Design of block brakes

problems on single and double block brakes

1. Unit - V Design of Brakes By Dr.S.Senthil &Mr. B.Balavairavan Mechanical Engineering Kamaraj College of Engineering and Technology Virudhunagar

2. 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 a machine. In the process of performing this function, In Automobiles, the kinetic energy of the moving vehicle is absorbed by the brake. In hoist and elevators, the potential energy released by the objects during the braking period is absorbed by the brakes. The energy absorbed by brakes is dissipated in the form of heat.

3. Introduction The design or capacity of a brake depends upon the following factors : The unit pressure between the braking surfaces, The coefficient of friction between the braking surfaces, The peripheral velocity of the brake drum, The projected area of the friction surfaces, and The ability of the brake to dissipate heat equivalent to the energy being absorbed.

4. Materials for Brake Lining It should have high coefficient of friction with minimum fading. In other words, the coefficient of friction should remain constant over the entire surface with change in temperature. It should have low wear rate. It should have high heat resistance. It should have high heat dissipation capacity. It should have low coefficient of thermal expansion. It should have adequate mechanical strength. It should not be affected by moisture and oil.

5. TYPES OF BRAKES

6. Block or Shoe Brake Let P = Force applied at the end of the lever, R N = Normal force pressing the brake block on the wheel, r = Radius of the wheel, 2 θ = Angle of contact surface of the block, μ = Coefficient of friction, and F t = Tangential braking force or the frictional force acting at the contact surface of the block and the wheel. SINGLE BLOCK OR SHOE BRAKE

7. Block or Shoe Brake

8. CLOCKWISE ROTATION OF DRUM When the line of action of tangential braking force ( F t ) passes through a distance ‘ a ’ below the fulcrum O Brake wheel rotates clockwise ∑M = P . l – R N . x + F t . a = P . l – R N . x + μ . R N . a = 0 P . l – R N (x – μ .a) =

9. ANTI CLOCKWISE ROTATION OF DRUM When the line of action of tangential braking force ( F t ) passes through a distance ‘ a ’ below the fulcrum O Brake wheel rotates clockwise ∑M = P . l – R N . x – F t . a = Remind F t = μ . R N P . l – R N . x – μ . R N . a = P . l – R N (x + μ .a) =

10. Block or Shoe Brake When the frictional force helps to apply the brake with no external force, 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 . The self-locking brake is used only in back-stop applications. The brake should be self-energizing and not the self-locking.

11. Single Block or Shoe Brake Problems Example 1 A single block brake is shown. The diameter of the drum is 250 mm and the angle of contact is 90°. If the operating force of 700 N is applied at the e n d o f a l e v e r a n d t h e coefficient of friction between the drum and the lining is 0.35, determine the normal force pressing the brake block on the wheel, tangential braking force, a n d t o r q u e t h a t m a y b e transmitted by the block brake.

12. Single Block or Shoe Brake Problems

13. Single Block or Shoe Brake Problems

14. Single Block or Shoe Brake Problems Example 2 A single block brake is shown in Fig. The diameter of the drum is 180 mm and the angle of contact is 60deg. If the operating force of 400 N is applied at the end of the lever and the coefficient of friction between the lining is 0.3, Determine ( i ). The torque that may be transmitted by the block brake. (ii). The rate of heat generated during the braking action, when the initial brake speed is 300 rpm and (iii). The dimensions of the block if the intensity of pressure between the block and brake is 1MPa. The breadth of the drum is twice its width.

15. Single Block or Shoe Brake Problems

16. Single Block or Shoe Brake Problems

17. Double Block or Shoe Brake

18. Double Block or Shoe Brake Problems Example 1 . The block brake shown in fig. Is set by a spring that produce as a force S on each arch equal to 3500 N. The wheel diameter 350 mm and the angle of contact for each block is 120deg. Take coefficient of friction as 0.35, determine ( i ). The maximum torque that the brake is capable of observing, and (ii). The width of the brake shoes, if the bearing pressure on the lining material is not to exceed 0.3MPa.

19. Double Block or Shoe Brake Problems

20. Double Block or Shoe Brake Problems

21. Double Block or Shoe Brake Problems

22. Double Block or Shoe Brake Problems Example 2. A double shoe brake, as shown, is capable of absorbing a torque of 1400 N-m. The diameter of the brake drum is 350 mm and the angle of contact for each shoe is 100°. If the coefficient of friction between the brake drum and lining is 0.4; find : 1. the spring force necessary to set the brake; and 2. the width of the brake shoes, if the bearing pressure on the lining material is not to exceed 0.3 N/mm 2 .

23. Double Block or Shoe Brake Problems

24. Double Block or Shoe Brake Problems 1. Spring force necessary to set the brake Taking moments about the fulcrum O 1

25. Double Block or Shoe Brake Problems 1. Spring force necessary to set the brake Taking moments about the fulcrum O 2

26. Double Block or Shoe Brake Problems 1. Spring force necessary to set the brake Remind, T B = ( F t 1 + F t 2 ) r 1400 × 10 3 = (0.776 S + 1.454 S) x 175 1400 × 10 3 = 390.25 S ∴ S = 3587 N

27. Double Block or Shoe Brake Problems

28. Double Block or Shoe Brake Problems