NHEHS Newsleter Christmas 2011 w.pub
- juliet learmouth senior team maths challenge in the west london round of the senior team maths challenge which took place at imperial college in november the nhehs team of four girls from years 12 and 13 did particularly well in the ‘cross number
- creditable 7th helen critcher duke of edinburgh awards mayor of ealing
- 5 chemistry in action amelia powell chrystall prize sorrel evans we year 12 chemistry students were given the fantastic opportunity of attending a day of lectures at london university
- year 13 visit to the sir john soane museum juliet learmouth on the last thursday of term the french department took a group of year 11 girls who
- Format PDF
COMPREHENSIVE SUMMARIES OF UPPSALA DISSERTATIONS FROM THE
FACULTY OF SCIENCE AND TECHNOLOGY 537
Friction and Contact Phenomena of
Disc Brakes Related to Squeal
ACTA UNIVERSITATIS UPSALIENSIS
UPPSALA 2000ENCLOSED PAPERS
The thesis comprises the following papers:
Paper I Mikael Eriksson, Filip Bergman and Staffan Jacobson,
Surface characterisation of brake pads after running under silent and
squealing conditions, Wear 232 (1999) 163-167.
Paper II Filip Bergman, Mikael Eriksson and Staffan Jacobson
Influence of disc topography on generation of brake squeal, Wear 225-
229 (1999) 621-628.
Paper III Mikael Eriksson, Filip Bergman and Staffan Jacobson
A study of initialisation and inhibition of disc brake squeal, Accepted for
publication in proceedings of Brakes 2000, Leeds, UK
On the nature of tribological contact in automotive brakes, Submitted to
Wear, Dec. 1999
Paper V Mikael Eriksson, Anna Lundqvist and Staffan Jacobson
A study of the influence of humidity on the friction and squeal generation
of automotive brake pads, Submitted to Journal of Automobile
Engineering, March 2000
Paper VI Mikael Eriksson and Staffan Jacobson
Friction behaviour and squeal generation of disc brakes at low speeds,
Submitted to Journal of Automobile Engineering, March 2000
Paper VII Mikael Eriksson, John Lord and Staffan Jacobson
Wear and contact conditions of brake pads - dynamical in-situ studies of
pad on glass, Accepted for publication in proceedings of Nordtrib 2000,
Paper VIII Mikael Eriksson and Staffan Jacobson
Tribological surfaces of organic brake pads, Submitted to Tribology
International, March 2000m
Mikael Eriksson 7
THE AUTHOR'S CONTRIBUTION TO THE PAPERS
1 INTRODUCTION 8
1.1 Outline of the thesis 9
1.2 Fundamentals of friction 9
1.3 Automotive brake systems 11
1.4 Brake pad materials 13
1.5 Sound in general and brake squeal in particular. 14
1.6 Squeal testing 15
2 TRIBOLOGICAL CONTACT IN BRAKES 19
2.1 Microscopic contact situation 19
2.2 The influence from plateau growth and degradation
on the coefficient of friction 29
3 DYNAMIC BEHAVIOUR OF THE CONTACT SURFACES 31
3.1 Rapid processes 31
3.2 Slow processes 32
4 FRICTION AND SQUEAL IN BRAKES 35
4.1 Variations in the coefficient of friction 35
4.2 Influence of humidity on the coefficient of friction. 38
4.3 Correlation between and brake squeal 39
4.4 Critical contact conditions 42
5 SUMMARY 43
6 ACKNOWLEDGEMENTS 45
7 REFERENCES 468 Friction and Contact Phenomena of Disc Brakes Related to Squeal
When Henry Ford introduced his model T in 1908, cars had been produced like he knew
them for over 25 years, with a combustion engine in front of the passengers, four wheels
and rear wheel drive. Even if the design was traditional, the model T was revolutionary.
It was the first mass-produced car ever and with it, cars became more accessible to
No one knows if it is true that Henry Ford once said; “You can paint it any color, so
long as it's black." . Nevertheless, what is true, is that black was the only available
colour on the model-T between 1914 and 1926. It is also true that the reason for this was
that the black enamel was the fastest drying paint available at the moment. Production
time could be reduced with a quick-drying paint and with it, also cost. This low price
philosophy founded what would become the largest family company in the world, the
Ford Motor Company.
The model T weighed 550 kg, had a 20-hp engine and a top speed of approximately
65 km/h see Fig 1. It was equipped with a band brake system, a cotton textile band
wound around a drum inside the planetary gearbox. The cotton band was lubricated with
the oil from the gearbox and in order to avoid over-heating, the driver was instructed to
apply the brake in short intervals only.
Fig. 1. Ford model-T, the first mass-produced car in history. It was equipped with a
cotton band brake, applied to a drum within the gearbox.
(Henry Ford Museum)Mikael Eriksson 9
Eighty-three years later Mercedes-Benz reintroduced the 600 S-class (the name was also
used in the 60's). Designed to be the best and most comfortable car in the world, it
weighed over 2 tons, with a 400-hp engine and an electronically limited top speed of
250 km/h (for safety reasons). The maximum kinetic energy was now 54 times higher
than 80 years earlier, putting enormous demands on the brake performance. The
model–T’s single band brake was replaced by four disc brakes and no one even thought
about giving the driver special instructions on how to take care of the them. Nowadays,
it is taken for granted that the brake systems should always work perfectly, despite
careless users, extreme speeds and difficult environments.
On sports cars, the brake performance demands are sometimes even higher than on the
Mercedes 600 S. For example, the Ferrari 550, one of the fastest cars on the market, has
a top speed of 320 km/h. This results in a 40% higher maximum kinetic energy, as
compared to the Mercedes.
1.1 Outline of the thesis
The aim of the work presented in this thesis has been to increase the understanding of
the contact and friction phenomena in the brake pad and disc couple. This understanding
is needed to analyse and ultimately solve the brake squeal problem. The thesis is mainly
a review of the appended papers. In cases where the presented information is not found
in the papers, the source is referred to using a number in square brackets .
The work comprises a number of investigations correlated to the tribology of brake pad
materials and squeal generation in brake systems. It is presented according to the
• First of all, we will start with a fundamental introduction to the friction and the
contact between two rubbing surfaces. This is followed by a general description of
the brake system and brake squeal testing.
• Chapter 2 comprises the main part of the thesis, describing the specific contact
situation found between a brake pad and a disc.
• This description is followed by chapter 3, where the dynamical behaviour of this
contact is discussed.
• Chapter 4 outlines a number of friction phenomena typical of brakes. These
phenomena are correlated to the contact situation and to the generation of brake
• A summary of the thesis is found in Chapter 5.
1.2 Fundamentals of friction
One of the most interesting and most important physical phenomenon related to brake
systems is the lateral force between two rubbing surfaces, i.e. the friction force. If a
block is dragged over a horizontal floor, the lateral force required to move the block is
equal to the friction force between the two surfaces.m
10 Friction and Contact Phenomena of Disc Brakes Related to Squeal
In the 1490's, Leonardo da Vinci found that when the normal force on the block
increases, the friction force also increases . He furthermore discovered that the
friction force between to rubbing surfaces is independent of the apparent, nominal,
contact area, see Fig. 2.
Two hundred years later Amonton rediscovered what da Vinci already had observed and
he formulated "Amontons' laws of friction":
1. The force of friction is directly proportional to the applied load.
2. The force of friction is independent of the apparent area.
These relations between the normal force, F , and the lateral force, F , can beN L
mathematically formulated as:
Where is the coefficient of friction. For many materials this relation is true, within
limited load intervals.
Fig. 2. When a block is dragged over a horizontal surface, the lateral force F requiredL
is equal to the friction force between the block and the surface. According to
Amonton and da Vinci, the lateral force is independent of the nominal contact
In order to explain why the friction force is independent of nominal contact area, one
must study the two facing surfaces. All technical surfaces have a roughness, even if
some appears very smooth. If two rough surfaces are pressed against each other, only
small parts of them will actually contact each other. Consequently, the area of real
contact will be very small. As a matter of a fact, the normal load and hardness of the
two materials will define the area of real contact . An increased hardness or a reduced
load will lead to a reduced contact area, see Fig. 3. Thus, for a given material
combination, the real contact area depends on the normal load only and has no
correlation to the nominal contact area. If the load is doubled, the area of real contact
will also be doubled.Mikael Eriksson 11
Fig. 3. Contact situation between two rough surfaces. Only small parts of the surfaces
are in real contact with each other, encircled. The area of real contact increases
with increased load and with decreased hardness.
a) Low load and/or high hardness. b) High load and/or low hardness
In general, the area of real contact is very small. If a 100x100x100 mm steel cube, with
a hardness of 3 GPa, rests on a flat steel plate, the nominal contact area is, of course,
2 210 000 mm . The area of real contact, however, is only 0.03 mm , a factor 300 000
times smaller !
Now, if the friction force is identified as the force required shearing the real contact
between the two surfaces, it can easily be understood that the nominal contact area does
not affect the friction force. It can also be understood that a doubled normal load,
resulting in a doubled area of real contact, will lead to a doubled friction force.
1.3 Automotive brake systems
An automotive brake system can be divided into three main parts
1. The rotor, as the name is indicating, is rotating with the wheel. It is the first part in
the friction couple. Rotors made of grey cast iron have always dominated the
market. The last couple of years, other materials, although still having only a small
commercial importance, have been introduced. Some examples are SiC-reinforced
aluminium, carbon-SiC composites and sintered carbon.
2. The brake lining is the second, stationary, part of the friction couple. During a
brake application, the pad is pressed against the rotor with a hydraulic piston. The
friction forces between the stationary lining and the rotating disc will turn the
kinetic energy of the vehicle into heat.
3. The hydraulic system transfers and amplifies the brake force from the brake pedal
to the hydraulic piston pressing the linings against the rotor. In modern brakes the
hydraulic system also includes the ABS-system (Anti-Blockier System, German)
and different kinds of traction systems.12 Friction and Contact Phenomena of Disc Brakes Related to Squeal
As mentioned in the introduction, a number of different vehicle brake systems has
existed over the years. Today two types reign the market, the disc brake and the drum
brake. Drum brakes, being an earlier design, dominated until the 1960´s in all kinds of
vehicles. Today, it is predominantly used in trucks and buses. Just recently, disc brakes
have been introduced in heavy vehicles as well and will probably have a large share of
this market within a few years.
The main difference between the two designs is the geometry of the rotor and linings.
The hydraulic systems are similar. Figure 4 shows a schematic picture of a brake system
with one drum and one disc brake.
In the disc brake, the linings (also called pads) clamp the disc from opposite sides. The
friction force between the pads and the disc are perpendicular to, and does not affect,
the normal forces of the pads. Thus, the braking force will depend linearly on the
applied normal force, with the premise that the coefficient of friction between the two
parts is constant. The result is a superior pedal feel as compared to the drum brake. The
lower weight is another benefit with the disc brake.
Fig. 4. Illustration of the two brake systems dominating the market, the disc brake and
the drum brake. The brake drum and the calliper have been cut open to reveal
the pads and shoes. The friction material is cast onto the back-plate, forming
the pad. (Karl Åstrand /FZ)Mikael Eriksson 13
In drum brakes, where the pads (shoes) are pushed outwards against the inside of a
drum, the friction force will affect the normal load. Causing the brake to have either a
self-locking tendency or the opposite. In either case, the brake system will get a poor
linearity and thus a weak pedal feel. The foremost benefit of the drum brake is the
insensitivity for harsh environments, such as water, dirt or road salt.
Most heavy vehicles use a pneumatic instead of a hydraulic system to apply the brake
force. The brake pedal is connected to a gas valve instead of a piston, controlling the
pressure drop from the storage tanks to the brake cylinder. Pneumatic brakes only
require a very low pedal force to apply a high braking force, which is needed to stop a
truck or a bus. The drawbacks are poor pedal feel and the size of the system. It requires
both an air pump and a storage tank.
1.4 Brake pad materials
As mentioned, the first brakes consisted of a rope winded around the back axle of a
horse carriage or a piece of wood pressed against the rim of the wheel. When more
effective brake materials were needed, an asbestos yarn was spun around the rope,
which had been impregnated with tar . Modern lining materials show many
similarities with these primitive ropes. Most of them are based on a metal fibre
reinforced organic matrix and are called organic. There are, however, also other types
of lining materials, categorised into metallic, semi-metallic and carbon.
This thesis comprises organic pad materials exclusively. This type will be further
Organic pads are generally a compound of a number of different materials. Sometimes
up to 20 or 25 different components are used. These components include a:
• Binder, that holds the other components together and forms a thermally stable
matrix. Thermosetting phenolic resins are commonly used, often with the addition
of rubber for improved damping properties.
• Structural materials, providing mechanical strength. Usually fibres of metal,
carbon, glass, and/or kevlar are used and more rarely different mineral and ceramic
fibres. Before its prohibition in the mid 80's, asbestos was the most commonly used
• Fillers, mainly to reduce cost but also to improve manufacturability. Different
minerals such as mica and vermiculite are often employed. Barium sulphate is
another commonly used filler.
• Frictional additives, added to ensure stable frictional properties and to control the
wear rates of both pad and disc. Solid lubricants such as graphite and various metal
sulphides are used to stabilise the coefficient of friction, primarily at elevated
temperatures. Abrasive particles, typically alumina and silica, increase both the
coefficient of friction and the disc wear. The purpose of the latter is to offer a better
defined rubbing surface by removing iron oxides and other undesired surface films
from the disc.
You may also like