Screw feeders are disclosed including a conical feed screw rotatably disposed in a conical screw tube, and a drive shaft mounted at one end of the conical feed screw with a pair of bearings mounting the drive shaft, a fixed ring mounted on the drive shaft, and an axially adjustable sleeve mounted on the drive shaft, with the pair of bearings disposed between the fixed ring and the axially adjustable sleeve in order for the pair of bearings to be pretensioned therebetween.