The widely used rotating single screw or auger is a familiar and straightforward approach. Material enters the screw channel and is moved to discharge by a helically wound screw, or captured and transported more forcefully with a closed-flight auger.
Variables of design include diameter, screw/auger pitch, tube length, auger shaft diameter (relative to tube), auger clearance (relative to tube), possible progressive pitch, material(s) of construction, and finish.
With a given material, element geometry and speed range, the envelope of possible volumetric feed rates becomes set. Some manufacturers, however, offer interchangeable elements to expand the feeder’s range and/or enhance its handling capability.
Note that screw/auger feeding is not precisely linear, as conveying efficiency falls somewhat at higher screw speeds. Thankfully however, under closed-loop loss-in-weight control, the feeder automatically compensates for this effect.
One danger of element mis-selection is the possibility that when handling pellets, some may become lodged between the element and its tube, possibly resulting in stoppage or damage. An increase in clearance between the element and its tube is called for.
Another danger is that some sticky or cohesive materials could tend to build up on the metering element over time, slowly reducing its volume-discharged-per-revolution. In gravimetric loss-in-weight application the first sign of this condition is a gradual increase in feeder speed as it attempts to maintain set rate. Such materials are better fed using self-wiping twin screws, profiled in the following screen.