Section 3.4 Steering Gear Flashcards
What is the alarm system for a steering system that is hydraulically operated.
- Manually activated by pressing the button (stopping alarm.)
- Fluid Leakage, low-level alarm.
- Power failure alarm. (Blackout or Power trip)
- Temperature alarm.
Types of steering gear system commonly used on vessel.
The steering gear systems used are usually either hydraulic rams or rotary vane steering systems, unless on larger vessel the use of thrusters as steering are also an option.
- The hydraulic ram system is most commonly used. The basic system in general use on the small vessels is a very basic set and operating without power and being driven by hand. The system shown relies on oil being transferred from one side of a cylinder assembly to the other, with the pump being operated by moving the wheel.
Note that the system is shown with a shock valve or relief valve to protect the system from damage. Remember that fluids cannot be compressed.
- The operation relies on a bi-directional, hand-operated rotary pump. (Rotary vane steering system) The helmsman turning the wheel displaces oil on one side of the hydraulic circuit and drawing oil from the
returning oil from the other side. This is called a closed-loop system and a top-up of oil to compensate for expansion and some leakage is from the header tank above the rotary pump. The cylinder, as shown, is a double-acting type and can move the same on each side because of the problem with air inclusion. The rotary pump is fitted with bleed vents to allow are to escape that may have entered the circuit. - The most common type of steering system is the electro-hydraulic steering system, this system would be used on a small vessel such as a fishing ship.
Operation of hydraulic ram system.
- An electrical signal from the control box located in the wheelhouse is connected to a solenoid-operated directional control valve located adjacent to the steering ram of rams.
- The directional control valve or DCV is fitted with a spring control that moves from the centre position and moves right or left to direct the flow of hydraulic fluid to the left or right-hand side of the cylinder. When either solenoid is energised by an electrical current,
the shuttle returns to the centre or neutral position. - The end of the ram control rod is connected to the rudder arm to the stock and rudder.
- The ram position alters and is held in position in a locked position unless the shuttle moves.
- Feedback linked to the system operates the shuttle solenoids once the new position is reached. This is when the switch(magnetic solenoid) is deactivated.
The basic description of a uniflow and a bi-directional hydraulic system.
The operation is using a unidirectional pump driven by an electric motor; the change in direction is completed using the directional control valve (DCV).
The direction of fluid flow uses magnetic solenoid control to shift a shuttle in the valve either way as required. The relief valve, in this case, is located as previously described.
A control box that could be used, as shown, this case could be by using a lever of a wheel to alter the angle of the rudder. You will note that on this system, there is a ‘feedback’; this simply means that when the rudder is in the matching position with the bridge signal, the solenoid coil is OFF the directional valve returns to the centre position. The rudder position will not change until another bridge signal is received.
A second and more common system uses a bi-directional pump that can be referred to as an axial flow pump or a variable delivery pump. This system operates in a similar manner to the uniflow system but does not require the use of a directional control valve. The pump selected can deliver that hydraulic fluid in either direction by the control lever actuated by the bridge to the steering flat control system.
Compliance with requirements is important and the SOLAS requirements are listed below to comply with requirements.
SOLAS requirements on steering gear
SOLAS Chapter II-1, Regulation 29 - Steering Gear
- All ships must have two steering systems; they could be both the same but must work independently of one another. This is essential if, when one system fails, the other should be able to operate.
- All components in the system should be of sound construction and this includes the rudder stock and fixing bolts (palm bolts) if fitted; the pintle and pintle bush are also to conform to Class standards.
- The hydraulic system is protected by a relief valve set at 1.25 times the working pressure.
- The main gear and stock to be capable of steering the vessel at maximum speed ahead and putting the helm hard over to 35 degrees on one side, 30 degrees on the other side in 28 seconds.
- The steering system should not be damaged if the vessel travels in the astern direction at the maximum astern speed.
- If an auxiliary steering system is fitted instead of two identical systems, then the auxiliary system should be capable of moving the rudder 15 degrees on one side to 15 degrees on the other in no more than 60 seconds. At half ahead or at 7 knots, whichever is the greater. The auxiliary system must be power-driven.
- Control is to be provided on the bridge and in the steering flat.
- Control systems for both steering systems are to be separated as much as possible and should be able to be fully isolated in the steering flat. This normally requires the systems to be separated on the main switchboard and the conductors to be run, usually, port and starboard to the steering flat.
- The systems should be capable of being started and stopped on the bridge and be connected to the alarming system with audible sound and flashing lights.
- Communications should be fitted to connect the bridge and the steering flat. This must be a fixed system.
- The oil levels in the hydraulic tanks should be connected to the alarm system to record low.
Details on rudder stock and blade assembly
Rotary vane steering system - The steering hydraulic machinery is attached either directly to the rudder stock.
Ram steering system - The steering hydraulic machinery is connected by rapson slides to the rudder arm.
The rudder stock is vertically mounted and is fitted through a neck bearing at the bottom and a carrier bush and stuffing box at the top.
The rudder stock connection to either the tiller arm or the internal rotor of the rotary vane system is made using a tapered key and keyway that correctly arranges the rudder angle equal to both sides. Usually, this is an operating combined angle of 70 degrees, equating to 35 degrees each side of the midpoint. The bottom flanged connection of the rudder stock allows for one or more types to be used to connect the rudder mating connection. This connection is referred to as a ‘palm’ and is fitted with close tolerance fitted bolts. The fixing in these areas are fitted with additional locking devices to avoid them coming loose when the vessel is in service. These are often a locking bar welded over the hexagon heads of the fitted bolts.
The rudder blade can be arranged as either unbalanced semi-balanced (generally the preferred option with larger vessels), or balanced. The semi balanced type is often arranged using a rudder horn assembly and a horn pintle. This allows the rudder to be supported on the stern frame and does allow the bottom section of the hull to be of a balanced design. The rudder requires less power to move through the full seventy degrees of movement with the least effort.
When the rudder is removed during a docking, there are a number of items that are inspected or tested and details are supplied to Class:
- The pintle and the pintle bush are measured and checked; this will generally require crack detection in the pintle and the clearance between the bush and the pintle to be recorded.
- The watertight condition of the rudder is checked. When the vessel is taken out of the water, the top and bottom plugs are removed. It can often be noted that if the rudder is cracked, water will leak from the crack. But the rudder is left empty on most vessels. Treatment to avoid internal corrosion is generally a simple tannin treatment.
- The palm bolts are removed. These are often specified as HT stainless steel and are checked using a dye penetrant test, although the use of X-ray is often warranted.
- If the rudder stock is removed, the use of magnetic particle testing is considered to be a positive response to the problem with rudder stock repair work.
- The use of cathodic protection is universally used on the rudder blade and the condition and replacement of the anodes is conducted at each docking.
Becker Rudder System
A chosen design for rudders on vessels servicing out of ports where tugs are not available.
The vessels fitted with this type of rudder are able to turn on a smaller turning circle owing to the hinged tip of the rudder.
The Becker rudder is well known for the ease of operation and the reliability of the system.
Azipod System
The Azipod system is used on offshore vessels and at present, at least one vessel of this type is used on the northwest shelf oil field in Western Australia.
This system is operated as a diesel-electric system with the driving pods fitted externally on the hull are connected directly from the motor to the propeller.
The vessels using this system are dynamic positioning vessels. This system is also commonly used on some of the very best and biggest cruise ships. These are able to enter and leave ports without the aid of towage vessels.
Voith Schneider Steering System
The system operates on the controlled variation in the direction of thrust and the use of a skeg that holds the vessel in a direct line.
The Voith system was used on offshore vessels, tugs and ferries.
The Voith Schneider system is also used as the motive power for bow and stern thruster devices.
Azimuthing Stern Drive (ASD) Steering System
ASD drive systems are used mainly on offshore and towage vessels.
The specialist type of vessel in common use include the self-propelled cranes and production platforms used in the oil industry.
Some tugs have been especially fitted with a third engine to provide even more manoeuvrability for the vessel.
The benefit of this type of steering is that the propeller can turn over 360 degrees in all cases.
