Hydraulic Braking System

Hydraulic Braking System

A hydraulic braking system transmits brake-pedal force to the wheel brakes through pressurized fluid, converting the fluid pressure into useful work of braking at the wheels. A simple, single-line hydraulic layout used to operate a drum and disc brake system is illustrated in Fig. 28.36. The brake pedal relays the driver's foot effort to the master-cylinder piston, which compresses the brake fluid. This fluid pressure is equally transmitted throughout the fluid to the front disc-caliper pistons and to the rear wheel-cylinder pistons. As per the regulations a separate mechanical parking brake must be incorporated with at least two wheels. This provision also allows the driver to stop the vehicle in the event of failure of the hydraulic brake system.

Fig. 28.36. Hydraulic single-line braking system.

In a hydraulic braking system the braking force is directly proportional to the ratio of the master-cylinder cross-sectional area to the disc or drum-brake wheel-cylinder cross-sectional areas. Therefore these cylinder diameters are appropriately chosen to produce the desired braking effect. The wheel-cylinder cross-sectional areas of the front and rear disc-and drum-brakes respectively may be chosen to produce the best front-to-rear braking ratio. Hydraulic fluid is incompressible provided there is no trapped air in the system. If air is present in the braking circuit, the foot-brake movement becomes spongy. In a hydraulic system the internal

friction exists only between the cylinder pistons and seals. The friction is caused by the fluid pressure squeezing the seal lips against the cylinder walls as the piston moves along its stroke. A hydraulic braking system is suitable only for intermittent braking applications, and a separate mechanical linkage must be incorporated for parking brakes.

The hydraulic system offers the following advantages over the mechanical layout, (a) This provides equal braking effort on all wheels. (6) This requires relatively less braking effort to deliver the same output.

(c) This is a fully compensated system so that each brake receives its full share of the pedal effort.

(d) The efficiency of the hydraulic system is greater than that of the mechanical layout.

(e) This system is suitable for vehicles having independent suspension.

(/) It is easy to alter thrust on shoe because the force exerted on a piston depends on the piston area. The larger the area, the greater the thrust on the trailing shoe, so a larger piston can be used.

28.8.1.

Various Components

Various components and their functions in a hydraulic braking system are as follows.

Brake Pipes.

These are steel pipes which form part of the fluid circuit between the master-cylinder and the wheel-cylinders. These pipes transfer the fluid along the body structure and rigid axle members. Flexible hoses connect the sprung body pipes to the unsprung axle wheel-brake units, to allow for movement (Fig. 28.36).

Master-cylinder.

This converts foot-pedal force to hydraulic pressure within the fluid system by means of the cylinder and piston (Fig. 28.36).

Disc-brake.

This comprises of a disc bolted to the wheel hub. This is sandwiched between two pistons and friction pads. The friction pads are supported in a caliper fixed to the stub-axle (Fig. 28.36). When the brakes are applied, the pistons clamp the friction pads against the two side faces to the disc.

Drum-brake.

This uses two brake-shoes and linings supported on a back-plate. The back-plate is bolted to the axle-casing. These shoes pivot at one end on anchor pins or abutments attached to the back-plate (Fig. 28.36). The other free ends of the both shoes are forced apart when the brakes are applied. The shoes expand radially against a brake-drum positioned concentrically on the wheel hub.

Wheel-cylinders.

As the hydraulic line pressure acts on the cross-sectional area of the disc and drum cylinder pistons (Fig. 28.36) in wheel cylinders, the hydraulic pressure is converted into braking effort. This braking effort either presses the friction pads against the side faces of the disc or forces the shoe friction linings against the inside of the drum.

28.8.2.

The Mechanics of a Hydraulic Braking System

To appreciate the machines of the hydraulic braking system, a simple analysis is presented to show how a suitable force ratio is obtained between the foot-pedal and the wheel-cylinder pistons. A braking system shown in Fig. 28.36 is considered.

Example 28.11. In a hydraulic single line braking system force on foot-pedal is 100 N, pedal leverage ratio is 4, cross sectional area of master cylinder is 4 cm2, cross sectional area of front

pistons 20 cm2, cross sectional area of rear piston 5 cm2, and distance moved by effort is 1 cm calculate,

(a) Front-to-rear brake ratio,

(6) Percentage of front and rear braking,

(c) Total force ratio,

(d) Distance moved by output,

(e) Cylinder movement ratio, and if) Total movement ratio.

Fig. 28.38. Girling master-cylinder.

Construction.

The cast-iron piston in the master-cylinder is shaped like a cylindrical plunger with a hollow stem at one end. A spring retainer in the form of a thimble-shaped steel pressing ff fitted over the piston stem end and clipped in place. The valve stem has a enlarged head, which rests in the hollow piston, and the valve itself is housed on the valve spacer near the reservoir inlet port.

A rubber ring acts as a lip seal and is fitted at each end of the piston. The rubber cup, called the primary seal, is installed near the return-spring. The cup is subjected to the line pressure and forms a fluid-tight piston end. A secondary seal, installed at the push rod end, prevents any leakage of the fluid from the rear end of the piston through the primary seal. A rubber boot, paced over the back end of the master-cylinder and around the push-rod, prevents the cylinder wall from dirt contamination.

Operation.

When the driver pushes down the foot-pedal to apply the brakes, the push-rod is forced against the piston. The initial piston movement pushes the edge of the spring-retainer around the mouth of the piston-stem central hole away from the valve stem head. Simultaneous­ly, fluid trapped in the hollow piston stem is momentarily pressurized and therefore pushes the valve-stem assembly towards the inlet port. The valve assembly and seal consequently close the inlet port disconnecting it from the reservoir. Further movement of the piston forces fluid to pass through the outlet port into the pipeline system to clamp the discs or expand the shoes against the drums (Fig. 28.38B).

When the brakes are released, the disc-brake piston seals or the drum-brake retraction springs retract the wheel-cylinder pistons so that fluid is displaced back to the master-cylinder. The master-cylinder piston return-spring moves the piston to its outermost position. But just before the piston reaches the end of its stroke, the spring-retainer clipped to the piston stem catches and pulls the valve stem and valve assembly away from the inlet port. Fluid then flows freely between the reservoir and the pressure chamber (Fig. 28.38A).

Compression-barrel Master-cylinder (Girling).

A compression-barrel master-cylinder incorporates a stationary primary recuperation seal held in the body, with the plunger moving through the middle to displace and apply pressure to the fluid. There are four small radial compensation ports in the plunger which, when the brakes are released, bypass the recuperation seal to allow movement of fluid between the reservoir and the cylinder (Fig. 28.39A). When the foot-pedal is pressed, the recuperation seal covers the radial compensation ports so that the fluid is trapped in the pressure half of the barrel. The brake pipeline is consequently pressurized (Fig. 28.39B).

The recuperation-seal shim permits free flow of the fluid between the horizontal recupera­tion ports in the body and the back of the recuperation seal when the brakes are released. This also safeguards the seal against pressing into the recuperation ports when under pressure. The recuperation-seal support holds the seal in place and limits its travel when the pressure is released. The secondary seal is placed at the push-rod end of the plunger. It is a wiper seal and prevents any fluid seeping out of the cylinder. Usually in drum-brakes a residual-pressure check-valve installed at the outlet port provides a small line pressure when the brakes are released.

When the system is pressurized for braking, the central conical valve is pushed open, so that additional fluid is transferred past the valve to the pipelines. Releasing the brakes reverses the process. This time the central valve is closed and the whole valve body is pushed away from

the outlet-port face. This action causes the fluid to escape back into the master-cylinder chamber. The stiffness of the plunger return-spring limits the minimum pipeline pressure at which the check-valve closes.

Wheel-cylinder Shoe-expanders

Hydraulic braking systems having drum-brakes use wheel-cylinder shoe-expander units. The wheel-cylinders transmit the hydraulic pressure to the brake-shoes, either through the single-piston system, which is common in front-drum-brake vehicles, or of the double-piston type that are incorporated in rear-drum brakes.

Double-piston Wheel-cylinder Shoe-expanders.

These units incorporate a cylinder body, two pistons, seals, seal-spreads and a retainer-spring (if cup-type seals are used), rubber dust-boots, and sometimes separate expander tappets (Fig. 28.40). The cast-iron wheel-cylinder body has an extended spigot portion for fitting into a hole in the back-plate to which it is secured usually by two studs. This attachment with the back-plate must be sufficiently rigid to absorb the braking-torque reaction during application of brakes.

A cylindrical hole in the body accommodate the two pistons, seals, and seal-spreaders and a retainer-spring (if fitted). Annular grooves are formed at both ends of the cylinder to install the rubber dust-boots. A bleed-screw valve is located at the centre of the cylinder, usually at the highest point, for purging of air from the chamber.

Fig. 28.40. Double-piston wheel-cylinder.

The two pistons installed in the wheel-cylinder convert the hydraulic pressure into brake-shoe tip load. The diameters of these pistons are based on the required brake load for the front and rear brakes. The piston outer end normally receives the shoe toe web for acting directly against the shoes. Sometimes, the shoes are pushed outwards by push-rods or screw tappets or abutments located between the pistons and the shoe tips.







In case of cup seals, a retainer-spring pushes the cup seals against the piston heads and the cylinder walls. Consequently, the fluid does not seep past the piston and air does not enter the wheel-cylinder when the brakes are released. The ring lip seals sit in grooves around the pistons and the natural elasticity of the rubber preloads the seal lip radially against the bore. A rubber boot or cap is fitted over each piston's exposed end, to safeguard the cylinder walls from brake-lining dust and dirt.

Brake Master cylinders

The brake master-cylinder contains a cylinder and a piston whose function is to produce hydraulic pressure in the pipeline. This pressure is subsequently converted to force to actuate the wheel-cylinder disc-pads or shoe-expanders. The master cylinders are either (i) residual-pressure type or («') non-residual-pressure type.

Residual-pressure Master-cylinder (Lockheed).

Construction.

The master-cylinder has a cylinder pressure chamber and a reservoir chamber! The reservoir takes up any fluctuation in the volume of the fluid in the system due to temperature change and for a limited amount of fluid leakage (Fig. 28.37).

The middle region of the master-cylinder piston has a reduced-diameter and is always full of fluid. Rubber lip seals are fitted at the both ends of the piston to prevent leakage of fluid. A high pressure cup seal known as the primary seal is fixed to the return-spring end of the piston and a low-pressure ring seal known as the secondary seal, which slips into a recess groove around the piston is fitted to the push-rod piston end. A thin washer is placed between the cup seal and the piston to prevent the cup being drawn into the recuperation holes, drilled around the piston head. A rubber boot encloses the push-rod end of the cylinder to keep the cylinder bore free from the dust.

Drum-brakes use a residual-pressure check-valve at the end of the pressure cylinder opposite the push-rod. Once the brakes are released this check valve develops a low line pressure of 49 to 98 kPa, which offers the following services :

(a) It provides a minimum pedal free-travel by opposing the brake-shoe retraction springs.

(b) It maintains a light contact of the wheel-cylinder seal lips with the cylinder bore to avoid entry of air.

(c) It prevents the re-entry of fluid into the master-cylinder during the bleeding operation. This ensures a fresh charge of fluid at each stroke of the brake pedal and a complete purge of air from the system.

Unlike drum brakes, disc-brakes must have no residual pressure in the pipeline. This permits a complete release of the pads from the disc, avoiding overheating of discs and rapid wear. To achieve this, a small restrictor hole is provided in a conical check-valve. This causes complete pressure release, and the system can still be cleared by fairly rapid pumping of the pedal during bleeding (Fig. 28.37D).

Operation.

When the foot-pedal is applied, the push-rod pushes the master-cylinder piston along its bore. Immediately the bypass or compensation port is sealed 'off, and fluid ahead of the piston is trapped. The pressure developed in the master-cylinder pushes the lips of the check-valve cup away from the metal body so that fluid is displaced into the pipelines. This forces the caliper or shoe wheel-cylinder pistons, causing the discs or drums to be braked. (Fig. 28.37B).

Fig. 28.37. Lockheed master-cylinder.

When the foot-pedal is released the master-cylinder return-spring moves the piston back against its stop washer and circlip faster than the return of fluid from the disc or drum wheel-cylinders. It therefore causes a depression in the master cylinder. As a consequence the primary seal is drawn away from the piston head distorting it, thereby uncovering the recuperation holes. Fluid from the annular space around the piston then flows through the recuperation holes and removes the temporary pressure difference between the two sides of the piston head (Fig. 28.37C).

At the same time fluid returning from the brakes, being under load from the disc-brake piston seals or drum-brake retraction springs, pushes the whole check-valve body away from its rubber seat and so flows back into the master cylinder. The fully returned piston then uncovers the bypass on compensation port (0.7 mm diameter) so that any excess fluid created by the expansion of the heated fluid is released to the reservoir from the pressure chamber. Fluid always fills the annular space formed between the piston and cylinder by way of the large feed port (Fig. 28.37A).

Non-residual Pressure Master Cylinder (Girling).

This master cylinder also contains the pressure chamber and an end fluid reservoir. The piston operates in the pressure chamber whereas the reservoir permits additional fluid to enter into or return from the system to maintain a constant volume during temperature changes and any seepage of fluid in the system (Fig. 28.38).