Page 71 - Reciprocating Compressors Operation Maintenance

58 Reciprocating Compressors: Operation and Maintenance

Materials commonly used for compressor pistons are aluminum, cast
iron, and steel.
Aluminum is used when lightweight pistons are required in order to
balance reciprocating weights or to reduce inertia forces so they do not
exceed rated frame load limits. The aluminum used is a special alloy with
a tensile strength of 40,000 psi and a hardness of 100-110 Bhn. It may be
given a surface anodizing treatment to achieve a hardness of 370-475
Bhn; this improves wear resistance. Applications are limited to approxi-
mately 200°F and a differential pressure of 125 psi for castings.
Cast Iron is the most common piston material due to its high strength
and good wear and corrosion resistance. It is used in either the cast or
solid form, conforming to ASTM A275, class 40.
Steels are used for small bore, high differential pistons when strength
requirements are higher. They conform to ASTM A354 or A320. Steel is
also used in fabricating built-up type pistons in some designs.
Rider bands may or may not be used on large diameter pistons for
lubricated compressors. Generally, aluminum pistons do not use a rider
band, while cast-iron pistons will have a rider band, usually a high lead
bronze, such as "Allen Metal." In conventional non-lubricated or oil-free
compressors, rider bands are always used. Figure 2-16 shows a segmen-
tal piston with a rider band.

Piston-to-Piston Rod Connections

There are several methods commonly used to fasten the piston to the
piston rod.
Taper Fit. The end of the piston rod is machined with a tapered bore
in the piston. The end of the piston rod is threaded, and a nut draws the
piston to the rod and holds it securely (Figure 2-17).
Interference Fit. The piston bore is machined to accept the piston rod
with an interference. No threaded connection or nut is used to hold the
piston to the rod, and the piston is "peened" at the rod end (Figure 2-18).
Single Nut. The piston bore is closely fitted to the piston rod, and the
piston is held to a shoulder or collar machined on the rod. The piston is
held to the rod by a nut which is torqued to the proper value dependent
on material, diameter, and number of thread. This is the most common
fastening arrangement (Figure 2-19).