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.
Various components and their functions in a hydraulic braking system are as follows.
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).
This converts foot-pedal force to hydraulic pressure within the fluid system by means of the cylinder and piston (Fig. 28.36).
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.
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.
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.
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.
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).
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).
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).
Fig. 28.38. Girling master-cylinder.
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.
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.
Fig. 28.39. Compression-barrel master-cylinder (Girling).
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.
Single-piston Wheel-cylinder Shoe-expander.
Single-piston wheel-cylinder units are normally used on front drum-brakes, to provide higher braking efficiency. Two single-piston units are fitted diametrically opposite each other. The single-piston unit expands one shoe against the drum and acts as the anchor abutment for the other shoe, thus performs the dual functions. If the outward movement of both the single-piston units is in the direction of forward rotation of the drum, the combination is known as a two-leading-shoe brake (Fig. 28.41).
Similar to the double-piston units, the single-piston units are bolted to the back-plate. These units work in similar way to that of the double-piston ones, except that the cylinder has a blind end to form the anchor abutment for the other shoe. Either the ring-seal or the cup seal with a seal-spreader and retainer-spring is used to seal the piston.
Fig. 28.41. Single-piston wheelcylinders.
Combined Hydraulic/Lever Rear-wheel Shoe-expander
In this case the cylinder bore (Fig. 28.42A) supports both inner and outer pistons. The outer piston uses a pressed-steel dust-cover welded to it and is grooved to carry a rectangular-cross-section rubber dust-seal. The inner piston uses a cup seal with a seal-spreader and a retainer-spring to preload the cup seal against the cylinder wall in the released position of the brakes. The tapered end of the cranked lever is housed in a triangular slot formed in each piston. This lever is located and pivoted on a pin in the body.
During application of the foot-brake, the fluid pressure pushes the inner and outer pistons until the leading shoe is forced against the drum. Consequently, the hydraulic reaction of the fluid forces the cylinder body to slide in its slot on the back in the opposite direction, until the trailing shoe is engaged against the drum. Actually, the cylinder body and pistons float between both shoes and provide an equal shoe tip load to each shoe. Since the slots in the pistons have enough clearance (Fig. 28.42B), the movement of the pistons relative to the cylinder body does not interfere with the enclosed end of the cranked lever.
During application of the hand-brake, the cable pulls the cranked-lever end away from the back-plate. This causes the lever to rotate about the pivot pin mounted in the cylinder body until its tapered end contacts the outer-piston tapered face and pushes this piston against the leading shoe. Any further pull of the cable, at this stage, gives rise to an equal and opposite reaction thrust at the pivot point. Consequently, the cylinder body slides on the back-plate away from the outer piston and hard against the trailing shoe and drum. Again, equal expanding force is applied to both shoes without causing disturbance to the inner hydraulic piston and seal (Fig. 28.42C).
Fig. 28.42. Combined hydraulic I lever rear-wheel shoe-expander for small car.
A very similar arrangement is illuilrated in Fig. 28.43 in which the cylinder and piston both work on the foot-brake shoe-expander movement. In this system, however, a bell-crank lever engages a rectangular hole in the leading-shoe web. Application of the hand-brake pivots the lever due to which its short end forces out the leading shoe. Consequently the equal and opposite reaction acts on the pivot pin so that the cylinder body is moved in its slot in the back-plate to engage the trailing shoe.
Fig. 28.43. Combined hydraulic I lever rear-wheel shoe-expander for large cars.
Separate Rear-wheel Parking-brake Mechanism.
In this parking-brake shoe-expander, the hydraulic foot-brake cylinder body is bolted to the back-plate. A piston at each end actuates the shoes. A link-strut bridges the two shoes, one end connecting against one shoe web and the other end acting as the pivot point for the parking-brake lever attached to the other shoe. Two alternative lever layouts are presented in Fig. 28.44. It is perpendicular to the shoe in Fig. 28.44A and is parallel to the shoe in Fig. 28.44B. The cable is
joined to the free end of the lever. The cable pull, due to application of the hand-brake, pivots the lever. The strut, pushed by the lever one way, actuates the leading shoe and levers the trailing shoe in the opposite direction. The expanding force is shared equally between them as the link-strut floats between the two shoes.
Fig. 28.44. Separate rear-wheel hand-brake lever.
Fig. 28.45. Pressure-regulating valve.
Pressure Regulating Valve
This valve (Fig. 28.45) is installed in the rear brake line. The purpose of the valve is to limit the pressure acting on the rear brakes so that the risk of rear wheel skidding is reduced. The valve uses a spring-loaded plunger enclosed in a body. Since low fluid pressure can not overcome the spring, full pressure acts initially on all brakes when the brake is applied. As soon as a predetermined pressure is reached, the valve closes and disconnects the fluid flow to rear brakes. Subsequently, any further increase of pressure is only felt by the front brakes.
Brake Pressure Control Valve (Inertia Valve)
This is a pressure regulating valve. It is particularly designed to overcome the problem of the large load variation between front the rear wheels of front-wheel drive vehicles. The valve is installed in the rear brake line(s). It is an inertia sensitive pressure reducing valve. It operates when the vehicle decelerates at a predetermined rate. During this period, the valve temporarily closes the rear brake line and allows the front brake pressure to increase further. When a preset pressure is attained, the valve re-starts the pressure supply to the rear brakes, but at a rate much below the increase in pressure at the front brake (Fig. 28.46). The valve takes into account vehicle weight transfer and effect of attitude during braking. It is also sensitive to vehicle loading and road conditions.
Figure 28.47 illustrates the construction of a valve applicable for a normal rear brake circuit. The system has independent lines using two valves mounted side by side. The valve unit uses a cylinder, fixed to the car body at a given angle. The cylinder contains a stepped piston and a steel ball. At low deceleration rates of a vehicle, fluid enters the inlet port and passes around the ball. Then it flows through the piston drilling to the rear brakes causing equal pressure in both front and rear brake lines.
Fig. 28.46. Control valve performance.
Fig. 28.47. Brake pressure control valve (Inertia valve).
If the inertia force, produced by the rate at which the vehicle slows down, rolls the ball up along the sloping cylinder, then the ball stops the fluid supply to the rear brakes. During this period, the difference in piston area maintains the outlet pressure constant while the inlet pressure is being increased. At a particular point, depending upon the piston areas, an increase
in inlet pressure moves the piston providing a proportional pressure to the rear brakes. Pressure in the two lines at this stage is governed by the relation ; Inlet pressure x Small area = Outlet pressure x Large area.
Pressure Differential Warning Actuator
This warning device illuminates a brake failure warning lamp when the pressure difference in the two brake lines differs by more than a specified amount. When failure of one brake line occurs the pistons (Fig. 28.48) moves and operates the electrical switch. The switch remains closed until the pistons are reset.
This valve, within specified limits, provides hydraulic pressure to the rear brakes in proportion to the load carried by the rear wheels. Therefore this reduces the risk of rear wheel skidding when the rear of the vehicle is lightly loaded. Also, this arrangement ensures
Fig. 28.48. Pressure differential warning actuator.
good braking when the rear wheels are over loaded. One load-apportioning valve is sufficient for a single hydraulic line layout, but a separate valve is required in each line when a diagonal line system is incorporated. The valve housing is mounted on to a rigid part of the vehicle body. A spring working either in tension or compression senses the load on the rear wheels. This spring connects the valve-operating lever to a part of the suspension system that moves in proportion to the vehicle load.
The construction of the valve is shown in Fig. 28.49. The lever acts directly on a piston, which uses a ball valve. In the released position of the brake, the piston lies at its bottom position and the ball valve is held open by a push-rod attached to the valve body. This allows the fluid to pass freely between the inlet and outlet ports. As the hydraulic pressure is applied to the valve the upward movement of the piston takes place. This is accomplished by providing a larger area exposed to the fluid on the upper part of the piston than the area on the bottom part. The amount of hydraulic pressure, required to raise the piston and close the ball valve is governed by the force exerted by the external spring on the piston.
If the load on the rear wheels is light, only a small force is exerted by the spring on the piston. Consequently, a relatively low pressure is required to move the piston upwards to close the valve. If the pressure at this closure point is exceeded, full pressure cannot be applied to the rear brake. Therefore any further increase in pedal force causes the piston to control the valve to maintain a lower pressure that is also proportional to the pressure applied to the front brake.
As the load on the rear wheels increases, the suspension deflects and the force on the external spring increases. Therefore, to face this extra force exerted by the spring on the piston, a higher fluid pressure is created before the piston is able to rise. As a result full pressure on the rear brakes is maintained until a much higher pedal force is applied.
Fig. 28.49. Load-apportioning valve (Bedix).
The type of valve presented in Fig. 28.49 uses an adjusting screw between the lever and piston. This controls the point at which the valve comes into operation, determining the front/rear braking ratio for a given rear wheel load.
Besides any leakage of the seals, fracture of the external spring may be the possible faults in this arrangement. The breakage of the spring results in considerable reduction in the pressure supplied to the rear brakes through the valve. The complete valve unit is usually replaced, when it is found defective.
A brake fluid meets the international standards set in the United States by the Society of Automotive Engineers (SAE) and Department of Transportation Federal Motor Vehicle Safety Standard (FMVSS).
The major characteristics of a brake fluid include :
(a) Low viscosity. The brake fluid must flow easily over a wide temperature range and be able to operate in very cold conditions.
(b) Compatibility with rubber components. Besides resisting corrosion of metal parts, it must be chemically non-reactive to the rubber seals etc. It must be non-injurious to the system.
(c) Lubricating properties. It must reduce friction of moving parts, especially rubber seals.
(d) Resistance to chemical ageing. It should have a long storage life and be stable when in use.
(e) Compatibility with fluids. It must be compatible with other fluids of its type.
if) High boiling point. Most braking systems use a glycerin-alcohol (glycol) fluid with additives to meet the required specifications. Because of the availability of a number of different fluids, some being vegetable and some mineral based, the manufacturer’s recommendation should be referred before designing the system and also for subsequent refilling to avoid damage to rubber seals.
Boiling Point of Brake Fluid.
Glycol-based brake fluids are hygroscopic in nature and hence they absorb water from the atmosphere over a period of time. Presence of water lowers the boiling point and in extreme cases failure of the brakes takes place due to vapor locking. This situation arises when the temperature of the fluid in a part of the system rises above its boiling point so that the water in the fluid is vaporized. Once this happens, the elastic nature of the steam causes the pedal to reach its limit of travel before sufficient pressure is built up to apply the brakes effectively.
Because of the hygroscopic nature of most of the brake fluids, SAE and FMVSS specifications recommend the fluid to have a wet boiling point and dry boiling point in addition to the stated value. The ‘wet’ boiling point is the temperature at which a fluid containing 3 to 3.5 percent of water boils and produces steam bubbles. The ‘wet’ boiling point of typical brake fluids must be above 413 K. For safety reasons it is recommended to change the fluid in a brake system every year. A brake fluid absorbs about 5 percent water in this time period so that the boiling point is lowered to about half its original valve. Some new fluids have wet and dry boiling points of 453 K and 533 K so that the fluid renewal interval can be extended to 2 years.
Some special silicone-based fluids have been developed to overcome the hygroscopic problem, but these are costly and hence not commonly used. Brake fluids should be stored in sealed containers and should not come into contact with the paintwork of the car. If any fluid some how drips on to the paintwork, then it should be washed off with water immediately.
Bleeding the Brakes
Bleeding is necessary to remove air from the brake system whenever it enters. The main steps involved in the bleeding operation are briefly as follows, (a) Ensure that the reservoir is full with brake-fluid.
(6) Attach one end of rubber tube to bleeder valve and immerse the other end in brake-fluid placed in a jar.
(c) Open the bleeder valve and slowly operate brake pedal until air bubbles cease to appear. Close the bleeder valve as the pedal is being depressed.
(d) Repeat the above operation at all wheel cylinders, (c) Top up reservoir with brake fluid up to the mark.
The main faults in the braking system and their causes are as follows :
|Pedal requires pumping||Shoes require adjustment|
|Springy pedal||Air is present in the system|
|Spongy pedal (pedal creeps downwards)||Leakage is present in the system, e.g. fluid is passing through main rubber cup.|