Why did my belt weigher’s span change after belt work?

This guide covers a span that stepped on the date of the belt work. For a zero that keeps walking, see zero drift. If the belt weigher and the weighbridge stopped agreeing after a shutdown, see belt weigher vs weighbridge. The full calibration sequence is in what happens during a calibration visit.

Why does a span error cost more than a zero error?

Zero is a fixed offset, so it matters most when the belt runs light. Span is the gain: a percentage of everything that goes over the scale. At full rate, a span error is straight tonnage.

Example only: a belt running 1,500 t/h for 20 hours a day moves 30,000 t. A 1% span error is 300 t a day, every day, until someone recalibrates. For how the two fit together, see zero, span and repeatability explained.

Why does belt work change the span?

The span was set against the old belt. The new one loads the weigh frame differently:

• Mass and stiffness. A different carcass, ply count or cover changes the mass per metre and the stiffness. A stiffer belt carries some of the load past the weigh idlers instead of putting it on them.
• The splice. It’s thicker, heavier and stiffer than the rest of the belt, and it crosses the weigh idlers every revolution.
• Tension. Retensioning changes the sag between idlers, and sag decides how much load presses on the weigh idlers. Too tight and the belt can lift through the weigh area.
• Tracking. A belt running off centre edge-loads the weigh idlers. Tracking wanders, so this one can look unstable rather than like a clean step.
• Length and speed. A new belt is rarely the length stored in the integrator, and both figures go straight into the tonnage calculation.
• Disturbed weigh idlers. One knocked out of line costs accuracy that no calibration gets back. We realign weigh idlers to typically within about 0.3 mm. The exact target depends on the scale.

Why does the zero move too, and which comes first?

Zero is what the empty belt reads, and you’ve just replaced the empty belt, so the zero steps as well. Fix the zero first, because span is measured on top of it. Run a span against the old zero and the span correction soaks up the zero error: two wrong numbers that agree at one load and nowhere else. Zero over whole belt revolutions so the splice averages out, then span.

A new belt beds in over a week or two while the splice settles and tension and tracking get adjusted. Check the zero after the belt work and again once the belt has bedded in.

What should happen at the shutdown?

Tell us before the belt work, not after. What we can do depends on when we hear.

• Get it into the shutdown plan, so recommissioning happens while the conveyor is isolated and belt time is available. For the assets we maintain, AIMM, our asset management system, tracks the calibration schedule, which makes it easy to line up with your planners.
• Protect the weigh area. Ask the belt contractor to leave the weigh idlers and weigh frame alone. If they have to come out for access, they need realigning properly going back in.
• Record what was fitted: belt type, length, thickness, splice position, and anything else that moved. That tells us whether it’s a calibration job or an alignment job.
• Re-measure, re-enter, recalibrate. Re-measure the belt speed and length and re-enter them. Then zero over whole revolutions, then span.
• Prove it with material when the tonnes matter. Test weights show how the scale responds to a known force, not how it reads your product at flow. Where tonnes are reconciled, a belt cut test or a material test settles it.

The numbers we work to: repeat span runs within about 0.1%, and a routine span that has moved more than about 0.25% needs attention. A belt speed more than about 2% off a measured speed needs investigating. Your integrator’s own limits are in the manual for your model.

Who adjusts what. On our service visits we adjust the zero and span when the checks show it’s needed. Between visits, site should run a zero about every two weeks and after a big change in the weather, and check the span but record it rather than accepting it. We recommend a service calibration every quarter so the scale keeps up with the seasons.

What does the calibration history tell you?

Start at the date of the belt work and look either side of it.

• A clean step, then stable: the ordinary case. Recalibrate against the new belt and it’s done.
• Zero and span moving together: the belt work signature. Zero moving on its own, with the span holding, is more often build-up.
• Repeat span runs that won’t agree within about 0.1%: something is mechanically unstable. Look at tension, the belt lifting through the weigh area, or an idler out of line.
• A wobble timed to the belt revolution: the splice, or a thickness change along the belt. It averages out over whole revolutions, which is why zero runs cover several.

What can site check?

Work on or near a conveyor, or inside an electrical enclosure, needs isolation under your site’s procedures and trained, authorised people. If that’s not you, stop and call us on 1300 101 666.

From a safe position with the belt running, and again once it’s isolated:

• Tracking. The belt should run central through the weigh area. That’s fixed at the conveyor, not the integrator.
• Sag and lift through the weigh area. Both change the span, and both are conveyor jobs.
• The splice. Watch a few revolutions. One that bumps, lifts or slaps across the weigh idlers is worth reporting.
• Build-up and alignment. Material packed on the frame or under the weigh idlers is dead load. Sight along the idler line too: one sitting high or twisted needs realigning by someone with the right gear.

Need a manual for your equipment?

Talk to us about manuals for your equipment. Send the make, model, serial number and software version (a photo of the label is fine) and we’ll help you find the right document.

When should you call us?

Call us on 1300 101 666 as soon as belt work is planned. After the fact, call when the scale stepped on the date of the belt work and stayed there, when repeat span runs won’t agree, or when the tonnes stopped agreeing with the weighbridge or a stockpile survey. A survey is a useful cross-check, but it carries its own density and shape assumptions.

If a re-zero and re-span haven’t fixed it, something was probably disturbed and needs realigning, not recalibrating.

Frequently asked questions