Page 69 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

54 Practical Machinery Management for Process Plants

I
1000.0

c
0
-- 100.0
0
Q
tn
s
3
.- 10.0
c
c
c
0
0
.b- 0
5 1 .o
iE
0.1
0.2 50 100
misalignment (milslinch)
Figure 1-22. Estimated time to failure of rotating machinery due to misalignment.
ure 1-22 shows the estimated time to failure of rotating machinery due to
misalignment?

Assembly-related Problems

A surprisingly large number of pump bearings are assembled without
much attention to acceptable fits and tolerances. Table 1-7 shows bearing
bore and shaft diameter minimum and maximum values recommended for
centrifugal pump radial bearings. What is perhaps less well known is the
fact that mating shafts and radial bearings, which are at the high and low
ends of their allowable tolerances respectively, are likely to cause high oper-
ating temperatures.
More specifically, oil-ring-lubricated bearings will usually receive
enough oil for adequate lubrication, but rarely will this amount be sufficient
to preclude hot bearing housings if interference fits are at the high end of the
apparent allowable interference spectrum. Hot bearing housings invite oper-
ators to pour liberal amounts of water on the assembly; this causes the bear-
ing outer ring to contract and bearing internal clearances to vanish, with
rapid failures inevitable. But even if the operator resists the impulse to pro-
vide this detrimental means to supplemental cooling, increased oil tempera-
tures will lead to accelerated oxidation of the lubricant.
Angular contact bearings require even more care. Mounted back-to-back
and often provided with a small axial preload gap, these bearings are often
intended for shaft interference fits in the order of 0.0001 in. to 0.0005 in.
only. Sets of these bearings, FAG 73 14B UOI are frequently supplied in cen-