2B11K

CURRENT NEEDS AND DEVELOPMENTS IN

HYDRAULIC FLUID TECHNOLOGIES

George E. Totten' and John V. Sherman2

1. G.E. Totten and Associates, Inc., Stony Point, NY USA (GETotten @ aol.com)2. BASF Inc., Wyandotte, MI USA (shermaj @basf.com)

ABSTRACT

There are various governmental and industry pressures on the fluid power industry that necessitates substantial

improvements in fluids and materials used in the fluid power industry. For example, substantial improvements in

fluid lifetimes, especially oxidative stability at higher use temperatures and dramatic improvements inbiodegradability and toxicity for mineral oil fluids is vital. Another very important and continuing market and

technology thrust in the fluid power industry is in the development of alternative hydraulic fluids whose

performance will rival that of petroleum oil. Substantial improvements in the materials used in hydraulic systemswith respect to corrosion and antiwear performance is vital. In fact, it is impossible further ignore the fact thatthe hydraulic fluid must be considered as a design component of the hydraulic system. This itself is a relatively

recent focal point. An overview of these and other technology thrusts will be discussed. The focus of this

discussion will be on the development of biodegradable fluids.

KEYWORDS

Hydraulic fluids, biodegradability, antiwear, design

INTRODUCTIONPetroleum oil basestocks are highly complex

chemical mixtures whose composition,biodegradability and toxicity vary with the crude oilsource [1]. Although petroleum oils havetraditionally been the most commonly usedhydraulic fluids in the fluid power industry, they arebeing subjected to ever-increasing controlsparticularly due to the increasingly stringentgovernmental regulations regarding the impact ofhydraulic fluid spill and fluid leakage on theenvironment.[2] Improper disposal, even if it isincidental, may be the source of large penalties oreven litigation.[3]

Hydraulic fluid leakage has been identified as a

potential source of ground water contamination [4].Such concerns have led to a world-wide effort toidentify hydraulic fluids, which will exhibit reducedenvironmental and toxicological impact uponincidental contact with the environment [5,6].Specific industries where these environmentalconcerns are particularly important includeagricultural [7], forestry [8] and other off-highwayapplications [9]. (Note: The term "environmentalimpact" includes biodegradation, persistence andtoxicity.)

This report will provide a selected review ofvarious aspects of biodegradable fluid technology.

DISCUSSION

Biodegradable Fluids

Thus far, the most commonly cited base

stocks used for the formulation of "environmentally

friendly" hydraulic fluids are either vegetable oils

or synthetic esters [ 10,11,12]. Polyalphaolefin

basestock has also been shown to be biodegradable

[13].

The most common vegetable oils that have

been identified for hydraulic fluid formulations are:

canola oil [14], soybean oil, [15,16,17] rapeseed

oil [18], and high oleic sunflower oil [19].

Rapeseed oil and canola oil are common vegetable

oil basestocks for formulation of biodegradable

hydraulic fluids [20]. Canola oil derived hydraulic

fluids have been successfully used in Germany as

long as the bulk fluid temperatures were kept below

60•Ž.[21]

Soybean oil, because it is readily

available in the USA, is also being evaluated as a

basestock for hydraulic fluid formulation

[15,16,22-26]. Studies completed to date have

shown that soybean oil derived hydraulic fluids

exhibit excellent performance in various

commercial and agricultural applications.[27]

Another class of biodegradable fluid basestocks

that have been reported are polyol esters and diesters.

[14,28] The molecular structure of the fluid

Fluid Power.Fifth JFPS International Symposium (C) 2002 JFPS.ISBN4-931070-05-3

395

basestock will dictate the end-use performance

properties. For example, Konishi, et. al. have

reported the effect of diester structure on properties

such as co-efficient of friction and elastomer

compatibility [29].

The biodegradability properties of fire-resistant

hydraulic fluids including phosphate esters [30] and

water-glycols [31,32,33] have been reported but will

not be detailed here.

In addition to the basestock, a hydraulic fluid

typically contains additives. Additives are used to

enhance fluid properties which include: oxidative

stability, antiwear, reduced foaming and improved air

release properties. The presence of these additives

also affect hydraulic fluid biodegradation and

toxicological properties [34-37] . In many cases,

additives used for petroleum oil formulations [1] are

unsuitable, therefore since additive selection will

affect biodegradability. [38]. additives developed

specifically for use with biodegradable basestocks

must be used [38,39].

Physical Properties

To perform satisfactorily as a pressure medium

in a hydraulic system, the fluid must exhibit at least

targeted minimum physical properties. For example,

the fluid must exhibit acceptable flash point, air

release, foaming and demulsibility properties.

However, there are some properties that are

particularly critical and have been difficult for

biodegradable, particularly vegetable oil-based and

ester-based fluids, to achieve and maintain during use.

These properties include: hydrolytic stability,

oxidative stability, corrosion protection, desired

traction coefficients and seal compatibility.

Low Temperature Properties

Low temperature properties of biodegradable

hydraulic fluids, especially pour point and low

temperature stability are important, especially in

mobile equipment operation in cold weather. In

many end-use machinery applications, such as

forestry, hydraulic fluids are subjected to widely

varying operating temperatures from-35•Ž (-310F)

to 100•Ž (2120F) and operating pressures up to 450

bar (6500 psi)[7]. A viscosity index of at least 150 is

desired [40]. Typically, increasing molecular weight

of the acid and ester functional moieties will degrade

low temperature performance.

Omeis [41] and Konishi [42] have shown that

branched diesters exhibit significantly improved low

temperature performance. Increased branching

improves low temperature performance.[43]

Rhee, et. al. have shown that synthetic esters

exhibit a number of advantages over vegetable oils

such as a broader pour point temperature range,

typically, -40•Ž-150•Ž versus vegetable oil derived

fluids which typically exhibit a temperature range of

-10•Ž-90•Ž [44,45,46]. Without the use of additives,

many vegetable oils fractionally crystallize some of

the saturated derivatives at approximately -10-

-20•Ž .[47] This more limited range of operational

temperatures for vegetable oils often renders them

unsuitable for cold-weather applications.

Oxidative Stability

Hydraulic fluids used in off-highway

applications are often subjected to higher

temperatures than encountered in stationary

applications. Therefore, oxidative stability is

particularly important. The oxidative stability of

vegetable-based fluids is directly related to oleic

acid content. Increasing oleic acid content, relative

to di-and tri-unsaturated acids, provides

corresponding increases in oxidative stability

[25,26,48] . However, the oxidative stability for

fluids with increased oleic acid content is still often

insufficient and therefore are often only suitable for

restricted use [27,49]. In some cases, recommended

use temperatures of 60•Ž [28]-80•Ž [22] have

been reported or if used at higher temperatures, a

reduced oil change interval is recommended [22].

The amount of unsaturation in a vegetable oil is

climate-dependent [19]. Increasing amounts of

unsaturation results in increasingly less oxidatively

stable fluids [34]. For example, rapeseed grown in

colder northern climates contains less unsaturation

(canola oil) than when grown in warmer southern

climates. This is important because the level of

unsaturation affects physical properties, especially

oxidative stability [13,28]. This means that the

performance of vegetable oils as hydraulic fluid

basestocks is inherently variable.

Remmelmann and Murrenhoff have studied

the oxidative stability of biodegradable fluids using

the Rotary Bomb Oxidation Test (RBOT)

according to the ASTM D 2272 test procedure. In

this study, it was shown that oxidative stability,

while only minimally affected by water

contamination, was significantly affected by the

presence of some metals such as copper [47].

Interestingly, catalytic levels of iron exhibited

a much smaller catalytic effect. The presence of

iron and copper arise from corrosion [43].

Fortunately, biodegradable iron and copper

corrosion inhibitors are commercially available

[40]. The effect of water content on oxidative

stability was examined in detail by Remmelmann

and Murrenhoff [47].

In addition to the well known RBOT (ASTM

D 2272) test, the Baader test, modified IP 48 and a"dry" TOST (ASTM D 943) test have also been

used to evaluate oxidative stability of vegetable oils

and synthetic esters.[28,44,51 ](The test must be

run dry to avoid hydrolysis of the ester

functionality.) The dry TOST test has the

disadvantages of relatively large sample sizes (300

mL) and long testing times (2000 h).[28]

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Alternatively, a high-pressure differential scanningcalorimetry (HP-DSC) test may be used for rapidscreening.

Currently, polyol ester and diester-basedfluids are being promoted as more oxidatively

stable biodegradable basestocks than vegetable oil-based fluids [14,41,52]. The degree ofimprovement in oxidative stability is dependent onthe antioxidant seleced [28].

Hydrolytic StabilityIn mobile hydraulics applications, it is difficult

to exclude the possibility of moisture (water)contamination. Sources of water ingression into thesystem include: water contamination of fresh fluid,rain water entrance into the reservoir, storage inunsealed containers, unprotected breathers, improper

filling techniques, and moisture contamination bycontact with humid air [43]. In the presence oforganic esters such as vegetable oils and syntheticesters, this is a problem since esters hydrolyze,

particularly in the presence of a catalyst, to formorganic acids [40,47].

The generation of acidic by-products exhibit anumber of deleterious effects including: catalysis ofthe hydrolysis reaction and corrosion enhancement

[40]. Interestingly, water contamination of vegetableoils and synthetic esters exhibits only a minimaleffect on oxidative instability as shown in Figure 8

[41]. However, this was due to reduced hydraulicload since the system pressure and temperature werereduced to minimize excessive hydraulic wear.

Since hydrolytic instability cannot be controlled,water contamination must either be eliminated or itmust be removed before hydrolysis occurs [51].Recently, a redesigned reservoir which utilizes theheat from the fluid to volatilize water into theatmosphere was reported [43]. Generally, it isrecommended that water levels < 500 ppm bemaintained [52] although water levels of 100 200

ppm may be achieved using the modified reservoirdesign [43].

Corrosion PropertiesThe use of yellow metals (copper alloys) is a

common type of material used in construction ofrunning surfaces in high pressure hydraulic pumps

[53] . For example, yellow metals may be used forpiston slippers and the running surfaces of thecylinder barrel and valve plate or bushing. One

potential failure mode of hydraulic pumps usingvegetable oils and synthetic esters is corrosion ofthese yellow metal surfaces. It has been shown thesulfur, either organic or inorganic, containingvegetable oils exacerbated corrosion of yellow metalsurfaces [22,40,41].

Krauss, et.al. have described the developmentof the "Linde Test" which is used as a screening testfor yellow metal corrosion under both static and

dynamic conditions [54]. The "Static Linde Test" is

conducted by heating different materials for 100

hours in the biodegradable fluid at 120•Ž, 10%

mineral oil, and 1% water. The materials that are

screened are: steel, cast iron, brass, cast bronze, and

sintered bronze with different levels of tin.

Observation and comparison determine the relative

amounts of corrosion. The limitation of this test is

that it does not account for the type of corrosion and

the stability of the corroded layer.

The apparatus used for the "Dynamic Linde

Test" includes a thermostatically controlled hot plate,

and test container. The test fluid is 89% ester, 10%

mineral oil and 1% water and glass beads. Four test

specimens; brass, cast bronze, sintered bronze and

hardened bearing steel are mounted on the stirrer.

The stirrer is rotated at 200 rpm and the fluid is

maintained at 120•Ž for 100 h. After the test, the

weight loss is determined as g/m2 of surface area.

Although excellent correlations with field tests

were obtained, it was concluded that field tests were

still necessary to validate the final results of these

screening processes.

Elastomer Compatibility

Elastomer compatibility with biodegradable

fluids has been examined by a number of authors

[40,41,55]. Although nitrile rubbers (NBR) are

among the most commonly used materials with

petroleum oil fluids, they are generally incompatible

with various types of biodegradable fluids [41,55]. In

general, only fluoroelastomers and some

polyurethane materials are compatible with

biodegradable fluids above 80•Ž [40,55]. However,

since compatibility varies with each fluid-elastomer

combination and use condition, compatibility must

be assessed on an individual basis.

Lubrication Properties

The antiwear behavior of biodegradable

hydraulic fluids is typically determined in two steps.

The first is to obtain a preliminary assessment using a

bench test such as: FZG (DIN 51 354) [40], 4-ball

wear (ASTM D 4172) [40], flywheel testing

[38,49,80], Aachen twin disk test [41,47], oscillating

pin-on-plate test [42], or a Plint and Partner High

Frequency Friction Machine (TE 77B), using two

different contact geometries; a cylinder-on-plate and

ball-on-disk [39]. Typically excellent antiwear results

were obtained with formulated biodegradable fluids,

however, lower viscosity ester basestocks exhibited

somewhat higher coefficients of friction than did

longer-chain, higher molecular weight esters [40].

Mineral oil-based fluids exhibited higher coefficients

of friction than obtained with various biodegradable

fluids examined [29,42,47].

The best fluid candidates were screened by

hydraulic pump testing. In addition to testing with

Vickers V-104 vane pump (DIN 51 389)[40], other

397

hydraulic pumps have also been used including:Vickers 35VQ25 vane pump [39], KomatsuHPV35+35 axial piston pump [22, 41], Sauer-Sundstrand Series 90 axial piston pump [49],Mannesmann Rexroth A4 VSG 125 axial piston

pump, and others [58] Although the antiwear resultswere generally good, these tests showed thatvegetable oils exhibit poorer oxidative stability thanmineral oil based hydraulic fluids.

FiltrationSince it would be difficult to perform in-

line filtration without restricting the fluid flow in thesystem, off-line filtration is typically used [52, 59].When converting from petroleum oil to abiodegradable oil, precipitate formation is oftenencountered. The precipitate typically contains metalcontaminants due to the additives originally presentin the petroleum oil. The presence of this precipitateresults in significantly reduced filter life. However,this problem has been corrected with the use of ahybrid filter as an alternative to more commonlyencountered paper filters in an off-line configuration

[59]. In this way, It is possible to maintain ISO 16/12or better using off-line filtration [52].

CONCLUSIONSAlthough mineral oil based hydraulic fluids

exhibit some of the best overall properties, they havetwo significant deficiencies; fire resistance andbiodegradability which may preclude their continueduse in many applications. While there is little thatcan be done about the poor fire resistance of mineraloils, except engineering or hardware modifications,bviodegradability may be improved. In generalhydrogen reduction or removal of aromatic

compounds and increasing the overall parafiniccomposition will improve aquatic biodegradability,

but not to vegetable oil or synthetic ester levels.

However, even highly refined mineral oilcompositions are among the most persistent in soil

biodegradability tests.Two biodegradable alternatives to mineral oil

based hydraulic fluids are vegetable oils and

synthetic esters. They are more biodegradable than

mineral oil. Of the two basestocks, vegetable oils are

somewhat more biodegradable although that dependssomewhat on molecular structure. In soil tests,

vegetable oils are clearly more biodegradable. One

vegetable oil, sunflower oil, although biodegradable,

exhibited significant toxicity to seedling growth andyield.

Polyol esters, usually those based on trioleateesters of trimethylolpropane, exhibit excellentthermal stability relative to vegetable oils.Formulated fluids, although exhibiting excellentbiodegradability, exhibit relatively poor soilbiodegradability, a notable disadvantage in additionto their relatively higher cost.

REFERENCES

1. J. A. Henry,"Review - Composition andToxicity of Petroleum Products and TheirAdditives", Human & ExperimentalToxicology, 1998, 17, p. 111-123.

2. N. Jones,"Managing Used Oil", Lubes 'N'Greases, 1996, 2 (6), p.20-23.

3. M. M. Mustokoff, and J. E. Baylinson, "No Caseis too Small", Hydraulics & Pneumatics, 1995,February, p.35-37.

4. H. F. Eichenberger,"Biodegradable HydraulicLubricant - An Overview of CurrentDevelopments in Central Europe", SAETechnical Paper Series, Paper Number 910962,1991.

5. J. Meni,"Selection of an EnvironmentallyFriendly Hydraulic Fluid for Use in TurfEquipment", SAE Technical Paper Series, PaperNumber 941759, 1994

6. S. Ohkawa,"Rough Road Ahead foronstruction Machinery Lubes", Lubes 'N'

Greases, 1995, 1 (2), p.20-23.7. P.M. Ahola,"Biodegradable Hydraulic Fluids

in the Forest", SAE Technical Paper Series,Paper Number, 981517, April, 1998.

8. H. H. Harms and H.J. Meyer,"EcologicallyOriented Agricultural MachineryDevelopment", SAE Technical Paper Series,Paper Number 981992, September, 1998.

9. K. D. Erdman, G. H. Kling, and D. E. Tharp,"High Performance Biodegradable Fluid

Requirements for Mobile Hydraulic Systems",SAE Technical Paper Series, Paper Number981518, 1998.

10. T. Mang,"Environmentally FriendlyBiodegradable Lube Base Oils - Technical andEnvironmental Trends in the European Market",Adv. Prod. Appl. Lube Base Stocks, Proc. Int.Symp., 1994, p.66-80.

11. J. Y. Chien,"The Dirt on EnvironmentallyFriendly Fluids", Hydraulic & Pneumatics, 1995,May, p.47-48.

12. JH. Hydrick " Synthetic vs. Vegetable", Lubr.World, 1995, May, p.25-26.

13. S. J. Asadauskas, J. M. Perez, and J. L. Duda,"Suitability of Basestocks for Biodegradable

Lubricants", Prepr.- Am. Chem. Soc., Div.Pet. Chem., 1997, Vol.42, No.1, p.246-249.

14. T. F. Bunemann, M. C. Steverink-de Zoete, andR. P. van Aken, "Environmentally AcceptableHydraulic Fluids Based on Natural SyntheticEsters", SAE Technical Paper Series, PaperNumber 981489, April, 1998.

15. L. A. T. Honary,"Potential Utilization ofSoybean Oil as an Industrial Hydraulic Oil",SAE Technical Paper Series, Paper Number941760, 1994.

398

16. L. A. T. Honary," An Investigation of the Use ofSoybean Oil in Hydraulic Systems, BioresourceTechnology", 1996, 56, p.41-47.

17. R. A. Padavich, and L. A. T. Honary," AMarket Research and Analysis Report onVegetable-Based Industrial Lubricants", SAETechnical Paper Series, Paper Number 952077,1995.

18. S. D. Scott," Biodegradable Fluids for AxialPiston Pumps & Motors-ApplicationConsiderations", SAE Technical Paper Series,Paper Number 910963, 1991.

19. P. C. Naegly,"Environmentally AcceptableLubricants", 1992, Seed Oils Future, p.14-25.

20. F. H. Roberts,and H. R. Fife, U. S. Patent2, 425, 755 (1947).

21. P. Lammle,"Panolin HLP SYNTH-MoreThan a Decade of Experience", SAE TechnicalPaper Series, Paper Number 981492, April,1998.

22. S. Ohkawa, A. Konishi, H. Hatano, K.Ishihama, K. Tanaka, and M. Iwamura,." Oxidation and Corrosion Characteristics of

Vegetable-Base Biodegradable Hydraulic Oils",SAE Technica Paper Series, Paper Number951038, 1995.

23. L. A. T. Honary, L. A. T.,"Soy-Based HydraulicOil: A Step Closer", Off-Highway Engineering,1995, April, p.15-18.

24. Personal conversation with Dr. L. A. T. Honary,director of the ABIL (Ag-Based Industrial

Lubricants) program at the University ofNorthern Iowa, Waverly, IA, June 23, 1997 atASTM D. 02 meeting in Pittsburgh, PA.

25. J. L. Glancy, S. Knowlton and E. R. Benson," Development of a High Oieic Soybean Oil-

Based Hydraulic Fluid", SAE Technical PaperSeries, Paper Number 981999, September,1998.

26. L. Honary and R. Boeckenstedt,"Making aCase for Soy-Based Lubricants", Lubric. Eng.,1998, Vol.54, No.7, p.18-20.

27. "Environmentally Compatible Fluids forHydraulic Components", Mannesmann Rexroth,Technical Bulletin, Number 03 145/05. 91.

28. Von H. Grupp,"Biologisch AbbaubarSchmierstoffe-Eine Kritische Durchsicht derLiteratur and Betreibserfahrungen", VGBKraftwerkstechnik, 1997, Vol.77, No.1, p.56-60.

29. J. Rieglert and E. Kassfeldt," Performance ofEnvironmentally Adapted Hydraulic Fluids atBoundary Lubrication",

Elastohydrodynamics ' 96, Ed. D. Dowson,et. al., 1997, Elsivier Science, By, p.467-473.

30. M. J. Goode, W. D. Phillips and D. Placek,"Triaryl Phosphate Ester Hydraulic Fluids-A

Reassessment of Their Toxicity andEnvironmental Behavior", SAE Technical

Paper Series, Paper Number 982004,September, 1998.

31. W. E. F. Lewis, U. S. Patent 4,855,070 (1989).32. G. E. Totten, and G. M. Webster,"High

Performance Thickened Water-Glycol HydraulicFluids", Proc. of the 46th National Conferenceon Fluid Power, March 23-24, 1994, p.185-193.

33. F. A. Litt,"Standards for Environmentally-Friendly Hydraulic Fluids", Oral Presentation,National Fluid Power Conference, Chicago, IL,April 24, 1996.

34. D. G. Clark,"The Toxicology of Some TypicalLubricating Oil Additives", Erdol andKohle, /1978,.31 (12), p.584.

35. R. K. Hewstone,."Environmental Heath Aspectsof Additives for the Petroleum Industry",Regulatory Technology and Pharmacology,1985, Vol.5, p.284-293.

36. C. M. Cisson, G. A. Rausina, and P. M.Stonebraker,"Human Health andEnvironmental Hazard Characterization ofLubricating Oil Additives", Lubr. Sci., 1996,Vol.8 (No.2), p.145-177.

37. R. J. C. Biggin,"Additives for Lubricants withImproved Environmental Compatibility", Adv.Prod. Appl. Lube Base Stocks, Prod. Int. Sump.,1994.

38. C. Busch and W. Backe,"Development andInvestigation in Biodegradable HydraulicFluids", SAE Technical Paper Series, PaperNumber 932450, 1993.

39. S. Hery and N. S. Battersby,"Development andApplications of Environmentally AcceptableHydraulic Fluids", SAE Technical Paper Series,Paper Number 981493, April, 1998.

40. R. Navette and F. DeClercq,"TheDevelopment of Hydraulic Fluids forEarthmoving Machines Complying withEcolabel Requirements", SAE Technical PaperSeries, Paper Number 981490, April, 1998.

41. J. Omeis, W. Bock and M. Harperscheid," TheDevelopment of a Generation of HighPerformance Biofluids", SAE Technical PaperSeries, Paper Number 981491, April, 1998.

42. A. Konishi, S. Ohkawa, M. Nanba, N.Nakamoto and T. Yoshida,"Development of aHigh Performance Biodegradable HydraulicOil for Construction Equipment", SAETechnical Paper Series, Paper Number 971632,May, 1997.

43. C. Kempermann and H. Murrenhoff,"Reduction of Water Content in Biodegradable

and Other Hydraulic Fluids", SAE TechnicalPaper Series, Paper Number, 981497, April,1998.

44. In-Sik Rhee, C. Velez, and K. VonBernewitz,"Evaluation of EnvironmentallyAcceptable Hydraulic Fluids", TARDECTechnical Report No. 13640, U. S. Army Tank-

399

Automotive Command Research, Developmentand Engineering Center, Warren, MI 48397-5000, March 1995.

45. Anon,"Hydraulic Fluids Are Getting MoreFriendly", Fluid Power, 1992, 7, p.68-73.

46. Honary, L,"Performance of Selected VegetableOils in ASTM Hydraulic Tests", SAE TechnicalPaper Series, Paper Number 952075, 1995.

47. A. Remmeh ann and H. Murrenhoff,"Environmentally Acceptable Hydraulic

Pressure Media-New Changes for MobileHydraulics", SAE Technical Paper Series, PaperNumber 981515, April, 1998.

48. B. Rose and L. Honary,"A Report on the FieldTest Performance of a Soybean-BasedHydraulic Oil", SAE Technical Paper Series,Paper Number 982005, September, 1998.

49." Guide to Alternative Fluids", VickersTechnical Bulletin, Technical Bulletin, Number579, 11/92.

50."Cat Biodegradable Hydraulic Oil", Caterpillar(BIO HYDO) Product Data Sheet, NumberPEHP 1021

51. M. B. Tumbrick,"Fluid Care as a Tool toImprove the Life Time of BiodegradableFluids in Mobile Hydraulic Vehicles", SAETechnical Paper Series, Paper Number 981499,April, 1998.

52. S. Cartwright and C. Devonshire,"Filtration ofBio-Degradable Oils in the EarthmovingIndustry-10 Years of Specialist Experience inthe Demanding German Market", SAETechnical Paper Series, Paper Number 981498,April, 1998.

53. V. M. Cheng, A. Galiano-Roth, T. Marougy,and J. Berezinski,"Vegetable-Based HydraulicOil Performance in Piston Pumps", SAETechnical Paper Series, Paper Number 941079,1994.

54. Krauss, F. Megerlin and H. Oswald," The"Linde-Test"-A Screening Method for

Biodegradable Oils with Respect to Copper-Alloy-Corrosion", SAE Technical Paper Series,Paper Number 981516, April, 1998.

55. Thiel, D. Experiences With Sealing Materials,Hydraulic and Lubricating Oils andBiologically Decomposable Hydraulic Fluids,Gepgyartastechnologia, 1993, Vol.33 No.9-10), p.433-441.

56. D. G. Feldmann and J. Hinrichs,"Evaluation ofthe Lubrication Properties of BiodegradableFluids and Their Potential to Replace MineralOil in Heavily Loaded HydrostaticTransmissions", in Tribology of HydraulicPump Testing, ASTM STP 1310, Eds. G. E.Totten, G. H. Kling and D. J. Smolenski,American Society for Testing and Materials,19996, p.220-229.

57. V. M. Cheng, A. A. Wessol, P. Baudouin, M. T.

BenKinney, and M. J. Novick,"Biodegradableand Non-Toxic Hydraulic Oils", SAE TechnicalPaper Series, Paper Number 910964, 1991.

58. J. Reichel,"Biologically Quickly DegradableHydraulic Fluids", Sauer Sundstrand TechnicalApplication Information ATI 9101,(Status03/91).

59. Y. Kagami,"Contamination Control inEarthmoving Machines-Hybrid FilterElements Versus By-Pass Filters-Influence ofBiodegradable Oil on Filter Life", SAETechnical Paper Series, Paper Number 981501,April, 1998.

400