In a slow speed 2 stoke engine the primary concern is to have proper sealing between the
combustion area and the crankcase. The crankcase is separated from the cylinder and
scavenge space by the diaphragm plate on a two stroke crosshead engine so a provision
must be made for the piston rod to pass through the plate without oil from the crankcase
being carried upwards, or used cylinder oil contaminated from products of combustion being
carried downwards. It is also highly undesirable to allow the pressurized air in the scavenge
space to leak into the crankcase.
The Piston rod passes through a stuffing box and is bolted into the diaphragm plate. The
stuffing box casing which can be split vertically, as shown in the photo, contains a series of
scraper rings and sealing rings which are each made up of three or four segments. On the
outside of each set of segments is a garter spring which provides the tension to hold the
ring segments against the piston rod. There is a clearance between each segment to allow
for wear. The rings are either bronze or can comprise of replaceable cast iron lamella fitted
into a steel backing ring.
As the Piston rod passes up through the
stuffing box, the oil from the crankcase is
scraped off by the lower sets of rings and is
returned via drillings to the crankcase. Any oil
that passes this primary set is scraped off by
another set of rings, and is led away through a
drain to a tell tale open ended pipe into a dish
outside the engine from where it drains to a
recycling tank.
As the piston passes down through the stuffing box, the top set of scraper rings will scrape
off the contaminated oil into the bottom of the scavenge space, where it is drained away
via the scavenge drains. However, if these rings are faulty, then the oil may drain into the
recycling tank.
By observing the open ended tell tale referred to above, a guide to the condition of the rings
can be ascertained. If a large quantity of oil is draining out, then the lower set of rings are
faulty. If air is blowing out, then the upper rings are worn.
Oil in the recycling tank can be purified back to the crankcase. Filtek portable oil filtration
unit is capable to fine filtration of this contaminated oil which can be used for recycling.
Filtek offers a complete centrifuging system with tank, level switches, heater, 1 micron
polish filter and a transfer pump for transferring the filtered oil to the crankcase.
However, care needs to be taken on the overall oil viscosity if the cylinder oil if mixed with
the crankcase oil. It may be contaminated by used cylinder oil which if mixed with
crankcase oil causes an increase in viscosity of the crankcase oil. Calcium deposits in the
bearings lead to damage and the oil may carbonise and deposit on the underside of the
piston crown when used as a piston coolant. Often this contaminated oil is just landed
ashore or burnt.
Regular maintenance of the stuffing box will keep it in good condition. checking garter
spring tension, ring butt and axial clearances, and replacing worn rings are all part of the
overhaul procedure.
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