The total shipborne trade in non free-flowing bulk materials is currently estimated to grow considerably. Most of the total trade in these non free-flowing bulk materials is unloaded by wharf mounted grab unloaders. The world trade in metallurgical coal, a major non free- flowing bulk material is forecast to grow and there is also forecast large annual increase in world trade in bulk commodities.

BACKGROUND ART At present non free-flowing bulk materials are unloaded by gantry grab unloaders, level luffing jib cranes and continuous unloaders. Gantry grab unloaders are similar to container cranes and move the grab over the ship by means of a trolley on a horizontal track. They are suitable for a variety of bulk materials but particularly coal, iron and other metalliferous ores.

Typically gantry gas unloaders range in capacity from 600tph to 5000tph. Costs of gantry grab unloaders range up from A$12M for a 1000tph unit.

Level luffing jib cranes are smaller grabbing machines typically up to 700tph capacity and suitable for smaller vessels and smaller quantities of various materials. The jib of the machine is moved to either luff or slew the grab between the ship and the receival hopper.

Costs of level luffing jib cranes are in the order of A$6M.

Continuous unloaders are available in many forms and are generally material specific.

Continuous unloaders generally include some form of elevator which is lowered on a boom into the ship. Elevators on large 5000tph coal and ore unloaders utilise a series of steel

buckets. Smaller machines use screw elevators or sandwich belt conveyers for free-flowing bulk solids such as grain. Suction unloaders are a further option but are less desirable due to low capacity, high energy consumption and high wear. Costs of continuous unloaders range from A$5M for small grain unloaders to more than A$25M for the large machines.

There is a need for a less capital expensive, more productive unloader than the level luffing jib crane for developing industrial areas where smaller volumes of many different bulk products are required to be unloaded. There is a need for an unloader which has the potential to move up into the mid range, say 2500tph capacity and also potentially be suitable for dedicated unloading of specific materials such as coal.

DISCLOSURE OF THE INVENTION According to the present invention there is provided a ship unloader including a jib crane having a grab mounted on a holding rope, said holding rope passing over a pulley on the jib to raise or lower the grab, guiding means to slidably attach a guiding rope to the holding rope between said pulley and said grab, said guiding means being suspended from the jib by a suspending rope of variable length, crowding means connected with said guide rope to draw the holding rope so as to position said grab, and means to adjust the length of said suspending rope to control the position of said guiding means along the holding rope.

Preferably a the position of the guiding means is controlled during drawing of the holding rope so that the guiding means follows a substantially linear path. It is further preferred that the path is substantially horizontal.

Preferably the guiding means includes at least one guiding sheave that may be used to slidably mount the guiding rope. The guiding sheave is preferably drawn in or extended by a crowding winch connected to the a guiding rope to position the grab over a bulk material on a ship for charging the grab and subsequently over a hopper for discharge.

The unloader geometry can be affected by friction effects where wire ropes deviate around the guiding sheave. Control of the height of the guiding sheave may be achieved by simple

counter-weighting connected to the suspending rope however a variable tension support may also be used. The variable tension support can comprise a winch. Preferably, interactions between the grab closing rope and the guiding sheave are preferably avoided.

The control of the height of the slidable mount will permit the guiding ropes to be kept clear of the ship, but low enough to allow the grab winch closing ropes to operate in the free length between the slidable mount and the pulley mounted on the boom. A simple counter-weight of equal mass to the slidable mount may be used to control the height of the slidable mount, alternatively a variable tension winch system may be used.

Friction associated with the wire suspending ropes wrapping around the guiding sheave may cause some uncertainty in the statical equilibrium of the grab suspension system. These friction forces may be controlled so as to allow for both new and worn wire ropes, and on different wire rope types and an automatic control system which compensates for the range of uncertainty which friction introduces into the system may be used.

The grab is typically operated by the use of at least two wire ropes, a holding rope and a closing rope, both of which preferably pass through the guiding sheave. Frequently two ropes are used for each of the holding and closing ropes.

Each holding rope and closing rope preferably passes through fairleads on either side of the guiding sheaves. The fairleads will preferably accommodate the change in rope wrap together with some impact in case of contact between the grab and the sheave assembly.

The ship unloader of the present invention advantageously permits reduction in mass of moving components. Only the grab and slidable mount need to be accelerated/decelerated compared with the grab assembly on a gantry crane or the boom structure on a level luffing jib crane. Higher speeds and greater productivity are therefore provided.

The present invention also permits greater control of grab discharge. The grab may be hung over the discharge hopper and controlled by short wires and by splay in the guiding wires.

This permits hopper size reduction and housing and dust control at the point of discharge.

The ship unloader of the present invention may advantageously be used to control spillage

between the unloader and the ship and also to minimise dust emissions from the hopper. We have found that spill plates and a passive plenum pressure relief system may be used to recycle the dust plumes within the hopper. The dust control system may incorporate some supplementary dust collection systems.

Further, the ship unloader of the present invention permits considerable reduction of structure mass. The mass of the machine will typically be 300 to 500 tonnes less than an equivalent gantry grab unloader. This represents a capital expense reduction in the order of 25 %.

The invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig. 1 is an elevation of the ship unloader.

Fig. 2 is a plan view of the ship unloader.

BEST MODE FOR CARRYING OUT THE INVENTION Referring to the drawings the ship unloader 1 of this invention includes a jib crane 2 of the kind having a jib 3 pivotally mounted on a sub-structure 4. The sub-structure 4 is of substantially conventional type mounted on wheels 5 for movement along a dock 6. The jib 3 is of A-frame construction and is pivotally mounted with the sub-structure 4 by king pins (not shown) at 7 and 8. A tip portion 9 of jib 3 extends outwardly from the upper end. A grab bucket 10 of conventional type is attached to holding and closing ropes shown as composite 11 in Figure 1. The holding ropes 12 and closing ropes 13 are separately shown in Figure 2. These ropes pass around a pulley 14 mounted with the jib in the conventional manner. The holding and closing ropes 12 and 13 are then directed by sheaves to respective winches 15 and 16 in the convention manner. Luffing of the jib 3 is controlled by a luffing rope 17 operated by a winch 18 in the conventional manner. It will apparent that the above

described jib crane is of substantially known type and its operation will be readily understood by those skilled in the art.

A guiding sheave 20 is slidably mounted on the holding and closing wires 11 and attached to guiding wires 21. As best seen in Figure 2 guiding wires 21 diverge outwardly from the longitudinal axis of of symmetry and are anchored to a gantry 22 formed to the rear of the sub-structure 4. The guiding wires 21 extend through guiding sheave 20 to a further sheave 23 and crowding winch 24. Guiding sheave 20 is suspended from the tip portion 9 of jib 3 by suspending rope 25 which passes over a pulley 26 at the end of tip portion 9. Suspending rope 25 is directed via further pulleys 27,28 to a small winch 29.

In operation, the ship unloader 1 is used to remove material from a ship 30 and deposit the material in a hopper 31 for subsequent discharge into trucks 32 in the known manner.

Guiding rope 21 is drawn in and extended by crowding winch 24. This positions the grab 10 to any required position on the inner side of the pulley 14 at the end of jib 3. Suspending rope 25 is used to control the position of the guiding sheave 20 so that it follows a substantially linear path. This results in the grab 10, in turn, following a substantially linear path. In the optimum arrangement the path is substantially horizontal so that the grab 10 can be moved from a position above the required location on the ship 30 from which it is lowered to be filled to a discharge position over the hopper 31. The positioning of the pulley 26 at the end of tip portion 9 allows sag or slack in the guiding ropes 21 can be reduced and controlled.

As an alternative to the winch 29 to tension the suspending rope 25 a simple counterweight system can be employed. The use of a winch 29 which allows the tension in the suspending rope 25 to be varied as the guiding sheave 20 is moved however provides improved operation of the ship unloader. In particular the path of the grab 10 can be optimized when the tension is increased as the guiding sheave 20 moves outwardly toward the tip of jib 3.

The A-frame construction of jib 3 of the present invention also provides significant

advantages. In this regard the double boom or jibs of the A-frame allows the guiding sheave 20 to pass between the two arms of the jib 3 which in turn allows the jib support points 7,8 to be located lower in the sub-structure 4 for maximum structural efficiency. The A-frame configuration also overcomes the common problem of crane design of having to add structural mass to the jib to achieve adequate resistance to forces causing side sway.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.