Page 244 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 229

Example: Piston to cylinder bore clearance:
piston diameter = 20"
discharge temperature = 280°F
ambient temperature = 80°F
material of piston = cast iron

A L = (.000006) (280 - 80) (20) x 1.2 = .029"

CHECKING BEARING CLEARANCES

The use of feeler gauges to check bearing clearances, while common,
is subject to error due to the problem of inserting the gauge in the limited
space and also because of the round surfaces being checked. It is there-
fore recommended that other methods be used.

Defining Clearances by the Use of "Lead" or Plastigage

The clearance in a babbitt bearing, or the clearance between piston and
cylinder head, can be determined by opening up and inserting a soft lead
wire, then measuring the thickness of the lead with a micrometer after
the lead has been compressed in the bearing or between piston and cylin-
der head. Fuse wire is normally used, if it is soft. Plastigage is used in the
same manner and is preferred because it is softer and will not embed in
the soft babbitt of the bearing shell.

Measuring Clearances With a Dial Indicator

The most accurate method of measuring clearances is with a dial indi-
cator. This is also a quick method that does not require any disassembly.
Here is how it works.
Clamp the dial indicator on the connecting rod, with the pin resting on
the crankshaft and parallel with the connecting rod. Place a bar under the
lower bearing cap and bounce the bearing up and down to read the clear-
ance directly on the dial indicator. To avoid errors in the readings, the
crankshaft should be prevented from turning.
Depending on compressor type and design, some ingenuity may be
required to place the dial indicator in the optimum position. The indicator
should read bearing clearance by getting only the relative movement
between the bearing and the journal, without lost motion being added to
the reading.