Brakes

In modern motor vehicles fitted with shoe-type brakes the latter are almost invariably of the internal-expanding kind, hydraulically operated.

Hydraulic actuation is based on a natural law, namely, that a pressure exerted upon a liquid is uniformly transmitted in all directions: the load on the piston at the extreme left is 100 lb.; as a result of the transmission of the pressure through the liquid, which is an incompressible medium, the force developed at each of the other pistons (which are assumed to have the same area as the first) will also be 100 lb However, the travel of the eight right-hand pistons will be only one-eighth of the travel of the left-hand piston.

The internal-expanding brake, together with its hydraulic equipment, comprises a main cylinder with a reserve fluid tank, the wheel cylinders, and the connecting pipelines.

Application of the brakes involves the following operations: when the brake pedal is depressed, a piston in the main cylinder is moved and produces a pressure through the brake system. This pressure forces the two small pistons in the wheel cylinders apart. As a result, the shoes are thrust against the brake drum. The kinetic energy of the vehicle is transformed into heat energy by the friction which occurs at the brake linings.

This causes heating of the brake drum. In order quickly to get rid of as much heat as possible, the brake drums must have a large external surface area and be mounted in a place where the air has proper access to them. If a brake drum becomes too hot, it will expand excessively, and then the brake shoes will not press so tightly against the inside of the drum. In addition, the braking efficiency of the brake lining diminishes at elevated temperature, as heat reduces the frictional force developed.

Thus, poor heat dissipation greatly reduces the efficiency of the brake.

The central component of the brake system is the master cylinder. When the brake pedal is depressed, the piston travels to the right and produces a pressure in the chamber behind it. This pressure is transmitted by the hydraulic fluid through the pipes to the wheel cylinders.

To equalise any differences of pressure in the system (which differences may, for example, be caused by expansion of the brake liquid in the pipelines), a check valve is installed between the pressure chamber and the pipeline.

In addition, the master cylinder ensures uniform filling: when the piston is at rest, hydraulic fluid flows from the reserve tank through the compensating port into the pressure chamber. To prevent air being drawn in when the piston returns to the home position, the space behind the back of the piston is kept filled with fluid through the auxiliary port. At one end of the master cylinder is the stop-light switch in which the hydraulic pressure moves a small piston which actuates the electrical contact that switches on the light.