Layout and components
Most of the cars today use hydraulically operated foot brakes on all the four wheels with an additional hand brake mechanically operated on the rear wheels. An outline of the hydraulic braking system is shown in figure. The main components in this is the master cylinder, which contains Reservoir for the brake fluid. Master cylinder is operated by the brake pedal and is further connected to the wheel cylinders in each wheel through steel pipe lines, unions and flexible hoses. In case of Hindustan Ambassador car, on Front wheels each Brake shoe is operated by separate wheel cylinder (thus making the brake two show leading), whereas in case of rear wheels there is only one cylinder on each wheel which operates both the shoes (thus giving one leading and one trailing shoe brakes). As the Rear wheel cylinders are also operated mechanically with the hand brake, they are made floating. Further all the shoes in the Ambassador car are of the floating anchor type.
The system is so designed that even when the brakes are in the released position a small pressure of about 50 KPa is maintained in the pipelines to ensure that the cups of the wheel cylinder are kept expanded. This prevents the air from entering the wheel cylinders when the brakes are released. Besides, this pressure also serves the following purposes:
(i) It keeps the free travel of the pedal minimum by opposing the brake shoe retraction Springs.
(ii) During bleeding, it does not allow the fluid pumped into the line to return, thus quickly purging air from the system.
1. Master cylinder :-
This can be right named as heart of the hydraulic Braking System. There are two main Chambers viz The fluid Reservoir and compression chamber in which the Piston operates. The fluid in the Reservoir compensates for any change in the fluid volume in the pipelines due to temperature variations and to some extent due to leakage. To prevent leakage there are rubber seals on both ends of the Piston in the compression chamber. The reduced the diameter region of the piston is always surrounded by the fluid. A rubber boot covers the push rod end of the master cylinder to prevent the dirt from entering inside. Towards the brake lines side of the compression chamber, there is a fluid check valve with a rubber Cup inside. It serves to retain the residual pressure in the brake lines even when the brakes are released.
There are a number of holes in the Piston head on the primary (high pressure) seal side. Two holes connect the fluid reservoir to the compression chamber. The smaller one out of this is about 0.7 mm diameter and is called the bypass or compensation port. The second hole is called the Intake or the recuperation port.
The push rod is operated with the foot brake pedal through linkage. As the pedal is pressed, push rod moves Piston to left against the force of the spring, till it covers the by bypass port. Further moment of the pushrod causes building up of pressure in the compression chamber. Finally, when sufficient pressure has built up, the inner rubber cup of the fluid check valve is deflected, forcing the fluid under pressure in the lines. This fluid enters the wheel cylinder or the caliper and moves the Pistons there by applying the brakes.
When the brake pedal is released, the spring pressure in the compression chamber move the Piston to the right extreme position. This same force of the spring keeps the fluid check valve pressure on its seat for sometime and thereby delays the return of fluid from the lines into the compression chamber again. Some delay is also caused by the inertia of the fluid in the lines. This produces a vacuum in the compression chamber and unless this is destroyed immediately, there are all chances of air leaking into the system. Even a very small amount of air will render the brakes useless, the air being compressible. This problem is solved by having intake port as shown. As soon as some vacuum is formed, the atmospheric pressure in the fluid Reservoir forces the fluid through Intake port and holes in the Piston which deflects the rubber cup and enters the compression chamber, destroying the vacuum.
But by the time this vacuum is destroyed, the fluid from the lines come back into reservior by lifting the fluid check valve off its seat. This extra fluid now has to be accommodated somehow, because compression chamber is already full. If this is not done, the pressure in the lines will not be relieved fully and there are all chances of brake shoes rubbing with the drum. Once this happens, there will be more heat generated at the drum, which when transmitted to the wheel cylinders would cause the fluid to expand and exert still more pressure, causing the shoes to move still further towards the drum. In this way, a vicious circle with start, causing the brakes to jam ultimately.
This is avoided by means of bypass port. The extra fluid coming from the lines passes to the fluid Reservoir, where pressure is maintained atmospheric by providing an air vent.
In case Brake Linings or pads are worn so that there is excessive clearance between lining and drum or between pad and disc, a quick release of brake pedal will draw extra fluid from the reservoir into the compression chamber and there after a quick pressure of the pedal will send this extra fluid into the brake pipelines, thus taking up the excessive clearance. In this way a quick pumping up of the brake pedal helps compensate for the wear of the Brake Linings or pads.
Moreover, unlike drum brakes, disc brakes should not have any Residual pressure in the pipeline because in that eventuality the pads are not completely separated from the disc when the brakes are released. In a vehicle having either disc brakes or both the disc as well as drum brakes, therefore, a special fluid check valve having a small restrictor hole is used in the master cylinder.
Tandem master cylinder :-
Tandem master cylinder ensures reliability with not much extra cost. In this separate lines go to different sections of the brake system, say, the rear and front brakes and it is so arranged that if the front brake lines are damaged, the rear brakes will be still effective. Similarly if rear brake lines is defective, atleast front brakes will be applied.
Under ordinary conditions the brake fluid we transmit pressure both to front as well as to the rear brakes, when the brake pedal is applied. However, when, say, the front brake lines are damaged, Piston two will move till it comes up against stop 3. After this pressure will start building up in the space between pistons 1 and 2 and rear brakes will be applied. Similarly when the rear brake lines are damaged, no pressure will build up in the space between piston 1 and 2. So Piston one will move freely till it comes up against two. Further push at the brake pedal will move both Piston 1 and 2 together thereby applying the front brakes.
Such hydraulic brake systems which employ tandem master cylinder to operate two different sections of the vehicle brakes, are called split systems. The system described above, wherein the front and rear brakes are operated by different Chambers of the tandem master cylinder is called axle by axle split system and is generally used for light vehicles say upto 1 tonne example passenger cars. As the braking effect is proportional to the swept volume in the master cylinder chamber, the two different Chambers can be designed different in size according to the braking effect required on the two axles. In passenger cars, generally, larger braking effort is required at the front axle, so the larger Chamber of the master cylinder is connected to the front axle and the smaller chamber to the rear axle.
For medium duty vehicle (1-3 tonne rating) also the axle by axle spit system as described above, may be used. However, when all the brakes are two shoe leading type, it is preferable to employee the system shown in figure, in which one line from master cylinder goes to the top wheel cylinder in all wheels while the other line goes to the bottom wheel cylinders of all wheels. The advantages of such a system are better directional stability and the property to retain almost same ratio of brake torques on the front and rear axles, even after failure of any brake line. In this case all brakes are used as two shoe leading. In case of failure of any section, the two shoe leading brake becomes simply the common one leading one training shoe brake. Thus the braking effectiveness even after failure remains up to about 75% of the original value. The split system for rear two shoe leading (two wheel cylinders) and front one shoe leading (one wheel cylinder) brakes is shown in figure. In front drive vehicles such as Maruti 800, diagonally split system is commonly used.
All vehicles employing tandem master cylinder use a pressure differential switch with warning light and proportioning valves. When disc brakes are used at front with drum brakes at the rear, a metering valve is also required.
Pressure differential valve with warning light switch :-
This is employed to warn the driver of failure of one part of the hydraulic Braking System. One such as shown in figure. When both the halves of the braking system are working, pressure from both sides keeps the plunger in the centre so that the warning switch contact is in the released position, keeping the switch off. However, when one half of the system, say the rear one, fails, the pressure on that side drops to zero, which causes the plunger to move towards right, thereby pressing the switch contact so that the warning light is on.
Proportioning valve :-
This is employed to proportion the braking effect between the front and rear axles. Different types of brakes at the front and the rear and the transfer of weight during brake application require different forces to be applied to the wheel cylinder or calipers at the front and at the rear. This is done by the proportioning valve.
Metering valve :-
Drum brakes used return springs to bring back the shoes when the brakes are released. But there are no such springs in case of disc brakes. Therefore, if in a vehicle, disc brakes are employed at the front with drum brakes at the rear, front brakes would start applying long before the rear drum brakes if no suitable provision is used to rectify the same. This may cause the front wheels to lock when the driver applies the brakes even lightly, resulting in the loss of steering control. To avoid this to happen a metering valve is used, which delays the application of pressure to the front disc brakes till sufficient pressure has been built up in the rear brake lines to overcome the force of Return springs there, which is usually about 800 to 900 KPa. At this time metering valve is fully open. When the brakes are released, metering valve is closed and a return valve located inside the Assembly will open, allowing the fluid to return from the lines freely.
Quite often a single valve incorporating the pressure differential valve, the proportioning valve and the metering valve is employed. Such a valve is known as combination valve.
Wheel cylinder :-
Wheel cylinders in the brake system are meant to force the brake shoes against the drum. The construction is very simple. Each wheel cylinder is provided with Pistons, rubber seals (cups), Cup spreaders, Spring and dust covers (Boots). The brake line from the master cylinder is attached to the inlet port and a bleeder screw with a cover is provided to bleed air from the system whenever required. Wheel cylinders are mounted on the back plate.
When brakes are applied, the fluid under pressure from the master cylinder enters the inlet port and forces the Piston to move outward to push the shoes against the drum. Similarly, when the brakes are released, the brake shoe retractor springs force the brake fluid out of the wheel cylinder by pushing the pistons inward.
With the contamination of the brake fluid, the cylinder bore area between the rubber cups is subject to corrosion. With the gradual wear of the Brake Linings, the distance between the cups is increased due to which larger area of the cylinder bore gets corroded. In such a case if Brake Linings are replaced without first servicing the wheel cylinders, the rubber cups would be pushed back inward and would thus operate on the corroded area of the bores. This would cause damage to the cups and leakage of the fluid resulting ultimately in brake failure.
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