CYLINDERS AND PISTONS
The cylinders on pistons should be inspected only
AFTER the manufacturers technical manual has been
consulted. Be careful when removing heads, parti-
cularly where metal-to-metal joints are involved, to
prevent damage to the joint.
If replacement of piston rings is required because
they are worn or broken, take accurate measurements of
the cylinder liners. Standard size rings may be used in
oversize cylinders if the oversize does not exceed 0.003
inch per inch of cylinder diameter. The liner may also
need to be replaced if it is badly worn or out of round.
When replacing piston rings, first fit them to the cylinder
to check for proper end clearance. You can file the ends,
if necessary, to make them fit. The side clearance of the
rings should be such that the rings will fall easily into
the piston grooves, which should be deep enough for the
ring thickness. Ring splits should be staggered. After
you assemble the piston, wire the rings tight with a soft
copper wire so that they will enter the bore easily. This
wire can be removed through the valve ports after the
ring has started into the cylinder bore.
When reassembling the air cylinders and heads, be
sure they are all drawn down evenly, especially on
multistage compressors where the heads contain
cylinders for third and fourth stages. Otherwise, the
result will be excess wear on the cylinders and pistons.
When a compressor piston has been replaced, the
piston end clearance must be checked. This is done by
inserting a lead wire through a valve port or indicator
connection. Jack the compressor over. When the piston
has moved to the end of its stroke, the lead will be
flattened to the exact amount of clearance. The wire
should be long enough to permit a reading near the
center of the piston. These readings should be taken after
any adjustment or replacement of the main, crank pin,
wrist pin, or crosshead bearings. Methods of adjusting
the clearances vary according to the compressor design.
You should consult the manufacturers instructions for
From time to time other miscellaneous adjustments
are required on compressors, including those pertaining
to wrist pins, crosshead shoes, reduction gears,
couplings, and V-belt drives. The manufacturers
technical manual will give you specific information for
the care, adjustment, and replacement of all fitted
bearings. Refer to the manufacturers instructions for
detailed information on when and how to make these
Wrist pin bushings are replaced when necessary.
This is done when they are worn to the point of
becoming noisy. In making a replacement, be sure the
oil hole in the bushing is properly lined up with the oil
hole in the connecting rod. After being pressed into the
rod, the new bushing must be reamed.
Crosshead shoes are provided with shim or wedge
adjustment. Wear should be slight, but adjustment
should be made when the travel of the piston rod causes
a movement in the stuffing boxes.
Alignment of reduction gears and pinions should be
checked periodically, especially on a new compressor.
Misalignment may be caused later by settling, straining,
or springing of foundations; pipe strains on turbine-
driven compressors; bearing wear; or springing due to
heat from a turbine.
Flexible couplings require very little maintenance
when they are properly lined up. Some types require
occasional lubrication to prevent excessive wear of
springs and bushings. A noisy coupling is an indication
that the bushing is worn and requires replacement.
V-belt drives require adjustment for belt tension.
Belts generally stretch slightly during the first few
months of use. A loose belt will slip on the motor pulley
and cause undue heating and wear on the belt. A tight
belt will overload the bearings. Belts should be protected
against oil and high temperatures. To prevent rapid
deterioration, belts should not be used at temperatures
above 130°F. V-belts are usually installed in sets of two
or three. If a single belt is worn or deteriorated, the
complete set should be replaced to ensure that each belt
will carry its share of the load.
Proper care of a compressor lubrication system
includes the following:
-Keep the oil at a normal level in the reservoir
at all times to maintain proper oil temperature.
- Change crankcase oil periodically, and at the
same time clean and flush the crankcase and clean the
-Maintain proper lube-oil pressure by keeping
the oil pump in good working order and adjusting the
bypass relief valve.