BRAKES PPT Flashcards
A ________ 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 a machine.
brake
In the process of performing this function, the brake absorbs either ________________ or _________________ by objects being lowered by hoists, elevators etc.
kinetic energy of the moving member, potential energy given up
The energy absorbed by brakes is dissipated in the form of ______.
This _______ is dissipated in the surrounding air (or water which is circulated through the passages in the brake drum) so that excessive heating of the brake lining does not take place.
heat
The energy absorbed by the brake and transformed into heat must be dissipated to the surrounding air in order to avoid excessive temperature rise of the brake lining.
The temperature rise depends upon the mass of the brake drum, the braking time and the heat dissipation capacity of the brake.
When the temperature increases, the coefficient of friction decreases which adversely affect the torque capacity of the brake.
At high temperature, there is a rapid wear of friction lining, which reduces the life of lining.
Therefore, the temperature rise should be kept within the permissible range.
Heat to be Dissipated during Braking
The highest permissible temperatures recommended for different brake lining materials are given as follows :
For leather, fibre and wood facing =
65 – 70°C
The highest permissible temperatures recommended for different brake lining materials are given as follows :
For asbestos and metal surfaces that are slightly lubricated =
90 – 105°C
The highest permissible temperatures recommended for different brake lining materials are given as follows :
For automobile brakes with asbestos block lining =
180 – 225°C
In these brakes, the force acting on the brake drum is in radial direction.
Radial brakes
The radial brakes may be sub-divided into external brakes and internal brakes. According to the shape of the friction
element, these brakes may be __________ brakes and ______ brakes.
block or shoe, band
In these brakes, the force acting on the brake drum is in axial direction. The ________ may be disc brakes and cone brakes.
The analysis of these brakes is similar to clutches.
Axial brakes
It 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 as shown below.
The other end of the lever is pivoted on a fixed fulcrum O.
Single Block or Shoe Brake
The frictional force helps to apply the brake. Such type of brakes are said to be _______________.
self energizing brakes
When the frictional force is great enough to apply the brake with no external force, then the brake is said to be _______________.
self-locking brake
When 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.
But when the angle of contact is greater than 60°, then the unit pressure normal to the surface of contact is less at the ends than at the centre.
In such cases, the block or shoe is pivoted to the lever, instead of being rigidly attached to the lever.
This gives uniform wear of the brake lining in the direction of the applied force.
These brakes have more life and may provide a higher braking torque.
Pivoted Block or Shoe Brake
When a single block brake is applied to a rolling wheel, and 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 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 ___________ brake, the braking action is doubled by the use of two blocks and the two blocks may be operated practically by the same force which will operate one
Double Block or Double Shoe Brake
