Lubricating Oils - Marine Engineering Study Materials

Introduction

A lubricant reduces friction and wear of the metal surfaces. It keeps metal surfaces clean by carrying away possible deposits and providing a seal to keep out dirt. A lubricant carries away the heat generated in bearings and gears, etc. preventing overheating, seizure and possible breakdown.

Desirable Properties of Lubricating oils are:

Viscosity

Viscosity is the resistance of a fluid to motion. It is an internal friction. The load carrying capacity of the bearing depends on the viscosity of the lube oil, speed of rotation of the journal, and the load on the bearing.

Viscosity of the lubricant decreases with increase in temperature.
Heat generated in the bearing is proportional to viscosity.
The temperature rise in the bearing is proportional to the heat generated and inversely proportional to the heat flow from the bearing.
Rate of lubricant flow depends on viscosity. Hence viscosity is indirectly related to the temperature rise in the bearing.
Correct bearing clearance ensures the required flow of lubricating oil through the system in maintaining the bearing and the shaft at right temperature and lube oil at right viscosity.

Viscosity Index

The viscosity index of lube oil indicates the change of viscosity that occurs with a change of temperature.

A lubricating oil with a high viscosity index has only a small change in viscosity over a large temperature change.
Viscosity index improvers are used to improve viscosity index of lube oils.

Pour Point

Pour point of a lubricating oil is he temperature at which it ceases to flow.
'Pour Point Depressors' are additives used to lower pour point of lubricating oils.

Cloud Point

The cloud point of a lube oil indicates the temperature at which wax crystals are formed.
The cloud point is important because wax crystals formed may choke filters in the lube oil system causing loss of lubricating oil pressure.

Other properties include Density, Flash Point, Total Base Number (TBN), Total Acid Number (TAN), SAE Number, Ash Content, etc.

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