Screw Conveyor: Design, Use, and Selection Guide
For the controlled transport of powders, granules, and bulk raw materials over short and medium distances, the screw conveyor is one of the most widely used types of mechanical transfer equipment in industry.
Due to their simple appearance, screw conveyor systems are often viewed merely as a “rotating screw.” However, behind a properly functioning screw conveyor lie numerous interrelated parameters, such as:
- product density,
- particle size,
- flowability,
- abrasiveness,
- target capacity,
- screw diameter,
- pitch,
- rotational speed,
- fill ratio,
- incline,
- bearing arrangement,
- and motor and gearbox selection
.
An improperly selected screw conveyor:
- may fail to deliver the desired capacity,
- may over-compress the product,
- may cause the motor to operate continuously at high current,
- may rapidly wear out the suspension bearings,
- may cause product breakage,
- may create bridging under the hopper,
- may become clogged,
- and may result in maintenance costs significantly higher than expected.
Therefore, the correct question is not:
“What size screw (in millimeters) should we use?” but rather, “For this product and capacity, which diameter, pitch, speed, and fill ratio will provide the optimal operating conditions?”
It should be.
International screw conveyor design guides also outline the selection process as follows: first, defining the product characteristics; then, determining capacity and speed; performing power and torque calculations; and finally, selecting the equipment components.
In this guide, we will examine the screw conveyor in detail from the perspectives of design and process engineering.
What Is a Screw Conveyor?
A screw conveyor is a mechanical conveying system that operates on the principle of spiral blades attached to a rotating shaft or tube, which move material through the housing.
The most common equivalent in English technical terminology is:
Screw Conveyor
.
The system generally consists of the following components:
- screw shaft or main tube,
- spiral flights,
- U-shaped or tubular housing,
- inlet,
- outlet,
- drive motor,
- gear reducer,
- end bearings,
- intermediate support bearings (if required),
- covers, and fasteners.
The product is pushed forward inside the housing as the helical blades rotate.
How Does a Helical Conveyor Work?
When the helical shaft rotates, the blades exert an axial force on the product.
Although the material tends to rotate, the friction between the housing and the product prevents the product from rotating completely with the screw.
As a result, the product moves in the following direction:
inlet → screw flights → housing → outlet
.
The amount of product transported by the system is directly influenced by the following variables:
- screw diameter,
- screw speed,
- pitch,
- fill ratio,
- and the product’s bulk density.
For this reason, two different screws of the same diameter can have very different capacities in metric tons per hour.
Are Screw Conveyors and Screw Conveyors the Same Thing?
In Turkey;
- screw,
- screw conveyor,
- screw feeder,
- screw conveyor
are terms often used interchangeably to refer to the same family of equipment.
In English, however, the most common term is:
screw conveyor
.
However, there is another important difference in terminology:
Although a screw conveyor and a screw feeder may appear to be the same equipment, they are not used for the same purpose.
KWS’s engineering guide specifically distinguishes between a “screw conveyor,” which transports material fed in a controlled manner from one point to another, and a “screw feeder,” which operates with the inlet fully loaded with product (i.e., flood-loaded) and controls the discharge rate.
The Difference Between a Helical Conveyor and a Helical Feeder
This distinction is particularly important in applications under silos and bunkers.
Screw Conveyor
Primary function:
to transport the product.
The amount of product at the inlet is typically controlled by another piece of equipment or process.
Screw Feeder
Primary function:
to control the product discharge rate.
In most cases,
- the entire product volume fills the screw inlet at the bottom of a:
- silo,
- hopper,
or bunker.
For this reason, load distribution and inlet geometry become more critical in feeder design.
Accidentally installing a standard screw conveyor directly beneath a bunker can result in:
- uneven pull,
- bridging,
- product discharging only from a portion of the inlet,
- and motor overload.
Where Are Screw Conveyors Used?
Screw conveyors are particularly effective solutions for short- and medium-distance bulk material transfer.
Common applications include:
- under silos,
- under bunkers,
- Big Bag discharge outlets,
- bag unloading stations,
- mixer feeding,
- mixer discharge,
- under filters,
- dosing systems,
- packaging lines,
- elevator feeding,
- transfer between process machines.
Which Products Can Be Conveyed Using a Screw Conveyor?
When properly designed, the following can be conveyed:
- flour,
- starch,
- sugar,
- plastic pellets,
- feed,
- minerals,
- cement,
- chemical powders,
- ash,
- additives,
- powdered and granular raw materials
can be conveyed.
However, the notion that “a screw conveyor can handle any powder” is incorrect.
In particular, the product’s:
- cohesion,
- adhesiveness,
- abrasiveness,
- and particle size
can significantly alter the design.
The First Step in Screw Conveyor Design: Defining the Product
A professional selection process should not begin with a capacity calculation.
The first question should be:
“What are we conveying?”
KWS lists the fundamental starting parameters for screw conveyor selection as product type, maximum particle size, bulk density, desired capacity, conveying distance/incline, and operating conditions.
Why Is Bulk Density Important?
Bulk density is one of the key pieces of data that enables the conversion of screw conveyor capacity into metric tons per hour.
A screw conveyor essentially moves volume.
Therefore, the initial calculation is based on
volumetric capacity
.
For example, let’s assume the same screw conveyor moves 10 m³ per hour.
Product A
Bulk density: 400 kg/m³
Mass capacity:
10 × 400 = 4,000 kg/h
Product B
Bulk density: 1,200 kg/m³
Mass capacity:
10 × 1,200 = 12,000 kg/h
The metric tons per hour value for the same equipment can vary by a factor of three depending on the product density.
KWS emphasizes that capacity calculations are volumetric and that the correct bulk density data is required to convert the metric tons per hour value to volumetric capacity.
Why Is Particle Size Important?
Maximum particle size is one of the critical parameters in selecting the screw diameter.
This is because large particles must
- fit into the clearances between the screw shaft,
- the flight,
- and the housing.
Large and hard lumps of an inappropriate size can cause problems such as
jamming,
- flight deformation,
- and increased shaft load.
- International engineering tables clearly state that when determining the screw conveyor diameter, not only the capacity but also the maximum lump size must be considered.
International engineering tables clearly state that when determining the screw conveyor diameter, not only the capacity but also the maximum lump size must be taken into account.
Why Is Product Flowability Important?
A free-flowing plastic granule does not behave the same way as a moist, cohesive fine powder.
A free-flowing product:
- fills easily,
- moves easily,
- and causes less buildup in the housing.
A cohesive product, on the other hand:
- may stick to the blades,
- may accumulate in the housing,
- may restrict the outlet,
- and may increase motor torque.
Therefore, it is essential to understand not just the product’s name, but also its actual behavior under process conditions.
How Does Abrasiveness Affect Screw Design?
In abrasive products:
- the screw flight,
- the barrel,
- the bearing housings,
- and the inlet and outlet points
are subject to wear.
Therefore, if necessary:
- thicker flights,
- replaceable wear plates,
- hardfacing,
- and wear-resistant steel
can be considered.
Maintaining recommended trough loading levels at lower levels in abrasive bulk materials is also one of the design approaches used to control wear.
How Is Screw Conveyor Capacity Calculated?
In simplified terms, the theoretical volumetric capacity is related to the following parameters:
Q ∝ A × P × N × φ
Where:
- Q: volumetric capacity
- A: effective cross-sectional area of the screw
- P: pitch
- N: rotational speed
- φ: fill factor
In actual calculations,
- corrections must also be made for factors such as
- the center shaft diameter,
- screw geometry,
- pitch,
and specific flight type
.
KWS’s selection approach follows the sequence of determining the required volumetric capacity, selecting the appropriate trough loading percentage for the product, finding the suitable diameter, and then calculating the actual rotational speed.
Example Screw Conveyor Capacity Calculation
The following example has been prepared solely to illustrate the calculation logic.
Let’s assume:
Target capacity:
12,000 kg/h
Bulk density:
750 kg/m³
First, the required volumetric capacity:
12,000 / 750 = 16 m³/h
is calculated.
Thus, the auger’s minimum target is approximately:
16 m³/h
of volumetric transfer.
From this point onward;
- the recommended filling ratio of the product,
- the screw diameter,
- the pitch,
- the actual rotational speed,
- and the incline
must be calculated.
It would not be technically correct to simply state, “This diameter is sufficient for 16 m³/h.”
How Is the Screw Diameter Selected?
The screw diameter directly affects capacity.
As the diameter increases;
- the effective conveying cross-section increases,
- and the volume that can be conveyed in one revolution increases.
However, the diameter is not determined solely based on capacity.
At the same time:
- the maximum product particle size,
- operating speed,
- hull loading,
- and system length
are also taken into account.
Is a Larger Screw Always Better?
No.
An unnecessarily large screw can result in:
- higher equipment costs,
- a larger motor/gearbox,
- more product buildup,
- and cleaning difficulties.
The goal is not to use the largest possible equipment;
the objective is to select the optimal diameter that achieves the target capacity at an appropriate speed and fill level.
What Is Screw Pitch?
Pitch is the distance between two consecutive spiral flights along the axis.
In most standard screw designs, the pitch may be approximately equal to the screw diameter.
However, in different applications,
- full pitch,
- short pitch,
- half pitch,
- and long pitch
can be used.
KWS capacity data also shows that systems with shorter pitches, compared to the standard full-pitch system, provide different capacities at the same speed and fill rate. Reducing the pitch decreases the volume advanced per revolution.
When Is a Short Pitch Used?
A short pitch may be preferred for:
- inclined conveying,
- product control,
- certain feeder applications,
- and dense and difficult-to-handle materials.
However, capacity may be reduced compared to a full-pitch system.
What Is a Variable-Pitch Screw?
In screw feeder systems, particularly those located beneath silos or bunkers, the pitch can be adjusted along the length of the feed.
The goal is to draw material more uniformly along the entire length of the hopper, rather than just from the initial section of the inlet.
For example:
short pitch → medium pitch → full pitch
This approach can be used.
This design presents a different engineering challenge than a standard transfer screw.
How Is Screw Speed Determined?
In theory, capacity may increase as speed increases.
However, the “we need capacity, so let’s increase the speed” approach is not always correct.
High speed can:
- increase wear,
- increase product breakage,
- reduce the service life of suspension bearings,
- and affect power requirements.
KWS’s capacity guide states that the maximum speed tables should not be applied directly to every product and that lower speeds may be preferable, especially in industrial applications.
Low Speed or Large Diameter?
Two basic approaches can be considered to achieve the same capacity:
small diameter + high speed
or
large diameter + low speed.
In particular,
sensitive products, a larger diameter and lower speed may be advantageous.
However, the investment cost may increase.
Therefore, the selection should be made by considering both CAPEX and OPEX together.
What Is the Screw Fill Ratio?
It indicates how full the screw body is with product.
A 100% fill ratio is not suitable for every product.
Excessive fill can cause:
- increased torque,
- product jamming,
- suspension bearings operating within the product,
- and motor overload.
For this reason,
This is why it is important that the trough loading ratios recommended in international design tables vary depending on the product’s abrasiveness and flow behavior.
Why Does Capacity Decrease in Inclined Screw Conveyors?
As the screw inclines upward from a horizontal position, the material is transported against gravity.
As the incline increases:
- the product may slip backward,
- effective fill factor may decrease,
- and capacity may decrease.
Therefore, the capacity calculated for a horizontal screw conveyor should not be directly applied to an inclined system.
When the incline increases, design modifications such as:
- higher speed,
- different pitch,
- and larger diameter
may generally be required.
Are Vertical Screw Conveyors Used?
It is possible with a custom design.
However, a vertical screw conveyor operates differently from a standard horizontal screw conveyor and may require high speeds or special feeding methods.
For long vertical distances:
- bucket elevators,
- pneumatic conveying
must also be evaluated as alternatives.
Screw Conveyor or Bucket Elevator?
Criterion Screw Conveyor Bucket Elevator
Horizontal transport Very suitable Not suitable
Short distances Very suitable May be unnecessary
High vertical lift Limited Very suitable
System structure Simple More complex
Capacity Can be medium to high Can be very high
Product friction Higher May be lower
Especially for significant vertical rises of 15–30 meters, a bucket elevator may be the more appropriate alternative.
An internal link to the article Guide to Selecting Bucket and Chain Elevators should be provided from this section.
Screw Conveyor or Chain Conveyor?
Both systems are mechanical bulk handling solutions, but their application areas may differ.
Screw Conveyor
Advantages:
- compact design,
- controlled transfer,
- short distances,
- easy integration with process equipment.
Chain Conveyor
Advantages:
- long horizontal runs,
- heavy-duty operation,
- high capacity,
- low-speed transport.
For long-distance, high-tonnage applications, a chain conveyor may be more economical.
Screw Conveyor or Pneumatic Conveying?
Criterion Screw Conveyor Pneumatic Conveying
Short distance High Unnecessary
Long distance Limited High
Complex route Weak High
Moving parts Many None in the pipe
Enclosed transport Yes Yes
Product friction May be high Depends on speed
Energy May be advantageous on short lines Air generation required
Screw conveyors can be a highly efficient solution, especially for transfers of a few meters between process equipment.
Big Bag Discharge via Screw Conveyor
This is one of the most common process integrations.
Example:
Big Bag → Discharge Hopper → Screw Conveyor → Mixer
In this configuration, the screw conveyor:
- can transport the product to the mixer level,
- and can provide controlled feeding.
However, the product flow from the Big Bag hopper must be compatible with the screw conveyor’s capacity.
A Critical Consideration When Selecting an Auger for a Big Bag
If the product flows freely inside the Big Bag, the auger inlet may be overloaded uncontrollably.
Therefore, the hopper outlet must be controlled using a design featuring:
- a slide gate,
- a star feeder,
- or an appropriate screw feeder.
A link to the Big Bag Discharge Systems guide should be provided here.
Use of Screws at the Bag Discharge Station
Typical process:
Bag Discharge → Screen/Magnet → Hopper → Screw → Process
can be set up in this manner.
Here, the screw:
- transports the product to the process above or to the side,
- reduces the need for manual product handling by the operator.
For dusty products, a filter or extraction system should also be used at the station.
Feeding a Mixer with an Auger
One of the most effective applications of augers is feeding a mixer.
However, in a batch mixer process, it’s not just the tonnage per hour that matters;
it’s how many seconds it takes to feed a single batch.
For example:
Mixer batch:
1,000 kg
Target feeding time:
5 minutes
Required instantaneous feed rate:
1,000 / 5 × 60 = 12,000 kg/h
In other words, even if the plant produces an average of 4 t/h, the mixer feed screw may need an instantaneous capacity of 12 t/h.
This difference is of critical importance in the design.
Can Dosing Be Performed with a Screw Conveyor?
Yes, a screw conveyor can be used as a controlled feeder.
However, a screw conveyor is not the same as a high-precision dosing system.
For dosing, a screw conveyor can be integrated with:
- VFD control,
- load cells,
- a weighing system,
- and gravimetric control
.
Volumetric Screw Feeder
It operates on the principle of conveying approximately the same volume of product per revolution when the speed is constant.
However, if the product’s bulk density changes, the weight-based dosing may vary.
Gravimetric Screw Feeder
The system adjusts the screw speed by measuring the actual weight change of the product.
Therefore, more accurate dosing can be achieved with products of varying densities.
Screw Blade Types
Standard full-flight blades are not used in every application.
Different types are available:
- standard full flight,
- ribbon flight,
- cut flight,
- cut and folded flight,
- paddle flight.
Special blade geometries can be used for process functions such as:
- mixing,
- product aeration,
- and controlled conveying.
KWS’s engineering guide clearly states that special flight and pitch designs alter capacity and that a correction factor must be applied to the standard capacity calculation.
KWS’s engineering guide clearly shows that special flight and pitch designs alter capacity and that a correction factor must be applied to the standard capacity calculation.
When Should Ribbon Flight Be Used?
In ribbon-type screws, the spiral flight is not continuously connected to the main shaft.
It can offer advantages, particularly in certain;
- sticky,
- viscous,
- processes where the product tends to accumulate on the shaft.
However, torque and mechanical strength must also be evaluated.
Screw Shaft Types
U-type screw
Advantages:
maintenance access,
the ability to open the top cover,
ease of cleaning.
Tubular Screw
Advantages:
- compact design,
- more enclosed system,
- suitable for inclined applications.
The selection should be based on maintenance and process requirements.
Carbon Steel or Stainless Steel?
Carbon Steel
It is an economical solution for many industrial applications.
Stainless
It can be considered especially in processes involving:
- food,
- chemicals,
- corrosion risk,
- and frequent cleaning.
However, using stainless steel does not automatically make the system hygienic.
Weld quality, surface finish, bearing arrangements, and dead zones must also be evaluated.
Weld quality, surface finish, bearing arrangements, and dead zones must also be evaluated.
What Is a Hanger Bearing?
In long helical screws, it may not be possible to support the shaft at only both ends.
In such cases, hanger bearings are used at intermediate points.
Since hanger bearings operate directly within the product stream, they are susceptible to wear.
If the product is:
the selection of the bearing becomes critical.
Why Is Screw Length Important?
As the screw length increases:
- shaft torsion,
- bearing requirements,
- power,
- torque,
- and alignment
become more critical.
In some projects, using multiple conveyors may be more appropriate than a single, very long auger.
How Is Auger Motor Power Determined?
Motor power is not determined solely by capacity.
Essentially:
- product transport,
- mechanical friction,
- incline,
- bearings,
- idle and loaded operation,
- and start-up conditions
are all taken into account.
In KWS’s design methodology, after capacity and speed are determined, horsepower and then component torque are addressed as separate engineering steps.
Why Can Screw Torque Be More Critical Than Motor Power?
Especially if the screw is stationary while loaded with product, restarting may require high torque.
Even if the motor appears sufficient in nominal kW, the
- gearbox,
- coupling,
- and shaft
may have insufficient torque capacity.
Therefore, power and torque must be verified separately.
Why Does the Screw Conveyor Clog?
Main causes:
Overfeeding
The system receives more product than its design capacity allows.
Wet/sticky product
Build-up occurs on the blades.
Foreign object
It may become wedged between the shaft and the housing.
Incorrect discharge
If the product cannot exit the auger quickly enough, it accumulates backward.
Low Torque
The motor cannot drive the system under full load.
Why Does Screw Conveyor Capacity Decrease?
If the system no longer meets its initial capacity:
- the vanes may be worn,
- the screw speed may have decreased,
- the product’s bulk density may have changed,
- there may be insufficient product at the inlet,
- there may be buildup in the housing,
- or the outlet may have narrowed.
A reduction in the diameter of a worn blade can significantly affect actual capacity.
Why Do Screw Blades Wear Out?
The most important factors:
- product hardness,
- product speed,
- load,
- operating time,
- material selection.
Wear is often more pronounced on the outer diameter of the blade.
This area can be reinforced with a hard filler or wear plate if necessary.
What Causes a Shaft Breakage?
Main causes:
- excessive torque,
- foreign object,
- material jamming,
- improper bearing mounting,
- fatigue,
- misalignment.
It is incorrect to attribute a shaft breakage solely to the shaft being “too thin.”
The reason why the system experienced excessive torque must be investigated.
Screw Conveyor Maintenance Checklist
Periodically:
- motor current,
- gearbox,
- bearing temperature,
- coupling,
- suspension bearings,
- screw blade thickness,
- housings wear,
- shaft alignment,
- fastening bolts
must be checked.
Quick Troubleshooting Table
Problem Possible Cause
Low capacity Blade wear / low feed rate
High motor current Excessive product / jamming
Screw stops Insufficient torque / foreign object
Increased noise Support bearing / misalignment
Vibration Shaft deflection / imbalance
Product is backing up Output is blocked
Housing is wearing Abrasive product
Product is breaking High speed
Product is accumulating Sticking / improper flights
Bearing is hot Misalignment / excessive load
10 Common Mistakes in Screw Selection
1. Making a selection based solely on tons per hour
Bulk density must be known.
2. Ignoring the maximum particle size
Large particles can become jammed.
3. Assuming a 100% fill rate
This is unrealistic.
4. Disregarding the incline
Capacity may decrease as the incline increases.
5. Increasing the speed more than necessary
Wear and product damage may increase.
6. Installing a standard conveyor under the bunker as a feeder
This can cause irregular product flow.
7. Failing to perform a torque calculation
The system may not operate during a full-load startup.
8. Selecting suspension bearings independently of the product
Service life can be significantly reduced with abrasive products.
9. Considering cleanability only after the fact
This can be critical in food and chemical processes.
10. Failing to compare alternative conveying systems
In some projects, a chain conveyor, elevator, or pneumatic conveying system may be more appropriate.
Technical Information Required for a Screw Conveyor Quote
Data Required Information
Product name Raw material
Bulk density kg/m³
Capacity kg/h or t/h
Particle size mm
Maximum Lump Size mm
Flowability Free-flowing / cohesive
Abrasiveness Low / medium / high
Moisture %
Temperature °C
Conveying Distance m
Incline Degree
Feed Method Controlled / flood-loaded
Starting Point Silo, bunker, Big Bag
Destination Mixer, elevator, etc.
Operation Continuous / batch
Material Carbon steel / stainless steel
Cleaning Standard / hygienic
Without this data, it is not possible to create an accurate design based solely on the “5-meter screw conveyor price.”
Screw Conveyor Selection Checklist
Before making a technical selection:
- What is the product?
- What is the bulk density in kg/m³?
- Does the product flow freely?
- What is the largest particle size in mm?
- Is the product abrasive?
- Is the product brittle?
- Is the product sticky?
- What is the capacity in metric tons per hour?
- What is the distance in meters?
- What is the incline in degrees?
- Will it operate under a silo?
- Is a conveyor or a feeder required?
- Carbon steel or stainless steel?
- Will there be a change in the product?
- How often will cleaning be required?
- Is there a risk of starting with a full load?
Frequently Asked Questions
What is a screw conveyor?
A screw conveyor is a mechanical conveying system that transports powdered and granular bulk materials within an enclosed housing via the rotation of a shaft with spiral flights.
How is the capacity of a screw conveyor calculated?
Capacity is determined by evaluating the screw diameter, pitch, speed, fill ratio, and the product’s bulk density together.
How is the screw diameter selected?
It should be selected based on the target volumetric capacity, product particle size, operating speed, and the recommended trough loading ratio.
Can a screw conveyor operate on an incline?
Yes. However, as the incline increases, the actual capacity may decrease, and the design must be adjusted accordingly.
What is the difference between a screw conveyor and a screw feeder?
While a conveyor transports a controlled feed of product, a screw feeder can operate in flood-loaded mode to control the product flow rate from a silo or bunker outlet.
Why does a screw conveyor clog?
Overfeeding, sticky material, foreign objects, insufficient torque, or a blocked outlet are the primary causes.
Does increasing the screw conveyor’s speed increase capacity?
It may increase within certain limits; however, since wear, breakage, and mechanical loads may also increase, the design should not be based on the maximum speed.
When should a stainless steel screw conveyor be preferred?
It can be considered in processes where corrosion resistance, hygiene, or food and chemical compatibility are required.
Can dosing be performed with a screw conveyor?
Yes. It can be used as a volumetric or gravimetric feeder with a VFD and weighing system.
Is a screw conveyor or pneumatic conveying better?
Screw conveyors may offer advantages for short, controlled transfers. Pneumatic systems are more flexible for long and complex routes.
Conclusion: How to Choose the Right Screw Conveyor?
Screw conveyors are pieces of equipment used in industrial bulk material handling processes that may seem simple but can cause serious capacity and maintenance problems if selected incorrectly.
The correct design must be determined by evaluating the following factors together:
product characteristics + capacity + diameter + pitch + speed + fill level + incline + torque + material selection
In particular, the screw diameter should not be determined solely based on tons per hour without knowing the product’s bulk density value.
Similarly, simply increasing the speed to achieve higher capacity can lead to wear, product breakage, and bearing issues.
Similarly, simply increasing the speed to achieve higher capacity can lead to:
- wear,
- product breakage,
- and bearing issues.
For long horizontal transport runs, chain conveyors; for high vertical distances, bucket elevators; and for complex route geometries, pneumatic conveying should also be considered as alternatives.
The right screw conveyor;
is not the one with the largest diameter or the fastest speed, but the one that conveys the product at the desired capacity in a controlled and sustainable manner.
Let’s Evaluate Your Screw Conveyor Project with Technical Data
Share the name of the product to be conveyed, its bulk density, capacity, conveying distance, incline, and source/destination equipment with the MASTRON Process engineering team.
For your application:
- screw conveyor,
- screw feeder,
- chain conveyor,
- bucket elevator,
- pneumatic conveying
—let’s evaluate these alternatives through the same process.