## 4 Orthogonal Experiment Analysis
### Influence of Clearance  
The clearance is one of the most important factors affecting the bearing lubrication performance. Its empirical value is approximately one-thousandth of the bearing diameter. To consider the influence of different lubrication states as much as possible in the calculation, the bearing clearance was set as 0.02 mm, 0.04 mm, 0.06 mm, 0.08 mm, and 0.1 mm. We then calculated rough contact load ratios, minimum oil film thicknesses, and film thickness ratios under different bearing clearances and the trend of frictional power consumption with increasing clearances.  
Figure 11 illustrates the change in asperity contact load percentage over one cycle under different clearance conditions. Considering the orbital path (Figure 7) and the oil film pressure distribution at maximum pressure (Figure 9), it can be seen that the piston pin load on the bushing is primarily concentrated at 180 \({}^{\circ}\)CA, corresponding to the orbital path close to the lower part of the bushing. Owing to its elasticity as well as the forces of the bushing and pin boss, the piston pin is bent, which results in asperity contact being primarily concentrated around the two bushing edges. As the clearance increases, the mean  
Figure 10: (**a**) MOFT at different factors and levels. (**b**) Friction power at different factors and levels. (**c**) MOFP at different factors and levels.  
and maximum values of the asperity contact ratio decrease. This indicates that increasing the clearance within a certain range increased the overall hydrodynamic lubrication ratio.  
Figure 12 illustrates the MOFT trend during one cycle under different clearance conditions. According to the figure, MOFT decreases as the clearance increases, and the corresponding film thickness ratios are all considerably less than 3. This is because the MOFT is primarily concentrated at the maximum pressure moment and the lower part of the bushing on both sides, and its lubrication state is mainly subject to load and piston pin structure stiffness constraints. In conjunction with an increase in the clearance, the load-bearing capacity of the oil film decreased, thereby lowering the minimum thickness of the oil film at this location.  
Figure 13 illustrates the trends of the friction power in one cycle under different clearance conditions. The friction power generated by hydrodynamic lubrication increased as the clearance increased, as shown in Figure 13a. This is mainly due to the increase in the hydrodynamic lubrication area with increasing clearance, as well as the increase in oil film thickness, which results in an increase in frictional power resulting from the oil film shear stress. When the spring does not work, the rough contact frictional power consumption decreases as the gap increases; however, at other moments, as the clearance increases, the MOFT decreases and the local friction coefficient increases, whereas the asperity contact area decreases, resulting in the friction power generated by the contact first increasing and then decreasing, as shown in Figure 13b. The main friction power of this model originates from hydrodynamic lubrication, and the increase in clearance corresponds to an increase in hydrodynamic lubrication throughout the cycle, whereas the rough contact frictional power consumption does not change significantly. Therefore, a comprehensive consideration of the MOFT friction power should ensure that the small-end bearing clearance is between 0.02 and 0.04 mm.  
Figure 11: Asperity contact load ratio at different clearances.  
Figure 12: MOFT at different clearances.