The Brakes that exist
There are various brakes that exist to inhibit motion by absorbing energy from a moving system.
Types Of Brakes
They are obsolete now as service brakes but are used as parking or emergency brakes.
- Brake shoe operation– Brake shoes are operated using cam or toggle lever. To make both the shoes leading, when the brakes are applied, expander forces the arm of the upper bell crank lever. This transmits its motion through the vertical strut to the lower arm of bell crank lever and mover towards left. But the adjuster doesn’t move towards left and gives a reaction and the brake shoe at lower end moves towards the right and acts as a leading shoe. The disadvantage of both wheels becoming trailing on reverse can be remedied by using the arrangement on both the shoes. In reverse, the shoes bear (pivot) about expanding mechanism and become leading.
- Linkage– The outline according to which the effect of our pedal reaches the wheels is-
- Brake compensation–
- When near and offside linings have unequal wear the braking effect on the wheels will not be uniform and brake application will not be smooth. The compensator is used to obviate this. Consider the balance beam compensator. Rod A is operated by the brake pedal and is hinged to the balance beam D, rod B operates the offside brake, rod C operates the nearside brakes, both of them are hinged to opposite corners of rod D.
- Now if the wear of the offside shoe is more, When the brake is applied, the balance beam also moves parallel to its length till the nearside shoe touches the brake drum. Now the further movement of the rod A causes the balance beam to rotate about the point of contact of nearside shoe and drum till the extra clearance is filled. After this forces in rods increase and brakes are applied. *This type of compensator fails when the coefficient of friction of brake linings are different in different brakes, it may be due to the presence of oil in the drum. Compensator used between the front and rear brakes have the brake lever A pivoted on a shaft. Lever B is carried on the pin by lever A. Lower end B is attached through the pin to a rod for applying rear brakes, while upper-end B is similarly connected through member C to lever D, the lower end of which carries the operating rod for front brakes. When the brake is applied, A moves towards left causing lever B to move left, which causes the rods going to the front and rear to apply brakes. Then the rod will rotate towards the side where clearance is more and brake is applied afterward. Safety stops are present to prevent the excessive turning of rod A.
They are widely used in most of the cars today. Master cylinder contains the reservoir for the brake fluid it is operated by the brake pedal and further sends the fluid to all the wheel cylinders. A pressure of 50 kPa is maintained in the pipelines even when the brakes are released to ensure that the cups of the wheel cylinders are kept expanded, prevents the air from entering the wheel cylinders, keeps the free travel of the pedal minimum by opposing the brake shoe retraction springs and prevents air from returning into the line during bleeding and pushes it out.
The main components of hydraulic brakes are-
- Master Cylinder–
- It is divided into two parts, a fluid reservoir, and a compression chamber. The fluid reservoir fills any change in the fluid volume in the pipes due to temperature or leakage. The compression chamber has rubber seals on both the ends to avoid the leakage. A rubber boot covers the pushrod end of the master cylinder to avoid dirt from entering. A fluid check valve is located on one end of the compression chamber to retain the residual pressure in the brake line when the brake is released. The piston head has holes to avoid excess pressure. The fluid chamber and compression chamber are connected by two ports, bypass port, and intake port. The pushrod is operated with foot brake pedal linkage.
- When the brake pedal is pressed, the push rod moves piston left till it covers the bypass port, further movement causes the pressure to build up, when sufficient pressure has built up the fluid is sent to the wheel cylinders and brakes are applied. When the brakes have released the piston in the compression chamber takes its initial position. The delay in return of fluid in the compression chamber due to pressure or inertia causes a vacuum, which if not destroyed may cause the leakage of air in the system. As the air is compressible it will make the braking effect null. Till the vacuum is destroyed the fluid from the lines also returns but as the compression chamber is full the fluid remains in lines and the pressure is not released and the brake shoe keeps rubbing with the drum and more heat is generated it causes the fluid to expand and more pressure is built up and the shoes keep rubbing the drum. To avoid this a bypass port is used. Which will send the extra fluid to the fluid chamber, where the pressure is maintained by exposing it to the atmosphere via a vent.
- Tandem master cylinder–
- Here separate lines go to different sections of the brake system, so that if one side of the bakes are damaged other side will work. Consider the case where the front brakes are not functioning then the piston 2 will move freely till 3 and then the pressure will build up between 1 and 2, which will apply rear brakes. Similarly, when the rear brakes are faulty the space 1-2 will not have any pressure and after the piston 1 reaches piston 2 pressure will build up between 2 and 3. Even in case of failure, the effectiveness remains about 75% of original value. They are used in light vehicles like passenger cars, Moreover, the chambers can be designed in different size according to the braking effect required on the axles. The braking effect is directly proportional to the swept volume of the chamber.
- Pressure differential valve with warning light switch–
- It is used to warn the driver about the failure of one part of the hydraulic system. When both the sides of the braking system are working, the pressure keeps the plunger in the middle, However, when one half of the system fails it causes the plunger to move to either right or left thereby pressing the switch and the warning light is on.
- Proportioning valve–
- Due to weight transfer and presence of different types of brakes at front and rear, brake application requires different forces to be applied to wheel cylinder or caliper at the front and at the rear. This is done by the proportioning valve.
- Metering valve–
- It delays the application of pressure to the front brakes till sufficient pressure has been built up in the rear lines to overcome the force of return springs, which is 800-900kPa, at this time the valve is fully open. The valve is closed when the brakes are released. Return valve will open to allow the fluid to return the fluid from the lines.
- Wheel cylinder–
- They force the brake shoe against the drum. It consists of pistons, rubber seals, cup spreaders, spring and dust covers. Bleed screw with cover. They are mounted on a backplate. The fluid from the master cylinder enters the wheel cylinders from the inlet port and forces the piston to move out and push the shoe against the drum.
- Bleeding of drums–
- Air is compressible in nature and gets compressed when the pedal is pressed, it doesn’t transmit the fluid pressure and no brakes are applied. The procedure of driving air out of the braking system is called bleeding. For bleeding, the master cylinder is filled with the fluid completely and a pipe is connected to the bleeding valve nipple and dipped in a jar filled with fluid. Now the brake pedal is pressed several times and the bleeder valve is opened so that the air bubbles escape. This procedure is repeated until the air escapes completely. This is done on all wheels. After completing the procedure, the bleeding valve is covered with a vacuum cap.
Advantages of the hydraulic system–
- Equal pressure is exerted in all the lines.
- Wear is very less, due to the absence of joints and linkages.
- The system is self-lubricating.
Disadvantages of the hydraulic system–
- Leakage of air in the system makes it useless.
- Not suitable for parking brakes.
It consists of an electromagnet within the brake drum, which actuates the cam and expands the brake shoe. The braking effect is controlled by a rheostat (variable resistance), operated by the brake pedal.
Advantages of Electric brakes–
- No complex linkages.
- Very less time lag in the application.
Servo Brake Systems-
It refers to a mechanism that adds to the driver’s effort in applying the brakes. These mechanisms are required when the driver can not apply the required effort conveniently.
- Mechanical Servo Mechanism–
- Disc A is always rotating when the vehicle is in motion. It is usually connected to the propeller shaft. A disc B is pressed against disc A with force F. This force applies the torque on disc B. thereby pulling the rod D and applying the brakes equal to the amount equal and opposite to the torque on disc D. Then as the disc A stops disc B slips to its initial position.
- Disc Brakes with Servo Action–
- The brake drum contains two discs. Disc A has a cylindrical groove while disc B has a piston that fits in that groove and several grooves with balls. When the brake pedal is pressed fluid in the groove pushes the disc B against the brake drum and is set into rotation. Disc A is still stationary so the balls ride the sloping sides of the grooves and forces are introduced normally, which press the linings and provide desired servo action. The slope of the grooves decides the extent of servo action, but increasing the angle beyond a certain limit causes brakes to jam and also cause excessive wear of lining.
- Vacuum Servo Brakes– It uses engine suction from the inlet manifold for braking. They are of two types, both have piston-cylinder arrangement with suitable linkages for braking and a small vacuum reservoir to provide enough vacuum –
- Here both sides of the piston are exposed to the atmosphere when brakes are not applied and engine suction is applied to one side that moves the linkage due to differential pressure on both sides and the brakes are operated.
- Here both the sides of the piston are subjected to engine suction. To apply brakes one of the sides is exposed to the atmosphere to apply the desired force. It is called Suspended vacuum system. Its rapid in action and is more preferred.
- Vacuum Reservoir–
- It is connected to inlet manifold via a non-return valve between the carburetor and engine. The other side is connected to servo cylinder on both sides of the piston, one side is directly connected and other is through the control unit.
- Control unit–
- It also contains a piston with valves on both side, the upper side is connected to the atmosphere and left side is connected to the servo cylinder. This piston is controlled using a brake cylinder. When the brake pedal is free upper valve is closed and lower is open, so both the sides of the piston are exposed to engine vacuum. When the brake pedal is pressed the brake fluid presses the piston up, closing the left side valve and opening the upper valve. The left side of the servo piston is exposed to the atmospheric pressure and vacuum acts on the right side. The piston moves to the right and the brakes are applied. This reduces the braking force and the whole braking is by engine vacuum.
- Vacuum Booster–
- It is more commonly used servo cylinder operated directly through the brake pedal. It actuates the push rod to operate the master cylinder, placed in front of the booster unit. The piston is exposed to engine suction on both sides and when brakes are placed a valve opens and exposes one side to atmospheric pressure.
- Tandem Mastervac unit– It contains two suspended diaphragms. Working is the same as the single diaphragm.
Hydraulic Booster Brakes–
- They are called Bendix Hydromax brakes. They are compact and light and suitable for diesel engine vehicles. It consists of a hydro max booster and mini master cylinder. The fluid under pressure from the power steering system enters the inlet port of hydro max booster, flows through the pressure valve and power piston, the flow switch and exits from the return port. When the brakes are applied, the actuator closes the compensative valve and restricts the flow through the power piston.
- Further movement of the primary piston then creates the pressure in the primary chamber causing the secondary piston to move and close the compensating valve of secondary chamber and further movement increases pressure, which is distributed to different wheel cylinders to apply brakes. This results in differential pressure on power piston and it applies force on the master cylinder. The mini master cylinder has two chambers for tandem operation. Both chambers have a piston subassembly carrying a caged actuator spring and return spring. In the released position, actuators of both pistons are in contact with their respective compensative valve system, thereby maintaining the valve in open position and replenish the fluid displaced from the chambers.
In case the fluid from power steering pump is stopped due to any fault, an attached electric pump provides power for reserve stops. When both these systems fail brakes can still be applied manually but with greater force.
Engine Exhaust Brakes-
It consists of a pressure regulator, foot control valve for actuation and air cylinder where compressed air operates. When the foot control valve is pressed, compressed air from air tank enters the air cylinder, where it operates a linkage to close the butterfly valve at the exhaust manifold and also cut-off the fuel supply. This brake is very effective under 40 kmph, but it can’t stop the vehicle like service brakes.
They are similar to the hydraulic brakes but compressed air is used here. It is used in heavy vehicles. The compressor takes air from the atmosphere, it passes from the filter and goes to the reservoir through the unloader valve, which gets lifted at a predetermined pressure (about 900 kPa) and relieves the compressor of the load. The reservoir supplies air to various accessories and also the brake chamber (diaphragm units) at each wheel. When brakes are applied, the pressure in the reservoir decreases, when the pressure is below 700 kPa, the governor again cuts in the compressor to raise the system pressure.
The various components are-
- Unloader Valve– Excessive air pressure will cause fierce braking and low air pressure will cause inadequate braking.
- When the reservoir pressure increases, pressure also builds up in the hollow plunger, its end is sealed by the spring loaded inlet exhaust valve. The pressure lifts the diaphragm assembly against the pressure setting spring. When the preset pressure value is reached, the inlet exhaust valve closes the exhaust passage. This air pressure also acts upward on the governor plunger and opens the inlet. When the air pressure moves the unloader plunger against the force of its spring it unseats the unloader valve allowing the air from the compressor to pass through exhaust check valve.
- When the reservoir pressure decrease. The diaphragm assembly is depressed where the plunger closes the inlet valve and opens the exhaust valve so that the air pressure in the unloader plunger closes the inlet valve and opens the exhaust valve so that the air pressure in unloader plunger bore escapes to atmosphere through the exhaust check valve. The opening of this valve pushes the unloader valve in its seat and stops the air flow. This causes the pressure in the reservoir to increase.
- Reservoir– It is made up of a steel sheet and has a safety valve provided on top to regulate the air pressure. A drain plug is also located at the bottom to drain the reservoir, without it the lubricating oil and moisture from the air will form an emulsion and would damage the brake units like brake valve, chamber etc.
- Brake Valve–
- It controls the intensity of brakes in air brakes. It consists of a spring-loaded hollow piston, with inlet and exhaust valves, an air bleed hole and exhaust port. The compressed air from the reservoir enters from inlet valve and goes to the stop light switch from one side and brake chambers from other. It is operated by a rod pressed by foot, pushrod or by hydraulic pressure. When the brake pedal is pressed, the piston in the brake valve is pushed down against the force of graduating spring.
- This spring force gives the driver the feel of the intensity of braking. the movement of piston opens the inlet valve and closes the exhaust valve, this allows compressed air to go to the wheel chambers. When the air pressure exceeds the force on the piston, the piston is lifted up and the exhaust valve opens. When the pedal is released the inlet valve is completely closed and the exhaust valve opens to remove the air in it.
- Dual Brake Valve– It has two sections which respond independently. The primary part provides normal service air braking. The secondary part operates in case of brake fails. When the service air pressure failure occurs, the primary piston can’t reach the force balance and it depresses the secondary piston through its stem. The secondary piston meets the secondary inlet-exhaust valve and seals the inlet port. Further, it opens the exhaust port to allow the air pressure to escape and apply brakes. Secondary reservoir pressure remaining in the inlet side of the secondary piston forces it to balance the inlet-exhaust valve and enables secondary brakes application.
- Hand Control Valve– They are used in trucks to control the actuators and spring brakes during the secondary brake application and parking. This application and release of air pressure are achieved by two graduating elements in the valve body.
- Brake chamber– It converts the pressure energy of compressed air into mechanical energy. For front brake chambers with pushrods are used whereas for rear sliding forks are fitted to allow hand brake operation. The diaphragm is made up of nylon and neoprene bonded together for strength and flexibility.
- Single diaphragm brake chamber- The diaphragm divides the chamber into two parts, one side is pressure chamber and other is connected to the atmosphere through breathing holes. When brakes have applied the air under pressure enters the pressure chamber and pushes the side open to atmosphere, it operates the push rod which actuates the cam to apply brakes.
- Double diaphragm brake chamber- The primary diaphragm is for the application of the service brakes. When the secondary brake valve is applied, the air enters the secondary inlet and builds up the pressure in the cavity connected to the port but not in the large chamber as it is sealed by a diaphragm. When the pressure increases it overcomes the thrust of return spring and the diaphragm moves forward slightly and breaks the seal so that pressure builds up over entire diaphragm area to apply brakes.
- Triple diaphragm brake chamber- It has three diaphragms. The service brakes are applied through the primary diaphragm to apply brakes. For the operation of the secondary diaphragm, air enters through secondary inlet and pressure is applied to all three diaphragms.
- Diaphragm/piston brake chamber- It has both piston and diaphragm. When brakes are applied, compressed air builds up the pressure on the piston and moves the piston to apply brakes. When secondary brakes are applied the compressed air enters the port in the Centre of brake chamber and builds up the pressure on the diaphragm to apply the brake.
- Spring brake chamber- It employs diaphragm for service braking and a power spring for parking and secondary brakes. The action of service brakes remains the same. For all service brake applications, the power spring remains compressed. When the parking brakes are applied, compressed air from piston chamber is exhausted and the spring fully applies the brakes.
- Slack adjuster- It controls the clearance between the brake shoe and brake drum using a worm and wheel. The worm has internal splines and is mounted on s-cam. The adjusting nut is mounted on the shaft of the worm, turning it adjusts the clearance.
Advantages of air Brakes-
- They are much more powerful than ordinary brakes.
- It simplifies the chassis design, as it doesn’t matter where the components of the system are located as long as they are interconnected by pipelines.
- The compressed air is also used for various operations.
It operates independently of foot brakes and is actuated by a mechanical cable. A ratchet release handle to which the catch rod is attached and is hinged on handbrake lever. On the other end, a pawl is attached which slides in the fixed guide on the brake lever. The brake lever is hinged on a bracket attached to the chassis. The operating cable connects to the lower end of the brake lever.
Application of brake– To apply brakes, the ratchet is released by pressing the handle. Then the brake lever is pulled up, it pulls the cable which operates the rear brakes manually through linkages which operate the piston of the rear wheel cylinder. The ratchet release handle is now released and the pawl moves down with the spring action and engages with the ratchet keeping brake applied.
Hill holding device-
It is used to stop a car from slipping down while going uphill.
- Normal Forward Driving– When the clutch is in the engaged position, a ball C controls the passage between the valves B and ball C. If cam D operates the valve B through E, this gives free passage of brake fluid from the master cylinder to brakes.
- When the brakes are applied– When the car is stopped and the clutch is disengaged, the cam D is operated so that the valve B is pressed against the seat A, closing the passage of brake fluid. The passage between B and C still remains open and keeps the master cylinder in contact with brakes. When the brakes are released and the car is in uphill, the ball C due to gravity will stop the passage between C and B while the passage between B and A is already closed by cam D, this helps to maintain the pressure of the fluid in brakes. After this, the brakes will be released only when the car is started again and the clutch is engaged, opening the passage B and A by cam D.
Electric Parking Brake-
- Cable Puller Type– It has a central electro-mechanical actuator with an integrated electronic controller which actuates the conventional duo-servo brakes or integrated calipers.
- Caliper-integrated type– It uses combined calipers with integrated actuation and power electronics. This system is economical and light.
- Duo-Servo type– It has a central control unit which actuates the actuators on Duo-Servo brakes. A special ESC unit takes care of emergency brake function. It is lightweight, decentralized actuators and highest hill holding forces.
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