Page 200 - The Tribology Handbook

B8 Brakes

A brake has to develop the required torque in a stable and controlled manner, and must not reach temperatures high
enough to impair its performance, or damage its components. There are three main types of brake: band brakes, drum
brakes and disc brakes.
A brake is characterised by its Brake Factor which is defined as, for band and drum brakes, as the frictional force at the
drum radius divided by the actuating force, provided it is the same on both shoes (for drum brakes). For disc brakes the
brake factor = Zp, where p is the coefficient of friction.

BAND BRAKES

A flexible steel band lined with friction material is
tightened against a rotating drum. Because of its self-servo
action a band brake can be made very powerful. Positive
self servo occurs when the frictional force augments the
actuating force so increasing the torque, that is, the brake
has a high brake factor. The brake factor increases rapidly
with p and the angle of wrap 6, in the case of simple band
brakes, as shown in Table 8.1.
Too much self-servo makes the brake unstable and
likely to grab and judder (it is usual to work with 6 = 270"
and p = 0.3-0.4).
The relationship between drum diameter and torque Table 8.1 Brake factor for different 6 and p
capacity for band brakes of conventional proportions is
shown in Figure 8.2. P
If the drum rotates in the opposite sense to the actuating 6 degrees
force (negative servo) the brake factor is very small, but by 0. I 0.2 0.3 0.4 0.5
suitable design of the actuating mechanism the brake can
be made equally effective for both directions of rotation. 210 0.44 1.08 2.00 3.33 5.25
As band brakes require small actuating forces they are 240 0.52 1.31 2.51 4.34 7.13
generally suitable for manual operation, particularly when 270 0.60 1.57 3.1 1 5.59 9.55
1.85
they are used only intermittently. 300 0.69 2.16 3.81 7.12 12.72
330
0.78
9.00
16.82
4.63
In band brakes, the width of the rubbing path is 360 0.87 2.51 5.59 11.34 22.16
typically one fifth of the diameter.
Table 8.2 The various types of band brake
%e Brakefactor (p = 0.3) Uses QPe
Simple 3.1 1 Winches, hoists, excavators,
(i3
6 = 270" tractors. etc.

Reversible High; depends on lever ratio As above but where the brake
has to be equally effective in
either direction

B8.1