17. State about stern bearing made from special wood:

-This is traditional type made by special wood with the name Lignum Vitae, so that we can call Lignum vitae bearing.

-This type of bearing is cooled and lubricated by water (sea or fresh water- normally sea water)

-Lignum vitae wood characteristic:

+This is hard wood with good wear characteristic

+It is very compatible with water

a. Structure of lignum vitae bearing

- Lignum vitae bearing consists of a number of lignum vitae staves located in a gunmetal bush.

-These lignum vitae staves held by bronze retaining strips (or brass strips)

-The staves in the lower part of the bearing are cut and fitted so that the end grain is vertical to give the longest possible life.

-Staves in the upper part are cut with grain in the axial direction for economy.

-The staves are shaped with V or U grooves between them at the surface, to allow access for water.

-The length of secured plate is equal to length of the staves and thickness of this plate is about 60% of the staves.

-Two or three brass plates can be used to secure the staves.

-When the bearing is working with water-cooling, the staves will expand -oundary and create pressing force to each other

-The metal bushes in which the staves are mounted are pressed into cast iron stern tube.

-At the after end of the metal bush is provided with a shoulder and it is secured by a lock nut on the stern tube.

-Generally, two staves bush are fitted in the stern tube

+The aftermost bush has a length about 4 times the diameter of the shaft and it is the main bearing unit.

+The forward bush is short and acts mainly as a guide

-The center of the stern tube is connected to a seawater service line, which, together with ingress of water between the shaft and aft bush, provides lubrication

-To increase longevity of the propeller shaft, the brass sleeves must be fitted

             Clearance of fitting: D1 = 1.003D  + 1.0 (mm)

+Where D1 is inside diameter of the bearing, D is outside diameter of the shaft (including brass sleeve).

Hinh()

1.  Lignum vitae (horizon staves)

2.  Propeller shaft with brass sleeve

3.  Secured plate

4.  Lignum vitae (vertical staves)

5.  Screw

6.  Metal bush

b. Advantages of lignum vitae bearing

-This kind of bearing is very suitable for propeller shaft, which has a big diameter and low speed.

-It is safe in operation.

-Absorb vibration of the propeller shaft.

-Maintain clearance between shaft and bearing if optimum cooling and lubricating.

-Longevity is high (about 10 years)

c.  Disadvantages

-Lignum vitae are an uncommon material. It is very expensive.

-It is difficult in manufacturing

-Bearing is excessively worn if the ship is sailing in shallow and dirty water, especially when sand comes into clearance of the bearing.

d. Application

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