a modular screening apparatus has four parallel shafts arranged in a single plane mounted to each module . the shafts extend between the sides of the machine frame and are driven by a motor and timing belts which connect the shafts directly or indirectly to the drive motor . each shaft supports three beater bars which are equally spaced about the axis of the shaft . a screen bed defined by a perforated polyurethane plastic sheet overlies the plane containing the driven shafts . the sheet serves as a screen which is clamped at its upstream edge , the downstream end extending over a fixed bar and vertically downwardly , to suspend a weight . the sides of the screen abut adjustable side plates which are mounted to the machine frame and define the boundaries of the screen bed . the beater bars rotate on the shafts to strike the underside of the screen .