Vegetable- Based Oil as aGear Lubricant

IBmis IKrzan and Jloief Vi.zintinSummary

Universal tractor transmission oil (UTI1Q) irnulrifunctional tractor oil formulated for use intransmissions, final drives. differentials, wet brakes,

and hydraulic systems of farm tractors employing acommon oil reservoir. In the present work, the gearprotection properties of two formulated vegetable-based uno oils. one synthetic ester-based UTIOoil,. one synthetic ester gear oil and one mineral-

based UTIO oil are investigated.The data, presented. in this paper, have demon-

strated thai the formulated vegetable-basedUTIO oils nave high lubricity, high vi cosilyindexes and provide equivalent Of-in someaspects-superior gear protection performancecompared with the mineral-ba ed UTTO oil. Thehigh-oleic sunflower oil formulation derivedfrom the genetically modified plant ha shownbetter results than the rape eed-ba ed (canola-based) oil formulation.

IntroductionIt is generally recognized that mineral oil lubri-

cants represent a potential danger in many applica-tions because they are not readily biodegradable

and are toxic. The need for biodegradable and non-toxic lubricants has been recognized especially in

the areas where they can come in contact with soil,ground water, and crops.

Biodegradability is the ability of a ubstance

ia'ble1-iest Oils. JViscosity Imm2ls1 I·

Base stock Oil type v4ll"C vlI)(),c VI Oil code ,IRapeseed oil biodegradable uno I 48.8 10.4 209 R

I

High-oleic sunflower oil biodegradable uno 51.4 10.6 2.03 S ISynthetic ester biodegradable gear oil 101 17.8 195 G I

Synthetic ester biodegradable uno 51.3 10.9 211 H IMineral oil UTTO 55.1 9.2 150 M

ilihle 2-Fatly Acid Content In Test Vegetable Base Stocks.Fatty acid content (%)

Base stock Palmitic I Stearic Oleic Linoleic Linolenic OtllBrI

C 16:0 C 18:0 C 18:1 C 18:2 C 18:3High·oleic sunflower oil 4.7 3.7 72.6 17.0 I 2.0!Rapeseedoil 6.1 2.5 49.1 32.2 6.9 3.2

C X:.Y .... fatty acid chain of length X and containing Y double bonds; e.g. C 18:3 is an 18carbon- chain fatly acid with three double bonds.

to be decomposed by the action of bacteria intoC02, water, mineral compounds and bacterial

bodies. Biodegradability is influenced by numer-ous factors, of which the main are the molecular

structure and the chemical properties of organiccompounds and the environmental conditions of

biodegradability, such as the presence of oxygen,

the possible level of nutrition and the pH (Ref. I).Vegetable oils and synthetic esters are the most

common base stocks for biodegradable lubricants.Synthetic oils represent a fairly recent. developmentin the lubrication market. They can be made byreacting alcohols with fatty acid. Synthetic oilsoffer improved performance compared with allother lubricants, but at a price. Both vegetable oils

and synthetic esters are highly biodegradable andreadily available, but vegetable oils occur naturally,have a "greener" image, and are, in general, three

times cheaper. Properly balanced additives cancompensate for low temperature performance andoxidative stability of the vegetable oils and favorthem as the base stock of choice (Refs. 2-3),

A comparison of the simplified chemical struc-tures of mineral and vegetable oils shows great

similarities. The major difference is that vegetableoil is an ester, while mineral oil is a hydrocarbon.

The presence ofthe polar ester group impacts ev-eral propertie , making vegetable oil better than

mineral oil in reducing friction and wear. Thepolar group also makes vegetable oil a better 01-vent for sludge and dirt, which would be other-wise deposited Oil the surfaces being lubricated.Because of these properties, it maybe pas ihletoreduce the amount of friction modifiers. antiwearagents. and dispersants required to formulate veg-etable oil-based lubricants.

The agricultural equipment is ideally suited touse vegetable oil-based lubricants, because theequipment operates dose to the environment The

opportunity exists to create a continuous cyc\ein

which the agricultural equipment is lubricated bythe oil from a plant growing in the field being cul-tivated! by that same equipment (Ref. 2).

Universall tractor transmission oj] (urrO) imultipurpose nil widely used for agricultural, COIl-struction and other off-road vehicles. UTTO oil i

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Dr: Jozef V1ifintini a prole sor ill (heU"iw'rsily oJ Ljubljallo'smechanical engineering fac-ult» and is head of the Celli"for Tribolog)' and TechnicalDiagnostics. His I1IQjorresearch' interests art' I'tg-etable-based lubricants, ....!'Orresistanc« 01advanced material, and diagnostics alldprediction a/wear failllPl's illmechanical sy Il'rns. For his

flower base lock is, derived from a genetically spread over 'the flat specimen before eachtest. On doaorat», V;tilllin studi daltered plant and possesses a sigl1ificantly higher the SRV test rig. just the friction coefficient, at p01l'l'r losses in gears.

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pecially designed for lubricating the transmi -sions, final drives. wet brakes and hydraulic sys-terns employing a common oil re ervoir. UTIOoil has to meet some specific reqniremems tooperatein agricultural and construction equip-ment. The oil must provide the correct frictionalbalance to' prevent wet brake chatter and to allowsmooth transmission clutch engagement,

At tile same time, the oil must provide 'enoughclutch capacity for ,efficient power transrni '01'1 andenough brake capacity to stop the tractor ina reason-able lime and distance. urro oil mu t also provideufficient antiwear (AW) and extreme pressure (EP)

properties for tile whole transmi ion y tern. espe-cia1]y for the piral bevel ring and pinion gears in theaxle , The AWIEP additives must not be so active asto cause corro ion in the tractor's hydraulic sy tern.where pump containing alloy of copper can bepresent (Refs. 2 and 4).

Rape eed and uoflewer oil are currentlyused in Europe for the fonnulation of thebiodegradable lubricants. In the present work. theantiwear and extreme pressure propertie of for-mulated rapeseed and high-oleicsunflower-basedVITO oils, synthetic ester-based UTTO oil, syn-thetic ester gear oi], and mineral-based VITO oilare investigated on FZG test equipment. Tileselected formulated vegetable-based uno oilwa further te led on the helical gear res trig,

Sample PreparationOil samples. We have formulated two differ-

ent vegetable-based UITO oil for the investiga-tion . The fmt fortnulation is ba ed on tile rape-eed ba e lock, while the second ba e tock is

derived from a genetically modified ul'1lflowerplant with a high oleic content. The arne additivesystem is u ed for both formulation . The proper-lies of these two fully formulated vegetable-basedVITO oils were compared with a commerciallyavailable mineral-based UITO od,a fastbiodegradable synthetic-based UTIO oil and a. ynlhetk ester-based gear oil (see Table 1).

The test' TIO oils have a kinematk vi cosi-ly between 9' mm2/s and ~] mm2/ . at ]OO°C. Thisvi cosily offers ufflciem thickne s to promotegood gear protection and is tin suitable for 'thehydraulic sy tern. The synthetice ter G has a vis-co it)' two ISO grades higher than other test oilsand i mtable a a gear oil onJ:y.

The main difference between the vegetablebase rocks for R and S fonnulatioa lies in fa~tyacids contem (see Table 2). The high-oleic un-

0.35EE 0.3~

0.25I:co.~'Vi, 0.2co.E 0.15..uS, 0.1

t.4

E 1.2!.,~.2lm

O.BE,'"~

0.6''-'"0'\0 0.4'-'"II>~ 0.2

a Oleic Stearic Palmitic Linoleic 'linolenicfatty Acids

Hartzian Pressure, 3.17 .GPa; FrequencY,s 50Hz; A,mplitude,.1 ,000 11m; Speed, 0. I m/s;Temperature, SO'C; Duration, 120 min.

Figure 3-Antiwea,. propertief o/fatty acids.

(l) test ,pinion 141brace coupling m torque maasu ring coupling HOI shaft 1!2) lest gear 151locking pin (8) temperature sensor 1111shaft 2(3) slave gearbox (61load lever and weights (91tast gearbox 021 electric mater

Figure 4-Schematic section of.tlle FZG K,eartest rig.sliding motion was measured, The test rig config-uration and the test specimens are described inDIN 51 834 T2 (Ref. 5).

FIgure 2 shews the results of friction coeffi-cient measurement of the test oils. The mineral-based UTTO oil M shows the highest value ofaverage friction coefficient. The biodegradableoils exhibit less friction, especiallythe test oil G.Biodegradable UTIO oils with similar elemental,compositions of additives-R, S, and H-obtainalmost the same value for friction coefficient.

five different fatty acids contained in the veg-etable oils were tested OIl the SRV test device todemonstrate their antiwear properties (see Fig. 3).The oleic fatty acid proved to have the best anti-wear properties at the selected parameters,Antiwear properties and good oxidation stabilitymake the oleic fatty acid the most desired fattyacid in the vegetable lubricant oil formulation.

FZG test results. The FZG gear test rig iscommonly used to evaluate scuffing load capaci-ty, pitting resistance and slow-speed, high-loadwear resistance. Experiments are based on a fail-ure of a standard gear set, lubricated with thetestoil under specific test conditions, using the testrig illustrated in Figure 4 (Refs. 6-7).

The load-carrying capacity of lubricants wasinvestigated by using the standard FliG A/8.3/90test procedure. The test oil is subjected to a load ..increasing by stages, until the scuffing failure cri-terion bas been reached. Twenty mini meters oftooth scuffing indicate a test failure. The failureload stage is reported as a result (Ref. 6-7).

Investigations of the pitting resistance wereperformed on the FliG gear test rig in the standardpitting test CI8.3/90. After a two-hour run-in atload stage 6 (135..3 Nm), the test is run at loadstage 9 (302 Nm) until the failure criterion isrecorded. The number of pinion lead cycles whenthe critical damage of the tooth flanks occur isreported as a result (Ref 8).

UTIU oils are intended to Jubricatetransmis-sions and gearboxes of the tractors ..In such sys-tems, high temperatures, high loads and lowspeeds are very common conditions. Tile primarymode of failure observed witb the spiral bevelgearing is scuffing, while the planetary unitsencounter normal abrasive wear. There are a num-ber of methods to evaluate scuffing, but the pri-mary concern of this investigation is to simulatenormal. rubbing wear of the planetary gears. In theslow-speed, high-lead wear test, the C-type gearswere used to reduce the sliding velocity and con-sequently the probability of scuffing. The test pro-cedure is divided into two stages. The test gearsare weighed before and after each stage and theweight loss associated with wear is rec-orded as aresult which indicates the lubricant antiwear per-formance (Ref. 9-10).

The main FZG test conditions are summarizedin Table 3.

The results of the FZG tests are summarized inTable 4. The best results on the FZG test rig wereobtained with the synthetic ester-based oil G,which has a viscosity that's two ISO grades high-er than other oils.

The results of scuffing load capacity for UITOoils show better scuffing resistance for the formu-lated. high-oleic sunflower-based oil S, whichpassed the 11 th load stage while the other veg-etable-based formulation R and reference UITOoils passed the 10th load stage. Generally, UTTOoils exhibit a scuffing load stage between 9 and11; therefore all tested oils meet the e require-ments (Ref. 11).

The results of pitting investigations show supe-nor pitting resistance of the vegetable- and thesynthetic esters-based oils, compared with themineral-based UTTO eu The high-oleie sun-

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flower oil S showed very good pitting performanceamong the formulated UTIO oil .

The results of slow-speed, high-load wear inves-tigations indicate no significant difference in wearrates among the TIO oils. All te Ioils show lowwear rate in a low-speed. high-load fZO test

On the basis of the .fZG test results, we select-ed the formulated high-oleic sunflower-based for-mutation S for further investigations,

Uelicaw Gear 'TestHelical gear test rig. A helical gear lest rig was

used in order to demonstrate visco ity lability, anti-wear properties, oxidatienresi ranee and seaJ compat-ibility of the formulated high-oleic sunflower-basedUT1U oil labeled S in contmlledlaborarory condi-tions. Through periodic sampling of 'the lubricant andused oil analysis, the condition of thetestoil and partsofithe gearbox were detennined.

The helical gear test rig is schematically shownon Figure 5.. The AC drive motor with frequencyregulation runs a test gear-unit which is lubricatedwith the formulated high-oleic sumflower-basedtJTI10 oil S. For load simulation, as a brake. theDC generator and electric heaters are 1.1 ed, TheDm CK60 pinion and DIN CK45 gear, ea e hard-ened to 60-62 HRC and uot undercut, were usedduring the test. The e helical gears had face widthsof 30 mm, normal modules of 2.5 mm, and thedrive pinion had 31 teeth meshing in a 1:L5 rari o,The test rig was run continuously [2 hours per dayat a constant load of approximately 60' Nm oftorque. The oil temperature was maintained in therange of' 78-82°C. A pair of helical gears wasrotated until the lubricant deteriorated.

Oxidation of lubricants is normally measuredby total acid number (TAN) and viscosity increase.New oils have an initial. TAN, therefore theincrease over the initial value measures oxidation.If the TAN exceeds 2.0 mg KOHIg over the origi-nal value, the oil shouldbe changed. (TIle unit "mgKOHIg" is the quantity in milligrams of potassiumhydroxide (KOB) needed to neutralize the acidconstituents in one gram of lubrication oil) Astrong indicator of oil degradation is also itsincrease in viscosity. Normally a 20% increa e inviscosityis a warning that the oil. is reaching theend of its useful life,

Oil cOlJditiolJ. The top line on 'the graph inFigure 6 represents kinematic viscosity oflhehigh-oleic sunflower-based UITO oil S, measured

~~_WearTable, 3:--fZG Test Conditions.

u~ ScuffingParameters, PittingStage I Stage II

I Test gears type A c1111.08 d stage " 9 HI.. 302 37.2.6 372.6Circumferential speed m/s 8.3 8.3 0.35 0.20Pinion rotational speed rpm 2,170 2,170 '93 53

1/.,-one stage until failureBunning time hour 20 30Sump temperature 'C 90. at start 90 121..... incrementally increased toad.

Oil Slow speed wearC/0.35-0.25/120

Iwe.ight loss in mgl

Table 4--HG Tes~Results.Sc uffing PittingA/B.3/90 C/B.3!90

. _ (FZG load staOfl'1 ~ ~ycles 01 pinianlR 10 13.96 11(16

111 26.75 1'()6 19/2211sG :>12 30.00" 1,(16

1'0 l'5.1fi61OS--------

HI 7.70 lOSM* pining, test. was stopped, but critical failure did not occur.1) Stage IIStage III.

electrometer test gearbox. electricbrak,e

generater

.FigllJ';eS-ScI,emalic d4JgrallJ oj the helical gear lest rig.

IL_ - - - - .........IF'"

1.- - .- .- .1

IKinematic viscosity /~ -: TAN r

~t- • -- -. I

-l-1111"

IIii-

5

4

'CO--3 :I:,02OJ

2 .sz.s

0

;;; '50

~.§. 40u

i 30.~..,eu .20'Su.z::>

'" 10E'"c::;;;:

0a 150 300 450 !600

Operating hours750 900

Figure fi...-..Trelld values for viscosity and TAN,by a table value until the sudden rise at 650 oper-ating hours, which indicates the oil deterioration.

,Gearbox condition. Oi1in the gearbox couJd bea very u eful condition monitoring media. If wecan separate the debris from the oil, we can iden-tify and. track an abnormal wear condition withouttearing down the equipment. Wear particles con-tained in the lubricating oil carry detailed and

at 40°C. ArLer initial shear-down, the viscosity is important information about the condition of thetable until. 600 working, hours. when a slight oil-wetted cemponemsin the gearbox. If noe ces-

increase is observed. The bottom TAN line shows ive wear is observed, then this indicates that thethree distinct ection: initial increa e i .followed effective lubrication in the gearbox i maintained

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2000

1600...

1.- DL O-DS r--i--• • I !,til 1200 r. • I I0J • • • • •• li-DI

800 oj •0t 0400

.,,(')() ~

'f! u' v -v-v 9I (I

00 150 300 450 600 750 900

I Operating hours

Figur,e 7-Qltantitative Jerrography reading».

10,000 100

'. WPC WPCcr '" PLP J=•jft ..- .... • • • •• ~.......

I.., --

I-ot Running-in wear .....---- Normal wear --------. II I

80

60CL....JCL.

40

20

olOa o 750 900150 300 450 600Operating hours

Figl,re 8-Tr:elld values Jor WPC and PLP.during the operation.

The method used for the quantitative evalua-tion of the wear particle concentration was directreading (DR) ferrography. DR ferrography mag-netically separates wear particles from lubricantsand optically measures the relative concentrationof ferrous particles present in the oil sample. Theinstrument is able to detect particles in the lengthrange of 1-300 microns. The output of the DR fer-rography consists of two digital readings, a DL(density large) for large particles (> 5 um) and aDS (density small) for small particles (1-2 um)(Ref. 12).

Figure 7 shows the measured values for DLand OS for high-oleic sunflower-based oil S as afunction of operating time. These readings can beprocessed in several ways to allow easier identifi-cation of an abnormal wear mode. Two such waysare briefly described (Ref. 12):Total Wear Particle Concentration (WPC)

WPC=DL+DS (1)Percentage of Large Particles (PLP)

PLP= [(DL-DS)IWPC]*100 (2)Although the magnitude of the WPC is impor-

tant, the change from historical values is the iadi-

count. In normal operating conditions, a baselineof normal wear may be established and the aver-age of WPC values can be calculated.The average of Wear Particle Concentrationvalue (WPC",ea,,)

WPC",ean = l/n2;WPC* (3)where WPC* means that only normal wear dataare summed (outliers are excluded) .

The WPC value should not exceed the valueof an alarm limit-the critical wear particle con-centration that is based on the WPC"'.1l1Ivalue andstandard deviation of the normal samples (out-liers are excluded).The critical Wear Particle Concentration (WPCc,J

WPCcr = WPCmean + zo (4)where (J is a population's standard deviation.

The most informative method for DR resultsrepresentation is a plot of WPC and PLP in thesame graph, because an increase in both WPCand PLP is the best indication of an abnormalwear condition. Figure 8 shows (he calculatedvalues for WPC and PLP, which are plotted overtime. The WPC value shows an initial sharp risethrough a running-in process, during which thequantity of wear particles quickly increases andthen settles to a lower value after 350. operatinghours, when a normal wear period is beginning.The WPC and PLP values remain relatively con-stant in the normal running operation period,because the gearbox wear reaches a state of equi-librium in which the particle loss rate equals theparticle production rate.

An alarm limit for severe wear WPCcr can becalculated for the last six samples, because atleast three data in the normal wear period are nec-essary to determine WPCmean and a population'sstandard deviation cr before the WPCcr for thefirst point can be calculated. All WPC values arebeyond the alarm limit. therefore the gearboxoperates in the normal wear mode. The test wasended when the test oil started to deteriorate andbefore the gear failures occurred.

To avoid leakage problems. seals used in agearbox should be compatible with the test oil,Fluoroelastomer (Viton®)was used as a seal mate-rial for the test, The seals were inspected after thetest and no change in the geometry was found.

DiscussionTwo formulated vegetable-based UTIO oils,

two synthetic esters, and a mineral UTIO oilwere investigated with respect to their gear pro-tection properties. The study has shown. that the

cator of machine wear condition. Quantitative fer- gear protection properties of the formulated veg-rography is a trending tool and not a particle etable-based oils are comparable with the corre-

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spending mineral-based oil, However. the FZGinvestigations show significantly better results

for pitting re i lance for the vegetable-ba ed oilsand synLl1eti.c e ters.

The vegetable-based oils and synthetic esters

exhibit very good lubricity in boundarylubrica-tion conditions. because the synthetic esters con-

Lain organic straight chain compounds with polarend groups=fauy acid (see Table 2). The polarnature of the biodegradable oils gives them agreater affinity for metal surfaces than nonpolarmineral oil . The need for anti wear additives isreduced. Therefore, with lower concentration of

the additive , both the vegetable- arid synthetic-ba ed oil how lower fri.cLion coefficients thanthe mineral test oil (see Figs. 1-2).

The FZG Lest results indicate that oil viscosity

has 3. strong influence on cuffing performanceand pitting resistance (Ref. 13). The best resultsare obtained with the ynthetic-ba ed gear oil,which has a vi cosity two ISO grades higher than

the UTTO te t oils. At the arne time. this syn-thetic ester has the lowe t concentration of addi-tive . The gear oils are generally of higher vi -cosity than hydraulic oils. but UITO oil is a mul-tipurpose lubricant that has 10 meet boththe gearprotection and hydraulic system requirements.

Vegetable- and synthetic-based oil haveexcellent viscosity propertie . Their viscosityindexes (VI) exceed 1.95. while the V] for miner-aI UITO cil equals ISO (see Table I),. The high-er VI allow- for the formation of the thicker

lubrication film and for better separation of the

contact surfaces at working temperatures (Ref.~4-15). The UTIO oils are of the arne ISOgrade vis co ity, but tests=-e pecially for pitting

resistance-show a great. differentiation in theresults. Besides the lubricant viscosity. the lubri-cant base stock: has a great influence on pittingresistance, while the additive type and concentra-tion have onty a minor influence. The fZjG pit-

ting test conditions corre pond to a Henzian con-tact point pre sure of 1.65 GPa. while contactpres 1IIfe at the SRV te ti 2.0 GPa. The pittingtest results elo ely follow the SRV inve tigations.Measured friction coefficient (Figure 2), the fattyacid content {Table 2). and antiwear propertie offatty acids (Figure 3) determine the pining per-formance ..The higher number ofcycles until fail-ure is thus a function ofthe lower sliding frictionat the point. of contact and. consequeruly, [ower

tangential stresses on the surface. which can effi-ciently prevent fatigue failure associated w.ith

SummaryThe vegetable-based lJITO oil formulations have

the advantages of having a green ouree of oil andlower cost than a biodegradable ynthetic UTfO oils.Agricultural equipment. is ideally uited to lise veg-etable-based oil because the tractor is lubricated by oi Iderived from a plant growing in 'the field that has beencultivated by tile arne equipment.

The vegetable-based oil formulations exhibit[ow changes of viscosity with temperature, The

vi co it)' index improvers can be used to enhance

the viscosity index of mineral-based oil • but it isadvantageous to have a high viscosity index "buill.into" the base oil molecule itself.

The te ts how that the pitting resistance ofvegetable- and synthetic ester-based UTIO oil

formulations is significantly better compared withthe mineral UITO oil.

The investigations on the helical gear te t righave shown that the test high-oleic uaflower-based UITO oil formulation derived from thegenetically modified plant. provides ufflcientgearbox lubricationfor 600 operation hours at. amaintained oil temperature in the range of78-82°C·O

R IerencesI, Kabuja, 1\.. and J.I... Bozet, "Comparative Analysis of theLubricating Power Between (l Pure MUlCraI Oil and BiodegmdableOils oflhe Same Mean tSOGrnde." whrirum, and uwrirnrwll. 1995.pp.25-3O.2. Gapin ki, RE,. LE. Joseph, and B,D, Layzell. "A Vegetable OilBused Tractor Lubricant." lntemational Off-Highway