Improvement of processing friction facing testing results ... · PDF fileImprovement of processing friction facing testing results & Investigation of wear at clutches Internship at

Improvement of processing friction facing

testing results

&

Investigation of wear at clutches

Internship at ZF Sachs AG, Schweinfurt (Germany)

Master Automotive Engineering Science TU/e

DCT 2007.055

J.N.M. Schnackers 0508895

Tutors:

K. Neumann ZF Sachs AG, Schweinfurt

Dr. P.A.Veenhuizen Eindhoven University of Technology (TU/e)

April 2006 – June 2006

Abstract page 1

Abstract

During this report, it has become clear that developments concerning normal dry-disc clutches

have no stand still. Even in the pressure plate, as in the clutch disc improvements are possible

or things have to change, because of requirements regarding to environment, performance or

life span.

The friction facing is one of the most important parts. It is a sensitive part, which is dependent

of many factors to function well, but also has a large influence on performance and comfort of

the whole clutch assembly and vehicle.

Due to environmental demands, use of some materials is limited and so it is not possible to

get an ideal composition. Therefore, the need for experimentation is high, to create an as good

as possible composition, which can be used for friction facings. This facing can be tested by

real-time vehicle tests and test-rig tests. The most ideal situation is test-rig tests, because on

this way it is possible to test under the same conditions and to get on a fast way data. The

Standard Clutch Friction Test (SCFT) is such a test, which can lead to getting information on

a fast way and tries to get this information on a reliable way with a great link to the reality.

The SCFT is a series of tests, at what every test has a special goal or imitates a special

situation of the reality. These tests are ran at 3 specific temperatures, which correspond to real

working situations.

Till now it was not possible to make a good comparison between the test results of the

different friction facings, by the lack of a way to compare these on a decent way. By

implementing computer software, it is possible to convert the data from the test-rig into

readable data for a computer program like Microsoft Excel, which can express the test results

in better and more understandable graphics.

Important factors like judder, the value and behaviour of the coefficient of friction, can now

determined objectively out of the graphics, which makes it possible to set a ranking list,

which leads to a choice of the best friction facing on the basis of the test results. On the basis

of real-time vehicle tests can be compared if the results of the test-rig are comparable with

those of the real-time vehicle tests. Out of this, there is any similarity, but due to the

subjective mark-system (very good = 10, very bad = 1) of the real-time vehicle test, a perfect

comparison is not possible.

The second part of this report engages of the wear at the dry-disc clutch. Wear off happens on

the one hand at steel-steel contact in the press-on plate, where the parts due to

engaging/disengaging and vibrations move along each other and on the other hand, the wear

happens at the facing of the clutch disc.

By investigating a number of clutches on the amount of wear in combination with the

kilometre performance, possible a connection can appear. With this connection, it could be

possible to give a prediction about the life span or the maximum amount of kilometres, what a

clutch can cope.

Samenvatting page 2

Samenvatting

Gedurende dit verslag is duidelijk geworden, dat ontwikkelingen omtrent de normale droge

plaat koppeling niet stil staan. Zowel in drukplaat als in de koppelingsschijf zijn verbeteringen

mogelijk of moeten dingen aangepast worden om te voldoen aan verschillende eisen van

verscheidene aard, zoals milieu, performance en levensduur.

Het beleg van een koppelingsschijf is één van de belangrijkste onderdelen. Het is een gevoelig

onderdeel, dat afhankelijk is van vele factoren om goed te kunnen functioneren, maar ook

grote invloed heeft op prestatie en comfort van de gehele koppeling en het voertuig.

Door milieueisen is het gebruik van materialen gelimiteerd en kan dus niet een ideale

samenstelling worden geformeerd. Hierdoor is het nodig om veel te experimenteren om zo

goed mogelijke mixen te creëren, die gebruikt kunnen worden als koppelingsbeleg. Deze

belegen worden getest door middel van real-time voertuigtesten en proefstand testen. Het

meest ideale zijn proefstand testen, omdat op deze manier zo goed mogelijk onder gelijke

condities kan worden getest en tevens op een snelle manier gegevens zijn te verkrijgen. De

Standard Clutch Friction Test (SCFT) is zo’n test, waardoor snel gegevens te verkrijgen zijn

en wordt getracht, zoveel mogelijk naar de werkelijkheid te testen. De SCFT is een serie van

testen, waar iedere test een bepaald doel heeft of een situatie uit de werkelijkheid nabootst.

Deze testen worden allemaal op 3 bepaalde temperaturen doorlopen, die ook weer

overeenkomen met reële situaties.

Tot nu toe was het niet mogelijk om een goede vergelijking te maken van de testresultaten

van verschillende belegen, door het ontbreken van een manier om deze op een goede wijze

met elkaar te kunnen vergelijken. Door het implementeren van een tussenprogramma kunnen

de meetresultaten worden ingelezen door het programma Microsoft Excel, waardoor betere

(grafische) weergave mogelijk is.

Belangrijke factoren als judder, de waarde en het gedrag van de wrijvingscoëfficiënt kunnen

nu objectief bepaald worden uit de grafische weergaven, waardoor het mogelijk is een

ranglijst op te stellen, die leidt tot het beste beleg aan de hand van de testresultaten. Aan de

hand van de real-time voertuigtests wordt vergeleken of de testresultaten van de proefstand

overeen komen met die van de real-time voertuigtest. Hieruit blijkt dat er wel enige mate van

overeenkomst is, maar door het subjectieve puntensysteem (heel goed = 10, heel slecht = 1)

van de real-time voertuigtests, geen perfecte vergelijking mogelijk is.

Het tweede deel van dit verslag houdt zich bezig met optreden van verslijt bij de droge plaat

koppeling. Verslijt vindt aan de ene kant plaats bij de staal-staal overbrengingen in de

drukplaat, waar het materiaal door koppelen/ontkoppelen en trillingen langs elkaar beweegt

en aan de andere kant verslijt het beleg van de koppelingsschijf.

Door een aantal koppelingen te onderzoeken op de hoeveelheid verslijt in combinatie met het

aantal gelopen kilometers, kan als er sprake is van een verband, een mogelijke voorspelling

worden gegeven over de levensduur of het maximum aantal kilometers wat een koppeling aan

kan.

Uit dit onderzoek blijkt dat door verschillende factoren het moeilijk is om een daadwerkelijk

verband weer te geven.

Table of contents page 3

Table of contents

Abstract..................................................................................................................................1

Samenvatting..........................................................................................................................2

Table of contents ....................................................................................................................3

Introduction............................................................................................................................4

1 ZF Sachs AG ..................................................................................................................5

2 Theory of a truck-clutch..................................................................................................6

2.1 Different parts of a clutch........................................................................................6

2.2 Geometry and dimensions .......................................................................................8

2.3 Functions ................................................................................................................9

2.4 Design...................................................................................................................10

2.5 New trends............................................................................................................12

3 Coefficient of friction (COF) at clutches, an overview ..................................................13

3.1 Coefficient of friction [1] ......................................................................................13

3.2 Wear rates [1] .......................................................................................................13

3.3 History of friction facings [1]................................................................................13

3.4 Development criteria [1] .......................................................................................14

3.5 Materials [1] .........................................................................................................14

4 General info to the Standard Clutch Friction Test..........................................................16

4.1 The Standard Clutch Friction Test.........................................................................16

5 Problems and phenomenons..........................................................................................21

5.1 Judder ...................................................................................................................21

5.2 Friction coefficient................................................................................................24

5.3 Geometry change (Shielding) ................................................................................24

5.4 Value of the friction coefficient.............................................................................24

5.5 Facing surface.......................................................................................................25

6 Evaluation SCFT ..........................................................................................................26

7 Clutch torque at constant releaser position, the AMT-test..............................................30

8 Results SCFT and AMT-test .........................................................................................32

9 Validation SCFT and AMT-test ....................................................................................37

10 Inspection of clutches ..................................................................................................38

11 Schematic overview of wear-off places ........................................................................40

11.1 Schematic overview of wear-off places at a clutch cover assembly........................40

11.2 Wear profile ..........................................................................................................41

11.3 Methods of measuring...........................................................................................42

11.4 Results ..................................................................................................................43

11.5 Wear off at clutch discs.........................................................................................47

11.6 Improvements .......................................................................................................48

Conclusions/Recommendations ............................................................................................49

References............................................................................................................................50

Acknowledgements ..............................................................................................................51

Introduction page 4

Introduction

ZF Sachs AG in Schweinfurt, Germany is an industrial company in the automotive parts

sector. The two main products are clutch systems and suspension systems. During my training

period at the Research Centrum of ZF Sachs AG, dry-disc clutches for commercial vehicles,

like trucks, busses and transporters were subjects of interest. Although a clutch in today’s

setup is still developed since more than 20 years, research and development at clutches will be

of big interest. Criteria like performance and comfort are during the years changed in level of

importance. Commercial vehicles are grown. More load in less time is the demand of

transport-companies and with as less as possible maintenance. The result of this is demand on

higher torques and products with a lifespan of 1 million to 1.5 million kilometres.

The criteria comfort is close to the criteria performance and is of increasing importance

during the last years. No driver wants to drive off, with a shaking vehicle or loud vehicle

noise.

Another cause for development is increasing measures to decrease the load on the

environment. Today’s engines are of the Euro 4 type. Difference with the Euro 3 engines is

the more aggressive torque build-up, caused by the new techniques to decrease NOx and CO

pollution. This asks for better performance of friction facings, especially for the behaviour of

the coefficient of friction. Also the use of some polluting and health treating materials like

asbestos, lead and some solvents are forbidden in the course of time or will be forbidden

(lead). Due to a European law the lead content in passenger car facings had to be eliminated

in July 2003. Truck facings do not have this necessity yet. Despite this all truck manufacturers

plan to replace today’s materials containing lead. Therefore the former properties of lead

containing materials were the benchmark for future lead free friction materials

To this case, it is important to test new developments and materials in friction facings, to

investigate behaviour and value of the coefficient of friction and the related clutch torque.

This can be tested in different ways. One way is to build the parts in a truck and drive a

prescribed route. Advantages are the lifelike test conditions, but disadvantages are subjective

opinions of test-drivers, different drive behaviour of test-drivers and the long test-time.

Another way to test friction facings is on a test-rig. ZF Sachs AG has different test-rigs and

developed a test program “Standard Clutch Friction Test” (SCFT). While this test-program is

in a developing phase, with questions about comparability and rating, the next investigation is

done:

Build a Microsoft-Excel file to display the results of the SCFT, to compare the results of

different facings and to interpret the results. Collect data of previous tests and test more

friction facings to build a database. Set criteria to rate facings and compare the results to the

results of the test-vehicles.

A second subject, what is investigated during the training period is life-span measurements at

build-out parts of trucks. These parts come from test-vehicles or transport companies.

Measurements of performance to measure clamp load, release load and lift are done, but also

on wear off at hinge parts of cover assemblies. The problem definition is:

Find a possible relation between walked kilometres and wear off values to predict a lifespan.

This report starts with an overview of the company ZF Sachs AG. In chapter 2, the dry-plate

clutch will be discussed, to give a general overview about this subject.

In chapters 3 to 9, the Standard Clutch Friction Test will be discussed, with some general

information, an explanation about the SCFT, evaluation, results and validation.

In chapters 10 and 11, the second subject about wear will be treated.

Chapter 1 ZF Sachs AG page 5

1 ZF Sachs AG

For more than 100 years, ZF Sachs AG, the power train and suspension division of the ZF

group has been a globally active components and system supplier as well as a development

partner for the automotive industry.[5]

In 1895 Ernst Sachs and Karl Fichtel established “Schweinfurter Präzisions-Kugellagerwerke

Fichtel & Sachs”. Till 1930 they were specialised in roll bearings and bicycle components. In

1930 they sold the bearing division (today known as the company SKF) and commencements

their activities in automotive motors, clutches and shock absorbers.

After World War 2, the company has growth during the time with establishment and

acquisition of several companies in Germany and abroad.

At 1997 Fichtel & Sachs is renamed Mannesmann Sachs AG, but this was for a short time,

because in 2000 the Mannesmann group broke up. In 2001, ZF Friedrichshafen acquires

100% of the Mannesmann Sachs AG shares and the company is renamed in ZF Sachs AG.

Worldwide, ZF Sachs AG has 23 production companies distributed about the whole world

with approximately 15 500 employees.

Figure 1-1 Overview of stand places of ZF Sachs AG

The main products are clutch systems for passenger cars and commercial vehicles, torque

converters, automated gear systems, electric drives and suspension systems for car,

commercial vehicles and train applications.

The total sales of ZF Sachs in 2005 were 2,172 Mio. Euro. Most of the sales are earned in

Europe (68 %) and the main application was cars with 74%.

Customers are all large car and commercial vehicle producers.

Car producers are: Volkswagen, General Motors, DaimlerChrysler, Ford, BMW, but also

Asian producers like Toyota, Honda en Hyundai.

In the commercial vehicle market nearly all producers are customer, like DaimlerChrysler,

MAN, Scania, Paccar, Volvo, Fiat and a lot of smaller producers.

Chapter 2 Theory of a truck-clutch page 6

2 Theory of a truck-clutch

A clutch is one of the most important contents of a vehicle driveline. It is the connection

between engine and power train. Torque and speed transmission is the main task of the clutch,

on a way that performance but also comfort is on an acceptable level.

In applications like trucks, busses, transporters or special purpose vehicles this main task is

more difficult, because higher torques has to be transmitted. Therefore, the dimensions of a

truck clutch are larger, also to deal with the higher thermic energy. Higher clamp loads and

clamp areas are of importance to transmit the desired torque. Comfort is of less importance

than at cars, but still plays an important roll.

Before to start with the targets, a little information about the clutch, like function, geometry

and dimension, design, parts is helpful in understanding.

Figure 2-1 The commercial vehicle clutch [2]

2.1 Different parts of a clutch

The two main parts of a clutch are the clutch disc and the cover assembly.

The cover assembly (figure 2-2) consists of different parts, like the pressure plate, the

diaphragm spring and the cover assembly (figure 2-3).

The function of the pressure plate is to press the clutch disc against the flywheel. The

diaphragm spring is responsible for the clamp load and the cover assembly is the connection

between all parts. The stamped housing is connected with the pressure plate by tangential

straps (figure 2-7) and is mounted on the flywheel. The diaphragm spring is clamped between

the pressure plate and the stamped housing.

Figure 2-2 Different parts of a cover assembly [2]

Pressure plate

Diaphragm spring

Stamped housing


Figure 2-3 Different parts of a cover assembly disassembled

The most important parts of the clutch disc (figure 2-4) are the friction facing, the facing

damping and the torsion damper. The damping and cushioning parts in the clutch disc are for

more driving comfort and better performance of the clutch assembly. The cushioning is for

more comfort at engaging the clutch due to a certain dose range. Also thermal load can be

compensated and axial resonance of the flywheel or input shaft.

The torsion dampers are to decreasing the tangential flywheel and input shaft resonance due

to engine-vibrations caused by the combustion. To decrease these vibrations, weak damper-

springs are used, to get the eigen-frequency beneath idle-speed. This is preferable because this

speed is never reached during working. This will prevent take-off rattle and chassis-noise

during driving at certain motor speeds. Take-off rattle is shaking of the gearwheels at a certain

resonance-speed in the gearbox, which are not engaged. Vibrations at the chassis cause

chassis-noise.

The formula to calculate the eigen-frequency is m

c=ω . (Formula 1)

With ω = eigen-frequency

c = stiffness

m = mass

More weight is not preferable, because then also inertia rises. With weak springs, the stiffness

decreases and so the eigen-frequency decreases. Limiting factors to build in the springs are

strength of the clutch disc (enough space between the springs in tangential direction) and

diameter (space in axial direction).

The small pre-damper springs are for more comfort and a better behaviour during idle-speed

and to prevent the sound of idle-rattle.


The facing is the part, which is responsible for the torque transmission. It is a mixture of

carrier thread from glass and synthetics and a brass armature [6]. Till approximately 1990,

facings contained the material asbestos, but because of health reason this material is

forbidden. At this time, facing producers are trying to produce facings without heavy metals

(Pb, Cd, Hg, Cr-Vl) and toxic solvents because these will be forbidden in near future [6].

More about materials of friction facings and other specifications in chapter 3.

Figure 2-4 Schematic overview of a clutch disc [6]

2.2 Geometry and dimensions

Truck clutches are instead car clutches big and heavy. The general pressure plate has an outer

diameter of 433 mm and an inner diameter of 235 mm. The whole cover assembly weights

about 35 to 40 kg with a mass moment of inertia of 1.130 kgm2. The reason for the large outer

diameter is the demand on clamp area to transmit the torque. The inner diameter is not a free

chosen parameter. Also in the connection between outer diameter and inner diameter is an

optimum, to get an ideal clamp radius. A very large difference between outer and inner

diameter is not ideal, because then high thermic loads will accrues on the inner diameter.

Effect is shielding (paragraph 5.3) and the position of the clamp radius moves to the outer

diameter, which gives a negative effect on wear off and performance. A small difference

between outer and inner diameter is also not preferable, because then the clamp area is not

large enough to transmit the torque or to deal with the thermic load On the other side design,

mass and inertia criteria demand for limiting the dimensions.


Figure 2-5 Pressure plate surface

2.3 Functions

A clutch in the complete power train has 4 different functions: torque transmission, a smooth

drive on, giving the possibility to switch and torsional vibrations damping.

The basic function is transfer the torque delivered by the engine to the transmission. To

transmit the torque, the parameters coefficient of friction, clamp radius, clamp load and

number of surfaces are of influence. The clutch disc is responsible to deliver sufficient

coefficient of friction. The clutch disc is the medium between the flywheel and cover

assembly.

Clamp radius and clamp load are parameters, which are set by the cover assembly. The

strength of the diaphragm spring is the most important parameter, to set the clamp load. The

shape and the strength (material parameters) of the diaphragm fingers are parameters, which

can influence the clamp load.

Clamp radius is a more difficult issue to influence. Default, this is the radius to split the

clamp-surface into two equal parts (figure 2-5).

The formula to calculate this value is ( )( )22

33

3

2

io

io

mRR

RRR

−

−= . (Formula 2)

With:

Rm = Clamp radius

Ro = Outside radius

Ri = Inside radius

Due to temperature influences, the clamp radius can possibly change. Through this

temperature influence, the pressure plate deforms. The geometry of the clamp-area changes

due to shielding, which has influence on the position of Rm. More about this subject in

paragraph 5.3

A 1-disc clutch transmits torque by two facing-surfaces; one half from flywheel to left facing

surface and one half from pressure plate to the right facing surface (figure 2-6).

A 2-disc clutch transmits the clutch torque by 4 facing surfaces; one quarter from flywheel to

the left facing from the flywheel-side disc, one quarter from the intermediate plate to the right

facing from the flywheel-side disc, one quarter from the intermediate plate to the left facing of

the clutch-side disc and one quarter from the pressure plate to the right facing of the clutch

side disc.

O.D

I.D


Figure 2-6 Pull type clutch [2]

Other functions of a clutch are the behaviour to drive on and give the possibility to switch.

Goal is a smooth drive off, with no shocks. To get this behaviour, parameters like releaser

load and the lift of the tangential preload straps are important. The releaser load to disengage

has not be higher than a described certain limit.

The tangential straps (figure 2-7) are connected between the press-on plate and the clutch

cover. The main function is to pull the pressure plate from the clutch disc away at

disengaging, so that there is no connection anymore between pressure plate and clutch disc.

When the lift of the preload tangential straps is under a certain level, this can lead to problems

with disengaging.

Figure 2-7 Tangential pre-load straps

To damp out tangential vibrations, a torsion damper is build into the clutch-disc. This will

smooth engine vibrations to a more comfortable level.

2.4 Design

Clutches are in to 2 different designs, a pull-type clutch (figure 2-6) and a push-type clutch

(figure 2-9). The mean difference between these two types is the direction of releasing. At a

pull-type clutch, the releaser moves to disengage out of the clutch and a push-type clutch

moves into the clutch. The advantage of the push-type clutch is the limited space. A push-type

clutch uses less space than a pull-type clutch. Also a fixed connection between releaser and

diaphragm-spring is not necessary.

Disadvantage of this limited space are higher loads to release, while the lever-function of the

diaphragm-spring is less large (figure 2-8).


Most preferable are the pull-type clutches, because the releaser load is less large comparing to

the push-type clutch (figure 2-9). Disadvantages are the large mounting space, because the

releaser moves outside and the mounting of the declutching fork takes more room.

Also a pull-fixed connection between releaser and diaphragm-spring is necessary. This is

more difficult at dismounting and is more expensive to produce.

If due to design-criteria, not much space is available, often is chosen for a push-type clutch

design. Otherwise mostly is chosen for a pull-type clutch.

Figure 2-9 Push-type clutch [2]

The loads, produced by the diaphragm spring, are relative high. An average clamp load lies

between 34800 N and 37900 N. This high clamp load is to ensure that the clutch in normal

circumstances always can deliver the asked torque. The clutch can accept even more torque

than the motor torque for safety reasons.

Average release loads are maximum 7000 N. While 7000 N is too much to release only by

press down the pedal, there is a servo-system for an external helping load (figure 2-10)

A

B

A

B

Push-type Pull-type

Fr Fr

Fs

Fs

Fr: Release load

Fs: Spring load

A,B: Length of lever

Figure 2-8 Different lever length between a push-type and a pull-type


Figure 2-10 Servo-actuation [2]

2.5 New trends

The world of the clutch is still in full development. New techniques for more and more

efficient performance of the clutch are today an issue.

In future, trucks will be grown to road trains, like trucks in Australia. The expectation is that

there will be truck combinations of about 60 tons, about approximately 5 years. This demands

bigger truck engines and as result higher torques. Higher torques means that today’s

generation clutches must be adapted. A simple solution is to enlarge the clutch. But then

parameters like mass and moment of inertia also increase, which gearbox-manufactures don’t

like.

Solutions to transmit the clutch torque have to be found in better friction facings or multi-disc

clutches.

A new development to improve the clamp load behaviour during the clutch life is the cover

assembly with automatic wear compensation. With wear-off of the clutch disc, the pressure

plate moves to the left and also the position of the diaphragm spring changes like displayed at

figure 2-11.

Negative side effect of this change of position is the lost of clamp load. The automatic wear

compensation controls the position of the diaphragm spring, through a mechanical system

with an adjustment ring, which compensates the position on the outside of the diaphragm

spring.

Figure 2-11 Automatic wear compensation

Chapter 3 Coefficient of friction (COF) at clutches, an overview page 13

3 Coefficient of friction (COF) at clutches, an overview

In this chapter, an overview is given about used materials, wear off and developments of

friction facings and coefficient of friction. It will serve as a theoretic background to give more

understanding for the coming chapters with as subject friction facing testing

3.1 Coefficient of friction [1]

In trying to determine the µ of a particular friction material one must consider not only an

absolute characteristic number that can be applied to any new design, but also one, which is a

function of many factors:

- The composition and type of manufactured form of the friction material and opposing

plate (“counter-surface”)

- The temperature, speed and pressure at which µ is determined

- Environmental contamination and prior usage history of the friction application.

There are two types of µ, dynamic (µd) and static (µs), which affect the smoothness of

engagement in the transition from sliding to lock-up of the surfaces. They have traditionally

been comparatively referred to as the µd to µs ratio.

3.2 Wear rates [1]

Wear rates are dependent on friction material composition, method of material manufacture,

characteristics of counter-surface as well as energy, power, heat absorbed in the application

and the presence of contamination.

Wear rate is defined as the absorbed energy per volume wear of the facing:

Wear Rate = E/volume facing wear

3.3 History of friction facings [1]

Through history, developments in friction and development of overall clutch design went

hand in hand.

Early model T Ford clutches had woven cotton-lined bands, which operated one for each gear.

Only those bands delivered sufficient µ. Disadvantages was the rapid decrease of µ and

increase of wear of the cotton, which compromised vehicle performance and durability.

With the evolution of molding technologies and their ability to form compounds from

particles and fibers, friction faced discs became popular. This in turn inspired the

development of plate clutches. The first clutch facing materials consisted basically of a small

variety of ingredients like cotton and/or asbestos for a fibrous reinforcement and saw dust,

iron, cork, leather and graphite as additional ingredients with resin and/or linseed oil as binder

material.

In the early 1900s, also materials as asphaltic petroleum residues and rubberized coatings

appeared. Later years found different rubber, resin and filler systems with metallic wires

being used.

With the grew of engine sizes and increasing temperature requirements, asbestos fibre became

the mainstay of the friction material industry.

The use of asbestos revolutionized the clutch industry. It was a cost-effective material that

could be easily and variable processed. Asbestos produced compositions over µ=0.3, could

operate over 200ºC and caused less counter-surface wear than competing molybdenum/iron-


oxide mixtures. Asbestos compositions continued to dominate the industry till environmental

concerns in recent years began to motivate changes.

By the beginning of World War 2, molded sintered bronze friction materials combined

copper, iron-oxides, tin and other materials into higher-energy compounds for use in military

tank clutches. Following the war, sintered bronze gained increased usage in commercial

vehicles [1]. After World War 2, with the increase of the vehicle market, also the study to find

better friction facings was of increasing interest.

3.4 Development criteria [1]

The criteria in that time to develop new friction materials were:

1. Meet specific higher performance criteria

2. Minimize costs

3. Replace asbestos

Instead of a more scientific approach, repetitive trial and error formulating has been used.

This trend increased the industry’s dependence on dynamometer and vehicle testing to predict

properties of friction ingredients. This necessary reversal of classical scientific approach

results in common use of many different ingredients in a given friction composition which

have unclear functionality.

To achieve an acceptable performing clutch facing, a minimum of seven characteristics need

to be achieved:

1. Desired coefficient of friction which minimizes fading

2. Excellent durability

3. Satisfactory engagement quality

4. High centrifugal burst strength

5. Minimal rotating mass

6. Process ability of materials

7. Performance / cost effectiveness

The coefficient of friction varies with temperature, speed, pressure, length of engagement,

presence of wear particles, contamination, condition of mating surface, airflow in the

assembly and other parameters. So it is quite hard to develop a product, which can deal with

al these circumstances, to get a stable product.

3.5 Materials [1]

In formulating a clutch facing composition, most ingredients would fall into one or more of

several categories:

- Fibrous reinforcements or strength contributors

- Matrix resin binders

- Possibly curing agents lubricators

- Friction enhancers (friction modifiers)

- Heat sinks and/or heat dissipaters

- Mating member conditioners

Manufacturing methods often dictate which material components are used and limit those,

which can be used, because methods of preparing compositions and facings, even the physical

form of the ingredient, often have a dramatic impact on the clutch performance.


Today used materials are:

Graphite particles (inexpensive, increase wear rate, increase heat resistance)

Sintered/Ceramic Metals (high power and high power absorbing capacity, heat resistance)

Fibrous Reinforcements:

Fibreglass (heat resistance and processability)

Acrylics

Rayons

Aramids

Para-aramid (Kevlar, Twaron, extremely high tensile strength, moderate temperature

resistance)

Mineral fibres

Cotton fibres (good COF and non-abrasiveness at low and medium temperatures, low

cost)

Ceramic fibres

Carbon fibres

Metal fibres (heat resistance and conductivity of heat from the friction surface, high COF

at high temperature and load pressures, Copper, Aluminum)

Matrix Resin Binders: (adhesive interface with other materials)

Phenolic resins

Styrene butadiene

Nitrile rubber systems

Fillers: (particles that add cost-effective bulk to the friction materials

Clays

Carbon black

Wood particles

Iron oxide

Friction Modifiers:

Teflon

Molybdenum disulfide

Lead

Chapter 4 General info to the Standard Clutch Friction Test page 16

4 General info to the Standard Clutch Friction Test

Testing of clutch facings at most clutch and facing manufacturers is performed at a

standardized test procedure called BPS 97 (Belag-Prüf-Spezifikation, estabished 1997). All

tests concerning friction, wear, other functions (deformation, burst, rust etc.) are done at a 200

mm passenger car clutch. Due to possible influences of the manufacturing process and

different heat dissipation in the 430 mm truck clutch a special verification test for COF under

various conditions has to be developed to pre select facing materials especially for automated

manual transmissions (AMT).

This special verification test, the SCFT, is a test-cycle to inquire friction facings on

parameters like clutch torque and friction coefficient on different truthful situations. The

results-database “Standard Clutch Friction Test” is a database, which can display the results

of the different tests, ran at the Standard Clutch Friction Test (SCFT). More information about

this can be found in the paragraph “SCFT”. Also a theoretical background behind the SCFT

can be found in the next paragraphs.

4.1 The Standard Clutch Friction Test The SCFT consists of a cycle of 6 tests and will be run at three different temperatures: 20ºC,

100ºC and 180ºC.

This will be replayed at the second day to see the influence of the wear off during the first test

day. More about this subject in paragraph 5.5 about “friction surface”

The temperatures are not chosen arbitrarily. Each temperature represents a certain working

range.

The first temperature is 20º C, a cold temperature at the start of working. The second

temperature is 100º C, the normal work temperature during driving. Finally the 3rd

temperature is 180º C, a temperature level reached after a lot of manoeuvring of the vehicle.

At each temperature the behaviour of friction coefficient and clutch torque can be observed.

The cycle of 6 tests consists of:

1. Tear up new

2. Torque-build-up

3. Judder-sinus

4. Drive-off simulation

5. µ-Check

6. Tear up end

The SCFT runs on the K-DK-17 test-rig (figure 4-2). The test-rig consists of 2 electric motors,

which represent respectively truck engine and truck inertia. The output is connected to the

clutch disc and the clutch cover and flywheel to the input. For engaging and disengaging, the

cover assembly is connected to a releaser, which is controlled hydraulic by the test-rig. A

schematic overview of the test-rig K-DK-17 is to see on figure 4-1. Both motors can be

controlled on speed and torque. Maximum torque of this test-rig is 2700 Nm. To stay in the

range of possible coefficients of friction, a light cover assembly is used with a clamp load of

20500 N

With these test-conditions, the SCFT can be run without reaching this maximum torque.


At the “tear up” test, the clutch torque increases, with fully engaged releaser, till the clutch

starts to slip. The input speed is kept zero, and the output-speed is influenced by the torque-

build-up. At sufficient torque, the clutch starts to slip and the output speed rises. The

maximum output speed is controlled on –100 rpm. During this time, the clutch torque is in

balance. At t=11s, the releaser goes back to starting position. The start of the measurement of

clutch torque is at starting slip-speed and with this torque, the friction coefficient µ can be

calculated with help of formula 3 (paragraph 5.2). In this example (figure 4-3), there is less

difference between static and dynamic coefficient of friction. At start of slip, mostly a small

peak appears in the torque, because of a higher static friction coefficient.

This test will be run at the start and end of a cycle. This is to observe the influence of the

separate tests and the raising temperature during the test (increasing contact area and

increasing thermic energy).

INPUT

Motor 1

OUTPUT

Motor 2

Motor 2

Motor 1

Installation room

Figure 4-1 Schematic overview test-rig

Figure 4-2 Test-rig K-DK-17 [2]


Figure 4-3 Tear-up test

At the “torque build-up” test, the releaser goes with constant speed from 0% to 95 % releaser

position and back again till 0% in a time period of 10 seconds. The clutch torque is measured

and with formula 3 the friction coefficient µ can be calculated. At approximately 70%

releaser travel, the clutch fully engages. In this example falls the torque down after engaging

due to reactions on the contact surface. More about this subject in the paragraph “Contact

area”. Theoretical good is a constant behaviour.

Figure 4-4 Torque-build up test

At the “judder-sinus” test, an input-speed sinus runs at constant releaser position between 630

rpm and 30 rpm amplitude for a time of 20 seconds. The output speed is kept constant on

zero. A vibration of 0.25 Hz is simulated in this way, which is the approximately the same as

resonance frequency of the drive train. The amplitude of this frequency is on this moment still

in a comfortable level,


Through behaviour of the coefficient of friction, this frequency can become unstable or stable,

which cause respectively a larger amplitude level (unwanted) or damp out to a lower

amplitude level (wanted).

A preferable behaviour is when the clutch torque moves in the same phase as the input-speed

sinus. The friction coefficient behaviour is out-damping, like in figure 4-5

An unwanted behaviour is when the clutch torque moves in contra phase. The friction

coefficient behaviour is constructive

More about this subject in the in paragraph 5.1 “Judder”

Figure 4-5 Judder-sinus

At the “drive-off simulation”, the output-speed is rising till synchronisation with the input

speed has happen. Again a slip speed is running at the test-rig but this time constant over a

time of 2.5 seconds, the normal time of synchronisation in real-time. During this rising the

clutch torque is measured which is required to bring the output-speed to the synchronisation

point.

Figure 4-6 Drive-off simulation


At the µ-Check, a step in the releaser position is taken from 0% till 95% releaser position.

This position is kept for 2 seconds and afterwards it will be go back to 0% releaser position.

Purpose is the reaction of the torque during a sudden change of the releaser position.

Observed is the friction coefficient during these 2 seconds.

Figure 4-7 Mue-check

An extra test will be running at the end of the second day after a cooling period back to 20-40º

C. The clutch torque is measured at a start level of 250 Nm, 500 Nm and 1000 Nm at constant

releaser position during a time of 10 seconds. More information can be found in chapter 7

about this “AMT-Test”.

Chapter 5 Problems and phenomenons page 21

5 Problems and phenomenons

Like described in the introducing chapter about coefficient of friction, designing a perfect

friction facing is almost impossible due to the large number of possible influences, which can

change the behaviour of a facing. Although, it is possible to decrease this influencing factors

to a minimum, if the clutch disc is used on a proper way. In the following chapter, some

problems and phenomenons are described, which influences the coefficient of friction and the

working of the clutch.

5.1 Judder

The relationship between coefficient of friction and slip-speed is an important issue. It is the

basic working principle during the engaging process of a clutch. In figure 5-1, this principle is

drawn in a chart. The red line is a constant motor speed and the green line is the power train

speed, which is build up till it matches with the motor speed.

At certain motor speeds, unwanted vibrations can occur, due to engine behaviour and

resonances in other parts. This can also influence the drive train, which transmit this

behaviour to the clutch. These vibrations can be compared with a sort of stick-slip behaviour

of the clutch plate. During the process of synchronising speeds, it can be displayed, like if the

speed-transfer from 0% till 100% motor speed is not a nice curve but a trend-sinus (blue line).

This behaviour is called judder.

In normal situation, the amplitude of the vibration is too low, to feel anything at the driver

seat. But the behaviour of the friction facing plays an important part in the amplitude level of

the judder sinus.

When the amplitude is raised, it is felled in the truck cabin or in a bus like an unpleasant

shaking at drive on, which is uncomfortable.

Figure 5-1 Drive train vibrations


Preferable is that the amplitude of the sinus is not large and is out-damping.

To make this happen, a special behaviour of the friction coefficient is asked (figure 5-4).

When the amplitude of the sinus is positive, ∆N (∆ speed) is relative small and the clutch disc

and cover assembly interconnects for a small time. At this point of slip-speed a low

coefficient of friction is asked to reduce the interconnection between the parts with result that

the amplitude falls down.

If at that point the coefficient of friction is relative high, the interconnection stays and pulls

the amplitude high, which causes a higher amplitude and negative behaviour.

When the amplitude of the sinus is negative, ∆N is relative large and the clutch is slipping. At

this level of slip-speed, the coefficient of friction must now be relative large to enlarge the

interconnection between clutch disc and cover assembly and to pull the amplitude to neutral

behaviour.

If at that point the coefficient of friction is relative low, the clutch stayed slipping and pulls

the amplitude down, which causes a higher amplitude and negative behaviour.

The friction coefficient has out-damping slip behaviour (figure 5.2), if the friction coefficient

increases with the slip-speed. This behaviour of the friction coefficient is preferable.

Figure 5-2 Out-damping vibrations

If the friction coefficient decreases with the slip speed, the behaviour of the friction

coefficient is worse. The amplitude of the judder-sinus rises during the time. The coefficient

of friction causes a resonance. (figure 5-3).


Figure 5-3 Unstable situation

To see the difference between an out-damping behaviour (∇ (µ) = positive) and initiating

behaviour (∇ (µ) = negative), this is reflected in figure 5-4. A neutral behaviour is showed in

figure 5-1.

In the SCFT result database, the results of the judder-sinus test are expressed in the way of

figure 5-4. So it is easy to see if a facing delivers a positive, neutral or negative behaviour.

Most of the time, the negative behaviour happens at higher temperatures. At higher

temperature, most friction facings have a more aggressive behaviour with higher coefficients

of friction.

New facings with a relative bad contact area are most of the time insensible for judder. They

have relative low coefficients of friction and with the bad contact area less power to

interconnect.

Figure 5-4 Schematic overview of µ-behaviour


5.2 Friction coefficient

To calculate the friction coefficient with help of the clutch torque, the next formula can be

used:

mm

mm

RFz

T

RFzT

⋅⋅=→

⋅⋅⋅=

µ

µ

(Formula 3)

with:

T: clutch torque

z: number of friction areas (2 for single-plate clutch)

Fm: clamp load

Rm: clamp radius

5.3 Geometry change (Shielding)

Most of the time mm RFz ⋅⋅ is constant, but through temperature influence, the geometry of

the clutch plate and pressure plate can be changed. Through this the value of Rm will be

different. In the next figure, a clutch plate is shown, which is getting an angle. The Rm isn’t

correct anymore.

To the SCFT, the value mm RFz ⋅⋅ is kept constant.

5.4 Value of the friction coefficient.

The value of the friction coefficient is important. High friction coefficients are in general

good, while then also high clutch torques are possible, or a lower clamp load. But also an

aggressive matching of the clutch is possible.

Maximum friction coefficient at the SCFT is µ = 0.38, because the maximum torque of the K-

DK-17 measure machine is 2700 Nm ( at Fm = 20550 N, Rm = 0.172 m, z = 2)

38.0172.0205502

2700=

⋅⋅=

⋅⋅=

mm RFz

Tµ

Rm

Figure 5-5 Shielding


The minimum accepted friction coefficient is µ = 0.23. The standard value for µ is µ = 0.3.

With a safety factor of 1.3, this minimum friction coefficient can be calculated: 23.03.1

3.0=

If the friction coefficient is to low, to transfer the required clutch torque, the clamp load can

be increased. But this can be done limited, because the clutch temperature will be raised by

this action, which will decrease the lifetime of the clutch.

To the SCFT, the value mm RFz ⋅⋅ is kept constant, so: T~µ.

5.5 Facing surface

When the surface of a facing is observed (micro-level) (black line), like schematic is showed

in figure 5-6, it is to see that this is not really even. This has influence on the contact with the

pressure plate. With a new clutch disc, the contact of the pressure plate to the clutch disc is

only with the top parts (blue line). After some performance of the clutch disc, the facing has

worn off a little bit. Through this the contact-surface has changed into a larger area (red line).

Result of this larger contact-area is a larger clutch torque transmission.

This is the reason, why the SCFT, is replayed on the second day to observe the changes to this

contact-area changes with regard to friction coefficient and clutch torque transmission.

Figure 5-6 Change of the contact area

In the SCFT, it is clear to see the influence of the wear off. At the tear-up test at the first day

at the lowest temperature, the coefficient of friction is on a near level. During the tests on the

first day en the temperature raise, the coefficient raises to a constant level. On the second day

at lowest temperature, the tear-up test still shows a bad behaviour. The worn off particles of

the first day, are fell into the spaces between the top-parts. This causes a slippery area. After

some heat input during the second day and further wear off of the facing the coefficient goes

again to a certain constant level.

Chapter 7 Clutch torque at constant releaser position, the AMT-test page 26

6 Evaluation SCFT

To evaluate the results of the SCFT, the original data from the test-rig K-DK-17 is processed

from MES-files into DAT-file, with help of TurboLab, a Microsoft based program to view,

evaluate and documentate test results. These DAT-files are processed in TurboLab with a

TurboLab-script into 3 TXT-files, which are suitable to import into Microsoft Excel.

The names of the TXT-files are: [3]

-“Reibwertauswertung.txt” (coefficient of friction evaluation)

Contains the results of the cycle-tests “tear-up”, “torque build-up” and “µ-check”. The time of

the evaluation range from every cycle will be saved. From the evaluation range, an average

temperature value and a path of the COF during time ( µ=f(t) ) with 10 markered curves is

saved.

-“Rupfsinusauswertung.txt” (judder-sinus evaluation)

Contains the results of the cycle-test “judder-sinus”. The time of the evaluation range from

every cycle will be saved. From the evaluation range, an average temperature value and a path

of the clutch-torque during slip-speed ( Tc=f(nslip) ) with 100 markered curves is saved.

- “KonstPositionauswertung” (constant position evaluation)

Contains the results of the cycle-test “Drive-off simulation”. The time of the evaluation range

from every cycle will be saved. From the evaluation range, an average temperature value and

a path of the clutch-torque during time ( Tc=f(t) ) with 20 markered curves is saved.

Filtering of the measured data.

While it is the goal to know the path of the result-cycles, filtering of the specific signals is

necessary before calculating. The signals are filtered with a low-pass filter with a frequency-

ratio of filter-frequency to scanning rate from 1:50. The measuring data is measured with a

125 Hz scanning-rate. “Eck-frequenz” from the filter is 2.5 Hz.

These 3 TXT-files can be imported into Excel. A special Excel file is prepared, which

includes macro’s to copy the data out of the TXT-files in Excel direct on the right place. With

this it is ensured, that always all data are processed in the same way.

Also some calculation steps to calculate the time-step per markered curve, is done by a

Microsoft Visual Basic script developed at ZF Sachs AG.

After data-input and processing it in the wished form, it is possible to make charts of every

test in the SCFT-cycle. On the next pages, all charts are displayed and the possible features

like start- and end-point are described. The fat line in the figures is the line, which is

expressed in the Excel charts.


At the tear-up test, COF is displayed against time. Input-speed is kept zero and output-torque

is rising. When the output-torque is so high, that slip occurs, the measurement is started at a

slip-speed of 10 rpm. The measurement ends at slip-speed 90 rpm after the releaser goes back

to his origin at t = 11s (test procedure rule). The 4 to 5 seconds measured time is processed in

a chart and a clear overview of all tear-up tests at 20, 100 and 180°C at the two days are

displayed. Most of the times the results at lowest temperature are those with the lowest COF.

This is because of the limited contact area at new facings at the first day and at the second day

due to the dirt of the wear off during the first day. After the last test of the first day, these

particles cool down and are causing a sort of a lubricating layer, which is decreasing the COF.

Because the tear-up test is the first test during the cycle, the influence of these dirt particles is

the most. After some performance these particles are disappeared.

The slope in the first second can say something about the difference between the static and the

dynamic COF. This is important, because it is not preferred that the dynamic COF falls down

after the start of slipping, which means a decreasing transferred torque. Preferred is a neutral

or increasing slope.

© Z

F S

ach

s A

G

Test conditions:

- Input speed (cover assembly) 0 rpm- Output speed (clutch disc) 0 - -100 rpm

- Clutch torque (constant releaser position)

0 Nm - 2700 Nm- Temperature 20/100/180ºC

- Duration 10 s

COF Static/Dynamic)Trigger point start: slip-speed >10 rpm

Trigger point end: slip-speed <90 rpm

Figure 6-1 Evaluation tear-up test

At the torque-build-up test, COF is displayed against time. The start trigger point is the

release load below 200 N and end trigger point above 200 N at controlled releaser way.

During these 2 to 3 seconds the COF is measured. At the 3 temperatures, the level of the COF

has less difference and also between the two days is less difference.

Important is the slope during the time. Increasing COF means at increasing releaser position,

positive behaviour.

The decreasing COF at decreasing releaser position is most of time faster decreasing than

increasing in the first stage due to the heat input during the test.


© Z

F S

ach

s A

G

Test conditions:- Input speed (cover assembly) 400 rpm

- Output speed (clutch disc) 0 rpm- Clutch torque 0 Nm - 2700 Nm

- Temperature 20/100/180ºC- Duration 10 s

- Releaser: 0-5s -> 0-95%, 6-10s -> 95-0%

Engagement and

disengagement characteristicTrigger point start: Release-load = <200 N

Trigger point end: Release-load = >200 N

Figure 6-2 Evaluation torque-build-up test

At the judder-sinus test, clutch torque is displayed against slip-sinus. The start trigger-point is

at input-speed is larger than 400 rpm and end trigger-point is as the output-speed is smaller

than 50 rpm. The path of the curves determines positive or negative behaviour. A positive or

neutral slope means judder damping and a negative slope judder-initiating behaviour (see

chapter 5).

The tendency of the results is that at 20ºC most of the time a neutral or almost neutral

behaviour takes places. This is due to the low temperature. At low temperatures, the facing is

less aggressive.

At the higher temperatures judder-excitation behaviour is a problem. Only a few facings have

positive or neutral behaviour.

© Z

F S

ach

s A

G

Test conditions:

- Input speed (cover assembly) 630-30 rpmfreq. =0.25Hz

- Output speed (clutch disc) 0 rpm

- Clutch torque (constant releaser position) 100 Nm - 250 Nm

- Temperature 20/100/180ºC

- Duration 20 s

Torque versus slip speed

(exitation/dampening)Trigger point start: Input-speed = >400 rpm

Trigger point end: Output-speed = <50 rpm

X-parameter

Y-parameter

Figure 6-3 Evaluation judder-sinus


At the drive-off simulation test, clutch torque is displayed against time. Trigger-point is the

hysterese of the releaser position during the test. The output is kept zero and at a torque of 500

Nm the releaser position is kept constant, till the clutch synchronises. Goal is to observe how

the clutch torque behaves during this constant releaser position. Preferable is constant

behaviour, because this ensures a smooth drive off. The chosen releaser travel percentage is

46%, because than the clutch torque is approximately 500 Nm, which is a minimal

requirement, to drive-on at idle speed.

© Z

F S

achs A

G

Test conditions:

- Input speed (cover assembly) 800 rpm

- Output speed (clutch disc) 0 - 800 rpm


50 % of drawing value- Temperature 20/100/180ºC

- Duration 4 s

Torque versus slip speed during drive-offTrigger point: Hysterias releaser position <0.2 %

Figure 6-4 Evaluation drive-off simulation

At the µ-check, the COF is displayed against time. The start trigger-point is a releaser-

position larger than 90% and end trigger-point is as the releaser-position is smaller than 90%.

Goal is to observe the COF during a step in the releaser position. At 20ºC the COF is too low.

© Z

F S

achs A

G

Test conditions:

- Input speed (cover assembly) 900-1000 rpm

- Output speed (clutch disc) 500-700 rpm


1800 Nm - 2200 Nm- Temperature 20/100/180ºC

- Duration 4 s

Dynamic coefficient of frictionTrigger point start: releaser position = >90%

Trigger point end: releaser position = <90%

Figure 6-5 Evaluation µ-check


7 Clutch torque at constant releaser position, the AMT-test

After the SCFT, an extra test will run to measure the clutch torque at constant releaser

position. This test takes place after the second test-day, after a cooling period till 20ºC-40ºC.

This test is called “AMT-test”, because the results can give an opinion about the tuning-

dependency, which is important for AMT-gearbox vehicles. Low torque fluctuations are

resulting in better performance of the controller.

At idle input-speed, 1000 rpm and output-speed 500 rpm, the releaser releases till the clutch

torque is on a certain level. When this torque is reached, the releaser will be kept constant for

10 seconds. Now can be observed how the torque behaves during this period. Preferable is a

constant behaviour. This because at AMT-vehicles, the releaser position is electric controlled.

This control adapts and tunes to keep the torque on a constant level. With an as much as

constant behaviour, the controller doesn’t have to interfere to adapt the releaser till the asked

torque is reached.

Also at 1000 Nm, the torque can raise to a level that it can break down the engine, which is

not preferable.

This test will be passed at three different torques: 250 Nm, 500 Nm and 1000 Nm. Every

torque will be measuring 5 times, totally 15 measurements, which are walked in succession.

Every torque is measured 5 times to proof the results on stability. On the figure beneath the

original results of the measurements are displayed at clutch-torque = 500 Nm. The blue line is

the clutch torque. The measured signal of the clutch torque is due to influences of the test-rig

a vibrating signal. Before the results are processed into excel, the signal is filtered.

With the black circles the start and end trigger points are displayed. Only this part between the

2 circles from the clutch torque signal is used for processing into TXT-files. Start and end

behaviour, which are unimportant are at this way not used and don’t disturb the legibility of

the excel-chart.

Figure 7-1 AMT-test


After the converting process from the original test-rig measure-data into txt-files, the data can

be imported in Microsoft excel and processed into charts.

This gives a clearer overview about the results. In the chart beneath, the results of figure 7-1

are displayed, which is directly copied from the original database with on the right side a

picture of the original test-results, the test-conditions and the legend.

AMT-test 500 Nm

450

470

490

510

530

550

570

590

610

630

650

670

690

710

730

750

0 1 2 3 4 5 6 7 8 9 10 11 12

Time [s]

Clu

tch

to

rqu

e [

Nm

]

S620C t=52.8 -z1d6S620C t=60.8 -z1d7S620C t=67.8 -z1d8S620C t=73.5 -z1d9S620C t=78.6 -z1d10

Test conditions:

- Clutch torque (constant releaver position)

500 Nm

- Input speed 1000 rpm

- Output speed 500 rpm

- Temperature ± 75ºC

- Duration 10 s

Figure 7-2 Evaluation AMT-test

The results are showing a light increasing clutch torque during the time with a maximum of

40 Nm. This is a relative good behaviour. More about interpretation and criteria to rate the

results in chapter 8.

In the same way as the figure above, the results of the 250 Nm measurement and the 1000 Nm

can be displayed.

Chapter 8 Results SCFT page 32

8 Results SCFT and AMT-test

To make a judgement to rank the different friction facings, certain criteria are described to

make an objective choice. Because this ranking list is made to choose a lead-free facing for an

AMT-gearbox, tuning-dependency is chosen as the head criteria.

Judder sensibility is chosen on the second place, because comfort plays an important role.

Lifespan is the third criteria. This is a less important criteria, because today the facings

survive a very long time and at normal use, the life of a truck.

Costs are the fourth criteria, but will in this report not further discussed.

Nine facings will be discussed about these three criteria, with two facings used as benchmark

and two facings to inquire the difference between a used and new facing. The two-benchmark

facings are today’s used lead containing facings and the hope is that the lead-free facings are

achieving equal or even better.

Other test of the SCFT are not used in this ranking, but can be looked back at the SCFT

database, to give extra information in a decision making situation.

AMT-Test

0

100

200

300

400

500

600

1 2 3 4 5

10 0 0 N m Test

To

rqu

e f

luctu

atio

n [

Nm

]

Facing 1

Facing 2

Facing 7

Facing 3

Facing 4

Facing 5

Facing 6

Facing 8

Facing 9

Figure 8-1 Results AMT-test

The criteria tuning-dependency is rated by the results of the AMT-test. The results of the 250

and 500 Nm test are not taking into account to the ranking because the mutual differences

were to small to make an objective judgment. Therefore only the results of the 1000 Nm test

are taken.

Preferable is a constant behaviour. This means during the test time an as small as possible

slope, as less as possible fluctuations and an as small as possible difference between the

minimum and maximum value of the clutch torque. The difference between minimum and

maximum value is taken, to compare the different facings, because this gives the clearest

difference between the facings. The results of this are displayed at figure 8-1.


Judder-sensitivity is rated by the results of the judder-sinus test of the SCFT.

Like explained in paragraph 5.1, judder can be rated by the slope of the judder-sinus

characteristic (figure 5-4). A positive and neutral slope are positive behaviour and a negative

slope means judder initiating behaviour. To compare the different facings, the difference

between the minimum and maximum value is determined and the sign of the slope. A positive

difference in figure 8-2 means judder-damping behaviour, a negative slope means judder-

initiating behaviour.

Judder sinus

-70.00

-60.00

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

10.00

20.00

30.00

40.00

20 100 180 20 100 180

Temp erat ure [ °C ]

To

rqu

e f

luctu

atio

n [

Nm

]

Facing 1

Facing 2

Facing 7

Facing 3

Facing 4

Facing 5

Facing 6

Facing 8

Facing 9

J

u

d

d

e

r

i

n

i

t

i

a

t

i

n

g

Figure 8-2 Results judder-sinus

Lifespan is rated by the BPS97-test. This is a test procedure to determine the wear off per

added energy.

Test conditions are:

BPS97 Test (Low Energy Test)

Temperature 85°C

Energy 8.6kJ

Number of cycles 60000

BPS97 Test (High Energy Test)

Temperature 170°C

Energy 50kJ

Number of cycles 10000


This results in the next table:

Table 8-1 Results of the life-span test, the BPS97 test

Facing Wear off [mm³/MJ] Low Energy Test

Wear off [mm³/MJ] High Energy Test Mark

1,9 11 21 5

2 14 29 5

3 20 26.3 1

4 20 26.3 1

5 13 26 5

6 13 26 5

7 7 16 9

8 12 17 5

To make a ranking between the facings with the above-described criteria, a ranking system

and weight factors are set.

As benchmark the lead containing facing 1 is set. If a facing shows clearly better results, a

grade 9 is noted. If a facing shows the same results or just a little badly or well results, a grade

5 is noted. If a facing shows bad results, a grade 1 is noted.

The three criteria, tuning-dependency, judder-sensibility are weighted to importance.

Tuning-dependency gets weight-factor 9, judder-sensibility a weight-factor 5 and lifespan, a

weight-factor 1.

These weight-factors are set in dialogue with a customer.

In fact, the lead-containing facing gets at the end ( ) ( ) ( ) ( ) 8051515559 =⋅+⋅+⋅+⋅ points.

Table 8-2 Total ranking table

Criteria �� Tuning-independency

Judder-sensibility

Lifespan Costs

Importance 9 5 1 1 TOTAL

New Facing 1 5 5 5 5 80

New Facing 2 1 5 5 9 48

Used Facing 3 5 1 5 9 64




New Facing 7 1 5 9 9 52



Information to the table:

Facing 1 and facing 2 are benchmarks

Facing 3 and facing 4 are the same facings

Facing 6 is an improved version of version 5

Facing 9 is the same as facing 1 but used.


Out of table 8-2 different things can be concluded. The lead-free facings are not being able to

get the same results or even better results. Only two lead-free facings can reach the qualities

of the lead containing benchmark and this facing 8.

Another important fact is almost the same results between facing 1 and facing 9, which are the

same facings, but facing 1 was new and facing 9 was used. This will mean, that for further

testing new discs can be used without a vehicle test before to wear of the facing.

Point is that this is the result for only one facing. Further testing has to prove if this is a fact.

Important is if the results of the table correspond to the results of the real-time vehicle tests.

Only the judder behaviour is investigated and marked on scale from 0 (very bad) to 10 (very

good).

The test procedure of the real-time vehicle test is based on real life situations, like drive-off

and shunting. In the next scheme, a schematic view of the test are showed

cold hot

Before endurance test

cold hot

After endurance test

Drive-on

cold hot

Before endurance test

cold hot

After endurance test

Shunting

Judder?

Figure 8-3 Test-procedure real-time vehicle test

Per facing, these 8 tests are walked. For every test, a mark is given to rate the behaviour. The

average of this 8 marks are showed in figure

Judder judgement at real-time vehicle test

0

1

2

3

4

5

6

7

8

9

10

Facing 6 Facing 1,9 Facing 3,4 Facing 8 Facing 7

Judder-

mark

Figure 8-4 Results of judder judgement at the real-time vehicle test [4]


All the facings show quite the same results. This corresponds with the facts in the 4th

column

of table 8-2, with the results of the judder-sinus. On first eye it seems that the results of the

SCFT and the real-time vehicle test are comparable. But on the other way is it difficult to

compare objective results (SCFT) with subjective results (real-time vehicle test), because of

reason’s used before.

A similar comparison with results of the AMT-test is not available on this moment.

Chapter 9 Validation SCFT and AMT-test page 37

9 Validation SCFT and AMT-test

After testing different facings, set criteria and rate the facings on this criteria, it is the question

if the SCFT and AMT-test is acceptable and confidential on the basis of the test results.

Main point is repeatability of the results. By run a SCFT on a specific friction facing under

the same conditions more times, the same results are expected. If this is not the case, the

whole test-procedure could be taken in doubt, because a judgement about a specific friction

facing is not possible. The main goal of SCFT to compare different friction facings with each

other is still not really possible.

At the moment, there are no facings tested double, to proof repeatability of the results.

A second point is the comparison used facings and new facings. During the time clutch discs

with new friction facings are tested and clutch discs, which come out test-vehicles from ZF

Friedrichshaven (named: “used facings”). The difference is that after some performance of a

clutch disc, the specifications and behaviour of the friction facings changes till a certain stable

situation is reached. The cause of this is wear off of the friction facing and heat input during

the performance like described in the previous chapters. New facings have mostly a bad COF

in the beginning, because off the less contact area (paragraph 5.5).

Preferable is that results of SCFT of a used and new facing from the same type, are the same.

This means that for future facing test, only new friction facings can be used, instead of

“expensive” used facings. Test-results of new facings are faster in stock, because new facings

are direct after production available. Used facings must first build into a vehicle and run a

50.000 to 100.000 km.

A third point is to validate the results of the SCFT with the results, which are collected by

testing by real-time vehicle tests. These are a mixture of objective vehicle data and subjective

driver-experience. With these two together a view can build, which can be compared with the

SCFT results.

Preferable is correspondence of both test results. This gives an extra validation for the SCFT.

This doesn’t mean that if the results are the same, test trucks are superfluous. In practise,

nobody rely blind on theoretical testing and wants confirmation of lifelike test in a vehicle.

But a test-method on a test-rig like the SCFT and AMT-test, can give fast knowledge about

behaviour and performance if validation with real-time testing is given

Chapter 10 Inspection of clutches page 38

10 Inspection of clutches

At research and development, inspection of clutches is of large importance to inquire the

performance of a clutch. Two different types of clutches can be measured. On one side

clutches, which are tested on a test-rig, to prove extreme situations or performance and on the

other side, parts, which are build out of a truck and come from transport companies.

The first category is of importance in the development phase of a part. Parameters, which are

not in the region of acceptance can be adapted or rebuild.

The second category is mostly parts from test trucks, or complains from customers. These

parts are important to know how performance is in real-time and if the parts satisfy on the

expectations. At complains of customers mostly it is a question if the complain is a clutch

problem or that another part causes the problem, which transmits the unwanted behaviour to

the clutch. Bearing problems in the back-axle or transmission can cause unwanted vibrations

or torsional moves, which influences the clutch behaviour. This is also important in warranty

questions.

Typical complains are disengaging problems, judder and harsh engagement.

Problems with disengage can be caused by two failures. First one is that the lift of the press-

on plate is to less, caused by problems with the preload tangential straps. These can be

weakened through high thermal loads or damaged by bearing balls of a damaged releaser

bearing, which are flying through the bell housing.

Judder is unwanted vibrations in the clutch caused by stick-slip behaviour of the friction

facing. This is very uncomfortable during driving. Further explanation of judder is already

explained in Chapter 5.1.

Harsh engagement is a problem at AMT-vehicles. At synchronisation of a gear, it is possible

that two teeth’s of the gearwheels stands together. The control adapts the position of the

releaser a little bit for the delivering of more torque to get the teeth’s in the right position.

Most of the times this works, but if in the clutch are places, which are worn off and clamp to

each other, the control adapts the position of releaser more. The places, which are clamping

are jumping open and through this “bang” the vehicle moves 10 centimetre, which is unhappy

behaviour for the comfort of the driver and for people outside the truck, which will be

frightened.

To get an idea about the problems, the clutches undergo a standard measure procedure.

Standard measure procedure:

To get information about several parameters like clamp load, releaser load, lift, obliqueness of

the pressure plate (shielding), a standard measure procedure is defined.

Out of the results of this measure procedure can be determined if the measured values are in

between the prescribed ranges. If not, this can give a possible answer to problems, which are

observed in the vehicle. A complete overview of the test procedure is given in the next

scheme.

Chapter 10 Inspection of clutches page 39

Figure 10-1 Way of working at analysing clutches

All data and test-results collected during the above procedure will be collected into a test

report, which gives an overview about the problem, the tests and possible answers and

solutions to the problem.

A main problem is loose of load due to wear off and therefore in the next chapter this will

discussed more into detail.

Chapter 11 Schematic overview of wear-off places page 40

11 Schematic overview of wear-off places

11.1 Schematic overview of wear-off places at a clutch cover assembly

The process of coupling and decoupling takes places with some face to face contact points in

the clutch cover assembly between different metals without any lubrification or bearings.

Through intensive use and vibrations, these places can wear-off, which causes improper

working. Especially the contact point between the fulcrum of the press-on plate and the

diaphragm spring is important. Wear off can cause movement of the fulcrum, which causes

loose of clamp load.

In the underneath figure is a schematic cross-section of the clutch cover assembly. The

contact points between the different parts are marked.

Table 11-1 Wear-off places

Wear-off places

1 Ring (contact point with the stamped housing)

2 Ring (contact point with the diapragm spring)

3 Diaphragm spring (contact point with the ring)

4 Diaphragm spring (contact point with the press-on plate)

5 Fulcrum of the pressure plate (contact point with the diaphragm spring)

6 Diaphragm spring (contact point with the release bearing)

7 Diaphragm spring (contact point with the release bearing)

1 2 & 3 4 & 5 7

6

Stamped housing Pressure

plate Diaphragm spring

Ring

Figure 11-1 Schematic cross-section of a pressure plate


Goal is to find out a relationship between clutch kilometre performance and wear-off rate to

find out an estimate for lifetime of the different parts.

With this relationship, a proposal for improvement can be made.

To make this happen, a number of clutches with different performances are collected. These

clutches are measured with the normal test procedure to measure clamp load, release load, lift

a.o. and after that disassembled till the different parts were available to measure on wear.

The wear on the different places is not all caused by the same cause. Wear at point 1 is only if

the ring rotate in the stamped housing. This is very unusual and is only created under false

working circumstances.

Wear on point 2 and 3 is on first side created by the coupling/decoupling process. In this case

the rate of wear is not very high. Sometimes the diaphragm spring rotates relative to the clutch

cover. Through this rotation the wear rate increases a lot, due to this movement. This

movement of the diaphragm spring relative to the clutch cover is an unwanted behaviour and

caused by decoupling process. A possible reason is that in the decoupling process, the

diaphragm spring connected to the release bearing moves sometimes faster than the tangential

leaf springs, which lift the pressure plate. On this moment, the diaphragm spring has some

free play and can rotate, which causes the extra wear.

Point 4 and 5 are the places with the larges wear rate, due to the movement at

coupling/decoupling. When the diaphragm spring does not rotate, the wear on the diaphragm

spring is limited to the connected fulcrum points of the pressure plate. If the diaphragm spring

rotates relative to the pressure plate, a wear track is seen along the whole perimeter of the

diaphragm spring.

The fulcrum points of the pressure plate also wear off more due to this motion.

Point 6 wears due to the connection with the release bearing at decoupling.

Point 7 wears due to vibrations at the connection point between the release bearing and the

diaphragm spring.

Extra causes of wear in all points are vibrations, during working. These vibrations are causing

small movements between the parts, which increase the wear rate.

11.2 Wear profile

The shape of the wear groove often can tell something about the use of the clutch.

A wide and shallow groove shows a vehicle with a lot of coupling events and a relative low

kilometre performance (100 000 – 500 000 km). This represents a vehicle, which is typical for

distributing transport in cities with a lot of stop and go.

A small and deep groove shows a vehicle with less coupling events and a relative high

kilometre performance (>500 000 km). The smallness show less coupling events, but due to

vibrations the groove is deep. This type groove is typical for a long distance transport vehicle.

Figure 11-2 Different kinds of grooves


11.3 Methods of measuring

A prescribed method to measure the wear off is not available. It is the question, which sort of

measurement shows the ideal measure to rate the differences in wear off. Is it only the

deepness of the groove or only the wideness of the groove or a combination of both (volume-

factor).

Also, a reference point is point of discussion. Which point has to be chosen to give a good

view about the real wear rate? At some of the points this not a difficult question like points 4

and 5, but through the shape of the diaphragm spring is it much more difficult to find a

reference point at 6 and 7. More will be explained later.

A measuring instrument is an x-z 2D-measure machine (contourograf), which measures on a

scale of 10-5

m and sends the data to a computer. A schematic view is seen in the next figure:

Also different types of calliper gauges, a 1D measure instrument is used to measure the wear

off.

The wear-off on the ring is measured on the two damaged sides (points 1 and 2) (figure 11-4)

with a calliper gauge.

Points 3 and 4 on the diaphragm-spring are measured with the 2D-measure machine even as

point 5.

Points 6 and 7 are measured together with a calliper gauge.

All points are measured on 3 different places to exclude measurement mistakes. An average

of these 3 measurements are taken to show in the results.

x

z

Figure 11-3 Schematic overview of x-z measure machine


Damaged

diameter

Real diameter

11.4 Results

Before any results can be reflect, first has to be sure which parts of the measured data are

taken to compare with other clutches.

At the ring, the up and down side are worn off, the right and left side are undamaged and can

be used as a reference (figure 11-4). In this case, the wear off rate can set as the difference

between the real diameter and the damaged diameter.

At the points 3 and 4, it is more difficult to choose a suitable point. It is possible to choose

wideness, deepness or a combination of both. Chosen is the depth of the groove, because this

represents the kilometre performance the best.

As reference point is the virtual undamaged diaphragm spring taken like explained in the

following figure:

Figure 11-5 Wear profile of the groove at point 4

Figure 11-4 Ring


The deepest point of the groove is searched and perpendicular on this point, the depth is

measured with the help of the Pythagoras formula:

( ) 222cba =+ (formula 4)

To discover the wear on the fulcrum of the press-on plate, a reference measurement with an

undamaged new pressure plate has to be done. With this, the wideness or the height of the

wear off can be determined. In this case is chosen for the height, but it doesn’t matter if it is

the wideness or the height, because this gives the same results

Figure 11-7 Reference profile

H Reference Fulcrum

Actual Fulcrum

Figure 11-6 Wear profile fulcrum (point 6)


The wear off measurements of point 6 and 7 is off the same principle as the ring.

With a calliper gauge, both the grooves are measured in one time. A reference measurement

next to the grooves gives the reference thickness of the diaphragm spring. The difference

between the reference thickness and the damaged thickness gives the wear rate.

This results in the following figures with the results of measurements at different clutches.

In figure 11-8 is the listed wear off, of point 1 till 5 showed. With a linear regression, a

relation between the different points is to see. Unfortunately, the number of investigated

clutches on total wear off is only 5, so a clear proof for a linear relation cannot be given.

Figure 11-8 Wear off fulcrum

In figure 11-9, the listed wear off, of point 6 and 7 is showed

The wear off at the tongues of the diaphragm spring is not clear dependent of the kilometre

performance. This is because point 6 is dependent of the number of decoupling and point 7 of

the kilometre performance. So, a clear relationship between these two points is not there.

Figure 11-9 Wear off tongues diaphragm spring


A measure for thermic load is the deformation of the surface of the pressure plate (shielding).

The difference in height of the surface at the outer and inner diameter can be measured (figure

11-10)

Like explained in chapter 5, due to high temperatures, the surface of the pressure plate

deforms in the way of the dotted line in figure 11-10. This phenomena leads to a move of the

clamp radius and to more wear of the friction facing.

Figure 11-11 Deformation due to shielding

Rm

Difference

in height

Figure 11-10 Height difference due to shielding


11.5 Wear off at clutch discs

It is also possible to find a relation between the wear of the friction facing and the kilometre

performance.

Wear at friction facings depends on several causes like performance, the driver skills, the

route of the vehicle and heaviness of the road.

A clutch disc is completely worn off, if the thickness of the clutch disc is less than 3 mm of

the original thickness of the clutch disc. So like to see in figure 11-12, the maximum wear is 3

mm.

The measurements show a not clear relationship between kilometre performance and friction

facing wear.

Figure 11-12 Wear off friction facing


11.6 Improvements

The method of working to determine wear at clutch parts can use some adjustments.

First, a unity in the way of measuring has to define, like the places of measuring, unite scaling

of the data and definition of reference points. If at later times, new parts are measured, it has

to go on a same way like these parts are measured to get a more reliable comparison.

Also it is a possibility to test more clutches. Maybe these results still causes a connection

between wear en kilometre performance.

In case of the wear at friction facings, collect data from test results of previous parts

Exclude results of parts, which are damaged or worn, due to false use or handling. These

results are not realistic to use in this survey.

Try to find as much information about the vehicle. Maybe, it is possible to divide the vehicles

into certain classes, like long-distance traffic, or supplier traffic.

Conclusions page 49

Conclusions/Recommendations

The Standard Clutch Friction Test is developed to rate facings on equal test conditions. The

results of the test must give an objective view of the performance of a friction facing. With

the results-database, it is possible to compare different facings with each other, on base of the

most important tests and the set criteria described in chapter 8.

Like already noted in chapter 9, some questions can rise about repeatability, equality of the

results from used and new facings and the equality of the results with the real-time vehicle

tests.

Repeatability of the results is on this moment unclear. Facing 3 and 4 are equal type and both

used, but are showing opposite results. The question is, if this is caused by the test procedure,

or that one of these facings is of a different version. Furthermore, no more facings are tested

double, so a clear conclusion on this question cannot be given. A recommendation is to test a

facing more times to prove repeatability.

Equality of the results from used and new facings is only tested one time. Facing 1 was a new

facing and facing 9 was a used one. The comparison between these two facings shows a

positive result. Based on the criteria set in chapter 8, the results show equal. Like in the

previous question, only one time this equality is tested. A recommendation is to test more

facings in the new and used state to prove equality.

The comparison between the results of the SCFT and the real-time vehicle tests is difficult to

make, due to compare respectively objective results and subjective results. Although, the

results shows a reasonably equality between the results of the SCFT judder test and vehicle

judder test. Only at facing 3, the results were different.

A general conclusion out of this is, that a basis step is taken, to compare facings on an

objective way, but that further testing is very necessary to increase the reliability of the SCFT.

In the second part of the report, a possible relationship between kilometre performance and

wear off values to predict a lifespan is investigated.

The come out of this investigation is that only to the point “fulcrum” any sign of a

relationship is seen.

On other points, unfortunately no relationship can be seen. Possibly, there are too many

influences, which can make the results turbid. In paragraph 11.6, possible improvements are

given.

Conclusions page 50

References

[1] Chap. 12 Manual Transmission Clutch Systems. SAE AE-17, Ray Shaver, Chrysler

corp. 1997

[2] Media-database ZF Sachs AG

[3] Anleitung zur Auswertung des Standard-Kupplungs-Test mit dem TurboLab-Script K-

DK-Belag, J.Hooss, ZF Sachs AG

[4] Auswertung Rupfuntersuchungen ZFF, R.Kramer, ZF Friedrichshafen

[5] ZF Sachs AG, At a glance, Powertrain and Suspension Components, edition 2006

[6] Die Kupplungsscheibe im Antriebsstrang, Mr. Buer, ZF Sachs AG

Acknowledgements page 51

Acknowledgements

First of all, I would like to thank mister Klaus Steinel, who gave me the opportunity to fulfil

my internship at a the powertrain components department of ZF Sachs AG in Schweinfurt,

Germany.

Also, mister Klaus Neumann, I would like to thank. His coaching was very instructive and his

comments were always well thought-out and critical. I learned a lot during my period. He was

very experienced in all facets, which have to do with (commercial vehicles) clutches.

My coach at TU/e, dr. Bram Veenhuizen, for his critical and pleasant supervision. His

comments were always stimulating and I appreciated his helpful coaching.

Furthermore, I would like to thank:

All “collegues” at the Technology Centre in Schweinfurt, which were always very helpful.

They were very pleasant and interested people to work with.

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