There are various types of piston rings, which are all generally manufactured and classified based on usability and functionality. The primary function of a piston ring is to serve as dry-running seals and guides like those used in reciprocating compressors.
Most piston rings are manufactured primarily from cast iron, a strong alloy with varying amounts of manganese and silicon that make it capable of bearing a great deal of weight. Cast iron is the preferred material for fabricating machine components due to the simple fact that it doesn’t require a great deal of refining work. However, steel is used a bit more often for constructing applications.
This article highlights the unique types of piston rings and their applications.
What is a Piston Ring?
A piston ring is a specially-designed split ring used to create a proper seal between the piston and the cylinder wall in a reciprocating compressor. The significant properties that piston rings need are to be in their original shape while being able to withstand dynamic forces, heat, and massive loads.
These unique attributes are only obtainable by using a suitable metal. This is why cast iron is used chiefly in making piston rings.
To many, it may seem as though cast iron is an obsolete material choice for something as vital as a piston ring. But the truth is that cast iron is used as the primary source material for making piston rings for a specific purpose: it contains graphite.
Graphite, or black lead, when used in lamellar form – i.e., in very flat and thin layers that alternate with other materials – performs as a natural lubricant. Heat-resistant graphite occurs naturally as crystallized carbon but doesn’t react dangerously with other materials.
Types of Piston Rings
Here are the different types of piston rings you should know:
Straight Joint Piston Rings
Piston rings with straight joints are primarily used for sealing pressuring differences well above 15 bar. The leakage is somewhat higher with this gap than with scarf-joint piston rings. Since high compressor speeds – in revolutions per minute – are typically achieved daily, the loss of gas due to leakage only has minor effects on the overall performance of the compressor. In other words, gas leakage using piston rings with straight joints is negligible.
Gas-tight Joint Piston Rings
Piston rings with gas-tight joints achieve the best sealing effect. Since they are designed specially, piston rings with gas-tight joints significantly reduce leakage to the barest minimum. The differential pressure is always limited to a maximum of 15 bar.
However, you should note that during assembly, the piston rings with gas-tight joints achieve an excellent sealing effect in only one direction of pressure.
Scarf Joint Piston Rings
Piston rings with scarf joints are primarily used to seal pressure differences above 15 bar. Throughout the run-in period, the sealing effect of piston rings with scarf joints is somewhat better than those with straight joints.
Overlapped Joint Piston Rings
Piston rings with overlapped joints generally achieve a more beneficial sealing effect. As a result, they are primarily used for sealing gases with a particular lightweight.
Due to bending stress and the ensuing risk of breakage within the overlapping area, piston rings with overlapped joints should only be used in reciprocating compressors with maximum pressure differences of 15 bar.
Other Materials That Makeup Piston Rings
Piston rings usually come with coatings designed to protect the graphite and other materials. The coatings used in this manner are also composed of several alloys to provide appropriate corrosion resistance and ultimate strength.
Piston rings can also be made of the following:
- Considerable amounts of less-known metals
The 3 Kinds of Piston Rings
The three piston rings showcased in the following paragraphs have different functions within a reciprocating compressor. Here is a brief description of each kind of piston ring:
1. Compressor Ring
The compression ring is the piston ring usually closest to combustion gases. It is capable of withstanding chemical corrosion as well as massive operating temperatures. Compression rings can transfer nearly all the heat within the combustion chamber to the piston’s cylinder wall. It also provides an airtight seal of the chamber that contains the piston, efficiently preventing leakage of any gas.
Barrel-faced compression rings can provide throughput lubrication to the piston and cylinder wall. In addition, taper-faced compression rings provide a wiping action that prevents excess oil from reaching the combustion chamber.
2. Wiper Ring
The wiper ring provides a consistent and optimal oil film thickness that helps lubricate the exposed part of the compression ring. This is why the wiper ring is sometimes referred to as a backup compression ring, scraper ring, or Napier ring.
The core function of the wiper ring is to provide wiping action to the piston as it moves toward the crankshaft. It also captures and cleans off any traces of oil or mineral deposits from the work cycle. It helps keep the whole system moving efficiently.
3. Oil Control Rings
The oil control ring is a vital part of a piston system. The oil rings are positioned on either side of the expander. This one-piece entity is equipped with two skinny running rails or surfaces.
Oil control rings look somewhat different from the other kinds of piston rings discussed earlier. They don’t even look like rings as they are square-shaped and often contain a spring. The primary function of oil control rings within the casing or chamber is to help in gathering oil back to the system for re-use.
This piston ring also helps replenish the oil in the chamber, making the transfer of heat more efficient and safer while reducing friction when the piston is still functioning within its work cycle.
Piston rings used in reciprocating compressors are generally made from cast iron. The material is chosen due to its ability to withstand heat and its non-reactive attribute with other materials.
There are different types of piston rings, and each one comes with unique attributes to fulfill a specific purpose. They have been highlighted above, and you can use any piston ring according to your specific needs.