21.  State function and feature of propeller shaft?

Propeller shaft works in hard condition such as contacting with seawater and heavy load, wearing by friction so, it is required a high quality material and structure.

Traditionally, a non-controllable pitch propeller is fitted to the propeller shaft with key and taper. On the other hand, a controllable pitch propeller is normally fitted to a flanged propeller shaft because the operating mechanism is housed in the propeller boss. Of course, a non-controllable pitch propeller could be flange-mounted, however the propeller shaft cannot be drawn into the ship for inspecting and result in a larger boss diameter of the propeller.

The propeller is pushed tightly to taper by hydraulic jack, after that the propeller is secured to the propeller shaft by key and nut together with interference fit method.

The propeller shaft is connected to the intermediate shaft by flange or coupling. This flange may be cast either integral with the shaft or separately and secured to the shaft by key and nut.

In some cases, especially the propeller shaft lubricated by water, to increase longevity of the shaft, a brass sleeve is inserted to the shaft.

22.  List the name of all main components of stern tube? State structure specification of stern tube?

1- Propeller shaft

2- Stern tube

3- Metal bush

4,5- Stern bearing

6- Stern frame

7- Stern tube nut

8- Retaining ring

9- Gland cover

10- Packing

11- Aft peak bulkhead

12- Water pipe

23.  Describe the structure and feature of special wood stern bearing?

The traditional propeller shaft or stern bearing is lubricated by water. It consists of a number of lignum vitae staves located in longitudinal grooves in a metal bush. Lignum vita is a kind of hard wood with very good wear characteristics and it is compatible with water.

The staves with against the grain are used in lower part of the bearing. They have a higher longevity than staves with the grain. These staves are closely placed and secured by brass plate. 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 boundary 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 (unbushed part) 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).

24.  State advantages and disadvantages of special wood stern bearing?

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)

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.

25.  Describe the structure and feature of synthetic rubber stern bearing?

There are some differences in structure of synthetic rubber bearings but basically, the rubber staves are 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). The same with special wood, brass sleeves must be inserted to propeller shaft in case of rubber bearing. 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.

26.  State advantages and disadvantages of synthetic rubber stern bearing?

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.

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.

27.         Describe the structure and feature of white metal stern bearing?

A typical analysis of white metal is 3% copper, 7.5% antimony the remain is tin. The thickness depends on requirement of registry. Lloyd’s register recommends the thickness of 3.8 mm for shaft 300 diameters, up to 7.4 mm for shaft 900 diameters. The bearing clearance can be calculated:

D1 = 1.001D + 0.5 (mm)

Where: D1 - Inside diameter of the bearing, D - Outside diameter of the shaft.

White metal liner must be fixed to the stern tube otherwise it will be turned together with shaft and be soon damaged. The white metal bearing must be lubricated by lubricating oil.

28.  State advantages and disadvantages of white metal stern bearing?

Advantages of white metal bearing

- Propeller shaft without brass sleeve can be used.

- It can withstand heavy load with vibration and changing direction of the shaft.

- Corrosion of the shaft is prevented because seawater does not contact with the shaft.

- Wear of the shaft is very small. If it is properly lubricated, longevity of the bearing may last 7- 8 years.

- Because there are many good features, so nowadays almost big ships use white- metal bearing.

Disadvantages

- Replacing and making is complex.

- Sealing and lubricating system for propeller shaft is complex.

- Longevity of bearing depends on operating condition, installation standards and especially the sealing.

29.         Describe the structure, feature, advantages and disadvantages of gland packing seal type?

It consists of simple stuffing boxes filled with packing material; usually rove cotton imbrued with tallow or graphite as a lubricant. In the case of high duty packing the material may be white metal clad.

These packing are tight to the shaft that make the sealing function

Gland packing can be lubricated by water or lubricating oil

Grand packing are adjusted by tightening or loosening gland cover

Advantages:

Simple and cheap

Disadvantages:

Water always penetrates to the engine room

Longevity of the packing is low

Force of loosing and tightening must be suitable

Gland packing have to assembly onboard the ship…

Application:

This kind of shaft seal is only used onboard the small and the former ship.

30.         List the name of all main components and explain sealing principle of the simplex seal type?

List the name:

1-              Cover ring

2-              Sealing ring

3-              Inter mediate rings

4-              Oil inlet

5-              Flange ring

6-              Packing

7-              Stern tube

8-              Stern frame

9-              “O” ring

10-        Aft chrome steel liner

Function:

To prevent sea water penetrates to the engine room and leakage of oil from the stern tube

Structure:

The aft seal is composed of the casing fixed to the hull and the liner, which is fixed to the propeller boss and rotates with the propeller shaft together.

The casing consists of three kinds of metal rings (flange ring, intermediate rings and cover ring), which are separately tightened with bolts.

Individually assembled between these rings, four sealing rings are composed the leading edge (lips) touching the liner. These lips are pressed hard against the rotating liner by the water and oil pressure, elasticity of the rubber material and the tightening force of the springs to maintain sealing effect.

The four sealing rings are numbered by No.1, No.2, No.3, and No.3S from the seawater side. Seawater penetration is tightly protected by No.1 and No.2 sealing rings.

No.1 sealing ring, in particular, has an additional function to protect the inside of the stern tube from extraneous substances in seawater.

The oil pressure in the chamber between two (No.3 and No.3S) sealing rings located at forward is usually adjusted to be the same pressure as stern tube pressure, which is 0.2 - 0.3 kg/cm2 higher than sea water pressure. Therefore, there is no pressure loaded on No.3S sealing ring on the above conditions. Normally, lubricating oil in the stern tube is sealed by No.3 sealing ring.

However, if much oil leakage from No.3 and No.3S chamber would be found, No.3S sealing ring could be used to protect the above leakage instead of No.3 sealing ring by the easy valve operation handle inboard