STEERING GEAR:

Steering Gear

Guide Book for Marine Engineers

STEERING GEAR:The steering gear system is the most important system on board ship. The steering gear consists of the following:

Telemotor System: The telemotor system is a system, which transmits a mechanical order (turning the steering wheel) to a distant system called the Actuating System; which in turn works to turn the rudder by a corresponding angle. The telemotor system comprises of transmitter and receiver.

Actuating System: The actuating system is the gear which converts the power force into torque and hunting mechanism helps to maintain the rudder in the position ordered against the actions of water, wave and propeller force. The actuating system comprises the power or amplifying units, the actuating mechanism and the hunting `mechanism

Tiller: The solid type tiller is of cast steel of adequate strength and fixed to rudderstock with key. The end of the tiller slides in the universal bearing of ram-pin with which they are connected.

Hydraulic Cylinder and Ram: The steering gear has one or two pair of opposing hydraulic cylinders, which are of nodular cast iron, and a ram. Each pair of cylinders is tied together axially by a guide bar and adjacent cylinders are held together by distance pieces. The ram is of carbon steel and machined accurately. The special packing is fitted for oil-seal between hydraulic cylinder and ram. The ram is so constructed as to touch the mechanical stopper on cylinder bottom at 37o for 35o maximum working rudder angle.

Ram -pin: Ram-pin, which of special steel, is inserted in the middle of ram and ram-pin bush is provided between it and the fork part of tiller arm. Thrust force of ram is transmitted to the tiller arm through ram-pin and its bush.

Hydraulic Pump: The hydraulic pump used is a servo controller variable-stroke pump is called radial piston pump or axial piston pump, and equipped with the hydraulic units for the steering gear. The hydraulic pump, consists of Hele Shaw or Janny pump, an auxiliary gear pump, relief valves, by-pass valves, a servo controller and filters.

Axial Piston Pump or Swash Plate Pump or William Jenny Pump:

The name axial piston pump is given because the plungers reciprocate parallel to the axis of the pump i.e. in an axial direction and the swash plate or the tilting box is also displaced in an axial direction.

a 0 a

Pump Control Rod

Port "T"

Ball joint

Stationary

Valve plate

Shaft

Port "B"

Cylinder Plunger Socket ring

Description:

It consists of a revolving cylindrical barrel or block, which has a number of cylinder bores.

This barrel is kept pressed against a valve, which has segment shaped by a strong spring.

These ports end on the other side of the valve plate in circular ports, which are connected to the steering gear ram cylinders by pipes.

The cylinder barrel-driving shaft is splined and coupled to a unidirectional constant speed motor.

This splined shaft also supports a double universal joint, which carries a socket ring fitted inside a tilting box or swash plate.

The cylindrical barrel has 7, 9 or 11 bores machined in it, which are parallel to the axis of rotation and concentric with it. In each cylinder reciprocates a lapped plunger, which is hollow and ends in a ball head, which fits in a socket as a ball joint in the tilting box.

Working Principle:

The constant speed motor rotates the shaft along with the cylinder barrel and the socket ring.

When the tilting box or swash plate and therefore the socket ring is at zero position or vertical position, the cylinder barrel and the socket ring revolve in the same plane and the plungers have no relative motion. So they do not reciprocate in the cylinder bores, thus resulting no pumping action.

Tilting the swash plate causes the plunger to move in and out axially for each revolution of the motor. When the swash plate is tilted to right side, the top ports "T" become suction and bottom ports "B" become discharge. Again when the swash plate is tilted to left side, consequently the ports "T" become discharge and ports "B" become suction with the same direction of rotation.

During outward suction stroke, oil is drawn from the steering gear cylinders into the pump and is delivered to the cylinders of the other side during the succeeding pressure stroke when the plungers move in the opposite direction.

Flow is reversed when the angle of tilt is reversed. The stroke of the plunger depends on the angle of tilting of the swash plate, so the pump is variable type.

Maximum tilting angle is 15.5o.

Radial Piston Pump or Hele Shaw Pump:

The radial piston pump is given because its plungers are positioned radially to the main axis of the pump.

Gudgeon pin Hollow shaft

Slot Plunger

Fixed Shaft

Cylinder bore

Slipper

Actuating rod

Roller bearing

Suction / Delivery Port

Description:

The Hele Shaw Pump consists of a fixed and stationary shaft with a block having 2 ports, one at the top and other at the bottom which act as the suction / delivery ports.

Around the stationary shaft there is a hollow shaft, which forms the cylinder body or block, coupled to and driven by a constant speed unidirectional motor.

The cylinder block has a number of radial cylinder bores machined in it. There are usually 7 or 9 such bores. In the bores fit lapped plungers with gudgeon pins, which can move in and out of the bores restricted by the slot length.

The gudgeon pins have slippers fitted on top and bottom and these run in the annular groves of two circular floating rings. These floating rings are not rigidly fixed but are free move sideways on roller bearings. This sideways movement or radial movement of the floating rings is received as the actuation of the control rod or spindle, which is connected to the receiver output through levers and links.

Direction of Rotation

Actuating rod

E G

2, 1, 3 Position of Centre

Working Principle:

When the motor runs it rotates the cylinder block with it in the direction shown. When the centre of the block or shaft coincides with the centre of the floating rings, the plungers rotate at a fixed radius from the centre and there is no relative motion between the plungers and the shaft. Thus there is no pumping action.

If the floating ring is moved to the right by the actuating rod, the centre of the plunger shift from "O" to "A" which is eccentric to the centre of the shaft. This means the greatest distance of plunger gudgeon from "O" is "OG" and shortest distance is "OF". With the direction of rotation as shown in figure, in travelling round to "F" the plunger is moving in relative to the fixed shaft and hence the top port "T" act as discharge and when travelling from "F" to "G" the plunger is moving out relative to the fixed shaft and therefore, bottom port "B" acts as suction.

The floating ring is moved to left so that the centres of the plunger shaft from "O" to "C". then the shortest distance of plunger gudgeon from shaft centre is "OD" and greatest distance is "OE". While revolving from "D" to "E" the plunger is moving out and top port "T" is suction, and when revolving from "E" to "D" the plunger is moving in and hence bottom port "B" is acting as discharge port.

Thus as the stroke of the plungers depends on the movement of the slipper path horizontally and hence the eccentricity, the pump can be said to be of the variable delivery type.

Direction of flow depends on the movement of the rings to the left or right of the central position, therefore for a unidirectional pump, the flow is reversible.

Advantage of axial flow over its radial flow:

1. Produce greater torque than radial displacement type, most suitable for larger ship, which need greater torque in holding the rudder.

2. For power requirement, it gives reduced size, lower cost and lighter in weight.

3. No heating of oil since no churning effect.

4. Additional gear pump is not required.

5. Better weight-torque ratio than radial flow type.

6. Swash plate running temperature is less but incase of radial flow, radial bores churning the oil raises the temperature.

Auxiliary Pump:

The auxiliary pump is a trochoidal gear pump, which is intended for supplying the oil under pilot pressure to the servo controller, the transfer valve, the oil under boost pressure to the main hydraulic pump suction and so on.

Servo Controller (Servo valve and servo motor):

The servo controller serves to incline the swash plate of the hydraulic pump or keep it at standstill. The servo controller consists of valve case, valve, centering spring, piston, link, arm, control shaft and so on. The sleeve moves left and right through the arm when the control shaft rotates and the sleeve can open or close the two ports A and B. according to the opening or closing of the port, the oil under pilot pressure led from the auxiliary pump to the port.

Steering Gear: Two Independent And Separate Power Actuating Systems 100% Torque Each

Each Capable of 35o to 30o in 28 seconds

SRV SRV

100% 100% Torque Torque

By-pass By-pass

Valve Valve

SV SV

SV SV SV SV

Main Aux. Aux. Main

Pump Pump Pump Pump

LL LL

Transmitter

Telemotor pump

Telemotor Actuating Cylinder

Emergency

Steering

3

SV SV

2

1

4 4

Rudder

Angle Ind.

5 5

50% Torque 50% Torque

TWO IDENTICAL POWER ACTUATING SYSTEMS WITH 50% TORQUE EACH

Both Together Capable of 35o To 30o in 28 Seconds

Steering Gear with PID Controller:

SP (Course) Hand MV

Gyro

Compass NFU Integrator ( P S Weather

Rudder Multiplier

Rate Integrator

Rudder Order signal

Indicator Feed back

Telemotor

Telemotor Actuating Cylinder

Emergency

Steering

3

SV SV

2

1

4 4

Rudder

Angle Ind.

5 5

50% Torque 50% Torque

Telemotor Fluid:

Good quality mineral lubricating oil is used with the following properties.

1. Low pour point ( -30 oC.

2. Non sludge forming.

3. Non corrosive.

4. Good lubricating properties.

5. High flash point ( 150 oC.

6. Low viscosity to reduce frictional drag but not too thin to make gland sealing difficult ( 12 cSt @ 50 oC.

7. Density 880 kg / m3 @ 15.5 oC.

Safety Arrangement of Steering Gear:

1. Shock Relief Valve: Opens 15 ~ 20 % higher than working pressure

2. By-pass Valve : Opens 20 ~ 30 % higher than working pressure

3. Line Relief Valve : Opens 40 ~ 50 % higher than working pressure

4. Emergency power supplies to one motor.

5. Steering gear room bilge alarm.

6. Steering gear compartment drain valve spring loaded non-return.

7. Alarms: Alarms should be audible and visible in the both control stations (WH & ECR)

a) Single phasing alarm - SOLAS.

b) Motor overloading 100% - SOLAS.

c) Power failure.

d) Auto pilot failure.

e) Oil tank low level alarm

Cushioning Arrangement:

Ship moving through bad weather, the rudder is subjected to additional shock loading from waves, etc. The cushioning arrangement is provided by shock relief valve.

In bad weather, the shock loading on the rudder is transmitted to the ram through the rudderstock and tiller. The sudden increase of pressure of oil in one cylinder is relieved through the shock relief valve to the other cylinder. Therefore, allowing the rudder to yield without causing any damage to the steering gear.

Each cylinder is provided with a shock relief valve. The opening pressure of the shock relief valve is 15 ~ 20% more than normal working pressure of the system.

The shock relief valve can handle smaller volumes of oil.

Relief Arrangement:

1. Shock Relief Valve: Each cylinder is provided with a shock relief valve. In bad weather condition, shock loading on rudder is relieved through the shock relief valve. Shock relief valve can handle smaller volume and opens at 15 ~ 20% more than normal working pressure.

2. By-pass Valve: Each pair of cylinder is provided with a double acting by-pass valve, which acts as a by-pass valve in open position and acts as a isolating valve in closed position. The by-pass valve can handle larger volume and opens at 30 ~ 40% more than normal working pressure.

3. Line Relief Valve: The line relief valve takes care of any excessive pressure in the line created by overrunning of the pump or accidentally shutting off the cylinder isolating valve. The line relief valve is situated in the valve block of the hydraulic pump and communicates the two main hydraulic lines. Line relief valve usually opens at 40 ~ 50% more than normal working pressure.

Cause of Air entering into steering gear hydraulic system:

Air may get into the steering gear hydraulic system due to:

1. Defective gland of the ram cylinder: If the gland is not holding during the suction stroke of that cylinder, air may ingress into the system.

2. Oil tank low level.

3. Improper charging of oil into the system.

4. Union nut is slack due to vibration.

5. Gassing up of hydraulic fluid due to local high temperature in the system.

State with reasons how piston and cylinder wear in the pump effect the steering gear.

If the piston and cylinder wear in the pump the following problems occur in the steering gear system:

1. Sluggish action of rudder: Piston and cylinder wear will cause leakage of oil during the pressure build up stroke of the piston. Therefore, the pump will take longer time to build up required torque for turning the rudder. As a result, rudder will take more time to turn.

2. Hunting: Due to piston and cylinder wear, there will be slippage of oil through. Rudder can not be kept in helm as the pump can not provide effective hydraulic locking. So, rudder can be easily displaced from the required position by the action of waves etc. Hunting gear will take action to return the rudder in ordered position.

Why the steering gear pump is constant speed and variable stroke type?

The pump used for steering gear system is constant speed and variable type due to following reasons:

1. A positive pressure is always available irrespective of load changes. Variable stroke for variable pressure demand for all load conditions from 0 ~ 300 bar.

2. Less wear and tear takes place as the pump runs continuously at constant speed.

3. Quick and accurate response to pressure demand.

4. Quick reversal of pumping action over the full pressure range.

Purpose of Hunting Gear:

1. Hunting gear floating lever mechanism is required to bring the rudder to the ordered position.

2. Also this mechanism is required to position the rudder in its ordered position when the action of water, waves or propeller force displaces the rudder from its ordered position.

Working principle of Hunting Gear:

Let us consider the hunting gear floating lever as RPT. The pump-actuating lever is connected to the middle P of the floating lever. The top of this lever is R is connected to the output of the telemotor receiver through a linkage. The bottom of the lever T is connected to the tiller arm as a feed back link through a buffer spring. The floating lever can pivot at R and T.

When the receiver output signal pushes the point R of the floating lever to R' with RPT fulcrum about T; thus pump control point P move to P'. this would actuate the pump and the pump will start pumping causing the rudder to rotate. When this happens, tiller feedback point T start to move towards T' with RPT fulcruming about R'. As this happens, P' will start coming back towards P and pumping action will slowly reduce and stop when P' reaches P. Pumping having stopped, would stop the rudder at ordered angle.

R R' R R' R R R R R

T' T T T' T T' T T T' T

When the receiver output shift the point R' to R (midship), consequently pump actuating point P shift to P" in the other direction. Thus the pump is working in the reverse direction. The rudder start rotating in the opposite direction and the feed back point T' moves towards T causing P" coming back to P and the pump will stop at midship position and RPT would come back it's original vertical position.

In bad weather, when the rudder is displaced from its ordered position, say midship, the rudder feedback point T shift to T' with RPT fulcrum at R. this causes the pump actuating point P shifting to P', hence the pump works and rudder turns in the opposite direction and the point T' moving towards T. when T' comes back to T and P' to P, then the pump stops working and the rudder is brought back to its ordered position.

Therefore, the hunting gear brings the rudder to its ordered position against the water, wave etc.

Why spring links are incorporated in the hunting gear?

1 The spring called buffer spring is incorporated in the hunting gear links to take up any excess movement beyond the maximum stroke of the pump. This extra movement is stored by the compressed spring and reset when hunting gear approaches the no-effect point to prevent the mechanical damage of the pump.

2 Also buffer spring will take up the shock movement of rudder due to heavy sea, thus preventing excessive hunting action of pump.

How rudder movement is confined within port and starboard stops?

Rudder movement is confined within port and starboard stops by various limit switches and mechanical stops to keep rudder between 35o port and 35o starboard.

1. Bridge telemotor transmitter mechanical stop (rack travel limited).

2. Auto pilot mechanical stop.

3. Local control mechanical stop.

4. Actuating mechanism mechanical stops (limit on ram travel).

5. Stern-post mechanical stop (excessive movement of rudder is limited by stern post set at 36o on each side).

Why maximum angular movement of the rudder is generally limited to a comparatively moderate angle port and starboard?

The normal force acting on rudder area is resolved to lift force and drag force. The lift force is responsible for steering the vessel, where the drag force tends to retard the ship as its direction is opposite to that of ship's motion.

The magnitude of lift and drag force for a given rudder and ship's speed changes with the angle of rudder as shown below.

For rudder angle up to 38o, the lift force increases steadily and drag force also increases but a slower rate than that of lift force.

But beyond 40o of rudder angle, the drag force increases rapidly and lift force drops rapidly. So beyond 40o of rudder angle, this result in the stalling of the ship and the ship loses speed and does not respond to steering.

Therefore, rudder angle is normally restricted to within 35o to port and starboard to avoid stalling effect of rudder and to maintain high lift to drag force ratio.

Drag Force

Lift and Drag

Force

Lift Force

35 40 45 Angle of Rudder

Explain how limited amount of rudder drop is accumulated in steering gear?

1. Ram Type Steering Gear:

a) In ram type steering gear, the rudder drop is accumulated in the clearance space provided between ram crosshead and pivoted swivel bearing as shown in the figure.

b) On the rudder itself, clearances are also provided for the rudder to lift and wear down.

Ram crosshead

Rudder stock Stator flange

Tiller arm Top

Anchor

Clearance Synthetic rubber

Bottom Anchor bolt

12 ~ 19 mm

wear down

allowance

Ram Type Steering Gear Rotary Vane Type Steering Gear

2. Rotary Vane Steering Gear:

a) In rotary vane steering gear, the stator is anchored to the ship's structure to resist the torque and also free to move vertically as shown in the figure.

b) A vertical clearance is provided between the inside of the stator flanges and the top and bottom of the anchor bracket to allow for the vertical movement of the rudder stock due to wear and lift.

c) The clearance varies with the size of the unit but is approximately 38 mm. The top clearance is called rudder drop and the bottom clearance is called rudder lift.

Steering Gear (Two parts)

Telemotor System

(2 Parts)

Actuating System

(2 Types)

Transmitter

Single Motor System

Electric

(Old Type)

Receiver

Ward Leonard System

Rotary Van Type

Electro Hydraulic

Ram Type

4 Ram Type

2 Ram Type

Feed Back

Closed under normal running condition

Open after power failure

Main and Auxiliary Pumps: Fixed displacement (not variable) Constant speed and unidirectional

Main Pump: Screw type

Aux. Pump: Gear type

SV = Solenoid Valve

LL = Low Level Alarm

Motor

# 2

Motor

# 1

# 2 Control Panel

# 1 Control Panel

Main Control Panel

Under Normal Running Condition

Auto Solenoid Valves 1, 2, 3 are operational

By-pass Valves 4 closed

SV = Solenoid Valve

Receiver

Motor

M

M

Gyro Pilot Computer

Rudder Order Computer PID

Rudder Servo Amplifier

Motor

M

M

F

D

P

11 Md. Abdul Hamid