BRAKES DEFINITION
Brake is a mechanical component used to, with the help of fricon, stop a moving member or hold a member staonary. staonary. Figure 4. Simple Band Brake
SIMPLE BAND BRAKE
q
Figure 4 below illustrates illustrates a simple band brake.
Tension in the band !here,
F1
=e
f q
F2
F" # force on high tension side, lb, $ F # coe&cient of fricon
Brake Tor(ue Tor(ue )eveloped T
=
F1 F2 D 2
F" and F% in terms of the Tor(ue Tor(ue F1 = F2 e
!here,
F"
F% # force on low tension side, lb, $ q # 'ngle of contact, rad.
f q
a
F = F1 F2 r = r F1 f 1q e Te
=
r e
f q
2
f q
F2 =
1
T # brake tor(ue, in*lb, k$*m r # brake drum radius, inches, cm, m
T
r e
f q
1
) # brake drum diameter, inches, cm, m
F
Actuang Force Required on the Arm
a Fa
Taking the summaon of m oment at the pivot, Fa =
!here,
=
F1 A f
=
L
a L
F1 w r q
ma # maimum unit pressure between the band the drum surface, psi, ka 'f # friconal area # w r q, in%, m%
+aimum Stress in Band s =
F1
a
1
From Figure 4 above, consider the actuang arm.
+aimum -nit ressure Pmax
F
2
F
Fa
F%
w # band width, inches, mm, cm, m
!here, t # band thickness, inches, mm, cm, m
wt
q DIFFERENTIAL BAND BRAKE
Figure /below illustrates a di0erenal band brake F Figure /. )i0erenal Band B rake
a
F
Tension in the band
!here,
=e
2
b
1
F1
f q
F2
L
F" # force on high tension side, lb, $ f # coe&cient of fricon
Brake Tor(ue Tor(ue )eveloped T
=
F1 F2 D 2
F" and F% in terms of the Tor(ue Tor(ue F1 = F2 e
f q
F% # force on low tension side, lb, $ q # 'ngle of contact, rad.
F = F1 F2 r = r F1 f 1q e =
Te
r e
f q
!here, T # brake tor(ue, in*lb, k$*m r # brake drum radius, inches, cm, m
f q
1
and F2 =
T
r e
f q
1
) # brake drum diameter, inches, cm, m F
Actuang Force Required on the Arms
a
F
1onsider the actuang arm
F
2
F
a
1
b
a
L
Taking the summaon of m oment at the pivot, Fa =
a F2 b F1 L
!hen Fa is 3ero or negave, the di0erenal band brake is called as self*locking. F L
SIMPLE BL!K BRAKE
Figure below illustrates illustrates a simple block brake. h
Figure . Simple Block Brake
Brake Tor(ue
q 2 = P max w r q sin q
4 f F r r sin
T = f F r h =
q sin q
2
q
F !here, !here, T # brake brake tor(ue, tor(ue, k$*m, k$*m, in*lb in*lb Fr # radial force between the drum and each shoe, k$, lb a f # coe&cient of fricon
h # e0ecve moment arm of the fricon force, inches, 5, m r # radius of the fricon surface of the drum, inches, 5, m ma # ma maimum normal pre ss ssure be betwe en en block and drum, ka, ps psi
q # 'ngle of
contact, rad. w # aial wi width of th the bl block, inch, 5, 5, cm, m
rob. 6 "7 ' band brake is installed on a drum rotang at %/8 rpm, and a diameter of 988 mm. The angle of contact is "./ radians and one end of the brake is fastened to :ed pin while the other end to the to the brake arm "/8 mm from the :ed pin. The coe&cient of fricon is 8.%/ and the straight brake arm is "888 mm long and is placed pe rpendicular to the diameter bisecng the angle of contact. a; Sket Sketch ch the the ban band d br brake ake b; )etermine )etermine the minimu minimum m force force applied applied at the end of the the brake brake arm necessary necessary to stop stop the drum if if /8 k! is being being absorbed. absorbed. c; ' steel band band with a maimum maimum tensil tensile e stress stress of // +a and and < mm thick will will be used. used. !hat should should be its its width width in mm= rob. 6 %7 The di0erenal band brake shown has an angle of contact e(ual to %>8 o and is to sustain a tor(ue of ?888 in*lb. The band has a compressed woven lining@ bears against a cast iron drum of "4 inches diameter, and the coe&cient of fricon is 8.<8. Ather dimensions are a # %<8 inches@ b # " inch@ and c # / inches. )etermine the necessary operang force F in lb. rob. 6 < ' simple band brake has a ? cm diameter drum and :Ced with a steel band %D/ cm thick lined with a brake lining having s coe&cient of fricon of 8.%/. The arc of contact is %4/ o. The drum is aCached to a 8 cm hoisng drum, which sustains a rope load of >%8 kg. The operang force has a moment arm of "./ m and the band is aCached "% cm from the pivot point. Find a; the tor(ue Eust re(uired to support the load clockwise;. b; Find the force, F, re(uired if the direcon is reversed. c; Find the width of a steel band for tension stress of /8 +a.
a
