Page 395 - Analysis and Design of Machine Elements

Couplings and Clutches
Universal couplings, also known as universal joints or Cardan joints, permit substan- 373
tial angular deviation between mating shafts, with the maximum value reaching 35–45 ∘
[5]. The single universal joint consists of a splined-hub driving yoke 1, and driven yoke 3,
connected by a cross link 2 (or a spider) through pivot bearings. The two yokes are ori-
ented at a right angle to each other and are connected to the driving and driven shafts,
respectively. The yokes pivot on the arms of cross. Since there are two pivots, the two
shafts can be at any angle to one another and can still rotate and transmit power [2].
When the driving yoke rotates at constant angular velocity, the angular velocity ratio
between driving and driven shafts varies periodically, although the two shafts make
complete revolutions during the same time [2, 5]. The fluctuation of angular output
velocity implies acceleration, leading to undesirable dynamic loads and subsequent
noise and vibration. Therefore, most practical applications require a double joint, that
is, to use an intermediate shaft or Cardan shaft to connect two single universal joints.
During assembly, it is important to ensure that the driving shaft, driven shaft, and
intermediate shaft lie in the same plane, and the misalignment angles at two joints
are equal, that is, the double joint arrangement is adaptable to either parallel shafts
(Figure 13.2a) or intersecting shafts (Figure 13.2b). In these arrangements, the second
joint tends to compensate for the nonuniform oscillation of the first joint so that the
fluctuation in angular velocity ratio is very slight [1–3].
In a gear coupling, two identical, externally geared hubs 1 and 6 mesh internally with
flanged sleeves 3 and 5, respectively. The hubs are connected to the driving and driven
shafts by keys and are enclosed by bolted flanged sleeves. Torque is transmitted from
one hub to the other by the meshing of teeth. The teeth on the sleeves are usually longer
than the hub teeth, and there is a gap between two hubs. The hubs are, therefore, able
to move axially to ensure that teeth always maintain adequate contact for power trans-
mission. Clearance between the mating teeth in the hub and the flanged sleeve allows
gear couplings to accommodate end float, parallel offset and angular misalignments.
Crowned gear teeth are often used to accommodate larger angular misalignments with-
out excessive backlash.
A chain coupling is composed of two hubs with sprockets and a detachable, duplex
chain. The common duplex roller chain meshes with two sprockets with the same
number of teeth on the hub to transmit torque, providing a convenient and rapid
method of disconnecting shafts. End float, parallel offset and angular misalignments
are accommodated by the built-in clearances between the chain and sprocket, which
exist between the rollers and sprocket teeth, the rollers and bushings, the bushings
and pins, and the links and sprockets teeth. Chain couplings find wide applications in
mechanical equipment and are especially suitable for working in hostile environments
such as high-temperature, humid and dusty conditions. In practice, a cover is usually
incorporated to maintain lubrication and reduce wear, thereby increasing the life of
chain couplings.
Elastomeric couplings are couplings that contain nonmetallic elastomeric elements,
including pin and bushing couplings, elastomeric pin couplings, resilient couplings with
a spider and tyre couplings. The flexibility of these couplings is obtained from stretching
or compressing resilient materials, usually rubber or synthetic rubberlike material.
Therefore, they have excellent vibration dampening capabilities and misalignment
accommodation capacities, providing extended life for connected equipment.