Screw Feeders
WHAT IS A SCREW FEEDER?
A screw feeder utilises the principle of an Archimedes screw, a helix-shaped mechanism capable of propelling particulate-based solids or liquids when rotated. Here, we focus on bulk solids applications. Typically submersed within the bulk solid, the Archimedes Screw operates inside a specially designed hopper featuring angled walls to facilitate gravity-driven material flow. In many screw feeder designs, a trough is positioned at the hopper’s base within which the screw is located and serves to concentrate the screw’s action while minimising tangential movement of the bulk solids.
The screw is driven by a motor or other power source, causing it to rotate. As the screw rotates, the bulk solid falls or is propelled by an agitator into the pocket formed by the sides of the screw flights and the hopper trough. With each revolution, the pocket of bulk solid advances by a distance proportional to the auger screw’s pitch, and the speed rotation of the auger determines the amount of material moved over time. Reversing the direction of rotation changes the direction of flow. The diameter of the auger screw directly influences the flow rate, with larger diameter augers producing larger flow rates. All screw feeders operate on a volumetric basis at this level.
Two critical elements must be in place for reliable performance of a Screw Feeder operating volumetrically. The products must be able to enter the screw-trough pocket and, once in place, maintain a consistent bulk density. The first element is critical in any screw feeder; if the bulk solid does not enter the screw flights, the propulsion of material cannot occur. Secondly, once the material is in place, the bulk solids must maintain a consistent density to ensure accurate dosing and preserve the material’s characteristics. For easy-to-handle bulk solids that are non-compacting and free-flowing, only the storage hopper wall angles must be considered, as the angles must promote material flow while preventing product hangup along the hopper sides. For cohesive materials that are non-free-flowing, the Screw feeder is fitted with additional agitation to stir and disturb any settling materials to avoid bridging or ratholing from occurring.
BENEFITS OF A SCREW FEEDER
Accuracy – Ensures precise material flow, reducing variability and enhancing consistency.
Adjustability – Easily adjustable feed rates to meet varying production needs.
Versatility – Handles a wide range of materials, suitable for diverse industries.
Efficiency – Minimises material waste, reducing costs and promoting sustainability.
Hygiene – Easy-to-clean designs minimise contamination, ensuring product safety.
Maintenance – Quick and efficient maintenance with accessible, modular components.
WHAT DO METERING SCREW FEEDERS DO?
Metering screw feeders are precision-engineered devices designed to deliver bulk solids with the utmost accuracy, ensuring controlled dispensation either by weight or volume, depending on the application requirements. Operating on the principle of the Archimedes screw, metering screw feeders effectively regulate the flow of various materials, including fine powders, pellets, flakes, and granules, regardless of their diverse flow properties. Screw feeders are meticulously designed and configured to provide a consistent and reliable stream of material, which is vital for a multitude of processes across various industries. In pharmaceuticals, food processing, plastics manufacturing, and more, screw feeders play a crucial role in maintaining efficiency and product integrity.
DISTINGUISHING SCREW FEEDERS FROM SCREW CONVEYORS
The primary difference between a screw feeder and a screw conveyor lies in their function and application. Understanding the distinctions between a screw feeder and a screw conveyor is essential for selecting the right equipment for your specific material handling needs. Screw feeders are exceptional at controlling and metering the flow of materials, making them perfect for precise applications such as packaging and batching. They do not transport materials over long distances; instead, they control the rate at which materials enter subsequent equipment. Whereas Screw conveyors are designed to transport materials from one location to another over a distance. They are integral to bulk handling and material movement within a plant or system, capable of moving materials horizontally, vertically, or at an incline. Screw conveyors can be positioned at any point within a process where material needs to be relocated. Whether you require precise control for metering applications or versatile transport solutions for bulk materials, our comparison guide will assist you in selecting the right equipment to optimise your plant’s efficiency and productivity.
For a comprehensive comparison of screw feeders and screw conveyors, visit our dedicated “Screw Feeder vs. Screw Conveyor” page. Here, you will find detailed information on the unique functions, applications, and advantages of each type of equipment. Our page provides insights into how each system can enhance your material handling processes, helping you make an informed decision based on your specific operational needs.
WHAT IS THE IMPORTANCE OF THE HOPPER & FEEDER BODY SHAPE IN A SCREW FEEDER?
The volume of material held within the screw feeder’s storage hopper directly affects the bulk density of the material being loaded into the auger screw. Certain bulk solids exhibit compaction effects under the weight and the material stack, resulting in varying densities at different fill levels within the hopper. Consequently, this variable translates into different feed rates, which can be mitigated through strategic hopper profiling and design adjustments.
Achieving a uniform density of bulk materials within the screw pockets necessitates meticulous attention to the design of the vessel containing the material. Hopper configurations with steep wall angles prove indispensable for handling poor-flowing materials, ensuring a consistent flow into the auger screw pockets. Close coupling of the agitator and the feeder body or conditioning section is paramount to prevent product hang-ups and minimise residue buildup, especially during material run-out. As the diameter of the auger screw decreases, particularly for applications requiring lower delivery rates, the geometric intricacies become more complicated. The combination of feeder profile and agitator action also becomes critical and must be designed to work in unison to get optimal feeding accuracies.
In essence, the shape and configuration of the feeder hopper and body serve as foundational elements in ensuring the efficient and accurate operation of the screw feeders across a spectrum of bulk solid applications.
AGITATORS IN SCREW FEEDERS
Ensuring a smooth flow of material is paramount, particularly for challenging, poor-flowing bulk solids, so a method of product flow encouragement is essential. Various techniques can be employed, with vibration being a common method, often sufficient enough to shake the particles of a bulk solid and loosen their contact cohesion, allowing gravity to take over. Care must be taken when using a vibratory device, as excessive vibration can lead to consequences such as product separation and compaction. Typically vibration is often applied in controlled pulses with variable timing intervals regulated by an on/off timer to mitigate these risks effectively.
Mechanical agitation is often integrated with a correctly configured screw feeder to complement the handling of bulk solids. Either independently driven or powered with the same motor as the auger screw, the stirrer imparts movement into the bulk solids churning and blending the material to break bridges and dislodge rat holes. As with vibration, there can be problems with over agitation where the material is stirred too much, adversely affecting the product characteristics. In such circumstances, independently driven agitators offer the most controllable solution and can include variable speeds and variable mark space on time to reduce the amount the product is worked and churned.
Agitation mechanisms come in various forms, ranging from simple steel bars attached to a central rotating shaft, to more complex paddles or hammer arrangements designed for effective agitation and lump breaking. One of the most effective agitators is based on the geometry widely used in ribbon blenders. This configuration involves two short-length Archimedes screws mounted on the same rotating shaft but with opposite hands. By incorporating Left and right-handed partial scrolls, this design effectively moves the product from the screw feeder’s body sides and ends towards the centre. When positioned in close proximity to the metering screw flights, these agitators have the added benefit of pushing material into the top of the screw/trough pockets, facilitating a continuous flow of products with an even density. Addressing two critical elements, creating a well-functioning Metering Screw Feeder design.
WHAT TYPE OF ARCHIMEDES SCREW IS USED IN A SCREW FEEDER?
Selecting the optimal Archimedes screw to use for any particular application is best determined by testing due to the diverse characteristics of bulk solids as the nature of materials and the ease with which they can change characteristics makes predicting performance very difficult. While Theoretical feed rates can be calculated based on the volume between the augers screw flights, the weight of the delivered product is considerably affected by the bulk density of the material within the pocket and how full the pocket becomes. In general terms, a full-flighted Archimedes screw, where the flights are evenly spaced along a centre core at regular intervals, proves effective across a range of bulk solids. However, numerous geometric factors of an auger screw influence the feed rate, including the ratio of outside diameter and core, the pitch of the flights, the thickness of the flights and most importantly, the outside diameter. These adjustments ensure that screw feeders operate efficiently and effectively across a wide range of applications, from fine powders to larger pellets and granules, enhancing overall process performance and product quality.
Within the bulk solids industry, endless variations of Archimedes Screw are offered. Many are specifically engineered to perform a particular function in addition to metering products. Specialised designs can address challenging materials, performing extra actions such as breaking up material bridging, cutting up cohesive products, breaking up materials that tend to form lumps, and many more can be performed by specially shaped flights. In this section, we will cover the most popular variants.
Standard auger screw – In this design, the diameter of the flight is consistent throughout the length of the screw, matching the pitch distance. Most screw feeders use this type of as auger as the uniformity ensures a steady and reliable movement of materials, making it suitable for a wide range of bulk handling tasks.
Short pitch – Short pitch auger screws feature a flight pitch that is reduced to approximately two-thirds of the screw diameter. This design modification is particularly advantageous in screw feeders handling materials prone to fluidization, as it effectively prevents material flushing by controlling the flow rate. Short pitch screws are commonly used in screw feeders where precise and controlled movement of material is critical.
Centreless/coreless auger flights – Centreless or coreless auger flights are specifically designed for screw feeders handling highly fibrous or sticky materials that tend to build up or wrap around a central shaft. By eliminating the central shaft, these augers minimize clogging and ensure a smoother and more consistent flow of challenging materials, such as those found in the food processing industry.
Variable pitch/stepped pitch – A variable pitch auger screw features flights where the distance between them gradually increases along the length of the screw. This design allows for a uniform withdrawal of free-flowing materials from the hopper of the screw feeder, enhancing efficiency and reducing the risk of blockages. Stepped pitch augers operate on a similar principle but change the pitch in increments, such as every five flights. This incremental adjustment can be tailored to specific material handling needs in screw feeders, providing greater control over the flow rate.
Tapered flight – In a tapered flight auger screw, the pitch remains constant while the flight diameter decreases to one-third towards the outlet end. This design creates additional space for the fed material at the inlet point of the screw feeder, facilitating a more uniform and controlled dispensing of material. Tapered flight screws are particularly useful in screw feeders where consistent output is essential, such as in packaging and dosing systems.
Double and triple start flight auger screw – Double and triple start flight auger screws incorporate additional flights between the existing ones. This configuration promotes a more accurate and smoother material flow within screw feeders by reducing the ‘pulsing’ effect often observed in single-flighted screws. The increased number of flights enhances the screw feeder’s capacity to handle varying material types and flow rates, making it ideal for precision applications in industries like pharmaceuticals and chemicals.
Tapered core – A tapered core auger screw features a core that gradually decreases in diameter towards the outlet end. This design ensures an even drawdown of material from the screw feeder hopper, promoting consistent flow and reducing the likelihood of material bridging or clogging. Tapered core screws are widely used in screw feeders where maintaining a continuous and uniform material feed is critical, such as in extrusion and molding processes.
By meticulously designing and configuring screw feeders with the appropriate Archimedes screw, industries can achieve a consistent and reliable stream of material. This precision is vital for a multitude of processes across sectors such as pharmaceuticals, food processing, plastics manufacturing, and more. Whether it’s dosing precise amounts of ingredients in food production or maintaining strict quality control in pharmaceutical formulations, screw feeders play a crucial role in ensuring efficiency and product integrity.