Lesson 18Steering gear()

The steering gear is one of the most important services in the ship and its main function is to control the ships course.

There are four main types of steering gears employed for power operation of the rudder(),

(i) the steam steering gear, (ii) the hydraulic steering gear,(iii) the electric steering gear and (iv) the electro-hydraulic steering gear.

The total system may be considered made up of three parts: control equipment, a power unit and a transmission to the rudder stock().

RudderRudder stockTransmissionunitPower unitControl equipmentTiller

The first part conveys() a signal or desired rudder angle from the bridge (wheel house).

The second one provides the force, when required, to move the rudder to that angle.

The third is the means by which the movement of the rudder is accomplished.

According to the way the steering gear is operated, the steering gear may be divided into three kinds: hand-steering, followup () steering and automatic steering.

The electro-hydraulic gear is now the most widely applied.

A two ram() hydraulic gear consists of a hydraulic ram situated on the port side of the tiller() and another ram on the starboard side,

linked at their outer ends to the tiller arm by a crosshead and swivel block(),

RudderstockTiller armRamHunting gearCrosshead &swivel blockControl rod

Crosshead &swivel block

the other ends of the rams working inside their own hydraulic cylinders and pipes connect these cylinders to a hydraulic pump.

The tiller is firmly keyed to the rudder stock.

The pump is of special construction and may be axial or radial design.

The pump runs continuously in the same direction driven by an electric motor,

and the position of a movable plate or a floating ring inside the pump controls the suction and discharge of the oil.

When the plate or ring is in mid position, no oil is drawn;

when the plate or ring is moved in one direction from mid position, oil is drawn from one cylinder and discharged into the other;

when the plate or ring is moved in the opposite direction, the suction and discharge of the oil is reversed in direction.

The plate or ring is actuated by a control rod which is attached at its outer end to the hunting gear().

A four-ram hydraulic steering gear may be fitted in large ships for greater steering power, instead of the two-ram type.

It is simply a double two-ram unit, so that the force of two diagonally opposite rams can act on the tiller to produce double the turning effect.

The pump control is moved by the telemotor() through a floating lever.

The other end of this lever is connected through a safety spring link to the rudder stock or tiller.

The telemotor is the receiver of the hydraulic remote control system from wheel on the bridge.

The linkage through the floating lever of telemoter, pump and rudder stock forms the hunting gear.

The pump is only required to deliver oil when the steering wheel is moved.

The hunting gear returns the pump operating rod to mid position as soon as the helmsman() stops turning the wheel.

When the rudder has moved through the angle corresponding to the wheel position, it will remain there until the wheel and telemotor are moved.

If a heavy sea strikes the rudder, the shock is transmitted through the tiller to the rams, this cause a sudden increase in oil pressure in one of the cylinders and the system.

If the pressure in the system rises to about 10 per cent above normal, double spring-loaded relief valves allow the tiller to give way slightly by by-passing a little of the oil into the other cylinder.

The resultant displacement of the rudder, tiller and ram crosshead moves the pump control rod through the hunting gear and the tiller is automatically brought back to its proper position.

The control equipment of steering gearsTelemotor control The telemotor has become, on many vessels, the standby steering mechanism, used only when the automatic steering fail.

It comprises a transmitter(), a receiver, pipes and a charging unit().

The transmitter, which is built into the steering wheel console (), is located on the bridge and the receiver is mounted on the steering gear.

Two rams in the transmitter move in opposite directions as the steering wheel is turned.

The fluid in the cylinders of the transmitter is therefore pumped down one pipe line and drawn in from the other.

The pumped fluid passes through piping to the receiver and forces the telemotor cylinder unit to move.

The cylinder unit has a control spindle() connected to it by a pin.

This control spindle operates the slipper ring or swash plate() of the variable delivery pump, which controls the suction and discharge of the oil.

Electrical control The electrical remote control system is commonly used in modern installations since

since it uses a small control unit as transmitter on the bridge and is simple and reliable in operation.

The control box assembly is mounted on the steering gear.

Movement of the bridge transmitter results in electrical imbalance and current flow to the motor in the control box.

The motor drives, through a flexible coupling(), a screw shaft, causing it to turn.

A screw block on the shaft is moved and in turn moves the floating lever to which a control rod is attached.

The control rod operates the slipper ring or swash plate of the variable delivery pump.

A cut-off lever() connected to the moving tiller will bring the floating lever pivot() and the lever into line at right angles to the screw shaft axis().

At this point the rudder angle will match the bridge lever angle and the pumping action will stop.

For local manual control, the electrical control is switched off and a small handwheel is connected to the screw shaft.

Rotation of the handwheel will move the floating lever and bring about rudder movement as described.

Steering gear testing Prior to a ships departure from any port the steering gear should be tested to ensure satisfactory operation.

These tests should be:Operation of the main steering gear.Operation of the auxiliary steering gear or use of the second pump which acts as the auxiliary.

3. Operation of the remote control(telemotor) system or systems from the main bridge steering positions.

4.Operation of the steering gear using the emergency power supply.

5.The rudder angle indicator reading with respect to the actual rudder angle should be checked.

6.The alarms fitted to the remote control system and the steering gear power units should be checked for correct operation.

During these tests the rudder should be moved through its full travel in both directions and

and the various equipment items, linkages, etc, visually inspected for damage or wear.

The communication system between the bridge and the steering gear compartment should also be operated.

READING MATERIAL A. HYDRAULIC MOTORS

Most designs of positive displacement mechanism are capable of acting as pump or motor.

The principles of pumping have been described previously, but if instead of driving the shaft,

fluid is introduced into the inlet port at some pressure then in many designs the mechanism will rotate and in turn drive the shaft.

External Gear, Vane and Axial Piston geometries are generally reversible, with detailed design differences between pumps and motors to achieve optimum performance in each case.

Speeds achieved are comparable with pumps but without the practical restrictions() imposed by a prime mover, speeds up to 4,000 or 6,000 r/min are possible with smaller sizes.

Output torques vary between 21 n mile (15 lbf ft) and 1,360 n mile (1,000 lbf ft) for a typical range of piston motors.

Obviously, the relatively low output torque from the majority of pump/motor equipment results in the requirement for some form of speed reduction gearbox in many cases to achieve acceptable torque levels for winch drives for instance.

This arrangement can be entirely satisfactory but an alternative approach is also possible using equipment of inherently lower speed and higher output torque capacity.

Slow Speed High Torque Motors A number of different designs of 'slow speed' motors are available and are commonly used in marine systems such as winch drives.

Most are generally of much higher capacity per revolution than the pump/motor types described above.

One of these designs is radial piston unit, such as five piston radial slow speed high torque motor, with pistons and connecting rods, operated via a single throw eccentric().

Flow of fluid to and from pistons is through the central pintle().

In the latest designs of this motor, cam eccentricity() can be varied through an independent hydraulic supply, giving smoothly varying capacity.

An example of an alternative design is the radial piston multi-lobe() low speed high torque motor().

This is similarly radial in operation but the pistons operate inwardly()',

the oscillating() movement being obtained from an internal multi-lobed cam profile(, ) around the periphery.

In a typical example, eight pistons run against a six-lobe track().

Therefore, each of eight pistons makes six oscillations in each revolution.

One feature of this type of motor is that the number of working pistons can be reduced by suitable internal porting, thus obtaining the step change in capacity referred to earlier.

Typically, ranges of this type of motor are capable of developing between 500 n mile (350 lbf ft) to 40,000 n mile (30,000 lbf ft)

with maximum speeds of 500 and 100 r/min, respectively, with certain equipment, torques of 130,000 n mile (95,000 lbf ft) are available directly with a maximum speed of 16 r/min.

B HYDRAULIC CIRCUIT CONTROL VAVALEAny hydraulic circuit includes a variety of valves to regulate pressure or flow conditions to control force,torque, speed and/or the direction of movement of system output.

The large range of alternative proprietary equipment available can be considered for convenience in three separate groups: Flow control, Pressure control; Directional control.

In practice, all valves are available in alternative forms for mounting either directly into pipelines or on machined manifold blocks and baseplates.

Pressure Control Valve There are two main types of pressure control valve-relief and reducing, the basic difference being that the relief valve is closed by a spring, and the reducing valve opened by a spring.

Relief valves are used to protect the system from over pressure and are of two main types-direct acting and pilot operated.

The direct acting relief valve is used for controlling low flows and may be used for pressures up to 172 bar (2,500lbf/in2).

For higher pressures and larger flows, pilot operated valves are used.

This type of valve, in conjunction with a variable orifice and suitable circuitry(), can be used for bleed off() control.

The pressure at which the valve opens and controls can be adjusted by the preload of the pilot valve spring.

Reducing valves are used to limit the pressure in any particular portion of the circuit, and again may be of direct acting or pilot operated types.

Direct acting valves are used mainly for providing a low flow and reduced pressure for pilot operation of other valves and for certain types of remote or sequencing control.

Pilot operated units are used for greater flow, and to limit the pressure applied to certain equipment in the circuit.

When used in conjunction with a suitable orifice, they may also be used to control or limit flow as well as pressure.

Flow Control ValveThere are three main variants of this type of control.Restrictor(), a simple needle valve, creating a variable restriction, effectively controls flow.

A significant disadvantage of this control however, is that the flow is directly dependent upon pressure, I.e. load and hence no unique relationship exits between valve opening and flowrate.

Accurate control of part speed may be very difficult to achieve as the pressure is changed.

The disadvantage mentioned above can be overcome by using series flow control valves.

Series flow control valves are used where a number of operations have to be carried out from one pumping set. The valve functions in a similar way to the reducing valve.

With a orifice downstream, any variation in the flow through the orifice causes a variation in pressure drop across the both ends of the main valve piston and

and this, in turn, causes the main piston to take up a new position to restore the pressure drop across the valve opening and flow to the original setting.

The bleed-off flow control valve is basically similar to the pilot operated relief valve and is used in conjunction with an orifice in the supply line.

The valve can restore the pressure drop across the orifice to the original valve. Hence, the flowrate is not dependent upon working pressure level.

Direction Control ValveThese valves, as their name implies, control the direction of flow of the fluid in the system, and are of three main types:1. Positive seated type in which a ball or piston moves on or off the seat;

2. Rotary spool() type in which the spool rotates about its own axis;3. Sliding spool type in which the spool moves axially in bore. This is by far the most common arrangement.

Sliding spool type valve may be mechanically, manually, electrically or hydraulically operated, or even a combination of these.

A very common arrangement is to have a solenoid(, ) operated valve as a pilot in order to operate a main valve hydraulically.

Directional control valves can have from two to six ports, although three and four are the most common.

It is usual to give the number of positions of the spool and the number of flow paths provided in the extreme positions.

For example, one which is very often used is a three-position, four-way valve which has two extremes and one central position, with two flow paths in each extreme position making a total of four in all.

Moving the spool or piston from one extreme position to the other reverses the connections.

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