Lubricant ageing effects on wet clutch friction characteristics

(1)

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

Lubricant ageing effects on

wet clutch friction characteristics

Lubricant ageing effects on wet clutch

friction characteristics

Kim Berglund August 18, 2008

(2)

(3)

Preface

The work of this master thesis has been carried out during one instructive year as a Research Trainee at Luleå university of technology. I would especially like to express my appreciation to Mr. Pär Marklund and my supervisor Professor Roland Larsson for taking time to guide and help in many valuable discussions. Acknowledgments should also be made to Statoil Lubricants in Nynäshamn and Haldex Traction in Landskrona for both financial support and help with my work.

(4)

(5)

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 clutches have been well investigated by several authors. Studies have also been made in or-der to unor-derstand the ageing of wet clutches. However, most lifetime studies have been made for systems with paperbased friction discs and systems involving sintered bronze 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 order to get a better understanding of how this system can change over time, the study in this master thesis is focused on how frictional performance is affected by oxidation of lubricant, 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 examined using 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 for each level of oxidation, a friction performance test was run using a pin on disc ma-chine.

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

Since lubricant additives are vital to the performance of wet clutches the effect of reducing the additive concentration in the oil was also studied, in the range 10 to 100% 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 Haldex LSC is needed. This thesis has focused on lubricant ageing but no attention has been paid to wear and age-ing of friction discs. To investigate and relate ageage-ing of lubricant and friction discs is another important task for future research.

(6)

(7)

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 RR0 C= O Ketone

RO• Alkyloxy radical

RR0

R00

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

(8)

(9)

CONTENTS CONTENTS

1 Introduction

Wet clutches are often used in the drivetrains of vehicles today. As revealed by the name, a wet clutch operates under wet conditions and working conditions can vary greatly 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) cars today. In a primarily front wheel driven vehicle it transfers torque to the rear wheels while 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 sintered bronze and the separator discs are 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 togeei-ther by an electronically controlled hydraulic pump, see Fig. 1. Hence, torque transfer is determined by friction in interfaces of friction and separator discs. Wheel rotational speeds are retrieved from the vehicles anti-lock braking system (ABS) sensors to the electronic control system of the clutch in order to regulate torque transfer.

Hydraulic piston pump Wet multi−plate clutch

Clutch piston

Controllable 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 of wet clutches Mäki [3] developed a Limited Slip Clutch test rig. Friction performance of limited slip differentials involving sintered bronze was thoroughly explored. Mark-lund [4] developed a pin-on-disc method to evaluate the same kind of system, with advantages such as being simple and inexpensive, e.g. suitable for screening-tests.

(12)

phe-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 of the mechanical system of which the friction contact is part", as stated in [5]. Shudder is 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" while shudder is "self-induced vibration due to negative slope of the friction-velocity rela-tion" [6]. Hence, a positive slope of the µ − v curve is beneficial in terms of avoiding these phenomena, see Fig. 2.

v [m/s]

µ

[-]

Positive slope-Suppresses vibrations Negative slope-Induces vibrations

0 0.5 1 1.5 0 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 been thoroughly studied, in order to achieve smaller, lighter and more costefficient products a better understanding of how friction and performance changes over life is needed. Studies have also been made 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-slip and shudder, when ageing samples. The samples were both aged in testrig and by ox-idation of the oil. Similar tests were performed by Gupta et al [9] but here oil samples from actual cars were also collected and tested for friction characteristics. However, most studies are performed with systems incorporating paper-based friction discs and systems involving sintered bronze friction discs remain unexplored. In order to get a better understanding of how friction performance for these systems can change over time, this study will focus on how frictional performance is affected by oxidation of lubricant and additive content.

(13)

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.

(14)

(15)

2 LUBRICATION THEORY

2 Lubrication theory

Since wet clutches work under lubricated conditions, it is important to understand the basics 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, to know only lubricant properties is not sufficient, working condition parameters such as sliding speed, temperature and pressure are also needed. In the following two sections lubricants and lubrication regimes are introduced, for more information see [10].

2.1 Lubricants

In a wet clutch the lubricant is very important to the overall performance 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 to enhance and modify lubricant properties. Base oils can be divided into:

• Vegetable oils have benefits such as high biodegradability, good lubricity, high viscosity index1and flash point2. On the other hand they usually age rapidly and for low temperature they exhibit poor fluidity. Some examples are rapeseed oil, canola oil and sunflower oil.

• Mineral oils consist of mixtures of different hydrocarbons. They are refined from crude oil and are used for most lubricating oils today. Depending on chemical structure of their main components they are subdivided into paraf-finic and naphthenic oils. Most widely used are parafparaf-finic oils. Compared with naphthenic 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 the application is small, naphthenic oils are often used.

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

1_{Standard used to express viscosity-temperature dependency where a high index indicates smaller}

vis-cosity changes with temperature

2_{The lowest temperature to which a lubricant must be heated before its vapor, when mixed with air, will}

ignite but not continue to burn

3_{The lowest temperature that oil will flow under the influence of gravity}

4_{Specific gravity is the ratio of the density of a given substance to the density of water at the same}

(16)

2.1 Lubricants 2 LUBRICATION THEORY

• Compounded oils consists of mineral oil and and three to ten percent 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 mineral oil which leads to formation of a protective film.

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

• Friction modifiers physically adsorb on metal surfaces forming a layer which can be easily sheared, hence reducing friction. The process is reversible since there is no chemical reaction involved.

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

• Extreme pressure additives also form a protective layer by chemical reaction with the metal surface which is initiated by high (flash)temperatures in the micro contacts. The purpose is to increase the load at which scuffing or seizing can occur.

• Anti-oxidants purpose is to enhance lubricant life. Their function is to slow down 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 them dispersed and in suspension to avoid deposits.

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

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

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

(17)

2 LUBRICATION THEORY 2.2 Lubrication regimes

2.2 Lubrication regimes

There are three different lubrication regimes in sliding or rolling contacts, Boundary Lubrication (BL), Mixed Lubrication (ML) and (Elasto)-Hydrodynamic Lubrication (EHL). In boundary lubrication the load is carried mainly by mechanical contact which is commonly encountered for low velocities when the hydrodynamic pressure build up can be ignored. The lubricant’s purpose is here to reduce and/or control friction and wear plus to cool the contacting surfaces. In hydrodynamic lubrication the contacting surfaces are separated by a thin lubricant film which results in low coefficients of friction typically µHL≈0.001−0.01 compared with for the boundary case µBL≈

0.08−0.14. Between the boundary and hydrodynamic lubrication regime the load is

partly carried by hydrodynamic pressure and partly by mechanical contact. Hence, friction levels will be between boundary and hydrodynamic lubrication. The Haldex limited slip coupling operates mainly in the boundary lubrication regime. Because of the mechanical contact in this regime, the additives in the lubricant play a significant role.

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 efef-fects are described in brief, for more information see [11].

Many of todays challenges in lubricant formulation involves the process of oxidation since it generates lubricant changes like viscosity increase, sludge formation, additive depletion, base oil breakdown, rust and corrosion and varnish5formation. Oxidation is defined as a reaction where electrons are transferred from a molecule. An exam-ple of oxidation commonly encountered is iron rusting where the reaction takes place between oxygen and iron. In combustion a hydrocarbon reacts with oxygen forming water and carbon dioxide. When a hydrocarbon reacts slowly the typical final product of oxidation is an acid and this hydrocarbon oxidation is complicated and involves sev-eral steps where different compounds are produced. In hydrocarbon oxidation there are three basic steps: initiation, propagation and termination.

2.3.1 Initiation

During the initiation free radicals6are formed which are usually short-lived and highly reactive. The predominant source of free radicals is oxygen but there are several other such 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)

5_{A thin insoluble film that deposits on the internal surfaces of a lubrication system}

(18)

2.3 Lubricant oxidation 2 LUBRICATION THEORY

R − R+ Electrolytic → R • +R• (2.2)

R − R+ UV − light → R • +R• (2.3)

RH and R−R represent hydrocarbons that are either part of the base oil or additives

in the lubricant and R• and HOO• are free radicals produced by initiation reactions. The reactions are quite slow for room temperature but the rate increases rapidly with temperature. As these reactions continue, the concentration of peroxides (ROOH and

HOOH) 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 at higher rate. If catalysts are present, like copper and iron, the reaction can occur at lower temperatures but then the reaction rate is much slower. An increased amount of 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 the system 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. New free radicals and hydroperoxides can be formed and the cycle of radical formation continues. In the propagation sequence the number of free radicals remain the same unlike for the initiation phase where the number is increasing. Reaction 2.5 shows how the 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 restart the cycle. The hydroperoxide (ROOH) produced in this cycle can then also react with the lubricant as in reaction 2.4 and 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 as most commonly, ketones and aldehydes, see reaction 2.8.

(19)

2 LUBRICATION THEORY 2.3 Lubricant oxidation

RR0

R00

C − O• → R •+RR0

C= O (2.8)

The ketones formed in reaction 2.8 can then react with oxygen forming carboxylic acid, 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 of the termination step determines the extent of oxidation of the lubricant. In order to increase the efficiency antioxidants of various types can be formulated into the lubricant. Some of these can be 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 by absorption of a free radical 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 with the antioxidants than with the base oil or additives they protect the lubricant efficiently. The second most common 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 or hydroperoxides into alcohols or water.

(20)

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 and 2.13. 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 and hot 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 the oxidation process. High molecular size products are formed via Aldol and Claisen condensation 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)R

RO•

→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 these molecules will continue as the oxidation process continues, resulting in high molecu-lar weights and high viscosity. When more and more oxygen atoms are included into the hydrocarbon molecules also polarity increases. Eventually, the increase in size of 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 additives and base oil are under the effect of the oxidation and condensation reactions. Con-densation reactions can then result in varnish, sludge, deposit formation and viscosity increase.

(21)

3 METHOD AND MATERIALS

3 Method and materials

In a standard pin on disc test, an axial force is applied to the pin which is in contact with a rotating disc which is submerged in an oil bath. The friction force on the pin can be measured and hence the friction coefficient can easily be calculated. In the method developed by Marklund [4] the pin holds a small test specimen which in turn is 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 is sintered bronze of the same composition as in the Haldex limited slip coupling. The rotating disc is of the same steel as the separator discs for the same application. For this study a modified version of Marklund’s method is used. The maximum sliding speed for this study is three times higher, which introduces new problems such as difficulties to stop the oil from escaping the contact at high rotational speeds. To solve this problem an external oil pump is added, which collects oil from the bottom of the oil bath and then supplies it directly into the contact. For temperature measurements a thermocouple is inserted in the small sintered bronze test specimen so that temperature is measured about 0.3 mm from the contact. This is the temperature referred to when analyzing the results later on in this thesis. The oil temperature right before it enters the contact is also monitored. The pin on disc machine used for these experiments is a 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 essentially describes the electrical contact resistance between the two surfaces. A value of 50 mV

(22)

3.1 Test procedure 3 METHOD AND MATERIALS

corresponds to high resistance and completely separated surfaces, while a value of 0 mV 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 an interface temperature of 30◦

C and is then gradually heated up to 100◦

C. Every 10◦

C the sliding speed is increased from standstill to 1.5 m/s and then decreased in about 90 seconds, see Fig. 4(a). The interface temperature increase during one test cycle for a start temperature of 30◦

C can be seen in Fig. 4(b). Temperature follows sliding speed quite well, indicating that the measured temperature satisfactory represents the mean surface temperature. In Table 2 the test parameters are displayed.

t [s] v [m /s ] 0 30 60 90 0 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 90 30 31 32 33 34 35 36 (b) Temperature

(23)

3 METHOD AND MATERIALS 3.2 Lubricant ageing

Table 2: Test parameter specifications Variable 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 Stability Test) ASTM D 943 has been used to evaluate oxidational stability of oils [12]. In our case, oil samples are in contact with oxygen (3.5l/h) in the presence of an iron-copper catalyst at 120◦

C for a period of time. In order to investigate how different levels of oxidation affect friction performance a fully formulated lubricant for the Haldex lim-ited slip coupling will be oxidized using the modified dry-TOST. The oxidation time period will be in five steps from 48 hours to 408h and for each level of oxidation, a friction performance test will be run. The remaining concentrations of antioxidants is measured for each oxidation level using RULER (remaining useful life evaluation routine), a quantitative linear voltammetry method.

Also an oil aged in a testrig at Haldex Traction will be tested for friction perfor-mance. The test is constructed in order to verify that an oil-friction disc combination will last the lifetime of the application. Both applied disc pressure and power ef-ficiency in the discs can be adjusted to maximum permittable level. To accelerate ageing even further tests are run at a temperature of 100◦

C. Occurance of stick-slip and/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 to accelerate a flywheel. Between the flywheel and a braking device a whole coupling is installed. When the brake is applied a transfer of torque from the flywheel to the brake starts to occur. Both input and output axles decrease in rotation but at different rate so that a differential rotational speed arises. In this case, a maximum torque of 1200 Nm and maximum differential rotational speed of 75 rpm is reached. Each braking cycle takes 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 significant role in the performance of a wet clutch. Therefore we will also study the effect of reducing the additive concentration in the oil. The additive content will be varied from fully formulated to only 10% of the additive package added to the base oil. A complete overview of the lubricants tested can be seen in Table 3.

(24)

3.3 Materials 3 METHOD AND MATERIALS

Table 3: Lubricants tested

Oxidation level (Duration of modified dry-TOST) 48h

96h 192h 360h 408h

Additive level (Percentage of additive package) 100 75 50 25 10

3.3 Materials

The friction pair in the Haldex LSC is sintered bronze and steel, as mentioned in section 1. A main characteristic of sintered materials is the porosity. The pores in the material can act as reservoirs for the lubricant and capillary forces keep the fluid in place [13, 10]. An image of the sintered bronze surface taken with a scanning electron microscope (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 favorable corrosion resistance properties. An application where the bronzes are often used are in boundary lubricated bearings in the form of bushings.

(25)

3 METHOD AND MATERIALS 3.3 Materials

(26)

(27)

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. A complete overview of performed tests can be seen in appendix A.

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. For additive 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 additive package. v [m/s] µ [-] 10% 25% 50% 75% Fully formulated 0 0.5 1 1.5 0.08 0.09 0.1 0.11 0.12 0.13 0.14

Figure 6: Friction characteristics comparison of different additive concentration at 70◦

C

However, when the repeatability of the experiments is taken into account, this difference is not as clear, see Fig. 7. Each figure shows friction data from four separate experiments together with fitted curves. In Figure 7(b) it can be noticed that for an additive level of 25% the friction can vary between friction levels similar to fully formulated to levels similar to 10% of the additive package. The repeatability of the experiments decrease when additive content is reduced. In order to investigate if this could be due to surface effects, two test pieces run at the same additive level but with distinctive differences in friction characteristics were selected for further testing. For

(28)

4.2 Friction characteristics change with oxidation 4 RESULTS v [m/s] µ [-] Fitted friction Friction data 0 0.5 1 1.5 0.08 0.09 0.1 0.11 0.12 0.13 0.14

(a) 10% of additive package

v [m/s] µ [-] Fitted friction Friction data 0 0.5 1 1.5 0.08 0.09 0.1 0.11 0.12 0.13 0.14 (b) 25% of additive package v [m/s] µ [-] Fitted friction Friction data 0 0.5 1 1.5 0.08 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, friction characteristics at 70◦

C for two 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). Results show similar distinction in friction characteristics in both cases, indicating that surface characteristics of the sintered bronze surfaces are responsible for the difference in friction characteristics. However, this only occur for lower additive content.

v [m/s] µ [-] Test piece 1 Test piece 2 0 0.5 1 1.5 0.08 0.09 0.1 0.11 0.12 0.13 0.14

(a) Friction characteristics at 70◦

C v [m/s] µ [-] Test piece 1 Test piece 2 0 0.5 1 1.5 0.08 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 with oxidation. For the 408h and 360h oils the negative slope is significant in comparison to the others. The behavior for these two are also quite similar, which could indicate that the friction rate of change with oxidation is decreasing. In comparison to the friction change with additive content, see Fig. 6, the friction change with oxidation are larger and more severe. In Table 4 the results of the RULER measurements can be seen, note that the antioxidant level for 408h and 360h dry-TOST are less than satisfactory, and here we also see large changes in friction behavior.

(29)

4 RESULTS 4.3 Comparison ageing methods v [m/s] µ [-] 408h 360h 192h 96h 48h Fully formulated 0 0.5 1 1.5 0.08 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 levels Oxidation time Antioxidant level (Duration of modified dry-TOST) 48h 100% 96h -192h Low 360h Very Low 408h Very Low

4.3 Comparison ageing methods

In Figure 10 friction characteristics at 70◦

C for some test oils are displayed. An oil aged in a test rig at Haldex, two dry-TOST oils, one with 10% additives and one fully formulated are compared. It is noticable that the oil aged at Haldex shows a steep positive slope for low speed almost in comparison with the 408h dry-TOST oil. The maximum friction coefficient though is considerably smaller for the Haldex oil and also the negative slope is less severe. For higher speeds both the 96h dry-TOST oil and the 10% additive oil is showing similar behavior as the Haldex oil.

(30)

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 1.5 0.08 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 the 96h dry-TOST oil and the 10% additive oil are exhibiting nearly the same friction behavior as the oil aged at Haldex. However, this is true only for that temperature, 70◦

C. As can be seen in 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 in friction is obvious. In Figure 11(b) it can be noticed that the separation in friction between the different temperatures is less than for the fully formulated oil, the curves are closer together. Tendencies to the same behavior can be seen for the 96h dry-TOST oil, especially for higher speeds, see Fig. 11(c). For the oil aged at Haldex the curves are approaching each other in the same way only for lower speeds. At higher speeds they are not getting closer together, instead they are shifting, see Fig. 11(d). The separation of the friction curves are clear and the friction coefficent, at speeds above about 0.2 m/s, increases with increasing temperature, the opposite of what happens for a fully formulated fresh oil.

(31)

4 RESULTS 4.5 Mean contact potential change v [m/s] µ [-] 30◦ C 70◦ C 100◦ C 0 0.5 1 1.5 0.08 0.09 0.1 0.11 0.12 0.13 0.14

(a) Friction vs. velocity, fully formulated

v [m/s] µ [-] 30◦ C 70◦ C 100◦ C 0 0.5 1 1.5 0.08 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◦ C 70◦ C 100◦ C 0 0.5 1 1.5 0.08 0.09 0.1 0.11 0.12 0.13 0.14

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

v [m/s] µ [-] 30◦ C 70◦ C 100◦ C 0 0.5 1 1.5 0.08 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 speed ramp can be seen. The standard deviation and Total Acid Number (TAN) is also displayed.

The contact potential can be seen to vary very little with additive content even though tendencies to a decreased contact potential with decreasing additive content can be observed. In the case of the dry-TOST oils, the contact potential clearly de-creases with oxidation. Noticeable is that when the maximum friction coefficient is high, e.g. 360h and 408h dry-TOST, the mean contact potential is low. The total acid number shows an increase typical for an ageing oil and the contact potential decreases as TAN increases.

(32)

4.5 Mean contact potential change 4 RESULTS M ea n co n ta ct p o te n ti al [m V ] % of additive package 100◦ C 70◦ C 30◦ C 100 75 50 25 10 0 10 20 30 40 50

Figure 12: Contact potential change with additive content

M ea n co n ta ct p o te n ti al [m V ] Duration of dry-TOST [h] 100◦ C 70◦ C 30◦ C T o ta l ac id n u m b er [m g K O K /g ] TA_N 0 48 96 192 360 408Aged in test 2 2.7 3.4 4.1 4.8 5.5 0 10 20 30 40 50

(33)

4 RESULTS 4.6 Safe working range, R

4.6 Safe working range, R

In Figure 14 the friction coefficient derivate (dµ_dv) versus velocity (v) is plotted. This figure describes how the slope of the friction curve changes with speed. In previous research a negative slope of the friction curve has been proved to be bad since this may introduce phenomena like stick-slip and/or shudder. Knowing this, it is interesting to know at which speed the slope shifts from positive to negative. In Figure 14 a safe working 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 safe working range. If the safe working range exceeds the the working range, stick slip and shudder should be avoided. In Figure 15 the change of the safe working range with

v [m/s] d µ _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 idea of monitoring R in clutch life testing. In Figure 16 the influence of additive content on the safe working range is shown. The safe working range is reduced with additive content except for the case of 75% where the safe working range is high.

(34)

4.6 Safe working range, R 4 RESULTS

Dry-TOST oxidation level [h]

S af e w o rk in g ra n g e, R , [m /s ] 0 48 96 192 360 408 Aged in testrig 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 S af e w o rk in g ra n g e, R , [m /s ] 100 75 50 25 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

(35)

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 testrig the circumstances will not be the same. The lubricant will be sheared in the contact and here temperatures will be high while wear particles formed. The differences in operating conditions could then lead to other reactions being benefited and other reaction products formed. What is interesting though is that there are similarities between reducing additive content, oxidation and testrig ageing, all show a general friction increase. Since some of the additives like friction modifiers reduce friction, it makes sense that reducing them 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 of pores 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 of the sintered bronze surfaces. Variation in surface roughness and material makeup of test pieces could be a factor influencing whether the surface is destroyed or not.

The overall effect of lubricant oxidation is hard to predict since the lubricant has a complex composition of base oil and additives, which can interact in many various ways, as described in 2.3. Additive consumption and reaction products formed may play a significant role for the frictional behavior. However, for the oxidized oils the influence on friction is even bigger than for additive content reduction, and friction change follows oxidation quite well. Here it is important to note that additive content is reduced to 10% at the least, for lower additive content the friction change may reach the 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 for oxidation, levels of contact potential 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 and higher resistance could be due to a number of factors such as hydrodynamic film build up, films formed by non-additives (e.g. reaction products) and/or films formed by additives. Supporting the theory of additive activity is the lubrication regime which based on friction levels would be boundary lubrication. Another way to explain the decreasing contact potential with oxidation is through the total acid number. When the 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 check how suitable a lu-bricant is for a certain speed range of a wet clutch application. If R exceeds the speed range, it is known that in the means of avoiding stick-slip and shudder, the formulation is good. For future research it would also be interesting to monitor this parameter in clutch life testing.

(36)

(37)

6 CONCLUSIONS

6 Conclusions

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

• Lubricant aged in testrig shows significantly different friction characteristics with temperature than lubricants aged by dry-TOST implying that dry-TOST is not a sufficient method to evaluate lubricant ageing.

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

• A safe working range, R, has been derived which can be useful when evaluating a lubricants suitability for a certain speed range. It shows a decrease with both oxidation and additive content.

(38)

(39)

7 FUTURE WORK

7 Future work

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

• Clarify why thermal oxidation of a lubricant differs from ageing of lubricant in testrig. Find out which mechanisms that are responsible for the decomposition of lubricant.

• Investigate which factors or parameters that influence the ageing of a wet clutch the most and in what way.

• Find out how ageing of friction discs and lubricant are linked and if one of them are 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 friction characteristics results be different if the system recently was active compared to if the system had been at rest for a long time?

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

• Develop a definition of wet clutch life and also define the end of clutch life. Further, there is a need to find suitable parameters to survey in order to measure the consumption of life.

(40)

(41)

REFERENCES REFERENCES

References

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

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

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

[4] P. Marklund and R. Larsson. Wet clutch friction characteristics obtained from simplified pin on disc test. Tribology International, 41, issues 9-10, Nordtrib 2006:824–830, 2008. doi: 10.1016/j.triboint.2007.11.014.

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

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

[7] Timothy Newcomb, Mark Sparrow, and Brian Ciupak. Glaze analysis of friction plates. 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 its effects on LVFA fric-tion and the performance of a modulated torque converter clutch. SAE Technical

Papers, (982668), 1998.

[10] Anton van Beek. Advanced engineering design - lifetime perfomance 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 boundary friction of the permeability of sintered bronze. Tribology Letters, pages 1–8, 2008. doi: 10.1007/s11249-008-9330-5.

(42)

(43)

A FRICTION CURVES