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hi:: Pump Installation 8 Piping

Alignment' is a misnomer. We are not concerned about bringing the
coupling halves into alignment, we are only interested in ensuring
the shafts of the pump and its driver will rotate on a common axis. If
the shafts are not coaxial, the resulting moments will increase the forces
on the pump shaft and beatings, causing accelerated wear and
premature failure.

10.3.1 Shaft couplings
That does not mean that we should ignore the coupling. There are only
two types of couplings, both of which are capable of operating with
some degree of misalignment. One type accommodates misalignment
through the sliding of one element over another, while the other
accommodates misalignment through the flexing of one or more
components.
Any couplings that operate through the sliding of one element over
another will not only require lubrication, but may prefer some minimal
movement between the elements in order to keep the surfaces
lubricated. In other words, these couplings might even last longer with
some minimal shaft misalignment because, without any relative motion,
the lubricant will be expelled from the spaces between the element
surfaces, thus causing premature wear.
Those couplings that accommodate misalignment through flexing as
shown in Figure 10.10, will last longer and work better at zero
misalignment as there are no forces being transferred through to the
pump shaft and bearings.
What should be recognized however is that all couplings resist being
operated when misaligned, and the restoring forces and moments

Figure 10.9: Gear type coupling (Reproduced Figure 10.10: Rubber type coupling (Reproduced
with permission of the Falk Corporation) with permission of the Falk Corporation)

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