automotive brake[¦ȯd·ə′mōd·iv ′brāk]
An energy conversion device used to slow a vehicle, stop it, or hold it in position. The two systems are the service brake and the parking brake, both of friction type. The service brake includes a hydraulically operated brake mechanism at each wheel. These wheel brakes are controlled by movement of the brake pedal, providing braking proportional to the applied pedal force. The parking brake is a mechanical brake operated through a separate hand lever or pedal; it applies parking-brake mechanisms usually at the two rear wheels. Most automotive vehicles have power-assisted braking, where a hydraulic or vacuum booster increases the force applied by the driver to the service-brake pedal. See Brake
The two types of wheel-brake mechanisms are drum brakes and disk brakes (see illustration). Drum brakes are used at all four wheels on older vehicles, and at the rear wheels of many vehicles with front disk brakes. Some vehicles have disc brakes at all four wheels.
The four wheel brakes are hydraulically interconnected so they operate together from one control. When the driver depresses the brake pedal, pistons are forced into fluid chambers in the master cylinder. The resulting hydraulic pressure is transmitted through steel pipe and rubber hose to hydraulic cylinders in the wheel brakes. The pressure forces pistons in the cylinders to move outward, pushing brake friction material, or lining, into contact with the rotating drum or disk to apply the brakes. See Hydraulics
In a drum brake, two nonrotating curved steel shoes, faced with heat- and wear-resistant lining, are forced against the inner surface of a rotating brake drum as the driver depresses the brake pedal. When the pedal is released, return springs pull the shoes away from the drum.
In a disk brake, a nonrotating caliper containing one or more pistons and carrying two brake pads, or lined flat shoes, straddles the rotating disk. As the driver depresses the brake pedal, the piston and hydraulic reaction push the brake pads against each side of the disk. When the brake pedal is released, the piston seal, which was deflected as the piston moved out, provides piston retraction. Two types of caliper are the fixed or nonmoving, and the floating or sliding. The floating or sliding type depends on slight inward movement of the caliper, resulting from hydraulic reaction, to force the outer brake pad against the disk.
Power-assisted braking is provided by a hydraulic or vacuum booster. As the brake pedal is depressed, the booster furnishes most of the force to push a pushrod into the master cylinder. The power piston in the hydraulic booster is operated by oil pressure from the power-steering pump or from a separate pump driven by an electric motor. In the vacuum booster, a diaphragm usually is suspended in a vacuum supplied from the engine intake manifold or from a vacuum pump driven by the engine or an electric motor. Depressing the brake pedal allows atmospheric pressure to act against one side of the diaphragm. The resulting pressure differential moves the diaphragm and power piston, which forces the pushrod into the master cylinder.