design of Brakes system

mechanical eng

1. UNIT 2 BRAKES
ABrakeisdefinedasamechanicaldevice,whichisusedtoabsorbthe
energypossessedbyamovingdevice,systemormechanismbymeans
offriction.
Theprimarypurposeofthebrakeistoslowdownorcompletely
stopthemotionofamovingsystem,sucharotatingdrum,machineora
vehicle.

2. Brakes
The energy absorbed by brake is converted into heat energy
and dissipated to surrounding i.e.
Heat Dissipation is a serious problem in brake applications.
Brakes are classified into following3 groups
1.Mechanical brakes –operated by means of
levers, springs and pedals
2. Hydraulic and Pneumatic brakes –operated by fluid
pressure such as oil pressure or air pressure
3. Electrical Brakes –Operated by eletro-magnetic forces.`
TypesofMechanicalbrakes:Shoebrakes,Bandbrakes,
InternalandExternalexpandingbrakes

3. Energy Equations
The first step in design of a mechanical brake is to determine the
braking torque capacity . The braking torque capacity depends
upon the amount of energy to be absorbed by the brake.

4. Chapter 12

5. Chapter 12

6. Chapter 12

7. Chapter 12

8. Chapter 12

9. Numerical Problems

10. Chapter 12

11. Chapter 12

12. Block Brake with short shoe
Ablockbrakeconsistsofasimpleblock,
whichispressedagainsttherotating
drumbymeansofaleverasshownin
figure.Thefrictionbetweentheblock
andbrakedrumcausestheretardationof
drum.
Application:Onrailwheels.
Angleofcontactbetweenblockand
brakeisusuallysmall<45
O
.Since
angleofcontactissmall,itresultsin
uniformpressuredistributiononblock.

13. Free Body Diagram (Clockwise Rotation)
M
t–Braking torque (N-mm)
R –radius of brake drum (mm)
μ -coff. of friction
N –Normal reaction (N)
The dimensions of block are determined by
N= plw
Here p –permissible pressure between block
and brake drum (N/mm
2
)
l= length of block
w = width of block
A Narrow Block (lesser w)
drawback->Size –large
A Broad Block (Higher w)
drawback ->Pressure distribution is
non-uniform

14. Free Body Diagram (Clockwise Rotation)
Assumptions
The width of block should be optimum between
2 limits given by:
(1/4 )* drum diameter < w < (1/2)*
drum diameter

15. Free Body Diagram (Clockwise Rotation)
Considering the equilibrium of forces in vertical and
horizontal directions:
Taking moments of forces acting on lever about the
hinge point O,
Depending upon magnitude of coeff. Of friction (μ),
location of hinge pin (c), there are 3 different cases:
1.a>μc(Desirablecondition)Partiallyself
energising–Prequiredforbrake
2.a = μ c(Non-Desirable condition)
The break is self-locking. P=0
3.a < μ c (Non-Desirable condition) P= Negative
This is a dangerous operating condition, resulting in
uncontrolled braking and grabbing. The brake is
non-controllableby operator as he can not apply it.

16. Free Body Diagram (Anti-clockwise Rotation)
P (Braking effort) also depends upon the
direction of rotation of brake drum:
In design, the objective will be to
design for:
•Smaller braking effort
•Avoid dangerous and undesirable
braking conditions
The main disadvantage of block brake is
the tendency of brake drum to bend
under the action of normal force.
The remedy is to use two symmetrical
blocks at opposite sides of brake drum.

17. Fig. 12.5

18. Chapter 12

19. Chapter 12

20. Chapter 12

21. Chapter 12

23. Fig. Free Body Diagram of Forces

24. Chapter 12

25. Block Brake with Long Shoe
•The angle of contact in this case is > 45
0
.
•For the short shoebrake normal Reaction (R) and frictional
forces are assumed to be concentrated at the center of shoe.
•This assumption is not applicable for brake with long shoe
brake.

26. Block brake with long shoe
Equation similar to Block
Brake with short shoe

27. Fig. 12.16Internal Expanding Brake
Internal expanding brake
Application: Vehicle, conveyor, belt

28. Fig. 12.17Free Body Diagram of Forces
Internal expanding Force diagrams

29. Chapter 12

30. Chapter 12
Torque

31. Chapter 12
Anticlockwise direction

32. Chapter 12
FORWARD AND REVERSE MOVEMENT

33. Chapter 12
Advantages

35. Chapter 12

36. Chapter 12

37. Chapter 12

38. Chapter 12

39. Chapter 12

40. Fig. 12.19

41. Band Brake
A simple band brake consists
of a flexible steel strip lined
with friction matrial, which
is pressed against the
braking drum.
When one end of band passes through the
fulcrum of the actuating lever, the brake
is called simple band brake.
The working of steel band is similar to that
of a flat belt operating at the zero
velocity

42. Free body diagram
Ratio of band tensions are given by:
P1= Tension in tight side of the band (N)
P2= Tension in slack side of the band (N)
μ = coeff of friction between friction lining
and brake drum
Θ= Anlgle of wrap (rad)
Mt = torque capacity of brake (Nmm)
R = radius of brake drum (mm)
Considering the forces acting on lever and
taking moments about the pivot:

43. Differential Band Brake (a) Construction (b) Free Body
Diagram
A band brake is called differential band brake when
neither end of band passes through the fulcrum of
actuating lever.Such brakes are designed for the
condition of self locking

44. Chapter 12

45. Advantages of self-locking: Although self-locking is
undesirable in speed control brakes. It is used to
advantage in Back-stop mechanism. A back-stop
brake is device, which is used to prevent the reverse
motion of drum would have harmful effects:
Applications:
•Bucket conveyors
•Hoisting application
•Material handling

46. Fig. 12.24

47. Chapter 12

48. Chapter 12

49. Chapter 12
Disk Brake
Adiskbrakeissimilartoaplateclutchexceptthatoneshaftis
replacedbyafixedmember.Frontwheelofmotorcycle.
Venteddiscbrakeshaveasetofvanes,betweenthetwosidesofthe
disc,thatpumpsairthroughthedisctoprovidecooling
Brake Pad

50. Chapter 12
Caliper Disk Brake
Thediscbrakeissamelikethebrakesonabicycle.
Inadiscbrake,thecaliperwhichsqueezethebrakepadstherotorinsteadofthe
wheel,andtheforceistransmittedhydraulicallyinsteadofthroughacable.
Frictionbetweenthepadsandthediscslowsthediscdown.
Ex:lifttrucks,farmmachinery,frontwheelofmotorcycle
Singleplateclutchequationisusedfordiskbrakes.

51. Disk Brake Annular pad
Thedimensionsofannularpadareasfollows:
R
o=Outerradiusofpad(mm),R
i=Innerradiusofpad(mm),θ=angular
dimensionofpad(radians)
Sincetheareaofpadiscomparativelysmall,itisassumedthatpressureonthe
frictionliningisuniform,Thebrakingtorquecapacityaccordingtouniform
pressuretheoryisgivenby:
Two shapes of pads
1.Annular
2.circular

52. Chapter 12 Disk Brake with Annular Pad

53. Disk Brake with Circular Pad
Friction radius of circular pad is given by:
R
f= δe
e = distance of pad centre from axis of disk (mm)
And values of δare taken from table
R/e δ
0.0 1.0000
0.1 0.9833
0.2 0.9693
0.3 0.9572
0.4 0.9467
0.5 0.9375
R-radius of circularpad (mm)

54. Chapter 12
Thermal considerations
•Theenergyabsorbedbythebrakeisconvertedintoheat,whichincreasesthe
temperatureofrubbingsurfaces.
•Asthetemperatureincreases,thecoefficientoffrictiondecreasesthatadversely
affectsthetorquecapacityofbrake.
•Athightemperaturethereisrapidwearoffrictionliningwhichreducesthelifeofthe
lining
•Thetemperatureriseshouldbekeptwithinpermissiblelimits.
•Ifitisassumedthatalltheheatgeneratedduringbrakingoperationisabsorbedbythe
brakedrumassembly,then:
Δ
t= E/(mc)

55. Chapter 12

56. Chapter 12

57. Chapter 12

58. Chapter 12

59. Chapter 12

60. Chapter 12