DieselShip

The causes of broken rings may be due to striking hard edges, such as ports excessive wear of rings and/or grooves, allowing the rings to till slightly, poor quality fuel leading to excessive peak pressures or collapse of the piston ring. Heavy starting, which results in excessive cylinder pressures which often lift the cylinder relief valve, can also lead to ring breakage. Ring gap too small. This usually leads to ring breakage, but could ultimately lead to disastrous or very serious consequences Insufficient clearance behind ring. Radius at top and bottom of exhaust and scavenge ports in cylinder liner inadequate, the ring then receiving a shock when sliding past the port edges. As the design width of the port increases relative to the cylinder circumference the port edge radius becomes increasingly important. Insufficient radius leads to ring breakage. Ring collapse is probably the main cause of ring failure. This is due to insufficient gas pressure behind the ring. If the ring is not exerting sufficient pressure on the liner wall gas pressure will penetrate between the ring and liner causing the ring to collapse into the groove. This will eventually result in breakage. - Causes of ring collapse are: - Deposits in the ring groove. - Insufficient ring clearance. - Rings sticking in grooves. - Poor sealing between the ring and ring groove lower face. - Excessive chamfering on ring butts or ring edges. - Clover leafing of liner. During piston overhaul all the ring grooves should be thoroughly cleaned and the groove and gap clearances checked. The condition of the rings and grooves should be noted. If any rings are broken the location of the break, i.e. ‚opposite‛ ring gap ‚or‛ ring gap, should also be noted. Ring surfaces should normally be smooth but may also show signs of scuffing or abrasive wear. Rings gaps should be checked by fitting them alternately between left and right hand cuts, ensuring the gaps are spaced 180° apart 1. Ensure that rings are the correct ones for the job and that they are correctly fitted. 2. Ensure that the cylinder lubrication is maintained, at optimum level at all times. 3. Avoid overloading the engine, collectively and as individual units. 4. Avoid high peak pressures. 5. Ensure that combustion is as clean and crisp as possible (including correct fuel treatment) 6. Monitor ring condition regularly (via scavenge ports). As piston rings are subjected to severe conditions they require to be very carefully fitted. They must be sufficiently free in their grooves to ensure that they will function properly when heated to working temperature, and that the ends of each ring will be almost, but not quite, touching. Failure is likely to occur in various ways if a fault is missed in checking. If the ring gap is inadequate the ring may break due to a restriction on its free expansion when coming up to working temperature. Whether a ring breaks or not, it may scuff the liner due to the very heavy wall pressures causing the oil film to break down, or at least causing a large increase in liner wear. Scuffing may be such that the liner must be condemned. Piston rings fitted with insufficient vertical clearance will stick when they come up to working temperature. In turn they may break, jam in the groove on one side, and eventually allow blow, past, which could lead to piston seizure on trunk – piston engines and two- stroke cycle crosshead engines with large piston skirts. It can lead to scavenge fires in two stroke–engines of any type.Besides a risk of ring breakage, worn rings can lead to blowby and risk of scavenge fires. Blowby will destroy the oil film and lend to an increased wear rate. The localised overheating may also cause a cracked liner. Blowby is usually indicated by darkened areas on the ring. Two–strokes engine pistons work at a higher temperature than those of four–stroke engines, so that the clearance allowed in the former type must be greater. There is no-doubt that in many instances the best results are not obtained from piston rings because the engineer who decides upon the amount of clearance to be given is afraid the amount is insufficient, and gives a little more to be on the safe slide. Those who give a little more than is stated on the manufacturer’s drawing just because it doesn’t look very much, overlook the fact that the cylinder liner also becomes heated and expands. It is, of course, a serious matter if the ring butt and break, but there is not the slightest doubt that a great deal of the initial wear of piston rings and cylinder liners is due to the rings being given too much end clearance and allowing high – pressure gas to behind the rings. Once cylinder liners have begun to wear it is impossible to arrest the wear by careful attention to the fitting of piston rings, since the clearance to be given is determined by the diameter of the least worn part of the cylinder, and consequently. When the piston is at the position where it is subjected to the high pressure of the cyclic, the gap at the ends of the piston rings is many times greater than is necessary for that particular part of the cylinder.