Engineering Tribology Episode 1 Part 5 doc

Oxidation stability of greases ismeasured in a test apparatus in which five grease dishes 4 grams each are placed in anatmosphere of oxygen at a pressure of 758 [kPa].. The temperatureli

(ASTM D-566, ASTM D-2265) The schematic diagram of a drop point test apparatus is shown

in Figure 3.10 Although frequently quoted, drop point has only limited significance as agrease performance characteristic Many other factors such as speed, load, evaporation losses,etc determine the useful operating temperature range of the grease Drop point is commonlyused as a quality control parameter in grease manufacturing

Oil bath

Bath thermometer

Stirrer

Gas burner

Vent

Test thermometer

Grease sample is applied only to the walls of the cup and does not touch thermometer

FIGURE 3.10 Schematic diagram of a drop point test apparatus

· Oxidation Stability

The oxidation stability of a grease (ASTM D-942) is the ability of the lubricant to resistoxidation It is also used to evaluate grease stability during its storage The base oil in greasewill oxidize in the same way as a lubricating oil of a similar type The thickener will alsooxidize but is usually less prone to oxidation than the base oil Oxidation stability of greases ismeasured in a test apparatus in which five grease dishes (4 grams each) are placed in anatmosphere of oxygen at a pressure of 758 [kPa] The test is conducted at a temperature of 99°Cand the pressure drop is monitored The pressure drop indicates how much oxygen is beingused to oxidize the grease The schematic diagram of the grease oxidation stability apparatus

is shown in Figure 3.11

Oxidized grease usually darkens and acidic products accumulate in the same manner as in alubricating oil Acidic compounds can cause softening of the grease, oil bleeding, and leakageresulting in secondary effects such as carbonization and hardening In general the effects ofoxidation in greases are more harmful than in oils

· Thermal Stability

Greases cannot be heated above a certain temperature without starting to decompose Thetemperature-life limits for typical greases are shown in Figure 3.12 [27] The temperaturelimits for greases are determined by a number of grease characteristics such as oxidationstability, drop point and stiffening at low temperature

972, ASTM D-2595) The percentage weight loss is then determined.

· Grease Viscosity Characteristics

Greases exhibit a number of similar characteristics to lubricating oils, e.g they shear thin withincreased shear rates, the apparent viscosity of a grease changes with the duration ofshearing, and grease consistency changes with temperature

Apparent viscosity of a grease is the dynamic viscosity measured at the desired temperatureand shear rate (ASTM D-1092, ASTM D-3232) Measurements are usually made in thetemperature range between -53°C and 150°C in specially designed pressure viscometers.Apparent viscosity, defined as the ratio of shear stress to shear rate, is useful in predicting thegrease performance at a specific temperature It helps to predict the leakage, flow rate, andpressure drop in the system, the performance at low temperature and the pumpability Theapparent viscosity depends on the type of oil and the amount of thickener used in the greaseformulation

Shear thinning of greases is associated with the changes in the apparent viscosity of greasewith increased shear rates When shearing begins the grease’s apparent viscosity is high butwith increased rates of shearing it may drop to that of its base oil An example of this non-Newtonian, pseudoplastic behaviour in calcium soap based greases is shown in Figure 3.13.Shear duration thinning of greases is associated with the changes which occur in theapparent viscosity of grease with the duration of shearing As with oils, the greases whichsoften with duration of shearing and stiffen when shearing stops are called thixotropic.Depending on the type of grease a permanent softening or reverse effect of hardening canoccur In some applications this effect can be beneficial, in others it is detrimental Forexample, the permanent softening of a small quantity of grease in rolling contact bearingswill result in good lubrication, low friction and low contact temperatures On the other hand,the softening of the main bulk of grease will result in its continuous circulation and high

Drop point limit for synthetic greases

with inorganic thickeners

Oxidation limit for mineral

greases with unlimited oxygen

Lowest limit on synthetic greases imposed by high torque

Oxidation limit for synthetic greases

with unlimited oxygen present

;;;;;;;;;;;;;;;;

;;;;;;;;;;;;;;;;

Upper limit

imposed by drop point of

mineral greases depends on thickener

Lower limit on mineral greases imposed by high torque

depends on amount of oxygen

Oxidation of mineral greases in this region

FIGURE 3.13 Non-Newtonian behaviour of calcium soap based greases [64]

operating temperatures Thixotropic greases are particularly useful where there is a leakageproblem, for example, in a gear box The grease in contact with the gears will be soft because

of shearing, but outside the contact it will be stiffer and will not leak

Grease consistency temperature relationship describes the changes in the grease consistencywith temperature As has already been mentioned in a previous chapter the viscosity of oil is

very sensitive to temperature changes Relatively small temperature variations may result insignificant changes in viscosity There are only relatively small changes in grease consistencywith temperature until it reaches its drop point At this temperature the grease structurebreaks down and the grease becomes liquid The variation in grease consistency, expressed interms of penetration depth, with temperature for a sodium soap grease is shown in Figure3.14 [21].

FIGURE 3.14 Variation in grease consistency, expressed in terms of penetration, with

temperature for a sodium soap grease [21]

The structure of some non-soap greases will remain stable until the temperature rises to apoint where either the base oil or the thickener decomposes It has also been found that if agrease is heated above the drop point and then cooled it does not regain its grease likeconsistency and its performance is unsatisfactory [21]

Classification of Greases

The most widely known classification of greases is related to their consistency and wasestablished by the National Lubricating Grease Institute (NLGI) It classifies the greases intonine grades, according to their penetration depth, from the softest to the hardest [28], asshown in Table 3.3

Depending on the application a specific grease grade is selected For example, soft greases, No

000, 00, 0 and 1, are used in applications where low viscous friction is required, e.g enclosedgears which are slow, small and have a tendency to leak oil In open gears grease musteffectively be retained on the gear surface and tacky or adhesive additives such as bitumenare used in its formulation to improve adhesion Greases No 0, 1 or 2 are used depending onthe operating conditions such as speed, load and size of the gear In rolling contact bearingsgreases No 1, 2, 3 and 4 are usually used The most commonly applied is No 2 Hardergreases are used in large bearings and in applications where there are problems associatedwith sealing and vibrations They are also used for higher speed applications In plain, slowlymoving bearings (1 - 2 [m/s]) greases No 1 and 2 are used In general practice the mostcommonly used grease is Multipurpose Grease which is a grease No 2 according to the NLGIclassification, with aluminium or lithium soap thickeners

TABLE 3.3 NLGI grease classification [28].

or pour point of the base oil Examples of low temperature limits for selected greases areshown in Table 3.4 [21]

TABLE 3.4 Low temperature limits for selected greases [21]

Base oil Thickener Minimumtemperature [°C]

Mineral oil Calcium soap -20

Bentonite clay -30

Di-ester Bentonite clay -55

It is interesting to note that at temperatures above the drop point a grease may still provideeffective lubrication but it will no longer be a grease since it will have changed its phase andbecome a liquid

Environmental factors must also be considered in grease selection Industries such asmining, pharmaceuticals, food processing, textiles, aero-space and others operate in specificenvironments where different types of greases are required In some applications, due totheir semi-solid nature, greases are essential For example, in dirty environments such asmining, greases are ideal since they reduce the risk of fire and have good sealing properties

In the pharmaceutical and food industry they are widely applied because they seal against dirtand prevent leakages which might otherwise contaminate the product The type of thickenerand base oil that can be used in grease formulation is restricted and controlled in theseindustries, so that any accidental contamination of the product will not pose a health risk

In aerospace applications, greases are expected to operate in extreme conditions For example,aviation greases are expected to operate at the temperatures encountered by some of the highaltitude military aircraft which range from -75°C to +200°C Synthetic lubricants are used in

these applications In space, greases must have exceptionally low volatility to withstand highvacuum Evaporation losses in space are controlled by specially designed seal systems.

TABLE 3.5 Typical properties of selected greases [63]

Thickener Droppoint

[°C]

wear Thermalstability Life Anti-fretting

Anti-Average relative cost Sodium soap 185 medium medium fair-

medium fair medium fair veryquiet 1 Li/Ca

mixed soap 185 good medium good-excellent medium medium-good fair-medium veryquiet 1.4

Mecha-Water resista- nce

ing noise

Churn-Grease Compatibility

Two lubricating oils, provided that they are of the same type (i.e mineral, silicone, silane,diester, etc.), should not present any problems with compatibility when mixed The generalrule, however, is that two greases should not be mixed, even if they are formulated from thesame base oil and thickener, as this may lead to complete failure of the system [21] Theparticular risk is that an oil added may dissolve or soften the thickener

Degradation of Greases

Even though grease is prone to a greater number of degradation modes than oil, it is required

to spend a greater period of time as a functioning lubricant Grease remains packed withinthe rolling bearing, gear, etc., whereas oil is circulated from a sump Grease failure often doesnot occur immediately but small changes in operating conditions, particularly temperature,may cause problems associated with grease degradation

The modes of grease degradation are: base oil oxidation, separation of oil from the thickeningagent and breakdown of the thickening agent Base oil oxidation proceeds in a similarmanner to that already discussed for plain mineral oils Separation of the oil and thickeningagent, or ‘bleeding’, and breakdown of the thickening agent are peculiar to grease Even instorage, where oil can be stored in a sealed container almost indefinitely, greases mayseparate, soften or harden or even become rancid as in the case of some soap thickenedgreases [21] The composition and physical form of the soap control the likelihood of

‘bleeding’ or ‘loss of consistency’ Loss of consistency means that either the grease has becometoo soft or too hard for the intended application or that the rheological and tribologicalcharacteristics have deteriorated

The soap may be present in the oil as a tangled mass of fibres or as discrete crystals It is onlythese fibres or crystals that prevent either the oil separating from the grease or the greasedegenerating to a simple liquid If a grease liquefies, this is called ‘slumping’ and is a majorcause of grease failure As mentioned earlier, the soap fibres are vulnerable to temperatureand excessive mechanical working Elevated temperature attacks the grease in two ways:

· the base oil loses viscosity and therefore separates from the grease more readily,

· the soap fibres melt, in some cases even at quite low temperatures

If the soap fibres melt (or soften when there is no clear melting point), the greasedisintegrates Rolling bearings and gears can reach temperatures well in excess of 100°Cduring operation and special soaps, as opposed to the traditional calcium stearate, have beendeveloped to meet these demands An example is lithium hydroxy-stearate which does notsoften up to 190°C, and other greases capable of withstanding even higher temperatures arealso manufactured The lifetime of any grease declines with temperature For example, at40°C the lifetime of a lithium hydroxy-stearate grease is approximately 20,000 hours, whereas

at 140°C its lifetime is only 500 hours Grease failure in these circumstances is caused byhardening of the grease and formation of deposits on bearing surfaces

Most greases are reasonably resistant to damage by water in spite of their soap content Whilstlithium and aluminium based greases are scarcely affected by water, sodium based greases arequite vulnerable to it Calcium based greases, on the other hand, exhibit intermediate levels

of water resistance

Lubricant additives are chemicals, nearly always organic or organometallic, that are added tooils in quantities of a few weight percent to improve the lubricating capacity and durability ofthe oil This practice gained general acceptance in the 1940’s and has since developed toprovide an enormous range of additives Specific purposes of lubricant additives are:

· improving the wear and friction characteristics by provision for adsorption andextreme pressure (E.P.) lubrication,

· improving the oxidation resistance,

· control of corrosion,

· control of contamination by reaction products, wear particles and other debris,

· reducing excessive decrease of lubricant viscosity at high temperatures,

· enhancing lubricant characteristics by reducing the pour point and inhibiting thegeneration of foam

Carefully chosen additives are extremely effective in improving the performance of an oil.Perhaps for this reason, most additive suppliers maintain secrecy over the details of theirproducts One result of this secrecy is that the supplier and the user of the lubricant may onlyknow that a particular oil contains a ‘package’ of additives and this can often impede analysis

of lubricant failures Another result is that large companies very often use many differentbrands of lubricants which are effectively the same or have similar properties andcomposition This is quite costly to a company as a variety of lubricants must be stored andreplaced from time to time The secrecy surrounding additives also means that theirformulation is partly an art rather than a purely scientific or technical process The mostcommon package of additives used in oil formulations contains anti-wear and E.P.lubrication additives, oxidation inhibitors, corrosion inhibitors, detergents, dispersants,viscosity improvers, pour point depressants and foam inhibitors Sometimes other additiveslike dyes and odour improvers are also added to the oils

Wear and Friction Improvers

Additives which improve wear and friction properties are probably the most important of allthe additives used in oil formulations Strictly speaking these chemicals are adsorption andextreme pressure (E.P.) additives and they control the lubricating performance of the oil.Performance enhancing properties of these additives are very important since, if oil lackslubricating ability, excessive wear and friction will begin as soon as the oil is introduced intothe machine These additives can be divided into the following groups:

· adsorption or boundary additives,

· anti-wear additives,

· Extreme Pressure additives

The adsorption or boundary additives control the adsorption type of lubrication, and are alsoknown in the literature as ‘Friction Modifiers’ [32] since they are often used to prevent slip-stick phenomena The additives in current use are mostly the fatty acids and the esters andamines of the same fatty acids They usually have a polar group (-OH) at one end of themolecule and react with the contacting surfaces through the mechanism of adsorption Thesurface films generated by this mechanism are effective only at relatively low temperaturesand loads The molecules are attached to the surface by the polar group to form a carpet ofmolecules, as shown in Figure 3.15, which reduces friction and wear

Surface

Adsorbed

molecules

FIGURE 3.15 Adsorption lubrication mechanism by boundary additives

The important characteristic of these additives is an unbranched chain of carbon atoms withsufficient length to ensure a stable and durable film Specialized additives which combineadsorption or boundary properties with some other function such as corrosion protection arealso in use [32] Such additives are rarely described in detail in open literature, although themost frequently used are sulphurized fatty acid derivatives, phosphonic acids or N-acylatedsarcosines [3] Stearic acid derivatives such as methyl and ethyl stearates are also used.Adsorption or boundary additives are very sensitive to the effects of temperature They losetheir effectiveness at temperatures between 80°C and 150°C depending on the type of additiveused With increased temperature there is sufficient energy input to the surface for theadditive to desorb The critical temperature at which the additive is rendered ineffective can

be manipulated by changing the additive’s concentration, i.e a higher concentration results

in a higher critical temperature, but the cost is also increased

· Anti-Wear Additives

In order to protect contacting surfaces at higher temperatures above the range of effectiveness

of adsorption or boundary agents, anti-wear additives were designed and manufactured.There are several different types of anti-wear additives that are currently used in oilformulations For example, in engine oils the most commonly used anti-wear additive iszinc dialkyldithiophosphate (ZnDDP), in gas turbine oils tricresylphosphate or otherphosphate esters are used Phosphorous additives are used where anti-wear protection atrelatively low loads is required

These additives react with the surfaces through the mechanism of chemisorption, and theprotective surface layer produced is much more durable than that generated by adsorption orboundary agents

Common examples of these additives are zinc dialkyldithiophosphate, tricresylphosphate,dilauryl phosphate, diethylphosphate, dibutylphosphate, tributylphosphate andtriparacresylphosphate These additives are used in concentrations of 1% to 3% by weight.Zinc Dialkyldithiophosphate (ZnDDP) is a very important additive commonly used inengine oil formulations It was originally developed as an anti-oxidant and detergent, but itwas found later that this compound also acted as an anti-wear and mild extreme pressureadditive The term ‘anti-wear’ usually refers to wear reduction at moderate loads andtemperatures whereas the term extreme pressure (E.P.) is reserved for high loads andtemperatures Although some authors recognize this additive as a mild E.P additive, it isgenerally classified in the literature as an anti-wear additive The chemical structure ofZnDDP is shown in Figure 3.16

C H C H

H H

1/2 C

S Zn

FIGURE 3.16 Chemical structure of zinc dialkyldithiophosphate

By altering the side groups a series of related compounds can be obtained, an example ofwhich is zinc diphenyldithiophosphate These new compounds, however, are not aseffective as ZnDDP in reducing wear and friction The presence of zinc in ZnDDP plays animportant role The substitution of almost any other metal for zinc results in increased wear.For example, it was found that wear rates increased with various metals in the followingorder: cadmium, zinc, nickel, iron, silver, lead, tin, antimony and bismuth [56] Cadmiumgives the lowest wear rates but is far too toxic for practical applications Interestingly, nodefinite explanation for the role of metals in the lubrication process by ZnDDP has yet beenoffered

Like many other lubricant additives, ZnDDP is usually not available in pure form andcontains many impurities which affect lubrication performance to varying degrees [33] Thesurface protective films which are formed as the result of action of ZnDDP act as thelubricant, reducing wear and friction between the two interacting surfaces The lubricationmechanism of ZnDDP is quite complex as the additive has three interacting active elements,i.e zinc, phosphorus and sulphur Water and oxygen are also active elements, and theirpresence increases the complexity of the mechanism of lubrication All of these elements andcompounds are involved in surface film formation, and our current understanding of thesurface films produced is that they consist of a matrix of zinc polyphosphate with inclusions

of iron oxide and iron sulphide The thickness of these films is of the order of 10 [nm] [34] Ithas also been suggested that the films might be formed by spontaneous decomposition of theadditive on the worn surface since only a small amount of iron is found in the film [35].Even the effective film thickness under operating conditions is a matter of controversy In adifferent experiment the contact resistance measured between sliding surfaces lubricated byZnDDP was found to be higher than expected It indicated that a thicker surface film ofperhaps 100 [nm] thickness was in place, which is much greater than when lubricated bysurfactants which are boundary agents [33].

Care should be taken with the application of ZnDDP This additive is most suitable formoderate loads and was initially applied to the valve train of an internal combustion engine,giving significant reduction in wear and friction [36] For high loads applications ZnDDP mayactually increase wear beyond that of a base oil [34] It is also found that temperature canamplify these effects This is demonstrated in Figure 3.17 [34] where the wear rates decreasedwith temperature at low loads for ZnDDP containing oils but the converse was true at highloads

All tests were at 100 rpm for 2 hours

FIGURE 3.17 Influence of load and temperature on the effectiveness of ZnDDP on wear rates

(adapted from [34])

ZnDDP is a prime example of the empirical nature of much of the science of lubricantadditive development The problem of valve train wear and oil degradation in internalcombustion engines was solved by applying ZnDDP many years ago Scientific understandingand interpretation of the process has only recently become available

Tricresylphosphate (TCP) has been used as an anti-wear additive for more than 50 years LikeZnDDP, it functions by chemisorption to the operating surfaces, which is explained in detail

in the chapter on ‘Boundary and Extreme Pressure Lubrication’ It is very effective inreducing wear and friction at temperatures up to about 200°C Beyond this temperature there

is sufficient energy input to the surface for the chemisorbed films to desorb and it is believedthat the compound will then form less effective, much weaker, thick phosphate films withlimited load capacity [62]

Other anti-wear additives such as dilauryl phosphate, dibutylphosphate, diethylphosphate,tributylphosphate and triparacresylphosphate are also being used in lubricant formulation.They function in the same manner as ZnDDP or TCP by producing chemisorbed surface

films Some of these additives, e.g diethylphosphate, can even behave as a moderate E.P.additive, and these are discussed in the next section.

· Extreme Pressure Additives

These compounds are designed to react with metal surfaces under extreme conditions of loadand velocity, i.e slowly moving, heavily loaded gears Under these conditions operatingtemperatures are high and the metal surfaces are hot E.P additives contain usually at leastone aggressive non-metal such as sulphur, antimony, iodine or chlorine They react withexposed metallic surfaces creating protective, low shear strength surface films, which reducefriction and wear The reaction with the metallic surfaces is a form of mild corrosion, thusthe additive concentration is critical If the concentration of E.P additive is too high thenexcessive corrosion may occur If the concentration of E.P additive is too low then thesurfaces may not be fully protected and failure could result E.P additives, if they containsulphur or phosphorus, may suppress oil oxidation but decomposition of these additivesmay occur at even moderate temperatures Extended oil life at high temperatures is thereforenot usually obtained by the addition of E.P additives Extreme Pressure additives are notgenerally toxic but some early types were even poisonous, e.g lead naphthenates

There are several different types of Extreme Pressure additives currently added to oils Themost commonly used are dibenzyldisulphide, phosphosulphurized isobutene,trichlorocetane and chlorinated paraffin, sulphurchlorinated sperm oil, sulphurizedderivatives of fatty acids and sulphurized sperm oil, cetyl chloride, mercaptobenzothiazole,chlorinated wax, lead naphthenates, chlorinated paraffinic oils and molybdenum disulphide.There are also other types of E.P additives, e.g tin based organochlorides, but these are notvery popular because of toxicity and stability problems

Dibenzyldisulphide is a mild E.P additive which has sulphur positioned in a chain betweentwo organic radicals as shown in Figure 3.18

CH2 S S CH2

FIGURE 3.18 Structure of dibenzyldisulphide

Examples of this type of additive are butylphenol disulphide and diphenyl disulphide Thespecific type of hydrocarbon radical, e.g diphenyl, provides a useful control of additivereactivity to minimize corrosion

Trichlorocetane and chlorinated paraffin are powerful E.P additives but they are also verycorrosive, particularly when contaminated with water They are applied in extremesituations of severe lubrication problems, e.g screw cutting

Paraffinic mineral oils and waxes can be chlorinated to produce E.P additives They are notvery popular since the mineral oils are quite variable in their composition and usually apoorly characterized additive results from this procedure Such additives may have veryserious undesirable side effects, e.g toxicity and corrosiveness

Sulphurchlorinated sperm oil is an effective E.P additive, but is becoming obsolete because

of the increasing rarity of harvested sperm whale oil It is still, however, used in heavy dutytruck axles

Sulphurized derivatives of fatty acids and sulphurized sperm oil provide a combination ofExtreme Pressure and Adsorption lubrication [38] Sulphurization of fatty acids and sperm oil

(which is a fatty material) produces a complex range of products, so that names of individualproducts are not usually quoted An early example is sulphurized lead naphthenate whichhas been used as an additive in hypoid gears Although, in general, E.P additives are nottoxic, this particular additive is poisonous and largely for this reason it is gradually becomingobsolete These additives can still be found in gear oils and cutting fluids for metalworkingoperations.

Molybdenum disulphide provides lubrication at high contact stresses It functions bydepositing a solid lubricant layer on the contacting surfaces It is non-corrosive but is verysensitive to water contamination as water causes the additive to decompose

Anti-Oxidants

· Oil Oxidation

Mineral oils inevitably oxidize during service and this causes significant increases in frictionand wear which affects the performance of the machinery The main effect of oxidation is agradual rise in the viscosity and acidity of an oil This effect is demonstrated in Figure 3.19which shows the variation of viscosity and acidity of a mineral oil as a function of oxidationtime [39]