Screw Conveyor Types | Helicoid Screw Flight | Ribbon Flight Conveyor Screws

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 Helicoids

2. 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.

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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.

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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.

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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.

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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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Application of screw conveyors:

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Screw conveyors are employed for handling a great variety of materials which have relatively good flowability. Sticky and stingy materials are unsuitable for screw conveying.

Screw conveyors have a few favourable points to its credit for recommending its application in different industries. Simplicity of design, easy construction and maintenance, are the few points which are often mentioned in favour of its application. But material characteristics often dictate the choice of its selection.

Dusty, hot materials or materials having foul ordour are often conveyed by screw conveyors, because it can be easily made dust-proof by jacketing the trough.

Typical applications include grain storage plants, feed mills, cereal processing plants, chemical plants, and sand preparation shop of a foundry for delivery of clay to the mixing machines.

Screw conveyors are very suitable for being adopted as a device to control the volume of materials flow from the bottom of bins, hoppers, storage silos etc. when screw conveyors are used for this purpose they are termed as screw feeders. Screw feeders have wide applications for the operation of processing units like driers, hammer mills, oil expellers and other innumerable mills.

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Screw conveyors may be used to convey materials which can be subjected to thermal operation during transit. The conveyor trough is provided with jacket through which the heating or cooling medium may be circulated to effect the thermal operation.

Screw conveyors may be operated horizontally, on an incline, or vertically. Often inclined conveyors can solve a transportation problem effectively, but otherwise a combination of horizontal and vertical units should be installed. The angle of inclination of inclined conveyors is usually 10° to 20°. Vertical conveyors are usually complex in design and should be run at faster speed.

Screw conveyors do not always get favourable consideration due to high friction of the material against screw and trough, probability of material degradation due to crushing of material and due to higher power consumption resulting from higher friction. Therefore screw conveyors have limitation in capacity and distance of conveying. They are suitable for low and medium capacities for short distance. Capacity may be as high as 100m3/hr and distance of conveying is of the order of 30 to 40m typically.