The most common damage to a wedgewire screen is for part of the surface to be smeared
over from being rubbed by the screw. This rarely is bad enough to affect press performance.
Profile bar screens generally work satisfactorily with 30% or even more of their surface
smeared over. Smeared screens can be remedied by running a die grinder blade through the
slots. Small holes can just be patched with sheet metal to extend life. In cases of severe
wear or damage, it is common to patch a screen. Stainless sheet metal is used for this. The
reduction in drainage surface is of little consequence as the screens have ample open area.
Standard perforated screens have a hole size of 0.094" diameter, although material with
0.050", 0.033", down to 0.023" holes can be supplied. Surprisingly, usually there is little
difference in the degree of filtration achieved by either changing hole size or going to a
slotted profile bar screen.
Frequently, increased press capacity can be achieved by changing a perforated screen to one
with smaller holes. This unexpected result arises from a combination of factors: (1) smaller
hole screens are made of thinner sheet metal, so the press liquor has a shorter distance to
travel before it falls free from the screen, reducing the chance of blinding and (2) particles
which fall into and plug a larger hole will roll over a smaller hole.
Minor rubbing between the screw and screen is normal, although, obviously, hard rubbing
will cause wear and premature failure of the screen. With a clearance greater than 3/16", the
dewatering performance of the press can start to deteriorate; this depends a lot on the nature
of the material being dewatered.
The most common cause of screen failure ties to failure of the outboard support bushing. If
the bushing holding the end of the screw wears out, it can let the screw move enough to rub
against the screen.
Cord Cutter and Stripper Pins
Sometimes long stringy material will be pinched where the feeder portion of the screw goes
through the hole in the B-plate. This material will co-rotate with the screw and build into a
bundle which reduces flow through the press.
A groove, like a 3/8" deep keyway, is cut halfway through the hole in the B-plate. We call
this a Cord Cutter. Material trapped between the flight and the hole in the B-plate will pop
up slightly as it passes the Cord Cutter. The result is that the material is sheared loose.
A part called Brian's Stripper may be welded to the bottom of the B-plate, inside the inlet
hopper. It goes in a position so that the flight lightly kisses the stripper as it goes past. This
strips the material away. Strippers are made of square bar.
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