A screw conveyor is used to convey loose materials using the principle of a screw. And this article consists of a step by step guide on designing a horizontal or inclined screw conveyor.

Screw conveyors can be used in horizontal, vertical and inclined directions. But, the efficiency changes with the angle. Compared to vertical screw conveyors, horizontal and inclined screw conveyors have a higher efficiency. 

The biggest advantage of a screw conveyor is the controllability of the flow of materials. 

CAUTION: This guide is only to get a basic understanding. For practical applications, it’s better to use standard datasheets. 

Workable materials for screw conveyors

• All kinds of grains
• Sand 
• Heavy minerals
• Light minerals
• Graphites
• Wheat
• Cement
• Flour
• Hematite
• Leginite
• Ash and slag
• Gravel
• Clay

Note: Screw conveyors are made for a variety of materials. As a result, you cannot use the same screw conveyor for all the materials. And the list of workable materials is not just limited to the above list. For example, in some cases, screw conveyors are used to transport potatoes and limes.

The size of the screw conveyor depends on many factors, but for simplicity, we only look into two factors. 

The size of the screw conveyor is determined by a variety of parameters, but for simplicity, we will just consider two. Those are,

1. The capacity of the conveyor

2. Maximum dimensions of a piece of materials

Capacity of the screw conveyor Q

The maximum operational capacity of the machine is the desired capacity of the screw conveyor. As a result, you should define your specifications in order to compute the capacity.

Assume you need to transport V volume of stuff in t seconds. And the material’s bulk density is ρ.

Bulk density – ρ

If you’re transferring different kinds of materials to different densities, you need to take the highest density among them. Here is the bulk density of some materials. And some more data is on ‘Bulk Density of Different Materials – Data Sheet’.

Material	Bulk density (g/cm^3)	Bulk density (lb/ft^3)
Cement	1.3	81
Charcoal	0.6	37
Coal	0.9	56
Flour	0.6	37
Graphite	0.6	37
Gravel	1.8	112
Mortar	2.1	131
Sand	1.8	112

TIP: The best way to find the bulk density is manual measuring. For that, you can fill the material into a container which you know its volume. And then you can measure the weight using a weight scale.

Mass and Volume Flow Rate of the Screw Conveyor

Find the mass flow rate

Mass flow rate I_M,

V – Volume

ρ – Density

t – Time

Find the volume flow rate

Volume flow rate I_V,

V – Volume

ρ – Density

t – Time

I_M – Mass flow rate

Special equations of mass and volume flow rate of a screw conveyor

And the volume flow rate also equals to,

And the mass flow rate is equal to,

I_M – Mass flow rate

D – Nominal diameter of screw (m)

S – Screw pitch (m)

n – Screw speed in rpm

φ – Loading efficiency of the screw

ρ – Bulk density of the material (kg/m^3)

ψ – Inclination correction factor

If you’re country still use imperial units,

Screw pitch of a screw conveyor – S

Screw pitch is the measurement of the distance between two adjacent threads. It is normally 0.6 to 1 times the screw conveyor’s diameter.

S = 0.6 D

Loading efficiency of the screw – φ

Material	φ
Easier to flow	0.5
Easy to flow	0.4
Medium difficulty to flow	0.3
High difficulty to flow	0.2
Higher difficulty to flow	0.1

Inclination correction factor – ψ

The inclination factor of a screw conveyor depends on its angle to the ground.

Inclination angle (degrees)	Inclination factor ψ	Inclination angle (degrees)	Inclination factor ψ
0	1	11	0.78
1	0.98	12	0.76
2	0.96	13	0.74
3	0.94	14	0.72
4	0.92	15	0.7
5	0.9	16	0.68
6	0.88	17	0.66
7	0.86	18	0.64
8	0.84	19	0.62
9	0.82	20	0.6
10	0.8	21	0.58

Diameter and the Speed

To find the diameter and the speed of the screw conveyor, we need to build a relationship between the two. Therefore, you can use the mass flow rate equation to do it.

And then, we will get an equation with only speed and the diameter as the variables. By changing one variable, note down the possible values for the other variable. And then select the suitable diameter and the speed.

Diameter D	Speed n (RPM)
D1
D2

REMEMBER: If you need more controllability of the flow of materials, it’s better to select low diameter and high speed. But the frictional losses are comparatively higher in such cases. But, if you just want a continuous flow, it’s better to select a large diameter and low speed.  

Length of the screw conveyor – L

There is no specific procedure to determine the length of the screw conveyor. You can select a suitable length depending on the application.

Power calculation of the screw conveyor

Now we need to calculate the drive power of the screw conveyor. This is important to select the motor, shaft, and bearings.

The drive power of the loaded screw conveyor is given by the formula:

P_H = Power required for the material’s progression

P_N = Drive power at no load

P_St = Power due to inclination

Power required for the material’s progression – P_H

I_M – Mass flow rate

g – Gravitational constant

L – Length of the screw conveyor

λ – Progress resistance coefficient

Progress resistance coefficient – λ

Progress resistance coefficient is really a made-up frictional coefficient. You may choose a figure between 2-3 by considering the friction of the material.

Drive power of the screw conveyer at no load P_N

At no load, the screw conveyor’s drive power is,

D – Nominal diameter of screw (m)

L – Length of the screw conveyor

Power due to inclination – P_St

I_M – Mass flow rate

H – Height

g – Gravitational constant

Height, H is the higher of output end, from the input end in the vertical direction. If the output end is in a higher position than the input end, the higher is positive. And, if the output end is lower than the input end, the height is negative.

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