Page 256 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors

tion or frame. In this case, as well as in the preceding three, the analysis
should not be confirmed or acted on until all main bearings have been
inspected.

Maximum Deflection Specifications

The number of variables involved and the complexity of the problem
make it impossible for an equipment builder to predict the deflection at
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 machines have run for years
with deflections much higher than compressor builders' limits. Further-
more, some locations make it difficult to keep a machine 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 mak-
ing that decision.
In 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 recommended
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 an undesirable situation in that there is a reverse bend,
or "S," indicated by a change from plus to minus signs. The stress con-
centration in the throw between the change of signs can become pro-
nounced if the deflection is much above the manufacturer's standards.
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. On vertical compressors, the up position occurs when the peak pressure
exerts maximum force on the journal and tends to spread the webs apart.
Because the webs are already inward, the peak 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 maximum force is exerted.
It can be seen that it is difficult to assign a maximum deflection to any
reciprocating machine, but if the value specified by the builder is not