Screw Conveyor

Screw Conveyor

A screw conveyor is a mechanical device that utilizes an electric motor to drive a rotating screw, pushing materials to achieve transportation. It can convey materials horizontally, at an incline, or vertically. It features a simple structure, a small cross-sectional area, excellent sealing, convenient operation, easy maintenance, and facilitates enclosed transportation. Screw conveyors are categorized into shafted screw conveyors and shaftless screw conveyors based on their conveying form, and into U-shaped screw conveyors and tubular screw conveyors based on their external configuration. Shafted screw conveyors are suitable for non-adhesive dry powder materials and small granular materials (e.g., cement, fly ash, lime, grain, etc.), while shaftless screw conveyors are ideal for transporting adhesive and easily entangled materials. (e.g., sludge, biomass, waste, etc.) The working principle involves rotating spiral blades pushing materials forward. The force preventing materials from rotating with the blades is their own weight combined with the friction resistance provided by the conveyor housing. The screw blades are welded onto the conveyor's rotating shaft. Blade profiles vary based on material type, including solid, belted, or flat designs. The screw shaft features thrust bearings at the discharge end to counteract axial reaction forces from the material. Intermediate suspension bearings are required for longer conveyor lengths.

Working Principle

When the screw shaft rotates, the material moves forward along the conveyor trough bottom solely due to the combined force of its own gravity and the friction generated against the trough walls. This action resembles a non-rotating nut translating along a rotating screw [3]. The primary driving force propelling the material forward stems from the tangential upward and forward force exerted by the screw blades as they rotate axially.

To maintain the screw shaft in a more favorable tension state, the drive unit and discharge outlet are typically positioned at the same end of the conveyor, while the feed inlet is placed as close as possible to the opposite tail end. The rotating screw blades advance the material through pushing action. The forces preventing the material from rotating with the blades are its own weight and the frictional resistance exerted by the conveyor housing. Blade profiles vary based on conveyed materials, including solid-face, belt-face, and leaf-face types. The screw shaft features thrust bearings at the discharge end to counteract axial reaction forces from the material. Intermediate suspension bearings are required for longer conveyor lengths.

Features

1) Relatively simple structure with lower cost.

2) Reliable operation and straightforward maintenance management.

3) Compact dimensions with small cross-sectional size and minimal footprint. Easily navigates through hatches and vehicle compartments during port unloading operations.

4) Enables sealed conveying, suitable for dusty, hot, or odorous materials, reducing environmental pollution and improving working conditions for port personnel.

5) Convenient loading/unloading. Horizontal screw conveyors allow loading/unloading at any point along the route; vertical screw conveyors with relative screw-type loading devices offer excellent material pickup performance.

6) Capable of reverse conveying and enabling a single conveyor to transport material in two directions simultaneously—toward or away from the center.

7) Relatively high energy consumption per unit.

8) Materials are prone to crushing and abrasion during conveyance, while the screw blades and troughs also experience significant wear.

Structural Characteristics

(1) The screw blades of screw conveyors come in three types: solid screw surface, belt-type screw surface, and blade-type screw surface. The solid screw surface is referred to as the S-type method, where the pitch GX is 0.8 times the blade diameter. The LS-type screw conveyor is suitable for conveying powdery and granular materials. The belt-type screw surface, also known as the D-type method, has a pitch equal to the blade diameter and is suitable for conveying powdery and small block materials. The blade-type spiral surface is less commonly used, primarily for conveying highly viscous and compressible materials. During transportation, it simultaneously performs mixing and blending operations. Its pitch is approximately 1.2 times the diameter of the spiral blade.

(2) The spiral blades of screw conveyors have two rotation directions: left-hand and right-hand.

(3) Screw conveyors are categorized into horizontal fixed-type screw conveyors and vertical screw conveyors. Horizontal fixed screw conveyors are the most commonly used type. Vertical screw conveyors are used for short-distance material lifting, typically with a conveying height not exceeding 8m. They feature solid-surface blades and require horizontal screw feeding to ensure adequate feed pressure.

(4) At the material discharge end of LS and GX type screw conveyors, 1/2 to 1 full turn of reverse-direction blades should be installed to prevent powder material from clogging the end.

(5) Screw conveyors consist of three main components: the screw conveyor body, inlet/outlet ports, and the drive unit.

  The screw conveyor body comprises the head bearing, tail bearing, suspension bearings, screw, casing, cover plate, and base.

  The drive unit consists of the motor, reducer, coupling, and base.

Applications

Screw conveyors are widely used across various sectors of the national economy, including the grain industry, building materials industry, chemical industry, machinery manufacturing, and transportation. Screw conveyors are primarily used to transport various powdered, granular, and small lump materials. The bulk materials conveyed include grain products such as cereals, beans, and flour; construction materials like cement, clay, and sand; chemicals including salts, alkalis, and fertilizers; as well as bulk commodities such as coal, coke, and ore. Screw conveyors are unsuitable for transporting perishable, highly viscous, oversized, or easily caking materials. Beyond bulk materials, they can also handle various packaged goods. Additionally, screw conveyors can perform mixing, stirring, or cooling operations while transporting materials.