Types
- 2 Ram type(Not used any more)
- 4 Ram type
- Rotary vane type
Basic definitions:-
- Steering gear control system:-Equipments by which orders from bridge are transmitted to steering gear power units.
The steering gear control systems comprise of transmitters, receivers, hydraulic control pumps and their associated motors, motor controllers, piping and cables.
2. Main steering gear:-Machinery, rudder, actuators, steering gear power units(to apply torque to rudder stock).
3. Steering gear power unit:-Can be electric motor, electrical equipments, and connected pump.
4. Auxiliary steering gear:-All equipments other than main steering gear.
5. Power actuating system:-All hydraulic equipments which supplies power to turn the rudder stock along with piping & fittings.
6. Maximum ahead service speed:-Maximum service speed which the ship is designed to maintain at the summer load waterline at maximum propeller rpm and corresponding engine MCR.
7. Rudder actuator:-Components which directly convert hydraulic pressure in to mechanical action to move the rudder.
8. Maximum working pressure:– Maximum expected pressure in system when steering gear is operated to comply with the requirement of being able to put the rudder from 35° on one side to 35° on the other side with the ship at its deepest seagoing draft and running ahead at the maximum ahead service speed and under the same conditions from 35° on one side to 30° on the other side in not more than 28 seconds.
Materials used in steering gear:-
Ram cylinders, pressure housings of rotary vane type actuators, hydraulic power piping, valves, flanges, fittings, and all steering gear components transmitting mechanical forces to rudder stock(like tiller).
These above materials are to be of steel(forged or cast steel) or other approved ductile materials duly tested. In general such materials is to have an elongation of not less than 12% and a tensile strength in excess of 650N/mm². Special consideration will be given to acceptance of grey cast iron for valve bodies.
Steel:-Typically 2.1% of carbon by weight in steel alloy with other allowing elements to retard movements of dislocation thus controlling hardness, ductility & tensile strength.
Theory of steering gear:-
Depending on the rudder angle a lateral force is exerted on the ship at the aft end of the ship. This lateral force acts towards starboard side, when the rudder is turned to port. This turns the ship heads to port side.
Stall angle for conventional rudder is approx. 37°. At this angle the drag force increases and longitudinal force decreases(rudder acts as brake).
Steering modes:-
- Auto pilot
- Follow up steering
- Non follow up steering
- Auto pilot:-
Here the required course is set and the auto pilot controls the steering gear system to turn the ship accordingly. Once the vessel is on course the rudder returns to midship.
If the ship deviates from the course the gyrocompass sends an error signal to the auto pilot. Auto pilot maintains the course by sending a correct signal to turn the rudder to the port or starboard.
2. Follow up:-
Used in restricted water or in poor visibility. Here the helm order is given as required by the steering wheel or joystick. Rudder turns according to helm order. The position of the rudder is feedback automatically through the hunting gear to the steering gear control. The rudder movement stops when the decreased rudder angle is reached.
Hunting gear:-Hunting gear is the feedback mechanism of steering gear which repositions the floating lever of hydraulic pump as he tiller moves to the desired position. As name suggests hunting gear moves its positions continuously based on bridge requirements and movement of rudder due to the wave force on the rudder.
If frequency of movement coincides with the natural frequency the hunting gear rod tends to oscillate, putting the entire steering gear system in unstable position.
3. Non-follow up:-
Here no signal from steering gear due to faulty transmitter or hunting lever. Once order given rudder moves and stops only when steering is put on stop position.
Q. Why is rudder angle is limited to 35°?
Ans:-Above 35° the lift forces drops and drag force increases to a great extent(acting as brake).
Q. Which components prevents rudder from exceeding the maximum angle of 35°?
Ans:-The telemotor stop.
Steering gear pumps:-
- Radial piston pump
- Axial piston pump
- Radial piston pump:-The variable stroke pump having radial piston is driven by a constant speed electric motor. Its output is controlled by a simple push pull rod attached to the floating ring in the pump. Without stopping the pump the output can be moved from zero to maximum delivery in both directions. The fluid pressure increases without any shock load on pipeline
- Axial piston pump:-The axial piston pump is driven by a constant speed electric motor. The rate of delivery and the direction of oil flow are varied by the angular movement of the swash plate. Step less changes of pump delivery from zero to maximum in either direction is achieved through lever or servo controls.
Ram type steering gear:-
Comprises of hydraulic pumps, reservoirs, cylinders and rams. The pressurized oil is supplied to the cylinder, which acts on the ram. The linear motion of the ram is converted in to rotary motion of the rudder stock by the tiller. The rudder can be actuated by two rams and cylinder or four rams and cylinders.
Two independent steering motors provided one from main power and other from emergency power(emergency switchboard).
The four ram steering gear designed with 50% built in redundancy, can be operated with two rams in case of any failure.
The tiller converts the linear motion of the rams in to rotary motion of the rudderstock. The fork side is connected to the codpiece of the ram. The codpiece slides in slots, which are machined in to upper and lower jaw pieces of the tiller. The rudder stock is connected to the other end of the tiller. The fork tiller is made of forged steel.
Cylinder houses the ram and the ram reciprocates under inside the cylinders. Pressurized hydraulic oil is supplied to the cylinder to move the ram in and out of the cylinders. The cylinder is made of mild nickel-chrome steel & ram is made of ground finish forged steel.
The hunting gear in the steering gear system is a feedback mechanism, it transmits the position of the rudder to the pump control lever through the floating lever. One end of the floating lever is connected to the hunting lever and the other end is connected to the telemotor receiver. The pump control lever is connected to the middle of the floating lever. The levers are made of forged steel.
The rudder carrier bearing is designed to support the rudder weight, allow the rudder stock to turn freely and accommodate rudder stock deflection. The rudder carrier bearing is lubricated by grease and the surface is grooved for optimal spread of grease. The bearing surface is made of bronze. Modern bearing use high elastomers which have very low coefficient of friction.
Rudder stock
The rudderstock transmits the rotary motion to the rudder. The tiller is mounted on top of the rudderstock. Keyways are provided in the tiller and rudderstock. The bottom of the rudderstock is connected to the rudder. The rudderstock is made of stainless steel.
Common faults in steering gear.
- Oil leakages
- Cylinder ram seal in hydraulic ram type.
- Seals in chamber of a rotary vane type.
- Difference in actual rudder angle and ordered helm angle
- Due to improper adjustment of control & repeat back lever.
- Unsatisfactory steering
Malfunctioning safety valves or bypass valves in system(If steering not proper fuel oil consumption will increase due to deviation from course.
- Excessive noise from steering gear
- Occurs due to air entrapment in system after maintenance, vibration also accompany.
- High oil temperature
- Will reduce oil viscosity hence hamper operation of system, caused due to low oil level in system.
- Rudder movement is within or beyond limits
Solas requirement:- 35° to 35° with ship at its deepest seagoing draught & running at maximum ahead service speed.
Due to malfunction in the limit switch fitted on the repeat back unit or on the auto- pilot.
7. Rudder angle transmitter and tiller link failure
Pre departure steering gear should be check for proper functioning.
Mac gregorhatlapaGmbh& co. Germany
Structural features
- Electro-hydraulic driven.
- 4 cylinders.
- 2 rams.
- Rudder stock diameter 550-1100mm.
- Working torque 2130-12012KNm.
- Variable displacement hydraulic pump.
- Conical hydraulic shrink fit to rudderstock(Keyway on demand)
- Installation on steel or epoxy resin chocks.
- Fastening by stopper or reamer bolts.
- In emergency easy separation & operation with 2 cylinders(manually & automatically).
Maintenance
Daily checks
- Lubrication of sliding or moving parts inspection
- Check greasing mechanism for proper functioning.
- Inspect all linkages.
- Inspect seals, connectors, pipelines for leaks
- Oil level checks
- Check temperatures & pressures.
- Check ammeter reading of pump
- Check level of replenishing tanks.
Weekly checks
- Check alarms & emergency changeovers.
- Check bridge to steering room communication on portable devices and sound powered phone.
Monthly checks
- Check & clean hydraulic oil filters or replace
3 Monthly checks
- Try out emergency steering from local control station in steering & log down in logbook.
6 Monthly checks
- Samples of oil for shore analysis.
Drydock
As per CSM complete overhauling of steering gear system which includes replacement of ram seals, measurement of bearing weardown.
Air purging in steering gear
- Breakers off
- Turning bars inserted in holes of coupling.
- Air release purge screws on cylinders partially opened.
- Pump put on stop by pressing bi-directional valve.
- Move the coupling , level in tank will fall.
- Air will escape from vent.
- Stop pressing directional valve & stop rotating pump.
- Switch breaker & start motor on manual.
- Using directional valve move steering from one side to another till liquid escapes from vent.
- Tighten purge screw & put system to normal.
Steering gear troubleshooting
- Abnormal noise in pump
- Clogged filter
- Directional control valve stuck
- Pump base foundation mountings loose.
- Internal wear of working components.
- Piping vibrations
2. Sluggish rudder movement
- Air in system.
- Solenoid of directional control valve not functioning.
- Rudder carrier bearing not lubricated hence friction.
- Rudder breached.
- Pump performance below normal.
- Leaky bypass valve
3. No rudder movement
- Pump not developing pressure.
- Lubrication not sufficient.
- Rudder carrier bearing jammed.
- Twisted rudder stock.
- Leaky valves.
- Pump worn out.
Rudder clearances measurements
Measurements of clearance of all bearing to be carried out during rudder inspection. Clearances of sleeve & bush in longitudinal direction(F-A) & transverse direction(P-S) of rudder should be measured. Methods are below.
- By lifting rudder
Here we can see both pintle & bush. The OD of pintle with external calipers & ID of bush with internal calipers to be measured at top, middle & bottom.
2. Without lifting the rudder
Measure clearance using feeler gauge between bush & sleeve.
False clearance
When measuring the pintle clearance using a feeler gauge, the measurement of clearance at the end of the bush sometimes shows a smaller value while the actual value of clearance is bigger.
Standard clearance
Pintle:-
- 1.5mm standard clearance.
- 6mm maximum allowable clearance.
Neck bearing:-
- 4mm standard clearance
- 5mm(If exceeds this replace bearing)
Bush materials:-Phenol resins are used as they make excellent water lubricated bushes.
Slack of sleeve:-Hammer sleeve on all sides if slack is found. Over 2/3rd of all surface sleeve should be replaced.
Note:-
- Top clearance should be greater than pump clearances to protect steering gear from damage in event of grounding of rudder.
- Bottom clearances should be greater than reading washer clearance.
Article by Kevin (Rex ) Fernandis Marine Engineer with 8+ years of experience.
