Stern tube lubrication system consist of a drain tank which acts as a reservoir for the storage of oil.
Drain tank is a double bottom tank located in the aft of the engine room.
The system consists of two sets of oil pumps.
The purpose of having two pumps is that if one pump fails the other pump can take over and continue the operation.
Pump draw suction from the drain tank and sends it to the oil cooler.
The cooler cools the lubricating oil and maintains the temperature of the oil with acceptable limits.
The cooling medium used in air cooler is air or water.
The oil reaches at the full load condition header tank from where it overflows to balast condition header tank. Both the tank have an outlet valve located at the tank bottom.
Water flow from these tanks is led to the drain tank via a side glass.
Pressure head exerted by the oil level in the header tank and water draught is monitored by nothing in the pressure gauge valves.
Drain valve is fitted to check water intrusion.
Small pressure differential of air ensured water exclusion.
The lubricating oil passes through the Stern tube bearing.
The oil pressure sufficient to generate hydrodynamic fluid film lubrication.
Since static head generates oil pressure it is sometimes referred to as static lubrication.
The diagram also shows a lubrication system for the forward seal.
The oil circulation within forward seals is achieved by the natural convection.
Oil in the forward seal tank is cooled by a cooler.
In some cases, the tank is provided with radiation fins on the tank surface.
The air in contact with the fins cools the tanks which in in turn cools the oil.
The simplex Stern tube consist of forward and aft sales. The seal works in in conjunction with oil lubricated bearing
The aft sealing assemble consists of two main and one auxiliary sealing rings.
All sealing rings are spring loaded and are able to seal any position of the shaft.
The forward sealing rings prevent oil leakage to the sea by Spring load and oil pressure of the system.
lip seal sports oil within the Stern tube and accept slight misalignment.
The elastic lip of each nitrate rubber seal grapes short from steal lines.
Chrome Steel liners are located at aft and forward ends of the propeller shaft.
The act as rubbing surface lip seals.
The liner at the outboard end protects the Steel shaft from seawater and corrosion.
The lubrication system is connected between the two fwd seals.
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