Screw Conveyors | Screw Conveyor Working Principle | Screw Conveyor Systems

Screw Conveyors

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Screw conveyors were used as low height lift pump to irrigate the lands. It comprises of an inclined rotating casing with an internal helical screw thread, called flight. When such a casing is rotated with its lower end submerged in water, the water entrained in the pockets, formed by the pitches, is raised. The performance of such screws is limited by the angle of inclination of the casing. A modified version of such screws have been tried with a rotating shaft having helical screw thread moving inside a stationery casing for lifting fluids like sewage sludge. But due to various limitations the process achieved limited success in the field of transportation of liquid. However when a rotating screw shaft in a stationery casing is used to convey solids, it has commendable success.

Floor mill industry was probably the first to employ horizontal screw conveyor to convey corn and flour. These screws were made by fitted wood blades to a central shaft rotating inside a cylindrical casing. Even now such screw conveyors enjoy popularity in agro based industries, handling grains and flours, because of low friction, negligible cohesive strength of the material.

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A screw conveyor consists essentially of a shaft mounted screw rotating in a trough and a drive unit for running the shaft. The material is moved forward along the axis of the trough by the thrust of screw thread or flight. The trough is usually of the U-shape.

A helical blade is attached to a drive shaft which is coupled to a drive unit. The shaft is supported by two end bearings and intermediate bearings. The U-shaped trough has a cover plate with an opening for loading the conveyor. A discharge opening is provided at the bottom of the trough. The loading and discharging points can be located anywhere along the trough. More than one feed hopper and discharge hopper may be fitted according to the necessity.

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The principle of material movement along the trough is similar to the sliding motion of a nut along a rotating screw when the nut is not allowed to rotate. The weight of the material and the friction of the material against the wall prevent the load from rotating with the screw.

Conveyor Take Up Arrangement | Take Up Pulley | Screw Take Up Conveyor | Quick Take Up Master Cylinder

Conveyor Take-up Arrangement:

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All belt conveyors require the use of some form of take-up device for the following reasons:

  • To ensure adequate tension of the belt leaving the drive pulley so as to avoid any slippage of the belt
  • To ensure proper belt tension at the loading and other points along the conveyor
  • To compensate for changes in belt length due to elongation
  • To provide extra length of belt when necessary for splicing purpose.

Usually there are two types of take up arrangements.

  • Fixed take up device that may be adjusted periodically by manual operation
  • Automatic take up devices for constant load type

In a screw take up system the take up pulley rotates in two bearing blocks which may slide on stationery guide ways with the help of two screws. The tension is created by the two screws which are tightened and periodically adjusted with a spanner. It is preferable to use screws with trapezoidal thread t decrease the effort required to tighten the belt.

01-hydraulic take up device-pneumatic take up device-electrical take up device-self adjusting take up device-automatic take up device

The main problem with the use of manual take-up is that it requires a vigilant and careful operator to observe when take up adjustment is required. Perfect tension adjustment with this system is also not possible. For this reason these devices are used only in case of short conveyors of up 60 m length and light duty.

In automatic take up arrangement the take up pulley is mounted on slides or on a trolley which is pulled backwards by means of a steel rope and deflecting pulleys. The carriage travels on guide ways mounted parallel to the longitudinal axis of the conveyor, i.e., horizontally in horizontal conveyors and at an incline in inclined conveyors. Hydraulic, pneumatic and electrical take up devices are also used.

Automatic take-up has the following features:

  • It is self adjusting and automatic
  • Greater take-up movement is possible.