## 3 Results and Discussion
### Oil-Film Temperature  
#### 3.2.1 Oil-Film Temperature of S220 and M220  
Table 5 gives the operation time and the maximum temperature of both oils at stable operation states. The maximum temperature of oil increases with speed and load, but the operation time to the maximum temperature decreases. The rotational speed and load have great influence on lubricating properties of oil [24], and the test results under different load cases are obtained. Although there are different lubricating performances between S220 and M220, the difference gradually shrinks with the increase of speed and load. Ultimately, the lubricating performances of both kinds of oils with respect to temperature tend to be the same and ultimately reach equilibrium, and the maximum temperature is within the permitted range.  
Figure 4 illustrates the oil-film temperature variation with time at 200 rpm-6 MPa. The temperature of T15 is the maximum temperature in the load-carrying area and the temperature of T25 is the maximum in the unload-carrying area. As shown in Figure 4, oil temperature rises gradually with time, but the gradient of temperature rise decreases constantly. Ultimately, the temperature tends to be constant and stable. This indicates that oil-film temperature will not rise infinitely but tends to be steady. Owing to higher oil-film pressure and larger internal friction in load-carrying area, more heat will be generated. So the temperature in the load-carrying area is higher than that in the unload-carrying area. Meanwhile, the equilibrium temperature of both oils is the same. As for the operating time to reach the equilibrium temperature, S220 is a little longer than M220.  
The distribution of the circumferential temperature from the equilibrium stage in the mid-plane of bearing at 200 rpm-6 MPa is illustrated in Figure 5. The temperature was tested by thermocouples from T1 to T12 in the load-carrying area and by T21 and T25 in the unload-carrying area. As shown in Figure 5, the temperature distribution of both oils is the same. Because oil-film temperature in the load-carrying area generally represents the load-carrying capacity and the stability of oil film, the load-carrying capacity and stability of both are equivalent.  
The distribution of axial temperature from equilibrium stage in load-carrying area at 200 rpm-6 MPa is depicted in Figure 6. The temperature in the load-carrying area increases along the  
\begin{table}
\begin{tabular}{c|c c c|c c c} \hline \multirow{2}{*}{
\begin{tabular}{c} **Load Case** \\ **(rpm-MPa)** \\ \end{tabular} } & \multicolumn{2}{c|}{**S220**} & \multicolumn{2}{c}{**M220**} \\ \cline{2-7}  & **Maximum** & **Location with** & **Operation Time** & **Maximum** & **Location with** & **Operation Time** \\  & **Temperature of** & **the Maximum** & **to Maximum** & **Temperature of** & **the Maximum** & **to Maximum** \\  & **Oil (C)** & **Temperature** & **Temperature (b)** & **Oil (C)** & **Temperature** & **Temperature (b)** \\ \hline
100.6 & 47.21 & T15 & 8.02 & 48.69 & T15 & 8.17 \\
150-06 & 56.56 & T15 & 7.10 & 54.59 & T15 & 7.20 \\
150-12 & 60.01 & T15 & 5.33 & 59.42 & T15 & 6.10 \\
200-06 & 59.71 & T15 & 3.60 & 59.81 & T15 & 4.30 \\
300-06 & 60.00 & T15 & 0.83 & 60.01 & T15 & 1.10 \\
300-12 & 60.20 & T15 & 0.40 & 60.01 & T15 & 0.50 \\ \hline \end{tabular}
\end{table}
Table 5: Operation time and maximum temperature under different load cases.  
axial direction, but temperature difference exists in both oils. The difference is somewhat larger on both ends of the bearing and smaller in the middle region. The difference is within 1 \({}^{\circ}\)C. Therefore, the temperature distribution of both oils is similar along the axial direction.  
As known from above, both oils are extremely similiar in the operating temperature and the temperature distribution. Oil-film temperature reflects load-carrying performance and service life of oil-film bearing oil, so the load-carrying capacity of S220 is equivalent to that of M220.