# Frictional Behaviour of the Microstructural Surfaces Created by Cylindrical Grinding Processes
## 3 Tribological Performance of Structural Surfaces
### Friction Behaviour under Different Loads and Rotational Speeds  
The COF is mainly determined by the factors such as the material of the contact surface, the surface topography and roughness, the loaded force on the contact surface and the speed of the relative movement between the surfaces [21]. Therefore, it is necessary to assess the influence of the different loads and rotational speeds condition on the friction performance.  
The average COF at the stable stage of each test (i.e., at the end of 5 min test running) is used to represent the component friction result in corresponding to each load and speed condition. Average COF comparison of the components under each test condition is shown in Figure 6. The test results show that polish-ground surfaces do not give the lowest COFs compared with those microstructural surfaces.  
Figure 5: Results of no-load torque tests at 500, 1000 and 1500 rpm.  
Figure 6: Average COF values obtained under different test conditions.  
As shown in Figure 6, the samples under the lower load 30 N (test A) presents higher coefficients of friction, when they run at a high speed of 1500 rpm. Comparing with polishground smooth surface, the textured ground surfaces could reduce COF by more than 20% in the best case in the test range. For the tests of 100N load (tests B and C), most exhibited lower friction at the higher rotational speed. Meanwhile, the COF values of the structural surfaces were smaller than the polished surface. The greatest decrease in COF values compared with polishground samples is 20% at the rotational speed of 1500 rpm. On the contrary, the best improvement of friction at the rotational speed of 500 rpm were small, only slightly better (8%) than the polishground components.  
It is interesting to note that the COF values at low-load high-speed tests were much higher than that of other test conditions. The reason for such results may relate to the hydrodynamic effect in cylindrical bearing contact area during the shaft sample rotation. The workload in the bearing pair is balanced by hydrodynamic force, which is generated because of the wedge gap forming between the shaft and the hole. The hydrodynamic oil film wedge is characterized by the convergence ratio, which is a function of workload. When a bearing pair is under a higher load, the bearing contact gap will get closer to enable the hydrodynamic lift force increases to balance the increased load. As the gap decreases, the gap convergence ratio becomes larger and fluid flow rate would decrease and so did the resistance, resulting in the friction reduction. The higher the workload, the higher the convergence ratio. It has been demonstrated in engineering tribology that a higher convergence ratio will give a lower COF in a hydrodynamic bearing [21]. Therefore, a higher workload leads a lower COF.  
Considering moving speed influence, the COF values in a high-load high-speed situation were only slightly higher than that of a high-load low-speed situation, which means the speed influence is much smaller than the influence of the workload. Such a small difference could be due to the higher moving speed causing higher shear stresses in the lubricant, which may play an insignificant role in the tribology.  
In a word, the experiment results confirm that hydrodynamics play an important role in friction reduction. A particular microstructural surface created by suitably designed dressing and grinding conditions could reduce the friction coefficient significantly.