Grease as Lubricant

grease is used to describe semisolid lubricants possessing a higher initial viscosity than oil. Grease consist of an oil and/or other fluid lubricant that is mixed with another thickener substance, a soap, to form a solid. The term soap is used in the chemical sense, meaning a metallic salt of a fatty acid, 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. After sufficient force to shear the grease has been applied, 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.

Greases are used where a mechanism can only be lubricated infrequently and where a lubricating oil would not stay in position. They also act as valuable sealants to prevent ingress of water and dust. Grease-lubricated bearings have greater frictional characteristics due to their high viscosity. Under shear, the viscosity drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease.

Grease Constituents

In general grease has the following content:

Base Oils     75 - 95 %
Thickener      5 - 20 %
Additives       0 - 10 %

Advantages of Using greases for lubrication:

Greases stay put
Greases seal out contaminants
Greases do not need circulation systems
Greases decrease dripping, splattering and leakage
Greases suspend solid additives easily
Greases are suitable for intermittent operations
Greases work under extreme operating conditions
Greases seal for life
Greases reduce noise
Greased machinery tends to need less power

Greases may also bear some disadvantages:

Greases may not reach all places in need of lubrication

Greases do not have any cleaning effect
Greases do not work as cooling agent
Greases cannot be used at as high speeds as liquids

Consistency Grades for Lubricating Greases

As most people know, the consistency of oil lubricants is measured in terms of viscosity. The consistency of bearing grease, on the other hand, is measured in penetration levels.

The consistency of bearing grease can greatly affect bearing performance.

NLGI Grade	Worked Penetration Range mm/10	Consistency
000 00 0 1 2 3 4 5 6	445-475 400-430 355-385 310-340 265-295 220-250 175-205 130-160 85-115	Fluid Semi Fluid Very soft Soft “Normal” Grease Firm Very Firm Hard Very Hard

Worked & Unworked Penetration of Grease

Worked penetration: it means the grease is first churned for 60 round-trip strokes using a standard grease worker. Air is driven out of the sample and the penetration of the cone is again measured. Under mechanical shear, lubricating greases often change their consistency. Therefore, it is more reasonable to indicate the worked penetration. It is the consistency of a worked grease.

Most greases change significantly in consistency when worked (sheared or kneaded). Thus a worked penetration level is considered as significant as the unworked penetration level in regards to the service behavior of the grease.

Unworked penetration: The consistency of a grease or paste in the state of rest, i.e. in the state of material as supplied.

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