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We studied about the role of the piston and the various forces acting on the piston and piston ring arrangement in our previous section. Now we will proceed to study about the various materials which are used in the construction of piston rings so that they withstand the heavy forces and high temperatures to which they are subject while being in to and fro motion during the operation of the engine.
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Materials of Manufacture
Cast iron is one of the most important materials used in making piston rings and a large number of marine diesel engines use piston rings made out of cast iron. One reason for this popularity is that cast iron contains graphite in the lamellar form which acts as a lubricant in the dry state and helps during the sliding motion of the piston rings. Of course this is complemented by the lubricating oil.
The hardness of piston rings made out of cast iron lies in the region of 160 – 190 Brinnel, and these rings are made out of either drum casting in a static sand mould or centrifugal casting.
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Providing Spring Tension
The piston rings are of circular shape and open-ended and have to be made in such a manner so that when they are pressed inside due to the cylinder liner walls, they exert an equal outward radial pressure which helps to perform the functions of these rings such as sealing. Several manufacturing techniques are used to achieve such a state including methods such as machining them to the size of the cylinder bore and then splitting them open and applying rolling/peening till the desired gap is obtained.
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Alloying with other metals
There was also a trend to use Chromium alloyed rings, but it is not very popular currently since these rings caused frequent seizure problems when running in unhardened liners. Other materials alloyed with cast iron include nickel, copper, vanadium and titanium.
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Of course the above mentioned materials are generic in nature and many special material combinations are used by different companies for the different rings such as compression rings, oil rings and so forth. A typical example is the use of Silicon Manganese spring steel in the manufacturing of the top most ring, since it is subject to the maximum temperature and worst conditions than the other rings and play a major role in transferring heat of the piston to the cylinder walls where it is taken care of by the jacket water cooling system. Similarly other rings can have slight variations in their material composition and design to fulfill the specific conditions of their usage.