The purpose of piston rings is to prevent the blow-by of gas from one end of the piston to the other. Rider rings or wear bands support the weight of the piston, help guide the piston in the bore, and prevent rubbing of the piston on the cylinder wall. Some designers use nylon buttons in the piston skid to prevent contact with the cylinder on trunk-type pistons.
For many years, piston rings were made from non-abrasive, relatively soft metallic materials. Cast iron was the most common material, later largely replaced by bronze. Metallic rings were favored because of their good heat transfer characteristics.
However, much development of non-metallic piston rings and rider rings occurred when non-lube applications became common in the 1950’s.
Carbon-graphite was tried first, but was found to be brittle and did not have suitable wear characteristics. Phenolic and laminated plastics such as Bakelite and Micarta were used when temperatures were low. Although it is relatively weak with poor heat transfer properties, PTFE (teflon) with various filler materials eventually became the favored material, because of its excellent low-friction characteristics. Today PTFE is used almost exclusively for lubricated, as well as nonlubricated services. Bronze is still used on rare occasions for clean and dry lubricated service when good heat transfer is needed.
The shapes of piston and rider rings are shown in Figure 300-27. Some designs call for an inner ring or expander ring (not shown) to be fitted under the piston ring to energize the piston ring and keep it against the cylinder wall as wear takes place. However, the most popular and safest design employs gas pressure to energize the piston ring.
The “angle” cut is generally preferred, and is the most commonly used. For smaller lower-pressure cylinders, the “step” cut is used, although care must be taken in the design to avoid joint breakage. The “seal” cut provides the best seal, but is more expensive.
Pressure in the cylinder acts on the piston rings, and assuming that the ring does some sealing, there will be a pressure drop from one side of the ring to the other. This pressure difference results in a net “pressure induced force” holding the ring against the side of the piston groove and outward against the cylinder bore (refer to Figure 300-28).
Figures 300-29 through 300-31 provide some typical dimension ranges for piston rings and piston clearance. The latter is governed mainly by the coefficient of thermal expansion of the piston material. In general, the ring should not protrude from the piston groove by more than 25% of its thickness.
Rider rings and piston rings are almost always of the same material. Rider rings must be designed so that they do not act as a piston ring. Otherwise, wear will occur too rapidly. Solid rider rings are not prone to outward expansion, but cut rider rings must be vented with holes or slots to bleed off pressure. Figure 300-32 and Figure 300-33 are examples of typical thicknesses for solid and cut rider rings versus cylinder diameter.
Rider-ring width is determined by the bearing pressure. Figure 300-34 shows piston ring and rider ring arrangements on the rider ring. The bearing pressure is generally limited to five psi for PTFE in non-lube services and 10 psi for lubricated cylinders (see API 618). These pressures are based on the weight of the piston plus one-half the weight of the rod divided by 0.87 DW (where D is the piston diameter, and W is the width of all rider rings on the piston).