# Demand Analysis of Lubricating Oil in Spur Gear Pairs
## 4 Effect of Gear Parameters
### Effect of Transmission Ratio  
In this section, the transmission ratio of a gear is used as the variable to examine the influence of transmission ratio on the oil volume for lubrication and cooling. The transmission ratios are set to 1, 2.95, 4.7 and 9.3. The other gear parameters are identical to those described in Section 3. The variations in the oil volume for lubrication during the meshing process at different transmission ratios are obtained using a series of simulations, as shown in Figure 8a.  
This figure indicates that the oil volumes for lubrication in the first two tooth meshing and the second two tooth meshing areas are invariable, and their values are identical when the transmission ratio of the gear is 1. The oil volume for lubrication in the single tooth meshing area is half that in the two tooth meshing area, because the entrainment velocity and oil film thickness of each meshing tooth during the meshing process are invariable. Moreover, the distance between the start of the meshing and pitch point increases as the transmission ratio increases and the width of the single meshing area decreases. The rangeability of the oil volume for lubrication along the mesh line (i.e., the slope of the curve of the oil volume for lubrication) increases as the transmission ratio increases. The oil volume for lubrication at the beginning of meshing decreases and the oil volume for lubrication at the end of meshing increases as the transmission ratio increases, because a similar variation trend occurs in entrainment velocity and oil film thickness at different transmission ratios.  
A series of simulations used to examine the influence of different transmission ratios on the friction coefficient during the meshing process and the variations in the oil volume for cooling at different transmission ratios during the meshing process are shown in Figure 8b,c, respectively. These figures contain partially enlarged images.  
Figure 8b indicates that the friction coefficient curve is symmetric around the pitch point when the transmission ratio of the gear is 1 and the friction coefficients at the start and end of meshing are consistent. The friction coefficient during the meshing process decreases as the transmission ratio increases. However, the friction coefficient at the start of meshing increases as the transmission ratio increases.  
Figure 7: Oil volume for lubrication (**a**), friction coefficient (**b**), and oil volume for cooling (**c**) under different numbers of teeth during the meshing process.  
Figure 8: Oil volume for lubrication (**a**), friction coefficient (**b**), and oil volume for cooling (**c**) under different transmission ratios during the meshing process.  
Figure 8c illustrates that the oil volume for cooling decreases as the transmission ratio increases. However, the oil volumes for cooling at the start of meshing and at the intersection of the single meshing teeth and the second meshing teeth areas initially decrease and then increase as the transmission ratio increases because two teeth are meshed simultaneously at these meshing points. One of the meshing teeth is found at start of meshing, and the friction coefficient and oil volume for cooling of this meshing tooth increase as the transmission ratio increases. However, the friction coefficient and oil volume for cooling of the other meshing tooth decrease as the transmission ratio increases. When the increase in the transmission ratio is small, the increase in the oil volume for cooling of the meshing tooth at the start of meshing is smaller than the decrease in the oil volume for cooling of the other meshing tooth. Thus, the total oil volume for cooling of the gear decreases. Moreover, when the transmission ratio increases considerably, the increase in the oil volume for cooling of the meshing tooth at the start of meshing is greater than the decrease in the oil volume for cooling of the other meshing tooth. Hence, the total oil volume for cooling of the gear increases.