A critical element of a screw press is the screw-to-screen (s2s) clearance. This clearance is important because the action of the screw in pushing fiber over the screen. This movement wipes the screen clear, brushing the screen so as to permit the free passage of press liquor. Thus, the tighter the s2s clearance, the better the dewatering action.

With slimy materials, such as washed citrus peel, biomass digester sludge, manure, DAF sludge, potato peel, and spent brewers' grain, the s2s is critical to successful operation. With many other applications, such as primary clarifier underflow, spent coffee, corn silage, tobacco, most nutraceuticals, and egg shells, the s2s is far less important.

From the 1950's into the 1990's, the standard s2s was 1/16". This was difficult to achieve in manufacturing because the screen is typically fabricated in two half cylinders, up to 100" long.

The screw is machined to within a few thousandths of an inch. Thus the screw has presented a fixed reference diameter.

It is holding the inside diameter of the screen which has presented the challenge. Screens are made of fabricated (not machined) stainless steel. Rings, gussets, and honeycomb reinforcing are used for strength and rigidity. The welding of these elements is an inherent source of distortion in the finished part.

Recent improvements in welding fixtures, screen design, welding sequences, and press alignment procedures have allowed the s2s to be set at 1/32" (compared to the previous 1/16"). The use of machined stainless indexing rings, built into the frame of the press, has contributed a great deal. Also, in the small and mid-sized Series KP presses, the use of a cylindrical screen (instead of two 180 degree segments) has resulted in tight tolerances.

Vincent is proud of these improvements. Without addition to the cost of manufacture, they have resulted in better dewatering and overall performance of our presses.

Note on abrasive applications:
Many of Vincent's customers run abrasive or corrosive materials through their presses. These applications require the flights have a special hardsurfacing to ensure a cost-effective lifetime of the screw. This surfacing is welded onto the flights at the top and leading edges. Unfortunately, this heat of application bends the flights forward. This movement alters the OD of the screw and hinders good s2s clearance. Vincent is currently investigating other forms of hardsurfacing and abrasion-resistant coatings which would either not alter the screw and flight dimensions or could be turned down on a lathe.