Pie Cutting

March 8, 2000

"Pie cutting" is a term frequently used at Vincent. It refers to cutting end pieces from the screw of a press so as to reduce compression. The segments that are cut from the flight are roughly shaped like a piece of pie.

If a press jams with material it is generally found overpacked in one of the final stages of compression. Each stage of compression has two helicoid segments of flighting, each about 180º, welded to the screw shaft. By cutting a pie shaped piece from the end of this flighting, the opening will be enlarged enough to prevent jamming.

Pie cutting is recommended if a press is found to work best at a very low discharge cone pressure. Reducing the compression of the screw will allow operation within a more manageable air pressure range.

Sometimes pie cutting is performed in order to increase the throughput capacity of a screw press. This is done when low press cake moisture is not of primary importance.

The reason that pie cutting comes up so often is that it is easier to remove metal from the screw than it is to add. Pie cutting can be done in the field with simply unbolting the screens from the press. The opposite procedure, adding flight to a screw in order to increase compression, is not practical in the field. Consequently during start-up it is preferable to find a press that overpresses or draws too much horsepower.

Pie cutting can be performed with arc-air, plasma, or a hand- held grinder. It has also been done with oxyacetylene even though this works very poorly on the stainless steel flights. In one extreme case gasoline operated cement saws were used to cut the flights.

A more severe modification, resulting in the Sterile configuration, involves removing certain flights altogether from the screw shaft. The consequent reduction in jamming, compressing and horsepower draw is dramatic.

Standard drawings are available that illustrate pie cutting options and the Sterile configuration.

Issue 104