Semi-solid lubricants

Semi-solid lubricants (Greases)

Preparation
Greases are semi – solid lubricants obtained by thickening of lubricating oil by the addition of metallic soaps. Soaps are prepared by saponification of vegetable oils (or) fats with alkali (like as thickner (or) gelling agents).

Example
Preparation of Lithum grease
1step
Lithium soap is prepared by the saponification of vegetable oils (or) fats with lithium hydroxide.

II Step
Lubricating oil thickened by adding lithium soap.

The nature of the soap determines,
(a) The temperature upto which the grease can be used .
(b) It acts as a thickener.
(c) It is water and oxidation resistant.
(d) It enables the grease to stick to the metal surface firmly.

Properties and Uses of different greases

1. Sodium – soap grease (soda – base grease)

(i) Slighty soluble in water.
(ii) Dropping point is very high. It can be upto 175 deg C in ball bearing.

2. Calcium – soap grease ( lime – bases grease, cup grease)

(i) It is water resistant.
(ii) Beyond 70o deg C the grease separates into soap and oil.
(iii) Lesser dropping point than soda – base grease. It is a general purpose grease, used for
lubricating water pumps, tractors, etc.

3. Barium – soap grease

(i) It is water – resistant.
(ii) Possesses good adhesiveness. Used for lubricating automotives.

4. Lithium – soap grease

(i) It is a resistant to water and heat.
(ii) Expensive and superior to all other types. It is used at lower temperature: used for
lubricating engines.

5. Aluminium – soap grease

(i) It possess very high adhesiveness.
(ii) Expensive and water resistance. Used for lubricating chains and oscillating surfaces.

6. Axle – grease (Resin grease)
It is cheap and water resistant. Used for less delicate equipments under high load and low speed.

Grease are used under the following situations
(i) Where oil is squeezed out due to heavier load or low speed.
(ii) Where the bearing and gears that work at high temperature.
(iii) Where the bearing need to be sealed against the entry of dirt and dust etc.
(iv) Where frequent application of lubricant is inconvenient, as in automobile wheel bearings.