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| November 30,2005
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Grease Information
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G. P. Reeves has been engineering and fabricating highly successful lubricant dispensing systems since 1971. The following grease information is presented with a focus on production greasing.
WHAT IS GREASE?
Grease is a semi-fluid mixture of a fluid lubricant, a thickener, and chemical additives. The fluid lubricant that performs the actual lubrication can be petroleum (mineral) oil, synthetic oil, or vegetable oil. Finely ground solid lubricants such as graphite, molybdenum disulfide, and Teflon are often blended into grease for unique applications. The majority of greases are composed of mineral oil blended with soap thickener. The thickener gives grease it characteristic "higher than oil" viscosity and consistency and holds the oil and solid lubricants in place. Often the most important feature of a grease is its rigidity, viscosity, consistency, or stiffness. A grease that is too stiff may not feed into areas requiring lubrication, while a grease that is too fluid may leak out. Chemical additives enhance performance and protect the grease and lubricated surfaces.
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The word "grease" is sometimes used to identify the apparent viscosity
characteristics of other mixtures that are not engineered for lubrication.
These mixtures may require special equipment for automatic dispensing.
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COMMON PRODUCTION GREASE VISCOSITIES
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The National Lubricating Grease Institute (NLGI) has classified grease according to consistency.
The measure of consistency is called penetration.
Penetration depends on whether the consistency has been altered by handling or working.
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NLGI NUMBER
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Viscosity and Consistency
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00
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semi-fluid
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0
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very soft
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1
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soft
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2
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common grease
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WHAT IS THE FUNCTION OF GREASE?
Grease is engineered and blended to remain in contact with and lubricate and silence moving surfaces without leaking out under gravity or centrifugal action or being squeezed out under pressure. Grease is often applied to automotive hinges, seat tracks, and latches to silence these parts by minimizing movement of internal components. Grease is also used as a sealant to prevent leakage and entrance of corrosive contaminants and foreign materials. Grease applied during manufacture of automotive parts must often remain for life. Grease is used because oil would run off, necessitating frequent re-application.
WHY IS GREASE MORE DIFFICULT TO DISPENSE THAN OIL?
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The higher viscosity and consistency of grease inhibits grease from "seeking its own level". Air can be accidentally introduced into grease whenever grease is agitated, pumped, scooped, stirred, or transferred. The high viscosity and consistency of grease prevents that air from automatically rising to the surface for natural dispersion. Air will remain trapped in grease forever unless it is intentionally removed.
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The thickeners in grease cause it to be more compressible than oil. Compression during dispense results in later expansion that makes nozzles drip and drool.
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Grease has a higher viscosity index (V.I.) than its base oil because grease is engineered to thin when heated to release its oil to lubricate. V.I. is the measure of the rate of change of viscosity with temperature. This higher viscosity index requires that grease be measured using positive piston displacement to obtain and the accuracy and repeatability necessary for most production greasing.
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The soaps, fillers, and solid lubricants blended with oil to make grease can be separated from the mixture by pressure, gravity, centrifugal action, acceleration, and deceleration. Separated solids can clog filters, inhibit the operation of valves, and reduce the lubricating qualities of the grease.
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(c) 2005-2006 G.P. Reeves Inc.
Send Comments or questions to Lubelogic@gpreeves.com.
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