July 30, 1997
For three years Vincent has experimented with a variety of wipers. These have been both brushes and pliable strips that we have attached to the screws of Bennett Screens and Screw Presses. Their purpose has been to wipe the screens clear of blinding material so that liquid can pass through the screen.
The first wipers used were made of nylon bristle brush crimped into a spiral strip of stainless steel. Nylon with good memory and resistance to water absorption was chosen. One disadvantage was that bristles could break off, which precluded use in food processing applications. The principal disadvantage was that the brush tended to loose much of its effectiveness within a couple months, and replacement involved complete disassembly and a welding process.
The next evolutionary step involved attaching a circular strip of plastic to the outer edges of the flights of the screw. Initial field retrofits were achieved by welding anchor pads to the face of the flight. Each pad had a drilled and tapped hole. The wiper was positioned next to the pad and a clip was bolted so as to clamp the wiper to the screw.
Later, for wipers installed at the factory, holes were drilled through the screw flight. This allowed wipers, with reinforcing back-up strips, to be bolted directly to the face of the screw.
A high durometer polyurethane that is used as a scraper for conveyor belt applications was selected. These wipers worked very well, but they too had a relatively short life span. They also had a tendency to tear loose. At least the bolting arrangement facilitated their replacement.
There has been much debate over whether a wiper should be installed on the leading or trailing edge of a flight. The leading edge has been preferred because that configuration allows the wiper to be supported by the screw flight. This preference has diminished with the advent of stronger reinforcing strips that are used with the rear-mounted wiper. At present trials are underway with wipers mounted on both edges of the screw flight.
Another style of wiper is referred to as an axial wiper. This is a different configuration in which straight strips of wiper material are bolted to two opposite faces of a 1" square bar. A length of this bar is welded at the outer perimeter of the screw, parallel to the axis of the screw. It is placed so as to bridge from one flight to the next. The construction allows for dual wipers (leading edge and trailing edge).
This wiper design is definitely the easiest to install and replace. It is limited to use with very soupy materials, otherwise the design can cause co-rotation.
Wipers have been found more suitable for larger presses than small ones. This is because the wiper and its support system block the passage of material through small screw diameters.
KP-6 and KP-10 presses are being built both with and without wipers. On one hand they are a costly addition, and on the other, they help support the screw away from the screen. The jury is still out.
Overall we have been frustrated with wipers because the performance improvement they offer deteriorates within weeks. They are no longer available on rental presses because they tend to give results that are not sustainable in the long run.