Page 399 - Analysis and Design of Machine Elements

13.3 Introduction to Clutches Couplings and Clutches 377
13.3.1 Applications, Characteristics and Structures
A clutch is a device used to connect or disconnect two shafts end to end at any time
during operation. The two shafts rotating at different speeds are brought to a common
angular velocity after a clutch is actuated. Through a smooth and gradual engagement
or disengagement, clutches change the speed or the rotational direction of the driven
machine to adjust to different operating conditions. Acting as an on-off switch, clutches
control the flow of power to ensure the safe and effective operation of machinery. They
can also serve as a variable operating control to automatic react to the predetermined
torque, speed or direction [2]. Clutches are indispensable power transmitting compo-
nents in mechanical power transmission.
A clutch principally consists of two parts that are engaged or disengaged either man-
ually or by power driven device. Each part is connected to a driving shaft and a driven
shaft, respectively. Most of clutches are highly standardized and commercially avail-
able from manufacturers. Clutches can be either selected or designed according to the
application requirements.

13.3.2 Types of Clutches
The classification of clutches depends on actuation methods and torque transmission
principles. Clutches can be actuated mechanically, pneumatically, hydraulically, electro-
magnetically and automatically. Mechanically actuated clutches are controlled manually
by a lever or wedges. Pneumatic and hydraulic controlled clutches move the actuating
linkages by pneumatic and hydraulic forces. Electrically or electromagnetically oper-
ated clutches are engaged electrically and spring-released. Automatic clutches realize
engagement and disengagement automatically when predetermined conditions, such
as torque, direction or speed, are satisfied.
Clutches work following different physical principles. For example, jaw clutches rely
on the interlocking of teeth to transmit torque, while friction clutches depend on fric-
tion between contact surfaces. The typical types of clutches, their features and applica-
tions are schematically outlined in Table 13.3, followed by a brief introduction to the
configuration and operating principles. More specific design information is unique to
manufacturers and available through catalogues and design handbooks.
A jaw clutch is usually composed of two halves, connected to a driving shaft and a
driven shaft, respectively, either by a key or by a spline. The two halves of clutch have
a series of evenly spaced teeth with square, triangular, spiral, or toothed profiles (see
Figure 13.3) to provide either one direction, or smoother engagement, or prolonged
engagement time [10]. The teeth of mating sets of jaws are brought into engagement
by sliding one or both halves axially to form a rigid mechanical junction. Once the teeth
are engaged, shafts are effectively joined and torques are positively transmitted [2]. The
structure of jaw clutch ensures a strong mechanical coupling between two shafts with no
slippage, wear, or heat generation. However, the engagement is constantly accompanied
by shock. Therefore, jaw clutches are normally engaged while a system is stopped or is
running slowly.
Friction clutches utilize the force between contact surfaces to transmit torque from a
driving shaft to a driven shaft. They are the most common type of all clutches. Friction