Misalignment Types
Shaft misalignment comes in three forms
(see Definition of Misalignments diagram
on page 19).

• Axial- misalignment is the variation in
the distance between the machinery
shafts in relation to the neutral length
of the coupling.

• Angular- misalignment is the effective
angle between the two machinery
shafts and is usually quantified by
measuring the angle between the
centre lines of the shafts if they were
extended to intersect.

• Radial (parallel) - misalignment is the
transverse distance between the two
machinery shaftsand is quantified by
measuring the distance between the
centre lines of the shafts if they are
extended to overlap.

Misalignment may result from the
practical tolerance in machining, initial
alignment of the machines or movement,
settlement or operating thermal
variations of the machinery. In practical
applications, all the types of
misalignment are likely to be present as a
result of all the causes.

Misalignment in Disc couplings
Disc couplings handle misalignment by
deflection of the flexible discs. The discs
are bolted on a set bolt circle diameter,
being fastened alternately to the
"Driving" & "Driven" flanges. The
deflection then taked the form of a
bending (& twisting) of the beam
between pairs of adjacent bolts.

In practice, a single flexible disc will only
accomodate axial and angular
misalignments. Radial misalignments are
accomodated by the use of two flexing
discs, one at each end of a central spacer.

(Note:DS couplings do not accept radial
misalignment). The radial misalignment is
then quoted in "mm/mm between
flexing points" (i.e. the total amount of
radial displacement divided by the
distance between the centres of the
flexing discs.

Since the disc coupling only identifies
radial misalignment as an angular
misalignment at each flexing disc, any
angular & radial misalignment must be
evaluated as a "Combined
Angular/ Parallel" misalignment. The
result of this combination may not,
necessarily, be the same at both ends-
(see Angular & Radial Misalignment in
"combination"diagram on page 19)-
and the worst case should, hence be
considered. Representation may be in
terms of the combined angle (degrees) or
combined radial (mm/mm).

For a disc coupling, the relational
misalignment allowance is generally
represented graphically, (see Allowable
misalignments for Disc Couplings diagram
on page 19).

Since the level of acceptable
misalignment is stress related, there is an
effect due to both the torque being
transmitted and the rotational speed of
the coupling (these both introduce stress
into the discs). Hence it is normal for
Bibby Turboflex to present the allowable
misalignments for a disc coupling
specifically for a given application.
However, the tables on page 18 will give
guidance for all the units outlined. More
details are available upon request.

The information in the tables is presented
assuming units operating at 66% stated
rated torque and 50% of the maximum
stated speed since this represents a
practical assumption in most cases. The
figures are given for point a, b & c (see