# Influence of Contamination of Gear Oils in Relation to Time of Operation on Their Lubricity
## 4 Conclusions  
The current paper presents an analysis of the operational contaminants that may affect the deterioration of the lubricating properties of synthetic gear oils. The main objective of the research was not only to compare the properties of oils, but to also determine the relationship between the physical properties in order to determine the condition of the oil. Two models were considered in the present work. One was the path friction coefficient. The second was the relationship between the number of contaminants and the diameter of the contaminants. For each oil, these models were fitted to the empirical data.  
Based on the conducted research, the following conclusions were formulated:  
1. The used oils can be characterized by a significant number of large contaminant particles. According to normative requirements and operational experience, this indicates their inability to be used to their full potential. The results of the friction tests indicate that the exploitation use of oil 75W-140 occurred after 350,000 km of mileage. It also applies to oil 75W-90, which had a much lower mileage but worked in a gearbox. Moreover, the comparison of the results of the lubricity tests and the amount of particles due to the amount of contaminants does not indicate a correlation between the degree of oil contamination and lubricity. A higher share of particles that were several dozen micrometres in size was found in the used oils. It is possible that these large particles, which also present in other products of wear, are also suspended in oil.
2. In the light of the conducted research, it seems reasonable to hypothesize that the use of fluids in the expected operational runs does not cause a critical deterioration of the lubricating and anti-wear properties. The deterioration in the properties is non-catastrophic.
3. Unfortunately, the current research has not allowed us to check how long the liquid work time in a gearbox and axle must be in order to reach critical deterioration.  
The conducted research showed that the presence of contaminants is not catastrophic and that in order to fully examine the oils and to determine the critical moment, the oils with a much greater operational mileage should be tested in order to establish the relationship between the number of particles and their tribological properties. After testing a larger number of samples with different mileages, the second model supports the development of a classifier that allows the oil mileage to be estimated in technical devices. This will be the subject of further research.  
Conceptualization, L.G., D.P., K.P. and E.K.; methodology, L.G., K.P., D.P., K.G., E.K. and K.A.;; software, E.K. and K.A.;; validation, K.P., D.P., K.A. and K.G.;; formal analysis, D.P., L.G., K.P. and E.K.;; investigation, D.P., E.K. and L.G.;; resources, K.P., K.A., K.G. and P.I.; data curation, D.P., E.K. and P.I.;; writing--original draft preparation, D.P., K.P., L.G. and K.G.;; writing--review and editing, E.K., K.A. and K.P.; visualization, K.P. and K.G.;; supervision, K.A., K.P. and L.G.; project administration, K.A., L.G., K.P., D.P. and K.G.; funding acquisition, K.A. and K.P. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This project was financed in the framework of the project Lublin University of Technology-Regional Excellence Initiative, funded by the Polish Ministry of Science and Higher Education (contract no. 030/RID/2018/19).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.