1.5. Hydraulic Brakes
The most automotive service brakes are hydraulic brakes. In this system the brake is operated with the help of exerted oil pressure, while master cylinder and wheel cylinder is most important components. This system is very popular in four wheeler basically light and medium weight vehicles.
1.5.1. Construction of hydraulic brake:
The outline of hydraulic brake is shown in Fig. 1.2. The brake system (with master cylinder) fitted with brake oil chamber contains brake fluid/oil.
The main component 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. Generally, all the vehicles utilise tandem master cylinders. This type of master cylinder serves two independent hydraulic lines.
1.5.2. Working principle of hydraulic brakes:
Pascal’s law: “The pressure exerted anywhere in a mass of confined liquid is transmitted undiminished in all directions throughout the liquid.” All hydraulic system works under Pascal’s law.
Fig. 1.3 illustrates the principle components arrangement of the system.
1.5.3. Working/operation of hydraulic brakes:
Fig. 1.3 illustrates the principle components arrangement of the system.
1.5.3. Working/operation of hydraulic brakes:
The hydraulic action begins when force is applied to the brake pedal. When driver presses the brake pedal, the pistons in the master cylinder are activated, causing pressure / force act on the brake fluid. This force creates pressure in the master cylinder.
It serves to displace hydraulic fluid stored in master cylinder. The master cylinder converts pedal force into hydraulic pressure to operate the brakes. The master cylinder creates oil pressure and sent to wheel cylinder via pressurised brake lines. And then starts the brake actuation process.
The actuation of these mechanisms forces the Piston in wheel cylinder get expands due to oil pressure, brake pads and brake shoes linings expand against the rotors/disc (front wheel) or drums (rear wheel) and rub on it to stop the wheel so that brake will get apply.
When the brake pedal is released, return springs move the pistons back to their original positions. And brakes are released.
1.5.4. Advantages, disadvantage and applications of hydraulic brakes:
1.5.4.1. Advantages:
1. Equal braking effort to all the four wheels, due to fluid exerts equal pressure anywhere in the circuitry system.
2. Less rate of wear (due to absence of joints compared to mechanical brakes) or simple in construction.
3. Force multiplication (or divisions) very easily just by changing the size of one piston and cylinder relative to other.
4. The system is mostly self-lubricating.
1.5.4.2. Disadvantages:
1. Even slight leakage of air into the breaking system makes it useless.
2. The brake shoes are liable to get ruined if the brake fluid leaks out.
3. This system is suitable only for applying brakes intermittently. For parking purposes separate mechanical linkages has to be used.
1.5.4.3. Applications:
1. Maruti Suzuki 800, Maruti cars.
2. Many other vehicles use hydraulic brakes such as cranes and lifts or elevators, aero planes, mining vehicles, the uses and application for hydraulic brakes are endless and all use similar systems and components.
1.5.6. Main components of hydraulic brakes:
1. Master cylinder
2. Wheel cylinder
3. Brake fluid & Reservoir
4. Hydraulic pressure pipe-lines
5. Disc and drum brake assemblies
1.5.7. Master cylinder:
• It is the heart of the hydraulic system.
• It builds up hydraulic pressure to operate the brakes.
• It maintains a constant volume of fluid in the system owing to its reservoir.
• It serves as a pump to bleed or force air out of the hydraulic system.
There are the following two types of master cylinders:
(a) "Single master cylinder" for all the front and rear wheel cylinders.
(b) "Tandem master cylinder" containing separate units for front and rear wheel cylinders.
1.5.8. (a) Construction of single master cylinder:
The master cylinder consists of two main chambers or reservoirs: (i) The "fluid reservoir" (which contains the fluid to supply to the brake system), and (ii) the "compression chamber" (in which piston operates).
The reservoir supplies fluid to the brake system through two ports. The larger port is called the filler or intake port and is connected to the hollow portion of the piston (there are a number of holes in the piston on the primary or high pressure seal side) between the primary and secondary cups which act as piston sews.
The smaller port is called the compensating, bypass or relief port which connects the reservoir directly with the cylinder and lines when the piston is in the released position.
The reservoir is vented to the atmosphere so that atmospheric pressure causes the flow through the filler port. The vent is placed in the filler cap. The "boot" covers the push rod and the end of the cylinder to keep it free from foreign matter.
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.
The master cylinder consists of two main chambers or reservoirs: (i) The "fluid reservoir" (which contains the fluid to supply to the brake system), and (ii) the "compression chamber" (in which piston operates).
The reservoir supplies fluid to the brake system through two ports. The larger port is called the filler or intake port and is connected to the hollow portion of the piston (there are a number of holes in the piston on the primary or high pressure seal side) between the primary and secondary cups which act as piston sews.
The smaller port is called the compensating, bypass or relief port which connects the reservoir directly with the cylinder and lines when the piston is in the released position.
The reservoir is vented to the atmosphere so that atmospheric pressure causes the flow through the filler port. The vent is placed in the filler cap. The "boot" covers the push rod and the end of the cylinder to keep it free from foreign matter.
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.
1.5.9. Working of single master cylinder:
When the brake pedal is pressed piston of the master cylinder moves forward to force the liquid under pressure into the system. The relief port is sealed out of the system.
The liquid pressure is conducted to the wheel cylinders, where it forces the wheel cylinder pistons outwards. These pistons force the brakes shoes out against the brake drums and the brakes are applied.
As soon as the brake pedal is released, the return spring quickly forces the master cylinder piston back against the piston stop. Since the fluid in the lines returns rather slowly, a vacuum tends to form in the cylinder in front of the piston.
This causes the primary cup to collapse/deflect to allow the liquid to flow from the reservoir through the filler port past the piston to fill the vacuum.
When the pedal is in "off position", the liquid may flow from the reservoir through the relief port in the master cylinder, supply lines, and wheel cylinders to make up for any fluid that may be lost or to compensate for shrinkage cooling of the liquid.
In this way, a complete column of liquid is always maintained between the master cylinder piston and wheel cylinder pistons.
1.5.10. (b) Tandem master cylinder:
A simple master cylinder is mostly used in all small and medium type of vehicles. However, in some vehicles, the tandem master cylinder is used.
A "tandem master cylinder" is the master cylinder with two separate cylinders and reservoirs in the same master cylinder assembly, one operating front brakes and the other cylinder operating rear brakes.
This master cylinder avoids the possibility of all the brakes of a vehicle being put out of order by a leak or fracture in the pipe line leading to one wheel cylinder.
Fig. 1.5: Tandem master cylinder (line diagram)
It ensures reliability with not much extra cost. In this arrangement separate lines go to different sections of the brake system, say, the rear and the front brakes and it so arranged that if the front brake lines are damaged, the rear brakes will be still effective. Similarly if rear brake line is defective, at least front brakes will be applied.
Fig. 1.5 shows a single line diagram of a tandem master cylinder. Two pistons have been shown which are in line with each other. If line A fails, the piston A bottoms against the end of the cylinder while the piston B continues to develop pressure in the line B thus applying brakes to one set of wheels. If the line B fails piston B comes up against piston A, thus building up p sure in line A.
Fig. 1.6 shows the sectional view of a tandem master cylinder. As shown in diagram, the master cylinder contains two pistons, the rear being operated directly by the brake pedal. The space between the pistons is connected to the front brakes while the connection to rear brakes is made at the front end opposite to operating link.
Fig. 1.6: Sectional view of tandem master cylinder
Depressing the brake pedal moves the rear piston inward developing pressure in the operating cylinder for front brakes. Since the front piston is free to move along the cylinder, so it also moves ahead developing an equal pressure in the operating cylinder for rear brakes. The return motion of the front piston is limited by the stopper screw.
1.5.11. Wheel Cylinder (Slave cylinder):
The
wheel cylinders are made up of cast iron with a bore drilled and finishing
provide for the seals and pistons.
Hydraulic
pressure is transferred from the master cylinder to each of the wheel cylinder
through brake fluid. The
links or push rods force the shoes outward against the brake drum.
Refer
to Fig. 1.7, a wheel cylinder consists of a cylinder, two pistons, two rubber
cups and a spring. The fluid presses against pistons. The pistons move outward
in the cylinder. When the pistons come closer, the liquid is forced into the
master cylinder. The spring between the two pistons holds the rubber cups in
position.
Fig 1.7: Wheel cylinder
The
copper-coated, tin-plated annealed steel tubing and flexible hoses are used to
connect the master cylinder to the wheel cylinders. The hoses are used to
connect the lines to the front wheel cylinder to permit the front wheel to be
turned. Rear wheel cylinders are generally connected directly to a line
fastened to the rear axle housing.
1.5.12. Brake fluid & Reservoir:
Brake
fluid is a special liquid for use in hydraulic brake systems, which must meet highly
exact performance specifications. Hydraulic systems rely on incompressible
fluid to transmit force.
It
is designed to be impervious to wide temperature changes and to not suffer any significant
changes in important physical characteristics such as compressibility over the
operating temperature range.
The
fluid is designed to not boil, even when exposed to the extreme temperatures of
the brakes.
Brake
fluid is store in fluid reservoir, supply it whenever required. Brake
fluid, typically containing ethylene glycol, glycol ethers. One
of the important characteristics of brake fluid is its boiling point.
Viscosity,
lubrication properties, effect on rubber, corrosion action, storage stability etc.
are other important properties.
If
the brake fluid defective or boils (becomes a gas), it will waste most of its
ability to transmit force. This
may partially or completely disable the brakes.
For
this reason the brake fluid should be regarded as an expendable item and be replaced
at regular intervals.
During
refill or top-up the reservoir with new brake fluid it must check, and check
specification as per required manufacturer table.
One
of the worst things that can happen to your car is if the brake fluid becomes
contaminated, because the seals are designed to work with only pure brake
fluid.
System
contamination means that all of the piston seals and hoses are deteriorating,
and therefore must be replaced, a major expense. So, be very careful what you
put in the master cylinder reservoir.
Different
types of brake fluid are used in different systems, and should NEVER be mixed.
Types of brake fluids:
Glycol
based (absorb water):
DOT 3, DOT 4
Silicon
based (doesn’t absorb water): DOT 5
1.5.13. Drum brake:
A drum brake assembly consists of a cast-iron drum bolted to and rotates with the wheel. A fixed backing plate to which the shoes, wheel cylinders, automatic adjusters and linkages are attached.
As the drum rubs against the shoes, the energy of the moving drum is transformed into heat.
When the brakes are released, the shoes and hence the slave-pistons are pulled back by the return springs.
Fig 1.9: Drum brake assembly
The
use of drum brakes on the rear wheels to keep costs and weight down as well as
to simplify the provisions for a parking brake. As the front brakes perform
most of the braking effort, this can be a reasonable compromise.
The
modern automobile drum brake was invented in 1902 by Louis Renault. Fig
1.8 and Fig 1.9 shows hydraulically actuated drum brake type. While Fig 1.13
and Fig 1.14 shows completely Mechanical type.
1.5.14. Disc Brake:
These brakes are different from drum brakes in that the drum is replaced by a circular plate and the brake shoes are replaced by a caliper which supports a pair of friction pads, one on each side of the disc, as shown in Fig 1.10.
These pads are forced inward by the operating force and so retard the disc. Types of brake discs are according to caliper arrangement- swinging caliper and sliding caliper type, according to makes- solid and ventilated type.
1.5.14.1. Construction of disc brake:
As shown in Fig 1.11, a disc brake consists of a grey cast iron disc bolted to the wheel hub and a stationary housing called caliper. To stop the wheel, friction material in the form of brake pads, mounted on a device called a brake caliper, is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc.
The caliper is connected to some stationary part of the vehicle (such as stub axle or axle casing) and is cast in two parts, each containing a piston. In between the each piston and the disc, there is a friction pad held.in position by retaining pins, spring plates etc.
Fig 1.11: Typical disc brake assembly
Passages are drilled in the caliper for the fluid to enter or leave from each house. These passages are also connected to one another for bleeding. Each cylinder contains a rubber sealing ring between the cylinder and piston.
1.5.14.2. Working of disc brake:
When brakes are applied, hydraulically actuated pistons move the friction pads into contact with the disc, applying equal and opposite forces on the later. A disc brake which slows rotation of the wheel by the friction caused by pushing brake pads against a brake disc with a set of calipers.
When the brakes are released the rubber sealing rings act as return springs and retract the pistons and the friction pads away from the disc. Special types of disc brakes include the swinging caliper type and the sliding caliper type.
1.5.15. Advantage, disadvantages and applications of disc brake:
The advantages and disadvantages of disc brakes compared with drum brake as follows.
1.15.1. Advantages:
1. Lighter than drum brakes.
2. Better cooling (since the braking surface is exposed directly to air).
3. Offer better resistance to fade.
4. Uniform pressure distribution (since disc brakes have no self -servo effect).
5. Brake pads can be easily replaced.
6. These brakes are self-adjusting by design.
1.15.2. Disadvantages:
1. Costlier than drum brakes.
2. For stopping the vehicle higher pedal pressure is required.
3. There is no servo action in these brakes.
4. It is difficult to install an adequate parking attachment.
1.15.3. Applications:
1. Disc brakes development and use began in England in the 1890s. The first caliper-type automobile disc brake was patented by Frederick William Lanchester in his Birmingham, UK factory in 1902 and used successfully on Lanchester cars.
2. Disc brakes were most popular on sports cars when they were first introduced, since these vehicles are more demanding about brake performance. Discs have now become the more common form in most passenger vehicles, although many (particularly light weight vehicles).
3. Jaguar C-Type racing car-Reliable caliper-type disc brakes were developed in the UK by Dunlop and first appeared in 1953.
4. The first production car to have disc brakes at all 4 wheels was the Austin-Healey 100S in 1954 and in 1965 on the Chevrolet Corvette Stingray.
5. The first German production car with disc brakes was the 1961 Mercedes-Benz 220SE coupe.
6. The first motorcycles to use disc brakes were racing bikes, 1969 Honda CB750. Disc brakes are now common on motorcycles, mopeds and even mountain bikes.
Simple Line Diagrams (for exam point of view)
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| Hydraulic brake system |
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| Master cylinder two reservoir |
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| Disc brake |
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| Master Cylinder one reservoir |
air brake hose
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