Schematic of flip-flow principle used in the Liwell screen (). 

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... or trampoline-type screens were designed and developed to efficiently screen the smaller size fractions of coal. The innovative flip-flow dry screening technology was developed by the Hein Lehmann company at the end of the 1960s ( Meinel, 2010). Run-of- mine coal typically contains about 3 to 10% free moisture; however, the percent moisture increases with decreasing particle size so dry screening efficiency achieved with conventional screens decreases sharply when the screening cut point goes below six millimeters. Dry screening of fine coal at apertures smaller than this is considered difficult due to possible blinding and clogging of the screen surface by moist and sticky material. Polyurethanes have been widely used to maintain a desired screening surface that efficiently passes this wet material. They have good wear characteristics (soft, flexible, and pliable) that can be unlimitedly stretched and released without losing their original shape (Short, 2011). The flip-flow principle combines the flexible polyurethane-type screen with a new vibrating system that results in up to 50 times more force of acceleration on particles riding on the screen surface. A schematic of the flip-flow principle used in the Liwell screen is shown in Figure 6. The screen is made of two side frames that move against each other with a phase shift of 180 degrees in a linear motion driven from a single crank shaft mechanism. The polyurethane screen mats are mounted on the cross beams and the cross beams are alternately connected to the two side frames. The opposing direction of movement of the side frames causes simultaneous tensioning of some individual screen mats and relaxing of others. During any complete revolution, the distance between cross beams changes up to ±24 mm causing a trampoline-type movement of the screening surface, which results in collapsing and breaking apart any plugged and blinded holes in the screen ...