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Hydraulic brake

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The hydraulic brake is an arrangement of braking mechanism which uses hydraulic fluid, typically some type of light-viscosity petroleum oil, 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 brake, found on most automobiles, consists of a brake pedal, a master cylinder, hydraulic lines, a "slave cylinder", and the braking unit.

General operation

When the brake pedal is depressed, leverage multiplies the force applied from the pedal to a piston in the master cylinder.

As force is applied to lines to the slave cylinders.

The two most common arrangements of slave cylinder are a pair of opposed pistons which are forced apart by the fluid pressure, (drum brakes) and a single piston which is forced out of its housing (disc brakes.)

The pistons then apply pressure to the braking mechanism, whether shoes inside a drum, or pads which compress on a disc.

Component specifics

(For typical light duty automotive braking systems.)

The brake pedal is a simple lever. It is attached at one point to the framework of the automobile, a rod extends from a point along its length to the master cylinder, and the pedal is at the end of the lever.

The master cylinder is divided internally into two sections, each of which pressurizes a separate hydraulic line. The forward segment applies pressure to the front brakes, and the rearmost segment to the rear brakes. This arrangement is made partly to balance the braking effect between the front and rear wheel sets, and partly for safety reasons: If one system fails, all braking ability is not lost; the other set can stop the vehicle. A master cylinder may use differing diameters between the two pistons to allow for increased fluid volume to one set of slave cylinders or the other.

Many modern hydraulic brake systems have a "Vacuum-assist" module which is attached between the master cylinder and the brake pedal and multiplies the braking force applied. Typically described as a vacuum booster, 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. 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 atomospheric 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 lines 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, steel lines carry 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 flexible lines at the pivoting areas. Allowing steel lines to flex invites metal fatigue and, ultimately, brake failure.

Steel lines are preferred for most of the system for their rigidity; any amount of bulging or pressure induced distortion in the lines when pressure is applied results in less useful volume and pressure of fluid reaching the slave cylinders, and thus, reduced braking effectiveness. Bundy tube is often used in cars. Flexible lines are usually rubber hoses surrounded by steel braid, then coated with rubber to avoid weather damage to the steel.

Finally, the fluid pressure enters the Slave Cylinders and use one or more pistons to apply force to the braking unit.

Special considerations

Hydraulic systems are used where space restrictions must be considered. Air brake systems are bulky, and require air compressors and reservoir tanks for their operation. Hydraulic systems are relatively smaller and less expensive.

Hydraulic fluid must be non-compressible. Unlike air systems, where a valve is opened, and air is allowed to surge in to the lines and brake chambers until the pressure rises sufficiently, hydraulic systems rely on a single stroke of a piston to force hydraulic fluid through the system. If any sort of vapor is introduced into the system, it will compress, and fluid pressure in the system may not rise sufficiently to actuate the brakes. This can lead to loss of control of the vehicle.

Hydraulic braking systems are sometimes subjected to high temperatures during operation in extreme environments such as when descending steep grades. For this reason, hydraulic fluid must resist vaporization under temperature extremes. 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 the reason for the common use of light oils as hydraulic fluids; oil displaces water and coats metal parts, protecting them against corrosion, and it can tolerate much higher temperatures before vaporizing.

"Brake Fade" is a condition caused by overheating in which braking effectiveness fades, and ultimately is lost. It may occur for a number of reasons: The pads which engage the rotating part may become overheated and "glaze over" (Become so smooth and hard that they cannot grip the metal sufficiently to slow the vehicle), vapor may be introduced to the system by vaporization of the hydraulic fluid under temperature extremes, and thermal distortion may cause the pads 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 parts, resulting in a reduction in braking capability that is irreparable without complete replacement of the affected parts.

See also