Page 301 - Reciprocating Compressors Operation Maintenance

286> Reciprocating Compressors: Operation and Maintenance

2. The piston and piston rod should be inspected according to the
instructions for Inspection of Compressor Pistons discussed previ-
ously in this chapter.
3. Blue* the piston to piston rod collar. A minimum of 75% bearing con-
tact should be evident; if not, lap the surface of piston and collar until
this minimum of 75% contact is achieved. The bearing area should be
uniform around the circumference and across the bearing face.
4. Blue piston nut to piston. Again a minimum of 75% bearing contact
is required. If not, lap until achieved. Remove all traces of com-
pound by cleaning with an approved solvent.
5. Install piston on rod and coat the threads of the piston rod with an
anti-galling compound. Do not use a copper based compound since
seizing or galling of the threads may result. Apply a thin coating of
oil to the face of the nut.
6. Torque the nut to the proper torque value as shown in the compres-
sor maintenance manual.
7. If a suitable torque wrench is not available, another suitable method
that can be used is as follows:
* With the piston properly located on the rod, tighten the nut to 150
ft. Ibs. to obtain good metal-to-metal contact.
* Scribe a line (A) through the centerline of the piston rod, and
extend it out to the piston as shown in Figure 4-37A.
* Refer to the nomograph in Figure 4-38 to determine the number of
degrees the nut must be turned in relation to the piston rod.
* Measure from the original scribe line (A) the number of degrees
the piston nut be turned in relation to the piston and scribe a line
through point (B) and the centerline of the piston rod.
* Install socket on piston nut and mark adjacent to the first scribe
line (A) as shown in Figure 4-37B.
* Tighten the piston nut until the mark on the socket coincides with
the second scribe line.
8. Place assembly between centers or in V-blocks and indicate piston
rod behind collar or location where piston shoulders against rod.
Indicator reading should not exceed .001". If indicator reading
exceeds .001", the piston counterbore or face is not perpendicular
with piston bore or shoulder of piston rod is not perpendicular with
piston diameter. Because something is not perpendicular, torquing

*"BIuing" is the application of a thin layer of dye that allows the technician to
spot incorrect mating contact of mechanical parts.