How Vacuum Booster Brake Works?

Vacuum Booster:


Vacuum booster has the following parts arranged as shown in the diagram.
Servo Cylinder: It is a simple piston-cylinder arrangement in which one side of the piston rod is connected with the brake shoe.  The piston is subjected to vacuum at both sides in the brake released position. When the brake is applied one side of the piston is exposed to the atmosphere due to which an immediate braking force applied to a piston which pushes the brake shoe.

Vacuum Reservoir: When the engine is in running position the vacuum in the servo cylinder is created by inlet manifold but when the engine is stopped the vacuum reservoir is used to create a vacuum in the servo cylinder.

Control Unit: The control unit is used to apply the brake. It is also a piston-cylinder arrangement in which the upper valve is used to connect atmosphere to the servo cylinder and the lower valve is used to connect the vacuum reservoir to the servo cylinder. When the brake is applied the upper valve is in the open position while the lower one is in the closed position.

NRV: NRV which means non-return valve is used to connect servo cylinder to inlet manifold which allows the only one-sided flow of air from servo cylinder to inlet manifold.

Master Cylinder: The master cylinder is the same as used in hydraulic brake which is used to operate the control unit.


Its working can be easily understood by the following points.
  • When the brake is in release position both side to servo cylinder is exposed to vacuum. The upper valve of the control unit is in the closed position while the lower valve is in the open position.

  • Now when the driver presses the brake pedal,  the master cylinder comes into action which pushes the piston of the control unit in an upward direction. This movement of piston opens the upper valve of the control unit and simultaneously closes the lower one.

  • The opening of the upper valve allows entering air into the servo cylinder left side through the control unit. This will create a differential pressure in the servo unit. This will pushes the servo piston at the right side thus applied brake.

  • This arrangement of vacuum booster considerably reduces the effort applied by the driver and the whole braking force is applied by the vacuum. 
Vacuum brake is mostly used in railway as an alternative of air brake.

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