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NATIONALTRIBOLOGYCONFERENCE24-26 September 2003

THE ANNALS OF UNIVERSITYDUNREA DE JOS OF GALAI

FASCICLE VIII, TRIBOLOGY2003 ISSN 1221-4590

221

TRIBOLOGICAL PROPERTIES OF VEGETABLE BASED

UNIVERSAL TRACTOR TRANSMISSION OILJoe Viintin, Boris Kran

University of Ljubljana, Faculty for Mechanical Engineering,Center for Tribology and Technical Diagnostics, Bogisiceva 8, 1000 Ljubljana, Slovenia

[email protected]

ABSTRACTUniversal Tractor Transmission Oil (UTTO) is multipurpose tractor oil

formulated for use in transmissions, final drives, wet brakes and hydraulic systemsof tractors employing a common oil reservoir. In the present work the development

of biodegradable vegetable based UTTO oil has been described. The properties offormulated rapeseed and high oleic sunflower based oils were investigated in thestandard test procedures and compared with the commercially available mineralUTTO oil. Tribological performances of the fluids were demonstrated by using SRVhigh frequency test device, Four Ball test rig and FZG spur gear test. For final tests alaboratory hydraulic system and a spur gear test rig were used.

KEY WORDS: biodegradable oils, vegetable oils, hydraulic oils, gear oils, oxidative stability.

1. INTRODUCTION

Increasing attention to the environmental issuesand more restrictive environmental regulations drives

the lubricant industry to increase the ecologicalfriendliness of its products. For the last three decades,the industry has been trying to formulate

biodegradable lubricants with technical characteristicssuperior to those based on mineral oils. Vegetable oilsare a candidate for replacement of mineral oils due totheir inherent biodegradability, and excellentlubricity. Additionally, vegetable oils are renewableresource, and their cost is reasonable compared withthat of other alternative biodegradable fluids.

The agricultural equipment is ideally suited touse vegetable based lubricants, because it operatesclose to the environment where lubricant can easily

come into contact with the soil, ground water andcrops. The opportunity exists to create a continuouscycle in which the agricultural equipment islubricated by the oil from a plant growing in the field

being cultivated by that same equipment [5].Universal Tractor Transmission Oil (UTTO) is

multipurpose oil widely used in agricultural andworking machines such as tractors, harvesters, etc. Inthese vehicles, the multifunctional oil meet complexrequirements including cold starts, dirty environment,water ingress, massive loads, etc.

The main functions of UTTO are [4, 6]:

Lubrication of transmission, differentials, and

final drive gears, Transmitting power for steering and braking,

Implementing hydraulic drives,

Providing proper cooling and frictional propertiesfor wet brakes and power take-off clutches.

For all these demands, robust lubricant

performance is key to efficient operation free ofunexpected downtimes.

2. SAMPLE PREPARATION

2.1. Oil samples

Two different vegetable based UTTO oils wereformulated for the investigation, Fig 1.

The first formulation was based on the rapeseedoil, while the second base stock was high oleicsunflower oil (HOSO). The same additive system wasused for both formulations. The properties of these

two fully formulated vegetable based UTTO oils werecompared to the commercially available syntheticester UTTO and conventional mineral based UTTO.Most tractor lubricants have a kinematic viscosity at100C between 9 and 11mm2/s. This viscosity isfound to offer sufficient thickness to promote goodtransmission protection and antisquawk performance,yet still be of a suitable viscosity for the hydraulicsystem. As shown by Table 1, the viscosity characte-ristics of all three ester based UTTO reveal them to bethicker at 100C and have significantly higher visco-sity index compared with the mineral based UTTOlabeled M.

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Table 1. Test oils.

Kinematic viscosity [mm2/s]Base stock Oil typevisc. at 40C visc. at 100C

IV Oilcode

Rapeseed oil Biodegradable UTTO 48.8 10.4 209 R

High oleic sunflower oil Biodegradable UTTO 51.4 10.6 203 S

Synthetic ester Biodegradable UTTO 51.3 10.9 211 E

Mineral oil Mineral UTTO 55.1 9.2 150 M

2.2. Base fluids

The vegetable oils are composed of triglicerydesof various fatty acids.

Table 2 shows the fatty acid composition of thebase stocks for test formulations. Higholeic sunfloweroil (HOSO) possesses the 72.2% of oleic acid and lessthan 20 % of polyunsaturated fatty acids like linoleic

and linolenic acid. Considering that the susceptibilityto oxidation is correlated with the number of double

bonds in the fatty acid chain then the high oleicsunflower oil has the potential for better oxidativestability than rapeseed oil with 49.1 % of oleic acidand more than 40% of polyunsaturated fatty acids.

Table 2. Fatty acid composition of test vegetable basestocks.

Rapeseedoil

HOSO

Palmitic, C 16:0 6.1 4.7

Stearic, C 18:0 2.5 3.7Oleic, C18:1 49.1 72.2Linoleic, C 18:2 32.2 17.0Linolenic, C 18:3 6.9 /

Fattyacidc

ontent

%

Other 3.2 2.0C X:Y fatty acid chain of length X and containing Y double bonds; e.g. C 18:3 is an 18-carbon chain fatty acid with three double bonds

2.3. Additives

TheED-XRFspectrometry hasbeenusedto obtainthe elemental composition of additives for test oils. It

can be seen from figure 1 that the elemental composi-tion of additives is quite similar for the fully formula-ted vegetable based oils R and S, and reference syn-thetic ester E. The reference mineral based oil M con-tainssignificantlyhigher

levelofphosphorous, calciumand sulphur than ester based oils. Zinc and phospho-rous are parts of AW/EP additive package, whilecalciumisa

typicalelementinadetergent additive.Ester based oils are generally lower additivated

than mineral oils because they posses good lubricatingproperties due to their polar nature. This providesgood metal wetting attraction and also makes themgood solvents for sludge and dirt, which would

otherwise deposit on the metal surfaces. Because ofthese properties, it may be possible to reduce theamount of friction modifiers, antiwear agents, and

dispersants required, by formulating natural orsynthetic ester based lubricants.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

R S E M

Oil

Elementalcom

position[%m/m]

P Ca Zn S

0.66

Fig. 1. Elemental analysis of test oils.

3. TEST EQUIPMENT AND

PROCEDURES

3.1. Oxidation stabilityThe oxidation performance of test oils is

demonstrated by a modified Baader test according toDIN 51 554, Part 2. Test oils are aged for three daysin a glass vessel at temperature of 95C while dry airis introduced and a copper wire is immersed

periodically. At the end of the test the viscosityincrease by oxidation must not exceed 20 %.

3.2. Friction and wear measurements

Initial testing was conducted on the SRV high

frequency test device and on the four-ball wear testerunder boundary lubrication conditions.SRV is a high frequency, linear-oscillation test

device. The upper test specimen is rubbed against alower specimen, on which a few milligrams of test oilis placed. The upper specimen is a ball made of thesteel AISI 52100 (100Cr6), while the lower specimenis a 100Cr6 disc. The friction and wear test similar toDIN 51 834 was run at 50Hz frequency and 1mmstroke. After a 30 second break-in at 10N, the normalload is raised to 27N and run at that load for twohours. The coefficient of friction is continuouslyrecorded on a chart. The main, highest and lowest

values on the chart are reported. After test, theaverage wear scar diameter on the ball specimen ismeasured with the aid of a microscope.

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Four-ball is a simple test rig for testing the anti-wear properties of lubricating oils. Three stationary12.7mm diameter chrome alloy bearing steel balls areclamped together and pressed with a force against theupper one, which is held in the ball chuck and rotates

at a defined speed. The balls are immersed in the testoil. The load of 392N is applied to the balls byweights on a load lever. The test run was carried outat 1500 rpm with test duration of 60min. The test oiltemperature of 65C was the requirement of the OEMspecification Massey Ferguson M1139. For test 10-12ml of test oil is used in each run. The wear scardiameter that the rotating makes on the fixed balls wasmeasured at the end of the test using an opticalmicroscope. The test measures only sliding wear.

3.3. Gear tests

Universal tractor transmission oil has to providesatisfactory AW/EP performance to protect gearbox,differential, and final drive gears in the tractortransmission system. Scuffing load capacity, pittingresistance, and slow-speed wear performance of testoils were demonstrated by using the FZG back-to-

back gear test rig. Tests are based on a failure of astandard gear set, lubricated with the test oil underspecific test conditions.

The scuffing performance is determined in a stan-dard FZG test procedure A/8.3/90 according to DIN51354. Test gears type A are used at pitch line velo-city of 8.3m/sand 90C oil inlet temperature. The loadis increased in stages (3534Nm) with a running timeof 15min until scuffing occurs. Twenty millimetres of

pinion tooth scuffing indicate test failure. Results arereported in terms of the number of passed stages.

The pitting performance is evaluated in a pittingtest C/8.3/90, running C type test gears at 8.3 m/s

pitch line velocity and 90C oil inlet temperature. Arun-in of 2 h in load stage 6 (135 Nm) is followed bythe test run in load stage 9 (302 Nm) until the failurecriterion is reached. The number of pinion load cyclescausing damage of tooth flanks is recorded.

The contact stress of the planetary gears used intractor is simulated in the FZG test rig. The conditionsfor slow-speed wear test are chosen on the basis ofappropriate calculations based on film thickness andlubrication regime in which equipment operates [8].Practically designed C type test gears are run at lowspeed of 0.35 and 0.2m/s, causing thin lubricatingfilm, at 120C and FZG load stage 10 (373Nm). Theweight loss of pinion and gear is determined after 20hours and after a total running time of 50 hours. Thetest gear weight loss associated with wear indicatesthe lubricant antiwear performance.

3.4. Subsystem tests

Vegetable based lubricants tend to oxidation,especially if oil temperatures raise above 80C.Remmele and Widmann [10] investigated the thermal

load of the mineral and rapeseed based oils applied inthe eight agricultural machines. The thermal load wasmonitored with a temperature recording system asshare of operating hours in three four-temperatureclasses (below 60C, 60 to 80C, 80 to 100C and

over 100C). At the end of the investigation period themachines had in all more than 45 000 operating hours.They found out that in all monitored tractors andwheel loaders the rate of hydraulic oil temperaturesover 80C was lower than 10 %. In this study selectedoils were tested in a laboratory hydraulic system atconstant oil temperature of 70C and in a spur geartest rig where oil temperature was maintained at 80C.

3.4.1. Laboratory hydraulic system testUTTO is of vital importance for the performance

of tractor hydraulic system. It has to guarantee thepower and signal transmission, and the protection of

components against wear and corrosion. Behavior ofthe high oleic sunflower oil formulation S andcommercial mineral UTTO M was comparativelyevaluated in the application-related laboratoryhydraulic system (Fig. 2)[12].

1 gear pump, 2 l/min; 2 directional valve;3 pressure relief valve; 4 cylinder; 5 water

cooler; 6 filter, 10 m.Fig. 2. Circuit diagram of the laboratory hydraulic

system.

The tests were run simultaneously on the twoequal laboratory hydraulic systems for 2000 hours at15MPa. The system oil volume was 15 liters and oil

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tank temperature 70C. In regular time intervals oilsamples were taken and investigated for their changein viscosity and total acid number (TAN).

3.4.2. Spur gear test system test

Gearings and the bearings are machine elementsthat suffer high tribological stresses. A lubricant ingear application is used to control friction and wear

between mating surfaces, and to transfer heat awayfrom the contact area. The gear protection

performance of high oleic sunflower oil formulation Sand mineral UTTO M were demonstrated in thelaboratory spur gear test system.

1 electromotor; 2 test gearbox; 3 generator;4 electric break

Fig. 3 Schematic diagram of the spur gear test system.

The laboratory spur gear test system is shownon figure. 3. The AC drive motor runs a test gear-unit,which is lubricated with the test oil. For loadsimulation the DC generator and the electric brake areused. The DIN CK60 pinion and DIN CK45 gear,casehardened to 60-62HRC and non-undercut, areused as test gears. These spur gears had a face widthof 30 mm, a normal module of 2.5mm, and 39 teeth

drive pinion meshing in a 1:1.1 ratio. The test rig wasrunning continuously at a constant load ofapproximately 60Nm torque. Operating conditions arechosen in a way that oil temperature be maintained inthe range of 78-82C. A pair of test spur gears wasrun until the signs of lubricant degradation wereobserved.

During the tests in the laboratory systems oilsamples were continuously investigated for theirchange in viscosity and total acid number (TAN). It isgenerally agreed that viscosity is the single mostimportant physical property of any lubricating oil.

Normally a 10 % increase over the viscosity of unused

oil is a warning that the oil is reaching the end of itsuseful life. Another strong indicator of oil degradationis monitoring TAN increase. As lubricants breakdown they generally form acid by products which can

be corrosive to metal components. A change of morethan 2.0 mg KOH/g in acidity over the original valueis a warning of lubricant deterioration.

Kinematic viscosities were determined at 100Caccording to the ASTM D 445 using a Cannon-Fenskecapillary viscometer. Total acid numbers weredetermined following the ASTM D 664 using thetitrimetric analyser Mettler DL25.

The condition of the mechanical elements in

the gearbox was determined with wear particleanalysis. Wear particles contained in the lubricatingoil carrydetailedand important information about thecondition of the oil-wetted components in the

gearbox. If we separate the debris from the oil, we canidentify and trend an abnormal wear condition withouttearing down the equipment. The method used forquantitative evaluation of the wear particleconcentration in the test gearbox was Direct Reading

(DR)ferrography [3].

4. TEST RESULTS

4.1. Oxidation stability

The ester based oils R, S, and E gave higherviscosity increase than the mineral based UTTOlabeled M, but all test oils were within the upper passlimit of 10 % viscosity increase (Fig. 4).

0.8

2.5

2.9

5

0 2 4 6

M

E

S

R

Kinematic viscosity change [%]

Fig. 4 Oxidation performance.

Due to the lower unsaturation, the high oleicsunflower formulation S results in a better oxidationstability with respect to the rapeseed oil formulationR, Table 2.

4.2. Friction and wear measurements

Figure 5 shows SRV friction test results.

0.1

0.15

0.2

0.25

R S E M

Oil

Coefficientof

friction

Fig. 5 Coefficient of friction measurements.

The highest, the lowest and the mean values ofcoefficient of friction recorded in a computer database

during two hours test are reported. From the figure 5,it can be seen that ester based oils R, S, and E exhibitless friction than mineral based oil M. The coefficientof friction for oil M is of the highest value, but small

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difference between maximum and minimum valuepoints out a very constant friction with time.

In figure 6 the wear results from the SRV areplotted against the wear from the four ball tester.

0

0.1

0.2

0.3

0.4

0.5

0 0.1 0.2 0.3 0.4 0.5

Four Ball wear scar diameter [mm]

SRVwearscardiameter[mm]

R

M

S

E

Fig. 6 SRV and four ball wear results.

High oleic sunflower oil formulation S exhibitsalmost the same wear scar diameter on the SRV asrapeseed oil formulation R, but less wear on four ball.Synthetic ester E shows the higher wear rate thanvegetable based oils, especially on SRV. The mineraloil M exhibits improved wear properties over the estertype oils.

4.3. Gear tests

The results of scuffing investigations are shownon Fig. 7. High oleic sunflower formulation S is ratedat higher scoring load capacity and gave the 11thstage

pass. Other test oils R, E, and M gave the 10thstagepass and also meet the requirements for UTTO oilswhich generally exhibit a scuffing load stage between9 11 [7].

Next, the pitting performance was investigated.As shown in figure 8, all ester based oils show better

pitting resistance than mineral based oil M. High oleicsunflower formulation S demonstrates very good

pitting performance with 27 106cycles.

0

100

200

300

400

500

R S E M

Oil

Scuffingtorque[Nm]

stage pass

10 11 10 10

Fig. 7 Scuffing load capacity.

The plots of figure 9 show the weight loss ofpinion and gear after 20 and 50 working hours.During the test the wear rate decreased for alllubricants. The results of slow-speed wearinvestigations indicate no significant difference in

wear rate among the test oils.

0

5

10

15

20

25

30

R S E M

Oil

LoadcyclesofpinionxE6

Fig. 8 Pitting test results.

4.4. Laboratory hydraulic system test

Figure 10 represents kinematic viscosity andneutralization number for high oleic sunflower oilformulation S and mineral based oil M.

0

5

10

15

20

25

0 20 40 60

Working hours

Weightreduction[mg]

S R M

Fig. 9 Low speed FZG wear evaluation.

0

2

4

6

8

10

12

0 500 1000 1500 2000

Working hours

Viscosity@

100C[mm

2/s]

TAN[mgKOH/g]

S, viscosity M, viscosity

S, TAN M, TAN

Fig. 10. Change of physical and chemicaloil parameters in a laboratory hydraulic system at oilsump temperature of 70C.

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After initial shear-down, the viscosity is stable forboth oils, showing no evidence of oxidatively inducedthickening. The acid number for mineral oil M is quitestable, while TAN for high oleic sunflower oil S wasslightly increased during the 2000 hours steady-state

test. Since an increase in acid number by a factor oftwo is allowed before the oil is to be replaced, bothtest oils pass in this respect.

4.5. Spur gear test system test

The top lines on the graph in figure 11 representkinematic viscosity of the high oleic sunflower oil Sand mineral based oil M, measured at 100C. Afterinitial shear-down the kinematic viscosity for higholeic oil S is stable until 700 working hours when firstslight and than strong increase was observed. The

bottom TAN line for oil S shows three distinct

sections: initial rise is followed by the stable valueuntil rapid increase starting at approximately 750operating hours indicates the oil deterioration. Theresults for mineral oil M appear normal. Thekinematic viscosity and acid number have allremained stable during the whole running time.

0

3

6

9

12

15

18

0 150 300 450 600 750 900

Working hours

Viscosity@100C

[mm2/s]

TAN[mgKO

H/g]

S, viscosity M, viscosity

S, TAN M, TAN

over the

critical value

Fig. 11. Change of physical and chemical oilparameters in a spur gear test system at oil sump

temperature of 80 C.

10

100

1000

0 150 300 450 600 750 900

Working hours

WPC

S

M

Figure 7. Trend values for WPC.

Figure 12 shows the wear particle concentration(WPC) which is trended over time. The WPC valueshows an initial rise through a running-in process,during which the quantity of wear particles quicklyincreases and then settles to a lower value when a

normal wear period begins. The WPC values for bothtest oils were relatively constant, because the gearboxwear reached a state of equilibrium in which the

particle loss rate equals the particle production rate.No excessive wear was observed, which indicates thatthe effective lubrication in the gearbox is maintainedduring the operation.

5. DISSCUSION

Vegetable oils are by their chemical nature longchain fatty acid triesters of glycerol. The alcoholcomponent (glycerin) is the same in all vegetable oils.

The fatty acid components are plant-specific andtherefore variable. The fatty acids differ in chainlength and number of double bonds. Main fatty acidswith double bonds are linolenic, linoleic and oleic.The oxygen absorption rate is 800:100:1 respectively,therefore less double bonds in a carbon chain result in

better oxidation stability [9]. Oxidation performanceresults presented in Fig. 4 are in good agreement withthese data.

Vegetable based oils have excellent viscosityproperties. Their viscosity indexes (VI) exceed 200,while VI for mineral UTTO equals 150, Table 1. Thehigher VI allows the formation of the thickerlubrication film and better separation of the contactsurfaces at working temperatures [1]. The UTTO oilsare of the same ISO grade viscosity, however pittingresistance test shows a great differentiation in theresults. Beside the lubricant viscosity, great influenceon the pitting resistance has lubricant base stock,while the additive type and concentration have only aminor influence. The pitting test results also followthe SRV investigations. The FZG pitting test condi-tions correspond to a Hertzian point contact pressureof 1.65GPa, while contact pressure at start on theSRV test was 1.5GPa. The higher number of cyclesuntil failure for the high oleic sunflower oilformulation S in FZG pitting test is thus a function ofthe lower sliding friction at the point of contact, lowertemperature, and consequently, lower tangentialstresses on the surface, which can efficiently preventfatigue failure associated with surface-initiated cracks[2].

Wear particle concentration of mineral based oilon spur gear test system was found to be lowercompared to the high oleic sunflower oil formulation,(Fig. 12). Mineral oil also resulted in the lowest wearon the SRV wear test (Fig. 5). These data quite followthe results of the published wear studies under

boundary lubrication conditions [11]. Thisphenomenon is explained by competition betweenpolar base oil ester molecules and polar AW/EPadditives for the same space on the metal surface.

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Insufficient quantity of AW/EP additives on thesurface resulted in thermally decomposition of estermolecules attached in the contact surface. The organicacids are created which directly attack the uncoveredmetal surfaces and create an easily sheared and very

soft oxide layer that reduces friction and preventsseizure, but also accelerates the rate of wear [12].

6. SUMMARY

The following conclusions can be derived fromthis study: Ester based oils show lower friction coefficientthan higher additivated mineral based oils, but

promote higher wear. FZG gear test rig results show that gear

protection properties of the vegetable based oils arebetter or equivalent than of the mineral based UTTO.FZG pitting resistance investigations showsignificantly better results for the ester based oils,especially for the high oleic sunflower oilformulation. Better FZG test results for vegetable

based oils are in the relation with the relatively shorttest times, where oil ageing hardly occurs. Laboratory hydraulic system test results showthat the high oleic sunflower oil formulation couldmatch mineral based UTTO for applications whereoperating temperatures are reasonable (70 C insteady state).Investigations in a spur gear test rigshow better thermal oxidative stability for mineral

UTTO compared to the high oleic sunflower oilformulation. High oleic sunflower UTTO providessufficient gearbox lubrication for 700 operating hoursat the maintained oil temperature in the range of 78 to82C.

REFERENCES

1. Arnsek A., Vizintin J., 2000, Lubrication properties ofrapeseed-based oils,Lubr. Sc.,Vol.16(4), pp. 281-296.2. Arnsek A., Vizintin J., 2000, Pitting Resistance ofrapeseed-based oils, Proc. of the 12th Int. Coll. Tribology, TAE,pp. 143-148.3. Basic Analytical Ferrography, 1999, Concepts ofFerrography. Predict DLI.4. Boschert T., McCombs T., 2002, Outstanding in theirfield,LubesnGreases,Vol.8(5), pp. 44-49.5. Gapinski R. E., Joseph I.E., Layzell B. D., 1994, Avegetable oil based tractor lubricant, Int. off-highway & powerplant congress & exposition, Milwaukee, Wisconsin.6. Gapinski R. E., Kernizan C. F., Joseph I. E., 2000,Improved gear performance through new tractor hydraulic fluidtechnology,Proc. 12th Int. Coll. Tribology, TAE, pp. 2269-2276.7. Hubmann A.,1994, Chemie pflanzlicher le,Proc. of the9th Int. Coll. Ecological and Economic Aspects of Tribology, TAE.8. OConnor B. M., Winter H.,1992, Use of low speed FZGtest methods to evaluate tractor hydraulic fluids, Engine Oils andAutomotive Lubrication, ExpertVerlag, pp. 661-686.

9. Ravasio N., Zaccheria F., Gargano M., Recchia S., Fusi,A., Poli N., Psaro R., 2001, Environmental friendly lubricantsthrough selective hydrogenation of rapeseed oil over supportedcopper catalysts, Applied Catalysis A: General 233:1-6.10. Remmele E., Widmann B., 1998, Hydraulic Fluids basedon rapeseed oil in agricultural machinery suitability andenvironmental impact during use,Proc.11th Int. Coll. Industrialand Automotive Lubrication,TAE, pp. 179-187.11. Rieglert J., 1997, Lubricating performance of environ-mentally adapted hydraulic fluids, PhD thesis, Lulea University ofTech., Sweden.12. Sraj R., Vizintin, J., Svoljsak M., Feldin M., 2000,Rapidly biodegradable hydraulic fluids on the basis of rapeseedoil,Lubr. Eng.Vol. 56(4), pp.34-39.

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