A grease can consist of an oil and/or other lubricating fluid that is mixed with another thickener substance such as, a soap, to form a solid. The term soap is used in the chemical sense, fatty acid, metallic salt, which forms an emulsion with the oil.
Greases are a type of shear-thinning or pseudo-plastic fluid, which means that the viscosity of the fluid is reduced under shear. When sufficient force is applied to shear the grease, the viscosity drops and approaches that of the base lubricant, such as the mineral oil. This sudden drop in shear force means that grease is considered a plastic fluid, and the reduction of shear force with time makes it thixotropic. .
Soaps are the most common emulsifying agent used, and the type of soap depends on the conditions in which the grease is to be used. Different soaps provide differing levels of temperature resistance, water resistance, and chemical reactivity.
CONE PENETRATION
The procedures for unworked, worked, and prolonged worked penetration are applicable to greases having penetrations between 85 and 475, that is, to greases with consistency numbers between NLGI 6 and NLGI 000.
NLGI Grease Classification System
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NLGI Grade
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Worked Penetration based on ASTM D 217
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Flow
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000
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445-475
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Semifluid (pourable)
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00
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400-430
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|
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0
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355-385
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|
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1
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310-340
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Slightly soft grease
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2
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265-295
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The most used grade
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3
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220-250
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Stiff grease
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4
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175-205
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|
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5
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130-160
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Hard grease
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6
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85-115
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Very hard.
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GREASE DROPPING POINT
The dropping point of a lubricating grease is the temperature at which it passes from a semi-solid to a liquid state under specific test conditions. It is an indication of the type of thickener used, and a measure of the cohesiveness of the oil and thickener of a grease.
Dropping point is used in combination with other properties to determine the suitability of greases for specific applications. It is applicable only to greases that contain soap thickeners. Greases with other thickeners, such as many synthetic greases simply do not change state in this way.