a fixed position in air chamber 90a of feeder head 10 by an angle iron cross bar 104 mounted between beams 19 and secured to the aligned front edges of mounts 93 and 102 a second cross bar 106 shown in broken lines in fig9 supports the rear edges of mounts 93 and 102 additional support can be provided by fastening mounting brackets 107 to the beams 19 through apertures 108 as best seen in fig9 trough 90 is concaved in cross section having a relatively flat central portion 110 and opposed arcuate side portion 113 which curve outwardly and upwardly from the central portion of sieve 91 this enables trough 90 to encompass the lower portion of the circular periphery of the helical blade of auger 85 when the central shaft of auger 85 is supported at its end portions for rotation by pillow blocks not shown and is driven by motor m2 by the central flattened concaved contour of trough 90 the central portion 110 of trough 90 is closer to the rotating periphery of auger than the opposed concaved side portions 111 of trough 90 there should be a minimum vertical central clearance between the trough 90 and the auger 85 of about 0 2 inches 5 mm to about 1 25.