Brakes ppt system,dynamics and condition
- 1. BRAKING SYSTEM part 2
- 2. Vehicle braking systems – The purpose of the braking system is to slow down or stop the vehicle and, when the vehicle is stationary, to hold the vehicle in the chosen position. When a vehicle is moving it contains energy of motion (kinetic energy) and the function of the braking system is to convert this kinetic energy into heat energy. It does so through the friction at the brake linings and the brake drum, or the brake pads and the disc. Some large vehicles are fitted with secondary braking systems that are known as retarders. Examples of retarders are exhaust brakes and electric brakes. In all cases, the factor that ultimately determines how much braking can be applied is the grip of the tyres on the driving surface.
- 3. Types of brakes – Two basic types of friction brakes are use in vehicles – The Drum and Disc brake. The drum brake – Figure shows a drum brake as used on a large vehicle. This cut-away view shows that the linings on the shoes are pressed into contact with the inside of the drum by the action of the cam. In this case the cam is partially rotated by the action of a compressed air cylinder. The road wheel is attached to the brake drum by means of the wheel studs and nuts. A brake of this type has a leading shoe and a trailing shoe. The leading shoe is the one whose leading edge comes into contact with the drum first, in the direction of rotation. A leading shoe is more powerful than a trailing shoe and this shows up in the wear pattern, because a leading shoe generally wears more than a trailing shoe owing to the extra work that it does.
- 6. The disc brake – The road wheel is attached to the disc and the slowing down or stopping action is achieved by the clamping action of the brake pads on the disc. In this type of brake, the disc is gripped by the two friction pads. When hydraulic pressure is applied to the hydraulic cylinder in the caliper body, the pressure acts on the piston and pushes the brake pad into contact with the disc. This creates a reaction force that causes the pins to slide in the carrier bracket and this action pulls the other pad into contact with the disc so that the disc is tightly clamped by both pads.
- 9. Hydraulic operation of brakes – The main braking systems on cars and most light commercial vehicles are operated by hydraulic systems. At the heart of a hydraulic braking system is the master cylinder, as this is where the pressure that operates the brakes is generated. Principle of the hydraulic system- The small-diameter master cylinder is connected to the large-diameter actuating cylinder by a strong metal pipe. The cylinders and the pipe are filled with hydraulic fluid. When a force is applied to the master cylinder piston a pressure is created that is the same at all parts of the interior of the system. Because pressure is the amount of force acting on each square milli meter of surface, the force exerted on the larger piston will be greater than the force applied to the small piston.
- 10. With reference of example – The force of 100 newton's on an area of 400 square milli meters of the master cylinder piston creates a pressure of 0.25 newton's per square milli meter. The piston of the actuating cylinder has an area of 800 square milli meters and this gives a force of 200 newton's at this cylinder.
- 11. Component and lay out of Servo assist hydraulic breaking system -
- 12. Component and lay out of hydraulic breaking system -
- 13. The master cylinder – The part of the hydraulic braking system where the hydraulic operating pressure is generated is the master cylinder. Force is applied to the master cylinder piston by the action of the driver’s foot on the brake pedal. When force is applied to the push rod the piston moves along the bore of the master cylinder to take up slack. As soon as the lip of the main rubber seal has covered the bypass hole, the fluid in the cylinder, and the system to which it is connected, is pressurized.
- 14. When the force on the brake pedal and the master cylinder push rod is released, the return spring pushes the piston back and the hydraulic operating pressure is removed. The action of the main piston seal ensures that the master cylinder remains filled with fluid.
- 17. The brake fluid reservoir – The fluid reservoir is usually mounted directly on to the master cylinder. The fluid reservoir is normally made from translucent plastic and, provided it is kept clean, it is possible to view the fluid level by under-bonnet inspection. The reservoir is fitted with an internal dividing panel, ensuring that one section remains operative should the other section develop a loss of fluid. The fluid level indicator operates a warning light. Should the fluid level drop below the required level, the switch contacts will close and the warning light on the dash panel will be illuminated. Brake pipes and hoses – The hydraulic pressure created at the master cylinder is conveyed to the wheel brakes through strong metal pipes, where they can be clipped firmly to the vehicle, and through flexible hoses where there is relative movement between the parts, for example axles and steering and wheels and the vehicle frame.
- 19. The master cylinder is designed to ensure that the brakes are applied evenly. There are two pistons, a primary piston and a secondary piston. The spring (1) is part of the primary piston and it is stronger than the spring (2) that is fitted to the secondary piston. Application of force to the piston causes the spring (1) to apply force to the secondary piston so that both pistons move along the cylinder together. The two piston recuperation seals cover their respective cut- off ports at the same time and this ensures simultaneous build-up of pressure in the primary and secondary circuits. When the brake pedal force is released, the primary and secondary pistons are pushed back by the secondary spring and both cut-off ports are opened at the same time, thus releasing the brakes simultaneously. In the event of leakage from one part of the master cylinder, the other cylinder remains operative. This is achieved through the design of the stops and other features of the master cylinder.
- 20. Wheel cylinders - The hydraulic cylinders that push the drum brake shoes apart, or apply the clamping force in the disc brake, are the wheel cylinders. There are two principal types of wheel cylinders, a single-acting cylinder and a double acting cylinder. The single-acting wheel cylinder – In the single acting wheel cylinder, the space in the wheel cylinder, behind the piston and seal, is filled with brake fluid. Pressure from the master cylinder is applied to the wheel cylinders through pipes. Increased fluid pressure pushes the piston out and this force is applied to the brake shoe or brake pad. Double-acting wheel cylinder - In the double-acting wheel cylinder has two pistons and rubber seals. Hydraulic pressure applied between the pistons pushes them apart. The pistons then act on the brake shoes and moves the linings into contact with the inside of the brake drum.
- 22. Wheel cylinder inner view -
- 23. The brake servo - A brake servo is used as a means of increasing the force that the driver applies to the brake pedal. The servo is the device that allows the driver to apply a large braking force by the application of relatively light force from the foot. The amount of increased force that is produced by the servo is dependent on the driver’s effort that is applied to the brake pedal. This ensures that braking effort is proportional to the force applied to the brake pedal. On petrol-engine vehicles, manifold vacuum is used to provide the boost that the servo generates.
- 24. On diesel engine vehicles there is often no appreciable manifold vacuum, owing to the way in which the engine is governed. In these cases, the engine is equipped with a vacuum pump that is known as an exhauster. The master cylinder (1) is firmly bolted to the servo unit on one side and, on the other side, the servo unit is firmly bolted to the vehicle bulkhead (4). The brake pedal effort is applied to the servo input shaft and this in turn pushes directly on the master cylinder input piston. At this stage you should concentrate on the flexible diaphragm, the sealed container, the two chambers A and B, and the vacuum connection. The valve body and the control piston are designed so that when the brakes are off, the manifold vacuum will draw out air and create a partial vacuum in chambers A and B on both sides of the diaphragm. When force is applied to the brake pedal, the control piston closes the vacuum port and effectively shuts off chamber B from chamber A. At this stage, the control piston moves away from the control valve and atmospheric pressure air is admitted into chamber B. The greater pressure in chamber B, compared with the partial vacuum in chamber A, creates a force that adds to the pedal effort applied by the driver.
- 25. The servo output rod pushes directly on to the master cylinder piston, so that there is no lost motion and the resulting force applied to the master cylinder is proportional to the effort that the driver applies to the brake pedal. It is common practice to fit a non-return valve in the flexible pipe, between the manifold and the servo unit, This valve serves to retain vacuum in the servo after the engine is stopped and also prevents petrol engine vapours and ‘backfire’ gases from entering the servo.
- 29. Brake fluid – Brake fluid must have a boiling temperature of not less than 1908ᐤC, and a freezing temperature not higher than 2408ᐤC. Brake fluid is hygroscopic, which means that it absorbs water from the atmosphere. Water in brake fluid affects its boiling and freezing temperatures, which is one of the reasons why brake fluid needs to be changed at the recommended intervals. Brake fluid is normally based on vegetable oil and its composition is carefully controlled to ensure that it is compatible with the rubber seals and not corrosive to the metal parts. Some manufacturers use mineral oil as a base for brake fluids and their systems are designed to work with this fluid. It is important always to use only the type of fluid that a vehicle manufacturer recommends for use in their vehicles.
- 30. Anti-lock braking system(ABS) – The term ABS covers a range of electronically controlled systems that are designed to provide optimum braking in difficult conditions. ABS systems are used on many cars, commercial vehicles, and trailers. The purpose of anti-skid braking systems is to provide safer vehicle handling in difficult conditions. If wheels are skidding it is not possible to steer the vehicle correctly and a tyre that is still rolling, not sliding, on the surface will provide a better braking performance. ABS does not usually operate under normal braking. It comes into play in poor road surface conditions, such as ice, snow, water, etc., or during emergency stops. The ABS system, which gives an insight into the way that such systems operate. The master cylinder (1) is operated via the brake pedal. During normal braking, manually developed hydraulic pressure operates the brakes and, should an ABS defect develop, the system reverts to normal pedal-operated braking. The solenoid-operated shuttle valve (2) contains two valves, A and B. When the wheel sensor (5) signals the ABS computer (ECU) (7) that driving conditions require ABS control, a procedure is initiated which energizes the shuttle valve solenoid.
- 31. Valve A blocks off the fluid inlet from the master cylinder and valve B opens to release brake line pressure at the wheel cylinder (6) into the reservoir (3) and the pump (4), where it is returned to the master cylinder. In this simplified diagram, the shuttle valve is enlarged in relation to the other components. In practice, the movement of the shuttle valve is small and movements of the valve occur in fractions of a second. In practical systems, the solenoids, pump and valves, etc. are incorporated into a single unit. This unit is known as a modulator. This brief overview shows that an anti-lock braking system has sensors, an actuator, an ECU, and interconnecting circuits. In order that the whole system functions correctly, each of the separate elements needs to be working correctly. When deciding whether or not a vehicle wheel is skidding, or on the point of doing so, it is necessary to compare the rotational movement of the wheel and brake disc, or drum, with some part such as the brake back plate that is fixed to the vehicle. This task is performed by the wheel speed sensing system and the procedure for doing this is reasonably similar in all ABS systems, so the wheel speed sensor is a good point at which to delve a little deeper into the operating principles of ABS.
- 33. Block diagram of ABS -
- 35. The wheel speed sensor – The sensor contains a coil and a permanent magnet. The reluctor ring has teeth and when the ring rotates past the sensor pick-up the lines of magnetic force in the sensor coil vary. This variation of magnetic force causes a varying voltage (emf) to be induced in the coil and it is this varying voltage that is used as the basic signal for the wheel sensor. The particular application is for a Toyota but its principle of operation is typical of most ABS wheel speed sensors. The raw output voltage waveform from the sensor is approximately of the form shown in Fig. It will be seen that the voltage and frequency increase as the wheel speed, relative to the brake back plate, increases. This property means that the sensor output is a good representation of the wheel behavior relative to the back plate and, thus, to provide a signal that indicates whether or not the wheel is about to skid. In most cases, this raw curved waveform is not used directly in the controlling process and it has first to be shaped to a rectangular waveform, and tidied up before being encoded for control purposes.
- 36. If the brake is applied and the reluctor (rotor) starts to decelerate rapidly, relative to the sensor pick-up, it is an indication that the wheel rotation is slowing down. If the road surface is dry and the tyre is gripping well, the retardation of the wheel will match that of the vehicle and normal braking will occur. However, if the road surface is slippery, a sudden braking application will cause the reluctor rotor and road wheel to decelerate at a greater rate than the vehicle, indicating that a skid is about to happen. This condition is interpreted by the electronic control unit. Comparisons are made with the signals from the other wheel sensors and the brake line pressure will be released automatically, for sufficient time (a fraction of a second) to prevent the wheel from locking. In hydraulic brakes on cars the pressure release and re-application is achieved by solenoid valves, a pump and a hydraulic accumulator, and these are normally incorporated into one unit called the modulator. The frequency of ‘pulsing’ of the brakes is a few times per second, depending on conditions, and the pressure pulsations can normally be felt at the brake pedal.
- 37. With air brakes on heavy vehicles, the principle is much the same, except that the pressure is derived from the air braking system and the actuator is called a modulator. The valves that release the brakes during anti-lock operation are solenoid operated on the basis of ECU signals, and the wheel sensors operate on the same principle as those on cars. As for the strategy that is deployed to determine when to initiate ABS operation, there appears to be some debate. Some systems operate what is known as ‘select low’, which means that brake release is initiated by the signal from the wheel with the least grip, irrespective of what the grip is at other wheels. An alternative strategy is to use individual wheel control. Whichever strategy is deployed, the aim is to provide better vehicle control in difficult driving conditions and it may be that the stopping distance is greater than it would be with expert manual braking.
- 39. Bleeding the brakes - During repair work, such as replacing hoses and wheel cylinders, air will probably enter the hydraulic system. This air must be removed before the vehicle is returned for use and the process of removing the air is called ‘bleeding the brakes’.
- 42. THANKS