This is also called fluid-film, thick-film or flooded lubrication. This type of lubrication is done in those cases where liquid lubricants having minimum viscosity under working conditions are used and at the same time, it should remain in place and separate the moving or sliding parts of the machine. A thick film of lubricant is interposed between the surfaces of bodies in relative motion.
The frictional resistance to the movement of the sliding parts is only due to the internal resistance between the particles of the lubricant. In the fluid film lubrication, the liquid lubricant fills the grooves or irregularities of the sliding surfaces, and forms a thick layer in between them so that there is no direct contact between the sliding surfaces.
In this way, the lubricant reduces the wear and tear by reducing the frictional resistance produced by the movement of sliding parts.
The frictional resistance is independent of the load, But depends upon the area and relative velocity of the sliding parts and viscosity, thickness of the lubricants. Due to this reason, the light machines like watches, clocks, guns, scientific instruments etc. and delicate machines are provided with fluid film lubrication's.
Hydrodynamic lubrication- characteristics
Pressure within the fluid mass is greatest at some point approaching minimum clearance and lowest at the point of maximum clearance
- Increase in load decreases minimum film thickness
- Also increases pressure within the film mass to provide a counteracting force
- Viscosity increases as pressure increases
- With a given load and fluid, the thickness of the film will increase as speed is increased
- Fluid friction increases as the viscosity of the lubricant becomes greater
- Film thickness at the point of minimum clearance increases with the use of more viscous fluids
- With same load, the pressure increases as the viscosity of fluid increases
- 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