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Right finish level for screw conveyor system ensures optimum conveyor performance

A screw conveyor system is composed of a body (casing) and a rotating screw. The right combination of the two elements ensures that a product is conveyed by pushing it from point A to point B. To optimise this process, Van Beek has developed seven finish levels with two options: a rolled screw and a screw composed of separate screw blades. But what is actually the most ideal combination and why is a high gloss polished screw not necessarily suitable for the conveyance of all products?

Before we go into the different finish levels let us first briefly explain the difference between a rolled screw and a screw composed of separate screw blades. While the material type and mechanical quality of the two screws are similar, the difference lies in particular in the finish. For example the surface of a rolled screw is rougher and it can be produced more quickly than a screw composed of separate screw blades. In addition rolled screws have a tapered blade thickness and a screw composed of blades has a constant thickness. With a rolling process that Van Beek has developed itself a very dimensionally stable product is produced, because the screw blade is rolled directly round the inner tube, hence the name ‘rolled’ screw. During this process the starting material is forced between two conical rollers round the tube (rolling), as a result the blade on the inside is thicker than the blade on the outside. An innovative solution where the construction itself has no disadvantages.

Smoother is not always better
Where the production of rolled screws can be seen as something purely mechanical, the production of screws composed of blades consists of 90 percent manual work. Because the screw blades for this finish are cut from sheet material and then pressed by a hydraulic press to the right pitch, after fixing to the screw tube this gives a smooth surface. In the case of finish 7 this is a high gloss polished screw, that conveys products in a streamlined way. This high quality immediately gives rise to misconceptions. Other than you might think, this shiny surface is not necessarily suitable for conveying all products. “Customers often think ‘the smoother the screw, the better’, but the opposite is true. The smooth surface may in fact mean that a product sticks to the screw. A slightly rougher screw is in some cases better for preventing sticking”, says Eric de Jong, Manager of Operations at Van Beek.

Conveyor performance
The secret lies in the right combination of the screw and the body, which both help to convey the product. The screw pushes the product forwards, but the amount of movement is also based on the type of body. In short: conveyance of the product is the result of the combination between movement and friction. In addition a higher finish level can be achieved with screws composed of blades, when these are used for products that do not involve sticking or fouling. “The wrong choice definitely affects the conveyor performance”, says De Jong.

Every industry has its own screw
It is then big chemical groups like DSM and Shell and pharmaceutical companies that use our finish 7. The food sector on the other hand often chooses finish 5 or 6, because in addition to a flexible conveyance process, they also have a high regard for hygiene due to the finely sanded, smooth screws. This gives bacteria as little chance as possible of nestling in chinks and holes.

The story is very different for example for the rolled, rough, completely unsanded finishes 1 and 2 that are therefore particularly suitable for example for the recycling industry for chopped plastic flakes or conveying rubber granules. Finishes 3 and 4 are the finish levels mostly chosen as part of a complete Van Beek screw conveyor system. These screws are machined during the production process using respectively glass bead blasting and wet blasting, which makes the screw surface ideal for conveying semi-finished products.

Further development
Van Beek would not be Van Beek, if we were not continuously innovating. This certainly applies for the production of screws composed of blades, an intensive process that still consists of 90 percent manual work. De Jong comments: “At present we are working on the development of a machine for sanding and polishing these screws, a labour-intensive job that at present is largely done by manual work”.