disclosed is a ball cleaner for a screening machine in which the ball cages are presented by a molded plastic insert having integral intersecting longitudinal walls and cross walls . the weight of material being screened on the screen which overlies the insert is not carried by the insert but rather by a separate support beneath it . the support comprises a peripheral frame and joists within the frame which directly underlie at least the longitudinal walls of the insert . the insert completely isolates the frame from the erosive action of the cleaning balls and can be replaced separately from the frame .