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Discuss the mechanism of thick- film lubrication.
2 Answers
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In thick-film lubrication, the moving/sliding surfaces are separated from each other by a thick-film of fluid, (at least 1000 Å thick), so that direct surface-to-surface contact and welding of junctions rarely occurs.

The lubricant film covers/fills the irregularities of the sliding/moving surfaces and forms a thick layer in between them, so that there is no direct contact between the material surfaces.

This consequently reduces wear. The resistance to movement of sliding/moving parts is only due to the internal resistance between the particles of the lubricant moving over each other. Therefore, the lubricant chosen should have the minimum viscosity under working conditions at the same time, it should remain in place and separate the surfaces.

In such a system, friction depends on the viscosity, thickness of the lubricant, the relative velocity and area of the moving/sliding surfaces.

The coefficient of friction in such cases is as low as 0.001 to 0.003.

Hydrodynamic friction occurs in the case of a shaft running at a fair speed as well as in well-lubricated bearings with not too high load.

In a journal bearing, a film of the lubricating oil covers the irregularities of shaft as well as the bearings surfaces; and the metal surfaces do not come into direct contact with each other.

Thus, the resistance to movement is only due to the internal resistance of the lubricant.

Delicate instruments, light machines like watches, clocks, guns, sewing machines, scientific instruments, etc. are provided with this type of lubrication.

Hydrocarbon oils are considered to be satisfactory lubricants for thick-film lubrication.

Inorder to maintain viscosity of the oil in all seasons of year, ordinary hydrocarbon lubricants are blended with selected long chain polymers.

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It is the condition of lubrication where two surfaces of the bearings are completely separated by the film of liquid, in this type of lubrication, the load is taken completely by the oil film.

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