Last winter I stood in a cement mill in Inner Mongolia watching a 27 kW motor pull 38 A before the VFD finally tripped. The screw conveyor feeding fly ash to the blending silo had been running fine for 11 months – then it blocked solid in under four hours. When we cut the trough open, what we found wasn’t a single lump. It was a 1.8-meter column of compacted, semi-set cement wrapped around the hanging bearing. The flight edge was paper-thin. The discharge chute looked like someone had packed it with concrete.<\/p>\n\n\n\n
That’s what screw conveyor blockage<\/strong> actually looks like in a cement plant. Not a textbook diagram – a trough full of rock, a burned motor, and a 16-hour unplanned shutdown on a \u0645\u0635\u0646\u0639 \u062e\u0644\u0637 \u0627\u0644\u062e\u0631\u0633\u0627\u0646\u0629<\/a> that was supposed to be running clinker.<\/p>\n\n\n\n A screw conveyor (sometimes called a spiral conveyor) is a helical flight welded to a central screw shaft, turning inside a stationary trough, pushing bulk material from inlet to discharge. That’s the textbook version. On the floor, a cement screw conveyor is a wear-intensive, hygroscopic-material-handling, dust-leaking beast that lives between your silo discharge and your downstream conveyor. When it blocks, it doesn’t politely stop. It stalls the motor, torques the screw shaft, cracks the trough flanges, and – if you’re unlucky – twists the screw into a permanent banana shape that has to be cut out with a torch.<\/p>\n\n\n\n Material blockage is not one failure mode. It’s the symptom of a dozen upstream decisions: flight wear you ignored, a feed rate that exceeded the conveyor capacity, a \u0635\u0648\u0645\u0639\u0629 \u0627\u0644\u0623\u0633\u0645\u0646\u062a<\/a> that allowed flash-set because cement sat too long, a hanging bearing seized by abrasive fines. Most plants treat “blockage” as the problem. It’s the report. The root cause is almost always further back.<\/p>\n\n\n\n I’m not going to give you a generic list. These are the seven I’ve personally cut out of troughs, in order of how often I see them in cement service.<\/p>\n\n\n\n Worn screw flights are the #1 cause I see. Cement and fly ash are abrasive – not “moderately abrasive” the way some spec sheets say, but genuinely aggressive on carbon steel. Over 8 to 14 months of continuous working condition, the OD of the flight erodes. Here’s the trap: as the flight wears, a thin layer of compacted material builds up between the flight edge and the trough wall. It looks like the conveyor is still working – material is moving, motor current is stable – but the effective screw conveyor diameter has shrunk. The conveying capacity silently drops 15, 20, 25 percent. Then someone bumps the feed rate to “catch up,” and the conveyor that used to move 35 t\/h is now trying to move 40 t\/h with a worn screw. It backs up at the discharge end in a single shift.<\/p>\n\n\n\n Inspect the screw. Not the trough, the screw. Measure flight OD against the original drawing. Anything more than 3 mm of radial flight wear on a 323 mm conveyor is a planned replacement, not a “we’ll watch it.”<\/p>\n\n\n\n Every cement screw conveyor longer than 3 meters has at least one intermediate hanging bearing that supports the screw mid-trough. That bearing sits inside the material flow. It runs in abrasive fines, in dust, in moisture from fly ash. When it seizes – and it will seize, eventually – the shaft snaps or the screw whips. A whipping screw scours the trough, packs material at the discharge port, and you have a blockage that looks like it came from the discharge end. It didn’t. It came from a $400 bearing nobody greased last Tuesday.<\/p>\n\n\n\n Hanging bearings in cement service need a grease line routed to the outside, and they need to be greased every shift, not every week. If you’re still using a packed-grease design with no grease line to support the screw, replace it on the next shutdown.<\/p>\n\n\n\n Two failure modes, opposite ends. A starved inlet – where the silo discharge valve restricts flow – lets the screw run partially empty, which sounds fine but lets cement dust pack inside the conveyor between flights. Over days it compacts into a hard plug at the screw conveyor inlet. A choked inlet – where the silo discharges faster than the screw can move – floods the trough immediately. The motor current spikes, the VFD trips, and you’re standing in front of a conveyor full of material that has to be dug out by hand.<\/p>\n\n\n\n The fix isn’t just valve adjustment. It’s a matching problem: the inlet opening, the screw conveyor diameter, the screw speed, and the silo discharge rate all have to be sized to each other. When I audit a plant, this is where I find the mismatch 70 percent of the time. Cement entering the screw has to arrive at the rate the screw blade can drive it, no faster.<\/p>\n\n\n\nWhat “Blockage” Really Means in a Cement Screw Conveyor?<\/h2>\n\n\n\n
The Seven Field-Verified Causes of Blockage<\/h2>\n\n\n\n
1. Worn Screw Flights and the “Cushion” Illusion<\/h3>\n\n\n\n
2. The Hanging Bearing – Your Hidden Failure Point<\/h3>\n\n\n\n
3. Inlet Design – Starving vs. Choking<\/h3>\n\n\n\n
4. Hardened Cement and the Storage-Time Trap<\/h3>\n\n\n\n