Semisolid Lubricants:

The most important semi-solid lubricants are grease, Vaseline, waxes and other compounds of oil and fats. These are called semi-solid because they are neither solids nor liquids at ordinary temperatures.

Greases are prepared by saponification of fat with alkali followed by adding hot lubricating oil while under agitation. At high temperature the soap dissolves in the oil, where upon the inter-connected structure exist and & the grease liquefies and it becomes a stiff solid mass. To improve the heat resistance of grease, inorganic solid thickening agents are added.

They are used

• For high load, low speed, intermittent operation, sudden jerks, etc.

• In bearing & gears at high temperature,

• As sealing agent in bearing.

Important greases are

1. Calcium-based Grease :

These are emulsion of petroleum oils with calcium soaps.

They are cheapest, most commonly used and good water resistance and used up to 800C because above 800c oil & soap separate out.

2. Soda-Based Grease :

These are petroleum oil thickened by mixing sodium soaps. They are poor water resistance because sodium soap is soluble in water. However they can be used up to 1750C. They are suitable for use in ball bearings.

3. Lithium Based Greases :

These are petroleum oils thickened by mixing lithium soaps. They are water resistance & suitable for use at low temperature only.

4. Axle Grease :

They are water resistance & suitable for less delicate equipment working under high loads and at low speeds.