Page 328 - Machinery Component Maintenance

310 Machinery Component Maintenance and Repair

ror 2 will generally cause reading fluctuation in case of excessive tight-
ness, nonrepeating readings in case of excessive looseness. Error 6 may
be handled like 1, 3,4, and 5 if the looseness is eliminated by a set screw
(or similar) in the same direction after each indexing and retightening cy-
cle. If not, it will cause nonrepeating readings. Error 7 will not be dis-
covered until the rotor is checked without the end-drive adapter, presum-
ably under service conditions with field balancing equipment. The only
(partial) remedy is to reduce the runout in the shaft extension and the
weight of the end-drive elements to a minimum.
Balance errors from belt-drive pulleys attached to the rotor are consid-
erably fewer in number than those caused by end-drive adapters. Only
the pulley unbalance, its fit on the shaft, and the shaft runout at the pulley
mounting surface must be considered. Such errors are avoided altogether
if the belt runs directly over the part. Certain belt-drive criteria should be
followed.
Air- and self-drive generally introduce minimal errors if the cautionary
notes mentioned previously are observed.

Balance Errors Due to Rotor Support Elements
Various methods of supporting a rotor in a balancing machine may
cause balance errors unless certain precautions are taken. For instance,
when supporting a rotor journal on roller carriages, the roller diameter
should differ from the journal diameter by at least 10 percent, and the
roller speed should never differ less than 60 rpm from the journal speed.
If this margin is not maintained, unbalance indication becomes erratic.
A rotor with mounted rolling element bearings should be supported in
V-roller carriages (see Nomenclature, Appendix 6B). Their inclined roll-
ers permit the bearing outer races to align themselves to the inner races
and shaft axis, letting the rolling elements run in their normal tracks.
Rotors with rolling element bearings may also be supported in sleeve
or saddle bearings; however, the carriages or carriage suspension sys-
tems must then have “vertical axis freedom” (see Terminology, Appen-
dix 6A). Without this feature, the machine’s plane separation capability
will be severely impaired because the support bridges (being connected
via the rotor) can only move in unison toward the front and rear of the
machine; thus only static unbalance will be measured.
Vertical axis freedom is also required when the support bridges or car-
riages are connected by tiebars, cradles, or stators. Only then can couple
unbalance be measured without misaligning the bearings in each (out-of-
phase) back-and-forth movement of the support bridges. This also holds
true for hard-bearing machines, even though bridge movement is micro-
scopically small.