The manufacture of a grease involves dispersion of a thickener in the oil. The thickener or jelling agent in over 90% of greases is a metallic soap. The usual method by which a metallic soap thickener is formed can be described as follows:

Fat (Animal – vegetable) + Metal Hydroxide
Calcium Hydroxide
Sodium Hydroxide
Lithium Hydroxide
Monohydrate Metal Soap + Glycerine

This reaction is carried out by cooking the fatty material and alkali with normally some oil in order to get the soap well dispersed and also to make it softer and easier to stir. Such a procedure produces a wet soap partially dispersed in the mineral oil. It is then cut back with additional mineral oil to produce consistency desired in the finished grease. The soap oil mixture is a plastic mass with a tough fibrous structure. After the cut back operation. It is necessary to further process the grease by kettle milling or homogenization to modity its structure. The grease is then deaerated to remove entrained air and then filtered to remove any contaminants they may have been picked up during manufacture.

The manufacture of greases therefore involves the following steps, some of which may be accomplished simultaneously.

1. SAPONIFICATION
2. DE-HYDRATION
3. CUT – BACK
4. MILLING
5. DEAERATION
6. FILTERATION

The operations are carried out in vessel known as grease kettle, which is equipped with some form of agitation and with heating facility. The agitation is usually of the double acting type, that is it has a set of paddles rotating in one direction with a second set rotating in the opposite direction. The rate of cooling is very important in the development of the proper structure for many greases. Hence, close temperature control is required to manufacture a grease of desired properties. The structure of the grease can be modified by milling which can be carried out in the kettle itself during the cooling period or may be done outside the kettle. For out side milling, a high shear rate gear pump, homogenizer or colloid mill is normally used. Depending upon the types of grease, the total time required for manufacture of a grease batch in a kettle varies from 10 / 20 hours.

Grease differs from an oil in that it is usually plastic. It is the plasticity that gives a grease its special property of staying in a place where an oil would leak out. Greases can be of different kinds. They can be light or dark, hard, or soft or buttery or fibrous.

1. Appearance, Ash % by Wt. Nature of ASH Colour visual
2. Copper strip Corrosion Test.
3. Consistency : Cone Penetration, unworked Worked 60 X 10000 strokes, 100,000 strokes
4. Dropping point ºC
5. Free Alkali % by Wt. as – free acid as oleic % by wt.
6. Heat stability
7. Filler Content % By Wt.
8. Mineral oil Content % By wt. K. Viscosity at 40ºC K. Viscosity at 100ºC Viscosity Index
9. Flash Point C-o-C ºC
10. Soap content % By wt. type of soap
11. Water content

1. Roll stability test
2. Timken OK Load Test – EP
3. Four Ball EP test
4. Tilted wheel bearing test
5. EMCOR – Bearing Corrosion
6. Test Oxidation stability

It will be observed that there is a difference of 30 units in each NLGI grade Numbers. And in between 2 grades there is a difference of 45 units.

Consistency: Is a measure of the relative hardness of a grease. Working refers to the shear that takes place in a grease when one grease moves relative to another layer. This occurs whenever a grease is handled, stirred, mixed or moved as in a bearing. The consistency of grease usually changes with working, and the direction and degree of change depend upon the type and amount, of thickener in the grease and the shear rate. Normally, greases soften with working – they loose consistency.