Characteristics of johnson screens:
Easy clearing
The narrowest part of the aperture comes at the top and the steady opening out from that point promotes easy clearing.
Lateral strength
Increases rapidly with the width of the profile and sufficient is needed to avoid the likelihood of the wedge wires being bent sideways and the accuracy of the slot being thereby lost.
Load carrying capacity or vertical strength
Increases rapidly with the depth of the profile and is needed to a greater degree than is lateral strength
Percentage open area
For any given aperture size this depends on the number of apertures per foot width of johnson screen. The wider the profile the fewer the apertures, and the less is open area - likewise the smaller the number of pressed wedge wires per foot, the greater the open area.
Wearing life
With abrasive materials the top surface of the profile becomes worn away and inevitably the aperture must increase. With a small angle of opening the increase is gradual, but with a large angle it is rapid. The apertures quickly reach the oversize limit and the working life is reduced.