Disc brakes

Partial disc brakes: In this arrangement the brake disc is gripped pincer-wise between two circular or kidney-shaped brake pads fitted with friction linings. The large area of the disc assures excellent dissipation of the heat generated by braking. Two-circuit hydraulic systems, when applied to disc brakes of this kind, do not separately serve the front and the rear wheels respectively; instead, each circuit serves all four wheels. For this purpose the brake disc of each of the wheels is provided with four hydraulic brake cylinders.

Full disc brakes: In this form of construction, fixed pads with friction linings act from inside upon both sides of a rotating casing, which has to dissipate the heat in much the same way as a shoe-type brake.

In this case the self-servo or self-energising action is obtained by means of steel balls which mount the inclined faces of sockets when the two brake discs (one fixed and one moving) rotate in relation to each other. This forces the discs apart, causing them to be pressed even more firmly against the casing. The degree of servo action developed is dependent on the inclination of the faces of the sockets. If they are flatly inclined, a considerable servo action results, but this may involve a risk of jamming and locking of the brake, as the balls may be wedged so tightly that they cannot by themselves roll back to the bottom of the sockets.

Besides, the two discs must then undergo a considerable displacement in relation to each other before they are forced so far apart that they can develop a powerful braking action.

Wear of the friction linings necessitates an even greater amount of displacement of the discs in relation to each other. For this reason disc brakes having a high degree of self-servo action are particularly sensitive to lining wear.