Page 201 - The Tribology Handbook

Brakes B8

Table 8.2 (continued)

Type Brakefactor (p = 0.3) Uses
__
~ ~ ~~
Screw-operated 1.81 Winches, reeling brakes for
(reversible) 6 = 155" for each lining marine applications

Differential Very high; depends on lever Parking or holding brake only,
ratio as the brake is unstable

Double wound 15.9 In rugged installations such as
6 = 540" oil-well equipment where
operation is manual and
precise control not required

RIGID SHOE (DRUM) BRAKES impaired and the friction material has a good life. Table
8.5 shows the area required for different types of duty.
Shoes are lined with friction material, usually over an arc Smaller, wider drums of sufficient thermal capacity
of 90-1 10 deg. Shoes may be leading (positive self-servo) reduce bending stresses and give more uniform pressure
or trailing (negative self-servo). By suitable combination on the linings, and reduce the moment of inertia of the
of shoes, or by altering the geometry of the brake. the drum. Drums for internal expanding brakes are generally
amount of servo required for the actuating force available, stiffened with external circumferential ribs to avoid 'bell-
and the amount of stability required, can be obtained. mouthing' but can 'barrel' if ribs constrain ends but not
On most drum brakes the actuating force on each shoe the centre of the rubbing path. These fins also improve
is the same and the brake factor B is defined as: cooling; axial fins may develop high thermal stresses.
Total frictional force at drum radius P + P'
-
B= --
Actuating force F

and some values of B are shown plotted against p for
different brakes in Figure 8. I
Knowing the torque, type of brake, its diameter, and the
p, of the linings, the actuating force required can be
obtained from Figure 8.1 (k would normally lie between
0.3 and 0.4---a lining with the Eower p would give a more
stable brake and would last longer, but a larger actuating
force would be required).
Alternatively, knowing the torque required, the type of
brake, and the 1~. of the linings, the actuating force for a I-L
given diameter brake, or the diameter of the brake for a Figure 8.1 Typical variation of Brake Factor B with p
given actuating force, can be determined from Figure 8.1. for a LT (leading-trailing shoe), a 2LS (two leading
The rubbing path area must be adequate to keep shoe), and a duo-servo drum brake having linings of
temperatures within limits so that performance is not arc length 100"
B8.2