Stern Tube Seals
A
ship requires vast quantities of water to float and sail, yet this very
sea water should not enter the ship from any openings or leaks. Of
course the ship is made of strong material but certain points are
vulnerable and could act as entry points for sea water. Just imagine the
thick propeller shaft protruding out of the ship's hull and connecting
the propeller to the main engine inside the engine room. How do we ensure that water from outside does not enter the ship through this opening.
A stern tube, which is present at the bottommost part of the ship, connects the engine, inside of the ship to the propeller outside. A propeller shaft passes through this stern-tube and is used to connect the propeller to the engine. Stern tube bearings, generally oil lubrication glands, are used to prevent the ingress of sea water inside the engine room and also to support the weight of the propeller. But this is not it, the whole stern tube arrangement is packed and sealed from both the sides for additional protection of the stern tube. The packing material used for these sealing arrangement are known as stern tube seals.
Thus, these specially made seals, fitted at the inboard and outboard ends of the tail shaft are used for two main functions :
A stern tube, which is present at the bottommost part of the ship, connects the engine, inside of the ship to the propeller outside. A propeller shaft passes through this stern-tube and is used to connect the propeller to the engine. Stern tube bearings, generally oil lubrication glands, are used to prevent the ingress of sea water inside the engine room and also to support the weight of the propeller. But this is not it, the whole stern tube arrangement is packed and sealed from both the sides for additional protection of the stern tube. The packing material used for these sealing arrangement are known as stern tube seals.
- To provide extra protection for preventing the entry of sea water
- To prevent loss of lubricating oil from the stern tube bearings.
Types of Stern Tube Seals
In the older ships, where the timber bearings with sea water lubrication systems were used, conventional stuffing box and glands were used at the aft bulkhead of the ship. However, the modern ships which uses oil lubricated stern bearings mainly uses two types of seals :
- Lip seals
- Face seals
Lip Seals
Lip
seals are seals made of special kind of materials (elastic) , having a
projecting lip or edge. This peculiar shaped ring is held in close
contact with the shaft and the stern tube to prevent oil leakage or
water entry. The shape, size and number of lip seals depends on the size
and type of application.
Face Seals
Face
Seals are seals made in pairs. The seals consists of two mating radial
faces, which are forced against each other to prevent any kind of
leakage. Out of the two faces, one is rotating and the other is
stationary. The rotating seal is attached to the propeller boss and the
stationary seal is attached at the aft bulkhead. The arrangement also
consists of a spring which forces the two faces, one stationary and
other rotating towards each other in order to provide the adequate seal.
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