Thermal Effects in Braking
Friction processes generate heat because work is being done on the
moving objects that acts to slow them down. The particular interactions
occurring in the friction interface that do that work can be studied in
various ways and theoretical models may be developed to simulate them.
Similarly, we measure temperatures at the surface and inside friction
materials and model the energy dissipation and its effects on the
friction and wear performance.
 Dynamometer Studies
On
the CAFS subscale aircraft dynamometer, the geometry is identical brake
rings sliding as shown against each other. The dynamometer is
programmed to perform a series of accelerations and stops. The
rotational inertia, I, is chosen by adding large disks to the rotating
axis. The initial rotational kinetic energy is 
In braking, this energy is dissipated via friction at the brake pad
interface and in the machine itself (motor, bearings, etc.). CAFS
researchers know the importance of dissipating a known and constant
quantity of energy within the brake, so machine losses are accounted
for on each stop.
One method for measuring temperatures within the stator is the use
of thermocouples. This photo shows a series of thermocouples positioned
at various depths from the friction interface.
Heat is non-uniformly conducted into the ring, so simple
one-dimensional models can only provide a rough estimate of the
temperature distribution.
Another
technique that has been used by CAFS researchers is a high sensitivity,
high-speed infrared camera and a series of fiber optic inserts in the
stator to measure temperatures occurring on the surface of the rotor.
The figures below show the experimental setup and a plot of the
measured temperature along the radius of the brake ring (curve 1 is
near the outer radius and curve 10 is near the inner radius). Note that
the temperature is much higher near the outer radius. Contact was
occurring primarily at that location; and the heat diffused both across
the surface and into the brake ring.
  This work is published in Journal of Physics D: Applied Physics 34 (2000) 976-984.
More recently, CAFS researchers, in association with scientists at
Oak Ridge National Lab, have been using the same infrared camera to
directly observe thermal diffusion in brake materials and to measure
thermal diffusivity between any two points in an image.
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