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Screw Conveyor design calculations step by step

Design of screw feeders (capacity, size)

Page summary

1. Screw conveyor design : methods

2. Design procedure of a screw conveyor

3. Screw conveyor size and capacity calculation tool

Page summary
1. Screw conveyor design : methods
2. Design procedure of a screw conveyor
3. Screw conveyor size and capacity calculation tool

1. Screw conveyor design : methods

How to size a screw feeder ?

Screw conveyors are among the most widespread equipment for transporting and dosing bulk solids. It is thus required in many projects to calculate the size of a screw conveyor in order to reach a required capacity. It can be also required to check the capacity of an existing screw in case the material handled change or the capacity required increases. Few formula exist in the literature and are presented in this short Engineering Guide.

Warning : It must be noted that those formula aim at giving an idea of the size of the screw conveyor (which means its diameter) and the speed at which it will operate, based on some assumptions and some design decisions (choice of screw pitch, inclination). The formula can also be used to roughly check the capacity of an existing screw feeder. They are not meant for a detail design and for manufacture of a new screw, a task that must always be given to a specialized company (for example adhering to CEMA association in US, or SHAPA in UK).

This page is considering that the material is not lumping. It is important, especially when using the screw in mining, concrete industries where special precautions, not detailed here, should be taken.

A typical screw conveyor design is shown below :

Detailed engineering schematic of a screw conveyor showing the trough, screw flight, shaft, hanger bearing, inlet, and discharge outlets.

Figure 1 : Screw conveyor principle drawing and key components

2. Design procedure of a screw conveyor

How to calculate the size and capacity of a screw feeder ?

Known : capacity required of the screw conveyor, material
Unknown : screw size and characteristics

2.1 General formula

The capacity of a screw conveyor with a standard screw flight can be estimated the following way :

\[ Q = 60 \times \left(\frac{\pi}{4}\right) \times D^2 \times S \times N \times \alpha \times \rho \times C \]

Mathematical formula for calculating screw conveyor capacity: Q = (pi/4) * D^2 * S * N * 60 * alpha * rho * C

With :

\(Q\) = screw capacity in kg/h
\(D\) = screw diameter in m
\(S\) = screw pitch in m
\(N\) = screw speed in rpm
\(\alpha\) = loading ratio (trough fill factor)
\(\rho\) = material loose bulk density in kg/m³
\(C\) = inclination correction factor

Step 1 : define the requirement

Define the capacity required for the screw conveyor. The design of the screw must reach a capacity equal or greater than this value.

Example : the requirement for a screw conveying sugar is 3500 kg/h.

Step 2 : calculate the capacity of the screw conveyor

Assume a diameter D
Define the screw pitch (depends on the characteristics of the product to be conveyed, among other parameters) according to the diameter of the screw.

Pitch	Pitch length S
Standard	\(S = D\)
Short	\(S = \frac{2}{3} D\)
Half	\(S = \frac{1}{2} D\)
Long	\(S = 1.5 D\)

Estimate the loading ratio \(\alpha\) of the screw according to the flow properties of the solid to be conveyed

Material	Min loading ratio	Max loading ratio
Not free flowing	0.12	0.15
Average flowability	0.25	0.30
Free flowing	0.4	0.45

To be noted that these are orders of magnitude only. Values can be more or less high, in some cases the loading ratio can even reach 95% for flooded feeder screws under hoppers.

For inclined screw conveyor design : Define if the screw conveyor is flat (which is always preferable) or has to be inclined. Determine the correction factor corresponding.

Inclination in °	Correction factor C
0	1.00
5	0.90
10	0.80
15	0.70
20	0.65

Adjust the screw speed so that the capacity of the screw is higher than the requirement.

Example :

The diameter assumed is 0.1 m
No specific duty for the screw, the pitch is chosen standard, \(S = 0.1\text{ m}\)
Sugar is free flowing, a loading of 0.45 is selected
The screw is installed without inclination, \(C = 1\)
Sugar density is 800 kg/m³

The calculation gives 17 kg/h for 1 rpm. Adjusting the speed, 207 rpm are required to reach a capacity of 3500 kg/h.

Step 3 : compare the calculated capacity to the max screw speed

Some reference max screw speed are given in the table below :

Screw diameter in m	15%	30%A	30%B	45%
0.1	69	139	69	190
0.15	66	132	66	182
0.23	62	122	62	170
0.25	60	118	60	165
0.30	58	111	58	157
0.36	56	104	56	148
0.41	53	97	53	140
0.46	50	90	50	131
0.51	47	82	47	122
0.61	42	68	42	105

If the calculated speed at step 2 is < than the max speed for the screw diameter selected, the design can be kept.

If the calculated speed at step 2 is > than the max speed for the screw diameter selected, the design is not suitable and the calculation must be run again by changing a parameter, typically the diameter.

Note that when handling powders susceptible to lead to a dust explosion, the max speed should give a conveyor tip speed < 1 m/s.

Example :

The diameter selected was 0.1 m for which the max screw speed at 45% is advised to be 190 rpm
The calculated speed is too high, the calculation must be done by changing a parameter. We can select a diameter larger, \(D = 0.15\text{ m}\)
The calculation is run again and this time a speed of 62 rpm is calculated. It is less than the max recommended speed, the screw design can be selected.

2.2 Alternative formula given by CEMA

The CEMA association gives the volumetric capacity of a screw conveyor accounting for shaft pipe displacement as :

\[ C_v = \frac{0.7854 \times \left(D_s^2 - D_p^2\right) \times P \times K \times 60}{1728} \]

With :

\(C_v\) = capacity in ft³/h/rpm
\(D_s\) = Outer diameter of the screw flight in inches
\(D_p\) = Outer diameter of the center pipe/shaft supporting the screw flight in inches
\(P\) = pitch of the screw flight in inches
\(K\) = percent trough loading (e.g. 0.45 for 45%)

It is the capacity per 1 rpm. If the dimensions are known, in the case of an existing screw, the screw capacity per hour can then be found by multiplying by the operating RPM at which the screw is driven.

The formula gives identical results to the metric formula when shaft volume is taken into consideration.

⚙️ Practical Plant Engineering Rules of Thumb & Safety Limits

ATEX Ignition Prevention: For combustible dusts (St 1, St 2, St 3), keep flight peripheral tip speed strictly below 1.0 m/s (197 ft/min) to eliminate the hazard of friction-induced hot spots or mechanical sparks in Zone 20/21/22 equipment.
Trough Loading (Fill Factor \(\alpha\)): Never exceed 15% loading for highly abrasive or heavy materials (alumina, clinker, silica sand) to avoid rapid trough wear and hanger bearing seizure. Non-abrasive, free-flowing grains and powders (sugar, flour, plastic pellets) may operate safely at 30% to 45%.
Flooded Feeder Screws: When mounting directly below hoppers or silos without an upstream dosing valve, use a shrouded trough with variable pitch or tapered shaft to ensure material is drawn evenly across the full length of the inlet slot and avoid rat-holing.
Hanger Bearing Spacing & Deflection: Center shafts must be sized to limit radial deflection to less than 6 mm (0.25 in) over intermediate spans. When screw length exceeds 3 to 3.5 meters (10 to 12 ft), specify hardened intermediate hanger bearings or switch to a shaftless spiral design.
Inclination Derating: Conveyors inclined between 15° and 20° suffer a 30% to 35% volumetric throughput reduction. For steep inclinations (> 25°), specify short pitch flighting (\(S = \frac{2}{3} D\)) and tubular housings to minimize powder back-spill.

3. Screw conveyor size and capacity calculation tool

Interactive Screw Conveyor Sizing & Capacity Tool

CEMA Volumetric Flow Rate, Flight Pitch, RPM & ATEX Tip Speed Verification

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Select Unit System:

SI Metric (kg/h, m, mm, kg/m³) US Customary (lb/h, ft, in, lb/ft³)

Required Mass Flow Rate \(Q\) (kg/h):

Bulk Loose Density \(\rho\) (kg/m³):

Screw Flight Outer Diameter \(D\) (m):

Center Pipe / Shaft Outer Diameter \(D_p\) (mm):

Flight Pitch Type:

Trough Loading Ratio \(\alpha\) (Fill Factor, 0.05 to 0.95):

Conveyor Incline Angle:

Engineering Output & Sizing Results

Flight Pitch Length (\(S\)): 0.1500 m

Net Cross-Sectional Area (\(A_{net}\)): 0.01716 m²

Volumetric Capacity per 1 RPM: 0.0695 m³/h/rpm

Mass Capacity per 1 RPM: 55.61 kg/h/rpm

Operating Screw Speed (\(N\)): 62.9 rpm

CEMA Recommended Max Speed: 182 rpm

Screw Flight Tip Speed (\(v_{tip}\)): 0.49 m/s

Inclination Factor (\(C\)): 1.00

Download Screw Conveyor Sizing Excel Spreadsheet

Get the professional CEMA design template offline. Customize geometric parameters, calculate volumetric capacity, fill ratios, and check conveyor power requirements.

Download Excel Tool Requires Excel 2010 or newer (No Macros)

Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.

Frequently Asked Questions (FAQ)

How is screw conveyor capacity calculated?

Screw conveyor capacity is calculated by multiplying the cross-sectional area of the screw flight by pitch length, rotation speed, loading ratio, material bulk density, and inclination correction factor. The resulting value, usually measured in kilograms per hour (kg/h) or cubic feet per hour, establishes if the selected conveyor diameter can handle the required flow volume safely without overloading the center pipe shaft.

What is the loading ratio (alpha) in a screw conveyor?

The loading ratio (alpha) represents the percentage of the conveyor trough's total cross-sectional area occupied by the bulk solid. It is a critical coefficient derived from the powder's flowability and abrasiveness. Highly abrasive or sluggish materials require a lower trough loading ratio (typically 15% to 30%) to prevent excessive component wear and shaft deflection, whereas free-flowing, non-abrasive bulk solids can be run up to 45% loading.

How does conveyor inclination affect screw feeder capacity?

As the conveyor inclination angle increases, gravity forces the powder to slip backward over the flighting, significantly reducing overall volumetric capacity. Standard screw feeder design dictates applying a downward correction factor (C) to the baseline capacity. For instance, an inclination angle of 10 degrees results in a capacity loss of roughly 20%, requiring a correction factor of 0.8 to re-size the speed or diameter accordingly.