Steering gear is the machinery used onboard to control the movement of the ship from port side (left side) to the starboard (right side) while the vessel is moving. The steering gear doesn’t produce any effect while the vessel is stationary.
The steering gear provides a movement of the rudder in response to a signal from the bridge through the control equipment, which conveys the rudder ordered signal from the bridge and activates the steering gear to move the rudder to the desired angle.
The system can be of different types (hydraulic, electro-hydraulic, electric), but mostly on vessel you will find electro-hydraulic type. The most common steering gear type met on vessels are 2 rams, four cylinder Rapson slide type and electro-hydraulic rotary vane type.
In the first case, the steering gear is composed of two rams with four hydraulic cylinders that are powered by oil supplied by two electrically-driven pumps, one of which is a back-up pump. The pumps have axial pistons with variable displacement and separate hydraulic oil tanks. Each primary pump is linked directly to an auxiliary gear-type pump that supplies control oil to the servo system and also circulates the oil and cools the pump casing. Both pumps are capable of putting the rudder through the working angle in the specified time. In normal operation at sea, one pump is enough to run, but the second pump unit can be connected at any time by starting the motor. When maneuvering or operating in confined waters, both pumps should be in service.
In the second case the steering gear system consists of an actuator, power units, electro/hydraulic control system and associated electric components. The actuator has a vane rotor and the hydraulic force is transferred to the rudder stock/rudder by means of a frictional connection between the rotor hub and rudder stock taper. There are two sets of pressure and return chambers in the actuator/rotor housing. Internal oil distribution is accomplished through the use of channels in the cover that direct oil to the pressure chambers and away from the return chambers. Control valves open and close oil flow channels; there are four cross-connection valve blocks mounted on the cover, and all valves are open during normal full operation, but two are closed if the steering gear is divided into two units. The dividing walls of the oil pressure chamber act as mechanical stoppers for rotor movement and rudder angle limitations.
The rotor has two upper and lower radial bronze bearings, as well as one vertical bronze bearing rudder carrier which are lubricated by the hydraulic oil from the pressure chambers. The rotor has two vanes, and the housing is divided into two cylinders by two mechanical stoppers that are bolted into the housing. Spring-loaded cast iron sealing bars are used to seal the pressure chambers and the gland seals are used to connect the actuator and the rudder stock. On top of the actuator is a header/expansion tank that serves as an oil reservoir tank for the system oil. The system includes two motor and pump units which consist of an electric motor, a hydraulic pump, an oil reservoir, and filters. Electric solenoid valves and hydraulic spool valves directly mounted on the actuator housing control the rudder.
In case of four rams steering gear an automatic isolation system is built into the system. The hydraulic systems are linked by solenoid-operated isolating valves, which allow both systems to work together to produce the torque required to move the rudder under normal conditions. When one of the systems fails and there is a loss of hydraulic fluid, the float switches in the affected hydraulic tank are activated and sends a signal to the isolation system, which divides the steering gear into two distinct systems.
The faulty system is isolated and the pump is turned off, while the intact system continues to function normally and steering capability is maintained, but only half of the rudder torque is available. When the oil-hydraulic piping failure causes the ‘Low’ level switch to be actuated, the isolation valve goes into operation to separate the hydraulic oil circuit into No.1 system and No.2 system, thereby reducing the steering capability to 50%. Therefore, upon sounding of the ‘Low’ level alarm, either promptly reduces the ship speed or, if the ship continues going full ahead, limit the steering angle to within 15°. It is important to know that the oil tank ‘Low’ level switch for visible and audible alarms is provided in accordance with ship classification society rules and regulations, and the one for auto-isolation of the system are separately installed and are independent of each other. Both, nevertheless, are actuated upon reaching the same oil level.
In case of rotary vane steering gear type, the steering gear is designed to meet a requirement of double separation when in operation as well. It is possible to separate (set in bypass) one rotor vane and operate the steering gear with the other vane. It is also possible to separate (set in bypass) one stopper and operate the steering gear by means of the other stopper. These separation arrangements effectively give a single cylinder operation with pressure oil on one side of a vane, or stopper, and vent oil on the other side of the vane, or stopper. The separation of one vane is a fully automatic function, with activation being brought about by failure of the oil distribution system, but vane separation may also be done manually from controls located on the bridge. The steering gear can work as two single actuators, or as two actuators operating on different sides of the same stopper. In the wheelhouse panel there are MAN SEP1 and MAN SEP2 pushbuttons to allow for these forms of separation.
At the main panel in the steering gear room there is a Test/Function switch, which enables all the different arrangements to be tested. Each pump may be operated with separated vanes and separated stoppers. Normally the switch should be in position NORMAL operation. Other switch positions are available but these are for testing and simulating separation conditions only and should not be used for normal operation of the steering gear.
The autopilot control or hand steering from the wheelhouse are used to control the steering gear, from where all commands are transmitted electrically to the steering compartment and the actual position signal for the systems is supplied by steering gear feedback transmitters. Limit switches limit the rudder’s operational angle of movement to 35° port and 35° starboard, but mechanical stops are also installed to physically limit any movement greater than 37.5 degrees port or starboard.
The variable-flow pumps are controlled by an electric torque motor, which is linked to the servo valve via the control lever, which activates the servo piston of the tilting lever for the piston thrust (swash) plate. This moves the pistons out of neutral and causes oil to be discharged to one pair of hydraulic rams while suction is taken from the other. The feedback linkage causes the pump’s tilting lever to move as the rudder turns, reducing the pump’s stroke. When the rudder has reached the desired angle, the tilting lever is returned to neutral, and oil delivery from the pump is stopped.
In case of rotary vane type the electrical signals activate solenoid valves to direct pressure and vent oil to and from the hydraulic cylinders formed by the vanes, casing and stoppers. Steering gear feedback transmitters supply the actual position signal for the systems, allowing the bridge actuator to know the actual position of the rudder at all times. Under normal circumstances both vanes will be in use, with oil supplied by any one or two of the pump units.
When maneuvering in confined waters, it is compulsory that two pump units are running, in order to achieve the IMO required rudder movement of 35° one side to 30° the other side in 28 seconds (with one pump in 56 seconds), but usually the systems are designed to perform these movements in shorter time (e.g in 24 seconds or 48 seconds respectively).
In addition to the central steering console, the wheelhouse control panel is provided with an FU ((Follow up) Tiller unit and a NFU (Non Follow Up) Tiller unit. The bridge wings are provided with FU Tiller and NFU Tiller units, allowing for control of the steering from the bridge wings. The FU Tiller units allow for normal (follow-up) steering from the wheelhouse console or the bridge wings. The NFU Tiller units are emergency controls and allow for emergency (or non-follow-up) control of the steering gear.
The emergency procedure used depends upon the nature of the failure. If the bridge steering unit or transmission system has failed but the steering gear is still fully functional, steering may take place by means of the NFU Tiller unit in the wheelhouse or bridge wing panels. An alternative is to use the NFU tiller in the steering gear room.
If the steering control system has failed and the NFU tiller system is inoperative, the hydraulic system must be operated manually and this involves the local control of the pump directional pilot valves. The pushbuttons at the ends of the solenoid valves must be pressed or the lever for control oil pumps, one for port and one for starboard turning of the rudder.
Instructions must be transmitted to the steering gear room from the bridge by telephone. As the pump directional pilot valves are controlled individually by hand, it is not possible to ensure that control of two pumps would take place at exactly the same time. Only one pump unit must be operating when in emergency control from the steering gear room.
Company instructions regarding the operation and checking of the steering gear under all conditions are displayed at the steering gear local control panel and these must be complied with at all times.
These normal checks are:
- Check the oil level in the pump tanks and refill as necessary. The level should not exceed ¾ of the glass at the normal working temperature.
- Check the system pipework and valves for leakage.
- Check the lubrication system and replenish the grease injection pump tank as required.
- Note the temperature of the oil in the hydraulic system.
- Visually check the components such as the indicators and linkage arms.
- Check the gauges for any abnormal readings.
- Check for abnormal noise when the rudder is moving.
- Ensure that the rudder carrier bearing grease system is functioning correctly and that there is sufficient grease in the container.
- Check the tightness of all coupling bolts and pipe connections.
- Check the settings of limit switches.
- Carry out a function test on the entire alarm system (see the manufacturer’s operating manual for full details).
- Check that the rudder indicator is functioning correctly.
- Check the ram V-packing and pump control unit oil seals.
- Check the stop valves and isolating valves.
Even though the steering gear can be started and stopped from the bridge the duty engineer should be present at such times and when pre-departure tests are undertaken. Any defects which are not serious enough to activate an alarm may then be detected by the duty engineer.
Whilst at sea and the steering gear is being operated with one pump, the operating pump must be changed over daily so that all three pumps have equal running hours.
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