Screw conveyors can handle powders, granules, bulk solids, and some materials with sticky or fibrous characteristics. However, different materials require different conveyor configurations and operating parameters. During equipment selection, important material properties include particle size, bulk density, flowability, moisture content, adhesiveness, abrasiveness, corrosiveness, and temperature.
These properties affect the screw diameter, flight design, rotational speed, drive torque, trough clearance, and material selection. If the equipment is not properly matched to the material, problems such as reduced conveying capacity, material blockage, excessive flight wear, motor overload, and component corrosion may occur.
Particle Size and Lump Dimensions
Particle size is one of the basic parameters to consider when selecting a screw conveyor. Powders, granules, and bulk materials with larger particles place different requirements on the screw diameter, flight clearance, and conveying capacity.
Powders and fine particles: Materials such as cement, flour, and fly ash can generally be conveyed by screw conveyors. However, flowability, dust generation, moisture absorption, and caking behavior must also be considered.
Small and medium-sized granules: Sand, plastic pellets, and grain can be conveyed using suitable screw configurations based on particle size, bulk density, and required throughput.
Large lumps: Stones, slag lumps, and other irregular bulk materials require careful consideration of maximum lump size and the proportion of oversized particles. Large lumps may become trapped between the screw flight, center pipe, and trough.
Materials containing elongated contaminants: Fibers, ropes, and strip-shaped objects may wrap around the screw or become entangled, depending on the shaft design and anti-wrapping provisions.
There is no universal maximum particle size that applies to all screw conveyors. The allowable lump size depends on the screw diameter, center pipe dimensions, radial clearance between the flight and trough, and the proportion of oversized material in the feed. For large or irregular materials that cannot easily be broken down, provide the maximum lump size and particle size distribution before equipment selection.
Material Flowability and Bulk Density
A screw conveyor moves material by applying force through its rotating flights. Material flowability affects how the material fills the trough, the resistance during conveying, and the actual conveying capacity.
Materials with good flowability
Cement, dry sand, some grain products, and plastic pellets are generally easier to convey with conventional screw conveyors when the material condition is stable and the feed rate is uniform.
Materials with poor flowability
Materials with high moisture content, a tendency to form lumps, or strong cohesive properties may accumulate at the inlet, along the bottom of the trough, or on the screw flight. This can result in uneven conveying or blockage.
Depending on the material, possible solutions include adjusting the screw speed, changing the flight design, controlling the trough filling ratio, or installing an anti-bridging, mixing, or forced-feeding device.
Materials with varying bulk density
Bulk density directly affects the mass conveying capacity. Two materials with the same volumetric flow rate may have significantly different mass flow rates.
For this reason, equipment selection should not be based only on the required throughput in tonnes per hour. The material's bulk density must also be provided to determine the screw diameter, rotational speed, filling ratio, and drive power.
Material Adhesiveness and Moisture Content
Moisture content and adhesiveness affect how readily material sticks to the screw flight, center pipe, and inner surface of the trough. Some materials flow easily when dry but become sticky and accumulate inside the conveyor after absorbing water or mixing with oil or grease.
Examples include wet clay, sludge, certain chemical process materials, and oily powders. Their actual properties must be considered when selecting the screw conveyor configuration.
For materials that tend to adhere strongly to the center pipe, several design options may be considered:
Shaftless screw conveyor: A shaftless design has no central shaft or pipe running through the conveying section. This reduces the surface area where sticky material can accumulate and is suitable for some cohesive, fibrous, or entangling materials.
Ribbon screw flight: A ribbon flight changes the connection between the flight and center pipe, reducing the area where material can adhere to the central pipe surface.
Wear-resistant or easy-to-clean liners: Suitable trough liners and surface treatments can be selected according to the material's adhesion and wear characteristics.
Auxiliary cleaning systems: Cleaning or flushing devices can be installed when material tends to accumulate on the flight or trough.
A shaftless screw conveyor does not automatically solve every problem associated with sticky materials. Moisture content, adhesion strength, resistance to shear, and conveying distance still affect the required drive torque and actual conveying capacity.
Material Abrasiveness and Corrosiveness
Material continuously contacts the screw flight, trough, and support components during conveying. Abrasive materials can cause mechanical wear, while corrosive materials may attack exposed metal surfaces. Both conditions should be considered when selecting the conveyor materials and structural configuration.
Abrasive materials
Sand, gravel, mineral powder, slag, and certain metal powders may cause continuous wear on the screw flight and trough.
For abrasive applications, the following measures may be considered:
Select flight and trough materials with suitable wear resistance.
Increase flight thickness or install replaceable wear liners in areas subject to severe wear.
Apply hardfacing or other wear-resistant treatments to the working edges of the flights.
Adjust the screw speed and trough filling ratio according to the material properties to avoid unnecessary wear.
Corrosive materials
Some chemical powders, solid materials containing corrosive substances, and wet industrial residues may cause chemical corrosion.
The materials in contact with the conveyed product should be selected according to its chemical composition, concentration, temperature, and moisture condition. Options may include suitable stainless steel grades, corrosion-resistant alloys, or protective surface treatments.
Stainless steel is not suitable for every corrosive environment. Different chemicals have different effects on metal materials, so corrosion resistance cannot be determined simply by identifying a component as stainless steel.
For liquid media such as acid and alkaline solutions, it is also necessary to determine whether a screw conveyor is appropriate for the process. Conventional screw conveyors are primarily designed for bulk solids and should not be treated as general-purpose liquid transfer equipment.
Material Temperature
Material temperature affects the operating conditions of the screw flight, shaft, trough, bearings, seals, and lubrication system.
For materials at normal ambient temperatures, conventional carbon steel construction is sufficient for many standard applications. For hot materials such as boiler ash, calcined minerals, and furnace residues, heat-resistant materials and a dedicated equipment design may be required.
High-temperature applications require attention to the following factors:
Material temperature resistance: High temperatures may reduce the strength of certain metals and accelerate oxidation or corrosion.
Thermal expansion: The screw shaft and trough may expand when heated. Support arrangements, fixed points, and expansion compensation must be designed accordingly.
Bearing and seal protection: Heat transfer to bearings, seals, and the gearbox should be limited. Thermal insulation or an extended drive connection may be required.
Lubrication conditions: Bearings and gearboxes must use lubrication methods and materials suitable for the actual operating temperature.
Cooling measures: For applications with a high thermal load, water-cooled shafts, water jackets around the trough, or other cooling arrangements may be considered.
During equipment selection, provide the normal material temperature, maximum operating temperature, and expected temperature variations rather than specifying only an average value.
Other Special Material Properties
In addition to the basic properties above, some materials may absorb moisture, ignite easily, create explosive dust, generate static electricity, release harmful gases, or become compacted during conveying. These characteristics also affect conveyor design and safety requirements.
Materials prone to moisture absorption or caking: Consider trough sealing, cover design, and moisture protection to prevent the material from absorbing ambient moisture and blocking the conveyor.
Materials that generate static electricity: For powders that may accumulate electrostatic charge, equipment grounding and appropriate static control measures should be considered based on the risk assessment.
Combustible and explosive dust: The design should address enclosure, grounding, venting or suppression measures, and electrical and mechanical components suitable for the hazardous area, based on the dust explosion risk assessment.
Toxic or hazardous dust: Improve equipment sealing and dust collection to prevent material leakage and the release of harmful substances.
Materials prone to compaction or lump formation: High-torque drives, lump-breaking devices, or special flight configurations may be considered to prevent material compaction during conveying.
Fibrous or entangling materials: For fibers, wood shavings, and elongated materials, assess the risk of wrapping around the center shaft and the effect of intermediate hanger bearings on material flow. A shaftless design or a dedicated anti-wrapping configuration may be necessary.
For combustible, explosive, or toxic materials, equipment selection must address process safety in addition to conveying capacity. Sealing and protective measures should be determined according to the material hazard assessment and applicable safety requirements.
Installation and Commissioning Requirements
In addition to material properties, installation quality affects screw conveyor performance and service life. The trough, screw shaft, drive assembly, and support components should be inspected and installed according to the equipment drawings and manufacturer's instructions.
Alignment of the Trough and Screw Shaft
The conveyor trough sections must be connected in the correct positions. The complete assembly should be installed without significant bending or twisting. The screw assembly must be positioned so that the clearance between the screw flights and trough meets the specified requirements.
For conveyors with multiple screw sections, check the connection positions and flight phasing between adjacent sections. Incorrect alignment may cause local interference or abnormal wear.
Trough Connections and Sealing
Trough flanges, end plates, and inspection covers must be properly connected and tightened. For applications requiring dust containment, leak prevention, or protection against external moisture, suitable sealing arrangements should be selected according to the material and process requirements.
If gaps are found at the connections, first check the flange surfaces, seals, and fastening condition. Use sealing materials that meet the design requirements rather than adding extra gaskets simply to compensate for installation errors.
Clearance Between the Screw Flight and Trough
The required operating clearance must be maintained between the outer edge of the screw flight and the conveyor trough. This clearance is determined by factors such as screw diameter, shaft deflection, manufacturing tolerances, material particle size, and operating temperature.
Insufficient clearance may cause the flights to rub against the trough. Excessive clearance may reduce conveying performance or increase material residue. For high-temperature equipment, the dimensional changes caused by thermal expansion of the screw shaft and trough must also be considered.
Intermediate Hanger Bearings and Connecting Components
In conveyors with sectional screw shafts, intermediate hanger bearings support the connecting shafts and maintain the position and stability of the screw assembly.
During installation, ensure that each hanger bearing is located at the specified position between adjacent screw sections. Check the fit between the bearing and connecting shaft. An offset hanger bearing, incorrectly installed connecting shaft, or misaligned support may cause abnormal wear and increased operating resistance.
No-Load and Loaded Commissioning
After installation, remove tools, welding slag, and other foreign objects from inside the trough. Then inspect the bearings, gearbox, and other components that require lubrication according to the manufacturer's instructions.
During no-load testing, check whether the screw shaft rotates smoothly. Observe any abnormal vibration, friction noise, or localized heating. Once no-load operation is confirmed to be normal, conduct a loaded test gradually in accordance with the manufacturer's procedures. Check the feeding, conveying, and discharge conditions, and record the motor current, bearing temperature, and overall operating status.
Permissible bearing temperature and temperature rise should be evaluated according to the bearing type, lubrication method, ambient conditions, and manufacturer's technical specifications. Fixed temperature-rise limits of 20°C or 30°C should not be applied to all screw conveyors.
Material Information Required for Screw Conveyor Selection
To avoid a mismatch between the equipment and the actual operating conditions, purchasing and engineering personnel should provide the equipment supplier with as much material information as possible.
| Material parameter | Information to confirm | Effect on equipment selection |
|---|---|---|
| Material name and composition | Main components and possible contaminants | Determines material compatibility and equipment suitability |
| Particle size and lump size | Average particle size, maximum lump size, and particle size distribution | Determines screw diameter and internal clearances |
| Bulk density | Density in the loose or actual conveying condition | Used to calculate mass conveying capacity |
| Moisture content and adhesiveness | Water content, oil content, adhesion, and caking behavior | Affects shafted or shaftless design selection and flight configuration |
| Abrasiveness | Particle hardness and wear characteristics | Determines flight thickness and wear-resistant components |
| Corrosiveness | Chemical composition, concentration, and corrosion conditions | Determines the materials used for product-contact components |
| Operating temperature | Normal temperature, maximum temperature, and temperature variations | Determines material selection, bearing arrangement, and thermal protection |
| Required throughput | Mass or volumetric flow rate per hour | Determines equipment size and drive power |
| Special properties | Combustibility, explosibility, moisture absorption, static electricity, or toxicity | Determines sealing and safety protection requirements |
Conclusion
There is no single set of material requirements that applies to every screw conveyor. Powders, granules, bulk solids, sticky materials, and fibrous materials can all be conveyed using suitable screw conveyor configurations. The equipment structure and operating parameters must be matched to the material properties.
During selection, do not focus only on the material name and required throughput. Particle size distribution, bulk density, moisture content, flowability, abrasiveness, corrosiveness, and operating temperature must also be confirmed. For materials that are sticky, prone to entanglement, or handled at high temperatures, the flight design, shaft configuration, support arrangement, and protective measures require further evaluation.
Hansheng Automation provides CNC precision machining of non-standard metal components according to customer-supplied engineering drawings. Our machining work covers components for yacht equipment, RC racing cars, tobacco and packaging machinery, and filling machinery.
