-Hydraulic_notes.doc

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Typical hydraulic applications

Deck crane

The drawing shows a typical hydraulic deck comprising a hoisting mechanism toraise and lower the load; a luffing mechanism to raise and lower the jib and aslewing mechanism to rotate the crane on its base. All three applications haveslightly different hydraulic requirements in terms of control and the followinghydraulic drawing using standard symbols illustrates these differences

Chapter 6 !ydraulic control systems "

Slewing motor

Hoisting motor

Jib luffing

cylinder



This may at first appear to be highly comple# but by breaking the circuit downinto four basic systems it becomes much easier to understand.

". The pumping unit.

This supplies hydraulic oil to the three main systems and consists of a storage tankholding the supply of hydraulic fluid $oil%; a uni&directional pump; a pump reliefvalve to protect against overpressure and a pressure gauge for monitoringpurposes.

'. The slewing circuit

This comprises in ascending order(&

A pressure regulating valve to control the load that the motor can handleA flow regulator to control the speed of operation of the motor

A manually operated directional control valve to control the direction that themotor will run.

Chapter 6 !ydraulic control systems '

M

Slewing motor

Pumping

unit

Luffing cylinder Hoisting winch motor

A

A A

A A

A

B

B B

P P P

P

P

P P

T

T T

TTT

T

TP

A

P

A

X



). The luffing circuit


A pressure regulating valve to control the load that the luffing cylinder can handle

A manually operated directional control valve to control the direction that theluffing cylinder will operate.Two flow regulators with bye&pass check valves to control the speed of operation of the luffing cylinder separately in both directions.A pilot operated check valve to stop the jib lowering under load.

*. The hoisting circuit


A pressure regulating valve to control the load that the hoisting winch motor canhandle

A manually operated directional control valve to control the direction that thehoisting winch motor will operate.Two flow regulators with bye&pass check valves to control the speed of operation of the hoisting winch motor separately in both directions.A pressure regulating valve to act as a counterbalance device. This will lift and stallthe motor if the load e#ceeds the setting point of the valve.

The subject demonstrates how quite complicated hydraulic systems can besectionalised in order to make them more comprehensible.

Chapter 6 !ydraulic control systems )



Constant pressure hydraulic active stabiliser control

This circuit used to drive a pair of active stabiliser fins comprises a pumping unit;a pressure control valve used to set the ma#imum load on the fins; a flow controlvalve used to set the operating speed of the fins; a directional control valve.The system is also fitted with shock relief valves to relieve e#cessive pressure inthe system if the fins are subject to overload from an e#ternal source $waveaction%.An accumulator is also fitted to damp down pressure fluctuations in the system as aresult of rapid movement of the directional control valve.The directional control valve would be activated via the electrical solenoids fed

from an electronic motion sensor and control circuit.

Chapter 6 !ydraulic control systems *

M

Fin operting cylinders

!irectionl "ontrol #l$e

Flow "ontrol #l$e

Pressure "ontrol #l$e

Supply %nit "omprising

Pump&Motor' (elief #l$e'Tn)*

Accumultor

Shoc) $l$es



Steering gears

Introduction

+teering gears or steering systems are used to move the rudder against the

considerable force of the sea and of the vessel,s speed through the water. Asteering gear may be as simple as a manual tiller attached to the rudder or mayinvolve comple# electro&hydraulic systems linked to navigational equipment toprovide automatic directional control of the vessel.-or vessels above a certain sie it becomes impractical to steer manually so overthe years various power booster systems have been used to transmit informationfrom the bridge to the steering flat and to amplify the force available to move therudder. /n the early days steam was used to drive the rudder but this was graduallysuperseded by hydraulic power. The steam engine driving the rudder was eithercontrolled directly by rods and chains or by a hydraulic telemotor system

The drawing shows a typical

hydraulic telemotor system that

could be used to give direct

movement of the rudder on a

small vessel or via a power unit

to control a larger rudder.

As the steering wheel is turnedthe rack and pinion moves theplunger thereby compressing theoil and transmitting a signal tothe spring loaded receiver plungerin the steering flat.As hydraulic oil is effectivelyincompressible the smallestmovement of the wheel willproduce an identical movement of the control rod in the steeringflat.

Chapter 6 !ydraulic control systems 0



Two ram hydraulic steering gear

A variable delivery hydraulic pump supplies oil to two opposed cylinders andrams. As oil is supplied under pressure to one cylinder oil is drawn from theopposing cylinder and the two rams move together.

The rams end in cross heads that are bolted together the joined ends being boredvertically to form the top and bottom bearings for the swivel block. The tiller pinor arm runs through the swivel block and translates the lateral movement of therams into a rotary movement of the tiller.




The outer face of the cross head has slipper plates bolted to its side which in turnslide on a guide beam. The guide beam serves two purposes(

• to relieve the cylinder hydraulic glands of side loads and

• to brace together the cylinders which tend to be pushed apart by the

hydraulic pressure.

The cylinders are bolted to stools which support and mount the whole gear on thereinforced vessel,s structure.

A two ram steering gear powered by an open loop hydraulic system

The pump change over block is designed to operate automatically as the pumps areswitched. The directional control valve can be operated electrically from the

bridge or locally by push buttons giving emergency local control.


Pump chnge o$er

bloc) + solenoid

operted

!irectionl control +

solenoid operted

with mnul o$eride

Min pump Au,iliry pump

Shoc) relief

$l$es

"ylinders -

(pson slide



A two ram steering gear powered by an closed loop hydraulic system

The variable delivery pumps control the amount and direction of flow in the systemand are positioned by moving the swash plates. The system is maintained under ahead pressure from the tank via the two non&return valves which prevent back&flow into the header tank. The pumps and the rudder operating cylinders areprotected against shock loading by the system overpressure relief valves.

Four ram hydraulic steering gear

This steering gear arrangement is basically a doubled version of the two&ram

steering gear. The system uses a double 2apson slide crosshead connected to thetillers one tiller and one 2apson +lide being on opposite sides of the rudder stock.

The cylinders are braced by the girders as in the two&ram design but furthercross&bracing is employed between diagonally opposite cylinders to maintainalignment and prevent twisting.


System

o$erpressure

relief $l$es

Heder tn)

.on+return

$l$es

Min $rible

deli$ery

pump

/perting

le$er

Au,iliry

pump



The system also offers a wide range of fle#ibility in the event of component failureand various combinations of cylinder can be selected by adjusting the isolating andbye&pass valves.

4airs of cylinders can be isolated from each pump and from the other pair ofcylinders by selecting different combinations of valves. This arrangement gives"55 redundancy in the event of system failure.

-or e#ample from the previous drawing(

/f cylinder " fails then the system can be isolated to run on cylinders ) 7 * only byclosing valves " 7 ' and opening bye&pass ". 8r by closing valves " 7 ) both bye&passes and isolating valve ' the system can continue to run on cylinders ' 7 * usingpump ".

The rudder can also be locked in position by closing valves "& *.

Combined shock relief and bye-pass valves


Pump 0

Pump 1

#l$e 0 #l$e 1

#l$e 2 #l$e 3

Bye+pss 0

Bye+pss 1

4solting$l$e 04solting$l$e 1

"ylinder 0 "ylinder 1

"ylinder 2 "ylinder 3



The double spring and ball shock relief valves are set to lift at pressures of about3' to ""5 bar and are intended to allow the rudder to give way if subjected toheavy sea pressure. The valves function to open up a connection between the ramwith high pressure and the ram with lower pressure. The e#cess pressure forces therelevant ball valve off its seat and so opens up the cross connection. The manual

bye&pass valve is only opened when charging the system or in an emergency whenisolating part of the system.

unting gear

4osition control of a variable delivery steering gear pump is carried out by atelemotor receiver or more commonly by an electro&hydraulic operated servocylinder operating via a floating lever onto the swash plate of the pump.

The other end of the floating lever is connected via a compressed safety spring tothe rudder stock or tiller arm as shown in the following diagram.

Assuming the rudder is stationary at amidships and a signal is received from thebridge to move the rudder to port. The telemotor or servo operating rod will movethe floating lever and pivot about :A,. This will cause the pump operating rod tomove outwards and put the pump on :stroke, i.e. the pump will start delivering oilto the cylinders. The rudder will then start to move to port and in doing so willcause the floating lever to pivot about :C,. This will cause the pump operating rodto return to the neutral position the pump will stop pumping and the rudder willcome to rest at a new position determined by the amount of signal received fromthe bridge.

Chapter 6 !ydraulic control systems "5

#rible

deli$ery

pump

/perting

signl from

bridge $i

telemotor or

ser$o

5Floting6

le$er

Sfety

spring

Pump

operting

rod

(pson

slide nd

tiller

A

B

"



!ariable delivery pump anti-rotation device

/n closed loop variable pressure systems under normal operating conditions bothmain and au#iliary pumps remain connected to system regardless of which pump isrunning. This enables the stand&by pump to take over in an emergency without the

need to open and close isolating valves. To prevent the stand&by pump being:motored, by the running pump both pump shafts are fitted with locking devicesthat will allow the pump to turn in the pumping direction but will lock the pump ifit attempts to motor in the opposite direction.

The drawing shows such a device which consists of a simple rachet and series ofpawls.

Chapter 6 !ydraulic control systems ""



"otary vane steering systems

The rotary vane unit replaces the ' or * ram steering unit using a 2apson slide toturn the tiller and rudder stock.The main benefit of the rotary vane unit is that it is much more compact and

therefore saves space.A disadvantage is that the torque available with a rotary vane unit is constant aslong as the hydraulic pressure is constant thus when torque demands by therudder increase as it moves towards the ma#imum rudder angle of )0 then thevane gear relies on the steering gear pump to meet the demand. A ram gear doesnot have to rely solely on this increased pump demand as it has a mechanicaladvantage due to the ram lever effect acting on the tiller through the rapson slide.The hydraulic circuit serving a rotary vane unit is virtually identical to one used fora '<* ram unit.The usual arrangement of rotary vane unit is of three moving vanes fitted betweenthree fi#ed vanes. This enables a full rudder movement through an angle of 15 $ie)0 to 4ort to )0 to +tarboard%.

/f a larger rudder turning angle is required then a two vaned system is requiredwith two moving vanes travelling between two fi#ed vanes but this is unusual.All the vanes are sealed by strips of steel backed by synthetic rubber and laid inslots. The full weight of the gear is supported by an under slung rudder carrierbearing-urther fi#ture to the ships structure preventing the stator turning is by twoanchor bolts held in fi#ed anchor brackets with shock absorbing sleeves of rubber.These bolts are fitted with outer cast iron bushes to take the wear from thesteering gear flanges.

Chapter 6 !ydraulic control systems "'



Chapter 6 !ydraulic control systems ")



#mergency arrangements

The failure of the telemotor or servo unit can be overcome by controlling thepumps manually in the steering gear flat.The regulations state that an alternative steering arrangement must be provided

and this is normally done by having duplication of power units one of which is fedfrom the emergency switchboard so that it can be run off the emergencygenerator.!owever hydraulic steering gears can be rendered useless by loss of hydraulic oil.2ules now e#ist under +8=A+ that require a steering system to be so designed thata single failure of the hydraulic system would not leave the steering gearinoperable. This has led to duplicated piping systems or two interconnectedcircuits with automatic isolation of one from the other should there be a loss ofhydraulic fluid.

Further safety devices

8il header<storage tanks must be fitted with low level alarms that will sound onthe bridge.The steering mechanism must be protected against e#cessive travel by movementlimiters. These take the form of hydraulic locking devices built into the operatingsystem that will bring the rams or the vanes to rest before they reach themechanical limits of travel. 2am systems will also have some form of mechanicalstop fitted to the tiller arm to prevent e#cess travel and damage to the rams. +topblocks may also be fitted on the rudder itself.

Chapter 6 !ydraulic control systems "*



Testing of steering gears

The testing and checking requirements of a +teering >ear are based on the

2egulations of the +.8.=.A.+. $+afety 8f =ife At +ea% "91* Convention.

The requirements for steering gears are dealt with in 2egulation "9&' which statesthefollowing( &

• The ship?s steering gear is to be checked and tested within the "' hours prior

to departure by a responsible person of the ship?s crew.

• The testing procedure must include the operation of the following where

applicable(the main steering gearthe au#iliary steering gearthe remote steering gear control systemsthe steering position located on the navigating bridgethe emergency power supplythe rudder angle indicators in relation to the actual position of the rudderthe remote steering gear control system power failure alarmsthe steering gear power unit failure alarmsthe automatic isolating arrangements and other automatic equipment.

The checks and tests are to include the following( &

• full movement of the rudder according to the required capabilities of the

steering gear.

• a visual inspection of the steering gear and its connecting linkages.

• the operation of the means of communication between the navigating bridgeand the steering gear compartment.

$ritten instructions%

+imple operating instructions with a block diagram drawing showing the change&over procedures for remote steering gear control systems and steering gear powerunits. These instructions are to be permanently displayed on the navigating bridgeand in the steering gear compartment.

All ship?s officers concerned with the operation or maintenance of the steering gear

are to be familiar with the operation of the steering gear systems fitted on the shipand with the procedures for changing over from one system to another system.

/n addition to the checks and tests mentioned above the emergency steering drillsare to be carried out at least once in every three months in order to practiceemergency steering procedures

These drills must include the following(&

• direct control from within the steering gear compartment

• the communications procedure with the navigating bridge and where

applicable the operation of alternative power supplies.




The administration $which in this case covers all ships of @ritish registry in theepartment for Transport% may waive the requirement to carry out the checks andtests for ships engaged on voyages of short duration. These ships are to carry outthose checks and tests at least once a week.The date upon which these checks and tests are carried out and the date and

details of emergency steering drills are to be recorded in the 8fficial =og @ook.

Steering gear general arrangements

The general requirements of a steering gear are based on the regulations laid downby the +.8.=.A. +. $+afety 8f =ife At +ea% "91* Convention.

2egulations '9 and )5 state the following(

". Bvery ship is to be provided with a main and au#iliary steering gear. The mainsteering gear and au#iliary steering gear must be so arranged that the failureof one does not render the other inoperative.

'. All steering gear compartments are to be of a sound and reliable constructionand must satisfy the requirements of the administration $i.e. epartment forTransport and classification societies%. The design pressure for piping andother steering gear components is to be subjected to an internal hydraulicpressure of at least ".'0 times the ma#imum e#pected working pressurewhen under the sea&going conditions specified in paragraph $*%.

). An au#iliary steering gear need not be fitted when the main steering gear hastwo or more identical power units and is arranged such that on a single failurein its piping system or one of its power units steering capability can bemaintained. To meet this latter alternative the steering gear has to complywith the operating conditions mentioned in paragraph $*% i.e. /n the case ofpassenger ships while any one of the power units is out of action. /n the caseof large tankers chemical tankers and gas carriers the provision of two ormore identical power units is a mandatory requirement for the main steeringgear.

*. The main steering gear must be capable of steering the ship at the ma#imumahead service speed and be capable at this speed and at the ships deepestservice draught of putting the rudder from )0 on one side to )5 on theother side in not more than '3 seconds. $The apparent anomaly in the degreeof movement is to allow for difficulty in judging when the final position is

reached due to hunting gear feed back%.here the rudder stock e#cluding ice strengthening allowance is required tobe over "'5 mm diameter at the tiller the steering gear must be operated bya power unit.

0. The au#iliary steering gear must be capable of being brought quickly intooperation and be capable of putting the rudder from "0 on one side to "0on the other side in not more than 65 seconds with the ship at its deepestservice draught and steaming ahead at a speed greater than one half of thema#imum service speed or 1 knots.here the rudderstock e#cluding ice&strengthening allowance is over ')5mm diameter then the au#iliary steering gear unit must be power operated.

$That diameter being at the tiller%




6. The navigating bridge must have the facility of bringing into operation themain and au#iliary steering gear power units in the event of a power failureto anyone of the steering gear power units or to its associated controlsystems. An audible and visual alarm must be activated on the navigatingbridge

The power units must be so arranged as to re&start automatically on therestoration of the power supply.

1 Control over the steering gear must be provided on the navigating bridge andin the steering gear compartment where the main steering gear is situated.here the main steering gear comprises of two or more identical power unitsthen there must be two independent control systems both of which must beoperable from the navigating bridge. $This does not mean that two steeringwheels are required.%hen a hydraulic telemotor is employed for the steering control system asecond independent system need not be fitted e#cept in the case of a tankerchemical tanker or gas carrier of "5555 grt. and over.

Au#iliary steering gear control must be arranged in the steering gearcompartment. here the au#iliary gear is power operated control must alsobe arranged from the bridge and be independent of the main steering gearcontrol system.ithin the steering gear compartment it must be possible to disconnect anyof the bridge operated control systems from the steering gear it serves. /tmust be possible to bring the system into operation from the bridge.

3 !ydraulic power systems are to be provided with arrangements to maintainthe hydraulic fluids cleanliness.A lower level alarm must be fitted on each hydraulic fluid reservoir to give anearly audible and visible alarm on the bridge and in the engine room of any

hydraulic fluid leakage.4ower operated steering gears require a storage tank arranged such that thehydraulic systems can be readily re&charged from a position within thesteering gear compartment.The storage tank must be of sufficient capacity as to be able to recharge atleast one of the power actuating systems.

9 here the rudder stock is required to be over ')5 mm diameter at the tillere#cluding ice strengthening an alternative power supply capable of providingpower for the au#iliary steering gear as mentioned in paragraph $0% is to beautomatically provided within *0 seconds.

This must supply the power unit its control system and the rudder angleindicatorThis supply can be provided from either the ships emergency power source orfrom an independent source of power located within the steering gearcompartment and solely dedicated for this purposeThe capacity of this supply shall be such as to sustain power on the steeringgear for at least )5 minutes f or ships of "5555 >2T and over and "5 minutesfor other ships.

"5 2elief valves must be fitted to any part of the hydraulic system which can beisolated and in which pressure can be generated from the power source orfrom e#ternal forces




The setting of such relief valves must not e#ceed the design pressure of thesystem The valves must be of adequate sie and arranged to avoid an unduerise in pressure above the desired pressure of the system.

Ch t 6 ! d li t l t "3

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-Hydraulic_notes.doc