With compatible materials, well-designed volumetric screw feeders can produce extremely accurate dosing of bulk solids.
As the name suggests, this type of metering feeder delivers products based entirely on volume. In reality, all metering screw feeders, loss in weight, gain in weight or gravimetric operate on a volumetric basis, but these also include electronics to monitor the mass of product delivered.
All volumetric screw feeders are based on the Archimedes screw which converts rotary motion into an almost continuous linear motion. The rotating flights of the Screw auger push product in a direction perpendicular to its rotation. If bulk solids are surrounding the screw and can penetrate the gap between the auger flight and the auger centre, these solids can be propelled into a tube or trough which can control the direction of material flow.
While the principles of the auger feeder are quite simple, implementing these principles into a reliable and accurate dosing machine is more complex.
A products bulk density is the link between its volume and weight. Variations in bulk density affect the overall accuracy of volumetric machines and must be avoided. Materials must be processed carefully within the machinery of the powder feeder to create, then maintain, an even density.
Hopper geometry, feed screw profile and arrangement, action and arrangement of product agitators and hopper fill levels all contribute to maintaining this consistent bulk density.
Trantec metering feeders are designed to optimise these key features as well as other factors known to influence the density, for example, friction within the hopper, which further hone our machines accuracies.
The arrangement of flights on the auger becomes the next element in feeder design. Once consistent density is created in the products to be metered. The auger screw must propel product without upsetting this equilibrium. Conversely, where the products bulk density is difficult to maintain due to the characteristics of the material, the auger can be utilised to develop consistent bulk density in the feed screw tube by employing variations in the pitch of the flights.
Multi flight or multi start augers as they are often called, bring unique and powerful features to volumetric screw dosing, in particular, their ability to maintain a steady flow of product at low rotational speeds. This technique can significantly improve an overall feed range of an auger.
Single start auger screws comprise one flight wrapped around a centre core. The flight provides propulsion to the material and pushes it along the feed tube. At low speeds, a marked pulsing of product flow occurs as the end flight rotates at the outlet of the feed tube.
Adding a second flight in the middle of the existing flight (turned 180 degrees) creates another propulsion, immediately doubling the number of output pulses of product over the same time. This improved flow of bulk solids can be highly desirable in applications requiring a broad throughput range and a steady stream of material. Adding a second flight represents a twin start auger. Triple or even quadruple start augers can be manufactured by Trantec. Small multi-start augers are almost impossible to make using traditional fabrication techniques, and computer Aided manufacture is the only alternative to produce these screws.