INTRODUCTION AND NECESSITY OF BRAKES
(a) The braking system is one of the vital factors in the running and control of the vehicles. It is necessary to provide means for bringing the vehicle rapidly to rest from any speed. The means of slowing down or bringing to rest a moving vehicle in a short possible distance is called brake.
(b) The primary object of the braking system is to destroy the motion of wheels when vehicle is moving. The efficiency of brakes depends upon the lives and comfort not only of the driver and of the passengers but of other road users. The brakes convert the kinetic energy of the vehicles into heat energy by means of friction created between the brake linings and wheel drum and this heat is dissipated quickly by providing cooling ribs on the outer surface of the wheel drum or making the wheel drum as large as possible necessity of brakes are as follows.:
(i) To stop the vehicle as and when required quickly and efficiently without skidding.
(ii) To slow down the vehicle as required.
(iii) To hold the vehicle stationary even in an inclined position.
MANDATORY BRAKES AS PER THE LAW
The traffic law lays down certain requirements to govern the fittings and efficiency of braking system and all vehicles are expected to comply with them.
(a) Motor vehicles must be equipped with two independent and efficient braking system having two independent means of operation so that if one fails other can be operated e.g. service brake and parking brake.
(b) Every vehicle must be fitted with parking brake, which can be set in such a way that two wheels of a vehicle are affected by it.
(c) If a vehicle is fitted with a servo brake system which embodies a vacuum or high pressure air reservoir there must also be some means readily available (a gauge) from the driving seat, whereby the driver will be warned of any failure in the system.
TYPES OF BRAKE ACTUATION
The drum brakes works on mainly two principles as Internal expanding and external Contracting.
(a) Internal Expanding Brakes. Internal expanding brakes comprise two shoes lined with friction material, which are mounted on brake back plate. The shoes are fitted within a flanged drum, opened or expanded at one end by an expander unit, which may be a mechanically operated cam or a hydraulic cylinder and pivoted at the other end on an anchor pin. With the brakes in the ‘off’ position, the drum is free to revolve with the wheel and hub, as its internal diameter is greater than the overall diameter of the brake shoes. As the brakes are operated the expander pushes apart ends of the shoes and the friction lining material on the faces of the shoes is brought into contact with revolving drum and thus the rotation of the drum ceased.
(b) External Contracting. The external contracting brakes are also comprised of two shoes, lined with friction material, which are mounted around a revolving drum. One end of both the shoes are anchored in a common pin and the other ends are operated by a lever. When it is operated the shoes are contracted thus made to contact with the revolving drum against the tension of return spring and causes the drum to stop. This type of brake are generally applied on transmission as a parking brake.
(c) Simple Cam Operated. Two brake shoes are mounted on a fulcrum pin or pivot and their other ends which have flat faces bear against the sides of a double cam. The complete assembly is carried on a back plate rigidly mounted to the axle casing or stub axle. The shoes are retained in position out of engagement with the drum, by strong tension springs and they are surrounded by a brake drum which is fastened to and rotate with the hub of the road wheel. The brake shoes are made to bear against the drum by partial rotation of the double cam on one end of an operating shaft. The operating shaft is carried in phosphor bronze bearings and extends on either side of the back plate. A lever on its external end is connected by rods or cable to the foot and hand controls. A minimum clearance between shoes and drum in the ‘OFF’ position allows the drum and hub to rotate freely. A compensating arrangement is incorporated in the cable or rod lay-out to ensure an even distribution of braking effort.
(d) Disc Type Brake.
(i) Disc type brake consist of a disc which is mounted on the wheel hub and two shoe pads either side of the disc floating calliper assembly.
(ii) When the foot pedal is pressed then the fluid is forced out from the master cylinder. This fluid pressure will act in the calliper assembly and this fluid pressure in the caliper assembly causes the pad (1) on the piston side to press against the disc. At the same time, the floating type caliper body is moved to the right by the cylinder pressure, as shown in image “Disc Type Brake”, which pulls pad (2) against the disc and so brake the wheel. The disc brake has no servo assistance as in drum braking, and it is necessary to increase the working pressure of the piston and pad.
(iii) For this purpose, the wheel cylinder has a large bore. Even a little clearance, between the disc and pad has therefore large influence on the brake pedal stroke. It is necessary to have the clearance adjusted to the minimum at all times, by means of the piston (rubber) seal.
TRANSMISSION BRAKE OR HAND BRAKE
The transmission brake consists of two brake shoes (or bands) of external contracting type, hand operating lever and linkages. The brake is designed for use as a parking brake only. When the hand brake lever is pulled, the shoes, which operate in the drum bolted to the gearbox rear-coupling flange, closed in and contact the drum.
Foot brakes are the actual brake which is meant for slowing down or bringing the moving vehicle to rest position. Foot brakes are further classified as follows.
(a) Mechanical brakes.
(b) Hydraulic brake.
(c) Air assisted hydraulic brake.
(e) Vacuum assisted hydraulic brake.
(f) Pneumatic brake.
In hydraulic brake system, the actuation of the brake shoe is achieved by fluid pressure, which acts upon a piston or pistons in the wheel brake-operating unit and thus causes the brake shoes to expand. Because of the use of fluid as operating medium, hydraulic braking system is fully compensated.
BLEEDING A HYDRAULIC BRAKE SYSTEM
Procedure as follows. (a) Before starting bleeding operation, inspect the reservoir fill it up with fluid and keep replenishing it as necessary throughout the bleeding operation.
(b) Brake shoes first are adjusted for correct clearance.
(c) Bleeding valves are always on the wheel cylinder and sometimes on the master cylinder as well. When a bleeding valve is fitted on the master cylinder, this should be purged first.
(d) Attach the rubber bleeding tube to the bleeder valve to the farthest wheels in sequence of their being at far distance from master cylinder.
(e) Immerse the other end, in a glass container containing a little fluid and depress the brake pedal twice/thrice slowly throughout its stroke and by holding the pedal, loosen the bleeder screw and watch the air bubbles in the glass jar. Tighten the bleeder screw and release the brake pedal.
(f) Do not assist the pedal to return, but allow it to do so under the influence of the return spring.
(g) Repeat this operation with a slight pause between each depression to allow the time for master cylinder to recharge. Continue to pump the pedal until fluid pumped from the system carries on more air (air when present will rise through the fluid in the jar in the form of bubbles) and then tighten the bleeder valve securely and repeat the same operation until air bubbles cease to appear at the end of the bleed tube allowing only fluid to flow. This operation should be repeated at all wheel cylinders generally for the conventional method, as mentioned two persons are necessary
ADJUSTMENT OF BRAKE PEDAL FREE PLAY
(a) Loosen check nut 1 remove pin 2 fork from lever.
(b) Adjust length of link rod.
(c) Check that pedal free play of 10 to 20 mm obtained.
ADJUSTMENT OF PARKING BRAKE
(a) Vehicle should be in unloaded condition.
(b) Jack up rear axle
(c) Keep the flick valve lever in brakes ‘OFF’ position.
(d) Build up air pressure in the system up to 7 bar. Ensure that the piston rod of the spring break chamber comes out fully and the brakes are released.
(e) Adjust service brake as described earlier.
(f) Keep both left hand and right hand brake lever in the same position.
(g) Adjust the turnbuckles on the two linkages till the hand brake lever on the hand brake shaft is just at the point of moving forward.
(h) Loosen the two turnbuckles by ½ to ¾ of a turn and tighten lock nuts.
(j) Apply fail-safe parking brake a couple of time and check that there is no grabbing.