Best Practice 6.Grease Construction and Function

32 J U N E 2 0 0 8 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

he March Best Practices Notebook introducedreaders to important principles about thenature of lubricants and lubricant raw materi-

als and how machines create an oil film when theirsurfaces begin to interact. This month we’ll exam-ine the somewhat specialized area of grease con-struction, performance attributes and issues ofthickener compatibility. These topics are a preludeto lubricant selection, an issue we’ll treat more in-depth in a future article.

Grease composition, function and performanceSliding and rolling surface actions, coupled withdifferences in speed, load, component size andoperating environment, all influence the type of oilfilm that is expected to form and the viscometricand additive properties that must be present with-in the oil to protect machine components.

The oil itself must provide the lift and surfaceprotection functions in order for the grease lubri-

Article highlights:

■ Benefits and drawbacks

associated with grease

usage.■ How grease stiffn

ess ratings

are formulated.

■ Why chemistry affects key

grease performance charac-

teristics.

■ Problems associated with

mixing greases.

BEST PRACTICES NOTEBOOK

Understandinggrease constructionand function

Understandinggrease constructionand function

By Mike Johnson, CLS, CMRP

Contributing Editor T

A few key points about grease chemistry, ratingsand the specific properties needed for yourapplication.

32-2-38 bpnotebook 6-08 5/2/08 7:39 AM Page 32

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y J U N E 2 0 0 8 3 3

cant to be effective. All of the base oil and additivetype choices that were previously examined mustbe revisited when selecting the grease. In additionto those issues, the lubrication practitioner alsomust select from a variety of thickener systems,each with its own set of potential problems. Col-lectively, grease selection seems like a simple task,but if the reliability engineer wishes for the grease-lubricated components to have life cycles resem-bling oil-lubricated components, a significantnumber of variables must be fully considered.

Greases, as a category, fall into a classificationof materials called non-Newtonian fluids. All fluidsthat experience a change in viscosity with changein shear stress are considered non-Newtonian.Mayonnaise, ketchup and hair gel are examples ofcommon household non-Newtonian fluids. Greas-es that exhibit shear-induced thinning are referredto as thixotropic greases, and those that exhibitshear-induced thickening are referred to asrheopectic greases.

Both thixotropic and rheopectic responses aretime dependent, meaning the thinning or thicken-ing effect is more pronounced as the period ofshear stress increases. In other words, thixotropicgreases tend to liquefy as the elements in themachine squeeze, push and otherwise stress thefluid, and rheopectic greases tend to harden underthe same types of mechanical force.

It would be ideal for the grease to “work thin”only at the point of immediate shear stress(motion of the lubricated component) and theninstantaneously return to its original state themoment the stress forcestops. If this condition exist-ed, the grease would liquefyand function more like an oilat the point of motion andthen reform and create anisolating seal when the mo-tion stops. Unfortunately, thethinning and thickening ef-fects tend to be permanent.

Greases perform their lu-brication function over timeby gradually releasing oil in-to the working areas of thecontacting machine surfaces.This function has been com-pared to that of a spongegradually releasing its liquidover a period of time. A morepractical image would be of a concentration of mil-lions of microscopic sponges held inside themachine, close to the working machine compo-

nents, each one gradually releasing oil into thework zone. The greater the amount of sheer stressthat the grease experiences (likened to squeezingthe sponge) the faster the grease releases its holdof oil. Of course, once the sponge has released theoil, its usefulness is done. The oil and additivetypes contained within the sponge, and eventuallyreleased into the working areas of the machine, areselected based on the type of frictional conditionsexpected.

The permanent changes that the grease under-goes are accelerated by rising temperatures,increasing shear stress and mixture with othergreases. For these reasons the reliability engineermust replenish the grease at an optimum timecycle, with an optimum volume to arrive at areplacement state that resembles the effectivenessof an oil bath.

Grease: Pros and consPreviously we stated that machine lubricants haveto perform six key functions:

1. Separate surfaces

2. Minimize friction

3. Cool the machine part

4. Clean the working area

5. Prevent corrosion

6. Provide a means of hydro-mechanical energytransfer.

With those being the stated objectives, thereare benefits and drawbacks associated with usinggrease:

While it may be somewhat easier to identifypotential drawbacks for the use of grease vs. oil,

CONTINUED ON PAGE 34

Grease Advantages Grease DisadvantagesReduced frequency of relubrication. Loss of component cooling.

Decreased cost of machine design for lubrication. Loss of component flushing.

Improved startup after a prolonged idle time. Localized heat spikes/hot spots.

Improved sealing effectiveness (seal assistance). Increased risk of lubricant incompatibility failure.

Reduced risk of process contamination. Loss of contamination control functions (filtration).

More effective use of solid film additives. Increased risk of lubricant oxidative failure.

Improved protection in high load/low speed machines. Machine component speed limits vs. Oil.

Increased risk of new lubricant contamination.

Storage stability limitations.

Increased risk of product variability/batch variability.

Risk from relubrication practice: volume control.

Risk from relubrication practice: frequency control.

Risk from relubrication practice: viscosity selection.

Risk from relubrication practice: application failure.



there clearly are advantages that make the use ofgrease compelling.

Grease construction1

The word grease is derived from the Latin wordcrassus, meaning fat. Early examples of systematiclubrication with animal fats date back to circa 1,400BC, with efforts to reduce friction on wheel axels.From these very early roots, efforts to reduce fric-tion were dependent on relatively abundant animaland vegetable-based oils. Colonel William Drakeand his well-publicized oil well created a new wayto supply an arguably superior oil product, whichaccelerated the move toward the use of mineral oiland hastened the birth of the petroleum age.

Mineral oil, as it turns out, is a pretty good rawmaterial for lubricating surfaces. Taking the leadfrom the (hand, hair, laundry) type soap manufac-turing industry, which has been robust since theearly days of the period of the enlightenment, lubri-cant manufacturers adopted a soap manufacturingtechnique called saponification to produce thebasis for building stable, useful petroleum greases.Saponification is the chemical reaction that pro-duces soap, which becomes the grease thickener.

High school chemistry teaches that a mixture ofan acid and a base produces a salt and water. If theacid happens to be an organic or fatty acid, thenthe product called is soap. Saponification occursfollowing the mixing of a fatty acid with an alkalicomponent. The early fatty acids were produced bycooking animal and vegetable fats with water toseparate glycerin and the inherent fatty acid. Theearly alkali component for soap manufacture wasderived from soda ash, which comes from the alka-li remains of burned vegetable matter.

Owing to the benefit of many early chemicalindustry developments, namely large-volume pro-duction of organic acids (Stearic acid – C17H35COOHand Benzoic acid – C6H5COOH are both commonorganic fatty acids used in grease manufacture)and metallic hydroxides (lithium, aluminum, calci-um, sodium and barium hydroxides), grease manu-facturers learned to create specialized soaps intowhich mineral oils and additives were introducedto deliver highly specialized product functions.

In the manufacturing process the raw materialsfor forming the soap, and some portion of thelubricating oil itself, are combined in a mixing ves-sel and blended/agitated to initiate the chemicalreaction between the selected metallic thickenerand fatty acid. The types and volumes of the mate-rials used have a dramatic impact on the charac-teristics of the finished products.

Simple soap greases (primarily lithium, alu-

minum and calcium) are created if only one—along chain—fatty acid is used during the soap for-mation process. If the supplier wishes to prepare aproduct to deliver better temperature resistance,then he will add another—a short chain—fatty acidin an additional step. Complex soap greases (lithi-um complex, aluminum complex, barium complexand calcium sulfonate complex) are known forsuperior temperature resistance vs. their simplesoap counterparts.

Once the soap has been formed (using only aportion of the required lubricating oil) the balanceof the lubricating oil is added, along with theremaining additives that are required to fortify theproduct for optimum results. The grease is cooled,milled to assure that the thickener is uniformly dis-tributed through the other raw materials, and asample is removed for stiffness testing.

The grease stiffness rating is the primary differ-entiating property that people use in selecting agrease. Grease stiffness is the characteristic thatenables the grease to either move freely or sit stillonce placed into service. The higher the stiffnessrating the thicker the grease body. The NationalLubricating Grease Institute (NLGI) has devised anine-point scale that is used in conjunction withASTM D217 to grade grease stiffness. This methodprovides the user with a stiffness rating rangingbetween 000 and 6.

CONTINUED FROM PAGE 33

Figure 1. Grease penetration and consistency

(Source: The Lubrication Engineers Manual, Third Edition, (2007), The Association for Iron

and Steel Technology, Warrendale, Pa., p. 92)


In the testing process, the sample of grease iscooled to 77 F, placed in a cup and smoothed overand, as shown in Figure 1, a pointed cone is placedon the surface of the grease. The operator releasesa fastener and allows the cup to sink into thegrease cup. Depending on the stiffness of the fin-ished product, the cup may settle significantly oronly slightly. Once the cup comes to rest, the oper-ator measures the degree of drop based on the dialattached to the top of the rod supporting the cone.Figure 2 represents the profiles established byNLGI that dictate the grade or number of the fin-ished product.

The thickness number or NLGI grade of thegrease only has implications relative to its require-ment to either flow or not flow once applied to themechanical structure requiring lubrication. Withthe exception of calcium sulfonate complex thick-eners, the thickeners do not provide surface pro-tection in addition to that provided by the base oiland additives. Having a grease rated as an NLGINo. 2 does not make it inherently superior or infe-rior to an NLGI No. 1 grease.

Each soap type imparts different performanceproperties worth noting. Reputable manufacturerswill work vigorously to overcome weaknesses andenhance strengths of their respective products rel-ative to the selected application types.

Key differences include: (See micrograph pictures).

■ Aluminum and aluminum complex greasesare known to have strong high-temperatureperformance characteristics, including high


Figure 2. NLGI grease consistency values


Micrograph (A): Aluminum Complex GreasesUsing STRATCO® Contactor™ Reactor

Micrograph (B): Calcium Greases UsingSTRATCO® Contactor™ Reactor

Micrograph (C): Lithium Greases UsingSTRATCO® Contactor™ Reactor

Micrograph (D): Lithium Complex Greases UsingSTRATCO® Contactor™ Reactor


dropping points and very good oxidation-resistance. Aluminum-based greases alsotend to perform well in high-wash applica-tions, resisting the force of process watersand housekeeping wash hoses. These greas-es tend to be stringy, and this characteristicincreases with rising sustained applicationtemperature. These greases have been knownto stiffen with extended use.

■ Calcium, calcium complex, calcium sulfonatecomplex greases are best known for theirexcellent wash- and water-resistance proper-ties and can be fortified to also providestrong corrosion-resistance properties. Thecomplex and sulfonate complex forms arerespected for high load-carrying capabilitiesand have temperature limits on par withother complex soaps. Calcium (hydrous andanhydrous) are best used in low to moderatetemperature applications and have accept-able stability at moderate temperatures.

■ Lithium and lithium complex greases arevery widely used. These have strong proper-ties in a variety of categories. These greaseshave excellent long-term work stability,strong high-temperature characteristics andhave acceptable wash- and corrosion- resist-ance capabilities. With additive enhance-ment, the wash- and corrosion-resistancecan be improved. These also have good lowtemperature shear performance, makingthem suitable for extremely low temperatureapplications. The generally well-rounded per-formance of these greases has made them

the product of choice for general purposegrease relubrication in industrial and manu-facturing environments.

There is another category of grease thickeners,referred to generically as non-soap thickeners.These thickeners are made with a variety of prod-ucts and processes, and deliver a wide array of per-formance results. The clay-based (bentonite) prod-ucts and polyurea products represent the largestmarket volumes of non-metallic thickeners.

There are appreciable differences in the manu-facturing practices for non-soap greases. Bentoniteproducts are created by direct addition of thethickener to the base and additive mixture. Theseproducts require significant milling to assure uni-formity. Polyurea thickeners are a type of polymerformed by a reaction between amines and iso-cyanates, which occurs during the grease forma-tion process. Some of the common raw materialoptions are considered to be hazardous, requiringcareful environmental and health precautions fortheir manufacture.

Common performance criteria for these non-soap greases include:

■ Polyurea greases are preferred for use in ballbearing applications, giving rise to theirbroad-based acceptance in electric motorapplications. Polyurea greases contain littleto no heavy metals and have favorable hightemperature performance. Together thesetwo traits provide very good oxidation-resist-ance. Polyureas tend to have fair work stabil-ity, wash- and corrosion-resistance. Somepolyureas have a low level of compatibilitywith other soap and non-soap greases,including other polyureas. Nonetheless,there are individual products being manufac-tured that demonstrate strengths in all ofthese categories, including the importantissue of compatibility.

■ Bentonite greases were the original non-melting grease. Bentonite is a type of clay.The base oils tend to evaporate before theclay material becomes hot enough to melt.This is both a strength and weakness. Whenused for extended periods of time at elevatedtemperatures, bentonite grease residues maycause a filling of the housing that can makelong-term relubrication difficult. Bentonitegreases are incompatible with most othergrease types as well.

Figure 3 provides a breakdown of the general mar-ket distribution of several greases by thickener type.2




Aluminum and aluminum

complex greases are

known to have strong

high temperature per-

formance characteristics,

including high dropping

points and very good

oxidation-resistance.



In all grease products, the oil and the additiveshave the primary roles of lifting, separating andprotecting surfaces, and the soap has the primaryrole of holding the oil in reserve until it is neededby the machine components. The March Best Prac-tices Notebook suggested that lubricating oils canbe constructed with a wide variety of additive com-pounds and base oil types and that some of thoseraw materials may well compete with one another.It was further suggested, since the lubrication prac-titioner was not routinely privy to the ingredientsof any of the given products in use, that it would bebest to avoid mixing lubricant products. That state-ment should be strongly reiterated, as it also per-tains to the materials used as metallic thickenersto form the grease products.

Many studies have been conducted over the

years that provide similar enough results that onecan take away from the discussion of grease com-patibility a simple rule: Do not mix greases! Mixturesof greases may fail to perform vs. either of the non-mixed products in a variety of ways, including:

Not withstanding the changes that may occurwith incompatibilities with base oils and additives,practitioners could expect obvious and promptchange in the greases stiffness (NLGI rating) andloss of temperature resistance, with the otherchanges occurring more gradually.

It is possible to know all of the ramificationsthat may occur when two or more greases aremixed, but not without a fair degree of testing andanalysis. Unless one is going to expend the finan-cial resources and effort to effectively study the

Shear stability Increase or decrease in the firmness of the grease mixture.

Dropping point Decrease in the mixture’s temperature stability.

Oxidation-resistance Loss oxidation stability, increase in oxidation byproducts.

Wear-resistance Loss of AW and EP additive performance.Rate of oil dissociation Premature loss of oil reservoir leading(bleed) to increased hardening.Wash-resistance Loss of ability to withstand passive or

direct wash action of process solutions.


Figure 3. General market distribution of severalgreases by thickener type



degree of compatibility between different types ofgreases, one should avoid mixing if at all possible.

Figure 4 provides a useful general reference onthe issue of thickener compatibility that the practi-tioner may use.3

SummaryGrease manufacturing is a highly complex anddetailed process. There are many different types ofmaterials that may be used, each of which has someimpact on the grease final performance characteris-tics. The two broad categories of greases include

soap-thickened or non-soap thickened greases.While the thickener type does clearly have an influ-ence on the long-term behavior of the grease, themajority of the surface protection work is providedby the oil and the additive choices. The NLGI grease

stiffness rating system provides greasemanufacturers with a clear mechanismfor grading grease stiffness character-istics.

Mike Johnson, CLS, CMRP, MLT, is theprincipal consultant for Advanced MachineReliability Resources, headquartered inFranklin, Tenn. You can reach him [email protected].

References1. NLGI Lubricating Grease Guide, FourthEdition, (1996), National LubricatingGrease Institute, Kansas City, Mo., pp.1.09-1.12.

2. Mang, T., and Dresel, W. (2007), “Lubricants andLubrication, Second, Completely Revised andExtended Edition,” Wiley-VCH, p. 644.

3. Web reference: http://www.finalube.com/reference_material/grease_compatibility_chart.htm.

TLT


Figure 4. Grease compatibility chart


Best Practice 6.Grease Construction and Function

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