Page 101 - Electric Drives and Electromechanical Systems

94 Electric Drives and Electromechanical Systems

will, to varying degrees, minimise the effect of misaligned shafts. If not properly
compensated a minor shaft misalignment can result in unnecessary wear and premature
replacement of other system components.
In certain cases, flexible couplings are selected for other functions. One significant
application is to provide a break point between driving and driven shafts that will act as a
mechanical fuse if a severe torque overload occurs. This assures that the coupling will fail
before something more costly breaks elsewhere along the drive train. Another applica-
tion is to use the coupling to dampen the torsional vibration that occurs naturally in the
driving and/or driven system.
Currently there are a large number of flexible couplings due to the wide range of
applications. However, in general flexible couplings fall into one of two broad categories,
elastomeric or metallic.
Elastomeric couplings use a non-metallic element within the coupling, through which
the power is transmitted, Fig. 3.16A. The element is manufactured from a compliant
medium (for example rubber or plastic) and can be in compression or shear. Elastomeric
couplings have good vibration and shock absorption and are capable of accommodating
a greater misalignment than the equivalent metallic coupling. However they are not
torsionally stiff, have poor overload capability and are difficult to balance in high speed
applications.
In practice there are two basic failure modes for elastomeric couplings. Firstly, the
coupling can fail due to fatigue from cyclic loading when hysteresis that results in
internal heat build-up if the elastomer exceeds its design limits. This type of failure can
occur from either misalignment or torque beyond its capacity. Secondly the compliant
component can break down from environmental factors such as high ambient tem-
peratures, ultraviolet light or chemical contamination. It should be noted that all elas-
tomers have a limited shelf life and will in practice require replacement as part of
maintenance programme, even if these failure conditions a not present.
Metallic couplings transmit the torque through designs where loose fitting parts are
allowed to roll or slide against one another (for example in designs based on gears) or

FIG. 3.16 Cross sections of commonly used couplings. (A) Flexible elastomer coupling. (B) Metallic bellows
coupling.