HOW TO MAKE PISTON RINGS OF MARINES ENGINESPiston rings of marine engines are prepared with the help of pot casting method. In this method a short cylinder of oval cross section is made and the piston rings are then cut and machined from the cylinder. With this method a homogeneous and balanced casting is formed around the entire circumference of the ring.

The piston rings should have tension properties with which sealing effect is produced. In olden days, the tension in the engine room was created by hammering around the circumference of the circular ring.

Today, this tension is accomplished by two methods:-


  1. Thermally Tensioned Ring
  2. Oval Pot Cam Turning Method


  • Thermally Tensioned Ring


The thermally tensioned ring is one of the inexpensive methods to induce tension in the piston rings but is limited to smaller engines. In this way the piston ring is machined from the circular pot to the required cylinder diameter.


After constructing the ring, a gap is cut and a metal piece is inserted in the gap which expands the ring and induces a tension in the ring. After expanding, the ring and the distance piece is placed inside the oven to relieve any stresses induced during the process. The major drawback of this process is that the ring loses its tension due to the heat of the engine.


  • Oval Pot Cam Turning Method


The oval pot cam turning method is expensive but the rings created from this method retain their tension while working in heat of the engine. The rings are machined in a cam turning lathe. By altering the cam shape and the oval form, the pressure distribution around the ring is changed and a tension is induced.


After the pot has been machine a gap is cut in the ring of about 7-13% diameter and rings are grounded on the flanks to ensure good sealing in the piston ring grooves.


Plating of piston rings


The piston rings used in the marine engines should be harder than the liner material in which they are used. To provide additional strength to the piston rings, some materials like chromium, molybdenum, vanadium, titanium, nickel and copper are added to them.


Chrome plating is one of the most usual methods of surface treatment. It is generally used on the running surface of the piston rings and on the landing surfaces i.e. in ring grooves.


Chromium has the advantage of high wear resistance, low friction and corrosion resistance properties. The coating done should be of high quality and be able to operate under all conditions inside the engine without damaging or peeling off or breaking.



Plasma coating


It is also one of the methods used for plating the rings. In this method, a gas mixture is passed through an arc generated between the tungsten electrode and water cooled copper tube. A very high temperature is thus generated and the gas molecules start to disintegrate. The plasma state level carbides and ceramic are sprayed as fine powder which melts and coats the ring surface. This plasma coating provides better properties than those provided by the chrome plating.


The disadvantage of chrome and plasma coating is that the thickness of coating and the bonding to parent metal is limited.


A new method of laser hardening treatment is also done. This creates a wear resistant layer which is several times thicker than that of the conventional coatings. Sometimes, copper is plated directly over the chrome layer of the ring. The plating thickness is very thin and the life of this is long enough to cater for running in period. Plasma coated rings are graphite coated to cater with the running in periods.