One-piece pistons are made of cast iron, or steel for small diameters and high differential pressures. They are also sometimes used when it is necessary to add weight for balancing to reduce reciprocating shaking forces. (Figure 300-19 illustrates three styles of pistons.)
Two-piece designs are used for ease in casting and weight control. They are made of aluminum or cast iron, and are generally applied for diameters above 10 inches. Aluminum is used to reduce reciprocating mass.
The three-piece segmental design incorporates a ring carrier. They are used to facilitate installation of rider rings (wear bands) which, when required, are placed on each side of the carrier. In this way, the rider band can be thicker because it does not have to be stretched over the outside diameter of the piston.
Another piston design worth mentioning is the labyrinth piston for non-lube service. This piston has no rings, and is used only in vertical machines. It has a large number of labyrinth grooves to limit blow-by. The blow-by is slightly more than that of ringed pistons, but the advantage is in low piston maintenance. The labyrinth compressor is manufactured only by Sulzer in Switzerland, and is used in rather special services such as oxygen at relatively low pressures.