Steering Gear — Interactive One-Line Diagram

1. Interactive P&ID Line Diagram

A complete electro-hydraulic steering gear, from the wheel on the bridge to the rudder below the counter. The bridge console drives the telemotor transmitter; two copper pipes carry the command down to the receiver in the steering gear room, where the hunting gear converts it into a pump stroke. Two identical power units draw oil from the reservoir through suction strainers and discharge through non-return valves into a common pressure manifold. A relief valve protects the circuit, the directional control valve distributes the oil to the two rams, and the rams work on opposite ends of the tiller to turn the rudder stock. The cut-off link from the tiller closes the loop back to the hunting gear. Hover over any symbol to inspect its operation and its setting, and use the mouse wheel or the toolbar buttons to zoom and pan.

2. System Flow: From the Bridge Wheel to the Rudder

The steering gear is the one system on the ship that cannot be allowed to fail, and the whole design of the circuit follows from that. The arrangement below is the classic electro-hydraulic two-ram gear, and the order in which the signal travels is worth memorising exactly.

Stage 1: The Command Leaves the Bridge

The helmsman turns the wheel, or the autopilot supplies the signal. In the telemotor system the wheel works a piston through rack and pinion inside the transmitter, displacing oil into one of two solid drawn copper pipes. Because the oil is effectively incompressible, the receiver piston in the steering gear room follows the transmitter exactly, with no lost motion. On modern ships an electrical signal usually replaces the pipes, but the telemotor is retained as the stand-by control and is still the arrangement examiners expect you to describe. Note what does not happen: no power leaves the bridge. The bridge sends a position, and nothing else.

Stage 2: The Hunting Gear Sets the Pump Stroke

The receiver rod is connected to one end of the floating lever. The middle of the lever is linked through a spring link to the pump stroke control or the directional valve, and the third end carries the cut-off link from the tiller. When the wheel is turned the receiver moves the lever and puts the pump on stroke. As the rudder follows, the cut-off link pulls the lever back until the pump returns to neutral and the gear comes to rest. This is the hunting action, and it is why the helmsman does not have to judge when to centre the wheel: the rudder stops at the angle ordered because the feedback has already cancelled the command.

Stage 3: Two Power Units Raise the Pressure

Oil is drawn from the reservoir through coarse suction strainers by two identical power units, each a variable delivery swash plate pump on its own electric motor. A variable delivery pump does not run at a fixed output and spill the surplus; it is tilted on stroke so that it delivers exactly the oil the rudder needs, and at the ordered angle it is brought back to neutral and delivers nothing at all. That is why the gear can stand still without any valve having to hold pressure, and why it wastes almost no energy. Each pump discharges through a non-return valve so that neither unit can be back-driven by the other, and either unit alone is capable of the full steering duty.

Stage 4: Protection and Distribution

A spring loaded relief valve sits between the pressure manifold and the return. It lifts when the rudder is stopped suddenly, when a heavy sea strikes the rudder, or when the rudder is forced against its stops. It has a second and rather elegant duty: if a heavy sea does displace the rudder, oil passes through the relief valve, the hunting gear is moved by the rudder stock, the pump is put back on stroke and the rudder is restored to its previous position without the helmsman doing anything. The directional control valve then sends the oil to one end or the other of each ram, returning the displaced oil from the opposite end to the tank.

Stage 5: The Rams Turn the Tiller

The two rams are double acting cylinders, free to oscillate on chocked trunnions so they can follow the arc of the tiller. Their ram ends work through swivel blocks, which convert the straight line movement of the ram into the rotary movement of the tiller arm. Because the rams work on opposite ends of the tiller, one is pushing while the other is pulling, which balances the load on the rudder stock and removes the side thrust a single ram would impose. Each cylinder carries its own relief valves, set to limit the rudder travel to about 37 degrees each way from midships so the rudder cannot be forced against the outside stops.

Stage 6: Feedback and Emergency

The tiller position is taken back to the hunting gear as the cut-off link, and to the bridge as the rudder angle indicator. Both are taken from the rudder itself, not from the command, because what matters is where the rudder actually is. If the power supply or both power units fail, the isolating valves are used to lock the rudder and either a hand pump or relieving tackles are rigged; opening the bypass valve makes the two cylinder lines common so the rudder can be moved without forcing oil through a stationary pump. A four ram gear that loses one cylinder can be run as a two ram gear by putting the rudder amidships, isolating the circuit valves of the defective cylinder and its opposite number, and opening the vent on that cylinder.

3. Critical Components: Working Principles & Design Specs

4. Regulations: SOLAS Steering Gear Requirements

Steering gear is governed by SOLAS Chapter II-1 Part C, and the requirements are unusually prescriptive because the consequences of failure are so serious. The following are the points examiners return to.

Main and Auxiliary Steering Gear

• Every ship must have a main steering gear and an auxiliary steering gear, arranged so that the failure of one does not render the other inoperative.
• An auxiliary steering gear need not be fitted where the main steering gear has two or more identical power units and is arranged so that, after a single failure in its piping system or in one of the power units, steering capability can be maintained. In the case of passenger ships the remaining capacity must meet the operating conditions with any one power unit out of operation.
• For large tankers, chemical tankers and gas carriers, two or more identical power units for the main steering gear are mandatory.
• Where a separate auxiliary steering gear is fitted it must be capable of putting the rudder from 15 degrees on one side to 15 degrees on the other in not more than 60 seconds with the ship at its deepest draught running ahead at half the maximum service speed or 7 knots, whichever is greater.

Rudder Movement

• The main steering gear must be able to steer the ship at maximum ahead service speed and, at that speed and the deepest service draught, put the rudder from 35 degrees on one side to 30 degrees on the other in not more than 28 seconds. The apparent anomaly in the angle allows for the difficulty of judging when the final position is reached, because feedback from the hunting gear shortens the variable delivery pump stroke as the rudder approaches the ordered angle.
• On passenger ships the main steering gear must be able to move the rudder from 35 degrees on one side to 35 degrees on the other at maximum service speed.
• Where the rudder stock, excluding any ice strengthening allowance, is required to be 120 mm diameter at the tiller, the steering gear must be power operated.
• Where the rudder stock is required to be over 356 mm (14 in.) diameter, a separately fitted auxiliary steering gear must itself be power operated.

Power Supply and Control

• Two widely separated power circuits must be provided from the main switchboard, one of which may pass through the emergency switchboard. Only short-circuit protection is provided for the motors and power circuits — no overload protection that could take a steering pump off the board when it is most needed.
• Where the rudder stock is required to be over 230 mm diameter at the tiller, an alternative power supply capable of operating the rudder must be provided automatically within 45 seconds. It must supply the power unit, its control system and the rudder angle indicator, and its capacity must be at least 30 minutes for ships of 10 000 GT and over and 10 minutes for other ships.
• Control of the auxiliary steering gear, where power operated, must be arranged from the bridge and be independent of the main steering gear control system. It must be possible to disconnect any bridge control system from within the steering gear room, and to bring the system into operation from the bridge.
• A rudder angle indicator must be fitted on the bridge, driven from the rudder itself.

Hydraulic System Requirements

• Hydraulic power systems must be provided with arrangements to maintain the cleanliness of the hydraulic fluid.
• A low level alarm must be fitted on each hydraulic fluid reservoir, giving an early audible and visual indication on the bridge and in the engine room of any hydraulic fluid leakage.
• Power operated steering gears require a storage tank arranged so that the hydraulic systems can be readily re-charged from a position within the steering gear compartment, of sufficient capacity to recharge at least one power actuating system.

Testing Before Departure

Except for ships regularly engaged on short voyages, the steering gear must be thoroughly checked and tested within 12 hours before departure. The tests include the power unit and control system failure alarms, the emergency power supply where relevant, and the automatic changeover arrangements. The result is recorded. This is a routine item for port state control, and the drill is usually carried out at the same time as the emergency steering drill.

5. High-Frequency Oral Exam Questions

1. Sketch and describe an electro-hydraulic steering gear, and explain how the rudder stops at the angle ordered.

   The wheel on the bridge works a piston in the telemotor transmitter through rack and pinion, displacing oil along one of two solid drawn copper pipes to the receiver in the steering gear room. The receiver rod moves one end of the floating lever of the hunting gear. The middle of the lever is connected through a spring link to the pump stroke control or directional valve, and the third end carries the cut-off link from the tiller. Moving the lever puts the pump on stroke, and the pump discharges through a non-return valve into a common pressure manifold. A relief valve protects the manifold, and the directional control valve sends the oil to one end of each ram while returning the displaced oil from the other end. The two rams work on opposite ends of the tiller through swivel blocks, and the tiller turns the rudder stock and rudder. The rudder stops at the ordered angle because of the hunting action: as the rudder follows, the cut-off link pulls the floating lever back until the pump returns to neutral and delivers nothing. If a heavy sea displaces the rudder through the relief valves, the hunting gear is moved by the rudder stock, the pump goes back on stroke and the rudder is restored to its previous position.

2. What are the SOLAS requirements for the main and auxiliary steering gear?

   Every ship must have a main and an auxiliary steering gear arranged so that the failure of one does not render the other inoperative. An auxiliary gear need not be fitted where the main gear has two or more identical power units and can still steer after a single failure in its piping system or one power unit; for large tankers, chemical tankers and gas carriers, two or more identical power units are mandatory. The main gear must be able to steer at maximum ahead service speed and, at that speed and deepest draught, put the rudder from 35 degrees on one side to 30 degrees on the other in not more than 28 seconds; on passenger ships it must go 35 degrees to 35 degrees at maximum service speed. The auxiliary gear must put the rudder from 15 degrees on one side to 15 degrees on the other in not more than 60 seconds at half the maximum service speed or 7 knots, whichever is greater. Power operation is required where the rudder stock is required to be 120 mm diameter at the tiller, and a separately fitted auxiliary gear must be power operated where the stock exceeds 356 mm. Two widely separated power circuits must be provided with short-circuit protection only, and where the stock exceeds 230 mm an alternative power supply must cut in automatically within 45 seconds with a capacity of at least 30 minutes for ships of 10 000 GT and over and 10 minutes for other ships.

3. Why are two power units fitted, and how do they interact?

   Because steering is a vital service and a single pump failure must not leave the ship unable to steer. Two identical power units are fitted, each with its own electric motor, suction strainer, pump, discharge non-return valve and isolating valves. Each unit alone is capable of the full steering duty, so with one unit out of service the ship still steers, which is what allows an auxiliary steering gear to be dispensed with. The discharge non-return valves stop one unit feeding back into the other and driving it backwards, so either pump can run alone, or both can run together to double the rudder rate. The two motors are supplied by two widely separated power circuits from the main switchboard, one of which may pass through the emergency switchboard, with short-circuit protection only. The units can be started and stopped from the bridge and from the steering gear room, and it must be possible to disconnect the bridge control from within the steering gear room.

4. How is emergency steering carried out, and how is the gear lined up for it?

   Emergency steering is provided either by a hand pump in the steering gear room or by relieving tackles. In both cases the rudder first has to be locked and the two cylinder lines made common. The isolating valves on each ram line are closed to lock the rudder, which is what allows the tackles to be rigged and the rudder to be worked by hand. The bypass valve is then opened so that the two cylinder lines are common, which lets the rudder be moved without having to force oil through a pump that is not running. If a hand pump is fitted, the changeover is set to emergency and the pump draws from the same reservoir and discharges into the pressure manifold. A four ram gear that loses one cylinder can be run as a two ram gear by putting the rudder amidships, isolating the circuit valves of the defective cylinder and its opposite number — which must not be in a diagonal position — and opening the vent on that cylinder. The gear must be tested within 12 hours before departure except on short voyages, and the drill includes checking the power unit and control system failure alarms and the emergency power supply where fitted.

5. Why is a variable delivery pump used rather than a constant delivery pump?

   A variable delivery pump, normally a swash plate axial cylinder type, has its stroke set by the pump control, so it delivers exactly the quantity of oil the rudder needs and nothing more. At the ordered angle the swash plate is brought back to neutral and the pump delivers nothing at all, so the gear stands still without any valve having to hold pressure, and almost no energy is wasted. A constant delivery pump runs at a fixed output and its surplus oil has to circulate through a bypass until the control valve calls for it, which wastes energy and heats the oil, and it needs a control valve that either delivers full pump output to the gear or bypasses it completely. The constant delivery arrangement is simpler and cheaper and is used on smaller installations, but the variable delivery pump is the better engineering solution and is what the follow-up control is designed around.