Page 215 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating Gas Engines and Compressors 197

would be (-), and the highest separation of the webs would be in the
middle throw. This situation is not characteristic of one or more bearings
being wiped, because it is improbable that both end bearings would be
wiped, leaving the center high. A typical cause for this condition is for
the bond between the frame and grout at each end of the engine to have
broken loose. The horizontal couple forces cause the frame to move rela-
tive to the grout, which, over a period of a year, can actually wear it
down.
If this is the problem in Case 2 it can easily be checked by inserting
long feelers (about 8 in.) between the frame and grout. If the feeler thick-
ness is too great (up to .025 in.), the situation is actually worse than the
deflections indicate because the frame is not supported. There are many
installations in which feelers can be inserted all the way at the end of the
frame, but the gravity of the circumstance is determined by how far the
feelers can be moved from the end toward the middle once they are in-
serted. Regardless, the deflections are excessive in Case 2, and if there is
a loosening of the grout, with frame movement, the unit may have to be
regrouted. A common error is to tighten the foundation bolts to restrict
movement. Such tightening is useless because once the bond is broken
the foundation bolts cannot hold the engine down. The amount by which
the maximum deflection can be exceeded will be discussed in subsequent
paragraphs.
If the inspection just described indicates that the bond between the
frame and grout is satisfactory and the grout has not broken up, then the
bowed condition of the shaft could be caused by a change in the shape of
the foundation. There is a possibility that it may be cracked. This can be
verified by a thorough examination of the foundation. Almost all con-
crete structures have hairline cracks, which should be ignored; but open
cracks, regardless of the width, are a good indication of trouble. A sketch
showing the exact location of the open cracks is sometimes useful in cor-
relating their location to the crankshaft deflections.
In regard to Case 3, if the deflections were exactly the same as Case 2
but the signs were all plus (+), then the grout or foundation is in a bad
sag. Comments for this condition are the same as Case 2.
In Case 4, the changes in signs of the deflections show the shaft to be in
a reverse bend. This could be caused by bad bearings, grout, foundation
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 engine builder to predict the deflection at