# Influence of Contamination of Gear Oils in Relation to Time of Operation on Their Lubricity
## 3 Results and Discussion
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 with the size of several dozen micrometres was found in the used oils. It is possible that these large particles, that have also been found in other wear products, are suspended in oil. This is also confirmed by the information contained in PN-ISO 4406:2005 [22], where it is stated that particles that are larger than 4 micrometres are treated as a reference value for suspended substances. It is believed that in the case of suspensions, the presence of solid particles in a liquid additionally gives the liquid a non-Newtonian liquid character, which is associated with various types of viscosity anomalies [34]. However, the content of large particles, which was much higher in the tested oils with a history of exploitation, did not adversely affect the friction coefficient in the kinematic test pair. It is possible that the content of small particles from fractional parts with diameters from 1 micrometre to 5 um in size is important. In paper [35], it was stated that the share of oil contaminants with such dimensions can be as high as 96%. The share of these particles was the highest in the volume of all of the tested gear oils. A similar situation occurred in the fresh oils and in the oils with a history of \begin{table}
\begin{tabular}{c c c} \hline  & **Fresh Gear Oil** & **Oil after 220,000 km** \\ \hline \(\alpha_{1}\) & \(-\)0.00578 *** & \(-\)0.00181 *** \\ (Std. Error) & (0.00017) & (0.00007) \\ \(\alpha_{0}\) & 1.43028 *** & 1.40962 *** \\ (Std. Error) & (0.00599) & (0.00224) \\ Observations & 90 & 90 \\ \(R^{2}\) & 0.92529 & 0.89762 \\ Adjusted \(R^{2}\) & 0.92444 & 0.89645 \\ Residual Std. Error & 0.03792 (df = 88) & 0.01415 (df = 88) \\ F Statistic & 1.089.854 *** (df = 1; 88) & 771.511 *** (df = 1; 88) \\ \hline \end{tabular} Note: ***\(p<0.01\).  
\end{table} 
Table 4: Basic parameters of the linear regression model for oil SAE 80W-140. \begin{table}
\begin{tabular}{c c c} \hline  & **Fresh Gear Oil** & **Oil after 220,000 km** \\ \hline \(\alpha_{1}\) & \(-\)0.00578 *** & \(-\)0.00181 *** \\ (Std. Error) & (0.00017) & (0.00007) \\ \(\alpha_{0}\) & 1.43028 *** & 1.40962 *** \\ (Std. Error) & (0.00599) & (0.00224) \\ Observations & 90 & 90 \\ \(R^{2}\) & 0.92529 & 0.89762 \\ Adjusted \(R^{2}\) & 0.92444 & 0.89645 \\ Residual Std. Error & 0.03792 (df = 88) & 0.01415 (df = 88) \\ F Statistic & 1.089.854 *** (df = 1; 88) & 771.511 *** (df = 1; 88) \\ \hline \end{tabular} Note: ***\(p<0.01\).  
\end{table} 
Table 5: Basic parameters of the linear regression model for oil 75W-90. exploitation. Additives are often used during the operation of oils; these can be microscopic particles of soft metals ranging in size from 5 to 155 \(\upmu\)m. In study [36], a beneficial effect of such particle additives on the lubricating properties of gear oils is shown. It is possible that this factor had a decisive impact on the lack of deterioration of the lubricating properties of the tested oils. The fact emphasized in other papers, including [37; 38], that large particles with diameters ranging from a dozen to several dozen micrometres in size correspond to the dynamic clearance, thus determining the thickness of a lubricating film should also be noted. They are harmful because the dynamic clearance in gears that are in the contact area of the meshing teeth should be from 0.1 to 1 micrometres [39]. Large dirt particles may affect the continuity of a lubricating film.