Page 398 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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misalignment type and magnitude. For example, slider block couplings can tolerate
large radial misalignments while universal couplings are ideal for intersection shafts or
large angular misalignments.
13.2.1.2 Reliability and Operating Environments
Since flexible couplings without elastomeric elements rely on sliding or rolling to com-
pensate misalignments, lubrication and wear must be considered. Therefore, they are
less reliable than flexible couplings with elastomeric elements that rely on the flexure
of elastomeric elements to compensate for misalignments. Besides, couplings with non-
metallic elastomeric elements, such as rubber, are sensitive to temperature and corrosive
media.

13.2.1.3 Manufacturing, Installation, Maintenance and Cost Considerations
The selected coupling affects both the cost of equipment and the downtime for coupling
replacement or failure repair [4]. By careful selection, installation and maintenance of
couplings, substantial savings could be made in reducing maintenance costs and down-
time.

13.2.2 Coupling Size Selection
While selecting the size of coupling, the design torque T ca is established by multiplying
a service factor with the nominal torque T to cope with momentary peaks caused by the
fluctuation of either prime mover torque or load torque, that is,

T = K T ≤ [T] (13.1)
ca
A
The service factor K is affected by the starting inertia of load, the characteristics of
A
prime mover and driven machine. It can be selected by increasing the values in Table
2.1 by 20–30%. While specifying commercially available couplings, the rated torque of
selected coupling [T] should be greater than the design torque. Both couplings and
clutches can be selected from manufacturers’ catalogues according to the design torque
obtained from Eq. (13.1).
Another capacity needs to be considered in coupling selection is the limiting speed.
Since the power a coupling can transmit is the product of transmitted torque and rota-
tional speed, for a given size of coupling, the transmitted power increases as the speed
of rotation increases. And centrifugal effects determine the upper limit of shaft speed.
The rotational speed of coupling must satisfy
n ≤ [n] (13.2)
In most cases, each coupling accommodates a range of shaft diameters. The maximum
and minimum values of shaft diameter or a series of dimensions, are provided in manu-
facturers’ catalogues or design handbooks [8, 9]. The diameter of mating shafts should be
within the specified range of selected couplings. Besides, assembly and installation accu-
racy must be specified according to the misalignment tolerance of the selected coupling.
Finally, it is important that the selected coupling is compatible with the connected
equipment. The introduced coupling should cause little change to the operating con-
ditions of the system and introduce minimum stresses on the components of coupling.
When a coupling has an elastomeric element, the operating temperature of the coupling
should be lower than the maximum allowable temperature for the elastomeric material.