A grab bucket is widely used for moving and loading bulk and large-clastic materials, scrap and wood shavings, as well as long timber. It can be imagined as a large iron scoop assembled from two identical moving parts, jaws, which is attached to crane equipment for moving goods or to an excavator for excavating above or below the parking level. However, the scope of its application is not limited to this.
Grapple Capacity
The grab is most often used as attachments in excavators and cranes with mechanical or hydraulic drives. The main parameter characterizing its work is load capacity. Its scooping ability depends on the ratio of the mass of the cargo to the bucket itself. Therefore, all materials that are moved using a clamshell bucket are divided into several groups according to their bulk density.
The carrying capacity of the working equipment is determined immediately before the start of work with a trial scoop. It is made from a horizontal platform on which freshly sprinkled soil or material is placed. The seized cargo is poured onto a special surface and weighed. During operation, ropes and blocks must be protected from entrained material.
Rope grabs
Grabs, depending on the number of working ropes, are divided into one-, two-, four-rope and raking subspecies. The simplest version has a single-rope clamshell bucket. In it, for the movement of cargo in a vertical plane and its capture, only one cable is responsible. The main feature of the bucket of this subspecies is the presence of a lock that connects the yoke to the clip. Its main disadvantage is the small height of the material and the manual control of the opening of the jaws.
Two-rope grabs have a lifting and closing cable. The latter is necessary for opening and closing the jaws. Such a bucket can be equipped with a chain hoist into which the closing rope is packed, if it is necessary to increase the cutting force. The main disadvantage is the inability to quickly replace the bucket with other lifting equipment.
The four-rope clamshell bucket has two locking and lifting cables, which gives them a greater carrying capacity than other types. The pick-up type of working equipment is used to scoop up material in cramped conditions - from wagons, holds. Often the rope system in such a grab forms a horizontally oriented chain hoist.
Drive Grabs
In the drive grab, a separate mechanism is responsible for closing and opening the jaws, which is fixed on a hook clip with a winch. They have a larger mass compared to cable subspecies, however, the unloading control is simpler and the carrying capacity is higher.
The volume of the clamshell bucket with the drive mechanism depends on its load capacity, which, in turn, is determined by the type of mechanism that closes the jaw. It may be represented by:
- Electric hoist with chain hoist.
- Hydraulic cylinders.
- Lever mechanism.
The main advantage of this type of grapple is its small size, which allows them to be used on small sites. The main drawback is the displacement of the center of gravity of the bucket, which violates its stability when scooping material from the slope.
Excavator Grabs
Clamshell buckets for excavators can have a different number of jaws of different shapes. This type of working equipment can be installed on machinery with mechanical and hydraulic drives.
An excavator with a mechanical type of drive for installing a grab must have a lattice dragline boom. The weight of the bucket depends on the density of the developed soil. In accordance with this produce equipment of light, medium and heavy class. With an increase in the weight of the grab, its productivity decreases, since the excavator can lift a smaller amount of soil.
Hydraulic grab buckets for excavators are used to develop dense rocks. Typically, this equipment is mounted on a backhoe. Soil cutting is carried out by hydraulic cylinders driven by a special engine.
Magnetic and pneumatic grabs
The arrangement of pneumatic buckets is no different from the design of hydraulic grabs. But such equipment is driven by compressed air, which is pumped into the system using a compressor.
The principle of operation of a magnetic grab is based on the appearance of a magnetic field, which occurs when an electric current is supplied to the excitation coils. After the current is applied, the magnet approaches the grab and closes the jaw. As a rule, magnetic grabs have a large bucket, which not only captures bulk materials, but also attracts ferromagnets (iron, steel), which significantly increases the amount of material being loaded.