Types of Piston Rings and Piston Ring Maintenance
Piston rings are manufactured and classified on the basis of function and usability. Marine engines of higher capacity generally have all the three types of rings with little difference in their appearances. However, in smaller marine engines, different types of piston rings are used for dedicated purposes. In this article we will have a look at the different types of piston rings used in marine engines.
Types and Functions of Piston Rings
Compression Rings or Pressure Rings
The compression rings provide sealing above the piston and prevents the gas leakage from the combustion side. The compression rings are located in the top most grooves of the piston. However, this may differ according the design of the engine. The main function of these rings is to seal the combustion gases and transfer heat from the piston to piston walls.
Second or Intermediate Rings
The oil in controlled by shearing the layer of the oil left by oil ring, thus providing the top compression rings enough lubrication. Moreover, it also provides help to the top compression ring in sealing and heat transfer.
Oil Control / Scrapper Rings
The oil control rings controls the amount of lubricating oil passing up or down the cylinder walls. These rings are also used to spread the oil evenly around the circumference of the liner. The oil is splashed onto the cylinder walls. These rings are also called scraper rings as they scrap the oil off the cylinder walls and send it back to the crankcase. These rings do not allow oil to pass from between the face of the ring and the cylinder.
Things to be Checked During Inspection of Piston Rings
During Routine Inspection
During normal scavenge space inspection the piston rings are pressed with the help of a screw driver. This is done to check the spring action or tension of the rings. This also tells whether the ring is broken or not. If ring is broken there will be no spring action.
The rings are checked for their freeness in the grooves, as they might get stuck due to carbon deposits and might finally break causing a lot of damage to the liner. The clearance between the ring and the groove is also checked and the wear is calculated. The ring is checked for scuffing marks and damage and overall condition is also assessed.
During Major Overhaul
During major overhauls the rings are changed completely with the new set. But following steps needs to be considered for scrapping the rings:-
1) If the piston ring is found stuck inside the groove.
2) If the axial height of the rings is reduced and the clearance in rings and groove is large.
3) If the chrome layer is peeled off or damaged.
2) If the axial height of the rings is reduced and the clearance in rings and groove is large.
3) If the chrome layer is peeled off or damaged.
During overhaul the grooves have to be properly cleaned off carbon deposits and checked for damage in the ring grooves. Before putting the piston rings it should be rolled rounded first in the grooves. In this process the ring is moved completely inside the grooves. With this test we can check that the grooves are deeper than the radial width of the ring.
The piston rings are put inside the worn liner and the butt gap is also checked. While putting rings, they should be checked for markings as to which part is up or down and also different marking for different positions.
Rings should be put with the help of proper toll i.e. with the help of ring expander. The clearance between the ring and the groove is checked with the help of the feeler gauge.
The axial and radial clearance of old ring is checked and recorded to assess the amount of wear for number of running hours.
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