Conveyors

Conveyors, broadly speaking, are used to transfer materials one location to another. Depending on the transfer method, there are four major conveyor categories.

Index

1. Belt Conveyor

The continuously moving conveyor belt, arguably one of the most widely used types of conveyors, finds extensive application in transporting materials both horizontally and on inclines, whether going up or down. A belt conveyor has five primary components:

The belt: This component serves as the dynamic and supporting surface upon which the conveyed material travels.
The idlers: These components constitute the supports for both the carrying and return sections of the belt.
The pulleys: Pulleys play a crucial role in providing support for the belt and facilitating its movement, as well as controlling its tension.
The drive: The drive mechanism transfers power to one or more pulleys to set the belt and its cargo in motion.
The structure: This framework serves the dual purpose of supporting and ensuring the alignment of the idlers and pulleys while also providing support for the machinery responsible for driving the conveyor system.

Photo 1: Belt Conveyor

Practically all belt conveyors used for handling bulk solids utilize rubber-covered belts, where the inner carcass of the belt provides the necessary strength to pull and support the conveyed load. The carcass is shielded from damage by layers of rubber, which may vary in thickness depending on the specific application.

Belt conveyors exhibit the capability to transport materials across a broad range of rates, spanning from just a few kilograms per minute to several thousand metric tons per hour. These conveyors can accommodate a wide variety of materials. However, the size of lumps may pose limitations based on belt width, and handling dusty substances can be challenging. Wet or sticky bulk solids require special attention, and operating at high temperatures should be approached cautiously. Some materials can react with the rubber used in the belt, necessitating the use of a special covering.

The maximum incline over which a belt conveyor can effectively operate primarily depends on the characteristics of the conveyed product. In typical applications, it’s advisable to keep the inclination angle somewhat below the suggested maximum.

Standard troughing angles are commonly set at 0°, 20°, 35°, and 45°. The angle of surcharge is a material-specific property and can be compared to the dynamic angle of repose.

The power requirements for belt conveyors hinge on various variables, including the conveyor’s profile, the type of drive-pulley arrangement, belt tensions, belt speed, and idler spacing.

2. Screw Conveyor

A screw conveyor typically comprises a long-pitch steel helix flight mounted on a shaft, with support provided by bearings within a U-shaped trough. As the helix rotates, material supplied to it is propelled forward by the thrust generated by the lower section of the helix and is discharged through openings in the trough bottom or at the end. When used correctly, this conveyor type performs effectively, and its cost is often only about half that of other conveyor types. A screw conveyor is characterized by easy maintenance, cost-effectiveness in replacement, and the capability to be sealed against dust, making it the preferred choice for many applications.

Photo 2: Screw Conveyor

Screw conveyors can operate with an upward incline in their path, but the capacity decreases significantly as the angle of inclination increases. For instance, a standard-pitch screw inclined at 15° above the horizontal retains 70% of its horizontal capacity. If the screw is inclined at 25°, the capacity drops to 40%, and at a 45° incline, the material will move along the trough’s floor at a substantially reduced rate. To handle steep inclines, the helix may have a shorter pitch, and the trough can be made tubular to minimize the capacity loss. In the case of a jammed feed, such a conveyor can still deliver about 50% of its horizontal capacity at a 45° incline.

The allowable loading and screw speed are determined by the material’s characteristics. Light, free-flowing, nonabrasive materials can fill the trough deeply, allowing for a higher rotation speed compared to heavier and more abrasive materials.

3. Chain conveyor

Chain conveyors utilize continuous chains that span the entire length of the conveyor. They serve to transmit the pulling force from the driving unit and, in certain instances, bear the full weight of the transported material. The material can be conveyed directly by the chains, through the use of flights that are either pushed or towed by the chains, or by employing special attachments fixed to the chains. The naming of the conveyor types is often derived from the nature of the attachment. Examples include apron conveyors, flight conveyors, and drag-chain conveyors.

Photo 3 : Chain Conveyor

Chain conveyors are especially well-suited for systems that require complete enclosure, which is crucial for dust containment, as well as for scenarios where minimizing the cross-sectional dimensions of the conveyor housing is essential. They are also ideal for systems that need the ability to load or unload materials at various points along the same conveyor, handle combinations of horizontal and vertical paths, or manage materials at elevated temperatures.

4. Bucket Elevators

Bucket elevator is a conveyor designed for carrying bulk materials in a vertical or inclined path. It consists of an endless belt, chain or chains to which buckets are attached, along with head and boot terminal machinery, and a supporting frame or casing. In most cases, it is more cost-effective to convey horizontally, elevate the material separately, and then convey again, rather than attempting to perform all these functions simultaneously with an inclined bucket elevator.

Vertical bucket elevators can be categorized into four main groups based on the methods used to convey and discharge materials. In centrifugal-discharge elevators, material is released due to centrifugal action. These elevators consist of buckets attached to a chain or belt at regular intervals and typically operate at a minimum speed of 76 m/min. The size of the material being handled in these elevators is usually limited to no more than 50 mm.

Photo 4: Bucket Elevator

Continuous bucket elevators release material through gravity. Buckets are mounted back to back on a continuous chain or belt, and the elevator operates at a rate of 36.6 to 38.1 m/min. These elevators can effectively handle materials ranging from 50 to 100 mm in size.

Positive-discharge elevators are a type of spaced-bucket elevator in which the buckets are turned over by idler wheels. The buckets are held over the discharge chute long enough to allow for free gravity discharge. These elevators operate at speeds not exceeding 36.6 m/min and are typically used for handling sticky solids.

Hinged/pivoted bucket elevators are designed for a closed-circuit path in a vertical plane. They consist of a series of overlapping buckets pivotally suspended between strands of a chain, with supporting rails or guides, turn wheels, a drive mechanism, and a tripper or dumper mechanism to upend the buckets for discharge.