Losing Screen Efficiency: Blinding & Pegging

A screen that used to make spec quietly stops making it, and the calls I get run the same way every time: "the SCADA's fine, but the product's dirty." Screening is a mechanical separation dressed up with a little instrumentation, so the honest starting point is that most efficiency losses are not a setpoint problem. They're moisture, worn cloth, a stroke that's drifted, or discs wrapped in film. The controls help you see the loss and hold a steady feed, and once in a while the controls are the fix. But if you go chasing a HMI number before you've put eyes and a hand on the deck, you'll burn a shift. This guide is for the maintenance lead or screen tech standing at a blinded deck at 2am on an aggregate crushing plant or a materials recovery facility (MRF, the plant that sorts single-stream recycling), trying to decide what to touch first. Let me fix the vocabulary first, because the crew and the plant manager usually mean three different things by the same word, and that alone sends people to the wrong repair.

Say what you actually mean: blinding vs pegging vs carryover Blinding is fine, damp or sticky material coating the cloth and bridging across the openings. Think clay, crusher fines with moisture in them, dirt, dust that's picked up humidity, sludge on a MRF fines screen. It builds up as a skin over the wire and effectively shrinks or seals the aperture (the opening in the cloth). Blinding involves material much finer than the opening — it's a coating problem, and it usually shows up in patches or across a whole zone of the deck. Pegging (also called plugging) is near-size particles physically wedged into the aperture, one stone per hole. A near-size particle is one just a hair bigger than the opening, so it enters partway and jams. Pegging is a geometry problem, not a moisture problem, and you'll see it as individual stuck stones you could knock out with a bar, often worst where the material bed is thinnest and the near-size fraction is heavy. Carryover and contamination is the result, not a cause. Carryover is undersize material riding over the end of the deck and reporting to the wrong pile because blinding or pegging closed off the openings it should have dropped through. On aggregate you'll hear it as "oversize in my product" or a