Marine Auxiliary Machinery

Chapter 9 Lesson 5

Deck Machinery

Cargo Handling

By Professor Zhao Zai Li 05.2006

CARGO HANDLING (1)

� The duty of a deck winch is to lift and lower a load by means

of a fixed rope on a barrel, or by means of whipping the load

on the warp ends; to top or luff the derricks, and to warp the

ship.

� In fulfilling these duties it is essential that the winch should

be capable of carrying out the following requirements:

� (a) lift the load at suitable speeds;

� (b) hold the load from running back;

CARGO HANDLING (1)

� (c) lower the load under control;

� (d) take up the slack on the slings without undue stress;

� (e) drop the load smartly on the skids by answering the operator’s

application without delay;

� (f) allow the winch to be stalled when overloaded, and to start up

again automatically when the stress is reduced;

� (g) have good acceleration and retardation:

CARGO HANDLING (2)

� In addition when the winch is electrically driven the

requirementsare:

� (a) prevent the load being lowered at a speed which will damage the

motor armature;

� (b) stop the load running back should the power supply fail;

� (c) prevent the winch starting up again when the power is restored until

the controller has been turned to the correct position.

CARGO HANDLING (3)

� Hydraulic winch systems are now quite common but electric

drives for cargo winches and cranes are most widely used.

� For the conventional union purchase cargo handling

arrangement or for slewing derrick systems handling loads up

to 20 tonne, standard cargo winches are normally used for hoist，topping and slewing motions, the full load duties varying from

3-10 tonne at 0.65 to 0.3 m/sec．

CARGO HANDLING (4)

� For the handling of heavy loads, although this may be

accomplished with conventional derrick systems using

multipart tackle, specially designed heavy lift equipment is

available.

� Thewinches used with these heavy lift Systems may have

to be specially designed to fit in with the mast arrangements

and the winch duty pull may be as high as 30 tonne.

Cargo winches (1)

� It is usual to select the number and capacity of and to group the

winches in such a way that within practical limits，all hatches

may be worked simultaneously and having regard to their size

(and the hold capacity beneath them) work at each is carried out in

the same period.

� Reduction of the cycle time during cargo handling is best

accomplished by the use of equipment offering high speeds say

from 0.45 m/see at full load to 1.75 m/sec light, the power

required varying from 40 kW at 7 tonnes to 20 kW at 3 tonnes;

this feature is available with electro-hydraulic and d.c. electric

drives as they offer an automatic load discrimination feature.

Cargo winches (2)

� However, the rationalisation of electrical power supply on

board ship has resulted in the increased use of a. c. power and

the majority of winch machinery now produced for cargo

handling utilises the pole—changing induction motor.

� This offers two or more discrete speeds of operation in fixed

gear and a mechaincal change speed gear is normally

provided for half load conditions.

� Normally all modern cargo handling machinery, of the

electric or electrohydraulic type，is designed to ‘fail safe.

� A typical example of this is the automatic application of the

disc brake on an electric driving motor should the supply fail

or when the controller is returned to the ‘OFF’ position．

Derricks (1)

� Most older ships and some recent ones use winches in

conjunction with derricks for working cargo.

� The derricks may be arranged for fixed outreach working or

slewing derricks may be fitted.

� A fixed outreach system uses two derricks, one ‘topped’ to a

position over the ship’s side and the other to a position over the

hold.

� Figure 9.9 shows the commonest arrangement adopted, known

as Union Purchase rig.

Derricks (2)

� The disadvantages of the fixed outreach systems are

that firstly if the outreach requires adjustment cargo work

must be interrupted, and secondly the load that can be lifted

is less than the safe working load of the derricks since an

indirect lift is used.

� Moreover considerable time and man power is required to

prepare a ship for cargo working.

Figure 9.9

Union purchase rig

Derricks (3)

� The main advantages of the system are that only two winches

are required for each pair of derricks and it has a faster cycle

time than the slewing derrick system.

� The slewing derrick system, one type of which is shown in

Figure 9.10, has the advantages that there is no interruption in

cargo work for adjustments and that Cargo can be more

accurately placed in the hold; however in such a system three

winches are required for each derrick to hoist luff and slew.

Figure 9.10

Slewing derrick

Deck cranes (1)

� A large number of ships are fitted with deck cranes.

� These require less time to prepare for working cargo than

derricks and have the advantage of being able to

accurately place (or spot) cargo in the hold.

� On container ships using ports without special container

handling facilities, cranes with special container handling

gear are essential.

Deck cranes (2)

� Deck-mounted cranes for both conventional cargo handling

and grabbing duties are available with lifting capacities of

up to 50 tonnes.

� Ships specialising in carrying very heavy loads，however,

are invariably equipped with special derrick systems such

as the Stulken (Figure 9.11).

� These derrick systems are capable of lifting loads of up to

500 tonnes

Figure 9.11

Stulken derrick (Blohm and Voss)

Deck cranes (3)

� Although crane motors may rely upon pole changing for speed

variation, Ward Leonard and electro-hydraulic controls are those

most widely used.

� One of the reasons for this is that pole-changing motors can only

give a range of discrete speeds but additional factors favouring

the two alternative methods include less fierce power surges since

the Ward. Leonard motor or the electric drive motor in the

hydraulic system run continuously and secondly the contactors

required are far simpler and need less maintenance since they are

not continuously being exposed to the high starting currents of

pole-changing systems．

Deck cranes (4)

� Deck cranes require to hoist, luff and slew and separate

electric or hydraulic motors will be required for each motion.

� Most makes of crane incorporate a rope system to effect

luffing and this is commonly rove to give a level luff—in

other words the cable geometry is such that the load is not

lifted or lowered by the action of luffing the jib and the

luffing motor need therefore only be rated to lift the jib and

not the load as well.

Deck cranes (5)

� Generally, deck cranes of this type use the ‘ Toplis ’ three-

part reeving system for the hoist rope and the luffing ropes

are rove between the jib head and the superstructure apex

which gives them an approximately constant load,

irrespective of the jib radius.

� This load depends only on the weight of the jib, the

resultant of loads in the hoisting rope due to the load on the

hook passes through the jib to the jib foot pin (Figure

9.12(a)).

Figure 9.12

Rope lift cranes-resultant loads when hoisting

Deck cranes (6)

� If the crane is inclined 5 in the forward direction due to heel of

the ship the level-luffing geometry is disturbed and the hook

load produces a considerable moment on the jib which

increases the pull on the luffing rope (Figure 9.12(b)).

� In the case of a 5 tonne crane the pull under these conditions is

approximately doubled and the luffing ropes need to be over-

proportioned to meet the required factor of safety.

Deck cranes (7)

� If the inclination is in the inward direction and the jib is near

minimum radius there is a danger that its weight moment

will not be sufficient to prevent it from luffing up under the

action of the hoisting rope resultant.

� Swinging of the hook will produce similar effects to

inclination of the crane.

Deck cranes (8)

� In the Stothert ＆ Pitt ‘Stevedore’ electro-hydraulic crane the jib

is luffed by one or two hydraulic rains.

� Pilot operated leak valves in the rams ensure that the jib is

supported in the event of hydraulic pressure being lost and an

automatic limiting device is incorporated which ensures that

maximum radius can not be exceeded.

� When the jib is to be stowed the operator can override the

limiting device.

� In the horizontal stowed position the cylinder rods are fully

retracted into the rams where they are protected from the

weather .

Deck cranes (9)

� Some cranes are mounted in p airs on a common platform

which can be rotated through 360º .

� The cranes call be operated independently or locked together

and operated as a twin-jib crane of double capacity", usually to

give capacities of up to 50 tonnes.

� Most cranes can, if required, be fitted with a two-gear selection

to give a choice of a faster maximum hoisting speed on 1ess

than half load.

Deck cranes (10)

� For a 5 tonne crane full load maximum hoisting speeds in

the range 50-75 m/min are available with slewing speeds in

the range1-2 rev/min.

� For a 25 tonne capacity crane, maximum full load hoisting

speeds in the range 20-25 m/min are common with slewing

speeds again in the range 1-2 rev/min.

� On half loads hoisting speeds increase two to three times.

Drive mechanism and safety features (1)

� In both electric and electro-hydraulic cranes it is usual to find

that the crane revolves on roller bearings.

� A toothed rack is formed on them periphery of the supporting

seat and a motor-driven pinion meshes with the rack to provide

drive.

� Spring-loaded disc or band brakes are fitted on all the drive

motors.

� These are arranged to fail safe in the event of a power or

hydraulic failure.

Drive mechanism and safety features (2)

� The brakes are also arranged to operate in conjunction with

motor cut-outs when the crane has reached its hoisting and

luffing limits, or if slack turns occur on the hoist barrel.

� In the case of the electro-hydraulic cranes it is normal for

one electric motor to drive all three hydraulic pumps and in

Ward-Leonard electric crane systems the Ward-Leonard

generator usually supplies all three drive motors．