Types of screw flight:
The
screw of the conveyor may be right hand or left hand, the right hand
type being the usual design. The threads of the screw may be single,
double or triple.
The flight of the screws may be made in either of the two ways:
1. As Helicoids2. As Sectional flight
Helicoids Flight:
They
are formed from a flat bar or strip into a continues helix. The threads
are thinner at the outer edge and thicker at the inner edge.
Sectional flights:
Sectional
flights are formed from a flat disc and the thickness of the thread is
uniform throughout. A continuous helix is made by joining a number of
sectional flights together on a piece of pipe and butt welded them.
Various styles of screw flights are in use, depending on the service
required.
Some of the typical configurations are:
1. Short pitch or continuous flight:
If
the conveyor is required to handle dry granular or powdered materials
that do not pack, this style of flight may be selected. It is of regular
construction and recommended for inclined conveyors having a slope of
20 or more, including vertical conveyors. This style is extensively used
as feeder screw.
2. Ribbon flight:
If
the conveyor is to handle lumpy, clinging, sticky, gummy or viscous
substances, this type flight may be selected. It consists of continuous
helical flight formed from steel bar and secured to the pipe by
supporting lugs.
3. Cut flight:
In
this type of flight screws have notches cut in the periphery of the
flight. These notches supplement the conveying with moderate mixing
action. They are recommended for conveyors required to handle light,
fine, granular or flaky materials.
4. Cut and folded flights:
This
type of flight is characterized by notches as in cut flight, together
with folded segments. This type of flight creates agitation and aeration
resulting in better mixing. This type of flight is used to handle light
or medium weight materials having fine, granular or flaky materials.
5.
Some screw conveyors have cut flight with paddles mounted at regular
intervals. The paddles counteract the flow of material past the flight
resulting in greater agitation and mixing.
6.
Sometimes screws are made of stainless steel to suit special
requirements, like the sanitation requirements for handling food, drugs
and other hygienic materials.
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