Page 250 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 23S

accurate conclusions may be reached concerning the actual crankshaft
condition.

The following factors should be considered for accurate web deflec-
tion measurement.

1. Punch marks
Crisp, properly aligned punch marks on the webs are critical. A
hollowed-out punch mark, instead of a precise, conical punch mark,
may cause the measuring tool to move as much as one mil (.001'').
Good punch marks must be readily identifiable and used for each
future web deflection measurement. Lining out "extra or old" punch
marks is achieved simply by using a small cold chisel and brass
hammer. (Never grind out punch marks because this removes
weight from the balanced webs.)
2. Equipment condition
Web deflection measurements should be taken after a compressor
has operated for three to four days (72 to 96 hours) nonstop. This
allows adequate time for the entire compressor, foundation, frame,
bearings, and crankshaft to come up to normal operational tempera-
ture. This condition allows complete thermal expansion to occur and
will reflect the normal operating mode of the crankshaft. "Hot" web
deflection is recognized as the best method for measuring web
deflection. "Hot" is usually represented by machine temperatures of
12()°F and higher ("cold" is usually 119°F and less).
The instrument to be used for measuring deflection should also be
acclimated to compressor temperature. A cold instrument in a hot
crank will expand and reflect false readings.
3. Instrument
With each type of measuring instrument, appropriate precautions
are necessary.
a. Dial indicator
The dial indicator (strain gauge) has been the standard precision
instrument used in taking web deflection in the past. This instrument
is very accurate provided the following conditions are met:
* Calibrate yearly or as often as necessary, such as after a severe
blow to the instrument.
• All extensions for measuring varied throw widths are straight.