Page 381 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 2. 1 Braking systems

12.1.5 Materials design have a relatively high MOT to prevent thermal degra-
dation of the surface although, due to the nature of its
12.1.5.1 Materials requirements composition, the MOTof the pad material will always be
lower than that of the disc. A low conductivity for the
for braking systems
pad or shoe material is desirable to minimize conduction
of heat to other components of the system, in particular
In any conventional foundation brake, the relative rota- to the hydraulic fluid. The material should be reasonably
tion of the so-called ‘friction pair’ under the action of the wear resistant but not excessively so since wear can be
brake system activating force is responsible for generat- beneficial in promoting a uniform contact pressure
ing the frictional retarding torque required to slow the distribution and preventing ‘hot spotting’. Likewise
vehicle. Most friction pairs consist of a hard, usually the elastic modulus of the material should be relatively
metallic, rotating component and a relatively compliant low to give good conformity with a roughened or
‘friction’ material in the form of a brake pad or shoe. The thermally distorted rotor surface. Finally, as for the
materials requirements for the rotating and stationary rotor, the friction material should be cheap and easy to
components of the friction pair are therefore quite dif- manufacture.
ferent as discussed below. The friction material selected to meet the above re-
Any rotor material must be sufficiently stiff and strong quirements is invariably a complex composite consisting
to be able to transmit the frictional torque to the hub of a variety of fibres, particles and fillers bonded together
without excessive deformation or risk of failure. How- in a polymeric matrix such as phenolic resin. For many
ever, the stresses arising from thermal effects are much years, asbestos fibres were an important element of
higher than purely mechanical stresses and are more friction materials due to their excellent thermal and
likely to give concerns over disc integrity. Thus the rotor friction properties. For health and safety reasons, asbes-
material should have high volumetric heat capacity tos has now largely been replaced by other less harmful
ðr$c P Þ and good thermal conductivity (k) in order to fibres, e.g. Kevlar. The exact composition of any friction
absorb and transmit the heat generated at the friction material must be tailored to the application and knowl-
interface without excessive temperature rise. Further- edge of the formulation is proprietary to the supplier.
more the maximum operating temperature (MOT) of
the material should be sufficiently greater than the 12.1.5.2 Cast iron rotor metallurgy
maximum expected temperature rise to ensure integrity
of the rotor even under the most severe braking condi- The overwhelming majority of rotors for conventional
tions. Ideally the rotor material should have a low co- automotive brakes is manufactured from grey cast iron
efficient of thermal expansion (a) to minimize thermal (GI). This material, also known as flake graphite iron, is
distortions such as ‘coning’ of a disc. It should also have
cheap and easy to cast and machine in high volumes. It has
low density (r) to minimize the unsprung mass of the
good volumetric heat capacity due mainly to its relatively
vehicle. It should be resistant to wear since generally it is
high density, and reasonable conductivity due largely to
far easier and cheaper to replace the friction pads or
the presence of the graphite (or carbon) flakes. The co-
shoes than the rotor itself. Finally, and most importantly,
efficient of thermal expansion is relatively low and the
the rotor should be cheap and easy to manufacture. material has an MOTwell in excess of 700 C (but note

The brake pad or shoe represents the stationary part of that martensitic transformations at high temperatures can
the foundation brake assembly. Normally a proprietary
composite friction material is bonded to a steel backing
plate or shoe platform. The primary function of the
friction material is generally considered to be the pro- Table 12.1-2 Tensile strength and conductivity of some common
duction of a stable and predictable coefficient of friction cast irons
to enable reliable and efficient braking of the vehicle over Min. tensile Thermal conductivity
a wide range of conditions. In fact, it is the combined Grade strength (MPa) at 300 8C (W/m K)
tribological characteristics of both rotor and stator
materials (i.e. the ‘friction pair’) which are responsible 400/18 SG ) 400 36.2
for the generation of the frictional torque. As for the 250 GI 250 45.4
rotor, the friction material must have sufficient structural
integrity to resist the mechanical and thermal stresses. 200 GI 200 48.1
This is particularly important for the bond between the 150 GI 150 50.5
friction material itself and the steel structure which
supports it, as a complete failure here could have * Spherical graphite iron.
disastrous consequences. The friction material should

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