The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle.
The most common arrangement of hydraulic brakes (for passenger vehicles) consists of a brake pedal, a vacuum assist module, a master cylinder, hydraulic lines, a "slave cylinder", and a brake rotor and/or brake drum.
Typical passenger vehicles employ disc brakes on the front wheels and drum brakes on the rear wheels. However, four wheel disc brakes are becoming more popular.
When the brake pedal is depressed, leverage multiplies the force applied from the pedal to a vacuum booster. The booster multiplies the force again and acts upon a piston in the master cylinder.
As force is applied to this piston, pressure in the hydraulic system increases, forcing fluid through the lines to the slave cylinders. The two most common arrangements of slave cylinders are a pair of opposed pistons which are forced apart by the fluid pressure (drum brake), and a single piston which is forced out of its housing (disc brake).
The slave cylinder pistons then apply force to the brake linings (generally referred to as shoes for drum brakes and pads for disc brakes). The forces applied to the linings cause them to be pushed against the drums and rotors. The friction between the linings and drum/rotor causes a braking torque to be generated, slowing the vehicle.
The brake pedal is a simple lever. One end is attached to the framework of the vehicle, a rod extends from a point along its length, and the foot pad is at the other end of the lever. The rod either extends either to the master cylinder (manual brakes) or to the vacuum booster (power brakes).
The master cylinder is divided internally into two sections, each of which pressurizes a separate hydraulic circuit. Each section supplies pressure to one circuit. Passenger vehicles typically have either a front/rear split brake system or a diagonal split brake system.
A front/rear split system uses one master cylinder section to pressurize the front slave cylinders, and the other section to pressurize the rear slave cylinders. A split circuit braking system is now required by law in most countries for safety reasons; if one circuit fails, the other circuit can stop the vehicle.
The diameter and length of the master cylinder has a significant effect on the performance of the brake system. A larger diameter master cylinder delivers more hydraulic fluid to the slave cylinders, yet requires more brake pedal force and less brake pedal stroke to achieve a given deceleration. A smaller diameter master cylinder has the opposite effect.
A master cylinder may also use differing diameters between the two sections to allow for increased fluid volume to one set of slave cylinders or the other.
The vacuum booster or "servo" is used in most modern hydraulic brake systems. The vacuum booster is attached between the master cylinder and the brake pedal and multiplies the braking force applied by the driver. These units consist of a hollow housing with a moveable rubber diaphragm across the center, creating two chambers. When attached to the low-pressure portion of the throttle body or intake manifold of the engine, the pressure in both chambers of the unit is lowered. The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed. A return spring keeps the diaphragm in the starting position until the brake pedal is applied. When the brake pedal is applied, the movement opens an air valve which lets in atmospheric pressure air to one chamber of the booster. Since the pressure becomes higher in one chamber, the diaphragm moves toward the lower pressure chamber with a force created by the area of the diaphragm and the differential pressure. This force, in addition to the driver's foot force, pushes on the master cylinder piston. A relatively small diameter booster unit is required; for a very conservative 50% manifold vacuum, an assisting force of about 1500 N (150 kgf) is produced by a 20cm diaphragm with an area of 0.03 square meters'. The diaphragm will stop moving when the forces on both sides of the chamber reach equilibrium. This can be caused by either the air valve closing (due to the pedal apply stopping) or if "runout" is reached. Runout occurs when the pressure in one chamber reaches atmospheric pressure and no additional force can be generated by the now stagnant differential pressure. After the runout point is reached, only the driver's foot force can be used to further apply the master cylinder piston.
The fluid pressure from the master cylinder travels through a pair of steel brake tubes to a compensator, which performs two functions: It equalizes pressure between the two systems, and it provides a warning if one system loses pressure. The compensator has two chambers (to which the hydraulic lines attach) with a piston between them. When the pressure in either line is balanced, the piston does not move. If the pressure on one side is lost, the pressure from the other side moves the piston. When the piston makes contact with a simple electrical probe in the center of the unit, a circuit is completed, and the operator is warned of a failure in the brake system.
From the compensator, brake tubing carries the pressure to the brake units at the wheels. Since the wheels do not maintain a fixed relation to the automobile, it is necessary to use hydraulic brake hose from the end of the steel line at the vehicle frame to the caliper at the wheel. Allowing steel brake tubing to flex invites metal fatigue and, ultimately, brake failure. A common upgrade is to replace the standard rubber hoses with a set which are externally reinforced with braided stainless-steel wires; these have negligible expansion under pressure and can give a firmer feel to the brake pedal with less pedal travel for a given braking effort.
Steel lines are preferred for most of the system for their rigidity. Any pressure induced distortion in the lines results in less useful volume and pressure of fluid reaching the slave cylinders, with reduced braking effectiveness. Finally, the fluid pressure enters the Slave Cylinders and use one or more pistons to apply force to the braking unit.
Air brake systems are bulky, and require air compressors and reservoir tanks. Hydraulic systems are smaller and less expensive. Hydraulic fluid must be non-compressible. Unlike air brakes, where a valve is opened and air flows into the lines and brake chambers until the pressure rises sufficiently, hydraulic systems rely on a single stroke of a piston to force fluid through the system. If any vapor is introduced into the system it will compress, and the pressure may not rise sufficiently to actuate the brakes.
Hydraulic braking systems are sometimes subjected to high temperatures during operation, such as when descending steep grades. For this reason, hydraulic or brake fluid must resist vaporization at high temperatures. Water vaporizes easily with heat and can corrode the metal parts of the system. If it gets into the brake lines, it can degrade brake performance dramatically. This is why light oils are used as hydraulic fluids. Oil displaces water and protects metal parts against corrosion, and can tolerate much higher temperatures before vaporizing.
"Brake fade" is a condition caused by overheating in which braking effectiveness reduces, and may be lost. It may occur for many reasons. The pads which engage the rotating part may become overheated and "glaze over", becoming so smooth and hard that they cannot grip sufficiently to slow the vehicle, vaporization of the hydraulic fluid under temperature extremes, and thermal distortion may cause the linings to change their shape and engage less surface area of the rotating part. Thermal distortion may also cause permanent changes in the shape of the metal components, resulting in a reduction in braking capability that requires replacement of the affected parts.
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