An arrangement in grading and cleaning machines with a screen (15). The screen is associated with at least one sensor (24) in the flow of material passing through the screen for generating electric signals in dependence on material particles leaving the screen as throughput and impinging on the sensor. The sensor is operatively connected to adjusting means (26) through a function circuit (25) for the control of an operational parameter of the screen (15) in dependence on the impingement intensity.