Screw conveyour

Screw conveyour

Manual labor isn’t enough when moving around materials in an industrial setting – most plants require specialized conveyor systems that help keep products organized and flowing. These material handling devices allow for easy, quick, and controlled transfer of materials through the process line, and use various methods to do so. One popular type, known as the screw conveyor system, is still widely used even though it was invented circa 250 BC by none other than Archimedes himself. This conveyor type, its design variations, and its uses will be detailed in this article so readers can gain a better understanding of where they are specified. Through this investigation, this article also shows the benefits of certain screw conveyors over their other types, to help differentiate this varied class.

What are screw conveyors?

Before diving into screw conveyors, we recommend reading our article on understanding conveyor systems, as it gives a good introduction to this diverse class of machinery.

A screw conveyor (also known as an auger conveyor) is, unsurprisingly, the shape of a typical screw, albeit with some caveats. Firstly, the threads are exaggerated and the center may or may not be present (see Figure 1). However, they function almost identically to screws, where a drive motor turns the screw (also known as the auger), moving product up one pitch every revolution. The auger is typically in coupled sections and is supported by bearings on either end, where coupling bolts transmit the motor torque to power the conveyor. A tube or U shaped trough around the auger acts as the conveying surface, where powder bulk, granular, and even non-flowing material is pushed along the trough surface. There can also be optional covers for the trough, making the process of moving such materials cleaner and more contained. Material is fed through the screw conveyor’s inlet, passes through the conveyor via the turning auger, and is taken out via the discharge end. Screw conveyors can be configured in a horizontal, vertical, or inclined arrangement.

The simplicity of the screw conveyor design allows it to be used in many different applications. Besides transporting flakes, powder, grains, and granules, screw conveyors often find uses as agitators and blenders, as well as for bottling system conveyors. Mini screw conveyors work well in moving concrete and other non-flowing media, and larger augers are found in the agricultural industry as threshers and balers. In the next section, we will detail the specific differences between screw conveyor types to show how they can succeed in all of these applications.

Types of screw conveyors

The screw conveyor is one of the most popular choices for processing and manufacturing, meaning there are many, many iterations of this design. This section will go through a general classification of screw conveyors available and what sets them apart from other unique designs. Note that, because they are so widely used, screw conveyors can either be a blend of these types or have their own specifics that should be explained by your supplier.

Horizontal screw conveyor

The horizontal screw conveyor is the most distributed version of the screw conveyor, thanks to its simple and easy to use design. They can convey materials at 15-45% trough loading, defined as the maximum trough capacity divided by the specified trough load for a given material. A more viscous, sluggish material will require a lower trough load, while a lighter free-flowing bulk can be fed at a higher load. Figure 2 shows the typical arrangement of the horizontal screw conveyor, where the drive unit is at the discharge end. This causes the material to be pulled towards the discharge, putting the auger sections in tension, reducing fatigue and wear on the conveyor. They are great for both free-flowing and non-flowing media and are a cost-effective option next to traditional belt conveyors. They are also perfect for homogenizing mixtures and can be left open for inspection or totally enclosed, depending upon the environment.

Inclined screw conveyor

Inclined screw conveyors are functionally the same as most augers but are used at an incline between 0-45 degrees from horizontal. They perform much differently however, as each degree of incline reduces the efficiency of the conveyor and how much material flows through it. As more torque is required to pull material through the conveyor, it must be more closely designed for the conveyed product. The angle of incline should, therefore, be as low as possible to maximize efficiency and reduce the need for additional power.

Inclination angles lower than 10 degrees minimally affect efficiency and can be compensated for by increasing the auger speed. Inclination angles between 10 and 20 degrees reduce the auger efficiency by 10-40%, where a noticeable increase in horsepower is needed to compensate, lowering efficiency. Angles past 20 degrees reduce the efficiency from 30-90% depending upon the angle, where pitch efficiency is further reduced, and horsepower requirements significantly increase. Inclines of 10 degrees or more must be designed to operate under upset conditions – that is, when material falls back to the lower end of the conveyor, causing a simulated 100% trough loading at its lowest point. A common application of these inclined screw conveyors is in cement mixers, feedlot dispensers, and sugar and meat applications.

Shaftless screw conveyor

The shaftless screw conveyor (also known as a ribbon screw conveyor) is an improvement to the typical screw design, as it allows material to flow “through” the auger as well as around it. This may seem like a small change, but it makes a huge difference in practice as it increases the efficiency, flexibility, complexity, and range of conveyable products. They are inherently non-clogging and can transport materials not typically handled by screw conveyors such as high-moisture or non-flowing products. They sport a better efficiency than other conveyors and even other screw conveyors, and do not require the internal bearings needed for conventional designs. They are most often found in food and chemical processing, wastewater, mining, and other vital industries.

Vertical screw conveyor

The vertical screw conveyor is capable of elevating large volumes of bulk product and includes all screw conveyors with inclination angles greater than 45 degrees. They work best at transporting dry to semi-fluid materials, and they take up a small footprint. The method of feeding these conveyors depends heavily on the product being transported; for example, light material cannot be fed using a gravity hopper alone, as the auger blades act as a fan and blow the material out. Therefore, a screw-type feeder (shown above in Figure 5) is most often paired with these designs to regulate feeding flow and ensure the passage of a uniform volume of material into the conveyor. It also allows the vertical auger to remain operating at a constant speed, increasing efficiency and decreasing power consumption over time. The most common applications of vertical screw conveyors are in wastewater facilities, wood production, chemical and food, as well as mining operation

Live bottom screw conveyor

The best way to control the flow rate of bulk materials is via the live bottom screw conveyor. It is designed to meter large volumes of materials, typically that pack under pressure, using multiple parallel screws. This allows the material to be evenly discharged, even if it enters the conveyor at different densities/sizes/speeds from the inlet. They are typically mounted under large bins whereby material will flood into a shrouded trough and be drawn through the many augers. They come in many arrangements such as the variable pitch, mass flow cone, and tapered flight screw conveyors, which all have unique characteristics that help regulate flow.

Specifications and selection criteria

Below is the necessary information to bring to a supplier when specifying the right screw conveyor for the job. Note that this section only outlines the major specifications, but more exist.

Screw conveyor capacity

Calculate the volume per hour needed to be conveyed; this can be done by finding the desired cubic feet per hour, selecting the recommended trough loading percentage for the conveyor, selecting the right screw diameter, and determining the conveyor drive RPM. The maximum recommended screw conveyor capacity is often given with each screw diameter and is also affected by the type and size of pitches, as well as the material being transported. Your supplier will most likely have tables that will help calculate this capacity and is the best source of information.

Screw design & drive

There are many kinds of screw designs, each with their own advantages. Certain types, such as the paddled screw, are used to ensure extra mixing occurs, while the short pitch screw is often used for incline conveyers to maintain efficiency. Determine the screw design by the desired function of the screw conveyor and choose from the available options. Also, the screw design is pertinent to the drive, as there must be enough horsepower to pull material through the auger. Getting the right drive for the desired screw shape will help make sure it works as intended.

Load type, trough load, and trough design

What will be passing through the conveyor? Depending upon the material, there are options to choose from that will help make conveying it easier. For example, If the product is light, dry, and hazardous, consider a sealed, rounded trough that will provide high dust protection and regulated flow. Similarly, if the material is inherently non-flowing, consider a lower trough load percentage and a shaftless screw type. The shape of the trough will impact the conveyor capacity, so determine which trough design will align best with the desired flow rate.

Behavior, control, and environment

Finally, consider the external factors and behavior of the screw conveyor in your application. How much distance must be covered? Will it be increasing/decreasing in elevation, and will the material need to be separated from operators/the atmosphere? Also, will the conveyor run at a set speed, or will a variable speed drive be used to change the speed? Will the conveyor be part of a larger control system, and will sensors need to be embedded to ensure accuracy? Answering these questions will allow you to narrow your search parameters and find the most effective screw conveyor for your application.