A belt scale is a weighing system that continuously measures the quantity of material flowing on a conveyor without stopping the belt. In crushing and screening plants it is used for production tracking, calculating efficiency indicators such as kWh/ton, feeder control, blending and dispatch tracking. A correctly installed and regularly calibrated belt scale is like the plant’s “cash register”: if you don’t know how much you produce, you can’t calculate your costs correctly either.
In this guide we look at how a belt scale works, the installation conditions that affect accuracy, calibration methods, sources of error and maintenance practice.
How Does a Belt Scale Work?
A belt scale combines two basic measurements:
- Load measurement: One or more carrying idler stations are mounted on load cells. The weight of material passing over these idlers is measured as load per unit length of belt (kg/m).
- Speed measurement: A speed sensor (usually an encoder connected to the tail pulley or an idler shaft) measures belt speed (m/s).
The electronic integrator multiplies these two values to calculate the instantaneous flow rate (t/h) and totals it over time to give the total tonnage. The system consists of load cells, a weigh frame, a speed sensor and an integrator unit.
Uses
| Purpose | Description |
|---|---|
| Production tracking | Product quantity by shift, day and month |
| Efficiency | kWh/ton, wear part consumption (g/ton), fuel/ton calculations |
| Process control | Keeping the crusher at constant load by adjusting feeder speed |
| Blending | Mixing different materials in set proportions (cement, lime plants) |
| Dispatch and stock | Tracking loading; comparison with stock inventory |
Belt scale data is the basic input for measuring energy efficiency in a crushing and screening plant; we cover this in detail in our energy efficiency guide.
Correct Installation Conditions
The accuracy of a belt scale depends more on where it is installed than on the quality of the device. An expensive scale installed in the wrong place can give worse results than a simple scale in the right place.
- A straight, stable section: The scale should be placed on a straight part of the conveyor where the slope does not change. Concave or convex curves apply extra force to the weigh idlers because of belt tension.
- Distance from the loading point: The scale should be far enough from the loading chute and skirt rubber, at a point where the material has settled on the belt. Skirt rubber grips the belt and disturbs the measurement (skirt rubber guide).
- Constant belt tension: Changes in tension lead to measurement errors. Gravity take-ups keep tension more constant (take-up guide).
- Idler alignment: The alignment (height and line) of the weigh idlers and of several idler stations before and after them must be set precisely. A misaligned idler makes the belt press on the weigh idler with a different force.
- Wind and rain: On open conveyors, wind and rain can affect the measurement; protecting the scale area is recommended.
- Belt mistracking: The belt must run centred at the scale. For solving mistracking problems, see our belt conveyor guide.
- Vibration: A location away from crusher and screen vibration should be preferred.
Calibration Methods
| Method | How it is done | Note |
|---|---|---|
| Zero calibration (tare) | Readings are taken over a whole number of belt revolutions while the belt runs empty and the zero point is set | Should be done often; material stuck to the belt and temperature shift the zero |
| Static weight test | Known weights are hung on the weigh frame to check the span | A quick check; does not replace a material test |
| Chain test | A calibration chain of known weight per metre is laid on the belt and the belt is run | Closer to real conditions |
| Material test | A quantity of material is passed over the scale and weighed on a separate approved scale (e.g. a weighbridge) | The most reliable method; should be done periodically |
Calibration records should be kept and changes in the calibration factor monitored. Sudden changes in the factor can signal a mechanical problem (idler, load cell, tension).
Sources of Error
- Material stuck to the belt and dirt building up on the weigh idlers.
- Seized or worn weigh and approach idlers.
- Changing belt tension (temperature, a jammed take-up).
- The speed sensor slipping or getting dirty.
- Moisture and damage in load cell cables.
- The belt mistracking or undulating at the scale.
- Running far below or above the scale’s capacity; belt scales are most accurate within a certain load range.
The Difference Between Process Weighing and Trade Weighing
Belt scales used for in-plant production tracking are “process weighing” devices. Weighing systems used for commercial transactions such as sales and invoicing are subject to legal metrology regulations; they require an approved type, specific accuracy classes and periodic verification. If you plan to use a belt scale for trade purposes, you should therefore research the relevant regulations and approval processes in advance. Most plants use a weighbridge for dispatch, and keep the belt scale for production and process control.
Maintenance Checklist
| Check | Frequency |
|---|---|
| Cleanliness of the weighing area | Daily |
| Zero calibration | Daily or per shift (depending on material and climate) |
| Weigh and approach idlers turning | Weekly |
| Speed sensor connection | Weekly |
| Chain or static weight test | Monthly |
| Material test | Periodically (e.g. every three months) and after major maintenance |
| Idler alignment check | Always after an idler change |
For idler maintenance and replacement, see our conveyor idlers guide, and for belt cleaning, our belt scrapers guide.
A Field Example
Suppose there is a marked difference at a plant between the monthly production shown by the belt scale and the dispatches through the weighbridge plus the change in stock. The investigation shows that an idler in the scale area was replaced but its alignment was not checked, and that because the belt scraper was worn, a lot of material was sticking to the underside of the belt. After the idler alignment is corrected and the scraper renewed, a material test shows the scale has returned to acceptable accuracy. This example shows that the accuracy of a belt scale cannot be separated from the general maintenance of the conveyor.
Frequently Asked Questions
How accurate is a belt scale?
Accuracy depends on device quality, installation conditions and the calibration routine. Well-installed and well-maintained systems give reliable results for process tracking.
How many idlers should the scale have?
Multi-idler systems provide a longer weighing zone and are generally more accurate; however, installation precision also matters more. The choice should be based on the accuracy required.
Can a belt scale be installed on an inclined conveyor?
Yes; but the incline must be constant and the scale kept away from points where the slope changes. The angle of incline is taken into account in calibration.
Why should zero calibration be done often?
Material stuck to the belt and changes in temperature and tension shift the zero point; small zero errors turn into large tonnage differences over time.
Conveyor Parts That Affect Belt Scale Accuracy
| Part | Effect on accuracy | Guide |
|---|---|---|
| Carrying idlers (weigh and approach idlers) | Misalignment and seized idlers distort the measurement | Conveyor idlers |
| Belt | Thickness variation, splices and stuck material | Conveyor belt |
| Take-up | Tension changes shift zero and span | Take-up |
| Belt scrapers | Carryback disturbs the zero | Belt scrapers |
| Skirt rubber | Changes the load by gripping the belt | Skirt rubber |
| Tail and snub pulleys | Speed sensor connection and belt movement | Tail pulley |
For idler, belt, scraper and take-up parts for conveyors fitted with belt scales, see our belt conveyor spare parts page; you can get a quote by sending idler diameter, belt width and shaft dimensions on WhatsApp.
Installation Checklist
- Is the scale area straight and away from changes in slope?
- Is there enough distance to the loading point and skirt rubber?
- Are the weigh idlers and approach idlers of the same type and diameter and precisely aligned?
- Is the speed sensor connected to a clean pulley so that it cannot slip?
- Does the belt run centred in this area?
- Is there protection against wind and rain?
- Are cables and the integrator protected against moisture and dust?
- Has the first material test been carried out and recorded?
Glossary
- Load cell: A sensor that converts force into an electrical signal.
- Integrator: The electronic unit that processes load and speed signals to calculate flow rate and total tonnage.
- Zero (tare): The reference value measured with an empty belt.
- Span: The scale factor set by calibration.
- Material test: A calibration with real material compared against an approved weighing.
More Questions
Can a feeder be controlled automatically with a belt scale?
Yes. The belt scale signal can be used to adjust feeder speed to keep a crusher or screen at constant load; this improves energy efficiency and product quality.
Is calibration needed after a belt is replaced?
Yes. Because the weight and stiffness of the new belt may be different, zero and span calibration should be repeated.
Why do the belt scale and the weighbridge give different results?
Differences can arise from installation and maintenance errors, moisture differences, stock changes and measurement timing; they should be checked by regular reconciliation and material tests.
Support from CSP Mühendislik
CSP Mühendislik supports planning suitable belt scale locations in conveyor design and manufacture, arranging idler and take-up systems, and supplying conveyor spare parts. For idlers, pulleys, belts and scrapers, see our belt conveyor spare parts page, and for plant planning, our crushing and screening plant set-up guide.
