Cement screw-feeder pulsation usually comes from unstable inlet filling, cyclic mechanical drag, drive-speed variation or weighing-system interference. Trend batch mass, feeder speed and motor current together; inspect aeration, screw condition and load-cell freedom; then tune coarse/fine feed only after the mechanical flow is stable.
Record feeder speed, motor current and batch mass for several consecutive dosing cycles.
Check whether the screw inlet remains flooded or alternates between starvation and surge.
Inspect flights, casing, bearings, buildup, coupling and gearbox for cyclic drag.
Prove repeatable cement mass over consecutive batches before tuning the controller.
Capture the dosing pattern before changing settings
Record commanded and actual screw speed, motor current, cement mass versus time, target mass, overshoot or undershoot, bin level, aeration status, inlet gate position and refill timing for several consecutive batches. Note whether the fault appears at startup, during steady feed or near the final trim stage.
A repeated mass error at the same point in the dosing curve usually points in a different direction from a random error. Preserve the batch traces before changing calibration or controller settings so the original pattern is not lost.
Step-by-step screw-feeder pulsation workflow
- Check inlet filling. Confirm that cement reaches the screw inlet continuously. Bridging, rat-holing or intermittent collapse above the inlet can produce surge flow even when screw speed is stable.
- Verify aeration condition. Too little aeration can cause starvation and compaction; excessive aeration can fluidize the cement and create uncontrolled inflow. Compare the fault with aeration timing and silo level.
- Inspect screw flights and casing. Look for bent flights, local wear, packed cement, rubbing marks and uneven clearances. Cyclic drag once per revolution can show up as repeating motor-current variation.
- Check bearings and supports. Inspect hanger or end bearings where fitted. Binding supports, poor alignment or contamination can create intermittent torque peaks and unstable delivery.
- Verify drive and gearbox stability. Compare commanded and actual VFD speed, motor current and gearbox condition. A stable command with changing actual speed points toward drive or load variation rather than weighing calibration.
- Separate flow instability from weighing error. If motor current and screw speed remain smooth while indicated cement mass jumps, inspect load-cell freedom, flexible connections, venting and mechanical interference around the weigh hopper.
- Review coarse/fine feed logic. After mechanical flow is stable, check the fast-to-trim transition, low-speed setting and stopping delay. A late transition increases overshoot; an early one increases cycle time.
- Verify several consecutive batches. Do not accept one successful batch. Confirm repeatable dosing and stable mechanical behavior over a representative sequence.
Recommended as-found evidence record
Diagnose the pulsation pattern
Return-to-service acceptance checklist
- Feeder speed and motor current are stable at representative dosing conditions.
- The screw inlet remains consistently supplied without repeated starvation or surge.
- No cyclic rubbing, binding or abnormal bearing/gearbox behavior is present.
- Load cells and flexible connections remain mechanically free throughout the batch.
- Actual cement mass remains inside the plant’s approved tolerance over consecutive batches.
- Coarse-to-fine transition is repeatable without excessive overshoot or cycle-time penalty.
- The final speed, current, batch-mass trend and controller settings are saved as the new good baseline.
Frequently asked troubleshooting questions
Can constant screw speed still produce pulsating cement flow?
Yes. The inlet can alternate between flooded and starved conditions, or the screw can experience periodic mechanical drag even when the speed command is unchanged.
Should the scale be recalibrated when batch weights fluctuate?
Only after stable feeder flow and mechanical freedom are proven. Calibration cannot correct bridging, rubbing or a weigh hopper loaded by rigid pipework.
Why does overshoot increase when cement becomes more fluid?
More fluid cement can continue moving after the stop command, increasing in-flight material. Stabilize flow first, then verify the coarse/fine transition and stopping delay.
Engineering reference basis
Use the installed batching-plant, screw-feeder, VFD and weighing-system OEM manuals for final clearances, speed limits, calibration tolerances and controller settings. The diagnostic sequence above is intended to separate material-flow, mechanical, drive and weighing causes before adjustment.