A Screw Conveyor is a versatile and efficient machine for transporting bulk materials such as powders, granules, and semi-solids. It plays a vital role in industries including food processing, plastics, chemicals, construction, and agriculture. For buyers and engineers who plan to customize or optimize a screw conveyor, understanding how to calculate its capacity is essential.

This guide explains the working principle, the key parameters affecting capacity, the calculation formula, influencing factors, and practical examples. 

1. Understanding Screw Conveyor Capacity

The capacity of a screw conveyor refers to the amount of material transported per unit time, usually measured in kg/hr or m³/hr. Capacity depends on several factors, including screw diameter, pitch, rotational speed, fill level, and material density.

Core Concept

The conveyor’s capacity is determined by how much volume the screw can move in one revolution and how many revolutions it completes per hour. In other words:

Capacity = Volume per revolution × Rotational speed × Material fill factor × Material density

Each parameter in this equation influences the final throughput and efficiency of the conveying system.

2. Key Parameters Affecting Capacity

Several technical factors define a screw conveyor’s performance. Understanding these helps in both design selection and capacity optimization.

2.1 Screw Diameter (D)

The diameter of the screw directly affects the volume of material conveyed.
A larger diameter allows more material to be moved per revolution.

Common industrial sizes include Φ141 mm, Φ159 mm, and Φ219 mm, corresponding to typical capacities of 800 kg/hr, 1500 kg/hr, and 2000 kg/hr respectively.

2.2 Screw Pitch (P)

The pitch is the distance between two adjacent blades on the screw.
A longer pitch increases the conveying capacity but reduces mixing efficiency. The standard pitch is typically equal to the screw diameter (P = D).

2.3 Shaft Speed (N)

The rotational speed of the screw (measured in revolutions per minute, RPM) determines how fast material moves. However, excessive speed may cause material backflow or wear. Typical speeds range from 60 to 150 RPM, depending on the material and inclination.

2.4 Fill Factor (Φ)

The fill factor represents the percentage of the screw’s cross-sectional area that is filled with material.
It depends on the material type and inclination angle:

• Horizontal conveyors: 30–45%

• Inclined conveyors: 20–35%

• Vertical conveyors: 15–25%

2.5 Material Density (ρ)

Material density affects total weight transported per hour. For instance:

• Flour: ~600 kg/m³

• Sand: ~1500 kg/m³

• Plastic pellets: ~750 kg/m³

2.6 Conveyor Inclination (θ)

The steeper the angle, the lower the conveying efficiency. Inclination usually reduces the fill factor and requires more motor power to maintain throughput.

3. Screw Conveyor Capacity Formula

The general formula for calculating screw conveyor capacity is:

[ Q = \frac{\pi \times D^2}{4} \times P \times N \times Φ \times ρ ]

Where:

• Q = Capacity (kg/hr or m³/hr)

• D = Screw diameter (m)

• P = Pitch of the screw (m)

• N = Rotational speed (r/min)

• Φ = Fill factor (decimal, e.g., 0.35 for 35%)

• ρ = Bulk density of the material (kg/m³)

Example Calculation

Suppose a screw conveyor has the following specifications:

• Diameter (D): 0.159 m

• Pitch (P): 0.159 m

• Speed (N): 100 rpm

• Fill factor (Φ): 0.35

• Density (ρ): 800 kg/m³

Then:

[ Q = \frac{3.1416 \times (0.159)^2}{4} \times 0.159 \times 100 \times 0.35 \times 800 ]

[ Q ≈ 1500 \text{ kg/hr} ]

This aligns with real-world data for a Φ159×3 mm tube screw conveyor using a 3×4-pole motor, confirming the accuracy of the calculation.

4. Practical Design Considerations

4.1 Tube Size and Material

The tube (or trough) must be strong enough to handle load pressure and reduce friction. Common options include stainless steel for food-grade use and carbon steel for heavy-duty materials like cement or sand.

4.2 Motor Power

Motor selection depends on capacity and incline.
Typical configurations include:

A higher power rating ensures smooth conveying of dense or heavy materials.

4.3 Incline Adjustment

Incline angles above 45° significantly reduce efficiency. To maintain desired output, consider increasing screw speed, diameter, or using a twin-screw system.

4.4 Material Flow Behavior

Powders with poor flowability (like cement) require slower speeds and larger pitches to prevent clogging. Granules or pellets, on the other hand, allow higher speeds.

4.5 Discharge Control

To avoid overflow or blockage, ensure the outlet design matches downstream equipment capacity, such as mixers, silos, or packaging machines.

5. Performance Optimization and Maintenance

Once the system is installed, ongoing monitoring and preventive maintenance are essential to preserve capacity and avoid performance loss.

5.1 Optimize Load Conditions

Maintain a consistent feed rate. Irregular feeding causes surges and reduces efficiency. Installing a vibrating feeder or hopper gate ensures stable loading.

5.2 Regular Cleaning

Residual buildup in the tube can reduce the effective conveying area and increase motor load. Clean the screw and housing periodically, especially when handling sticky materials.

5.3 Lubrication and Bearing Care

Lubricate bearings according to the manufacturer’s schedule. Proper lubrication reduces friction, temperature, and noise, extending service life.

5.4 Monitor Wear and Blade Condition

Inspect screw blades every few months. Worn or deformed blades reduce capacity and may cause uneven discharge.

5.5 Safety and Operation

• Avoid running the conveyor empty for long periods.

• Never open inspection covers while operating.

• Use overload protection for motor safety.

With proper care, a screw conveyor can maintain over 90% of its rated capacity for years.

Conclusion

Calculating the capacity of a screw conveyor is essential for designing an efficient bulk handling system.
By applying the standard capacity formula and considering parameters such as diameter, pitch, speed, fill factor, and density, engineers and buyers can accurately determine throughput and select the right model for their process.

A properly sized and maintained screw conveyor ensures:

• Stable and consistent material flow

• Reduced downtime and energy cost

• Long service life under continuous operation

As a professional screw conveyor supplier, offering customized OEM and ODM designs allows clients to achieve optimal performance across diverse industries — from food and plastics to construction and energy.
Understanding capacity calculation is the first step toward a reliable and productive conveying system.