Belt weigh feeder

Belt weigh feeders are everywhere on a cement plant — raw meal proportioning, coal/fuel feeding, clinker and additive dosing. They're one of those instruments that runs quietly for months and then becomes the center of attention the moment a mix ratio goes wrong. Here's how they actually work, and how we calibrate and troubleshoot them in the field.

The Measuring Principle

A belt weigh feeder continuously weighs the material being carried on a short section of conveyor belt. That section rides on a weighing platform, supported by one or more load cells, sitting between two carrying idlers. As material passes over the platform, its weight is transferred through the idler onto the load cell, which outputs a voltage proportional to the load. That signal is amplified and digitized, then fed to the feeder's controller.

Since only part of the total belt load acts on the weighing idler (the rest is carried by the fixed idlers on either side), the controller accounts for this using the effective platform length:

Leff = Lg / 2 (for a single-idler platform)
where Lg is the total platform length.

From there, belt load is calculated as:

Q = QB / Leff
Q = belt load (kg/m), QB = weight on the platform (kg)

The second key input is belt speed, picked up by a speed sensor and converted to a pulse frequency. Combining load and speed gives the actual feed rate:

I = Q × v
I = feed rate (kg/s), v = belt speed (m/s)

Operating Modes

Gravimetric (controlled) mode — the controller actively adjusts belt speed to hold the actual feed rate at the setpoint. If the setpoint exceeds the feeder's rated capacity, the system flags an error rather than over-driving the belt.

Volumetric (uncontrolled) mode — belt speed is set proportional to the desired feed rate, but the system doesn't correct for changes in material bulk density or hopper conditions. Under stable, rated conditions the feed rate tracks the setpoint reasonably well, but it will drift if material characteristics change. Setpoint is typically limited to about three times nominal feed rate as a safety margin.

Volumetric synchronous mode — used during mode changeover. Instead of computing belt speed from nominal belt load, the controller uses the belt load measured just before the switch, so the transition happens without a sudden jerk in feed rate.

Calibration — Tare and Span

Before running a tare (zero) calibration, confirm:

  • Belt is empty — no material on the weighing section
  • Belt is running at normal operating speed
  • Belt is tracking centered, not drifting to one side
  • Feeder is in volumetric mode for the calibration procedure
  • Load cells and weighing platform are clean — no built-up material or debris affecting the reading

Once tare is set, span calibration is done with a certified check weight or chain applied to the platform, comparing the feeder's indicated reading against the known reference value and adjusting the span factor accordingly. If you're correlating against a weighbridge reading instead, the correction factor is calculated as:

New factor = (weighbridge reading / weigh feeder reading) × old factor

Common Field Faults

  • No power to the load cell or controller — check supply and wiring first before assuming a sensor fault.
  • Material stuck on or around the load cell/platform — adds false weight and skews readings; clean and re-zero.
  • Speed sensor or wiring fault — causes feed rate to read incorrectly even if the weight signal itself is fine, since feed rate depends on both weight and speed.
  • Faulty load cell — check output signal against expected range with no load and with a known test weight; replace if drifting or dead.
  • Belt drifting off-center — throws off the weight distribution across the platform and idlers; check belt tracking and alignment.
  • VFD problems — inconsistent belt speed control feeds bad speed data into the feed rate calculation even if weighing is accurate.
  • Material overload — exceeding rated capacity causes nonlinear response and calibration drift over time.
  • Rollers jammed — adds friction and mechanical resistance that skews the true material weight being measured.
  • Cable faults — loose or damaged load cell cabling causes noisy or intermittent weight signals; inspect connections and continuity.

Bottom Line

A belt weigh feeder is really just a precise scale mounted on a moving conveyor — accurate feed rate depends equally on getting the weight signal right and the speed signal right. Most of the faults above trace back to either a dirty/misaligned mechanical system or a degraded load cell/speed sensor signal, so a methodical check — mechanical first, then electrical, then calibration — usually gets you to the root cause quickly.

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