Lubricating fluid most commonly used is mineral oil. The choice of oil depends upon the intended application, and anything from the lightest spindle oil to the heaviest cylinder oil can be used, but usually a light or medium machine oil is chosen. Other fluids used for special purposes include dibasic esters such as dioctyl sebacate, the different types of silicone oils, and polyalkylene glycols.

The fatty material used to form grease making soaps are mainly of animal origin but also include vegetable and marine oils and fats. They can be used as the original glycerides or as the fatty acids split from the glycerides or fractions thereof recovered by filteration distillation or solvent crystallization. Hydrogentated oils are also used especially fish oils and castor oil, the latter being important as the source of 12- hydroxyl stearic acid. Most fats contain both saturated and unsaturated acids in the range C 12 to C 18 chain length, and in some cases (especially fish oil) to C 20 and C 22. The C 16 and C 18 acids preponderate. The type of material affects the melting point of the soap, and the degree of unsaturation may affect grease texture.

Fatty materials can be characterized chemically for grease manufacturing purposes by their:

1. Saponification value
2. Neutralization number
3. Iodine value

Saponification value: Is an indication of average molecular weight and hence approximate composition (if no unsaponifiable matter is present) and is a measure of the amount of alkali needed to convert it to soap.

Neutralisation Number: Measures the amount of the fatty acid present.

Saponification value and Neutralization number are expressed as Mg KCR/GM.

Iodine Value: Indicates the extent of unsaturation. Too much unsaturation leads to poor oxidation stability, so high Iodine values (0ver 60-70), are avoided and values of 35 or less are preferred.

Iodine value is expressed as No. of gms. Of Iodine absorbed by 100 gms. of product.

Many different alkalies have been used or proposed for saponifying the fatty material but calcium sodium and lithium hydroxides are the most commonly used.

Calcium Hydroxide: is used in the form of hydrated lime, which should be finely ground to pass 200 mesh and with a low content of carbonate which does not saponify easily, limits of 95% Ca(OH)2 minimum and 2.0 % CaCo3 maximum have been recommended.

Sodium Hydroxide: Usually supplied as solid or flake caustic soda but usually used in concentrated aqueous solution: should be at least 99% purity.

Lithium Hydroxide is supplied as the crystalline monohydrate LioH, H2o with LiOH 55% minimum and Carbonate and other alkali metals less than 1.0% by wt. each.

While most greases are made from soaps formed in situ, pre-formed or pre-made soaps supplied by chemical manufacturers are used to some extent.

The commonest are:

1. Lithium Stearate
2. Aluminium Stearate

Non stop thickners are also similarly available from manufacturers.

Various additives are used in greases in much the same way as in lubricating oils in order to improve the characteristics in particular respect.

Oxidation: Oxidation inhibitiors are widely used in greases in much the same way as in lubricating oils in order to improve the characteristics in particular respect e.g. to obtain longer storage and service life.

It is interesting to note that greases are apparently oxidised in different ways statically as in storage and dynamically as in service.

Amine and phenolic derivatives are effective inhibitors under static conditions and for service are effective high temperatures.

Dilauryl selenide and phenothiazine are effective in high temperature service.

Rust Inhibitors are mostly surface active materials of which a wide variety are used, and exert their function by forming absorbed films on the metallic surfaces.

EP Additives are necessary in cases where the greases has to sustain heavy or shock loading as in Roll-neck bearing of steel mill. Lead soaps, sulphurized oils alone or in combination are used for this purpose. Other additives containing sulphur, phosphorous or chlorine may be used.

Additives are also used to improve:

• Structure stability
• Resistance to bleeding
• Adhesiveness
• Stringiness
• Water resistance and so on.

Solid Additives (Fillers) Graphite.

Molybdenum disulphide are also used under very heavy loads or at high temperatures when the normal hydrodynamic lubrication provided by grease has for some reasons ceased to operate.