3 July 2016

SHIP STEERING GEAR SYSTEM-ETO

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Regulation of steering gear

There must be a main and an auxiliary steering gear, each independent of other. The reason for this is that, if one fails then other can be put on work. While where two identical power units are provided an auxiliary unit is not necessary. The power and torque capacity must be such that the rudder can be swung from 35° on one side to 35° to the other side with the ship at maximum speed. The time to swing from 35° on one side to 30° to the other side must not exceed 28 seconds. Steering gear must be power operated if rudder stock diameter is greater than 120 mm. Mostly we hydraulic power for operating the rudder post.

But one thing to see here is that power of the auxiliary steering shall be such that rudder can be swung from 15° on one side to 15° to the other at deepest draught and speed of 7 knots in 60 seconds. Steering gear must be protected from shock loading, short circuit and overload and audible and visual indicatorsmust be obtainable on bridge, engine control room and the steering gear room for motor running, alarms and trips.Low level alarm must be there for hydraulic oil tank. A tanker which carries oil of tonnage 10000 gross ton and more must have two self-governing steering gear systems where failure of one results in automatic changeover to the other within 45 seconds, along with alarm for indication. This is necessary from point of view that, if change does not take place then ship will keep in moving in one direction. The system must be protected from shock loading .Any of these failures should effect in audible and visual alarms on the bridge Control, power and transmission system consisted of hydraulic equipment, all connected with hydraulic tubes. Since ship is very long and the line carrying this hydraulic is also long, so this system posed a greater risk of steering gear failure due to leakage of oil or ingress of air into the system. Further developments replaced the hydraulic tele-motor with electric one and the effort required to turn the vessel became insignificant. Now all steering gears have electric command signal. 


Electric steering gear system of ship     
1) Electric tele-motor
When bridge wheel is turned, it moves rheostat B and disturbed current flows to rotate the control motor. This is same like we change the speed of our fan. Rheostat A will hunt back unless balance is restored which will stop the control motor. This same we do an experiment of electric current by using rheostat. Control motor drives a screw shaft via a flexible coupling in the control box and screw block moves through the floating lever and causes movement to actuating rod and displaces the pumps on stroke. This rod moves side by side and that what defines the movement of rudder. If the event of tele-motor failure occurs, steering gear can be controlled nearby by turning off control motor power and engaging with screw-shaft bevel gear with hand-wheel gear. 

2) Ward leonard principal
Steering gear functioning which depends on Ward-Leonard principle comprises of an electric control, electric power unit and electrical transmission work. The arrangement consists of a continuously running motor-generator set which has a straight coupled exciter to provide the field current of the generator. The main motor which drives the rudder has no input and thus is motionless. 

When the wheel on the bridge is turned and moves from one side to other side, and the rheostat contact moved, the control system is disturbed and a voltage occurs in the exciter field, the exciter, and the generator field. The generator produces power which turns the rudder motor and hence the rudder. As the rudder moves, it returns the rudder rheostat contact to the same position as the bridge rheostat, bringing the system into balance and stopping all current flow.

Power units of ship
Two types of hydraulically powered transmission units or steering gear are in common use, namely: 

a) Ram type. 
b) Rotary vane type.

Depending upon torque needs, two variations are possible the two-ram and the four-ram. The rams acting in hydraulic cylinders operate the tiller by means of a swivel crosshead carried in a fork of the rams. A variable delivery pump which is mounted on each cylinder and the slipper ring is related by rods to the have power over spindle of the tele motor receiver. 

1) Ram type steering gear
The changeable delivery pump is piped to each cylinder to enable suction or discharge from either. The whole hydraulic fluid is controlled by these motors. A replenishing tank is mounted close by and arranged with non-return suction valves which without human intervention provide make-up fluid to the pumps. A bypass valve is coupled with spring-loaded shock a valve which opens in the event of a very heavy sea forcing the rudder over. This maintains the position of the ship and load on the rudder. In moving over, the pump is actuated and the steering gear will return the rudder to its original position once the heavy sea has passes.

2) Four Ram steering gear
A spring loaded relief linkage on the tiller will avoid damage to the manage gear during a shock movement. During usual operation one pump will be running and other will work as stand by units. But if a faster response is required, for instance in confined waters, both pumps may be in use. The pumps will be in the no-delivery state, so that until a rudder progress is required by a signal from the bridge tele motor transmitter. The tele motor receiver cylinder will then move this will result in a movement of the floating lever which will move the floating ring or slipper pad of the pump, causing a pumping action. Fluid will be drawn from one cylinder and pumped to the other, thus turning the tiller and the rudder. 

The basic principles of operation of two-ram and four ram gear is similar except that the pump will draw from two diagonally opposite cylinders and discharge to the other two. Two discharge valves are present on one side while two valve of inlet are opposite side. The four-ram arrangement provides greater torque and the elasticity of different arrangements in the event of component failure.
Either pump can be used with all cylinders or with either the two port ( left hand side) or two starboard ( Right hand side) cylinders. Various valves must be opened or closed to provide these arrangements. The use of a direct valve block incorporating rudder shock relief valves, pump isolating valves, ram isolating and bypass valves, offers greater flexibility with a four-ram steering gear. In normal operation one pump can operate all cylinders. In an emergency situation, the defective cylinders could be isolated and operation of steering gear resumed.

3) Rapson slide arrangement
The crosshead arrangement on the four-ram type steering gear incorporates the 'Rapson Slide'. This provides a mechanical benefit which increases with the angle turned through. The crosshead arrangement may use either a forked tiller or a round arm tiller. The round arm tiller has a centre crosshead which is free to slide along the tiller. The straight line movement of the rams is thus transformed into an angular tiller movement. In the forked tiller arrangement the ram movement is transferred to the tiller through swivel blocks.

4) Rotary vane steering gear
This is equivalent to a two-ram gear, with torque capacities depending on size. An assembly of two rotary vane gears, one above the other, or with two independent hydraulic circuit with self closing lock valves provide the security of a four ram gear.
As you can see the diagram, rotor C is fitted and keyed to a tapered rudder stock A and the stator B is protected to the ship's structure. Fixed vanes, secured equidistantly in the stator bore and rotating vanes secured equidistantly in the rotor, form two sets of pressure chambers in the annular space between the rotor and stator. They are interconnected by a manifold. Three fixed and three moving vans are normal and permit a total rudder angle of 70°, i.e. 35° in each direction. The fixed and rotating vanes may be of spheroid graphite cast iron. This is done because the surface finish of cast iron. They are securely fixed to the cast steel rotor and stator by high tensile steel dowel pins and cap screws. Keys are also fitted along the length of the rotary vanes, for mechanical strength. 

The vanes fixing is well thought-out to be of enough strength to make them suitable to act as rudder stops. Their strength is to be more than the other parts because lot of load face by these parts. Steel sealing strips, backed by artificial rubber, are fitted in grooves along the operational faces of the fixed and rotary vanes, thus ensuring a high volumetric efficiency, of 96-98% even at the relief valve pressure of 100 bar or over. The anchor brackets are securely bolted to the ship. This clearance varies with each size of the rotary vane unit, but is approximately 38 mm in total and it is necessary that the rudder carrier should be capable of restricting the vertical movements of the rudderstock. But if vertical motion is large then it can damage the arrangement of steering gear present.

Type of controls in steering gear
There are three types of controls as following:

1) Follow up system
In Follow up system mode, movement of rudder follows the movement of steering controller. When the helm is applied on the bridge, order is transmitted to steering engine control. As a consequence, this control causes the steering engine to shift with it and swings the rudder associated with it. When the preferred rudder angle is achieved, the feedback without human intervention cuts off the steering engine power through the hunting gear. Thus the rudder will turn until the actual rudder angle is the same as desired rudder angle shown on steering pedestal. 


2) Automatic system
In Automatic system mode the steering is controlled by the signals received from the master compass so that the ship is automatically held on to a selected course. The compass is electronic equipment. System is arranged in such a way that when vessel is on course, rudder is amidships and balance is made in rudder position. As rapidly as the compass indicates the error, the auto pilot applies a quantity of rudder enough to bring the vessel back on route. By the time the vessel is on course again, the applied correcting rudder has been removed. With some systems, it is possible to alter the course without taking to hand steering and this can be achieved by a trim switch. 

3) Non-Follow up system
In Non-Follow up system mode, the gear will run and rudder will carry on turning while the steering wheel or the controller is moved from its central position. Rudder movement is stopped when steering control is centered once again or by the mechanical stops placed at 37o on either side. Controllers with non follow up systems take the form of wheel, tiller lever or push buttons.

Steering gear system testing
Prior to a ship's departure from any port the steering gear should be tested to ensure satisfactory operation. These tests should include:

1) Manoeuvre of the auxiliary steering gear or use of the second pump which acts as the secondary.
2) Manoeuvre of the remote control (tele motor) system or systems from the main bridge steering positions.
3) The rudder angle indicator reading with respect to the actual rudder angle should be checked.
4) During these tests the rudder should be moved through its full travel in both directions and the various equipment items, linkages, etc., visually inspected for damage or wear.
5) Operation of the main steering gear.
6) The communication system between the bridge and the steering gear compartment should also be operated proper functioning confirmed. 
7) Operation of the steering gear using the emergency power supply.
8) The alarms fitted to the remote control system and the steering gear power units should be checked for correct operation.



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