Tuesday, 26 June 2012

DECK CRANES

DECK CRANE
  1. A large number of ships are fitted with deck cranes.
  2. 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.
  3. On container ships using ports without special container handling facilities, cranes with special container  handling gear are essential.
  4. Deck-mounted cranes for both conventional cargo handling and grabbing duties are available with lifting capacities of up to 50 tonnes.
  5. Ships specialising in carrying very heavy loads,however, are invariably equipped  with special derrick systems such as the Stulken (Figure 9.11).
  6. These derrick systems are capable of lifting loads of up to 500 tonnes
 
 
  1. Although crane motors may rely upon pole changing for speed variation, Ward Leonard and electro-hydraulic controls are those most widely used. ( Induction motors with PWM control also have been developed)
  2. 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.
  3. Deck cranes require to hoist, luff and slew and separate electric or  hydraulic motors will be required for each motion.
  4. 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.
  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.
  6. 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)).
  1. 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
  2. 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.
  3. 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.
  4. Swinging of the hook will produce similar effects to inclination of the crane.
  5. In the Stothert & Pitt ‘Stevedore’ electro-hydraulic crane the jib is luffed by one or two hydraulic rains.
  6. 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.
  7. When the jib is to be stowed the operator can override the limiting device.
  8. In the horizontal stowed position the cylinder rods are fully retracted into the rams where they are protected from the weather .
  9. Some cranes are mounted in pairs on a common platform which can be rotated through 360ยบ .
  10. 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.
  11. 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.
  12. 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.
  13. 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.
  14. On half loads hoisting speeds increase two to three times.

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