BRAKES

Introduction-

A Brake is a device by means of which artificial frictional resistance is applied to a moving machine member, in order to retard or stop the motion of machine. In the process of performing this function, the brake absorbs either kinetic energy of the moving member or potential energy either given up by the objects being lowered by hoists, elevators, etc. The energy absorbed by brake is dissipated in the form of heat. This heat is dissipated in the surrounding air so that excessive heating of the brake lining does not take place. The capacity of brake depends upon the following factors:-

1. The unit pressure between the braking surfaces.2. The coefficient of friction between the braking surfaces.3. The peripheral velocity of the brake drum.4. The projected area of the friction surfaces.5. The ability of the brake to dissipate heat equivalent to the energy being absorbed.

Manufacturing and Materials-

Materials for brake lining:-

The materials used should have following characteristics:-

1. It should have high co-efficient of friction with minimum fading.2. It should have low wear rate.3. It should have high heat resistance.4. It should have high heat dissipation capacity.5. It should have adequate mechanical strength.6. It should not be affected by moisture and oil.

Materials commonly used are Cast iron on cast iron, Bronze on cast iron, Steel on cast iron, Wood on cast iron, Fibre on metal, Cork on metal, Leather on metal, Wire asbestos on metal, Asbestos blocks on metal, Metal on cast iron.

Types of Brakes

Brakes are classified upon:-

1. Hydraulic brakes e.g. pumps or hydrodynamic brake and fluid agitator.2. Electric brakes e.g. Generators and eddy current brakes.3. Mechanical Brakes.

Mechanical brakes according to the direction of acting force, maybe divided into the following groups:

1. Radial brakes: - In these brakes, the force acting on brakes drum is in radial direction. The radial brakes may be subdivided into external brakes and internal brakes. According to the shape of the friction elements, this brake may be block or shoe brakes and band brakes.

2. Axial brakes: - In these brakes, the force acting on the brake drum is in axial direction. The axial brakes may be disc brakes or cone brakes.

Block or Shoe Brake:-

Block or Shoe brake is divided into various categories as follows:-

1. Single block or shoe brake2. Double Block or Shoe Brake3. Pivoted block or Shoe Brake

Single Block or Shoe Brake:-

A Single Block or Shoe brake is shown in figure.

Types of Brakes

Hydraulics Electric Mechanical

Radial

External Internal Block or Shoe Band

Axial

Disc Cone

WorkingA single block or shoe brake consist of a block or shoe which is pressed against the rim of a revolving brake drum. The block is made of a softer material than the rim of the wheel. This type of brake is commonly used on railway trains and tram cars. The friction between the block and the wheel causes a tangential braking force to act on the wheel, which retard the rotation of the wheel. The block is pressed against the wheel by a force applied to one end of a lever to which the block is rigidly fixed as shown in figure. The other end of the lever is pivoted on a fixed fulcrum.

If the angle of contact is less than 60◦, then it may be assumed that the normal pressure between the block and the wheel is uniform.

Components of Brake:-

1. Block or Shoe:- It is pressed against the rim of the revolving brake wheel drum. It is made of softer material than rim of the wheel.

2. Rim3. Revolving Brake wheel drum

Application

This type of brakes is commonly used in:

1. Railway and trains- The brake shoe carries the brake block. The block was originally made of wood but is now usually cast iron. When the brake is applied, the shoe moves and presses the block against the tread of the wheel. As well as providing braking effort this also "scrubs" the wheel and keeps it clean. This scrubbing causes wear and tear on the wheel tread and often causes brake squeal. Tread brakes on passenger trains have now largely been superseded by disc brakes.

2. Automobiles- The brake shoe carries the brake lining, which is riveted or glued to the shoe. When the brake is applied, the shoe moves and presses the lining against the inside of the drum. The friction between lining and drum provides the braking effort. Energy is dissipated as heat.

3. Bicycles- This comprises a pair of rectangular open boxes which are mounted on the brake callipers of a bicycle and that hold the brake blocks which rub on the rim of a bicycle wheel to slow the bicycle down or stop it.