Engine Structure and Power Transmission Systems Flashcards
what is the purpose of the thruster
(bow and stern)?
The purpose of the thruster units is to turn the vessel when is operating at slow speeds or when is not under way, to keep the ship in position in a cross wind and to move the ship towards or away from a mooring position as required. The thrust is produced by rotation of a propeller unit which is housed in a transverse cylindrical ducting, where the propeller unit is rotated by means of a vertical electric motor via bevel gears. The propeller blade pitch is controllable in order to obtain the desired magnitude and direction of thrust. Power is transmitted from the electric motor through the flexible coupling, input shaft and bevel gears to the propeller shaft, rotating the propeller in a single direction.
The thruster comprises of a number of separate sections:
The electric motor unit with drive shaft and bevel gearing driving the propeller unit hub;
The propeller unit with blades mounted in the hub;
The hydraulic unit which changes the pitch of the propeller blades;
The control system which regulates the blade pitch in accordance with demand from the bridge.
with reference to bow and stern thrusters
what does the propeller unit consist of?
The propeller unit, usually consists of four propeller blades and a propeller hub. The propeller hub and gear case house a hydraulic servomotor and sliding block mechanism. The propeller blades are connected to blade carriers by blade bolts, and this ensures easy exchange of blades in the thruster tunnel. The gear case, which carries the propeller parts, is connected to the thruster tube by bolts and this ensures easy overhauling of all parts inside the thruster tube. The power transmission gear is located inside the gear case and consists of the vertical input shaft, the right angle reduction bevel gear and the horizontal propeller shaft, and serves to transfer the power from the prime mover to the propeller. The bevel gear and individual bearings are lubricated by the gravity oil filling the gear case
with reference to bow and stern thrusters
what does the hydraulic unit do?
The hydraulic power pack unit provides oil under pressure and this is used to change the pitch of the thruster unit blades. The oil is drawn from the gravity tank, through the suction filter and into the oil service pump. The pressurized oil is pumped to the solenoid valve via the check valve and the flow of oil is controlled by the solenoid valve.
The hydraulically operated solenoid valve is a changeover valve for the distribution of the hydraulic oil to the respective servo cylinders depending on the command entered at the active control panel. When the command is entered on the control panel, the solenoid valve is actuated and pressurized hydraulic oil is supplied to one of the hydraulic circuits down the oil tube, through the feed ring and oil entry tube to the servomotor, causing displacement of the cross head piston. The reciprocating movement of the piston is converted into a turning movement by the sliding block mechanism and this turns the propeller blades. The vent side of the servomotor piston drains to the oil bath in the thruster body via a solenoid valve. From this pressurized oil bath, oil returns to the header tank. The main actuator power pack pump takes oil from the header tank and supplies it to the thruster unit via the solenoid control valves. A shaft sealing mechanism is attached to the gear case in order to prevent leakage of oil out of the system. When a pitch change command is entered, the propeller will tend to move excessively. The pilot check valve prevents any excessive movement of the propeller whilst changing pitch.
with reference to bow and stern thrusters
how is the bevel gear and all the bearings inside the gear case lubricated?
The bevel gear and all the bearings inside the gear case are lubricated by the bath lubricating method. The lubrication oil in the gear case is slightly pressurized by the connection with the gravity tank which is positioned above the waterline to prevent sea water from leaking into the oil system.
with reference to bow and stern thrusters
when is the main motor started?
The main motor must only be started when the blades are in the neutral position (zero pitch), or in the allowable zone (blade pitch of ±3°). The system is interlocked to prevent the main motor from starting if the blade pitch is outside of the set limits. The interlock switches also prevent the main motor from starting when:
The cooling fan is stopped;
The power pack gravity tank level is low;
The control oil pressure is low
with reference to bow and stern thrusters
explain the operation of the bow thrusters?
Operation of the bow thrusters requires starting a large induction motor and the power requirement of this electric motor is high, requiring that additional generators are started in order to avoid the risk of a vessel blackout. It is important to note that, on modern vessels if there is insufficient power capacity available at the switchboard an additional generator is started by the power management system. The thruster drive motor cannot be started until sufficient power is available at the switchboard. Usually, the main switchboard includes a bow thruster control panel, which includes a control position selection switch, lock-out relay trip reset, motor control and an ammeter. A series of status indicators are also included for monitoring the condition of the VCB, gravity tank, pump pressure etc. If any warning lamp is illuminated, the cause of the fault should be determined and remedied before operation of the bow thruster. Feeder protection for the bow thrusters is achieved by means of the protection and monitoring panel located on the main switchboard bow thruster panel and offers both measurement and protection for the bow thruster drive motor. To ensure safe, reliable operation of the bow thrusters, limits are imposed on the vessel’s speed and draught. If there is insufficient draught, the thruster will suffer a reduction in performance along with cavitation and the possibility of air drawing. The result of this will be increased vibration which may cause damage. Similarly, at speeds greater than 5 knots there is a risk of drawing air into the thruster, particularly when operating at shallow waters. This will degrade the performance and can cause cavitation damage and it can be detected by hunting of the main motor ammeter and should be avoided. If the vessel’s speed is below 5 knots and air drawing is occurring, reducing the propeller pitch will prevent further air drawing from taking place. Under normal circumstances the main power supply is activated by the engineering department and after that the thruster operation and control is undertaken by the deck department from the bridge panels. The main switch at the local thruster control panel should be set at REMOTE in order to allow for this. Control of the thruster on the bridge is either at the wheelhouse control stand or the control stands on the bridge wings. It is important to note that, especially on bow thruster, when the hydraulic pump is started the fan is also started and the FAN RUN indicator in the panel will be illuminated. The main motor is interlocked with the fan and oil pump and will not start unless they are running. Also is important to remember that there are EMERGENCY STOP push-buttons in the wheelhouse panel, forward mooring station and in the bridge wing panels. The thruster unit includes a feedback system for transmitting the angle of the propeller blades to the remote control panel located on the bridge. As the oil entry varies, the stroke of the oil entry tube also varies. The movement of the oil entry tube causes movement of the feedback lever. This movement is transmitted via the feedback chain to the blade angle transmitter located outside the thruster gear casing. This mechanical movement is then converted to an electrical signal by the blade angle transmitter and transmitted to the angle indicator on the bridge and local control panels.
difference between medium and slow speed diesel engines
Medium-speed diesels, e.g. 250 to 750 rev/min, and slow-speed diesels, e.g. 100 to 120 rev/min, each have their various advantages and disadvantages for various duties on board ship. The slow-speed two-stroke cycle diesel is used for main propulsion units since it can be directly coupled to the propeller and shafting. It provides high powers, can burn low-grade fuels and has a high thermal efficiency. The cylinders and crankcase are isolated, which reduces contamination and permits the use of specialised lubricating oils in each area. The use of the two-stroke cycle usually means there are no inlet and exhaust valves. This reduces maintenance and simplifies engine construction. Medium-speed four-stroke engines provide a better power-to-weight ratio and power-to-size ratio and there is also a lower initial cost for equivalent power. The higher speed, however, requires the use of a gearbox and flexible couplings for main propulsion use. Cylinder sizes are smaller, requiring more units and therefore more maintenance, but the increased speed partly offsets this. Cylinder liners are of simple construction since there are no ports, but cylinder heads are more complicated and valve operating gear is required. Scavenging is a positive operation without use of scavenge trunking, thus there can be no scavenge Fires. Better quality fuel is necessary because of the higher engine speed, and lubricating oil consumption is higher than for a slow-speed diesel. Engine height is reduced with trunk piston design and there are fewer moving parts per cylinder. There are, however, in total more parts for maintenance, although they are smaller and more manageable. The Vee engine configuration is used with some medium-speed engine designs to further reduce the size and weight for a particular power.
purpose Couplings, clutches and gearboxes
Where the shaft speed of a medium-speed diesel is not suitable for its application, e.g. where a low speed drive for a propeller is required, a gearbox must be provided. Between the engine and gearbox it is usual to fit some form of flexible coupling to dampen out vibrations. Elastic or flexible couplings allow slight misalignment and damp out or remove torque variations from the engine. The coupling may in addition function as a clutch or disconnecting device. Couplings may be mechanical, electrical, hydraulic or pneumatic in operation. It is usual to combine the function of clutch with a coupling and this is not readily possible with the mechanical coupling. There is also often a need for a clutch to disconnect the engine from the gearbox. A clutch is a device to connect or separate a driving unit from the unit it drives. With two engines connected to a gearbox a clutch enables one or both engines to be run, and facilitates reversing of the engine.
with the aid of a sketch describe the operation of a hydraulic coupling?
The hydraulic or fluid coupling uses oil to connect the driving section or impeller with the driven section or runner (Figure 2.25). No wear will thus take place between these two, and the clutch operates smoothly. The runner and impeller have pockets that face each other which are filled with oil as they rotate. The engine driven impeller provides kinetic energy to the oil which transmits the drive to the runner. Thrust bearings must be provided on either side of the coupling because of the axial thrust developed by this coupling.
with the aid of a sketch describe the operation of a plate type clutch
A plate-type clutch consists of pressure plates and clutch plates arranged in a clutch spider (Figure 2.26). A forward and an aft clutch assembly are provided, and an externally mounted selector valve assembly is the control device which hydraulically engages the desired clutch. The forward clutch assembly is made up of the input shaft and the forward clutch spider. The input shaft includes the forward driven gear and, at its extreme end, a hub with the steel pressure plates of the forward clutch assembly spline-connected, i.e. free to slide. Thus when the input shaft turns, the forward driven gear and the forward clutch pressure plates will rotate. The forward clutch plates are positioned between the pressure plates and are spline-connected to the forward clutch spider or housing. This forward clutch spider forms part of the forward pinion assembly which surrounds but does not touch the input shaft. The construction of the reverse clutch spider is similar. Both the forward and reverse pinions are in constant mesh with the output gear wheel which rotates the output shaft. In the neutral position the engine is rotating the input shaft and both driven gear wheels, but not the output shaft. When the clutch selector valve is moved to the ahead position, a piston assembly moves the clutch plates and pressure plates into contact. A friction grip is created between the smooth pressure plate and the clutch plate linings and the forward pinion rotates. The forward pinion drives the output shaft and forward propulsion will occur. The procedure when the selector valve is moved to the astern position is similar but now the reverse pinion drives the output shaft in the opposite direction.
explain how an engine can be reversed
Where a gearbox is used with a diesel engine, reversing gears may be incorporated so that the engine itself is not reversed. Where a controllable pitch propeller is in use there is no requirement to reverse the main engine. However, when it is necessary to run the engine in reverse it must be started in reverse and the fuel injection timing must be changed. Where exhaust timing or poppet valves are used they also must be re-timed. With jerk-type fuel pumps the fuel cams on the camshaft must be repositioned. This can be done by having a separate reversing cam and moving the camshaft axially to bring it into position. Alternatively a lost-motion clutch may be used in conjunction with the ahead pump-timing cam. The shaping of the cam results in a period of pumping first then about 10° of fuel injection before top dead centre and about 5° after top dead centre. A period of dwell then occurs when the fuel pump plunger does not move. A fully reversible cam will be symmetrical about this point, as shown. The angular period between the top dead centre points for ahead and astern running will be the ‘lost motion’ required for astern running. The lost-motion clutch or servo motor uses a rotating vane which is attached to the camshaft but can move in relation to the camshaft drive from the crankshaft. The vane is shown held in the ahead operating position by oil pressure. When oil is supplied under pressure through the drain, the vane will rotate through the lost-motion angular distance to change the fuel timing for astern operation. The starting air system is re-timed, either by this camshaft movement or by a directional air supply being admitted to the starting air distributor, to reposition the cams. Exhaust timing or poppet valves will have their own lost-motion clutch or servo motor for astern timing. Medium- and slow-speed diesel engines will follow a fairly similar procedure for starting and manoeuvring. Where reversing gearboxes or controllable-pitch propellers are used then engine reversing is not necessary. A general procedure is now given for engine operation which details the main points in their correct sequence.
what is the transmission system on a ship?
The transmission system on a ship transmits power from the engine to the propeller. It is made up of shafts, bearings, and 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 tail shaft. These shafts are supported by the thrust block, intermediate bearings and the stern tube bearing. A sealing arrangement is provided at either end of the tail shaft with the propeller and cone completing the arrangement.
explain what a thrust block is?
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. 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. The casing of the independent thrust block is in two halves which are joined by fitted bolts. 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 three-quarters 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 thrust. 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.
what are shaft bearings?
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 tail-shaft. The other shaft bearings only support the shaft weight and thus have only lower half bearing shells. 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.
explain what a stern tube bearing does?
The stern tube bearing serves two important purposes. It supports the tail shaft and a considerable proportion of the propeller weight. It also acts as a gland to prevent the entry of sea water to the machinery space. Early arrangements used bearing materials such as lignum vitae (a very dense form of timber) which were lubricated by sea water. Most modern designs use an oil lubrication arrangement for a white metal lined stern tube bearing. Oil is pumped to the bush through external axial grooves and passes through holes on each side into internal axial passages. The oil leaves from the ends of the bush and circulates back to the pump and the cooler. One of two header tanks will provide a back pressure in the system and a period of oil supply in the event of pump failure. A low-level alarm will be fitted to each header tank. Oil pressure in the lubrication system is higher than the static sea water head to ensure that sea water cannot enter the stern tube in the event of seal failure.
