18. State about stern bearing made from synthetic rubber bearing

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a.Structure:

1.  Stern tube

2.  Metal bush

3.  Rein forced rubber stave

4.  Brass sleeve

5.  Propeller shaft

-In general this type of bearing has the same structure as the Lignum vitae type

-The rubber staves are also positioned in a metal bush

-A groove is made between two staves to let water flow in for lubricating

-The metal bush is fixed to the stern tube by screws (metal bush must not rotate)

-Brass sleeves also must be inserted to propeller shaft to increase shaft ‘s longivity.

-The friction coefficient between rubber bearing and brass sleeves in water is about 0.02 ¸ 0.007 depends on kinds and shapes of rubber bearing.

b. Advantage of rubber bearing.

-When working in water, friction coefficient is decreased.

-Abrasion resistance ability is high even in dirty water with much sludge and sand

-Noise and vibration is reduced. When rotating propeller shaft can center it self.

-It is cheaper than lignum vitae material.

-This kind of bearing is suitable for propeller shaft with medium and high speed.

c.     Disadvantages.

-Heat exchange coefficient is low, so working temperature of the bearing is limited under 65oC. If temperature is excessive the limit value about 200C, rubber may be deformed and if temperature is too low (about - 40oC), it will be hardened.

-Sulfur in rubber will corrode the brass sleeve of the shaft. Because of this when the ship staying in port for a long time, sometimes the shaft must be turned.

-Oil must be away from bearing if not, bearing will be worn very fast.

d.     Application

-A large number of vessels with this type of bearing are still in service and they continue to be installed