Grab refers to the special tool for cranes to grab dry bulk goods. The material space is composed of two or more bucket-shaped jaw plates that can be opened and closed. When loading, the jaw plate is closed in the material stack, and the material is caught in the storage space. When unloading, the jaw plate is opened in the suspended state of the material stack, and the material is scattered on the material stack. The opening and closing of the jaw plate are generally operated by the steel wire rope of the hoisting mechanism of the crane. Grab operation is not heavy manual labor, can achieve high loading and unloading efficiency and ensure safety, is the main dry bulk cargo loading and unloading tool in port. According to the type of goods, it can be divided into ore grab, coal grab, grain grab, wood grab, etc.
Mechanical grab itself is not equipped with an opening and closing structure, which is usually driven by the external force of rope or connecting rod. According to the operation characteristics, it can be divided into double rope grab and single rope grab, and the most commonly used is double rope grab.
The double rope grab is provided with support rope and open and close rope, which are respectively wound on the drum of the support mechanism and the opening and closing mechanism. The support rope and the open and close rope are lowered simultaneously, and the opening bucket mouth is inserted into the stack. Tighten the open and close ropes, close jaw plate, and grab materials. The support rope and the open and close rope rise simultaneously after the grab is closed. Support rope does not move, open and close rope drops, bucket mouth opens and unloads material. Double rope grab is reliable, easy to operate, high productivity, and widely used. After using two groups of double rope, it becomes a four rope grab, and the working process is the same as that of the double rope grab.
The single rope grab support rope and the open and close rope are used with the same wire rope. Through the special lock device, the wire rope can support and open and close in turn. The winding mechanism of a single rope grab is relatively simple, but its productivity is low, so it is rarely used in a large number of loading and unloading operations.
According to the bulk density of the material, the grab can be divided into three categories: light (such as grasping grain), medium (such as grasping gravel), and heavy (such as grasping iron ore); According to the number of jaw plate, it can be divided into double jaw plate grab and multi jaw plate grab, the most commonly used is double jaw plate grab. The multi jaw grab is suitable for large ore, scrap iron, and scrap steel, because it has the characteristics of multi-claws and sharp incision, and is easy to insert into the material pile, so it can get better-grasping effect. There is also a scissor grab (Fig. 2) which imitates the principle of scissor structure. Its grabbing force increases gradually with the closing of the jaw plate and reaches the maximum at the end of closing; The opening of the bucket mouth and the area of covering materials are also larger than that of the general grab, which improves the grabbing capacity and is conducive to cleaning the stockyard and cabin, but for large materials, the effect is poor because of its small initial grabbing force.
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