By Rohit B. Viegas
Fig 1.1 Solid Shaft Hopper (left) and Flexible Shaft Hopper (right)
Screw conveyors are used extensively to transport bulk solid materials from one location to another. They provide efficient and effective movement of these materials with little to no spillage or loss of material during conveying. Screw conveyors are most often used when materials have to be conveyed vertically but are just as useful horizontally. They are best suited when moving materials over a short distance. When moving materials over a long distance they are outdone by belt conveyors.
The design of screw conveyors is critical to maintain smooth consistent flow to the process. The design depends mainly on the size of particles transported, the transport rate (volume/min), physical and chemical properties of particles, and the viscosity of material. Like all moving parts, eventually the screw or the screw casing will have to be replaced. While designing these two components, one has to ensure that the time between replacements is maximized and performance characteristics are maintained. Replacing the screw or screw casing can be a laborious process. Hence, designing the screw by understanding the process of screw conveying can ensure that it lasts 20–30% longer and saves both time and money.
Fig 1.2 Uniform Diameter Screw (top) and Uniform Diameter Screw Loading (bottom)
The most common screw conveyors are the helical screw type. The helical screw conveyor may or may not have a solid shaft as shown in Fig 1.1. The advantage of using a helical screw without a shaft is that it allows the conveyor to be flexible and can be bent within a certain limit. This provides the customer with reasonable flexibility when it comes to locating the conveyor, and it can be used in places where there are space constraints. However, this can result in concentrated wear of the casing and inconsistent flow rates. These conveyors have their drive motors mounted on the wall or ceiling of the shop, which can be a disadvantage in terms of maintenance and service.
Screw conveyors with a solid shaft do not provide the same flexibility as the one previously discussed. They have fixed dimensions and are heavier than screw conveyors with no shaft. However they provide higher flow rates for the same speed of rotation of the drive and wear at a slower rate. This kind of screw conveyor is particularly useful when moving highly abrasive particles, while flexible conveyors are the best option for moving fine powders. The screw conveyors with shafts have found most application in the mining and minerals industry, while screw conveyors without shafts are better suited for the food and pharmaceutical industry. These conveyors cost approximately the same and provide excellent value depending on the application.
Fig 1.3 Tapered Diameter Screw (top) and Tapered Diameter Screw Loading (bottom)
For shaftless screw conveyors, it is the casing that is most vulnerable to wear, as it is usually made of a polymer plastic. While for shaft screw conveyors it is the shaft that wears fastest, even more than the helical flighting. It has been observed that the most wear on the screw is near the area where the material is loaded onto the screw conveyor. The process is illustrated in Fig 1.2. You will notice that as the screw conveyor moves material, it is basically moving only the material that was collected within the space of the first few flights and, henceforth, just pushed forward. So new material is added only to the first few flights every time and when this new material comes in contact with the moving shaft it causes wear. As a result, the shaft is most prone to wear in that region.
Fig. 1.2 shows how material gets loaded unevenly in the first space between the flights, which results in excessive wear of the shaft. Due to this concentrated loading of the screw, the wear is localized and life of the screw drastically reduced. The identical rectangular sections indicate that the quantity of material that is pushed by the screw remains constant all along its length and loading only takes place at the start. A small adjustment in the screw design (i.e. using a tapered screw as shown in Fig. 1.3) can help reduce such problems. Fig 1.3 shows how the screw gets loaded uniformly along its length and eliminates localized loading and wear. The increasing rectangular sections indicate the extra amount of material pushed by the screw with every rotation. This design change will show a marked improvement in longevity, especially for smaller screws used for metering purposes. However, this adjustment loses value as the screw gets longer because the taper becomes insignificant.
In conclusion, screw conveyors continue to offer the most effective and economical option to deliver bulk material. We continue to learn and improve equipment design through our experience and by sharing our knowledge.
Rohit B. Viegas is a mechanical engineer in Houston, TX. He can be reached at [email protected].