Mech theory 2 Flashcards
What is the function of the gearbox?
Reverse the direction of the propeller shaft to make the ship go astern
Allow the prime mover to be slowed to a more manageable speed(15000rpm to 250rpm)
Types of gear teeth:
Straight cut/spur
Helical
Double helical
Worm and wheel
What is double helical
Two helical gears back to back, angle of gear cut opposing directions.
Cancels out thrust and negates any need for thrust bearings.
Can be interchanged with spur gears without any need for additional bearings.
How do you change the speed of a gear wheel
Adding a second different sized wheel such as a pinion, depending upon the size of the wheel this will effect the speed and torque.
What is the maximum single gear reduction ratio
15/1
Double gear reduction
Increase the amount of meshing gears, increases reduction. This reduces overall size of gears and the gearbox.
How do you change the speed of a gear wheel
By adding different size gear wheels, different speeds or torques can be produced.
What is the driving gear called
Primary pinion
What is the driven gear called
Primary wheel
Double gear reduction- dual tandem
loading is halved alowing smaller pinions and therefore smaller gear wheels which saves weight
What are quill shaft
Maintains alignment of the loading
Acts as a torsional spring
What is the purpose of the shaft brake
Used to stop and hold the propeller shaft
Shaft brake checks- on application of brake
- Ensure no fire risk exists
- All Brake Pads are APPLIED
- The shaft is stationary
Shaft brake checks- on removal of brake
- All Pads are clear/released
- No fire danger exists
- The shaft is turning
Thrust block
A large thrust bearing, at the gearbox end of the propeller shaft, designed to absorb the force of the propeller so that the ship moves astern in the water.
What is turning gear
Used to slowly rotate the shaft and propeller for docking and maintenance purposes. It uses a worm and wheel-the worm is always the driving gear.
Motor driven pump
Used to prime gearbox luboil system
Used to drain oil
Acts as back up to the gear driven pump
Gear driven pump
Only works when the gearbox is turning
Serckstat
Directs hot oil to the cooler, allows cold oil to bypass the cooler. Sets the oil temperature to an optimum temperature using coolers and exchangers.
Tuning valves
Provide boundary lubrication by directing high pressure oil to components that require specific oil pressure
Forced lub- cut ins
If oil pressure drops below 18.5PSI the pressure switch will signal the MD pump to take over. Checked and tested every 24 hours.
Hazards associated with gearboxes
- Noise
- Heat/Fire hazard
- Rotating Parts – guards must always be in place and secure
Purpose of Plummer blocks
used to support the weight of the shaft, within the ship, without absorbing any thrust
Components of the Plummer block
A plummers block has bearings, a casing, a dipstick, journal pads and an oil cooler.
Lubricated by dynamic lubrication.
Purpose of Bulkhead glands
The purpose of the bulkhead glands is to maintain watertight integrity. Lubricated with grease.
Purpose of stern seal
Maintains a water tight seal as the shaft passes through the ships hull, only works when the propeller shaft is stationary.
Purpose of Main and Intermediate A Brackets
Support the weight of the propeller shaft
Lubrication of the bearings in the brackets is by sea water using the Hydro-Dynamic principle
Purpose of SFK coupling
A hydraulic coupling Designed so that the tail shaft can be separated from the intermediate shaft without leaving a flange.
High pressure hydraulic oil is forced between the two sleeves to allow for connection and disconnection between the tail and intermediate shafts.
The tail is most likely to sustain damage.
What is a fixed blade propeller and what are its benefits and drawbacks
The blade angle/pitch cannot be changed
Highly efficient at full power
loss of efficiency at low power and when moving astern
Must have a reverse gear
Controllable pitch propeller
Alter the angle/pitch of the propeller blades using hydraulics to give forward and astern motion.
Controllable pitch propeller
alter the angle/pitch of the propeller blades using hydraulics to give forward and astern motion.
Methods of embarking fuel
Replenishment at sea (RAS) Road bowser/tanker Jetty storage tanks Lighter barge Fixed jetty pipeline
Displacement tanks/Displaced tanks
A fuel tank that will contain some salt water. Act as fuel storage and ballast for ship stability. Only ever filled to 90%
Un-Displaced/Non Displaced Tanks
Only holds fuel (can be dirty)
Never filled with water as ballast
The fuel then passes through a centrifuge
Centrifuge
Separates oil/fuel from water, using centrifugal force. Once the fuel is clean it is stored in a service tank for use.
What is fuel tested for
Water
Dirt
Surfactants
MBG
Reasons for testing
Good quality fuel is clear and bright
Sodium can cause corrosion
Water in a tank can encourage MBG
MBG is a fungus and is found in most fuels, Moisture and warmth encourage multiplication of the spores.
Dirt in the fuel will cause filtration devices to become blocked.
Methods of testing fuel
Visual
Water reaction test-a mixture of 20ml distilled water and 80 ml, no more than a maximum of 2ml cloudy area
Colour test-Darker fuel is older
Diesel filtration test kit- shows how quickly a filter will block/clog-must pass 250ml of fuel before pressure reaches 15 PSI
Hazards of a fuel system
Fire and explosions
toxicity
Dermatitis
Pollution
What is the order of flow from the tanker to ships engines
- Tanker
- Filling Trunk
- Manifold
- Displaced tanks
- Centrifuge
- Undisplaced tanks
- Fuel Transfer pump
- Service Tank
- Fuel Boost Pump
- Fuel Boost Ringmain
What is the filling trunk?
Fuel enters the trunk and fills until it reaches the weir height. The fuel continues to fill until it overfills past the weir height, it then cascades over into the manifold.
Order from the gearbox in the propeller shaft
1) Gearbox
2) thrust block
3) plummer block
4) bulkhead gland
5) plummer block
6) stern seal
7) intermediate A-bracket
8) SFK coupling
9) Main A-bracket
10) propeller
