Page 216 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

198 Major Process Equipment Maintenance and Repair

which shaft failure will occur. Therefore, a very tight maximum figure
has to be assigned to any shaft so that all situations will be covered. It is
for this reason that failures have happened to shafts with deflections
slightly above specifications while other engines have run for years with
deflections much higher than engine builders’ limits. Furthermore, there
are some locations that make it very difficult to keep the engine level
enough to stay within the limits. The problem is to decide how far one
can go beyond recommendations. The following discussion might help in
making that decision.
In regard to Case 1 , the change in deflection from throws No. 2 to No.
3 is very abrupt. In that case the web stress is very high, and it is recom-
mended that the specified maximum deflection not be exceeded. This can
be demonstrated by holding adjacent main bearings of the wire-model
shaft and creating a bending motion. This would break the shaft quicker
than by holding it at the end main bearings.
Case 4 is also a very undesirable situation in that there is a reverse
bend, or “S,” indicated by a change from plus to minus signs. The stress
concentration in the throw between the change of signs can become very
pronounced if the deflection is much above the engine builder’s stan-
dards.
Case 2, which is a bow (all plus), should allow more deviation from
standards than the other examples, because the stress concentration, as in
the case of the sag, is not as dangerous. Also, a bow is better than a sag
because in the former the deflection is minus. Where a minus reading is
involved, the webs are inward from the neutral position when the throw
is up. The up position is when the peak firing pressure exerts maximum
force on the journal and tends to spread the webs apart. Since the webs
are already inward, the peak firing pressure does not contribute as much
to web stress as it does in the situation of a plus reading, where the webs
are spread apart before the firing force is exerted.
It can be seen that it is difficult to assign a maximum deflection to any
engine, but if the value specified by the engine builder is not exceeded
under any conditions, experience has shown that the shaft should not
break. It is always wise to consult the manufacturer when deflection lim-
its a~e reached.

Crankcase Inspectlon
The preceding paragraphs have covered the foundation, grout, frame,
crankshaft, and main bearings. It should be noted that those important
items can be checked without disassembly of any parts, except for re-
moval of the crankcase doors. Once the doors have been removed, the
operator should take advantage of one of the most revealing inspections
available to him, i.e., the crankcase inspection.