Screw feeders are useful to assist bin / hopper unloading and in producing uniform / metered feed. The pitch of the screw is kept variable in order to produce uniform draw of material across the entire hopper opening.
The design of a Screw feeder resembles to that of a Screw Conveyor closely but unlike a screw conveyor, a screw feeder is required to be very precise and accurate. While in a Screw conveyor the screw is filled to the extent of 30% to 40%, in a screw feeder the material completely fills the screw. This is the reason why the material is discharged with uniform and controlled rate and the rate can be varied by controlling the speed of rotation of the screw.
Apart from metered feed, one of the important applications for using a Screw Feeder is where the material is dusty or toxic and hence requires an enclosed device.