Grab ship unloader is mainly used to unload bulk cargo. Its working principle is as follows:
1. Initial positioning
At the beginning of work, the bridge of the ship unloader spans between the ship and the dock, and the grab bucket is moved to the appropriate position above the ship's hatch through the trolley running mechanism on the bridge. The trolley running mechanism is like a "small train" running on the bridge track, which can accurately control the position of the grab bucket in the length direction of the hatch. This positioning process is achieved through an electrical control system. The operator can use the operating handle or computer control system to issue instructions according to the hatch layout and cargo distribution of the ship to make the trolley run to the target position.
2. Grab bucket descent and grab
Descent process: After the grab bucket position is positioned, the lifting mechanism starts to work. The lifting mechanism is mainly composed of an electric motor, a reducer, a drum and a wire rope. The power of the motor drives the drum to rotate after the reducer decelerates and increases the torque. When the drum rotates, the wire rope is released, and the grab bucket slowly descends into the cabin. During the descent, the speed can be controlled and is generally adjusted according to the depth of the cabin and the stacking of the cargo.
Grabbing action: When the grab comes into contact with the bulk cargo, the opening and closing mechanism of the grab begins to work. For hydraulic grabs, the hydraulic system drives the grab's petals to open and close; for rope grabs, the opening and closing of the grab is controlled by a special rope device. The grab is inserted into the cargo pile, and then the petals are closed. Using the shape and mechanical structure of the grab itself, a certain amount of bulk cargo is tightly grasped inside the grab. The amount of grabbing can be determined based on the size of the grab, the characteristics of the bulk cargo (such as density, particle size, etc.), and the operator's experience and control instructions.
3. Grab lifting and moving
Lifting process: After the grab grabs the cargo, the lifting mechanism works in reverse. The drum rolls up the wire rope, driving the grab and the cargo to rise together. During the lifting process, the lifting mechanism must overcome the gravity of the cargo and the weight of the grab itself, while ensuring the stability of the lifting speed to prevent the cargo from falling. This process requires the motor of the lifting mechanism to output sufficient torque, and the lifting speed and height are precisely controlled by the control system.
Movement process: When the grab bucket is lifted to a certain height, the trolley running mechanism starts again to move the grab bucket horizontally to the top of the unloading funnel on the dock shore. This movement process also needs to ensure the stability of the grab bucket to prevent the grab bucket from shaking and causing the cargo to spill.
4. Unloading process
After the grab bucket moves to the top of the unloading funnel, the opening and closing mechanism of the grab bucket works again, opens the grab bucket, and the bulk cargo falls from the grab bucket into the unloading funnel under the action of gravity. The unloading funnel is usually connected to conveying equipment such as a belt conveyor. After the cargo enters the conveying equipment through the funnel, it is continuously transported to the dock yard or other subsequent processing facilities. After the unloading is completed, the grab bucket is closed, and the trolley running mechanism and the lifting mechanism work together again to send the grab bucket back to the top of the ship's hatch, starting the next round of grabbing and unloading cycle until all the cargo on the ship is unloaded.
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