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 :
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.

4. Materials for Brake Lining
1. 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.
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 low coefficient of thermal expansion.
6. It should have adequate mechanical strength.
7. 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,
 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.
SINGLE BLOCK OR SHOE BRAKE

7. Block or Shoe Brake

8. CLOCKWISE ROTATION OF DRUM
When the line of action of
tangential braking force (Ft)
passes through a distance ‘a’
below the fulcrum O
Brake wheel rotates clockwise
∑M = 0
P . l – RN . x + Ft . a =0
P . l – RN . x + μ. RN . a = 0
P . l – RN (x – μ .a) = 0

9. ANTI CLOCKWISE ROTATION OF DRUM
When the line of action of
tangential braking force (Ft)
passes through a distance ‘a’
below the fulcrum O
Brake wheel rotates clockwise
∑M = 0
P . l – RN . x – Ft . a =0
Remind Ft = μ. RN
P . l – RN . x – μ. RN . a =0
P . l – RN (x + μ .a) = 0

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.

12. 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.

13. Single Block or Shoe Brake Problems

14. Single Block or Shoe Brake Problems

15. 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.

16. Single Block or Shoe Brake Problems

17. Single Block or Shoe Brake Problems

18. Double Block or Shoe Brake

19. 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.

20. Double Block or Shoe Brake Problems

21. Double Block or Shoe Brake Problems

22. Double Block or Shoe Brake Problems

23. 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/mm2.

24. Double Block or Shoe Brake Problems

25. Double Block or Shoe Brake Problems
1. Spring force necessary to set the brake
Taking moments about the fulcrum O1

26. Double Block or Shoe Brake Problems
1. Spring force necessary to set the brake
Taking moments about the fulcrum O2

27. Double Block or Shoe Brake Problems
1. Spring force necessary to set the brake
Remind, TB = (Ft1 + Ft2)r
1400 × 103 = (0.776 S + 1.454 S) x175
1400 × 103 = 390.25 S
∴ S = 3587 N

28. Double Block or Shoe Brake Problems

29. Double Block or Shoe Brake Problems