Screw conveyor

Screw conveyor design: Essential guide to those beautiful curves!

Screw conveyor design: Essential guide to those beautiful curves!

If conducted, it would be the most sought out title in the conveyor world! “The beauty contest of conveyors”. And who would win the glorious crown? It would be none other than the SCREW CONVEYOR.

With her beautiful curves, controlled and disciplined “gait” and the smooth delivery, who else could beat her?

In this guide to screw conveyor and screw conveyor design we will see the following aspects in detail:

  • What is a screw conveyor?
  • History of screw conveyor design and its current applications
  • Common types of screw conveyor design types
  • Principle of Operation
  • Special applications and material types conveyed
  • Screw conveyor design calculations
  • Design of inclined screw conveyor
  • Advantages and disadvantages of screw conveyor
  • Maintenance and cleaning of screw conveyor

First, we will look at the operation of a screw conveyor:

 

 

What is a screw conveyor?

A screw conveyor is a bulk material conveyor that comprises of a rotating helicoidflighting’ that is mounted on a shaft inside a pipe. It is used to transport a wide range of free-flowing materials from food items to explosive dust.

It is also known as auger conveyor, helix conveyor or spiral conveyor.

The screw blades that carry the material forward are called flights.

A driving motor, mostly mounted at the discharge end of the conveyor, rotates the screw blade. This, in turn, moves liquid or free-flowing granular materials in an industrial process.

History of screw conveyor: The magical Archimedes’ screw!

The screw conveyor design is attributed to the great Greek scientist Archimedes. He designed a machine, called Archimedes screw, to pump water from low-lying areas for irrigation purposes.

It was an internal helical conveyor which effectively scoops and carries water upwards. It pours water out of the hollow pipe through the outlet.

This screw pump drive system was using hand or connected to a windmill.

 

Illustration of Archimedes screw
The great Greek scientist Archimedes designed the first screw conveyor. This is an illustration of Archimedes Screw.

Where do they stand today?

Presently, screw conveyors occupy an exceptional position in industrial material handling. This is due to the efficiency and economy of these conveyors. Contemporary technological advancements in process controls have contributed to the automation of screw conveyor mechanisms.

Accurate dosing control of the conveyed material is possible today in batch mixing applications. In this regard, modern screw conveyor mechanisms are commonly used to meet bulk material-handling needs in different industries.

Common industries are

  • Food production
  • Waste management
  • Mining
  • Agriculture

Horizontal, vertical, and inclined installations are possible depending upon the industrial need.

Screw conveyor used for dry sugar transport
Tubular screw conveyor used in dry sugar transport in a food processing plant

Common types of screw conveyor design

Different applications use different types of screw designs.

Following are most common types:

  • Helicoid conveyor screws
  • Sectional conveyor screws
  • Sectional flights
  • Ribbon flights
  • Paddle conveyor screws
  • Cut & cut folded screws
  • Internal Collars
  • Coupling Bolts and Quick Release Keys.

Principle of Operation

A helical shaft or a ‘shaftless-spiral’ is coupled with a driving motor on the driving end. This shaft rotates inside a “U-shaped” trough, tube, or conduit.

During rotation, the flights on the screw carry the product through the tube forward.

The opposite end of the trough is free to allow the material exit during rotation. The material volume transferred is proportional to the rotational rate of the shaft.

Screw conveyor animation
Screw conveyor animation. Source: Wikicommons

The weight and friction of the conveyed material and the trough walls prevents the material from rotating with the shaft. The smooth finish of the screw also helps in preventing this.

In practical industrial control applications, a driving unit with a variable feed rate controls the rate of rotation. The rate will be as per the desired delivery rate specified for each process stage.

Based on this principle, screw conveyors can accommodate operation in inclined positions. So screw conveyors are helpful in elevation or conveying applications. The elevation operation is economical. However, inclined operation suffers a limitation. As the angle of inclination increases, the capacity decreases.

Rotation of the screw conveyor may be either right hand or left hand. The direction of rotation depends on the form of the helix blade used in the shaft.

We need to look at the end of the screw shaft to determine the screw hand direction.

For a right-hand screw conveyor, when conveyed material flows, the direction of flow away from the intake end is counter clockwise. Whereas left-hand screw conveyor turns in a clockwise rotation.

Special Applications and types of material

The design considerations and construction of screw conveyors depend on the application. Diverse industries use different screw conveyor types.

Common applications involve the use of screw conveyors as feeders, collectors, distributors, or mixers equipped with either heating or cooling systems while they operate.

During the screw conveyor design stage, for specific applications, special considerations are taken. This makes the selection of components based on the type of material conveyed.

For example, if gentler treatment of the product is required, shaftless screw feeder conveyor with specially designed flights are used. An excellent example is shown in the following video:

Applications involving the following material need special screw conveyor design considerations.

Corrosive materials:

When conveying such material, conveyor components are from alloys that are corrosion resistant, or coats of protective substances.

Contaminable materials:

Such materials warrant the use of oil dry-type bearings that are impregnated and sealed. Similarly, the end shafts are sealed thus preventing the entrance of external contaminants. Screw conveyors for such applications need frequent cleaning. Here conveyor components are to be convenient to disassemble.

Abrasive materials:

Conveyors handling abrasive material have their troughs, housings, and shafts constructed from abrasion resistant alloys with hardened screws. Rubber or special resin lining for all exposed surfaces helps to reduce abrasive damage.

Hygroscopic materials:

Feeders handling such materials need the conveyors to have strong seals and packing. This prevents the exterior atmosphere from entering the housing.

Usage of jacketed trough/housings for the extremely hygroscopic material is common. The jackets will have an appropriate circulating medium to maintain the temperature.

Viscous/ sticky materials:

Ribbon flight conveyor screws are used to convey these materials. Alternatively, the standard components of the conveyor can be specially coated to improve material flow.

Harmful vapors /dust:

Screw conveyor with sealed troughs in plain tubular housings or gasket-flanged tubular housing safely handles such dust.

Blending in transit:

Screw types like ribbon, cut flight, paddle, or a combination of all these are used in blending lines. The selection depends on the required grade of blending, aeration or mixing.

Explosive dust:

A screw conveyor handling such materials are designed to have proper sealing and non-ferrous installations. This will cut the risks involved with handling such material.

Screw Conveyor design calculations

Capacity

The capacity of a screw conveyor is given in cubic feet per hour (ft3/hr.) at one revolution per minute. The capacities of various sizes of screw conveyors, under five cross-sectional loadings, are available in manufacturers manuals.

The five cross-sectional loadings used in capacity tables are 15%, 30% A, 30% B, 45%, and 95% of the total screw conveyor cross section.

screw conveyor capacity chart
One of the capacity chart considering 45% cross-sectional loading. Source: Screw Conveyor Catalogue and Engineering Manual (1996) from Continental Conveyor Company

The capacity values presented in capacity manuals can account for most industrial applications. The maximum capacity for any screw conveyor size in given loading conditions along with material type available in cubic feet per hour. The maximum recommended speeds are also available.

Screw conveyor Speed

The formula for screw conveyor speed (having screws with a standard-pitch helical flight) is:

Screw conveyor speed formula

Where N is revolutions per minute of the screw

Remember, this is applicable for only screw conveyor with normal helical flights.

Some special screw types are short pitch, cut flights, ribbon flights screws, and cut & folded flights screws. Calculating conveyor speeds that have special types of screws is a bit different.

In this case, the required capacity will consider capacity factors for the screw type. The required capacity will be multiplied by these capacity factors to find the design capacity or equivalent capacity.

This design capacity will be used to find the conveyor speed and diameter.

The following formula summarises this relation:

screw conveyor design capacity
Formula for design capacity of screw conveyor with special screw types

Where CF1 relates to screw pitch; CF2 relates to the type of flight; CF3 relates to mixing paddles within the flight pitches.

For a detailed study of the calculations, please refer Screw Conveyor Catalogue and Engineering Manual (1996) from Continental Conveyor Company, Canada. The chart images provided in this article is sourced from this manual.

This manual contains complete tables and formulas and discusses screw conveyor capacity, speed, shaft size, bearings etc. Another example of the table is given below. This table helps to code your product depending on the properties of the same. A detailed, almost complete, list of bulk materials along with material code (industrial standard) is available in table 2 of the same Engineering manual.

screw conveyor material classification
Material classification table from the Screw conveyor catalogue and Engineering manual

Lump Size Limitations

Besides conveyor capacity, the size of a screw conveyor also depends on the size and proportion of material lumps to be handled. Material lump size is given by is the maximum dimensions of the material in its transverse cross-section. Accordingly, long dimension or lengths determine the lump size.

Considerations are made for the characteristics of the handled material lump. Some materials may have hard lumps that do not break during transit through a screw conveyor. The following provisions are made to classify material lumps:

Class 1 (Class ratio 1.75):

Lump mixtures with less than 10% of lumps. This ranges from the greatest size to one-half of the greatest; with 90% of lumps smaller than one-half the greatest size.

Class 2 (Class ratio 2.5):

Lump mixtures with less than 25% of lumps. This ranges from the greatest size to one-half of the greatest; with 75% of lumps are smaller than one-half of the greatest size.

Class 3(Class ratio 4.5):

Lump mixtures with 95% or more lumps. This ranges from greatest size to one-half of the greatest size; while 5% or less of the lumps below one-tenth of the greatest size.

Horsepower Requirements

The horsepower required for operating screw conveyors depends on proper machine installation and regular conveyor feed rate. The total horsepower capacity of a screw conveyor system is calculated using 4 important factors.

  • The horsepower required to overcome friction (HPf)
  • The horsepower required to transport material at a specified rate (HPm)
  • The overload factor Fo
  • The total drive efficiency e

The formula is as follows:

Screw conveyor Horsepower calculation

Formula for Screw conveyor horsepower calculation

Screw conveyor flight

Flight calculations are determined using the screw pitch, length, and shaft diameter. Screw conveyor flights are made to be either “helicoid” or “sectional”.

Helicoid flights are made from a flat bar strip that is wound into a continuous helix. A helicoid flight is thinner on the edge than on the base owing to material stretch.

Sectional flights are made from individual round plates. They are welded end-to-end, forming a continuous helix. The “sectional” flights have a constant flight thickness. Each end of a screw shaft is reinforced with a flight lug which is mounted at the backside (non-product carrying side) of the flighting.

Design of inclined screw conveyor

Below an inclination angle of 15 degrees, the screw conveyor operates like a normal horizontal conveyor with minimal losses in capacity. The design of an inclined screw conveyor system requires:

  • More horsepower for lifting product and “reconveying” product that falls back
  • Elimination of hangers to counter the need for longer screw designs due to “dead flow”
  • Tight clearance between the trough and screw in cases of granular/free flowing products
  • Higher speeds to increase the product’s forward momentum while reducing fall-back.
  • Shorter pitch screw flights to improve the angle between flights and conveyed product
  • Tubular trough/shrouds that surround the screw to prevent product fall-back at the top which occur due to increased rotational speed

Advantages and disadvantages of screw conveyor systems

Well, it is a great machine. A versatile bulk material handling equipment which could handle a wide range of products. When compared to other bulk handling conveyor systems like drag chain conveyors, screw conveyors have some definite advantages.

Advantages Disadvantages
Proper designs of covers and gaskets make screw conveyor systems weatherproof, dust tight, and even rodent proof The intermittent rate of discharging conveyed products
Compact designs offer an easy fit for these conveyors into restricted areas where it would otherwise be impossible to install most types of bulk material handling equipment.  

No self-cleaning system available

Per foot, these conveyors are arguably one of the most economical bulk material handling equipment commercially available. Conveying capacity decreases by increase in the inclination
Highly durable and low maintenance  If the product is stuck up inside the conveyor, it is very difficult to unstick the screw (dependent on the cross-sectional loading percentage).

Furthermore, we will see the maintenance and cleaning facts about the conveyor.

Maintenance and cleaning of screw conveyor

The screw conveyors are highly durable machines. The maintenance requirements vary according to the product being conveyed. However, the fact that only a gear motor and bearings are responsible for the smooth operation, makes the screw conveyor a low maintenance machine.

The following things need to be checked/maintained regularly.

How to clean a screw conveyor?

The cleaning system of screw conveyors varies drastically depending on the installation type. In some installations, it would be necessary to remove the whole unit out. Whereas in some plants, the flexible inlet and outlet system allows cleaning the unit from the installation area itself.

In the case of tubular screw conveyors, water and ventilation cleaning is applied for convenient cleaning. Water cleaning involves the use of a water jet pumped from the inlet in order to flash our material inside the pipe.

Ventilation cleaning involves the use of blowers to pump in air, which in turn blows away residual material in the tube.

For trough-type screw conveyors, the trough casing enclosing the conveying channel is taken down by unscrewing the fastening bolts before manual removal of residual material. Air may also be blown through the trough to clean it.

Conclusion

We have reached the end of this guide. If you are looking for a versatile bulk material handling device an economical and of low maintenance option would be that of a screw conveyor. There are a number of screw conveyor manufacturers with excellent track record and quality products.

With the information above and detailed links provided below, you are in a position to make a selection depending on your product and requirement.

If you genuinely liked the article, it would be a huge help if you shared the word. It seems insignificant, but it helps more than you might think. Let us know your views in the comments below.

References and further reading:

  • “FLEXIBLE SCREW CONVEYORS Principle of Operation”. com. N.p., 2016. Web. 13 July 2016.

https://www.flexicon.com/Bulk-Handling-Equipment-and-Systems/Flexible-Screw-Conveyors/Principle.html

  • “Screw Conveyor Catalogue and Engineering Manual (1996).” Continental Conveyor & Machine Works Ltd. 470 St-Alphonse Street East Thetford Mines, Quebec. Canada

http://www.continentalconveyor.ca/images/product_pdf/Screw_Conveyor_Catalogue.pdf

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