When two machines are directly coupled via a flexible coupling, any misalignment between their centerlines of rotation can result in vibration which can produce premature wear or even catastrophic failure of the machines’ bearings, seals, the coupling itself, or other rotating components.

“Short-Flex” Coupling Tolerances
This is one of the most common ways of defining
alignment tolerances. The offset tolerance
simply describes the maximum separation that
can exist between two centerlines of rotation at
a specific location along the shaft axes, usually
the coupling center.

Since most flexible couplings have two flex
planes (or points of articulation), these spacer
coupling tolerances may safely be used with all
such couplings, even the ones usually considered
“short flex” couplings. The best criterion to
make the distinction between a short flex and
a spacer coupling is the relation between the
diameter of the flex planes and the distance
between them. Anytime the distance between
flex planes (axial span) is greater
than the working diameter
of those flex planes, call it a
spacer coupling.

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