Conveyor Belt Scale Product

Overview

A conveyor belt scale (or belt weigher) is an in-line weighing device installed in a conveyor system to measure the mass flow rate of material moving across it. Rather than stopping the belt and placing material on a static platform, the belt weigher measures weight continuously while the material is in motion, reporting instantaneous flow rate in tons per hour (t/h) and cumulative total in tonnes.

The system comprises three key subsystems: (1) a Weigh Frame Assembly containing instrumented Load Cell Idlers (conveyor rollers with embedded load cells); (2) a Speed Sensor Module that measures belt velocity via an optical or magnetic encoder; and (3) a Flow Rate Integrator that combines weight and speed signals to compute flow rate using the formula: Flow (t/h) = Weight on belt (kg) × Belt speed (m/s) × 3.6.

Used extensively in mining, quarrying, cement production, and grain handling, belt weighers allow operators to monitor and control material throughput in real-time, optimize process balance, and maintain custody transfer accounting.

How It Works

Material is fed onto the moving conveyor belt upstream of the weigh frame. As the material passes over the instrumented weigh section, its weight is measured by load cells integrated into the Load Cell Idlers. Typically, three idlers are instrumented: inlet (where material enters the weigh zone), center (average point), and outlet (exit point). Averaging multiple points reduces error caused by uneven material distribution across the belt width.

Each [[belt-weigher-idler-inlet|idler with a load cell]] has a shear-beam compression load cell embedded in its core, measuring the vertical force exerted by the belt and material. The three load cells output 4–20 mA signals proportional to their individual loads. These signals are cabled to the Flow Rate Integrator where a Summing & Conditioning Card sums them (or averages them) and applies calibration coefficients.

Simultaneously, a Speed Sensor Module (an incremental Incremental Encoder mounted on a sprocket or fixed to the conveyor frame) counts belt motion. The encoder generates A/B quadrature pulses: each pulse represents a fixed increment of belt travel (e.g., 1 mm per pulse if the encoder has 1000 PPR and is on a 314 mm diameter pulley). The Integrator CPU Module calculates belt velocity (v = pulses/second × distance/pulse) and updates the flow rate.

The Integrator CPU Module continuously computes: Flow (t/h) = Stable weight (kg) × Current belt speed (m/s) × 3.6 + running total. The result is displayed on the Flow Rate Display as both instantaneous (e.g., 247.3 t/h) and cumulative (e.g., 1543 tonnes since start of shift).

Mechanical Design & Installation

The Weigh Frame Assembly is a rigid welded steel structure (Grade 50 I-beams or box section) that isolates the instrumented idler section from the rest of the conveyor. This isolation is crucial: if the weigh frame were rigidly bolted to the main conveyor frame, vibration and dynamic loads from the drive motor, other idlers, and material impact would corrupt the load cell signals.

Instead, the weigh frame is mounted on elastomeric Base Isolation Pads pads (natural rubber, ~20 Shore A hardness) that absorb vibration at frequencies above 10 Hz while supporting static load. This filtering is essential for accuracy; without isolation, a rumbling 5 metric ton load cell on an unbalanced conveyor system can read ±500 kg as the frame oscillates.

The Frame Beams are typically 1.8 m long and support four Bearing Blocks (ductile iron pillow blocks with self-aligning ball bearings). The bearing blocks hold the idler shafts and allow precise axial and radial alignment.

Two Tensioning Mechanism adjusters (one at each end) allow the belt tension to be dialed in. Correct tension is critical: under-tensioned belt slips and gives false low readings; over-tensioned belt strains the load cells and adds friction. Typical belt tension is 2–5 metric tons of pull per meter of belt width.

Electrical & Control

The Flow Rate Integrator is wall-mounted near the conveyor in a NEMA 4X stainless steel enclosure, IP65 rated. Inside are:

• CPU Module: ARM Cortex-M4 or equivalent, running real-time firmware that reads the Summing & Conditioning Card output (0–10 V) and the Incremental Encoder quadrature signal at kHz rates.
• Display: 7" LCD showing weight and flow rate, updated 1–4 times per second.
• Filtering & Damping: The CPU applies digital signal processing: a Kalman filter or exponential moving average (EMA) smooths the noisy weight signal (reflecting material lumpiness and belt vibration), and the flow rate is typically time-averaged over 5–30 seconds.
• Totalizer Relay: A Totalizer Output Relay can be configured to close a dry contact when accumulated tonnage reaches a preset (e.g., every 100 tonnes, trigger a gate valve to redirect material to a new bin).

Power consumption is modest (100–200 W), so the integrator can run on a standard 230 V AC single-phase outlet. Some systems add Ethernet output for remote monitoring or integration with a plant SCADA system.

Calibration & Accuracy

Belt weighers are calibrated using a two-step procedure:

1. Static Weight Check: Place Test Weights (ASTM Class 1 certified masses: 10, 25, 50 kg) on the stationary conveyor directly over the weigh section. Record the displayed weight and compare to the nominal. Trim the Summing & Conditioning Card gain and offset until the reading matches the known mass at 5–10 points across the load range.

2. Speed Calibration: Rotate the belt at a known speed (measured with a handheld Speed Reference Standard tachometer or reference encoder) and verify that the encoder pulse rate matches the actual belt speed. The Integrator CPU Module has a calibration constant (distance per pulse) that may need adjustment.

Accuracy is typically ±1–2% of full scale; this is limited by belt speed stability (variations in motor speed, slip, and thermal expansion) and by uneven material distribution (a lump on one side of the belt reads heavier than distributed fines). Repeatability is excellent (±0.5%) because the load cells are stable and low-drift.

Standards such as ISO 4411 (conveyor belt scales for industry) define acceptance criteria: linearity within ±2% over the 10–100% load range, hysteresis less than 0.5%, and zero drift less than 0.2 kg/hour.

Maintenance

Load cells are the highest-maintenance components; they have no moving parts but can fail due to:

• Water ingress: Seals wear or cable glands crack, allowing moisture into the strain gauge bridge.
• Mechanical overload: Sustained load above rated capacity stretches the strain gauges permanently.
• Vibration fatigue: Repeated flexing of the load cell stem can cause the weld connecting the sensing element to rupture after 10+ years.

Annual inspection should include:

• Visual check of load cell seals and cable glands.
• Verification of belt alignment (centered on the idlers).
• Cleaning of the weigh frame to remove dust and spillage.
• Re-verification of Base Isolation Pads pad elasticity (should not be hard or cracked).
• Spot-check of calibration using test weights.

The Incremental Encoder is robust but can become misaligned if the conveyor is subjected to lateral shock; monthly visual check of the encoder gap is recommended.

Applications

• Mining & Ore Handling: Measuring ore throughput from excavator to mill; controlling crusher feed rate.
• Quarrying & Aggregates: Monitoring sand/gravel production and blending different size fractions.
• Cement Plants: Weighing raw meal, coal, and clinker on kiln feed conveyors.
• Grain & Flour Mills: Measuring grain intake and flour production flow.
• Recycling: Sorting and weighing commingled waste streams.
• Power Plants: Monitoring coal feed and ash removal rates.
• Chemical & Fertilizer Production: Controlling continuous reaction feedstock.

Related Reading: Weigh Frame Assembly, Load Cell Idlers, Speed Sensor Module, Flow Rate Integrator.