What Are Mechanical Brakes? Explain Drum Type & Disc Type Brake

MECHANICAL BRAKES

               Generally mechanical type brakes are available in following construction –
1.      Drum type brake.
2.      Disc type brake.

Ø DRUM TYPE BRAKE –

These are generally of two types ;
                           I.          Internal expanding drum brake.
                          II.          External contracting drum brake.
ü INTERNAL EXPANDING DRUM BRAKE –

·        It consists of brake drum, stationary plate, two shoes hinged at the anchor pins, and cam system to expand the shoe and a retracting spring.
·        Brake shoe is generally made of cast iron, aluminum, or pressed steel. Brake linings are fixed at the outsides of the shoe. When brakes are applied, the cam is turned; the shoes with brake linings are forced against the drum, thereby stopping or slowing down the vehicle. When brake is released, the retracting spring brings the shoes back to its position.
·        This type of brake is used in most of the modern vehicles. The mechanism used for internally expanding brake may be of mechanical i.e. cam operated pneumatic type or hydraulic type.
ü EXTERNAL CONTRACTING DRUM BRAKE –

·        This type of brake is used only for parking the vehicle. The system consists of drum, band with lining, operating lever with adjusting lever and push rod with returning spring.
·        When push rod is operated by hand or foot operated lever, then the lined brake band fitted around the drum is tightened to lock or slow down the drum.
·        When the brake is released, the return spring brings the band back to its initial position. The system remains the air opened; therefore the dirt gets deposited between the rubbing surfaces, which reduce the efficiency.
Ø DISC TYPE BRAKE –

1.      Disc brake consists of the cast iron disc bolted to the wheel hub and a stationary housing called as ‘calliper’. The calliper is connected to some stationary part of the vehicle, like stub axle and is cast into two parts each containing a piston.
2.      In between each piston and the disc there is a friction pad held in position by retaining pins, springs etc. passages are drilled in the calliper for the fluid to enter or leave the cylinder.
3.      The brakes are operated when friction pads are forced against the disc by hydraulic pressure from the master cylinder, thereby engaging the braking system.
4.      When the hydraulic pressure on the piston is released then the friction pads are also released thus the system is disengaged.
5.      To avoid the radial forces on the disc the system is balanced by using two callipers diametrically opposite to each other. In this way the brake torque can also be doubled.
Ø ADVANTAGE OF DISC BRAKE –
a)      Better heat dissipation as braking surface is directly exposed to air.
b)     Flat friction pads compared to curved friction lining in drum brakes gives uniform wear and wide choice of friction material.
c)      Under the most difficult condition these brakes fade due to better anti-fade characteristic.
d)     Pad wear adjustment is automatic.
e)     Renewal of pad is quick and easy.
Ø DISADVANTAGE OF DISC BRAKE –
a)      In case a vehicle is not equipped with servo assistance, to operate the brake slightly higher pedal pressure is required.
b)     In comparison with drum brake of similar capacity, rate of pad wear is more.
c)      The hand brake mechanism is not very convenient.