Review Reports - Enabling Ultra-Stable Bearing Performance: Design of a Self-Lubricating PI Composite Retainer

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study conducted a meticulously designed research on the development of self-lubricating polyimide (PI) based composite retainers for rolling bearings. This work demonstrated a clear integration of material and structural innovations, achieving corresponding results in terms of thermal stability and reduction of wear. However, the paper lacks some experimental details. It is suggested to address the following issues to enhance the rigor and persuasiveness of the manuscript:

(a) The raw material ratios are not clearly defined: Section 2.1 only lists the basic parameters of PI, PTFE, and Al₂O₃, but does not provide the mass or volume ratios when the composites are formed. Different ratios have a significant impact on the material properties. The lack of ratios makes the experiments unable to be replicated. It is recommended to provide specific ratios and the basis for determination.

(b) Inconsistent pre-treatment process: The results and discussion mentioned that the bearings were "immersed in oil at 100°C and then wiped", but the soaking time and type of oil were not specified. Different oil contents can lead to differences in lubrication performance. It is recommended to standardize the pre-treatment process and supplement the key parameters of the oil.

(c) Missing calculation of contact pressure: During the experiment, axial loads of 700N and 1500N were applied, but the corresponding Hertz contact pressure was not calculated. The contact pressure is a key factor affecting frictional heat and wear, and it is recommended to supplement the calculation process of the Hertz contact pressure.

(d) The comparison conditions for YSU-PA1 and YSU-PI1 are inconsistent: In Section 3.1, the comparison between YSU-PA1 (2412 r/min, 7.5h) and YSU-PI1 (1376 r/min, 3h) shows that the differences in rotational speed and time make the comparison unfair (high speed may increase heat dissipation, and long duration may accumulate wear). It is recommended to conduct supplementary comparison experiments under the same rotational speed (such as 1376 r/min) and time (such as 3h) to ensure the reliability of the conclusion.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Dear Distinguished Reviewer,

We are writing to submit the revised version of our manuscript entitled "Enabling Ultra-Stable Bearing Performance: Design of A Self-Lubricating PI Composite Retainer" (Manuscript ID: lubricants-3904834). We wish to extend our heartfelt thanks to you for the thoughtful and insightful suggestions, which have significantly strengthened our paper.

We would also like to take this opportunity to extend our best wishes to you. China is currently observing a public holiday encompassing both our National Day and the Mid-Autumn Festival. Consequently, we hope for your kind understanding if our responses to any urgent follow-up communications are not as prompt as usual during this period.

We believe that we have addressed all the points raised by the reviewers. A detailed, point-by-point response to each comment is presented in the following sections, along with the corresponding changes made in the manuscript.

(a)The raw material ratios are not clearly defined: Section 2.1 only lists the basic parameters of PI, PTFE, and Al₂O₃, but does not provide the mass or volume ratios when the composites are formed. Different ratios have a significant impact on the material properties. The lack of ratios makes the experiments unable to be replicated. It is recommended to provide specific ratios and the basis for determination.

Response: We sincerely thank the reviewer for raising this critical point. We agree that the precise composition is fundamental to the study and apologise for this omission.

In response to the reviewer's comment, we have now clearly provided the specific mass ratios (wt%) for all composite materials within the manuscript. To directly link each material's composition with its corresponding performance data and discussion, we found it most logical and reader-friendly to integrate this information at the Part 3: Results and Discussion. The added content has been highlighted in red font for easy identification.

(b)Inconsistent pre-treatment process: The results and discussion mentioned that the bearings were "immersed in oil at 100°C and then wiped", but the soaking time and type of oil were not specified. Different oil contents can lead to differences in lubrication performance. It is recommended to standardize the pre-treatment process and supplement the key parameters of the oil.

Response: We sincerely thank the reviewer for this critical comment regarding the standardization of the pre-treatment process. We apologize for the omission of these key parameters in the original manuscript and fully agree that they are essential for reproducibility.

We have now revised the manuscript to provide a detailed and standardized description in Section 2.3 with font. The specific parameters are supplemented as follows:

Lubricant Type and Specification: The oil used was L-AN46 mechanical oil (Grade A) conforming to the Chinese National Standard GB 443-89. This is a standardized paraffinic mineral oil widely used in general machinery lubrication. Its key property, a kinematic viscosity of 46 mm²/s at 40°C, is now explicitly stated.

Soaking Time: The bearings were immersed in the oil bath at 100 °C for a standardized duration of 2 hours. This time was determined to be sufficient for the oil to fully penetrate and wet the internal components of the bearing.

Wiping Procedure: After immersion, the bearings were gently wiped with clean, lint-free cloths to remove all excess oil from the external surfaces (outer ring, inner ring, and faces) until no visible oil film or droplets remained. This ensures a consistent initial condition by relying on the lubricant retained within the bearing's internal spaces.

The purpose of this pre-treatment was to ensure all tested bearings started with an identical and well-defined initial lubrication state. We believe these precise clarifications have fully addressed the reviewer's concern and significantly enhanced the reproducibility of our work.

Response: We are deeply grateful to the reviewer for this insightful and professional comment. The reviewer is absolutely correct that the Hertzian contact pressure is a critical parameter for analyzing tribological behaviors like frictional heat generation and wear mechanism. We sincerely apologize for this omission in the original manuscript.

In response to this valuable suggestion, we have supplemented the manuscript with an estimation of the maximum Hertzian contact pressure. We fully acknowledge the reviewer's point that obtaining a highly precise value requires exact internal geometrical parameters of the bearing (e.g., precise raceway curvature coefficients) and complex iterative calculations, which was a challenging task. Therefore, following the classical Hertzian contact theory for point contact, and based on the known parameters of the angular contact ball bearing (7206C) and a contact angle of 15°, we performed a reasonable engineering estimation.

The calculation process and results have been added to the manuscript. Briefly, the estimated maximum contact pressure on the inner ring is approximately 1100 MPa under the 700 N axial load and 1400 MPa under the 1500 N axial load. These values provide a quantitative assessment of the stress conditions in our experiments.

The detailed explanation, including the reference to the classical formula has been incorporated into the Results and Discussion of the revised manuscript. The additions are highlighted in red font for the reviewer's convenience.

We believe this addition significantly strengthens the discussion of our results by linking the applied load to the resulting contact stress. Once again, we thank the reviewer for pushing us to improve the manuscript. We are open to further guidance on this matter.

Response: We thank the reviewer for this critical observation. Upon careful consideration, we agree that the comparison under non-identical conditions could lead to misinterpretation. To ensure the clarity and robustness of our conclusions, we have chosen to remove that specific comparative statement from the Section. The revised text now focuses on demonstrating the excellent inherent thermal stability of the YSU-PA1 bearing based on its own performance data, which shows a stable temperature plateau under extended operation. We believe this revision strengthens the manuscript by eliminating any potential ambiguity.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript ID: lubricants-3904834

Review of the article

«Enabling Ultra-Stable Bearing Performance: Design of a Self-Lubricating PI Composite Retainer»

Authors: Zhining Jia, Caizhe Hao

This paper addresses the problem of increased temperature, operational instability, and premature failure of rolling bearings caused by high friction in cages. A porous composite self-lubricating cage has been developed, manufactured, and tested. According to the authors, it provides "ultra-stable operation" for the bearings. The authors have achieved results that expand the application potential of high-speed rolling bearings in machines and mechanisms operating at speeds at which conventional rolling bearings are unable to operate due to high temperatures.

The following comments are provided below.

From the introduction, it follows that the authors are the first to use porous cage materials, which significantly improves bearing lubrication. However, other studies have been conducted on this type of bearing.

For example, Zhuangya Zhang and other authors, "Study on Mechanics and High-Temperature Tribological Properties of Porous Bearing Cage Material," September 2023, Journal of Reinforced Plastics and Composites 43(19-20):1165-1178, DOI: 10.1177/07316844231201479/

The literature review should have been expanded to indicate how the porous separator studied by the authors differs from known porous separators.

In Section 3, the authors compared the temperature properties of their separator with a separator made of a solid-state material. However, it would have been more logical to compare it with porous separators, which, as it turns out, exist. This would have allowed the properties of the proposed separator to be evaluated among similar separators.
The authors selected a material for the manufacture and subsequent study of the properties of their separator, but completely ignored the basis for choosing this material, and not another.
The authors attribute the contradictory properties of their cage, with lower temperatures than expected during high-speed bearing operation, to some kind of ultra-high "intellectual capacity." This, of course, is an exaggeration unacceptable for scientific research. Indeed, a porous cage is incapable of being "intelligent," that is, possessing the ability to create new knowledge. In reality, the observed effect demonstrates new properties of a high-speed porous bearing that a solid cage does not possess.
The authors discovered a uniform film with a cotton-like morphology on the raceway surface, which reduces friction during operation and protects the raceway from premature failure. They attribute this to the exceptionally low coefficient of friction of the filler, which promotes the formation of a self-lubricating layer on the contact surface due to the rigid reinforcing particles. These particles allegedly act as "microbearings," changing the contact mode from pure sliding to a combined rolling-sliding mechanism. This is a rather strange conclusion, as hydrostatic sliding has the lowest coefficient of friction.
The conclusions expressed in the paper concern the special properties of porous separators compared to solid-state separators. However, this was previously known. How is this separator better than other porous separators? What is its novelty? This cannot be understood from the paper.

There are other comments regarding the presented research results. Overall, the material presented is relevant and may have experimental evidentiary value, but only if the properties of porous separators are compared correctly. The article needs to be significantly improved in this area. Only after such revision can it be accepted for publication.

Author Response

Dear Distinguished Reviewer,

From the introduction, it follows that the authors are the first to use porous cage materials, which significantly improves bearing lubrication. However, other studies have been conducted on this type of bearing.

The literature review should have been expanded to indicate how the porous separator studied by the authors differs from known porous separators.

Response: We sincerely thank the reviewer for bringing this highly relevant reference (Zhang et al., 2023) to our attention. We apologize for this omission in our initial literature review; it was an oversight during our literature search, and we appreciate the reviewer's diligence in identifying it.

Upon careful examination of the cited work by Zhang et al., we have revised the introduction of our manuscript to provide a more comprehensive background. The revised text now acknowledges this prior research on porous cage materials and clearly delineates the novel aspects of our present study.

We believe that incorporating this discussion not only addresses the reviewer's valid concern but also strengthens the manuscript by more precisely framing our research objectives and innovation within the current state.

In Section 3, the authors compared the temperature properties of their separator with a separator made of a solid-state material. However, it would have been more logical to compare it with porous separators, which, as it turns out, exist. This would have allowed the properties of the proposed separator to be evaluated among similar separators.

Response: We sincerely thank the reviewer for raising this insightful point. We agree that comparing the proposed retainer with existing porous retainer is a valuable and necessary step to fully benchmark its performance.

In our experimental design, the primary comparison with the solid-state retainer made of the identical base material was intentionally conducted to serve a fundamental purpose: to unequivocally demonstrate the intrinsic advantage of the porous structure itself, independent of material composition. This comparison under identical testing conditions provides the most direct evidence that the performance enhancement is attributable to the engineered oil-containing pores rather than a change in the matrix material. We believe this foundational comparison is a critical and logically prior step in the scientific inquiry process.

However, we fully acknowledge the reviewer's valid perspective that positioning our retainer within the broader context of similar technologies is essential. Therefore, we have enriched the discussion in Section 3 by adding a comparative analysis with the porous PEEK retainer reported by Zhang et al. [25]

The authors selected a material for the manufacture and subsequent study of the properties of their separator, but completely ignored the basis for choosing this material, and not another.

Response: We thank the reviewer for this critical comment, which provides us with an opportunity to clarify the rationale behind our material selection. The choice of the specific composite system (PI matrix with PTFE and nano-Al₂O₃ fillers) was not arbitrary but was based on a deliberate strategy to meet the demanding requirements of aerospace bearing retainers. We have now expanded the manuscript (Section 2.1, Materials) to include the following justification:

Selection of Polyimide (PI) Matrix: PI was chosen as the matrix material due to its exceptional combination of properties, which are crucial for high-performance bearings. These include:

Outstanding Thermal Stability: PI maintains its mechanical integrity and performance across a wide temperature range (-269 °C to 400 °C), far exceeding that of other engineering plastics. This ensures reliability under the high frictional heat generation expected in high-speed bearings.

High Mechanical Strength and Modulus: PI possesses superior strength, stiffness, and creep resistance compared to many polymers, which is essential for the structural integrity of the retainer under centrifugal and impact loads.

Inherently Good Tribological Properties: PI has a low inherent friction coefficient and good wear resistance, providing a solid foundation for a self-lubricating material.

Selection of Functional Fillers (PTFE and Al₂O₃): The PI matrix was modified with specific fillers to create a synergistic effect and tailor the composite's properties:

PTFE as a Solid Lubricant: PTFE is one of the most effective solid lubricants known, with an extremely low friction coefficient. Its primary role is to form a durable transfer film on the counterface, significantly reducing friction and wear, especially during the start-up or under starved lubrication conditions.

Nano-Al₂O₃ as a Reinforcing Agent: While PTFE improves lubricity, it can reduce the mechanical strength of the composite. To counter this and enhance wear resistance, nano-sized Al₂O₃ particles were incorporated. They act as hard reinforcing phases, improving the composite's load-bearing capacity, hardness, and dimensional stability.

Continuity from Prior Research: This specific material formulation is the culmination of our group's long-standing research into high-performance tribological materials. It represents a logical progression from fundamental material-level studies to the current system-level engineering application, aiming to bridge the gap between laboratory research and practical implementation in bearings.

We apologize for this omission in the original manuscript and believe that the added explanation now provides a clear and compelling basis for our material selection. All supplements have been marked in red font.

The authors attribute the contradictory properties of their cage, with lower temperatures than expected during high-speed bearing operation, to some kind of ultra-high "intellectual capacity." This, of course, is an exaggeration unacceptable for scientific research. Indeed, a porous cage is incapable of being "intelligent," that is, possessing the ability to create new knowledge. In reality, the observed effect demonstrates new properties of a high-speed porous bearing that a solid cage does not possess.

Response: We sincerely thank the reviewer for this profound and insightful comment, which raises a crucial point regarding the precision of terminology in scientific writing. We fully agree with the reviewer that attributing literal, cognitive "intelligence" or "intellectual capacity" to a material is not scientifically accurate, and we appreciate the reviewer's vigilance in upholding this standard.

Our intention in using the term "intellectual capacity" was not to imply the cage possesses cognitive abilities, but to employ it as a metaphor to vividly convey the remarkable, self-adaptive lubrication behavior we observed. Similar to how an intelligent system responds dynamically to its environment, the porous structure—through the synergistic effect of centrifugal force, temperature rise, and capillary action—autonomously regulates the storage and release of lubricant, achieving a stable thermal state that appears "well-managed." We apologize if this metaphorical expression caused any misunderstanding regarding its literal meaning.

In direct response to the reviewer's entirely valid concern, we have revised the relevant descriptions throughout the manuscript to enhance scientific rigor. We have replaced the term "intellectual capacity" with more precise descriptors such as "self-regulating capability" .

We believe this revision successfully addresses the concern by eliminating potential misinterpretation, while still effectively communicating the core innovative concept of our work—that the porous retainer exhibits a functional behavior analogous to a regulatory loop, exactly as the reviewer astutely points out in the final sentence. We are grateful to the reviewer for prompting this improvement in the clarity and precision of our manuscript.

The authors discovered a uniform film with a cotton-like morphology on the raceway surface, which reduces friction during operation and protects the raceway from premature failure. They attribute this to the exceptionally low coefficient of friction of the filler, which promotes the formation of a self-lubricating layer on the contact surface due to the rigid reinforcing particles. These particles allegedly act as "microbearings," changing the contact mode from pure sliding to a combined rolling-sliding mechanism. This is a rather strange conclusion, as hydrostatic sliding has the lowest coefficient of friction.

Response: We sincerely thank the reviewer for this insightful comment and would like to provide a more nuanced explanation of our proposed mechanism. The reviewer is correct that hydrodynamic sliding represents an ideal low-friction state. We also acknowledge the user's astute point that a rolling bearing exhibits complex contact mechanics, where macroscopic rolling inevitable involve microscopic sliding within the Hertzian contact zone, especially under starved lubrication.

Our proposed "micro-bearing" mechanism is best understood at this microscopic scale. Under boundary lubrication conditions, the contact between the cage and raceway is not ideal. It is precisely at these discrete, localized micro-sliding interfaces that the nano-Al₂O₃ particles are postulated to act. Their role is not to replace the macroscopic motion but to modify the microscopic interactions:

They carry a portion of the load, reducing the direct adhesion and ploughing between the polymer composite and the steel surface.

More importantly, at these localized sliding points, the particles can roll, effectively changing the friction mode from pure sliding to a combination of rolling and sliding at the microscale.

This mechanism operates synergistically with the PTFE, which forms a low-shear-strength transfer film. The particles, embedded in this film, act to reduce friction in the micro-sliding regions This provides a coherent explanation for the observed friction reduction without contradicting established lubrication theories.

We have revised the manuscript to clarify that this effect is primarily targeted at optimizing friction within microscopic slip zones, presenting a more precise description of the mechanism.

The conclusions expressed in the paper concern the special properties of porous separators compared to solid-state separators. However, this was previously known. How is this separator better than other porous separators? What is its novelty? This cannot be understood from the paper.

Response: We sincerely thank the reviewer for this critical comment, which prompts us to more clearly articulate the novel contributions of our work beyond the established concept that porous retainers generally outperform solid ones. We apologize if this was not sufficiently emphasized in the original manuscript.

The novelty of our work lies not in rediscovering the basic advantage of porosity, but in demonstrating and elucidating a higher-order performance metric— "ultra- stable operation" —through a novel material system and a system-level validation approach. Specifically, the key advancements are:

From Material Properties to System-Level Performance: While prior studies on porous retainer materials[25] have excellently characterized material-level tribological properties , our work shifts the focus to the bearing system level. We demonstrate how our PI-based porous retainer achieves unprecedented thermal stability within a functioning angular contact ball bearing under continuous operation. This transition from component testing to system-level validation is a significant step towards practical application.

Introduction of a Critical Performance Metric: We introduce and validate "ultra-stable operation" as a crucial indicator of bearing reliability, moving beyond the conventional focus solely on the absolute value of temperature or friction coefficient. This metric directly addresses the issue of operational instability caused by temperature fluctuations.

A Novel and High-Performance Material System: We present a porous retainer based on a Polyimide (PI) matrix. Compared to other polymer systems, PI offers superior thermal stability, making our bearing retainer particularly suited for more demanding applications. The synergistic formulation with PTFE and nano-Al₂O₃ is engineered specifically to balance lubrication and mechanical strength under these conditions.

In the revised manuscript, we have significantly strengthened the Introduction and Conclusions to explicitly highlight these points of novelty. We are grateful to the reviewer for this suggestion, which has helped us better frame the significance of our findings.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The article can be published.

Author Response

We sincerely thank the reviewer for this critical suggestion. We have undertaken a comprehensive language polishing of the entire manuscript to enhance the clarity, fluency, and academic tone of the text.