The construction of the shaft bearings is important, as this holds the complete weight of the propeller shafts. There are two main types of bearings- the full case bearing located at the stern, and the half case bearing located at the other positions.
The full casing provides a complete bearing for the weight of the shaft and is an integral part. The reason it is located at the stern is to account for both catenary weight forces, and also to counteract any buckling or reverse thrust forces felt at the aft due to the motion of the propellers. This bearing is also known as the aftmost tunnel bearing, as it encases the shaft just like a tunnel.
The other shafts only account for the weight, and hence do not require an upward casing unit. These bearings must be designed of high strength metals that do not easily buckle or deform under high strains. In addition, low levels of tolerances are expected during the manufacturing stage.
Special bearing pads are fit into slots on the connecting inner face of the bearing, such that it allows for smooth rotation. To lubricate the shaft bearing, an oil dip arrangement is carried out. By coating the rotating surface with oil from an oil thrower ring at regular intervals, a thick coat of lubrication is maintained at all times.
The coolant used to prevent overheating and subsequent damage are water circulated about the shaft bearing. This is stored in specialized tubes that run along the bearing and shaft. Tanks stored above the engine platform house coolant that is circulated around the propulsion machinery and systems.
Intermediate Shaft Arrangement The thrust blocks are used primarily to dampen and absorb forces from the rotating propeller shafts. These forces are redirected into specialised frames that make up the bed of the engine compartment. The energy in these frames is further distributed into the surfaces of the hull through the hull girders.
The hull girders serve as the framework on which the hull of the ship is constructed. The thrust blocks must be rigidly mounted in place to prevent any form of vibration during the course of the journey. Also, the primary thrust block can either be an independent unit that is built separately or can be integrated into the marine engines itself.
By integrating the block into the engine, it reduces the space requirements and maintenance costs while sailing. However, maintenance, while berthed, can be an issue as it would require opening the engine block casing. The casing that makes up the thrust blocks are built in two parts- an upper half that is detachable, and a lower half that supports the shaft.
The shaft is laid onto the lower block, and the upper half is then bolted into place using specialized fasteners that can absorb shock. To lubricate the rotating shaft, oil is regularly coated on to the rotating surface. This is achieved in a manner similar to that of the shaft bearings.
An oil thrower and deflector are put in place to maintain a constant supply of oil from a storage unit located on the lower half of the thrust block.
The operating temperature is controlled using cooling coils that circulate a chosen type of coolant throughout the block. It also draws coolant from the central propulsion cooling system. To absorb the vibrations and shocks, bearing pads are attached to the blocks.
They can be of two types- tilt pads, or pivotal pads, both of which are held in specialized holders built into the thrust block. The thrust pads transfer energy to the lower half of the casing that is constructed to withstand larger amounts of shock.
A thrust collar is also used to absorb thrust from the propeller shaft. The thrust blocks incorporate integral flanges that primarily help in bolting the block to other surfaces.
For instances, the block can be connected to the gearbox or engine using this flange. It can also be used to connect the engine thrust shaft to the intermediate shafts using these flanges. In case the thrust block is built into the engine block, it is made of the same casing material that the engine base plates are manufactured from.
In addition, they directly use the lubrication and coolant from the engine components itself. The integrated block is similar to the normal thrust blocks in most other features. It is interesting to note that the thrust block is integrated into the engine in most ships, except for the smaller boats that have space constraints.
The shafts themselves must be built from robust materials with high yield strength, and lower probability of buckling. Each shaft starting from the thrust shaft must be built into small and manageable components that can be disassembled when the need arises.