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Cargo handling gears - Use of deck cranes
deck cranes use for hatch cover opening
fig 1: Handling of hatch covers
by deck cranes

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-mounted cranes for both conventional cargo handling and grabbing duties are available with lifting capacities of up to 50 tonnes. Ships specializing in carrying very heavy loads, however, are invariably equipped with special derrick systems such as the Stulken (Fig 2). These derrick systems are capable of lifting loads of up to 500 tonnes.

Although crane motors may rely on pole-changing for speed variation, Ward-Leonard and electro-hydraulic controls are the most widely used. One of the reasons for this is that pole-change 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 are required 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.

Stulken derrick (Blohm and Voss)
Figure 2: Stulken derrick (Blohm and Voss)

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 3 a). 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 3 b).

In the case of a 55 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. 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.

In the Stothert & Pitt 'Stevedore' electro-hydraulic crane the jib is luffed by one or two hydraulic rams. 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 cannot 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.

Some cranes are mounted in pairs on a common platform which can be rotated through 360 deg. The cranes can 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.

Rope lift cranes - resultant loads when hoisting. (Stothert &
Pitt Ltd)
Figure 3: Rope lift cranes - resultant loads when hoisting. (Stothert & Pitt Ltd)

Most cranes can, if required, be fitted with a two-gear selection to give a choice of a faster maximum hoisting speed on less than half load. For a 5 tonne crane full load maximum hoisting speeds in the range 50-75 m/min are available with slewing speeds in the range 12 rev/min. For a 25 tonne capacity crane, maximum full load hoisting speeds in the range 2025 m/min are common with slewing speeds again in the range 12 rev/min. On half loads hoisting speeds increase by two to three times.

Drive mechanism and safety features

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 the 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. 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.

Summarized below some of the basic operation of deck machinery and maintenance guide :
  1. Powering deck machinery -Systems and components

  2. Pump and motor systems are used for powering deck machinery such as winches and windlasses. Pump and actuating cylinders are normally employed for hatch covers. One or more pumps will be used to supply the volume of fluid at the pressure required to operate one or more motors. ......

  3. Mooring equipment for general cargo vessel

  4. The operation of mooring a vessel has traditionally required the attendance of a large number of deck crew fore and aft. Supervision of the moorings was also necessary to maintain correct tension through changes due to the tides and the loading or unloading of cargo. ......

  5. Hydraulic systems for deck machinery and cargo equipment

  6. The three essential components for a hydraulic circuit, are the hydraulic fluid held in a reservoir tank, a pump to force the liquid through the system and a motor or cylinder actuator to convert the energy of the moving liquid into a working rotary or linear mechanical force. Valves to control liquid flow and pressure are required by some systems. ......

  7. General cargo ship deck machinery electric drives

  8. Electric motors on vulnerable deck areas may be protected against ingress of water by being totally enclosed in a watertight casing. Vents are provided on some winches, which must be opened when the motor is operating in port. ......

  9. Handling deck machinery- Anchor windlasses,Anchor capstan & mooring winches

  10. The windlass cablelifter brakes must be able to control the running anchor and cable when the cablelifter is disconnected from the gearing when letting go'. Average cable speeds vary between 5 and 7 m/s during this operation. ......

  11. General cargo ship deck deck crane

  12. 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. ......

  13. Mechanically operated steel hatch covers

  14. Hatch cover equipment like the other deck machinery, has to exist in a very hostile environment and the importance of regular maintenance cannot be over-emphasized. Drive boxes and electrical enclosures should be checked regularly for water-tightness. ......

  15. Derricks and cargo winches -Ship cargo handling gears

  16. 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. ......

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