Development of the Cost-Based Model for Monitoring the Lifetime of the Earth Moving Machines
, Milos Tanasijevic 2,*, Stevan Djenadic 2, Dragan Ignjatovic 2 and Ivan Jankovic 2
Round 1
Reviewer 1 Report
In this manuscript, the authors present the model for the identification of reference points on the lifetime curve of engineering systems. The curve commonly represents the increase and decline of failure in relation to time. A direct correlation between failure rate and costs is hypothesized in their paper, therefore, statistical and empirical analysis of costs provided reference points. Their approach is used for positioning stages of the engineering system lifetime with a minimal number of failures and costs, regardless of whether these are acceptable or not. A case study for their model is presented for the dozers operating on lignite open pits. It has been shown that reference points are not the same for two types of machines from different manufacturers.
In my opinion, the topic is interest. Besides, in order to expedite the processing of the revised manuscript, the authors had better provide a detailed point-by-point response letter and all the changes have been highlighted in some color in the new revised version. Finally, I would recommend a minor revision.
Minor comments
ü The author's main work is not clear in the abstract.
ü The literature review of the author is chaotic, not only did not write clearly the main work of the existing literature, but also did not write clearly the difference between their work and the existing results.
ü Style of References are not coherent.
Author Response
Answer to Reviewer 1:
Thank you for your well-intentioned comments and suggestions. We found it very useful and think that they will improve quality of our paper. We accept all yours comments and suggestions. Changes in the paper according to your comments and suggestions are made in red. Also, 4 new references have been added [15, 17, 18, 19], old ref [2] is written off and three refences are translate.
Reviewer 2 Report
This paper showed cost-base model considering the life-time of the machines. However, the presentation is not clear and hard to understand the meaning. I think there are many points to correct. Therefore, I assessed to reject this paper. I attached a PDF file of comments for this paper.
Comments for author File: 
Comments.pdf
Author Response
Answer to Reviewer 2:
Thank you for comments. We found some of them useful. All of your comments, according to our opinion, are of technical nature.
Comment 1:
We change K’ to Kr
Comment 2:
Equation (2) is taken from [x4].
Equation (1) and (2) are used to explain from what the costs of ownership consist of.
Comment 3:
Nature of write-off coefficient is explained in detail in further text.
Comment 4:
You are right that in the reliability theory F(t) is failure function and f(t) is its density. Also 1-F(t) is reliability function and it is denoted by R(t) not by (t).
Comment 5:
We think that „average cost function“ and „cost per unit of time“ means the same.
Comment 6:
We made typing mistake, brakets should not be written in t(E). We change it.
Meaning of tE is explained in page 8, raws 264-268.
tE1 is for machine type 1 while tE2 is for machine type2. tE1 and tE2 are calculated from inverse functions of linear regression. We thought that is obvious.
Comment 7:
We obtained f(t) on page 8 for machine type 1 and 2 using polynomial regression in standard way. We thought that it is not necessary to explain procedure in detail.
f(t) represents cumulative total costs, while figure 3 shows total cost per year, so they can not be the same.
Comment 8:
We accept suggestion and change notation in text and on pictures.
Comment 9:
You are right, but we think that it is the question for person who finally prepares paper for publication.
Changes in the paper according to your accepted comments are marked with color.
Author Response File: Author Response.pdf
Reviewer 3 Report
The research addresses the interesting theme of identifying the transition points of a machine’s economic lifetime. Although the theme is attractive, the paper has moderate scientific soundness. Moreover, content and presentation can be improved. Following are some considerations and suggestions:
1. The research focuses on specific mining machines; thus, some assumptions are valid for this category of machine. I think you should specify the application already in the title to better frame the problem
2. Lack of references (lines): 25-28, 41-42, 61-76, 113-115, 146-151 (are the definitions of the transition points personal definitions of the author? Can one find some references in literature?), 193-198, 206-207, 208-210 (who else uses this formulation? Is it valid for the specific machine?)
3. I suggest dedicating a paragraph to the assumptions of the model to clarify to the reader the field of the application. For instance, the model seems to be valid if the function is monotonically decreasing and the function f(t) is monotonically increasing.
4. I have some doubts about eq. 5. The function , that the author calls average cost, is monotonically increasing, therefore it is only integrable in a closed interval (). The average of a continuous function in one variable in the interval is :
I’m not clear how the author obtains .
5. R2 in type 1 regression (Figure 3) is low, thus the linear regression model might not be valid to make predictions on Y. However, the value of R2 is a necessary but not sufficient condition for the goodness of the regression model. I would suggest verifying and showing if the residuals distribution is linear and if their variance is homogeneous. If these two hypotheses are not verified, the linear regression model is not enough reliable to predict the reference point of economic lifetime.
6. I’m not clear on how the author derives the polynomial function f(t). I would suggest clarifying this aspect.
Author Response
Answer to Reviewer 3:
Thank you for your well-intentioned considerations and suggestions. We found it very useful and think that they will improve quality of our paper. We accept all yours comments and suggestions. Changes in the paper according to your comments and suggestions are made in green.
We only want to clarify comment no. 5
Formula of regression line in figure 3, for machine type 1, was wrong. We made mistake.
By recalculating the linear regression for machines type 1, new this time correct, results were obtained, which can be easily verified.
The new correlation coefficient is in the interval 0.71-0.9, which according to [16] represents a "high" correlation.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Although I assessed to reject this paper, I checked the paper again. Because the authors corrected following reviewer's comments, I recommend accepting it.
Reviewer 3 Report
Thank you to the authors for accepting my tips. All suggested corrections have been made and all required clarifications have been added to the text. Thus, I accept the manuscript in its present form.
