12-06-2014, 02:05 PM
Hydrodynamic Lubrication
Hydrodynamic Lubrication.ppt (Size: 241 KB / Downloads: 351)
Also called fluid-film, thick-film, or flooded lubrication
A thick film of lubricant is interposed between the surfaces of bodies in relative motion
There has to be pressure buildup in the film due to relative motion of the surfaces
Fluid friction is substituted for sliding friction
Coefficient of friction is decreased
Prevalent in journal and thrust bearings
Hydrodynamic lubrication- characteristics
Fluid film at the point of minimum thickness decreases in thickness as the load increases
Pressure within the fluid mass increases as the film thickness decreases due to load
Pressure within the fluid mass is greatest at some point approaching minimum clearance and lowest at the point of maximum clearance (due to divergence)
Viscosity increases as pressure increases (more resistance to shear)
Hydrodynamic condition- Fluid velocity
Fluid velocity depends on velocity of the journal or rider
Increase in relative velocity tends towards a decrease in eccentricity of journal bearing centers
This is accompanied by greater minimum film thickness
Bearing characteristic number
Since viscosity, velocity, and load determine the characteristics of a hydrodynamic condition, a bearing characteristic number was developed based on the effects of these on film thickness.
Increase in velocity increases min. film thickness
Increase in viscosity increases min. film thickness
Increase in load decreases min. film thickness
Therefore
Viscosity x velocity/unit load = a dimensionless number = C
C is known as the Bearing Characteristic Number
The value of C, to some extent, gives an indication of whether there will be hydrodynamic lubrication or not