Page 193 - Machinery Component Maintenance

Machinet-! Alignment 175

priate section of this text. Many times, high-performance
couplings require interference fits as high as .0025 in. per
in. of shaft diameter.
Coupling cleanliness, and for some types, lubrication, are
important and should be considered. Sending a repaired
machine to the field with its lubricated coupling-half un-
protected, invites lubricant contamination, rusting, dirt ac-
cumulation, and premature failure. Lubricant should be
chosen from among those recommended by the coupling
manufacturer or a reputable oil company. Continuous run-
ning beyond two years is inadvisable without inspecting a
grease lubricated coupling, since the centrifuging effects
are likely to cause caking and loss of lubricity. Certain lu-
bricants, e.g., Amoco and Koppers coupling greases, are
reported to eliminate this problem, but visual external in-
spection is still advisable to detect leakage. Continuous
lube couplings are subject to similar problems, although
such remedies as anti-sludge holes can be used to allow
longer runs at higher speeds. By far the best remedy is
clean oil, because even small amounts of water will pro-
mote sludge formation. Spacer length can be important,
since parallel misalignment accommodation is directly
proportional to such length.

Alignment Tolerances

Before doing an alignment job, we must have tolerances to work to-
ward. Otherwise, we will not know when to stop. One type of “toler-
ance” makes time the determining factor, especially on a machine that is
critical to plant operation, perhaps the only one of its kind. The opera-
tions superintendent may only be interested in getting the machine back
on the line,fast. If his influence is sufficient, the job may be hurried and
done to rather loose alignment tolerances. This can be unfortunate, since
it may cause excessive vibration, premature wear, and early failure. This
gets us back to the need for having the tools and knowledge for doing a
good alignment job efficiently. So much for the propaganda-now for the
tolerances.
Tolerances must be established before alignment, in order to know
when to stop. Various tolerance bases exist. One authority recommends
‘h-mil maximum centerline offset per in. of coupling length, for hot run-
ning misalignment. A number of manufacturers have graphs which rec-
ommend tolerances based on coupling span and speed. A common toler-
ance in terms of face-and-rim measurements is .003-in. allowable face