Page 255 - Reciprocating Compressors Operation Maintenance

24O Reciprocating Compressors: Operation and Maintenance

throws and better from No. 3 to No. 6, and all signs are minus (-). A con-
dition such as this means that the shaft is in a continuous bow. This can be
verified by bending your wire model crankshaft into a bow and by rotat-
ing it as is done in taking the readings. It will be seen that all signs 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 relative 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
thickness 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 inserted. 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 dis-
cussed 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 that should be ignored. However,
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 correlating their location to the crankshaft deflections.
In 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. Com-
ments 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, founda-