Marine Auxiliary Machinery

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．

Marine Auxiliary Machinery