User:KDS4444/Hydraulic brakes (redone)

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, motorcycles, scooters, and mopeds, consists of the following:

A glycol-ether based brake fluid usually fills the system and manages the transfer of force/ energy between the brake lever and the wheel.
 * A brake pedal or lever
 * A pushrod, also called an actuating rod
 * A master cylinder assembly containing:
 * A piston assembly (also called a plunger assembly) made up of:
 * Either one or two pistons (or plungers)
 * A return spring
 * A series of gaskets/ O-rings
 * A fluid reservoir
 * Reinforced hydraulic lines
 * A brake caliper assembly usually containing:
 * One or two hollow aluminum or chrome-plated steel pistons called caliper pistons
 * A set of thermally conductive brake pads secured by steel springs or clips
 * A rotor (also called a brake disc) or a drum attached to a wheel

At one time, passenger vehicles commonly employed disc brakes on the front wheels and drum brakes on the rear wheels. However, because disc brakes have been shown a better stopping performance and are therefore generally safer and more effective than drum brakes, four-wheel disc brakes have become increasingly popular, replacing drums on all but the most basic vehicles. Many two-wheel vehicles designs, however, continue to employ a drum brake for the rear wheel.

For simplicity, the braking system described hereafter uses the terminology for a disc brake.

System Operation
Within a hydraulic brake system, as the brake pedal is pressed/ brake lever is squeezed, a pushrod exerts force on the piston(s) in the master cylinder causing fluid from the reservoir to flow into a pressure chamber through a fluid intake and return port. As the piston passes this port, it creates a fixed volume of fluid which it then begins to push out of the master cylinder and into the hydraulic lines toward one or more calipers. Here it acts upon one or two caliper pistons secured by one or more seated O-rings which prevent the escape of any fluid from around the piston.

The caliper pistons for a drum brake are a pair of opposed cylinders which are forced apart by the fluid pressure, while for a disc brake a single piston is forced out of its housing.

The brake caliper piston(s) then apply force to the brake pads. This causes them to be pushed against the rotating metal of the rotor, and the friction between the pads and the rotor causes a braking torque to be generated, slowing the vehicle. Heat generated from this friction is usually dissipated through vents and channels in the rotor, through the brake fluid, and through the pads themselves which contain a layer of specialized heat-tolerant materials (kevlar, sintered glass, et al.).

Subsequent release of the brake pedal/ lever allows the spring(s) within the master cylinder assembly to return the assembly's piston(s) back into position. This relieves the hydraulic pressure on the caliper allowing the brake piston in the caliper assembly to slide back into its housing and the brake pads to release the rotor. By design, at no point is any of the brake fluid intended to leave the system.

Component specifics
(For typical light duty automotive braking systems)

The brake pedal is a simple lever. One end is attached to the framework of the vehicle, a pushrod extends from a point along its length, and the foot pad is at the other end of the lever. The rod either extends to the master cylinder (manual brakes) or to the vacuum booster (power brakes).

In a four-wheel car, 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 (the master cylinder in a motorcycle or scooter may only pressurize a single unit, which will be the front brake).

A front/rear split system uses one master cylinder section to pressurize the front caliper pistons, and the other section to pressurize the rear caliper pistons. 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 caliper pistons, 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 caliper pistons or the other.

Power brakes
The vacuum booster or vacuum servo is used in most modern hydraulic brake systems which contain four wheels. 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 movable 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 "run out" is reached. Run out 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 pressure differential valve, sometimes referred to as a "brake failure valve", which performs two functions: it equalizes pressure between the two systems, and it provides a warning if one system loses pressure. The pressure differential valve 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 pressure differential valve, 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.

Special considerations
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 plastic 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.

Patents

 * . Kinchin 1956-05-22
 * . Dubois 1952-04-08
 * . Martin 1951-03-13
 * . Bryant 1949-10-08
 * . Johnson Wade C, Trishman Harry A, Stratton Edgar H. 1949-04-12
 * . Fitch 1947-02-12
 * . Lambert Homer T. 1946-08-06
 * . Lambert Homer T. 1945-05-15
 * . Forbes Joseph A. 1944-12-26
 * . La Brie 1938-12-20
 * . Poage Robert A. and Poage Marlin Z. 1937-06-15
 * . Avery William Leicester 1936-02-21
 * . Buus Niels Peter Valdemar 1934-05-15
 * . Norton Raymond J 1934-04-10
 * . Boughton Edward Bishop 1929-07-16
 * . Borgwar Carl Friedrich Wilhelm 1940-09-06
 * . Hall Frederick Harold 1932-07-28
 * . Rubury John Meredith 1932-01-06