Page 90 - Reciprocating Compressors Operation Maintenance

Design and Materials for Reciprocating Compressor Components "77

that no part of the piston which is in contact with the frame-lubricating
oil will pass into the cylinder.
Generally an oil deflector assembly is used on the piston rod to pre-
vent any oil from "creeping" up the piston rod by capillary action and
entering the cylinder.
The cylinder bore is honed to an 8-16 RMS finish to reduce wear of
the piston rings and rider band. In addition to the normal honing to
achieve this sort of finish, the non-lube cylinder is given an additional
treatment of "teflon honing." In this treatment, blocks of virgin teflon are
used in place of the abrasive stones on the hone sets, which impregnates
the pores of the iron bore of the cylinder.
The piston rod surface finishes through the packing travel are very
important. On a lubricated compressor, the piston rod is finished to 16
RMS, while on the non-lube cylinder the piston rod is micro finished to
8-10 RMS.
In addition, the piston rod will have a surface hardness in the packing
travel area of 50-55 Re.
Piston rod alignment and eccentricity must very carefully be checked
in order to minimize packing wear. Excessive lateral movement or runout
exceeding .003" will prevent packing from sealing.

PISTON AND RIDER RINGS

The pistons shown in Figures 2-34 and 2-35 are of the two-piece
design which is the preferred construction; however, some compressors
will use pistons of the one-piece design, and some piston used in larger
bores use three separate pieces, as illustrated in Figure 2-36.

Three Piece

FIGURE 2-36. Three-piece piston design (Source: Dresser-Rand, Painted
Post, New York),