Explained: Thrust bearing working principle and where it is located inside the ship?

 Principle:

If the direction of rotation is reversed, the propeller generates a thrust in opposite direction, and the ship moves in an astern direction. The propeller blades set at an angle.

The after the end of the ship, water ahead of the propeller blade and the ship in front affect the water flow over the propeller blades. The shape of the blade is like an aerofoil which increases the velocity of water at the top of the blade. This reduces the pressure at the back and exerts greater pressure on the front face of the propeller. This pressure difference generates a force perpendicular me hg see  to the blade.

The axial component ofl the force exerts trust on the propeller shaft. Thrust block mounted on the shaft transmits the thrust to the ships hull.

Operation:

The casing of the independent thrust block is in two halves, and fitted bolts join the halves.

Thrust pads, which pivot around the thrust collar surface carry the thrust load. The pads are mounted in holders and coated with white metal.

An oil deflector deflects the oil fitted by the thrust collar and directs the oil onto the top cover.

Oil from the top cover cascades over the breast pads, bearings and fall back to the sump. The top cover acts as a pad stop.

The thrust shaft transmits the thrust onto the trust collar.

Hydrodynamic fluid film wedges separate the thrust pads and thrust collar. The trust pad transmits trust to the lower half of the casing. The lower half of the casing is connected to the ships Hull.

The ship moves when the trust is transferred to the ship's Hull.

Construction and working:

At the aft portion of the ship we have a propeller and then a propeller shaft which comes inside the ship through the stern tube bearing.

There is an intermediate shaft that connects the propeller shaft and crankshaft. To support the intermediate shaft there is an intermediate shaft bearing.

The aft portion of the main engine, we can see the thrust bearing and it is an integral part of the main engine.

After the trust bearing, there is a crankshaft which is supported by the main bearing. main bearing and thrust bearing are supported by the main engine bed plate which is bolted down to the ships hull.

The intermediate shaft bearing is also bolted down to the ship's Hull hand stern tube bearing forms a part of the aft peak bulkhead.

As the propeller rotates it creates the thrust which is called propeller thrust, which is taken up by the propeller shaft and is passed onto the intermediate shaft. This happens because the propeller shaft and the intermediate shaft are connected together and the thrust is further avoided by the thrust block. 

The main reason why the thrust block is placed at the aft-most part of the engine is the crankshaft should be saved from this propeller thrust. The crankshaft at any cost should never be subjected to the propeller thrust. This thrust block has thrust pads, when the propeller thrust is offered in a head direction these pads (on the fwd side of thrust bearing) take the propeller thrust thus they do not offer any further motion to the crankshaft and when the ship is going in the astern direction these pads ( aft side of thrust bearing) takes up the propeller thrust.

So thrust pads absorb the propeller thrust and since the thrust pads are located inside the thrust block which is bolted to the main engine bed plate through the holding down Bolts are bolted to the ships hull the ship starts to move.

Location:

To know where must be the thrust block should be located, first, we need to know about nodes and anti-nodes.

• Node: it is a fixed point where the amplitude of vibration is minimum or zero.
• Anti-node: It is the opposite if the node and has a maximum amplitude of vibration. It occurs between two nodes.

So the thrust block must always be placed at the nodes, not at the anti-nodes. Thrust block can be placed at any of the nodal points along the shafting system.

Thus is always better to position the thrust block at nodal points where vibration is zero or minimum.

Thus the thrust blocks are fitted within the engine or outside the engine at the immediate nodal points at the end of the crankshaft.

The chain or gear drive for the camshaft is located at the nodes or near nodal points.

If the drive is taken from the nodal point, the turning moment is even.

This is desirable as this will prevent any undue stress from being developed in the chain drive or get drive system.

Where to fit dampers and de tuners?

We can see the vibration curve in the diagram with a red line the thrust block is located at the node or near nodal point.

In the same manner, the vibration curve raises and the damper is fitted at the antinode or near the antinodal point.

If the detuners and the dampers are fitted at nodes, they will not function effectively as the amplitude of vibration is minimum or zero.

If they are fitted at the antinodes. Then they will function effectively. But it is not practically possible to fit exactly at antinodes, and thus often fitted at the forward end of the engine.

Why can't the Thrust block is located at the stern tube?

If thrust block is placed near the stern tube there is less area for the propeller thrust to act on and since the area is very small we require more additional stiffening members to withstand such a huge thrust and there is also one more reason the number of nodal points available near stern tube is comparatively very less.

When thrust block is placed near the main engine the cross-sectional area is comparatively larger for the propeller thrust to act on and it doesn't require any additional stiffening member, as the thrust block forms an integral part of the bedplate itself.

• The thrust force from the propeller when taken up by a thrust block, located near the Stern tube, have a lesser area to distribute to the hull.
• The number of nodal points is fewer near the stern tube.
• The cross-sectional area of the ship near the main engine is large when compared the area near the stern tube.