Transmissions Flashcards
what is the main function of the gear box
- Allow the speed of the prime mover (eg 15000 rpm) to be reduced to a proportional, more manageable speed eg (250 rpm) for the propeller shaft
- Reverse the direction of rotation of the propeller shaft to allow ahead or astern
what is a straight cut/ spur
single tooth contact- only suitable for low speeds and low torque applications where making noise is not important.
helical
Due to the teeth being cut at an angle there is more tooth contact which makes these gears stronger and quieter. However, due to the angle of the teeth thrust is created which means thrust bearings/collars are required to prevent shafts pushing apart. Also, due to the sliding action between teeth an extreme pressure gear oil (or an oil additive) is required.
double helical
These are made of two helical gears back to back, with the angle of gear cut in opposing directions. This cancels out any thrust and negates the requirement for thrust bearings. Double helical gears could be interchanged with spur gears without the need for different/additional bearings.
worm and wheel
The worm and wheel is used on a non-parallel axis. The worm resembles a screw and is always the driving gear. The wheel resembles a straight cut gear and is always the driven gear. Very large gear reductions are possible with this arrangement, giving a very slow but high torque output.
what are gear reductions
a change of direction can be changed by adding a second gear to the first
what is added when changing speed
by adding a different size gear wheel, different speeds can be achieved.
gear reduction =
small driving big = more torque and less speed overdrive
big driving small= more speed and less torque
how is changing speed and direction achieved
the driving gear which is called the primary pinion
the driven gear is called the primary wheel
and all the gears are called wheels
what is singe gear reduction
A single gear reduction can manage a maximum reduction of approximately 15:1 due to minimum and maximum size limitations of the pinion and wheel.
what is double gear reduction
by increasing the amount of meshing gears we can increase the the overall reduction across the gear train. this reduces the overall size of wheels and in turn the size of gear box.
the gear wheel is driven by the primary pinion becomes the secondary pinion, this in turn drives the secondary gear wheel.
what is double gear reduction - duel tandem
duel tandem gear trains mean the load will be halved, therefor smaller pinions can be used. this is a benefit as with smaller gears and pinions means less weight.
quill shafts maintain the alignment of the loading if any misalignment is witnessed. (this allows a certain amount of twist and flex to keep the gears pushed together.) the quill shaft acts as a torsional spring.
what is a shaft break
the purpose of a shaft break is to stop and hold the prop shaft, basically like a hand break on a car.
the shaft break is a break disk attached to the shaft and has 3 evenly spaced break callipers (with 2 break pads in each) to clamp the break disk causing the prop shaft to stop.)
the break is applied by HP air 80-100 bar
what are the shaft break checks
upon aplication of the shaft break
* Ensure no fire risk exists
* All Brake Pads are APPLIED
* The shaft is stationary
and upon removal of the break
* All Pads are clear/released
* No fire danger exists
* The shaft is turning
what is the thrust block
the thrust block is a large bearing, at the gearbox end of the prop shaft, designed to absorb the force of the propeller so the ship as a whole moves ahead in the water not just the propeller shaft moving
it consist of a machined collar on both side of which are fitted white metal pads lubricated by the michelle principle
what is the turning gear
the turning gear is used to slowly rotate the shaft and propeller in a controlled manner for docking and maintenance purposes..
it is mounted on to of the gear case and provides drive onto the upper quill shaft in the gearbox.
it uses a worm and wheel, the worm is always the driving gear.
- It will self-lock if power is lost
- It will not work if the wheel/spur gear is powered
- (If drive is attempted the wrong way (wheel driving worm) the whole thing will lock up solid)
what is the forced lubrication system
it provides lubrication to all the areas of the gear box including gearings and bearings.
what is the motor driven pump
this is used to prime the gearbox luboil system. this can be used to drain the oil and acts as a back up for the gear driven pump if it fails or drops in pressure.
what is the gear driven pump
the gearbox’s main pump and will only work when the gearbox is turning.
what is the serckstat
this directs the hot lub oil through the cooler (heat exchanger) but allows cold oil to bypass the cooler. oil temperature is controlled to the optimum temperature by the serckstat using coolers and heaters
what are the tuning valves
they provide a boundry lubrication by directing high pressure oil componants that require specific oil pressure
what are forced lub cut ins
if the oil pressure drops bellow 18.5psi the pressure switch will signal the MD pump to take over. FL cut-ins are checked / tested every 24 hours
what are the hazards of gear boxes
noise
heat/fire hazard
rotating parts
overalls are to be warm correctly and the man bellow tally procedure is to be followed
what is the plummer block
the plummer blocks are used to support the weight of the shaft, within the ship without absorbing any thrust.
they are lubricated by dynamic lubrication, using oil from an oil bath. A plummers block has bearings, a casing, a dipstick, journal pads( these are individual and supports with white metal bearings) and an oil cooler.
what is a bulkhead gland
the purpose of the bulkhead glands are to maintain a watertight integrity as the shaft passes through the compartments, without absorbing any thrust.
it has white metal lined bearings and they are lubricated with greased nipples.
what is the stern seal
this maintains the watertight integrity as the the shaft passes through the ships hull. this is fitted to the inboard end of the stern tube and is a mechanical seal. in the case of an emergency an inflatable seal can befitted if the stern seal fails.
what are the main and intermediate A brackets
the brackets take no actual thrust but support the weight of the propeller shaft. lubrication of the bearings in the brackets is sea water using the hydro-dynamic principle (50rpm required to maintain hydro-dynamic lubrication).
what is the SKF coupling
A hydraulic coupling, which is designed so that the tail shaft can be split from the intermediate shaft without leaving a flange, enabling the tail shaft to be withdrawn through the stern seal and A-brackets.
High pressure hydraulic oil is forced between the two sleeves to allow for connection and disconnection between the tail and intermediate shafts.
what is the propeller
a fixed blade
the blade angle/pitch cannot be changed
* Highly efficient at full power (high speed)
* Loss of efficiency at low power and when moving astern
* With a fixed propeller the gearbox must have a reverse gear
