DYNAMIC MEASUREMENT OF THE FORCES IN THE FRICTION ...

In the analyses of the brake disc is also apparent that the distribution of the surface pressing
brake disc shielding will change by a blow or DTV.
Thermal effects as e.g. the shielding
of a brake disc (Fig. 5) let expect
changes of the surface pressing of the
clamping force as well.
initial
state
after 3
seconds
after 7
seconds
Figures 2 to 5 show that inconsistent/not uniform surface pressing can be expected in a disc
brake. According to (4) this would lead to a slanted wear of the brake pads. Furthermore, a
higher energy expenditure results on the positions with higher surface pressing which leads to
higher temperatures. On the other hand, according to (3), higher temperatures drop to lower
friction coefficients and this reduces the efficiency of the brake. According to (4), the comfort
of the brake is also reduced, since sloping worn pads increase the squealing sensibility.
In order to avoid or reduce the problems mentioned, different measures are used in serial
brakes. Nowadays piston offsets, cuttings in the metal sheet, shiftings of the centre of pressing
of the piston, drawn pads (push-pull principle), two-piston calipers with different piston
diameters and even further measures are used among other things to counteract these effects.
In order to determine the influence of these measures and the actual surface pressing various
methods were developed. State of the art are the tension-optical ball pressing process,
pressure indication films of the company Fuji and electrical pressure indication films of the
company Tekscan (18), for example.
In the tension-optical ball pressing process of the company Girling (4) one 1.5 to 2 mm thick
plastic plate with known tension-optical properties and a ball cage with balls with a diameter
of 3 mm between brake pad and brake disc are laid. By increase of the hydraulic pressure of
the brake system the balls are pressed into the plastic material. Tension curves, which help
one to make statements about the transmitting force, can be seen under polarized light.
Photographs taken by the authors with electrical
pressure indication films show an non-uniform
surface pressing of a brake pad by a hydraulic
pressure of 50bar and a stationary brake disc. The
contact pressure distribution of the clamping force
and the point of application of load calculated by the
red-white rhomb are to be derived.
Further researches with electrical pressure indication
films can be found in (1 and 9), for example.
- 4 -
Fig. 5: Shielding of a brake disc (3)
Fig. 6: A photograph of the fotostress-plastic
plate for the ball pressing process of the
company Girling under polarized light after
load. The number of the interference rings on
the particular ball indentation is a measure for
the pressing dominating there. (4)
⎡ g ⎤
⎢ 2
⎣ cm ⎥
⎦
Fig. 7: Surface pressing distribution
under a brake pad (stationary brake disc)