Hydraulic Brakes – Parts, Working, Diagram, Advantages and Disadvantages

Hydraulic Brake : 

The brakes which are actuated by the hydraulic pressure (pressure of fluid) are called hydraulic brakes. Hydraulic brakes are commonly used in automobiles. Principle Hydraulic brakes work on the principle of Pascal’s law which states that “pressure at a point in a fluid is equal in all directions in space”. According to this law when pressure is applied on fluid it travels equally in all directions so that uniform braking action is applied on all four wheels.

Construction and Working of Hydraulic Brakes 

When the brake pedal is pressed, the force is transmitted to the brake shoes through a liquid (link). The pedal force is multiplied and transmitted to all brake shoes by a force transmission system. The figure shows the system of a hydraulic brake of a four-wheeled automobile. It consists of a master cylinder, four-wheel cylinders, and pipes carrying a brake fluid from the master cylinder to a wheel cylinder.
Working Of Hydraulic Brake -Components Of Hydraulic Brake System 
Working Of Hydraulic Brake -Components Of Hydraulic Brake System
The most common arrangement of hydraulic brakes for passenger vehicles, motorcycles, scooters, and mopeds, consists of the following:
a) Brake pedal or lever
b) A push rod (also called an actuating rod)
c) A master cylinder assembly containing a piston assembly (made up of either one or two pistons, a return spring, a series of gaskets/ O-rings and a fluid reservoir)
d) Reinforced hydraulic lines
e) Brake caliper assembly usually consisting of one or two hollow aluminum or chrome-plated steel pistons (called caliper pistons), a set of thermally conductive brake pads, and a rotor (also called a brake disc) or drum attached to an axle.
hydraulic braking system diagram
hydraulic braking system diagram

The master cylinder is connected to all the four-wheel cylinders by tubing or piping. All cylinders and tubes are fitted with a fluid that acts as a link to transmit pedal force from the master cylinder to wheel cylinders. Brake Fluid The fluid-filled in the hydraulic brake system is known as brake fluid. It is a mixture of glycerine and alcohol or castor oil and some additives. The Master cylinder consists of a piston that is connected to peal through the connecting rod. The wheel cylinder consists of two pistons between which fluid is filled. Each wheel brake consists of a cylinder brake drum. This drum is mounted on the inner side of the wheel. The drum revolves with the wheel. Two brake shoes that are mounted inside the drum remain stationary. Heat and wear resistant brake linings are fitted on the surface of the brake shoes. 

Working of Hydraulic Brakes 

When the brake pedal is pressed to apply the brakes, the piston in the master cylinder forces the brake fluid. This increases the pressure of the fluid. This pressure is transmitted in all the pipes and up to all-wheel cylinders according to Pascal’s law. This increased pressure forces out the two pistons in the wheel cylinders. These pistons are connected to brake shoes. So, the brake shoes expand out against brake drums. Due to friction between brake linings and drums, wheels slow down and brakes are applied.

Working of master cylinder of Hydraulic brake:

1) Brakes applied:
When the brake pedal is operated, the pushrod moves the piston against the action of the spring force. When sufficient pressure is built up, the rubber cap of fluid check valve deflects and the high-pressure fluid enters the wheel cylinder through fluid lines and operates the brake shoe against the revolving drum.
2) Brakes released:
When the pedal is released, the piston return towards its initial position due to the spring force and closes the fluid check valve for a short time to avoid entry of any air. The fluid from the lines also comes back in the compression chamber by lifting the check valve off its seat.

Method of Bleeding of hydraulic brakes:

1) Pressure Bleeding
a) Using air b) By forcing brake fluid
2) Manual Bleeding
3) Gravity bleeding
1) Pressure bleeding:
Pressure bleeder is a device used for bleeding procedure that is attached to the master cylinder. The pressure bleeder supplies pressurized brake fluid to the master cylinder.
When the bleeder screw is opened, the pressure force air and brake fluid out of the bleeder screw. With a pressure bleeder, you can bleed the hydraulic system without any helper. The pressure used in pressure is usually 104 to 138 KPa.
pressure bleeding
pressure bleeding
2. Manual bleeding of Hydraulic brakes:
Two service technicians are needed for the manual bleeding. One technician opens a bleeder and the other technician depresses the pedal, to force out air and brake fluid from the bleeder screw. To bleed the system following procedure is adopted.
a) Attach a bleeder hose to bleeder screw at the wheel cylinder and insert the other end of the hose into the clean plastic container which is partially filled with clean brake fluid.
b) Loosen the bleeder screw at least one full turn.
c) Have an assistant to depress and hold the brake pedal and then tighten the bleeder screw.
d) Have your assistant to release the brake pedal.
e) Repeat steps b,c & d until the fluid flow in the container is free of air bubbles. Periodically check the brake fluid level in the master cylinder and brake fluid of correct grading to keep the reservoir filled.
f) Repeat this procedure at each wheel.
Manual bleeding of hydraulic brakes
Manual bleeding of hydraulic brakes

3) Gravity bleeding:

Gravity bleeding is the method of bleeding that uses the earth’s gravity to bleed air from the hydraulic system. No external force is applied to brake fluid. To bleed the system following procedure is adopted.

a) At the wheel, the cylinder loose the bleeder screw at least one full turn.
b) Remove the cover from the master cylinder reservoir. The level of brake fluid to flow from the bleeder screw.
c) Watch the bleeder hose when brake fluid flows from opening and tightening the screw.
d) Repeat this procedure at each wheel in sequence and it should be changed.

Types of Hydraulic Braking System : 

The hydraulic braking scheme is categorized as, 

1. Based on the frictional contact system, there are two kinds of hydraulic pumps–

(i)drum brake or inner hydraulic valve expansion.
(ii) Hydraulic disk brakes or internal hydraulic brakes.

2. Based on the brake force allocation-there are two kinds of hydraulic brakes-

(i) single-acting hydraulic pumps
(ii)dual-acting hydraulic pumps

Hydraulic Disc Brake:

Construction of Hydraulic Disc Brake :

A disc brake consists of a rotating disc and two friction pads that are actuated by a hydraulic braking system. The friction pads remain free on each side of the disc when brakes are no applied. They rub against the disc when brakes are applied to stop the vehicle.

Hydraulic disc Braking System Diagram
Hydraulic disc Braking System Diagram

Working of Hydraulic Disc Brake :

In a disc brake, the fluid from the master cylinder is forced into a caliper where it presses against a piston. The piston in turn crushes two brake pads against the disc that is being attached to the wheel, making it to stop or slow down. The main advantage of disc brakes is their resistance to wear as the discs remain cool even after repeated brake applications.

Difference between Mechanical Braking System and Hydraulic Braking System 

Mechanical Brakes vs hydraulic brakes – Comparison between Mechanical Braking System and Hydraulic braking system is as follow,
Sr. no.Mechanical Braking SystemHydraulic Braking System
1.Less Braking efficiency.More Braking Efficiency.
2.Poor Anti fade characteristicsBetter Anti-fade characteristics
3.Complicated due to more parts.Simple in construction
4.It do no self compensateSelf-compensating system
5.Construction is less flexibleConstruction is more flexible
6.Low mechanical advantageHigh mechanical advantage
7.External Lubrication is requiredSystem is self-lubricating
8.No leakage problemLeakage may take place
9.No hydraulic oil useHydraulic oil is used
10.More effort required for braking
Less effort required for braking operation
12.Ex: Motor CyclesEx: Cars and Jeeps.

Difference between Hydraulic and Pneumatic Braking System:

Hydraulic vs Pneumatic braking System – Comparison between Hydraulic and Pneumatic Braking System

Sr. no. Hydraulic Braking SystemPneumatic Braking System
1.Braking Fluid used as a working mediumCompressed air is used as a working medium
2.Simple in construction & less expensive.Complicated in construction and expensive.
3.Occupied less space as compared to Air brakeOccupied more space as compared to Hydraulic brake
4.The system is self-lubricatingNeed to lubricate mechanical parts
5.Bleeding is necessaryNo need for bleeding
6.The increased braking effort, but less powerful than air brakes.Most powerful than Hydraulic brake
7.Low maintenance costMaintenance cost is more
8.Mostly used in passenger cars, LMVsMostly used in heavy vehicles like buses and trucks.

Advantages and Disadvantages of Hydraulic Braking System : 

Hydraulic Braking System has some advantages and limitations over other Braking System i.e. Mechanical, Pneumatic braking System ; 

Advantages of Hydraulic Braking System : 

The advantages of the hydraulic braking system are as follows,

(a) Equal braking action on all wheels.
(b) Increased braking force.
(c) Simple in construction.
(d) The low wear rate of brake linings.
(e) The flexibility of brake linings.
(f) Increased mechanical advantage.

Disadvantages of Hydraulic Braking System : 

Disadvantages of the hydraulic braking system are as follows,

(a) The whole braking system fails due to leakage of fluid from brake linings.
(b) The presence of air inside the tubings ruins the whole system
(c) Maintenance is more than other braking systems.
(d) Construction is complex.

See Also: Working Of Pneumatic /Air Braking System 

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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