is threadedly engaged with the rear vertical screw 44 located inside the rear leg 16 this screw 14 is also secured at the bottom of leg 16 to allow rotation but no vertical movement the inside end of beater assembly 28 is also secured to the frame means by way of an additional pair of vertical screws 46 and 48 which at their lower ends are stationarily secured to horizontal bracket 50 which in turn is secured to the inside end of beater assembly 28 vertical screws 46 and 48 are nonrotatable and have threadedly engaged on them respective nuts or collars 52 and 54 which preferably are of the ball screw type as are the earlier mentioned collars or nuts 34 and 42 collars 52 and 54 are secured to respective l shaped brackets 56 and 58 which in turn are secured at their foot portion to the base 60 at the central portion of the frame the upright portions of brackets 56 and 58 being respectively secured to beams 62 and 68 which form part of boom 10 threaded collars 52 and 54 also have secured to them sprockets 70 and 72 respectively at the other end of the boom the other two vertical screws 36 and 44 have secured to them respective sprockets 74 and 76 entrained around these four sprockets 70 72 74 and 76 is a chain 78 which is driven by motor 80 via a gear reducer 82 and sprocket 84 when motor 80 turns sprocket 84 thereby driving chain 78 the four corner sprockets 70 76 are rotated simultaneously at the outer end since sprockets 74 and 76 are fixedly secured to the vertical screws 36 and 44 movement of the chain causes those vertical screws to rotate.