Belt scales are used to measure the weight carried on a belt conveyor and are used extensively in the mining, quarrying and materials handling industries.
A typical belt scale consists of one or more roller sections mounted directly onto one or more load cells or a flexure with a single load cell. This is then mounted into the belt system in place of existing rollers. Belt speed is typically measured by means of a pulse generator combined with a friction wheel attached to the belt.
Multiplying belt loading by belt speed gives flow rate and then by integration within the control electronics, the total weight of material conveyed in a given time can be calculated.Flow rate = m/s x s/t
where m/s = weight of material per unit length of belt and s/t = belt displacement per unit time
The accuracy of belt scales depends on:
By controlling the belt speed and belt loading, a belt conveyor scale effectively becomes a weighfeeder which can be used to provide a continuous gravimetric flow of material. Unlike conventional belt scales, weighfeeders can be designed to optimize performance by controlling factors such as belt uniformity, alignment, tension and cleaning. The layout of belt feeders is determined by the required flow rate and the characteristics of the material being conveyed. The infeed devices to weighfeeders are usually selected to maintain a smooth flow of material even under varying conditions. Where necessary devices such as vibrating hoppers, aeration devices and discharging belts are used. Controllers for weighfeeders can provide information on flow rate, conveyed quantity, belt loading, test values and faults.
Weighfeeders are used in blending plants, for mixing and batching and in feed systems for furnaces such as in the cement industry to ensure the correct combustible mixture. In a typical blending plant such as that used in the fertilizer industry, the required quantities of raw materials can be continually fed from storage hoppers by weighfeeders to the mixing drum. Feed back systems can be used to vary and balance throughput to meet varying demands.