Power brake
In mechanical braking system,
when the braking force is applied by the driver by pressing the brake pedal due to the mechanical advantage combined with braking force the vehicle is stopped.However, when the vehicle’s weight and speed are increased, the mechanical braking system will be no longer capable of providing sufficient braking force to stop the vehicle immediately. Hence for providing sufficient power to the mechanical braking system and to increase the braking force we use any one of the following support as a supplemental force.This type of brake is called as power brake.This power brake is classified according to the following categories as
1. Hydraulic brake
2. Air brake
3. Vacuum brake
4. Electric brake
5. Servo brake
Hydraulic brake system
The schematic view of the hydraulic braking system can be seen in Figure.
The hydraulic braking system works based on the principle of Pascal’s
law. It is simple in construction and can provide more braking force. The hydraulic brake is capable of delivering the braking force evenly to all the wheels at a time,smoothly and consistently for all the wheels. Therefore most of the cars and some of the two wheelers are employed with hydraulic braking system.
Construction and working principle of hydraulic brake system
The construction of the hydraulic brake system could refered from the Figure. The hydraulic brake system consists of a master cylinder and wheel cylinders which are the two important components of the system. The master cylinder is used to store the brake fluid. One side of the master cylinder is connected with brake pedal and on the other side there
is a pipe line which connects the wheel
cylinders that are kept separately on the back plates.When the brake pedal is pressed, the brake fluid inside the master cylinder is pressurized and sent to the wheel cylinders through four pipe lines and pushed the
pistons inside the cylinder in outward direction. At that time the brake shoes which are connected to the pistons get expanded and force the rotating brake drum to stop which stops the rotation of the wheel.When the brake pedal is released the pressure of the brake fluid in the master cylinder is reduced. This causes the piston in the master cylinder to come back to its original position due to the spring’s action. Thus, pressure in the brake
lines is reduced completely. Due to the drop in pressure of the brake fluid, the piston inside the wheel cylinder is move inward. Due to this action, the brake shoes get contracted by the return spring and moved away from the brake drum. Thus, the wheels are allowed to rotate freely. By this way, returning action of piston in the wheel cylinder makes the brake fluid to return back to the master cylinder.
Merits and demerits of hydraulic brake system The following are the merits and demerits of the hydraulic braking system used in automobiles.
Merits
1. Unlike the mechanical brakes it
is simple in construction with no
mechanical lever and linkage.
2. The braking force in this system is
uniformly distributed to all the wheels.
3. Parts of this system are self-lubricated.
4. It occupies less space.
5. More braking efficiency could be
achieved.
6. The braking force is equally shared for. all the four wheels.
7. Even for small force of the driver on the brake pedal, the master cylinder multiplies the pressure and gives increased mechanical advantage.
8. Due to the possibility in varying the diameter of the brake pipe lines from
the wheel cylinders, variable braking efficiency could be achieved.
Demerits
1. Even a small leakage in the brake lines makes the entire system failure.
2. The air entering into the system reduces the braking efficiency.
3. There is a chance of oil leakage from wheel cylinder. Due to this the
frictional force and braking force get reduced in between brake drum and
brake shoe.
Quality of the brake fluid
The brake fluid is an important part which acts as a brake force transmitter in the hydraulic braking system. The compression of the brake fluid increases the pressure in the system. However the volumetric capacity of the fluid does not change. The pressure applied at one point on the fluid is equally distributed in all directions, based on the scientific law Pascal by that way the hydraulic braking system works. The brake fluid is generally
made up of glycol with different alcoholic additives. In addition, the brake fluid can also be prepared by good quality silicon based fluids.
In general the quality brake fluids such as DOT3, DOT4, DOT5, DOT5.1 are commonly used in modern vehicles. These brake fluids are certified by the
Society of Automotive Engineers (S.A.E).The DOT3 type brake fluid is widely used in both cars and heavy vehicles.Requirements of a brake fluid
The following are the important requirements of a brake fluid to be used
in automobiles.
1. It should withstand high temperature.The boiling point of the brake fluid
should be approximately 300 deg C.
2. Even at high temperatures the brake fluid should not lose it’s viscosity and
lubricity.
3. It should lubricate the various parts of the braking system.
4. It should not react and corrode with the rubber and metal parts of the
braking system.
5. It should not lose its properties when it is stored for very long time (upto the maximum of 3 years).
Parts of the hydraulic braking system
The hydraulic braking system of an automobile consists of the following parts
1. Brake pedal
2. Master cylinder
3. Brake Fluid pipelines
4. Hose pipes
5. Wheel cylinder
6. Brake shoes
7. Brake linings
8. Return springs
The parts listed above are discussed
in the following paragraphs.
Master cylinder and its type
Master cylinder acts as the heart
of the hydraulic braking system. By pressing brake pedal, master cylinder
supplies the required amount of the pressurized brake fluid to the wheel
cylinders for expanding the brake shoes of the wheel cylinder. There are three different types of master cylinders used in automobiles. They are
1. Simple master cylinder
2. Tandem master cylinder
3. Center valve type master cylinder
Among the above types, the first two are mostly used in automobiles.
The details of construction and working principle of the above two cylinders are
discussed below.
Simple type master cylinder Construction
The master cylinder is generally made up of cast iron material. The master
cylinder consists of a reservoir tank for storing brake fluid and a compressor unit for operating the braking system. In these
two parts are completely filled with brake fluid. In the top of the reservoir unit there is a filler cap fitted for pouring the brake oil and it is closed tightly. In the filler cap a small air vent hole is provided for maintaining the atmospheric air pressureb in the reservoir. The sectional view of
the master cylinder can be seen in Figure
The piston travels front and back inside the reservoir chamber. The push rod and brake pedal are connected with each other.As the return or helical spring is placed in front of the piston, the piston always stays in the direction where the
push rod stays. With this arrangement a check valve is also connected with four wheel adapter for transferring brake fluid to four wheels. In addition to this operating system a brake light switch is also connected. There are two ports called as inlet port and bypass port located on the wall which separates reservoir tank and compressor unit. The inlet port is made as slightly larger, whereas the bypass port is smaller in their construction. In the front face of the piston small feed holes are provided. In front of the feed holes a primary rubber cup is also present. These feed holes allow the brake fluid to move from backside of the piston to the front. A check valve and a return spring are provided in front of the primary rubber cup. A secondary rubber cup is also located at the back side of the piston for avoiding brake fluid leakages. In this arrangement, the check valve and primary rubber cup arrangement act as one way valve.
Working of master cylinder
When the brake pedal is pressed
Normally, the piston inside the master cylinder is positioned in the backward position due to the action of the tensile force of the return spring. Hence due to the opening of inlet and bypass ports
the brake oil is filled at the front and back side of the piston. When the brake pedal is pressed by the driver, the piston rod connected to the brake pedal pushes the piston in the forward direction. Because of this action, the two holes are sealed and fluid is pressed well. The compressed fluid is then passed through the one way valve to the wheel cylinder through the pipe lines connected with it. Now the two pistons inside the wheel cylinder are pushed outward. Because of this action the brake shoes are expanded with help of the piston’s push rod which forces the brake drum to stop from rotation and stops the wheel from the rotation.
When the brake pedal is released
The pressure inside the master cylinder is reduced. At the same time due to the tensile force of return spring brake shoes are relieved from the brake drum and pulled towards in. Hence the pistons inside the wheel cylinders are moved inward. Hence the brake fluid which is staying in the pipes is forced backwards and sent to the master cylinder through the test valve.
In the master cylinder when the piston moving in backward direction, the brake fluid is initially sent to the reservoir through bypass port. At the same time through the feed holes in the piston the brake fluid is moved to the front side of the piston. Hence the required fluid for the next piston operation is made available at the front side of piston.
Tandem master cylinder
In normal master cylinders only one outlet is generally provided. Moreover, from the location point of view, the master cylinder is located very near to the front wheels and the rear wheels are far away from the master cylinders. Hence, while braking the time taken by the brake fluid to travel from the master cylinder to reach the rear wheel cylinder becomes long. Hence the applied braking force is varied
for both the front and rear wheels. Hence, Tandem master cylinder was designed to overcome the above said problem. The photographic view of the tandem master cylinder can be seen in Figure.
In this cylinder there are separate outlets available for front and rear wheels.The brake fluid is passed to the wheel cylinders through separate pipelines and the braking action is performed. In this type of master cylinders, initially the brake fluid is sent to the rear wheels and then to the front wheels. By that way equal amount of braking force is applied at a time for both the wheels. Hence the
chance for the faulty operation or faulty situation during braking is eliminated. In the modern vehicles mostly tandem type master cylinder is used. The construction and working principle of the Tandem master cylinder can be seen in Figure.
While pressing brake pedal
Similar to the normal master cylinder, tandem master cylinder also has pistons, primary rubber cap, return spring with test valves – all are in two sets, therefore when the driver press the brake pedal, the piston inside the cylinder is moved by means of the push rod and hence the brake fluid is discharged initially to the rear wheels. In addition, while pressing the brake pedal the second piston is also finely moved and through another way the brake fluid is passed to the wheel cylinder of the front wheels and the brakes are
activated. When the legs are removed from the brake pedal, the brake fluid pressure is reduced. Hence the brake fluid in the wheel cylinders is pushed backward by means of pistons and comes back to the master cylinders.
Through the pipelines the brake fluid reaches the inlet port and pushes the
pistons out there. Hence the both brake shoes are expanded against tensile force caused by the return spring and mate with brake drum which create friction and tend
to stop the wheels. Also by releasing brake pedal the fluid pressure is got reduced. Furthermore, by means of return spring the brake shoes are pulled and the pistons get back to its original positions.
