Generally speaking the piston ring must be harder than the liner in which it runs. Rings are cast and machined from grey cast iron. This may include some additions such as chromium, molybdenum, vanadium, titanium, nickel and copper. In some engines nodular or spheroidal graphite iron is used, which has greater hardness and tensile strength, as has vermicular or compact graphite cast iron, the use of which is becoming more widespread. Alloying elements such as titanium and vanadium form hard insoluble carbides within the matrix.
The primary sealing effect of the piston ring is achieved by the inherent tension of the ring which makes it contact the cylinder wall around its whole circumference. Rings in early engines were hammered around their circumference to induce a tension in a circular ring. Two methods are used today; the thermally tensioned ring and the oval pot cam turning method. The cheaper method employed mainly for small engines is the Thermally Tensioned Ring. In this method the ring is machined from a circular pot to the cylinder diameter. A gap is cut in the ring into which a distance piece is inserted, thus expanding the ring. The ring and distance piece is then placed in an oven to relieve the induced stresses. This method suffers from the disadvantage the rings lose their tension in the heat of the engine.
The advantage of this method is that when the ring is in storage, it is under no artificially induced tensions. Also when the ring is expanded for fitting over the piston the stresses induced in the back of the ring are not in addition to any previous built in stresses. A ring cast by this method always tends to return to its natural stress free state by pressing against the cylinder walls.
Plating of Rings
Piston rings need to combine strength with wear resistance. To enhance the wear resistant properties the rings can be plated with a layer of different material.
Plasma coating provides a running surface with better properties than chrome plating. A gas mixture is directed through an electric arc generated between a tungsten electrode and a water cooled copper tube . Very high temperatures are generated, the gas molecules atoms dissociate and into this plasma state metal, carbides, or ceramics are sprayed as a fine powder, these melt, and coat the ring surface.
Soft Coatings for Running In
Ring Pack Designs
2 Stroke Ring Packs
4 Stroke Ring packs
Two Manufacturers of Marine Diesel Engine Piston Rings are Daros