Introduction
With the development of world industrial economy, the amount of bulk cargo transported by ocean shipping has increased immensely. To take full advantage of greater transporting capacities and the low cost of ocean shipping, bulk cargo ships are now larger and have been specially developed to facilitate an increase in capacity which has resulted in significant economic benefit. To improve the transporting system, and shorten the parkingnperiod of ships, unloading equipment has also become bigger and increasingly more efficient, especially in the case of bulk transport.
Amongst the many varieties of ship-unloading equipment, many users in the world choose to use grab ship-unloaders. Today, shipunloaders require roughly several hundred grab ship-unloaders, and the price of each ship-unloader is more than $5 million.
Between the 1970’s and the end of the 1980’s some of the top ports around the world in the USA, Holland, Belgium, Germany, Japanese and etc. used ship-unloaders to improve the operation capacity. One of the top ship-unloaders in the world can be found at the Port of Rotterdam. Its maximum hoisting weight is 85t, the maximum capacity is 5,100t/h, and this ship-unloader can unload a 250,000, DWT ship. Up to now, the main technical data of the existing grab ship-unloaders include:
• Increase of the girder out reach to 50m from 25m
• Increase of the hoisting height to 53.5m from 25m
• The speed of the main mechanism has also been improved, for example, hoisting speed has been increased to 180 m/min and trolley traveling speed has been increased to 270 m/min
• The operation cycle time has been shortened to 40-50 seconds from 55-60 seconds.
Different designs of bridge grab ship-unloaders
In the last twenty years, the need for bridge grab ship-unloaders in China has greatly increased. In recent years the market demand has been at an all time high. Both international and Chinese manufacturers have been supplying many machines of this kind. Chinese users’ understanding and requirements of different types of ship-unloaders has changed a lot. It has forced manufacturers to make new improvements in their design and fabrication. Because of the generalisation of different manufacturer, users’ understanding of different types is not comprehensive or sufficient.
In the 1980’s in China, the bridge grab ship-unloaders with a capacity over 1000t/h were mostly imported, and most of them were the self-traveling trolley type (hoisting/closing/trolley traveling machinery integrated in trolley). At the start of 1990’s under the generalisation of SPMP, Chinese users have gradually realised that the rope pulling type has become the mainstream unloader the in world market and have accepted the comparison in Table 1. In this period, they accepted that the main-auxiliary trolley form was the most popular rope pulling type. Thus, more designs of this type have been developed:
a) Two ropes (hoisting and closing) with one trolley design
b) Conventional rope scheme with main and auxiliary trolley design
c) Four ropes with planetary winches design
d) Four ropes electrical synchronization design
Below is a comparison of the advantages and disadvantages of the four different designs of rope pulling type grab ship-unloaders, corresponding (a to d) with the above mentioned design types.
Rope reeving system and main motion feature
a) One hoisting rope and one closing rope fastening on the drums, then through the alternate sheaves and sheave group on the grab. The dead ends of the ropes fasten on the storage drums. The hoisting/closing motion and trolley travelling can be driven individually. They can also do composing action.
b) Two hoisting ropes fastening on one drum, then through the alternate sheaves, and sheave groups of main trolley and auxiliary trolley, finally fastening on the grab. Two closing ropes are the same fleet through, then reeving the grab sheaves and fastening. The hoisting/closing motion and trolley travelling can be driven individually. They can also do composing action.
c) Hoisting mechanisms are driven by a motor with a planetary gearbox connecting with two drums. The closing mechanism is the same construction. Four ropes are fastened on four drums individually. The hoisting rope fleet through the bend sheaves, and fasten on to the grab. The closing ropes are the same fleet through, then reeving the grab sheaves and fastening. The trolley motion driving motor drives two planetary gearboxes. Two ropes of two front drums pull the same distance as two ropes of two rear drums releasing.
The grab load overcomes the resistance of motion in front direction and the same condition in the opposite direction. The hoisting/closing motion and trolley travelling can be driven individually. They can also do composing action.
d) Four ropes pulled by four drums driven by four motors with standard gearboxes. Two ropes on the front two drums pull the same distance as the two ropes on the rear two drums releasing. The grab load overcomes the resistance of motion in front direction and the same condition in the opposite direction. When the four ropes are pulled up or down, the lifting and lowering activities can be carry on. The hoisting/closing motion and trolley travelling can’t be driven individually. These four ropes are controlled by electrical synchronisation.
