Screw Conveyor

Screw Conveyor

A screw conveyor is a type of continuous conveying machinery that utilizes rotating helical blades to push materials along a fixed casing. It is one of the most fundamental pieces of equipment for transporting powdery and granular materials in industrial production, finding wide application across sectors such as mining, metallurgy, coal, electric power, ports, chemicals, construction materials, grain processing, fertilizers, and environmental protection. Screw conveyors are particularly well-suited for applications where installation space is limited, there are strict requirements for leak-free and loss-free transport, conveying distances are short, and operating environments are demanding.

A screw conveyor consists of three main components: the conveyor body, the inlet and outlet assemblies, and the drive unit. During the conveying process, the forces preventing the material from rotating along with the helical blades are derived from the material's own weight and the frictional resistance exerted by the casing. Capable of horizontal, inclined, and vertical transport, screw conveyors offer advantages such as simple construction, excellent sealing, ease of operation and maintenance, and low cost; they serve as core equipment for linking production stages and enabling the enclosed transport of materials. Depending on material characteristics and process requirements, screw conveyors are classified into two basic types—shafted and shaftless—and are further distinguished by their structural form as either U-shaped or tubular conveyors.

Structural composition of the equipment

The screw conveyor is primarily composed of the following core components:

(1) Screw assembly (auger): This is the core working component, consisting of a screw shaft and helical flights welded onto it. The flights come in various types—such as solid, ribbon, or paddle—depending on the material being conveyed. In shafted screw conveyors, the assembly consists of a screw (with a central shaft) and flights; shaftless screw conveyors utilize a shaftless screw (a thick ribbon-like flight), resulting in a simpler structure.

(2) Casing (trough): This serves as the container for the material and comes in two shapes: U-shaped trough and tubular. U-shaped conveyors use an open U-shaped trough, which can be fitted with a top cover; tubular conveyors use an enclosed cylindrical pipe, offering superior sealing. The casing typically consists of three sections: the head section, intermediate section, and tail section.

(3) Drive unit: Includes the electric motor, gearbox, coupling, etc. The drive unit is installed at the head end to power the rotation of the screw shaft. Power ratings generally range from 1.1 to 73.3 kW.

(4) Bearing assembly: Includes head bearings, tail bearings, and intermediate hanger bearings. Head bearings are located at the drive end, while tail bearings are at the discharge end. For longer conveyors (typically exceeding 3–4 meters), intermediate hanger bearings are required to support the screw shaft and prevent sagging due to its own weight. Hanger bearings are usually of the sliding type and are equipped with dust-proof seals.

(5) Inlet and outlet devices: These include the feed inlet and discharge outlet, positioned at appropriate locations on the casing. Some models feature a cleaning device at the discharge end.

(6) Sealing and auxiliary devices: Tubular screw conveyors utilize flange connections and are equipped with labyrinth seals. The unit is available in either carbon steel or stainless steel.

Application scope of the equipment

Screw conveyors have a wide range of applications. In fertilizer production, they serve as essential conveying equipment for organic, compound, and BB fertilizer lines, enabling the enclosed transport of powdery or granular materials; within these production lines, they often act as "micro-circulation" links connecting crushers, screening machines, and granulators. In the chemical industry, they are used for the enclosed transport of chemical powders and raw materials. In the grain and food sectors, they handle materials such as grains, cereals, and animal feed. They are also utilized in environmental protection and sludge treatment for conveying sludge, biomass, and waste, as well as in the construction materials and power industries for transporting cement, fly ash, and lime. Additionally, the equipment can be used to feed quantitative packaging scales.

Types of usable raw materials

Screw conveyors are suitable for transporting non-sticky, granular, powdery, or small-lump materials. Examples include fertilizers (organic fertilizer powder, compound fertilizer granules, raw materials for blended/BB fertilizers), grains (wheat, corn, rice, cereals), construction materials (cement, lime, fly ash), chemical products (fertilizer raw materials, chemical powders), and environmental materials (sludge, biomass, waste). Shaftless screw conveyors are particularly well-suited for transporting sticky or easily entangled materials (such as sludge). Care should be taken to prevent hard, large particles from mixing with the conveyed material, as this could cause the screw flighting to jam.

The working principle of the equipment

The operating principle of a screw conveyor is based on the core mechanism of "helical blades pushing the material." Material is conveyed as the rotating helical blades push it along the interior of a stationary casing. Specifically, the drive unit rotates the screw shaft, causing the helical blades to turn. Material enters the casing through the inlet and settles onto the rotating blades. Due to the material's own weight and the frictional resistance exerted by the casing, the material does not rotate with the blades; instead, it is continuously pushed forward—much like a non-rotating nut moving axially along a rotating screw. The material travels axially along the casing to the discharge outlet, where it is unloaded. Depending on the conveying direction, screw conveyors can operate horizontally, at an incline (typically ≤20°), or vertically.

Equipment operating instructions

(I) Pre-start Inspection: Conduct a comprehensive inspection before operation. Check for any loose fastening bolts across the screw conveyor assembly. Ensure the inlet and outlet are unobstructed. Verify that the gearbox oil level meets specifications. Check the lubrication status of all bearings.

(II) No-load Start-up: The screw conveyor must be started without a load—that is, with the casing empty of material. Material feeding should only commence after the machine has started. Starting the machine while loaded is strictly prohibited to prevent motor burnout due to overload or bending of the screw shaft.

(III) Operational Monitoring: Gradually increase the feed rate until the rated conveying capacity is reached. Ensure the feed is uniform; uneven feeding can easily lead to material blockages and drive unit overload. Monitor the equipment for normal operation and check for any abnormal noise. Check that the temperature rise of the bearings remains within normal limits.

(IV) Shutdown Procedure: Stop the material feed first, and cut off the power supply only after all material has been cleared from the casing.

Equipment maintenance methods

(I) Daily Inspection: Inspect machine operation daily, paying attention to any abnormal noise. Check for seal leakage. Check all fasteners for looseness; if any are loose, stop the machine immediately and retighten them. Inspect the screw flights for wear; if wear is severe, perform repair welding promptly.

(II) Periodic Maintenance: Lubricate the drive unit's gearbox according to the manufacturer's instructions. Use No. 30 machine oil for the gearbox in winter and No. 45 machine oil in summer. Change the oil after 2,000 hours of operation.

(III) Lubrication Management: Use lithium-based grease for the bearing housings at both ends; inject approximately 5 grams every two weeks. Regularly lubricate the screw conveyor's hanger bearings. Periodically apply lubricating oil to all drive components.

(IV) Precautions: Do not overload the conveyor; failure to discharge material can cause the screw shaft to bend and the casing to rupture due to pressure buildup. When conveying high-temperature materials, monitor the casing for thermal expansion and contraction.

Solutions to Equipment Problems

Material Clogging: Maintain the feed rate within the rated range to ensure uniform feeding. For materials with high moisture content, pre-drying is required, or a shaftless screw conveyor should be used. Clear any blockages at the discharge outlet; installing a hinged access door on the discharge end cover facilitates cleaning.

Difficulty Starting: Check the power supply for phase loss and restore the missing phase immediately. Remove accumulated material from the casing before startup. Forced starting is strictly prohibited to prevent motor burnout.

Abnormal Noise: Stop the machine immediately to inspect and troubleshoot by opening the casing. Check for contact between the screw flights and the casing, and adjust the clearance. Inspect the hanger bearings for wear and replace them promptly.

Material Leakage: Inspect and replace damaged sealing components. Tighten flange connection bolts. Check the integrity of the access door seal.

Application of the equipment in the production line

Screw conveyors play a pivotal role in the enclosed transport of materials across various production lines. In organic and compound fertilizer production lines, they typically function as "micro-circulation" units, linking crushers, screening machines, and granulators. Within batching systems, multiple screw conveyors transport different raw materials to the mixer, while in quantitative packaging systems, they are used to feed material precisely to the packaging scale. Their fully enclosed conveying capability makes them particularly suitable for applications where installation space is limited and strict leak-proof standards are required.