Page 99 - Mechanical Engineer's Data Handbook

88 MECHANICAL ENGINEER’S DATA HANDBOOK

drum. In this type of brake the force is provided by a Far
f
(c
Pb)
spring which normally keeps the brake applied. Fur- Friction torque T= -
ther compression is necessary to release the brake.
This type of brake is used for lifts, for safety reasons. A~~~~~~ pressure Pa = T
1 2pwr2 sin 9
Friction torque T=Farp - Maximum pressure pm = Kp,
[ (c + pb)+&]
Fa 2.8.3 Internally expanding shoe brake
Maximum pressure p = ~
(c - @)A
This type is used on vehicles and has two shoes, lined
where: F= spring force. with friction material, which make contact with the
The brake is released by a force greater than F. inside surface of a hollow drum. For rotation as shown
in the figure:
KpFar
Torque for left-hand shoe T --
L-(b+Kpc)
KpFar
Torque for right-hand shoe T --
’- - Kpc)
(b
Total torque T- TL + TR
with K as previously.

Maximum pressure pm = TR
2pwr2 sin 9
Average pressure p, = Pill -
K

Block brake with long shoe
Here the friction force is applied around a large angle.
The torque is increased by a factor K which is a
function of the angle of contact. The shoe subtends an
angle of 28 and is pivoted at ‘h’ where
4 sin 8
h=Kr; K=
(28 + sin 28)
2.0.4 Disk brake

Let:
F = force on pad
r = mean radius of pad
A=pad area

Torque capacity (2 pads) T= 2pFr
F
Pad pressure p=-
A