined by the minimum cell size and the requirement that there are five cells between the inner/outer ring and the rolling element. This allows for stable numerical calculations. A study to investigate the influence of gap width on the result was not conducted, as it can be assumed from the extant literature that the influence of the gap on the hydraulic losses of the overall system is negligible.

 

Comment 2: The guidance method of the selected bearing is not mentioned in the manuscript. The friction torque caused by factors such as friction between the cage and the guiding surface, as well as differential sliding, is not considered. It is recommended to supplement relevant explanations in the manuscript.

Answer 2: Thank you for your hint. We added the information that the bearing has an outer-ring guided cage (line 130). Torque, which results from load-dependent losses, is not part of the result from CFD simulation . For details, check lines 21-32 and Figure 1.

 

Comment 3: In Section 2.1.2, it is stated that "specific bearing parameters can be found in Table??". Is "Table??" a formatting error? Additionally, the manuscript lacks the table presenting specific bearing parameters, and relevant content should be supplemented.

Answer 3:  Thank you for pointing this out. The formatting error has been corrected. A sketch of bearing 22320 and a table with the measured geometric dimensions have been added to the manuscript. See Figure 3 and Table 3.

Comment 4: In Section 2.2.3, should the definition and duration of the "rotation cycle" be explained? It is recommended to add supplementary descriptions.

Answer 4: We added a more detailed description of the revolutions of the rolling element set (rotation cycle) and the time in seconds for one revolution in chapter 2.2.3. line 268

Comment 5: Compared with Reference [1], what are the innovations of the research method in this manuscript? Is it merely the difference in the selected bearing type?

Answer 5: In addition to investigating different bearing types, we developed a two-phase simulation model (whereas [1] considered only single-phase simulations), enabling us to study the influence of oil level on hydraulic losses. Furthermore, we investigated the effect of oil temperature on hydraulic losses. Additional information has been added in line 138.

Reviewer 2 Report

Comments and Suggestions for Authors

This work is a comprehensive study combining numerical simulation and experiment to analyze the hydraulic losses of a roller bearing under various lubrication conditions. The relevance and novelty of the research are sufficiently substantiated. The authors set clear objectives: the development of a reproducible CFD-based methodology and the validation of total losses using experimental data. The structure, methods, and results do not raise any questions or require additional clarification. However, the authors need to address minor shortcomings in the manuscript:

1. In Figure 2, use markers of the same color for the components of the test rig. Mirror the rig's schematic to improve the figure's readability.
2. In line 95, the reference or a part of the text is missing.
3. The reference to Figure 4 is missing.
4. Table 4 is located too far from where it is first mentioned in the manuscript text.
5. Table 2 is positioned significantly above its mention in the text.
6. Section 2.2.4 should be entirely merged with Section 2.2.1, as Section 2.2.1 partially discusses the computational mesh.
7. In the provided mathematical model, add explanations for the symbols used.

Author Response

Comment 1: In Figure 2, use markers of the same color for the components of the test rig. Mirror the rig's schematic to improve the figure's readability.
Response 1: We agree, and Figure 2 has been updated ; see line 79.

Comment 2: In line 95, the reference or a part of the text is missing.
Response 2: Thank you for pointing this out. The formatting error has been corrected.

Comment 3: The reference to Figure 4 is missing.
Response 3: Thank you for the feedback. We added the missing reference.

Comment 4: Table 4 is located too far from where it is first mentioned in the manuscript text.
Response 4: By merging sections 2.21 and 2.2.4 and restructuring, Table 4 is now located closer to where it is mentioned .

Comment 5: Table 2 is positioned significantly above its mention in the text.
Response 5: Table 2 was the missing reference from question 2. It is located in chapter 2.1.1 because it is referred to for the first time here. We have omitted the table in 2.2.1 to save space.

Comment 6: Section 2.2.4 should be entirely merged with Section 2.2.1, as Section 2.2.1 partially discusses the computational mesh.
Response 6: We agree with your comment and have merged the sections.

Comment 7: In the provided mathematical model, add explanations for the symbols used.
Response 7: We checked all formulas and added more explanations in Formulas 4, 5, 10, and 11.

Reviewer 3 Report

Comments and Suggestions for Authors

The study addresses an important gap in CFD-based hydraulic loss modeling for spherical roller bearings and shows strong experimental validation for total losses. The methodology is well-structured, and results are physically consistent. However, there are a few items needed to be addressed:

1. Add quantitative validation results in the abstract and conclusion; clearly state limitations.
2. Strengthen justification for modeling simplifications and discuss uncertainty sources.
3. Expand discussion on practical implications for bearing design and lubrication strategies.
4. Improve figures clarity (error bars, units, legends) and include absolute values in Table 5.

 

Comments on the Quality of English Language

Correct minor typos and ensure consistent terminology and reference formatting.

Author Response

Comment 1: Add quantitative validation results in the abstract and conclusion; clearly state limitations
Response 1: Agreed. We added validation in the abstract and stated the limitations of the simulation model in lines 491-498.
Comment 2: Strengthen justification for modeling simplifications and discuss uncertainty sources
Response 2: Thank you for your comment. We agree and added an explanation of the assumption for the removal of roundings and chamfers, which is based on [16], where the removal of rounding radii showed no significant impact on power/hydraulic losses. See lines 133-136.
Comment 3: Expand discussion on practical implications for bearing design and lubrication strategies.
Response 3: The objective of the present investigations is not to derive new bearing designs or to propose improved lubrication strategies. Such conclusions would also be beyond the scope of the applied methodology, since the verification of adequate lubrication requires elastohydrodynamic (EHD) simulations in addition to CFD to resolve the lubricant film structure.
Instead, the CFD results are intended to provide hydraulic loss inputs for subsequent rolling-element kinematic analyses within a multi-body simulation framework, as outlined in the conclusions. We therefore refrain from discussing design or lubrication strategies, as this would not be consistent with the focus and scope of the paper.   Comment 4: Improve figures ' clarity (error bars, units, legends) and include absolute values in Table 5   Response 4: Absolute values were added to Table 5. Figure 8 was updated with an improved description, and its font was adjusted to match the rest of the manuscript. Typos in Figure 5 were corrected. The legend of Figure 11 was expanded to improve clarity. Error bars were not added because each experiment was performed only once.