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M A S T E R’S THESIS 2008:152 CIV Kim Berglund MASTER OF SCIENCE PROGRAMME Mechanical Engineering Luleå University of Technology Department of Applied Physics and Mechanical Engineering Division of Machine Elements 2008:152 CIV • ISSN: 1402 - 1617 • ISRN: LTU - EX - - 08 /152 - - SE Lubricant ageing effects on wet clutch friction characteristics
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Page 1: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

MASTER’S THESIS

2008:152 CIV

Kim Berglund

MASTER OF SCIENCE PROGRAMME Mechanical Engineering

Luleå University of Technology Department of Applied Physics and Mechanical Engineering

Division of Machine Elements

2008:152 CIV • ISSN: 1402 - 1617 • ISRN: LTU - EX - - 08 /152 - - SE

Lubricant ageing effects on wet clutch friction characteristics

Lubricant ageing effects on wet clutchfriction characteristics

Kim Berglund

August 18, 2008

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Preface

The work of this master thesis has been carried out during oneinstructive year as aResearch Trainee at Luleå university of technology. I wouldespecially like to expressmy appreciation to Mr. Pär Marklund and my supervisor Professor Roland Larsson fortaking time to guide and help in many valuable discussions. Acknowledgments shouldalso be made to Statoil Lubricants in Nynäshamn and Haldex Traction in Landskronafor both financial support and help with my work.

Kim Berglund Luleå, July 2008

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Abstract

Somewhere in the transmission of vehicles today, a wet clutch can often be found.Characteristic of this type of clutch is that they operate under lubricated working con-ditions. In earlier research, friction characteristics and performance of wet clutcheshave been well investigated by several authors. Studies have also been made in or-der to understand the ageing of wet clutches. However, most lifetime studies havebeen made for systems with paperbased friction discs and systems involving sinteredbronze friction discs remain unexplored.

Friction discs of sintered bronze is used in the Haldex limited slip coupling (LSC),an all-wheel drive system used in cars from many different manufacturers. In orderto get a better understanding of how this system can change over time, the study inthis master thesis is focused on how frictional performanceis affected by oxidation oflubricant, testrig ageing and additive content. This work has been conducted in coop-eration with Haldex Traction in Landskrona and Statoil Lubricants in Nynäshamn.

The oxidation effects on friction performance was examinedusing a modified dry-TOST (Waterless Turbine Oil Oxidation Stability Test) on a fully formulated lubricant.The oxidation time period was divided into five steps from 48 hours to 408h and foreach level of oxidation, a friction performance test was runusing a pin on disc ma-chine.

Also an oil aged in a clutch disc testrig was tested for friction performance. Thetest is constructed in order to verify that an oil-friction disc combination will last thelifetime of the specific application.

Since lubricant additives are vital to the performance of wet clutches the effectof reducing the additive concentration in the oil was also studied, in the range 10 to100% of the standard additive formulation.

Results showed that a general friction increase can be seen for oxidation, addi-tive reduction and testrig ageing. Lubricant aged in testrig shows significantly dif-ferent friction characteristics with temperature than lubricants aged by dry-TOST im-plying that dry-TOST alone is not a sufficient method to evaluate lubricant ageing.

Further research has to be made in order to understand the ageing of wet clutches.A better understanding of which mechanisms that are responsible for the decompo-sition of a lubricant in a wet clutch system such as the HaldexLSC is needed. Thisthesis has focused on lubricant ageing but no attention has been paid to wear and age-ing of friction discs. To investigate and relate ageing of lubricant and friction discs isanother important task for future research.

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Nomenclature

ω Rotational speed

µ Friction coefficient

v Sliding speed

AH Antioxidant

A• Antioxidant radical

H2O Water

HO• Hydroxy radical

O2 Oxygen

R−R Hydrocarbon

RH Hydrocarbon

RC(O)R Ketone

RCH2C(O)CH2R Ketone

RR′C = O Ketone

RO• Alkyloxy radical

RR′R′′C−O• Alkyloxy radical

(RCH2)2C = C(R)C(O)C(R) = C(CH2R)2 Unsaturated aldol condensation product

HOO• Hydroperoxy radical

R• Alkyl radical

RC(O)OH Carboxylic acid

RC(O)OR Ester

RCH2C(O)CH(R)C(O)R Condensation product

ROH Alcohol

ROO• Alkylperoxy radical

ROOH Alkyl hydroperoxide

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CONTENTS CONTENTS

Contents

Nomenclature vii

1 Introduction 11.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Lubrication theory 52.1 Lubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Lubrication regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3 Lubricant oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 Initiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.2 Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3.3 Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3.4 Ester formation and condensation reactions . . . . . . . .. . 9

3 Method and materials 113.1 Test procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2 Lubricant ageing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4 Results 174.1 Friction characteristics change with additive concentration . . . . . . 174.2 Friction characteristics change with oxidation . . . . . .. . . . . . . 184.3 Comparison ageing methods . . . . . . . . . . . . . . . . . . . . . . 194.4 Influence of temperature . . . . . . . . . . . . . . . . . . . . . . . . 204.5 Mean contact potential change . . . . . . . . . . . . . . . . . . . . . 214.6 Safe working range, R . . . . . . . . . . . . . . . . . . . . . . . . . 23

5 Discussion 25

6 Conclusions 27

7 Future work 29

References 31

A Friction curves 33A.1 Friction vs. velocity, fully formulated . . . . . . . . . . . . .. . . . 33A.2 Friction vs. velocity, 75% of additive package . . . . . . . .. . . . . 34A.3 Friction vs. velocity, 50% of additive package . . . . . . . .. . . . . 35A.4 Friction vs. velocity, 25% of additive package . . . . . . . .. . . . . 36A.5 Friction vs. velocity, 10% of additive package . . . . . . . .. . . . . 37A.6 Friction vs. velocity, 48h dry-TOST . . . . . . . . . . . . . . . . .. 38A.7 Friction vs. velocity, 96h dry-TOST . . . . . . . . . . . . . . . . .. 39A.8 Friction vs. velocity, 192h dry-TOST . . . . . . . . . . . . . . . .. . 40

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CONTENTS CONTENTS

A.9 Friction vs. velocity, 360h dry-TOST . . . . . . . . . . . . . . . .. . 41A.10 Friction vs. velocity, 408h dry-TOST . . . . . . . . . . . . . . .. . . 42A.11 Friction vs. velocity, Oil aged in testrig . . . . . . . . . . .. . . . . 43

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1 INTRODUCTION

1 Introduction

Wet clutches are often used in the drivetrains of vehicles today. As revealed by thename, a wet clutch operates under wet conditions and workingconditions can varygreatly depending on function in the transmission. The Haldex limited slip coupling(LSC) is an all-wheel drive system used in many modern all wheel drive (AWD) carstoday. In a primarily front wheel driven vehicle it transfers torque to the rear wheelswhile in a primarily rear wheel driven vehicle it transfers torque to the front wheels.The clutch consists of multiple separator and friction discs which are alternately posi-tioned. Friction material on friction discs are of sinteredbronze and the separator discsare of steel. Friction and separator discs are connected by splines to different shafts, ei-ther input or output, and torque is transmitted when the clutch pack is pressed togetherby an electronically controlled hydraulic pump, see Fig. 1.Hence, torque transfer isdetermined by friction in interfaces of friction and separator discs. Wheel rotationalspeeds are retrieved from the vehicles anti-lock braking system (ABS) sensors to theelectronic control system of the clutch in order to regulatetorque transfer.

Hydraulic piston pumpWet multi−plate clutch

Clutch pistonControllable throttle valve

Figure 1: Schematic figure Haldex LSC

The friction characteristics and performance of wet clutches has been well investi-gated by several authors [1, 2]. In order to further investigate friction characteristics ofwet clutches Mäki [3] developed a Limited Slip Clutch test rig. Friction performanceof limited slip differentials involving sintered bronze was thoroughly explored. Mark-lund [4] developed a pin-on-disc method to evaluate the samekind of system, withadvantages such as being simple and inexpensive, e.g. suitable for screening-tests.

Commonly encountered in literature are investigations of two troublesome phe-

1

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1 INTRODUCTION

nomena, stick-slip and shudder. Stick slip is described as "the phenomenon of un-steady sliding resulting from varying friction force in combination with elasticity ofthe mechanical system of which the friction contact is part", as stated in [5]. Shudderis a phenomenon similar to stick-slip. However, stick slip is "induced by a discontin-uous friction coefficient change on transition from static friction to dynamic" whileshudder is "self-induced vibration due to negative slope ofthe friction-velocity rela-tion" [6]. Hence, a positive slope of theµ−v curve is beneficial in terms of avoidingthese phenomena, see Fig. 2.

v [m/s]

µ[-

]

Positive slope-Suppresses vibrationsNegative slope-Induces vibrations

0 0.5 1 1.50

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Figure 2: Friction vs. velocity

Although friction characteristics of wet clutches has beenthoroughly studied, inorder to achieve smaller, lighter and more costefficient products a better understandingof how friction and performance changes over life is needed.Studies have also beenmade in order to understand the ageing of wet clutches, e.g. Newcomb et al [7] devel-oped a methodology to evaluate worn or damaged friction material plates. Devlin et al[8] investigated the loss of friction control in automatic transmissions, like stick-slipand shudder, when ageing samples. The samples were both agedin testrig and by ox-idation of the oil. Similar tests were performed by Gupta et al [9] but here oil samplesfrom actual cars were also collected and tested for frictioncharacteristics. However,most studies are performed with systems incorporating paper-based friction discs andsystems involving sintered bronze friction discs remain unexplored. In order to get abetter understanding of how friction performance for thesesystems can change overtime, this study will focus on how frictional performance isaffected by oxidation oflubricant and additive content.

2

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1 INTRODUCTION 1.1 Objectives

1.1 Objectives

The aims of this thesis is to:

• Determine how friction characteristics change with additive content.

• Determine how friction characteristics change with oxidation of lubricant.

• Determine how friction characteristics change with test rig ageing of lubricant.

• Relate and compare these three ageing methods.

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1.1 Objectives 1 INTRODUCTION

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2 LUBRICATION THEORY

2 Lubrication theory

Since wet clutches work under lubricated conditions, it is important to understand thebasics of lubrication. Each lubricant is an often tailor-made complex mix of compo-nents, each with a specific purpose. Therefore, an understanding of lubricant com-position and components is necessary to understand frictional behavior. However, toknow only lubricant properties is not sufficient, working condition parameters such assliding speed, temperature and pressure are also needed. Inthe following two sectionslubricants and lubrication regimes are introduced, for more information see [10].

2.1 Lubricants

In a wet clutch the lubricant is very important to the overallperformance of the system.A lubricant consists typically of two parts, base oil and additives. To put it simply,the base oil determines base properties, e.g. viscosity, while additives are added toenhance and modify lubricant properties. Base oils can be divided into:

• Vegetable oils have benefits such as high biodegradability, good lubricity, highviscosity index1 and flash point2. On the other hand they usually age rapidlyand for low temperature they exhibit poor fluidity. Some examples are rapeseedoil, canola oil and sunflower oil.

• Mineral oils consist of mixtures of different hydrocarbons. They are refinedfrom crude oil and are used for most lubricating oils today. Depending onchemical structure of their main components they are subdivided into paraf-finic and naphthenic oils. Most widely used are paraffinic oils. Compared withnaphthenic oils they exhibit higher resistance to oxidation, higher pour point3,higher viscosity index, low volatility, high flash points and low specific grav-ities4. When a low pour point is acquired and the temperature range for theapplication is small, naphthenic oils are often used.

• Synthetic oils, are usually superior to mineral oils. Theyare produced by chem-ical synthesis from petroleum or vegetable oils. Common types are polyal-phaolefins, polyisobutylenes, polyalkylene glycols, phosphate esters, syntheticesters and silicones. They have higher viscosity index, better oxidation stabil-ity and a much lower pour point than mineral oils. However, observe that theproperties vary a lot between the different types. They differ as much from eachother as from mineral oil.

1Standard used to express viscosity-temperature dependency where a high index indicates smaller vis-cosity changes with temperature

2The lowest temperature to which a lubricant must be heated before its vapor, when mixed with air, willignite but not continue to burn

3The lowest temperature that oil will flow under the influence of gravity4Specific gravity is the ratio of the density of a given substance to the density of water at the same

temperature

5

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2.1 Lubricants 2 LUBRICATION THEORY

• Compounded oils consists of mineral oil and and three to tenpercent of fatty oil.The purpose of adding fatty oil is to obtain a reduction in coefficient of friction.The fatty oils adhere to metal more extensively than mineraloil which leads toformation of a protective film.

Other than the base oil additives play a significant role in the performance of alubricant. Following are some examples of different types of additives and their func-tions.

• Friction modifiers physically adsorb on metal surfaces forming a layer whichcan be easily sheared, hence reducing friction. The processis reversible sincethere is no chemical reaction involved.

• Anti Wear additives purpose is to protect the metal surfacethrough formation ofa protective film. This can be achieved either by adsorption or a mild chemicalreaction with the metal surface. Contrary to the friction modifiers, the anti wearadditives do not form a layer that is easily sheared.

• Extreme pressure additives also form a protective layer bychemical reactionwith the metal surface which is initiated by high (flash)temperatures in the microcontacts. The purpose is to increase the load at which scuffing or seizing canoccur.

• Anti-oxidants purpose is to enhance lubricant life. Theirfunction is to slowdown the oxidation process, for more information see section 2.3.3.

• Viscosity index improvers purpose is to reduce the viscosity-temperature de-pendency by affecting the viscosity at elevated temperatures.

• Dispersants have the task to envelope solid and liquid particles like dust, water,combustion products and oxidation products and to keep themdispersed and insuspension to avoid deposits.

• Detergents counteract formation of deposits on the component parts exposed tohigh temperatures. They also contribute by being alkaline reserves in the oil, sothat acid byproducts produced by oxidation can be neutralized.

• Corrosion inhibitors are used to protect metals from corrosion by forming a filmon the metal surfaces and hinder acid formation.

• Anti-foam additives purpose is to prevent foaming by reducing surface tension.

• Demulsifiers facilitate water separation from oil in orderto drain off water.

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2 LUBRICATION THEORY 2.2 Lubrication regimes

2.2 Lubrication regimes

There are three different lubrication regimes in sliding orrolling contacts, BoundaryLubrication (BL), Mixed Lubrication (ML) and (Elasto)-Hydrodynamic Lubrication(EHL). In boundary lubrication the load is carried mainly bymechanical contact whichis commonly encountered for low velocities when the hydrodynamic pressure build upcan be ignored. The lubricant’s purpose is here to reduce and/or control friction andwear plus to cool the contacting surfaces. In hydrodynamic lubrication the contactingsurfaces are separated by a thin lubricant film which resultsin low coefficients offriction typically µHL ≈ 0.001−0.01 compared with for the boundary caseµBL ≈

0.08−0.14. Between the boundary and hydrodynamic lubrication regime the load ispartly carried by hydrodynamic pressure and partly by mechanical contact. Hence,friction levels will be between boundary and hydrodynamic lubrication. The Haldexlimited slip coupling operates mainly in the boundary lubrication regime. Because ofthe mechanical contact in this regime, the additives in the lubricant play a significantrole.

2.3 Lubricant oxidation

When friction characteristics change with time is of interest, lubricant oxidation ef-fects need to be taken into account. In this section, these effects are described in brief,for more information see [11].Many of todays challenges in lubricant formulation involves the process of oxidationsince it generates lubricant changes like viscosity increase, sludge formation, additivedepletion, base oil breakdown, rust and corrosion and varnish5 formation. Oxidationis defined as a reaction where electrons are transferred froma molecule. An exam-ple of oxidation commonly encountered is iron rusting wherethe reaction takes placebetween oxygen and iron. In combustion a hydrocarbon reactswith oxygen formingwater and carbon dioxide. When a hydrocarbon reacts slowly the typical final productof oxidation is an acid and this hydrocarbon oxidation is complicated and involves sev-eral steps where different compounds are produced. In hydrocarbon oxidation thereare three basic steps: initiation, propagation and termination.

2.3.1 Initiation

During the initiation free radicals6 are formed which are usually short-lived and highlyreactive. The predominant source of free radicals is oxygenbut there are several othersuch as nitro-oxides, ultraviolet light and flow electrification (electrostatic discharge).In reaction 2.1, 2.2 and 2.3 some of these reactions can be seen:

RH+O2 → R•+HOO• (2.1)

5A thin insoluble film that deposits on the internal surfaces of a lubrication system6Molecular fragments having one or more electrons accessible to easily react with other hydrocarbons

7

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2.3 Lubricant oxidation 2 LUBRICATION THEORY

R−R+Electrolytic→ R•+R• (2.2)

R−R+UV− light → R•+R• (2.3)

RH andR−Rrepresent hydrocarbons that are either part of the base oil or additivesin the lubricant andR• andHOO• are free radicals produced by initiation reactions.The reactions are quite slow for room temperature but the rate increases rapidly withtemperature. As these reactions continue, the concentration of peroxides (ROOHandHOOH) increases, which leads to a secondary initiation scheme, see reaction 2.4.Reaction 2.4 usually requires high temperatures around 120◦C and higher to occur athigher rate. If catalysts are present, like copper and iron,the reaction can occur atlower temperatures but then the reaction rate is much slower. An increased amountof wear metal ions in the lubricant catalyze the amplified formation of free radicals,resulting in additional oxidation. Temperature will affect any reaction in two ways.For the reaction to occur a certain threshold energy is needed and if this energy in thesystem is sufficient, the reaction rate will about double every 10◦C.

ROOH→ R0•+HO• (2.4)

2.3.2 Propagation

Next, free radicals produced are now able to propagate the oxidation process. Newfree radicals and hydroperoxides can be formed and the cycleof radical formationcontinues. In the propagation sequence the number of free radicals remain the sameunlike for the initiation phase where the number is increasing. Reaction 2.5 shows howthe peroxy-radical is produced and in reaction 2.6 it then reacts with the base oil or ad-ditives regenerating the alkyl-radical which will restartthe cycle. The hydroperoxide(ROOH) produced in this cycle can then also react with the lubricant as in reaction 2.4and 2.7 to initiate the production of oxidation compounds, alcohol (ROH) and water(H2O).

R•+O2 → ROO• (2.5)

ROO•+RH→ R•+ROOH (2.6)

RO•+RH→ R•+ROH (2.7)

Radical decomposition can generate additional oxidation related products such asmost commonly, ketones and aldehydes, see reaction 2.8.

8

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2 LUBRICATION THEORY 2.3 Lubricant oxidation

RR′R′′C−O•→ R•+RR′C = O (2.8)

The ketones formed in reaction 2.8 can then react with oxygenforming carboxylicacid, see reaction 2.9.

2RC(O)R+2O2+2H2O→ 4RC(O)OH (2.9)

2.3.3 Termination

The cycle is then finally terminated and here the efficiency ofthe termination stepdetermines the extent of oxidation of the lubricant. In order to increase the efficiencyantioxidants of various types can be formulated into the lubricant. Some of these canbe seen below:

• UV absorber

• peroxide decomposer

• chain breaking electron acceptor

• chain breaking electron donor

Most commonly phenolic and aromatic amine antioxidants (primary antioxidants)are used and these are of the chain breaking type. They work byabsorption of a freeradical forming a stable radical, see reaction 2.10 and 2.11.

ROO•+AH → ROOH+A• (2.10)

ROO•+A•→ Inert products (2.11)

Since the oxidation reaction rates normally are faster withthe antioxidants thanwith the base oil or additives they protect the lubricant efficiently. The second mostcommon type of antioxidants are the peroxide decomposers (secondary antioxidants).These typically include sulfur and phosphorous chemistries such as ZDDP, alkyl phos-phates, alkyl phosphites, phenothiazines. These work by destruction of peroxides orhydroperoxides into alcohols or water.

9

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2.3 Lubricant oxidation 2 LUBRICATION THEORY

2.3.4 Ester formation and condensation reactions

Major reaction products of oxidation in the lubricant are ester, see reaction 2.12 and2.13. Reaction 2.12 usually occurs in the hot zones of the lubricant, where the perox-ide can decompose to free radicals while reaction 2.13 can occur both in the cold andhot zone of the fluid.

ROOH+RC(O)RAcid→ ROH+ RC(O)OR (2.12)

ROH+RC(O)OH→H2O + RC(O)OR (2.13)

When a lubricant is oxidized usually an increase in viscosity can be observed.This is due to additional side reactions occurring involving reaction products from theoxidation process. High molecular size products are formedvia Aldol and Claisencondensation reactions, see reactions 2.14 and 2.15.

3RCH2C(O)CH2RAcid→ (RCH2)2C = C(R)C(O)C(R) = C(CH2R)2 + 2H2O (2.14)

2RCH2C(0)OR+RC(O)RRO•→ROH+ RCH2C(O)CH(R)C(O)R (2.15)

The aldol condensation products can increase even further in size by polymer-ization when initiated by free radicals from the propagation step. Growth of thesemolecules will continue as the oxidation process continues, resulting in high molecu-lar weights and high viscosity. When more and more oxygen atoms are included intothe hydrocarbon molecules also polarity increases. Eventually, the increase in sizeof the polar materials will be enough to exhibit poor solubility in the nonpolar hy-drocarbons making up the lubricant and insoluble material is formed. Both additivesand base oil are under the effect of the oxidation and condensation reactions. Con-densation reactions can then result in varnish, sludge, deposit formation and viscosityincrease.

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3 METHOD AND MATERIALS

3 Method and materials

In a standard pin on disc test, an axial force is applied to thepin which is in contactwith a rotating disc which is submerged in an oil bath. The friction force on the pincan be measured and hence the friction coefficient can easilybe calculated. In themethod developed by Marklund [4] the pin holds a small test specimen which in turnis in contact with the disc. In Figure 3 the experimental setup is displayed.

ω

Figure 3: Pin on disc setup

The test piece is by spark erosion cut out of friction discs. Friction material issintered bronze of the same composition as in the Haldex limited slip coupling. Therotating disc is of the same steel as the separator discs for the same application. Forthis study a modified version of Marklund’s method is used. The maximum slidingspeed for this study is three times higher, which introducesnew problems such asdifficulties to stop the oil from escaping the contact at highrotational speeds. To solvethis problem an external oil pump is added, which collects oil from the bottom of theoil bath and then supplies it directly into the contact. For temperature measurements athermocouple is inserted in the small sintered bronze test specimen so that temperatureis measured about 0.3 mm from the contact. This is the temperature referred to whenanalyzing the results later on in this thesis. The oil temperature right before it entersthe contact is also monitored. The pin on disc machine used for these experiments isa Phoenix Tribology TE67. The resolution of the measurements are shown in Table 1.Another feature of this pin on disc machine is the contact potential, which essentiallydescribes the electrical contact resistance between the two surfaces. A value of 50 mV

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3.1 Test procedure 3 METHOD AND MATERIALS

corresponds to high resistance and completely separated surfaces, while a value of 0mV corresponds to very low resistance and complete contact.

Table 1: Test rig specifications

Measurement Resolution

Temperature 0.2 [◦C]Sampling Rate 10 [Hz]

Rotational speed 1 [rpm]⇒⇒ Sliding speed 0.0016 [m/s]

Friction force 0.015 [N]

3.1 Test procedure

Before testing starts the surfaces are run in at ambient temperature 25◦C for 20 min-utes. The sliding speed is 0.15 m/s and surface pressure is 3 MPa. The test starts at aninterface temperature of 30◦C and is then gradually heated up to 100◦C. Every 10◦Cthe sliding speed is increased from standstill to 1.5 m/s andthen decreased in about 90seconds, see Fig. 4(a). The interface temperature increaseduring one test cycle for astart temperature of 30◦C can be seen in Fig. 4(b). Temperature follows sliding speedquite well, indicating that the measured temperature satisfactory represents the meansurface temperature. In Table 2 the test parameters are displayed.

t [s]

v[m

/s]

0 30 60 900

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

(a) Sliding speed

t [s]

T[◦

C]

0 30 60 9030

31

32

33

34

35

36

(b) Temperature

Figure 4: Speed and interface temperature during one test cycle

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3 METHOD AND MATERIALS 3.2 Lubricant ageing

Table 2: Test parameter specificationsVariable Value

Temperature 30-100 [◦C]Sliding Speed 0-1.5 [m/s]

Surface pressure 3 [MPa]Oil flow 200 [ml/min]

3.2 Lubricant ageing

In earlier research a modified dry-TOST (Waterless Turbine Oil Oxidation StabilityTest) ASTM D 943 has been used to evaluate oxidational stability of oils [12]. In ourcase, oil samples are in contact with oxygen (3.5l/h) in the presence of an iron-coppercatalyst at 120◦C for a period of time. In order to investigate how different levels ofoxidation affect friction performance a fully formulated lubricant for the Haldex lim-ited slip coupling will be oxidized using the modified dry-TOST. The oxidation timeperiod will be in five steps from 48 hours to 408h and for each level of oxidation, afriction performance test will be run. The remaining concentrations of antioxidantsis measured for each oxidation level using RULER (remaininguseful life evaluationroutine), a quantitative linear voltammetry method.

Also an oil aged in a testrig at Haldex Traction will be testedfor friction perfor-mance. The test is constructed in order to verify that an oil-friction disc combinationwill last the lifetime of the application. Both applied discpressure and power ef-ficiency in the discs can be adjusted to maximum permittable level. To accelerateageing even further tests are run at a temperature of 100◦C. Occurance of stick-slipand/or shudder are performed in form of noise control which is carried out periodi-cally at temperatures between 25◦C and 100◦C. First, an electric motor drive is used toaccelerate a flywheel. Between the flywheel and a braking device a whole coupling isinstalled. When the brake is applied a transfer of torque from the flywheel to the brakestarts to occur. Both input and output axles decrease in rotation but at different rate sothat a differential rotational speed arises. In this case, amaximum torque of 1200 Nmand maximum differential rotational speed of 75 rpm is reached. Each braking cycletakes about five seconds and a total of 53000 cycles were run.

As described in section 2.2 the additive package in a lubricant plays a significantrole in the performance of a wet clutch. Therefore we will also study the effect ofreducing the additive concentration in the oil. The additive content will be variedfrom fully formulated to only 10% of the additive package added to the base oil. Acomplete overview of the lubricants tested can be seen in Table 3.

13

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3.3 Materials 3 METHOD AND MATERIALS

Table 3: Lubricants testedOxidation level (Duration of modified dry-TOST)

48h96h192h360h408h

Additive level (Percentage of additive package)10075502510

3.3 Materials

The friction pair in the Haldex LSC is sintered bronze and steel, as mentioned insection 1. A main characteristic of sintered materials is the porosity. The pores in thematerial can act as reservoirs for the lubricant and capillary forces keep the fluid inplace [13, 10]. An image of the sintered bronze surface takenwith a scanning electronmicroscope (SEM) can be seen in Fig. 5.

Bronzes are made up of copper and other elements such as tin, zinc, aluminium,silicon and nickel. They exhibit good tensile properties and generally also favorablecorrosion resistance properties. An application where thebronzes are often used arein boundary lubricated bearings in the form of bushings.

14

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3 METHOD AND MATERIALS 3.3 Materials

Figure 5: Scanning electron microscope image of the sintered bronze surface

15

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3.3 Materials 3 METHOD AND MATERIALS

16

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4 RESULTS

4 Results

During experiments a large amount of experimental data was collected. In the fol-lowing sections the outcome of this experiments will be displayed and explained. Acomplete overview of performed tests can be seen in appendixA.

4.1 Friction characteristics change with additive concentration

Figure 6 shows curve fitted frictional behavior for different additive concentrations.Generally, a friction increase with reduced additive content can be noticed. The in-clination of the curve at low velocities increases with reduced additive content. Foradditive levels of 75%, 50% and 25% the friction characteristics are very similar,though at higher velocities the negative slope increases with reduced additive content.A distinct difference in friction can also be seen between 25% and 10% of the additivepackage.

v [m/s]

µ[-

]

10%

25%

50%

75%

Fully formulated

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

Figure 6: Friction characteristics comparison of different additive concentration at70◦C

However, when the repeatability of the experiments is takeninto account, thisdifference is not as clear, see Fig. 7. Each figure shows friction data from four separateexperiments together with fitted curves. In Figure 7(b) it can be noticed that for anadditive level of 25% the friction can vary between frictionlevels similar to fullyformulated to levels similar to 10% of the additive package.The repeatability of theexperiments decrease when additive content is reduced. In order to investigate if thiscould be due to surface effects, two test pieces run at the same additive level but withdistinctive differences in friction characteristics wereselected for further testing. For

17

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4.2 Friction characteristics change with oxidation 4 RESULTS

v [m/s]

µ[-

]

Fitted frictionFriction data

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(a) 10% of additive packagev [m/s]

µ[-

]

Fitted frictionFriction data

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(b) 25% of additive packagev [m/s]

µ[-

]

Fitted frictionFriction data

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(c) 100% of additive package

Figure 7: Friction data and fitted curves

an additive content of 10% of the additive package, frictioncharacteristics at 70◦C fortwo separate testpieces are shown in Fig. 8(a). After the full test ranging from 25◦

to 100◦C, a new test is performed at 25◦C with the same sintered bronze test pieces,but this time with new countersurfaces and fresh lubricant,see Fig. 8(b). Resultsshow similar distinction in friction characteristics in both cases, indicating that surfacecharacteristics of the sintered bronze surfaces are responsible for the difference infriction characteristics. However, this only occur for lower additive content.

v [m/s]

µ[-

]

Test piece 1Test piece 2

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(a) Friction characteristics at 70◦C

v [m/s]

µ[-

]

Test piece 1Test piece 2

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(b) Friction characteristics at 25◦C after full test

Figure 8: Comparison test pieces at 10% of additive package

4.2 Friction characteristics change with oxidation

In Figure 9 it can be noticed that the general friction coefficient levels increase withoxidation. For the 408h and 360h oils the negative slope is significant in comparison tothe others. The behavior for these two are also quite similar, which could indicate thatthe friction rate of change with oxidation is decreasing. Incomparison to the frictionchange with additive content, see Fig. 6, the friction change with oxidation are largerand more severe. In Table 4 the results of the RULER measurements can be seen, notethat the antioxidant level for 408h and 360h dry-TOST are less than satisfactory, andhere we also see large changes in friction behavior.

18

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4 RESULTS 4.3 Comparison ageing methods

v [m/s]

µ[-

]

408h

360h

192h

96h

48h

Fully formulated

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

Figure 9: Friction characteristics comparison of different oxidation levels at 70◦C

Table 4: Antioxidant levelsOxidation time Antioxidant level

(Duration of modified

dry-TOST)

48h 100%96h -192h Low360h Very Low408h Very Low

4.3 Comparison ageing methods

In Figure 10 friction characteristics at 70◦C for some test oils are displayed. An oilaged in a test rig at Haldex, two dry-TOST oils, one with 10% additives and one fullyformulated are compared. It is noticable that the oil aged atHaldex shows a steeppositive slope for low speed almost in comparison with the 408h dry-TOST oil. Themaximum friction coefficient though is considerably smaller for the Haldex oil andalso the negative slope is less severe. For higher speeds both the 96h dry-TOST oiland the 10% additive oil is showing similar behavior as the Haldex oil.

19

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4.4 Influence of temperature 4 RESULTS

v [m/s]

µ[-

]

408h

96h

Aged in testrig

10% of additive package

Fully formulated oil

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

Figure 10: Friction characteristics comparison of different ageing methods

4.4 Influence of temperature

From Figure 10 in section 4.3 one can be led to believe that the96h dry-TOST oiland the 10% additive oil are exhibiting nearly the same friction behavior as the oilaged at Haldex. However, this is true only for that temperature, 70◦C. As can be seenin Fig. 11 friction behavior is varying significantly with temperature. Figure 11(a)shows a fully formulated oil and friction characteristics for 30◦C, 70◦C and 100◦C.The friction coefficient is reduced with increasing temperature and the difference infriction is obvious. In Figure 11(b) it can be noticed that the separation in frictionbetween the different temperatures is less than for the fully formulated oil, the curvesare closer together. Tendencies to the same behavior can be seen for the 96h dry-TOSToil, especially for higher speeds, see Fig. 11(c). For the oil aged at Haldex the curvesare approaching each other in the same way only for lower speeds. At higher speedsthey are not getting closer together, instead they are shifting, see Fig. 11(d). Theseparation of the friction curves are clear and the frictioncoefficent, at speeds aboveabout 0.2 m/s, increases with increasing temperature, the opposite of what happensfor a fully formulated fresh oil.

20

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4 RESULTS 4.5 Mean contact potential change

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(a) Friction vs. velocity, fully formulated

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(b) Friction vs. velocity, 10% of additive package

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(c) Friction vs. velocity, 96h oxidation level

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.50.08

0.09

0.1

0.11

0.12

0.13

0.14

(d) Friction vs. velocity, oil aged in test at Haldex

Figure 11: Friction behaviour with temperature

4.5 Mean contact potential change

In Figures 12 and 13 the mean contact potential over one speedramp can be seen. Thestandard deviation and Total Acid Number (TAN) is also displayed.

The contact potential can be seen to vary very little with additive content eventhough tendencies to a decreased contact potential with decreasing additive contentcan be observed. In the case of the dry-TOST oils, the contactpotential clearly de-creases with oxidation. Noticeable is that when the maximumfriction coefficient ishigh, e.g. 360h and 408h dry-TOST, the mean contact potential is low. The total acidnumber shows an increase typical for an ageing oil and the contact potential decreasesas TAN increases.

21

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4.5 Mean contact potential change 4 RESULTS

Mea

nco

ntac

tpot

entia

l[m

V]

% of additive package

100◦C

70◦C

30◦C

100 75 50 25 100

10

20

30

40

50

Figure 12: Contact potential change with additive content

Mea

nco

ntac

tpot

entia

l[m

V]

Duration of dry-TOST [h]

100◦C

70◦C

30◦C

Tota

laci

dnu

mbe

r[m

gKO

K/g

]

TAN

0 48 96 192 360 408Aged in test

2

2.7

3.4

4.1

4.8

5.50

10

20

30

40

50

Figure 13: Contact potential and Total acid number for aged oils

22

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4 RESULTS 4.6 Safe working range, R

4.6 Safe working range, R

In Figure 14 the friction coefficient derivate (dµdv) versus velocity (v) is plotted. This

figure describes how the slope of the friction curve changes with speed. In previousresearch a negative slope of the friction curve has been proved to be bad since this mayintroduce phenomena like stick-slip and/or shudder. Knowing this, it is interesting toknow at which speed the slope shifts from positive to negative. In Figure 14 a safeworking range, R(m/s), is shown for a fully formulated oil and a 408h dry-TOST oil.Knowing the speed working range of an application, it can be compared to the safeworking range. If the safe working range exceeds the the working range, stick slip andshudder should be avoided. In Figure 15 the change of the safeworking range with

v [m/s]

dv

R

R

Fully formulated, 70◦C

Oxidation level 408h, 70◦C

0 0.5 1 1.5-0.03

-0.02

-0.01

0

0.01

0.02

0.03

Figure 14: Safe working range, R

oxidation is shown. It is clear that the oxidation reduces R and this supports the ideaof monitoring R in clutch life testing. In Figure 16 the influence of additive contenton the safe working range is shown. The safe working range is reduced with additivecontent except for the case of 75% where the safe working range is high.

23

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4.6 Safe working range, R 4 RESULTS

Dry-TOST oxidation level [h]

Saf

ew

orki

ngra

nge,

R,[

m/s

]

0 48 96 192 360 408 Aged intestrig

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Figure 15: Safe working range, R, for different oxidation levels

% of additive package

Saf

ew

orki

ngra

nge,

R,[

m/s

]

100 75 50 25 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 16: Safe working range, R, for different additive levels

24

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5 DISCUSSION

5 Discussion

Measurements of friction and its variation with temperature indicate that the dry-TOST ageing method is not comparable with test rig ageing performed at Haldex.The dry-TOST thermally ages the lubricant while in a testrigthe circumstances willnot be the same. The lubricant will be sheared in the contact and here temperatureswill be high while wear particles formed. The differences inoperating conditionscould then lead to other reactions being benefited and other reaction products formed.

What is interesting though is that there are similarities between reducing additivecontent, oxidation and testrig ageing, all show a general friction increase. Since someof the additives like friction modifiers reduce friction, itmakes sense that reducingthem will increase friction.

When additive content is reduced, the repeatability of the experiments decreases.This could be due to destruction of the sintered bronze surface such as clogging ofpores when material is smeared across the surface. Less additives will lead to a re-duced protection of the contacting surfaces. In turn, this could lead to destruction ofthe sintered bronze surfaces. Variation in surface roughness and material makeup oftest pieces could be a factor influencing whether the surfaceis destroyed or not.

The overall effect of lubricant oxidation is hard to predictsince the lubricant hasa complex composition of base oil and additives, which can interact in many variousways, as described in 2.3. Additive consumption and reaction products formed mayplay a significant role for the frictional behavior. However, for the oxidized oils theinfluence on friction is even bigger than for additive content reduction, and frictionchange follows oxidation quite well. Here it is important tonote that additive contentis reduced to 10% at the least, for lower additive content thefriction change may reachthe same levels as oxidation.

One way to interpret the mean contact potential is additive activity. For low ad-ditive content the contact potential is still high while foroxidation, levels of contactpotential which are quite low are reached. The oil aged at Haldex also show con-tact potential which are still quite high, indicating additive activity still taking place.Note however that the contact potential is based on the resistance in the contact andhigher resistance could be due to a number of factors such as hydrodynamic film buildup, films formed by non-additives (e.g. reaction products) and/or films formed byadditives. Supporting the theory of additive activity is the lubrication regime whichbased on friction levels would be boundary lubrication. Another way to explain thedecreasing contact potential with oxidation is through thetotal acid number. Whenthe number of polar components in the lubricant increases the conduction increases,leading to a decreased contact potential.

The safe working range, R, provides a good parameter to checkhow suitable a lu-bricant is for a certain speed range of a wet clutch application. If R exceeds the speedrange, it is known that in the means of avoiding stick-slip and shudder, the formulationis good. For future research it would also be interesting to monitor this parameter inclutch life testing.

25

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5 DISCUSSION

26

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6 CONCLUSIONS

6 Conclusions

• Reduced additive content, oxidation and testrig ageing all increase the frictioncoefficient.

• Lubricant aged in testrig shows significantly different friction characteristicswith temperature than lubricants aged by dry-TOST implyingthat dry-TOST isnot a sufficient method to evaluate lubricant ageing.

• Lubricant aged in testrig shows significantly different friction characteristicswith temperature than lubricants with reduced additive content implying thatthis is not a satisfactory way to simulate lubricant ageing.

• A safe working range, R, has been derived which can be usefulwhen evaluatinga lubricants suitability for a certain speed range. It showsa decrease with bothoxidation and additive content.

27

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6 CONCLUSIONS

28

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7 FUTURE WORK

7 Future work

Although this work has answered some questions, issues of interest for future researchhave also appeared, as can be seen below.

• Clarify why thermal oxidation of a lubricant differs from ageing of lubricant intestrig. Find out which mechanisms that are responsible forthe decompositionof lubricant.

• Investigate which factors or parameters that influence theageing of a wet clutchthe most and in what way.

• Find out how ageing of friction discs and lubricant are linked and if one of themare more important than the other.

• Determine how time and history of the frictional system influence friction char-acteristics. As an example, for a specific set of parameter values, could frictioncharacteristics results be different if the system recently was active compared toif the system had been at rest for a long time?

• Investigate friction characteristics of lubricant aged in test vehicles with resultsobtained in this thesis and how different types of tests can be correlated.

• Develop a definition of wet clutch life and also define the endof clutch life.Further, there is a need to find suitable parameters to surveyin order to measurethe consumption of life.

29

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7 FUTURE WORK

30

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REFERENCES REFERENCES

References

[1] Y. Kato and T. Shibayama. Mechanisms of automatic transmissions and theirrequirements for wet clutches and wet brakes.Japanese journal of tribology,1994.

[2] Mikael Holgerson. Influence of operating conditions on friction and temperaturecharacteristics of a wet clutch engagement.TriboTest, 7(2):99 – 114, 2000.

[3] Rikard Maki. Wet clutch tribology - friction characteristics in all-wheel drivedifferentials.Tribologia, 22(3):5 – 16, 2003.

[4] P. Marklund and R. Larsson. Wet clutch friction characteristics obtained fromsimplified pin on disc test.Tribology International, 41, issues 9-10, Nordtrib2006:824–830, 2008. doi: 10.1016/j.triboint.2007.11.014.

[5] F. Van De Velde and P. De Baets. The relation between friction force and relativespeed during the slip-phase of a stick-slip cycle.Wear, 1998.

[6] Y. Kato, R. Akasaka, and T. Shibayama. Experimental study on the lock-upshudder mechanism of an automatic transmission.Japanese journal of tribology,39(12), 1994.

[7] Timothy Newcomb, Mark Sparrow, and Brian Ciupak. Glaze analysis of frictionplates.SAE Technical Papers, (2006-01-3244), 2006.

[8] M.T. Devlin, et al. Fundamentals of anti-shudder durability: Part II - fluid effects.SAE Technical Papers, (2003-01-3254), 2003.

[9] G. K. Gupta, et al. ATF bulk oxidative degradation and itseffects on LVFA fric-tion and the performance of a modulated torque converter clutch. SAE TechnicalPapers, (982668), 1998.

[10] Anton van Beek. Advanced engineering design - lifetimeperfomance and relia-bility, 2006.

[11] Dave Wooton. The lubricant’s nemesis - oxidation.Practicing oil analysis,March, 2007.

[12] Mayte Pach, et al. Aged environmentally adapted lubricants - part I: Procedures,techniques and affected properties for aged oils.Proceedings of the 15th Inter-national Colloquium Tribology, Esslingen, 2006.

[13] Pär Marklund, Kim Berglund, and Roland Larsson. The influence on boundaryfriction of the permeability of sintered bronze.Tribology Letters, pages 1–8,2008. doi: 10.1007/s11249-008-9330-5.

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REFERENCES REFERENCES

32

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A FRICTION CURVES

A Friction curves

A.1 Friction vs. velocity, fully formulated

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.1: Test 1

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.2: Test 2

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.3: Test 3

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.4: Test 4

33

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A.2 Friction vs. velocity, 75% of additive package A FRICTION CURVES

A.2 Friction vs. velocity, 75% of additive package

Figure A.5: Test 1v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.6: Test 2v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.7: Test 3v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

34

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A FRICTION CURVES A.3 Friction vs. velocity, 50% of additivepackage

A.3 Friction vs. velocity, 50% of additive package

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.8: Test 1

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.9: Test 2

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.10: Test 3

35

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A.4 Friction vs. velocity, 25% of additive package A FRICTION CURVES

A.4 Friction vs. velocity, 25% of additive package

Figure A.11: Test 1v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.12: Test 2v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.13: Test 3v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.14: Test 4v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

36

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A FRICTION CURVES A.5 Friction vs. velocity, 10% of additivepackage

A.5 Friction vs. velocity, 10% of additive package

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.15: Test 1

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.16: Test 2

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.17: Test 3

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.18: Test 4

37

Page 48: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

A.6 Friction vs. velocity, 48h dry-TOST A FRICTION CURVES

A.6 Friction vs. velocity, 48h dry-TOST

Figure A.19: Test 1v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.20: Test 2v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

38

Page 49: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

A FRICTION CURVES A.7 Friction vs. velocity, 96h dry-TOST

A.7 Friction vs. velocity, 96h dry-TOST

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.21: Test 1

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.22: Test 2

39

Page 50: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

A.8 Friction vs. velocity, 192h dry-TOST A FRICTION CURVES

A.8 Friction vs. velocity, 192h dry-TOST

Figure A.23: Test 1v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.24: Test 2v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

40

Page 51: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

A FRICTION CURVES A.9 Friction vs. velocity, 360h dry-TOST

A.9 Friction vs. velocity, 360h dry-TOST

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.25: Test 1

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.26: Test 2

41

Page 52: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

A.10 Friction vs. velocity, 408h dry-TOST A FRICTION CURVES

A.10 Friction vs. velocity, 408h dry-TOST

Figure A.27: Test 1v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.28: Test 2v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

42

Page 53: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

A FRICTION CURVES A.11 Friction vs. velocity, Oil aged in testrig

A.11 Friction vs. velocity, Oil aged in testrig

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.29: Test 1

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.30: Test 2

v [m/s]

µ[-

]

30◦C70◦C100◦C

0 0.5 1 1.5

0.06

0.08

0.1

0.12

0.14

Figure A.31: Test 3

43

Page 54: Lubricant ageing effects on wet clutch friction characteristics1027724/FULLTEXT01.pdf · 2016-10-04 · Lubricant ageing effects on wet clutch friction characteristics Lubricant ageing

A.11 Friction vs. velocity, Oil aged in testrig A FRICTION CURVES

44