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Ships propeller shaft -Thrust block & Shaft bearings
The transmission system on a ship transmits power from the engine to
the propeller. It is made up of shafts, bearings, and Finally the propeller
itself. The thrust from the propeller is transferred to the ship through
the transmission system.
The different items in the system include the thrust shaft, one or more
intermediate shafts and the tailshaft. These shafts are supported by the
thrust block, intermediate bearings and the sterntube bearing. A sealing
arrangement is provided at either end of the tailshaft with the propeller
and cone completing the arrangement. These parts, their location and
purpose are shown in Figure below.
The thrust block transfers the thrust from the propeller to the hull of
the ship. It must therefore be solidly constructed and mounted onto a
rigid seating or framework to perform its task.
Fig: A solid fixed-pitch propeller
It may be an independent
unit or an integral part of the main propulsion engine. Both ahead and
astern thrusts must be catered for and the construction must be strong
enough to withstand normal and shock loads.
Fig: Thrust block arrangement
The casing of the independent thrust block is in two halves which are
joined by fitted bolts (Figure above). The thrust loading is carried by
bearing pads which are arranged to pivot or tilt. The pads are mounted
in holders or carriers and faced with white metal. In the arrangement
shown the thrust pads extend threequarters of the distance around the
collar and transmit all thrust to the lower half of the casing. Other
designs employ a complete ring of pads.
An oil scraper deflects the oil
lifted by the thrust collar and directs it onto the pad stops. From here it
cascades over the thrust pads and bearings. The thrust shaft is
manufactured with integral flanges for bolting to the engine or gearbox
shaft and the intermediate shafting, and a thrust collar for absorbing the
Fig: Propeller shaft arrangement
Where the thrust shaft is an integral part of the engine, the casing is
usually fabricated in a similar manner to the engine bedplate to which it
is bolted. Pressurised lubrication from the engine lubricating oil system
is provided and most other details of construction are similar to the
independent type of thrust block
Shaft bearings are of two types, the aftermost tunnel bearing and all
others. The aftermost tunnel bearing has a top and bottom bearing shell
because it must counteract the propeller mass and take a vertical upward
thrust at the forward end of the tailshaft. The other shaft bearings only
support the shaft weight and thus have only lower half bearing shells.
An intermediate tunnel bearing is shown in Figure below.
Fig: Tunnel bearing arrangement
journal bush is here replaced by pivoting pads. The tilting pad is better
able to carry high overloads and retain a thick oil lubrication film.
Lubrication is from a bath in the lower half of the casing, and an oil
thrower ring dips into the oil and carries it round the shaft as it rotates.
Cooling of the bearing is by water circulating through a tube cooler in
the bottom of the casing.
Oil lubricated sterntube bearing, sterntube seals & shafting arrangement
Function of solid fixed pitch propeller & Propeller mounting
Controllable pitch propeller
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