Effective grease lubrication depends on the grease's consistency

Tag : aluminium sealant

T he American Society for Testing and Materials (ASTM) defineslubricating grease as: "A solid to semifluid product of dispersionof a thickening agent in liquid lubricant. Other ingredientsimparting special properties may be included" (ASTM D 288, StandardDefinitions of Terms Relating to Petroleum).
Grease Anatomy
As this definition indicates, there are three components that formlubricating grease. These components are oil, thickener andadditives. The base oil and additive package are the majorcomponents in grease formulations, and as such, exert considerableinfluence on the behavior of the grease. The thickener is oftenreferred to as a sponge that holds the lubricant (base oil plusadditives).

Figure 1. Grease Anatomy Base Oil
Most greases produced today use mineral oil as their fluidcomponents. These mineral oil-based greases typically providesatisfactory performance in most industrial applications. Intemperature extremes (low or high), a grease that utilizes asynthetic base oil will provide better stability. Thickener
The thickener is a material that, in combination with the selectedlubricant, will produce the solid to semifluid structure. Theprimary type of thickener used in current grease is metallic soap.These soaps include lithium, aluminum, clay, polyurea, sodium andcalcium. Lately, complex thickener-type greases are gainingpopularity. They are being selected because of their high droppingpoints and excellent load-carrying abilities.
Complex greases are made by combining the conventional metallicsoap with a complexing agent. The most widely used complex greaseis lithium based. These are made with a combination of conventionallithium soap and a low- molecular-weight organic acid as thecomplexing agent.
Nonsoap thickeners are also gaining popularity in specialapplications such as high-temperature environments. Bentonite andsilica aerogel are two examples of thickeners that do not melt athigh temperatures. There is a misconception, however, that eventhough the thickener may be able to withstand the hightemperatures, the base oil will oxidize quickly at elevatedtemperatures, thus requiring a frequent relube interval. Additives
Additives can play several roles in a lubricating grease. Theseprimarily include enhancing the existing desirable properties,suppressing the existing undesirable properties, and imparting newproperties. The most common additives are oxidation and rustinhibitors, extreme pressure, antiwear, and friction-reducingagents.
In addition to these additives, boundary lubricants such asmolybdenum disulfide (moly) or graphite may be suspended in thegrease to reduce friction and wear without adverse chemicalreactions to the metal surfaces during heavy loading and slowspeeds.

Table 1. NLGI Consistency Function
The function of grease is to remain in contact with and lubricatemoving surfaces without leaking out under the force of gravity,centrifugal action or being squeezed out under pressure. Its majorpractical requirement is that it retains its properties under shearforces at all temperatures it experiences during use. Applications Suitable for Grease
Grease and oil are not interchangeable. Grease is used when it isnot practical or convenient to use oil. The lubricant choice for aspecific application is determined by matching the machinery designand operating conditions with desired lubricant characteristics.Grease is generally used for:
Machinery that runs intermittently or is in storage for an extendedperiod of time. Because grease remains in place, a lubricating filmcan instantly form.
Machinery that is not easily accessible for frequent lubrication.High-quality greases can lubricate isolated or relativelyinaccessible components for extended periods of time withoutfrequent replenishing. These greases are also used insealed-for-life applications such as some electrical motors andgearboxes.
Machinery operating under extreme conditions such as hightemperatures and pressures, shock loads or slow speed under heavyload.
Worn components. Grease maintains thicker films in clearancesenlarged by wear and can extend the life of worn parts that werepreviously lubricated by oil. Functional Properties of Grease
Grease functions as a sealant to minimize leakage and to keep outcontaminants. Because of its consistency, grease acts as a sealantto prevent lubricant leakage and also to prevent entrance ofcorrosive contaminants and foreign materials. It also acts to keepdeteriorated seals effective.
Grease is easier to contain than oil. Oil lubrication can requirean expensive system of circulating equipment and complex retentiondevices. In comparison, grease, by virtue of its rigidity, iseasily confined with simplified, less costly retention devices.
Grease holds solid lubricants in suspension. Finely ground solidlubricants, such as molybdenum disulfide (moly) and graphite, aremixed with grease in high-temperature service or in extremehigh-pressure applications. Grease holds solids in suspension whilesolids will settle out of oils.
Fluid level does not have to be controlled and monitored. Characteristics
As with oil, grease displays its own set of characteristics thatmust be considered when being chosen for an application. Thecharacteristics commonly found on product data sheets include thefollowing:
Pumpability . Pumpability is the ability of a grease to be pumped or pushedthrough a system. More practically, pumpability is the ease withwhich a pressurized grease can flow through lines, nozzles andfittings of grease-dispensing systems.
Water resistance. This is the ability of a grease to withstand the effects of waterwith no change in its ability to lubricate. A soap/water lather maysuspend the oil in the grease, forming an emulsion that can washaway or, to a lesser extent, reduce lubricity by diluting andchanging grease consistency and texture.
Consistency. Grease consistency depends on the type and amount of thickenerused and the viscosity of its base oil. A grease's consistency isits resistance to deformation by an applied force. The measure ofconsistency is called penetration. Penetration depends on whetherthe consistency has been altered by handling or working. ASTM D 217and D 1403 methods measure penetration of unworked and workedgreases. To measure penetration, a cone of given weight is allowedto sink into a grease for five seconds at a standard temperature of25°C (77°F).
The depth, in tenths of a millimeter, to which the cone sinks intothe grease is the penetration. A penetration of 100 would representa solid grease while a penetration of 450 would be semifluid. TheNLGI has established consistency numbers or grade numbers, rangingfrom 000 to 6, corresponding to specified ranges of penetrationnumbers. Table 1 lists the NLGI grease classifications along with adescription of the consistency of how it relates to commonsemifluids.
Dropping point. Dropping point is an indicator of the heat resistance of grease.As grease temperature increases, penetration increases until thegrease liquefies and the desired consistency is lost. The droppingpoint is the temperature at which a grease becomes fluid enough todrip. The dropping point indicates the upper temperature limit atwhich a grease retains its structure, not the maximum temperatureat which a grease may be used.
Oxidation stability. This is the ability of a grease to resist a chemical union withoxygen. The reaction of grease with oxygen produces insoluble gum,sludges and lacquer-like deposits that cause sluggish operation,increased wear and reduction of clearances. Prolonged exposure tohigh temperatures accelerates oxidation in greases.
High-temperature effects. High temperatures harm greases more than they harm oils. Grease,by its nature, cannot dissipate heat by convection like acirculating oil. Consequently, without the ability to transfer awayheat, excessive temperatures result in accelerated oxidation oreven carbonization where grease hardens or forms a crust.
Effective grease lubrication depends on the grease's consistency.High temperatures induce softening and bleeding, causing grease toflow away from needed areas. The mineral oil in grease can flash,burn or evaporate at temperatures greater than 177°C (350°F).
Low-temperature effects. If the temperature of a grease is lowered enough, it will becomeso viscous that it can be classified as a hard grease. Pumpabilitysuffers and machinery operation may become impossible due to torquelimitations and power requirements. As a guideline, the base oil'spour point is considered the low-temperature limit of a grease. References

1. Pirro, Wessol. Lubrication Fundamentals . New York: Marcel Dekker, 2001.
2. U.S. Army Corps of Engineers. Engineering and Design - Lubricants and Hydraulic Fluids . EM 1110-2-1424 CECW-ET, 1999. Please reference this article as:
Jeremy Wright, "Grease Basics". Machinery Lubrication Magazine. May 2008 Issue Number: 200805
Machinery Lubrication
Back Page Basics