A lubricating grease is a solid or semi-fluid lubricant containing a thickening agent in a liquid lubricant. Grease can be likened to a sponge soaked in lubricant. In cases of a soap based grease the soap can be likened to the sponge soaked in a mineral oil as the lubricant.
They are usually made

Grease can be divided into 6 general categories or types:
MINERAL OILS MIXED WITH SOLIDS
Specialised heavy equipment lubricant

HEAVY ASPHALTIC-TYPE OILS BLENDED WITH LIGHTER OILS
More a thick heavy oil, lighter oil blends improve pour point.

EXTREME-PRESSURE GREASES
Contains additives that improve wear resistance under extreme pressures. A chemical reaction between the metal and the chemical brought about by the heat generated by pressure as the faces come together.

These aditives include compounds containing Chlorine, Phosphorous, Sulphur and phosphates.Zinc, lead or even asbestos may be added as a filler to cushion shock loading in gears

ROLL NECK GREASES
Semi solid grease exclusively used to lubrictate journal bearings. Typically the grease bar is profiled

SOAP THICKED MINERAL OILS
This is the largest category and is split up into groups stermined by the base oil or thickener

Sodium-base (Sodium Hydroxide) greases are also general-purpose greases. They are used higher temperature stability is required having a drop point 300 to 350 0F
Sodium-soap greases have a spongy or fibrous texture and are yellow or green in color. They may be used for luricating rolling element bearings more typically they are used in rough, heavy duty appilcations.Tend to be water soluble

Barium-soap greases are general-purpose types, valued for their ability to work over a wide temperature range. The drop point is over 3500F although it is not recommended to use them continously over 2750F
They are used in many applications although the high soap content makes them unsuitable for high speed or low temperature applications. They have a buttery of fibrous texture and are reddish-yellow or green in color.

Lithium-soap (Lithium Hydroxide) grease is suitable for high and low temperature applications. It has a drop point of 350 0F and may be used at coninuous tempeatures over 3000F
They may be made with oils having a very low viscosity to make them suitable for very low temperature applications
They have very good stability; good water resistance, and are also readily pumpable. They have a buttery texture and a brownish-red color. More expensive than sodium soap

Calcium-soap(calcium Hydroxide) are inexpensive general purpose greases with a relatively low drop point of 800C due to water content drying out, also called lime-soap greases. Uses include axle grease, water pump grease and general machinery applications and have the major advantage of water resistance
Calcium complex greases has a very high heat resistance of over 500oF making it particularly useful to extreme pressure applications although lack of mechanical stability has limited their usage
Calcium-soap greases are yellow or reddish in color, and have a smooth buttery texture.

Aluminum-soap are special purpose greases which are inherently very sticky

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