Braking system

 Introduction 

•  A brake was a mechanical device that inhibits motion.

                 A brake was a device by means of which artificial frictional resistance was applied to a moving machine members, in order to retard and stop the motion of machine. In this process, performing this function, the brake absorbs either kinetic energy of the moving member or potential energy given  by objects being lowered by hoists, etc. The energy absorbed by brakes was dissipated in  form of heat energy. This heat is dissipates into the surrounding air (or water which is circulated through the passages in the brake drum) so that excessive heating of the brake lining doesn't take place.
The capacity of  brake depends upon the below 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.

CLASSIFIED AS

                The brakes, according to the means used for transforming the energy by the braking  elements, are classified as :

1. MECHANICAL BRAKES          

•  DRUM BRAKES 
•  DISC BRAKES  

      2. HYDRAULIC BRAKES
      3. POWER BRAKES
                  • AIR BRAKES
                  • AIR HYDRAULIC BRAKES
                  • VACCUM BRAKES
                  • ELECTRIC BRAKES

Mechanical brake

A)   The mechanical brakes, according to the direction of acting force, may be divided into the          following two groups :

       (a) Radial brakes. In these brakes, the force acting on the brake drum is in radial direction.

            The radial brakes may be sub-divided into external brakes and internal brakes. 

            According to shape of  the friction elements, these brakes may be block or shoe brakes and

            band brakes

       (b) Axial brakes. In these brakes, the force acting on the brake drum is in axial direction. 

            The axial brakes may be disc brakes and cone brakes. The analysis of these brakes is      similar to clutches.

B) The mechanical brakes, according to the application in automobiles, may be divided into the          following two groups:

        a)  DRUM BRAKES 

        b) DISC BRAKES

DRUM BRAKES

  1. Single Block or Shoe Brake

             A single block or shoe brake  consists of a block or shoe which is pressed against the rim of a revolving brake wheel drum. The block is made of a softer material than the rim of the wheel. This type of a 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 and the other end of the lever is pivoted on a fixed fulcrum O.

2. Double Block or Shoe Brake

When a single block brake is applied to a rolling wheel, an additional load is thrown on the shaft bearings due to the normal force (RN). This produces bending of the shaft. In order to overcome this drawback, a double block or shoe brake, as shown in Fig. 19.9, is used. It consists of two brake blocks applied at the opposite ends of a diameter of the wheel which eliminate or reduces the unbalanced force on the shaft. The brake is set by a spring which pulls the upper ends of the brake arms together. When a force P is applied to the bell crank lever, the spring is compressed and the brake is released. This type of brake is often used on electric cranes and the force P is produced by an electromagnet or solenoid. When the current is switched off, there is no force on the bell crank lever and the brake is engaged automatically due to the spring force and thus there will be no downward movement of the load. In a double block brake, the braking action is doubled by the use of two blocks and these blocks may be operated practically by the same force which will operate one. In case of double block or shoe brake, the braking torque is given by      

                       TB = (Ft1 + Ft2) r 

                       where Ft1 and Ft2 are the braking forces on the two blocks

 Simple Band Brake

A band brake consists of a flexible band of leather, one or more ropes,or a steel lined with friction material, which embraces a part of the circumference of the drum. A band brake, as shown in Fig. 19.11, is called a simple band brake in which one end of the band is attached to a fixed pin or fulcrum of the lever while the other end is attached to the lever at a distance b from the fulcrum. When a force P is applied to the lever at C, the lever turns about the fulcrum pin O and tightens the band on the drum and hence the brakes are applied. The friction between the band and the drum provides the braking force. The force P on the lever at C may be determined as discussed below :

 Let T1 = Tension in the tight side of the band, 

      T2 = Tension in the slack side of the band,

      θ = Angle of lap (or embrace) of the band on the drum,

      µ = Coefficient of friction between the band and the drum,  

      r = Radius of the drum,

      t = Thickness of the band, and

     re = Effective radius of the drum =  r +( t/2)

                                               Differential Band Brake

In a differential band brake, the ends of the band are joined at A and B to a lever AOC pivoted on a fixed pin or fulcrum O. It may be noted that for the band to tighten, the length OA should be greater than the length OB.

The braking torque on the drum may be obtained in the similar way as discussed in simple band brake. Now considering the equilibrium of the lever AOC. It may be noted that when the drum rotates in the clockwise direction,

 (a), the end of the band attached to A would be slack with tension T2 and end of the band attached to B would be tight with tension T1. On the other hand, when the drum rotates in the anticlockwise direction

(b), the end of the band attached to A would be tight with tension T1 and end of the band attached to B would be slack with tension T2. Now taking moments about the fulcrum O, we have

                                               Band and Block Brake
The band brake may be lined with blocks of wood or other material, The friction between the blocks and the drum provides braking action. Let there are ‘n’ number of blocks, each subtending an angle 2θ at the centre and the drum rotates in anticlockwise direction.

                                            Internal Expanding Brake 

An internal expanding brake consists of two shoes S1 and S2 . The outer surface of the shoes are lined with some friction material (usually with Ferodo) to increase the coefficient of friction and to prevent wearing away of the metal. Each shoe was pivoted at one end about a fixed fulcrum O1 and O2 and made to contact a cam at the other end. When the cam rotates, the shoes are pushed outwards against the rim of the drum. The friction between the shoes and the drum produces the braking torque and hence reduces the speed of the drum. The shoes are normally held in off position by a spring . The drum encloses the entire mechanism to keep out dust and moisture. This type of brake was commonly used in motor cars and light trucks.

DISC BRAKES

•  In a disc brake, the fluid from the master cylinder was forced into a caliper where it presses against a piston. 

• The piston in turn squeezes two brake pads against the disc (rotor), which was attached to wheel, forcing it to slow down or stop. 

 The brake pads squeeze the rotor instead of the wheel, and the force was transmitted hydraulically instead of through a cable.  

Thank you

          source : 1)Theory of machines --R.S.KURMI

                        2)WIKIPEDIA