Brake A traditional clasp brake the brake shoe

Brake

A traditional clasp brake: the brake shoe (brown) bears on the surface (tyre) of the wheel (red), and is operated by the levers (grey) on the left 24/11/2020 NAS 2

What? �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 24/11/2020 NAS 3

Brake Capacity �The capacity of a 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, and 5. The ability of the brake to dissipate heat equivalent to the energy being absorbed. 24/11/2020 NAS 4

Brake Lining Material �The material used for the brake lining should have the following characteristics : 1. It should have high coefficient of friction with minimum fading. In other words, the coefficient of friction should remain constant with change in temperature. 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. It should not be affected by moisture and oil. 24/11/2020 NAS 5

Classification 1. ◦ 2. ◦ 3. Hydraulic brakes e. g. pumps or hydrodynamic brake and fluid agitator Electric brakes e. g. generators and eddy current brakes, and Mechanical brakes 24/11/2020 NAS 6

Mechanical brakes �block or shoe brakes �band brakes 24/11/2020 NAS 7

Single Block or Shoe Brake 24/11/2020 NAS 8

�F = Force applied at the end of the lever, � RN= Normal force pressing the brake block on the wheel, � r = Radius of the wheel, � 2θ = Angle of contact surface of the block, � μ = Coefficient of friction, and � Ft = Tangential braking force or the frictional force acting at the contact surface of the block and the wheel. 24/11/2020 NAS 9

�Braking torque, TB �Self locking is calculated when Force applied at the end of the lever (P) is zero. �Kinetic energy of the drum = Power 24/11/2020 NAS 10

�Number of revolution made before coming to rest from the instant the brake is applied �the time taken before coming to rest from the instant the brake is applied 24/11/2020 NAS 11

Example 1 The block brake shown in Figure provides a braking torque of 360 Nm. The diameter of the brake drum is 300 mm. The coefficient of friction µ = 0. 35. Find i. The force P to be applied at the end of the lever for the clockwise and anticlockwise rotation of the brake drum. ii. The location of the pivot or fulcrum to make the brake self-locking for the clockwise rotation of the brake drum. 24/11/2020 NAS 12

24/11/2020 NAS 13

Example 2 �A brake drum of 440 mm in diameter is used in a braking system as shown in figure. The brake lever is inclined at an angle 20 with the horizontal axis. A vertical force of 400 N magnitude is applied at the lever end. The coefficient of friction is 0. 35. the brake drum has a mass of 160 kg and it rotates at 1500 rpm. Determine the i. Braking torque. ii. Number of revolution made by the drum and the time taken before coming to rest from the instant the brake is applied 24/11/2020 NAS 14

Example 3 24/11/2020 NAS 15

Solution 24/11/2020 NAS 16

24/11/2020 NAS 17

Band brakes 24/11/2020 NAS 18

�Kinetic energy of the drum = Power 24/11/2020 NAS 19

�Number of revolution made before coming to rest from the instant the brake is applied �the time taken before coming to rest from the instant the brake is applied 24/11/2020 NAS 20

Example 3 The simple band brake, as shown in Figure is subjected to a force of 100 N at the end of the lever. If the coefficient of friction is 0. 2 and the brake drum diameter is 240 mm, find the torque transmitted due to this force. 24/11/2020 NAS 21

Example 4 The brake shown in Figure has a coefficient of friction of 0. 3 and is to operate using a maximum force P = 500 N. The drum is rotating in clockwise direction. Find the bend tensions and the braking torque if a = 150 mm, c = 200 mm and the diameter (D) of the brake drum is 250 mm. How long is the braking time from 1200 r. p. m. if the rotor moment of inertia (I) is 1. 5 kg. m 2? 24/11/2020 NAS 22

24/11/2020 NAS 23