A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. In the USA it is commonly called a brake booster.
It is used on virtually all vehicles which use hydraulic brakes for their primary braking circuit. Vacuum servos are not used on vehicles which use cables, rods (or other mechanical linkages), or pressurized air systems for their primary brake circuits.
It uses a stored vacuum to multiply (assist - it is not a multiplier of force) braking force applied by the driver to the brake pedal, before applying the transferred force to the brake master cylinder.
The vacuum is generated in two distinct methods, dependent on the type of internal combustion engine, or other motive force (as in electric vehicles). In petrol engines, the manifold vacuum is utilized, whereas in diesel engines, a separate vacuum pump is used. The vacuum is transferred to the servo along semi-rigid plastic lines, and is stored in the servo by using a non-return valve.
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 run out point is reached, only the driver's foot force can be used to further apply the master cylinder piston.
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