Fig. 6.1.1. A flat belt conveyor with drive control
(b) Troughed Belt Conveyor: In this conveyor, comparatively wide flat belt is supported on
troughed carrying rollers or shaped supporting surface so that the two edges of the active
side of the belt are elevated from the middle part to form a trough. This provides a greater
carrying capacity than a flat belt of equal width for conveying bulk materials or those
materials which would slide off flat belts. These conveyors are used in handling bulk materials
of different classes. The return side of the belt is generally kept flat supported on
cylindrical rollers.
The troughed conveyors which are used within a plant for moving bulk materials from one
point to another, are generally termed as ‘‘normal’’ or ‘‘transfer’’ conveyors. These are
comparatively of shorter lengths, and path of movements are in straight line in a horizontal
or an inclined plane. The stresses in the belts being within limits of cotton fabric belts.
However, troughed belt conveyors are often used for transportation of bulk materials over
long distances, by means of a series of conveyors, over paths that are combination of inclines,
declines and horizontal sections, following the natural contours of the ground. These
are generally termed ‘‘long-centre’’ conveyors. There is no clear demarcation between a
normal or long-centre conveyor. Long center conveyors are those where belt tension is
high warranting use of high tension quality belts with less belt stretch, and low acceleration
through gradual starting controls for the drive. By using a number of conveyors in series, it
is possible to change the direction of materials movement at the junction of two conveyors,
called ‘‘transfer terminal’’.
Long-centre conveyors are used for jobs like: (i) transportation of the output of mines to
the processing plants, (ii) materials from shipping ports to the storage/transport loading
sites, (iii) materials from outdoor storage yards to inside plants, (iv) movement of materials
between plants etc.
(c) Closed Belt Conveyor: In a closed belt conveyor, the specially fabricated belt, after being
loaded with the material, can be wrapped completely around the load. It essentially forms
a closed tube moving along with the material. The advantages of a closed belt conveyor
are: (i) it can handle fragile materials safely and without breaking by reducing inter particle
collision, (ii) it can handle fine bulk materials without being swept by air (however, it is not
really air tight at loading and unloading points), (iii) ability to handle corrosive and reactive
materials without contamination and (iv) the tubed belt can travel around bends in
more than one plane and hence versatile in layout.