Page 403 - Analysis and Design of Machine Elements

Table 13.3 (Continued) Couplings and Clutches 381
Feature and
Types Figures applications

Centrifugal Speed sensitive clutch.
clutches
Suitable for accelerating large
masses of inertia to high
rotational speeds by a relatively
small driving motor.

Overrunning Direction sensitive clutch.
clutches
Examples of applications are in
bicycle hubs, centrifugal pumps
and conveyors, elevators.

Safety clutches, also called overload clutches, are torque sensitive clutches. When the
applied torque exceeds the predetermined value, clutches will automatically disengage
or slip and thus save driven machines from costly breakdowns. This overload protection
is the mechanical equivalent of an electric fuse [2]. Torque limitations can be realized
by different mechanical designs. When the torque reaches the specified value, shear
pins may break in safety clutches; connected shafts may separate in jaw clutches, and
friction discs may slip in friction clutches, thus protest important components from
damage.
A safety clutch with shear pins relies on one or more pins to shear at the predeter-
mined torque. As shown in Table 13.3, pins 3 are placed in the holes on flanges 1 and
4 of the two clutch halves. They are usually made of medium carbon steel, heat treated
to 50–60 HRC, with precut notches to assist snap. To avoid the sheared pins scratch-
ing surfaces, pins are installed in hardened steel sleeves 2. The sheared pins have to be
replaced manually before the coupling can operate again.
Centrifugal clutches are speed sensitive clutches. When the rotational speed reaches
or exceeds the predetermined value, centrifugal clutches will automatically engage
or disengage. A typical structure of a centrifugal clutch in Table 13.3 composes of an
input spider 1, an output drum 2 and asbestos blocks 5 connected to the input spider
1 by bolts 3. The input spider deliver power from a prime mover. When the input shaft
rotates, asbestos blocks 5 are thrown radially outward by centrifugal force against the
inner surface of drum 2. Torque is thus transmitted through friction between asbestos
block 5 and drum 2. When the drum 2 reaches operating speed, the clutch acts simply