Belt weigh feeder Zero and Span Drift

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Fault Log · Field Report

Schenck Weigh Feeder — Zero and Span Drift, Two Different Root Causes

Plant: Cement  |  System: Schenck weigh feeder, belt scale, hopper load cells  |  Downtime: None

Symptom

A Schenck weigh feeder started showing both zero drift and span drift at the same time. Two separate accuracy problems showing up together made this a two-part investigation from the start — zero and span errors don't always share the same root cause, even when they appear together.

Investigation — Zero Drift

Started with the load cell loop and mechanical side of the belt scale:

  • Load cell loop — checked the full loop, cleaned the load cell area.
  • Idlers/rollers — checked the rollers for wear or misalignment.
  • Belt tracking — with the belt empty, noticed it was drifting to one side. Zero was being tared in that empty, drifted position. Once material loaded onto the belt, the belt tracked back closer to center — meaning the zero reference had been taken in a position the belt didn't actually run in under load.

Adjusted the belt tracking so the empty belt sits centered, matching the position it settles into under load. With the zero taken at a position that actually matches running conditions, the zero drift issue cleared.

Investigation — Span Drift

Span drift kept recurring even after the zero side was fixed, and it needed its own separate investigation:

  • Weigh feeder parameters — checked configuration parameters for anything abnormal. Found correct.
  • VFD parameters — checked the VFD driving the feeder for any parameter drift affecting belt speed reference. Found correct.
  • Span reference source — the span calibration reference for this feeder is the hopper's loss-of-weight measurement, which meant the hopper load cells themselves needed checking. The hopper has three load cells; checked all three and found one was giving a reading, but an incorrect one — not failed outright, just inaccurate enough to throw off the loss-of-weight reference the span calibration depended on.

Root Cause

Two unrelated issues causing what looked like one combined problem: the zero drift was a belt tracking issue — the empty-belt taring position didn't match the belt's actual loaded-running position. The span drift was a bad hopper load cell feeding an inaccurate loss-of-weight reference into the feeder's span calibration — the feeder itself was fine, but the reference it was being calibrated against wasn't trustworthy.

Fix

  • Adjusted belt tracking so the empty-belt zero position matches the loaded-running position — resolved zero drift.
  • Replaced the faulty hopper load cell — resolved span drift, since the loss-of-weight reference used for span calibration was now accurate.

Lesson

When a weigh feeder shows both zero and span problems together, don't assume they share one cause — chase them as two separate investigations. And when span calibration depends on an external reference like a hopper's loss-of-weight, remember that the feeder can be completely healthy while still giving a bad span result, because the problem is in the reference, not the feeder. A calibration is only as good as the reference it's checked against — verify the reference itself before condemning the equipment being calibrated.

Chased a weigh feeder drift back to a bad reference instead of the feeder itself? Drop a comment — always good to compare notes.

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