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[EHD Theory]
Lubrication can be described in three steps as follows:
In the stationary state, a bearing and a shaft are in full contact due to gravity, and there is little lubricating oil between these two surfaces. The lubrication occurring in this state is called “Boundary Lubrication”. Boundary lubrication instantaneously occurs when the shaft starts rotating in the stationary state and its section shows the largest coefficient of friction.
While the rotation is in the progress, the lubricating oil gradually penetrates between the bearing and the shaft’s surface. Its section is called “Mixed Lubrication”. The friction coefficient gradually decreases due to the penetrated lubricating oil in this section.
After passing through the mixed lubrication section, the bearing and the shaft’s surface are completely separated by the oil film formed between the bearing and the shaft. Its section is called “Hydrodynamic Lubrication” The reason that friction coefficient increases according to the speed increment in the hydrodynamic lubrication state is because the viscous lubricating oil has a greater resistance as the rotational speed increases. However, in reality, the resistance does not continue to increase according to the speed increment and after a certain speed, the curve of friction coefficient becomes gradual and finally converges. This is because the viscosity of lubricating oil decreases due to the heat caused by rotation. Since RecurDyn’s EHD does not reflect the change in the viscosity of lubricating oil due to the heat, the actual results may differ when the heat generated by high-speed rotation is significant.
The equation and calculation process are as follows.
[Example Model]
Here is an example model to verify the validity of RecurDyn's EHD calculation results.
The model was implemented in RecurDyn based on the structures and parameters specified in the paper.
You can see that the results from RecurDyn's simulation and results from the paper are similar.
