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This article will help you understand the basics of screw conveyors, the components, the industries they are used, and the materials they typically convey. A screw conveyor is a popular method of conveying materials in various industries due to their versatile design and compact nature.
Screw Conveyor Basics
Screw Conveyor Components
Configurations of Screw Conveyors
Important Considerations when Using Screw Conveyors
Leading Screw Conveyor Manufacturing Companies
Summary
Screw conveyors are used in industry to move large amounts of bulk solids, granular solids, liquids, slurries, and non-flowing solids. Basic screw conveyor design consists of a shaft with a screw mounted to it, placed in a trough. When the screw rotates, it pushes the material along the trough towards the discharge of the conveyor. Screw conveyors can move materials at any angle from the horizontal to the vertical.
Screw conveyors are one of the oldest pieces of industrial equipment, having been invented by Archimedes in 250 BCE or so. He used them to move water from low lying areas up to irrigation ditches. A screw conveyor is also known as an auger conveyor in some places and is even sometimes referred to as an auger screw conveyor.
Different materials present different challenges to conveying equipment. Some materials can be abrasive, freezing, toxic, or unable to flow freely. There are different types of screw conveyors for moving different types of materials. The versatility of the screw conveyor makes it a great option for difficult applications.
Although different manufacturers may alter the design, most types of screw conveyor have the same basic components. However, not all screw conveyors have each of the components listed here. The information below is based on common assemblies in the bulk material handling industry.
The conveyors screw is the mechanical component that pushes the material through the trough of the conveyor. It is usually made of a long shaft with a blade wound spirally around it. The spiral-wound blade is called the “flight.” The screw does not travel along its axis. Instead, it stays in place and rotates. The rotating screw pushes the material along its axis. The material travels the distance of one pitch (the axial distance between two crests of the flight) in one rotation.
There are many different types of conveyor screws.
Standard Pitch: With these, the pitch is equal to the screw diameter. These screws are most often used in horizontal conveyors, or those with only a slight incline, and they can handle most kinds of material.
Variable Pitch: These have a longer pitch at the inlet side than the outlet, so that material has extra space as it is fed into the conveyor. They are used when a controlled volumetric flow rate is needed.
Short Pitch: Short pitch conveyor screws have a pitch equal to two thirds of the screw diameter. They are used in vertical and steeply inclined conveyors, and to transport free-flowing materials, which can get overly agitated in longer pitches.
Half Pitch: The pitch of these screws is equal to half the screw diameter. They are used in vertical and inclined conveyors.
Long Pitch: Long pitch screws have a pitch that’s one and a half times the diameter. They are used for rapid transport of liquids.
Double Flight: There are two flights on the shaft of this type of conveyor screw, to accelerate transport and to ensure a uniform flow rate. The pitch of these screws is standard.
Tapered Flight: While the pitch stays constant, flight width increases from two thirds of the diameter on the intake end to full diameter at the outlet. This arrangement draws the material off evenly. They are cheap substitutes for variable pitch screw conveyors.
Mass Flow: These screws are conical with a constant pitch at the intake end. The discharge end is at a variable pitch. They promote even material flow and sweep a volume that increases incrementally with each flight.
Helicoid Flight: The flight has a greater thickness at the base than at the edge. These flights are specially constructed from steel bars to be extra strong and hard. They are used for the transport of moderately abrasive materials.
Sectional Flight: Sectional flight screws are made from steel discs that have a constant thickness. They are used for the transport of extremely abrasive materials.
Cut Flight: The outer edges of the flights of these screws have notches cut in them at regular intervals. This agitates and mixes the material as it travels. These screws are used to transport materials that tend to clump up and pack.
Cut and Folded Flight: These are like cut flight screws, but the flight edges are folded as well as notched. This more thoroughly mixes the material, aerates it, and keeps the temperature more even.
Ribbon: Ribbon screws have gaps cut between the shaft and the inner edge of the flight. Single flight ribbon screws are used to transport viscous and/or sticky materials. Double flight ribbon screws are used for that and to mix dry materials.
Standard Pitch with Paddles: This kind of screw is made of paddles placed spirally around the shaft. It is the best design for mixing materials, but the least efficient for transport.
All screws have a handedness. A right-handed screw that is rotated clockwise will pull material toward the end of the screw. A left-handed screw that is rotated counterclockwise will do the same. They can reverse material flow when rotated in the other direction, but this is not recommended, as the screws are designed with a specific carrying side. This concept and design allows for special use as well, combining the two (left and right), allowing a screw to discharge on opposing sides simultaneously when loaded in the middle.
The trough supports and channels the material to be moved as it is the component that houses the conveyor screw. It is either supported by feet or by saddles. The trough is covered along its length which serves a variety of different functions, including but not limited to: weather protection, dust emissions, and safety against accidental contact with the rotating shaft/screw.
The inlet of the trough is connected to a feeder mechanism that controls the rate and volume at which material is added to the trough. This is typically a hopper, silo, or feed conveyor. The outlet is either connected to a storage device such as a hopper or silo or to further processing equipment. As mentioned above, a screw conveyor can even feed into another conveyor, which may assist in changing direction or pitch, depending on the application that the process is engaged.
There are a few different types of trough:
U-shaped: This is the most common type, as well as the most versatile. They’re easy to make so they tend to be the most cost effective as well.
Rectangular: Rectangular troughs are used to transport abrasive materials. Because of the shape, they have a gap between screw and trough walls on the bottom. This gap is filled with a static layer of the material being transported, which insulates the trough walls from the moving abrasive material, protecting them from damage.
Tubular: This design tightly contains the material so it can operate at higher speeds. This type is often used in vertical and inclined screw conveyors.
Jacketed: Made from concentric troughs with a hot or cold liquid between them, this design is used to heat or cool the material as it is transported.
Hangar bearings support the screw when shaft deflection can occur. This is typically seen in longer screw conveyor or when multiple screws need to be connected in one trough, while allowing it to rotate freely. They are usually suspended from the trough and bolted to the housing directly beneath the lid of the trough that fits over the trough to prevent dust, weather, and accidental contact to the rotating equipment. These bearings can be made of hardened steel, wood, brass, or polymer based.
The couplings join sections of the screw together. They compensate for any misalignment between the sections and typically incorporate a hanger bearing assembly to support the joint. These can also be referred to as Coupling Shafts when they are used to join two screws. Induction hardened or Tungsten coated couplings may be furnished where highly abrasive materials are being conveyed.
Internal collars, which are sometimes referred to as bushings, are used to reduce the inside diameter of the center pipe of a screw to match standard CEMA shaft sizes and to increase the torque rating of the CEMA bolted connection.
End lugs are put on the ends of the screw. They are used to provide extra support to the first and last flight of a screw section and are located on the non-carrying side of the flight. End lugs are manufactured from heavy-gauge steel and continuously welded to the flight and center pipe of a screw section. End lugs are designed to provide maximum support with the least obstruction of material flow.\
The screw is driven by an electric, hydraulic, or pneumatic motor and gearing mechanism, placed on the outlet end of the trough. Typically, a shaft-mounted gearbox and motor configuration is used, but pulleys and sheaves can also be used in a drive assembly. The drive assembly is mounted directly to the trough/housing in most applications.
These screw conveyor configurations are each best suited to a different type of flow path.
This is the simplest type of screw conveyor to make and use. They can handle many different types of materials, and can be easily put together with multiple intakes and outlets. They can have the trough loaded anywhere from 15% of the way to 45% of the way, depending on the characteristics of the material being transported. Loading it any further results in excessive wear on the screw conveyor components.
Inclined screw conveyors can be used to transport materials at any angle up to 45 degrees. The steeper the angle, the lower the load capacity and the higher the power requirement. To compensate for this effect, reduced pitch screws are used, because they reduce fallback of the material.
Vertical screw conveyors are used to transport materials at angles from 45 to 90 degrees. These conveyors are housed in a fully sealed, round, trough assembly. This type of screw can be used to convey dry to semi-fluid materials. Vertical screws are often used when material needs to be elevated in applications with limited space.
Special Configurations include:
These conveyors have no shaft or bearings. This reduces flow obstruction and makes these conveyors ideal for transporting viscous and/or sticky substances.
These conveyors are installed on the bottoms of silos, bins, and hoppers to control the volume of material coming from them. They have multiple variable, tapered, or mass flow screws to handle fluctuating intake volumes. These screws also assist in the distribution of materials within the bin.
Flexible screw conveyors have shaftless screws and flexible, tubular troughs. They are ideal for transporting materials through obstructed, cramped areas and along variable elevations. Flexible Screw Conveyors are called by many different names, including a flexible auger, flexible screw conveyor, flex conveyor, flexible spiral conveyor, helix or helical conveyor.
The layout of the operation in need of a screw conveyor is one of the two biggest considerations when it comes to selecting or designing a particular conveyor. The number of intakes and outlets, the distance the material must be transported, and the elevations and obstacles it must overcome will all determine your conveyor’s design. The area in which the conveyor is to be installed must also be considered, especially in retrofit applications where other equipment may be in the path of the conveyor.
Besides layout, the other biggest consideration in conveyor design is the nature of the material being transported. Free flowing materials flow in an uncontrollable manner when they are overly agitated. They require conveyors with shorter pitches and/or slower speeds.
Viscous and sticky materials need something to prevent buildup of the material, which will otherwise clog up the conveyor and require a shutdown for cleaning. Such materials need a shaftless or ribbon flight screw conveyor.
Abrasive materials need to be transported by a conveyor that is made of hard materials that can resist abrasion. Hardened steel and stainless steel are two of the best choices. A screw conveyor transporting abrasive materials should also have a rectangular trough when the application allows. Having a low angle of inclination is best practice, but not always permitted due to application differences.
Structural requirements and screw sizing are determined by other properties of the material being transported, such as particle size (for granular solids) and density.
Although ACT manufactures custom screws, sometimes options are what you need! Below are the companies The ACT Group recommends when we don’t carry or make you screw you need:
· Conveyor Engineering and Manufacturing
Screw conveyors are used in industry to move large amounts of bulk solids, granular solids, liquids, slurries, and non-flowing solids. Basic screw conveyor design consists of a shaft with a screw mounted to it, placed in a trough. When the screw rotates, it pushes the material along the trough towards the discharge of the conveyor. Screw conveyors can move materials at any angle from the horizontal to the vertical.
Screw conveyors are one of the oldest pieces of industrial equipment, having been invented by Archimedes in 250 BCE or so. He used them to move water from low lying areas up to irrigation ditches. A screw conveyor is also known as an auger conveyor in some places and is even sometimes referred to as an auger screw conveyor.
Different materials present different challenges to conveying equipment. Some materials can be abrasive, freezing, toxic, or unable to flow freely. There are different types of screw conveyors for moving different types of materials. The versatility of the screw conveyor makes it a great option for difficult applications.
Screw conveyors can be classified by the flight design and the pitch. These types include standard pitch, variable pitch, short pitch, long pitch, half pitch, mass flow, double flight, and tapered flight.
There are also several special types that are classified by the design of the screw. These include ribbon screw, cut flight screw, cut and folded screw, and standard pitch flight screw with paddles. Screw conveyors can be classified by the way the flights are constructed. These types include the sectional flight screw and the helicoids flight screw.
· Screw conveyors can also be classified by flow path, such as horizontal, inclined, or vertical.
· Other types of screw conveyor include shaftless and live bottom.
· Screw conveyors can be either left or right-handed. Handedness and direction of rotation together determine the direction in which the material is moved. Sometimes, they can be combine to convey the material in two directions at once, with one screw and drive!
· There are multiple kinds of troughs. They include U-shaped, rectangular, and tubular. U-shaped is the most common. Tubular troughs are used for inclined and vertical conveyors.
· Screw conveyor design is determined by application and the nature of the material to be transported. The most important factors are mass feed rate, volumetric feed rate, density, maximum particle size, and path of flow.
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