Design and Development of a Planetary Gearbox for Electromechanical Actuator Test Bench through Additive Manufacturing

Round 1

Reviewer 1 Report

Dear Authors,

I really like your paper, probably because planetary gear design is my hobby. 
It is a pity that instead of comparing your 3K planetary gear (Wolfram planetary gear) with harmonic gear, you have not presented this interesting way of loading the braking motor. With such comparison, it is necessary to determine the meshing efficiency of both gear transmissions.
It was also possible to show the cross-section of the gear (2D), because this view (tearing out) in 3D is hardly legible.
Specific remarks:

1) analysis of planetary gears begins with the determination of DOF. For your gearbox to have DOF = 1, you must support shafts A and B. Please use the formula:
W = 3 * N - 2 * p5-p4
2) the adoption of constance value of modules for gears in first and seconds stages is a big mistake (especially at gear ratio> 100) in times when gear optimization is used (it could be mentioned after general considerations that you adopted them in the test bench)
3) In this 2D drawing you could show how to mount the wheel with herringbone teeth)
3) you provided the formula for zmin for straight teeth, using helical gears (in the case of helical teeth zmin also depends on the beta angle).
4) to facilitate the interpretation of the impact of the number of teeth of individual wheels on the gear ratio, the formula for gear ratio could be simplified.
5) with velocity distributon (Figure 3) it was possible to perform one more action (one formula) to determine the gear ratio using simple kinematics rules for checking.

These are minor corrections.
Sincerely Reviewer

Author Response

Dear Sirs,

As requested in your previous mail (2020-04-16), our paper has been modified in order to meet the reviewers’ suggestions. In this regard, I take this opportunity to thank the reviewers for their valuable suggestions, that have certainly contributed to improving the quality of our work.

Please find enclosed the revised version of our manuscript " Design and development of a planetary gearbox for Electromechanical Actuator test bench trough additive manufacturing" which I have submitted to your journal "Actuators" on 07 April 2020.

In particular, the following actions have been performed:

Review Report Form 1

• Reviewers’ Request: It is a pity that instead of comparing your 3K planetary gear (Wolfram planetary gear) with harmonic gear, you have not presented this interesting way of loading the braking motor. With such comparison, it is necessary to determine the meshing efficiency of both gear transmissions.
  • Authors’ Response: Thank you so much for your suggestion. As requested, we added considerations regarding efficiency of the transmission (lines 116 to 121). However, a more in-depth study of efficiency, including an experimental measurement, will be published in a future work. In addition, some further detail on the use of the test bench for collecting health monitoring data has been added: possible failures to inject are listed, as well as possible sensors to install in the gearbox (lines 83 to 92). The work “Brassitos, E; Jalili, N. Design and Development of a Compact High-Torque Robotic Actuator for Space Mechanisms. Journal of Mechanisms and Robotics. Transactions of the ASME. DECEMBER 2017, Vol. 9” has been added to the references, since it proposes an electromechanical actuator with a similar design (lines 73 to 76).

• Reviewers’ Request: It was also possible to show the cross-section of the gear (2D), because this view (tearing out) in 3D is hardly legible.
  • Authors’ Response: Thank you so much for your suggestion. As suggested, a 2D cross section has been added to Figure 4, showing the rolling interfaces and the layout of the herringbone and helical gears.

• Reviewers’ Request: Analysis of planetary gears begins with the determination of DOF. For your gearbox to have DOF = 1, you must support shafts A and B. Please use the formula: W = 3 * N - 2 * p5-p4.
  • Authors’ Response: Thank you very much for your suggestion. According to the reviewer’s request, the computation of degrees of freedom has been added (lines 127 to 155), alongside with some references to justify the lack of a planet carrier or bearings on the input and output shafts. In facts, the full joints that would be required to support input and output shafts are replaced by:
    1) rolling surfaces for radial loads (i.e. the satellites act as bearing rollers),and             
    2) by the opposed helical teeth (or herringbone teeth) for axial loads (since a herringbone meshing constrains also the DoF related to axial displacement, as opposed to a straight or helical meshing. 

• Reviewers’ Request: The adoption of constance value of modules for gears in first and seconds stages is a big mistake (especially at gear ratio> 100) in times when gear optimization is used (it could be mentioned after general considerations that you adopted them in the test bench).
  • Authors’ Response: Thank you very much for this observation. In order to justify our approach, we added some supplementary considerations regarding the choice of the common modulus of the gears, despite different moduli could result in more optimized gear ratio (lines 163 to 172).

• Reviewers’ Request: In this 2D drawing you could show how to mount the wheel with herringbone teeth.
  • Authors’ Response: Thank you so much for your suggestion that allows us to enhance the efficacy of our work. About that, a 2D cross-section, showing the rolling interfaces and the layout of the herringbone and helical gears, has been added to Figure 4.

• Reviewers’ Request: You provided the formula for zmin for straight teeth, using helical gears (in the case of helical teeth zmin also depends on the beta angle).
  • Authors’ Response: Thank you so much for your suggestion: as requested, we modified the paper using the formula for zmin related to helical gears, according to ISO 6336 (line 183).

• Reviewers’ Request: To facilitate the interpretation of the impact of the number of teeth of individual wheels on the gear ratio, the formula for gear ratio could be simplified.
  • Authors’ Response: Thanks for this suggestion; based on your request, we have rearranged the equation (4) to simplify its interpretation (lines 176 to 179).

• Reviewers’ Request: With velocity distribution (Figure 3) it was possible to perform one more action (one formula) to determine the gear ratio using simple kinematics rules for checking.
  • Authors’ Response: We thank the reviewer for this suggestion: we have tried to implement this hint by adding the computation of gear ratio from velocity distribution. To this purpose (and also in relation to the previous point of this list), the equation (4) has been rearranged to simplify its interpretation (lines 176 to 179).

In conclusion:

• We carefully evaluated the reviewers' comments and tried to best implement their suggestions in our paper ;
• As requested, we modified our paper using the "Track Changes" function of Microsoft Word to highlight any revisions and make them easily visible to the editors and reviewers.

We thank you again for your support and, hoping to have correctly understood and satisfied all the requests and suggestions, remain at your disposal for any future request.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper deals with design and manufacturing of a planetary gearbox for the electromechanical test bench, in order to simulate different operating scenarios and environmental conditions for an EMA, and to collect operating parameters of the actuator both in nominal conditions and under the effect of incipient progressive faults.

The diagnosis and prognosis of failures in reduction and mechanical transmission systems is a recurring problem which is faced with the lack of data and systems allowing the experimental validation of algorithms. The test bench presented in this paper will therefore contribute to providing researchers with an experimental validation tool. The paper is well written and easy to understand, however, the following points can be improved:

• The introduction of the paper can be enriched with papers proposing methods and reviews for Fault diagnosis and failure prognosis of gearboxes such as: Condition monitoring and fault diagnosis of planetary gearboxes: A review, and “Vibration based condition monitoring and fault diagnosis of wind turbine planetary gearbox: A review: Mechanical Systems and Signal Processing 126 (2019) 662–685.Other method of data generation for the validation of prognosis algorithms such as for example: "Data-driven approach augmented in simulation for robust fault prognosis.(2019). "Engineering Applications of Artificial Intelligence. V. 86. pp. 154-164.: which proposes to increase the experimental databases by generating in simulation all trajectories of possible degradations. Or other work using physical models, such as "Hybrid method for remaining useful life prediction in wind turbine systems."(2018). Renewable Energy. V. 116. pp. 173-187: which is based on a multi-physical model of the bond graph type for the simulation of normal, degraded or faulty operating situations.
• The mechanism for introducing degradation into the gearbox is not detailed enough in my opinion, perhaps for intellectual property considerations. If the authors can detail this mechanism it will give more interest to the paper.
• What are the degradation phenomena that can be emulated by this system? eccentricities of the transmission shaft?, fouling, Cracks? ....
•  
• The gearbox will operate in a complex environment which causes and accelerates the process of degradation of the guearbox. How the influence of this environment (thermal influence for example ...) is taken into account in this testbensh.

Author Response

Dear Sirs,

As requested in your previous mail (2020-04-16), our paper has been modified in order to meet the reviewers’ suggestions. In this regard, I take this opportunity to thank the reviewers for their valuable suggestions, that have certainly contributed to improving the quality of our work.

Please find enclosed the revised version of our manuscript " Design and development of a planetary gearbox for Electromechanical Actuator test bench trough additive manufacturing" which I have submitted to your journal "Actuators" on 07 April 2020.

In particular, the following actions have been performed:

Review Report Form 2

• Reviewers’ Request: The introduction of the paper can be enriched with papers proposing methods and reviews for Fault diagnosis and failure prognosis of gearboxes such as…
  • Authors’ Response: We first want to thank the reviewer for his suggestions. According to reviewer's hints, we extended the review of the literature by adding the following works (lines 48 to 54 and 367 to 394):
    • Lei, Y.; Lin. J.; Zuo, M.J.; He, Z. Condition monitoring and fault diagnosis of planetary gearboxes: a review. Measurement: Journal of the International Measurement Confederation, 2014, volume 48, Issue 1, pp. 292–305.
    • Wang, T.; Han, Q.; Chu, F.; Feng, Z. Vibration based condition monitoring and fault diagnosis of wind turbine planetary gearbox: A review. Mechanical Systems and Signal Processing, 2019, Volume 126, pp. 662–685.
    • Djeziri, M.A.; Benmoussa, S.; Benbouzid, M.EH. Data-driven approach augmented in simulation for robust fault prognosis. Engineering Applications of Artificial Intelligence, 2019, Volume 86, pp. 154-164.
    • Djeziri, M.; Benmoussa, S.; Sanchez, R. Hybrid method for remaining useful life prediction in wind turbine systems. Renewable Energy, 2018, Volume 116, pp. 173-187.

• Reviewers’ Request: The mechanism for introducing degradation into the gearbox is not detailed enough in my opinion, perhaps for intellectual property considerations. If the authors can detail this mechanism it will give more interest to the paper.
  • Authors’ Response: Thank you very much for this observation. It is a crucial topic and, in our opinion, the proposal of new solutions that allow carrying out experimental activities, evaluating progressive quantifiable, controllable, and repeatable degradations, is nowadays of great interest among experts. In this regard, we have added a description of the possible failure modes to be injected into the gearbox and a brief examination of the potential sensors to be used for the detection of these faults (lines 83 to 92).

• Reviewers’ Request: What are the degradation phenomena that can be emulated by this system? eccentricities of the transmission shaft?, fouling, Cracks? ....
  • Authors’ Response: At least in part, this request can be linked to the previous one. The emulation of progressive degradations is crucial to guarantee reliable experimental evidence. In this regard, we added a brief description of the most critical failure modes that can be considered for our gearbox testbench (lines 83 to 92).

• Reviewers’ Request: The gearbox will operate in a complex environment which causes and accelerates the process of degradation of the guearbox. How the influence of this environment (thermal influence for example ...) is taken into account in this testbensh.
  • Authors’ Response: Thank you very much for this observation. It should be noted that our test-bench will be employed to collect data for training new diagnostic/prognostic algorithms for the detection of incipient faults and the estimation of the related Remaining Useful Life (RUL). It is possible to inject prodromal faults and, subsequently, observe the propagation of these gearbox damages in different environmental or operating conditions, but given the different properties of the materials used for the gearbox (PLA instead of metal) data for RUL estimation may not be representative of a real operating scenario (lines 58 to 63).

In conclusion:

• We carefully evaluated the reviewers' comments and tried to best implement their suggestions in our paper ;
• As requested, we modified our paper using the "Track Changes" function of Microsoft Word to highlight any revisions and make them easily visible to the editors and reviewers.

We thank you again for your support and, hoping to have correctly understood and satisfied all the requests and suggestions, remain at your disposal for any future request.

Author Response File: Author Response.pdf