Chapter 7AUXILIARY MACHINERY
(MRC) for the proper methods and tools required
for removing the heads. The following is general
information on removing a piston, fitting new
piston rings, and checking piston end clearance
for a vertical compressor.
To remove trunk pistons from vertical com-
pressors, it is first necessary to remove the cylinder
heads. In the case of three- or four-stage com-
pressors, the third and fourth stage cylinders will
also have to be removed. Next, you must turn the
compressor by hand to top center and remove the
lower half of the crank pin bearing, or the entire
crank pin bearing box, if so directed. Pull up the
piston and connecting rod through the cylinder.
In removing pistons from compressors fitted with
cross heads and piston rods, the general procedure
is to loosen the piston rod locknuts adjacent to
the cross head, then unscrew the piston rod from
the cross head and lift the piston and the rod out
of the cylinder.
If the piston rings are worn or broken and
require replacement, take accurate measurements
of the cylinder or liner to determine the exact
diameter. Standard size rings may be used in over-
size cylinders if the oversize does not exceed 0.003
inch per inch of the cylinder diameter. If the
cylinder or liner is worn beyond the manufac-
turers recommended limits, it should be replaced.
On compressors not equipped with liners, such
as air-cooled or large pneumatic service com-
pressors, one reboring is allowed.
When piston rings are being replaced, they
should first be fitted to the cylinder to check for
proper end clearance. If necessary, the ends
should be filed until the manufacturers recom-
mended clearance is obtained. Rings should then
be fitted to the piston grooves, making sure that
the side clearance of each ring is such as to allow
it to fall into the groove by its own weight. The
thickness of each ring should be checked to make
sure that the groove is deep enough for the ring
thickness. In replacing the rings, care must be
taken to ensure that they are staggered so that the
ring splits are not in line. After the piston is
assembled, it is advisable to wire the rings tightly
with a soft copper wire so that they will enter the
bore without difficulty. As each ring enters the
bore, the binding wire can be removed through
the valve ports. Most cylinder liners are beveled
at the top to permit the rings to compress and
enter the bore easily. In certain designs of
differential pistons, it is necessary to loop a wire
around the top of the lower liner, using valve ports
for access, and squeeze each ring closed as it
approaches the top of the liner. One technique
is to slip the rings over a thin piece of shim stock
which, in turn, is slipped over the piston. This
prevents overstretching and other possible ring
damage.
After a piston has been replaced in a com-
pressor and the head has been drawn down
evenly, it will be necessary for you to check piston
end clearance. NOTE: The piston end clearance
must always be measured and adjusted after
replacing the pistons, or after any adjustment or
replacement of the main, crank pin, wrist pin, or
cross head bearing has been made. This is ac-
complished by inserting a lead wire through a
valve port or indicator connection, and then jack-
ing the compressor over so that when the piston
has moved to the end of its stroke (top dead center
(TDC)), the lead will be flattened to the exact
amount of clearance at the end of the stroke. The
lead wire should be long enough so that the
reading can be taken at a point near the center
of the piston.
In differential piston compressors in which
compression takes place in two stages during the
same stroke, the clearance must be measured for
both stages. In double-acting compressors, or in
those compressors in which compression in any
stage takes place on the reverse stroke, a second
reading of the clearance at the bottom of the
stroke (bottom dead center (BDC)) of that
cylinder must be taken.
After taking these readings, you can adjust the
piston end clearances. Proper clearances for a
specific compressor are usually found in the
manufacturers instruction book or on the
blueprints. If neither of these documents is
available, then you should adjust the clearance
so that the TDC stroke is approximately 1/64 inch
greater than the BDC stroke. This is necessary to
allow for the expansion of the running parts when
the compressor is in operation.
The method of adjusting the piston clearance
varies with different compressors or designs.
Adjustments may be accomplished by one of the
following methods: (1) by adding or removing
shims in the connecting rod between the lower end
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