Unproper Belt Scale Results?

Belt Weighing

Unproper Belt Scale Results?

10 reasons, why your perfect belt scale might under-perform
Belt weighing is common practice in lots of bulk solid handling operations. Although it’s a long established technology, there are still bits and pieces regarding installation and maintenance which might lead to erroneous results. Following you can find the reasons why.
(ed. wgeisler - 31/3/2017)
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Determining the Impact: Every Situation is different

You also need to remember there are limits to how precise any one impact can be quantified. Every application is different – and will yield different results.

And most importantly, because a belt scale is part of a larger dynamic application involving lots of moving parts, several of the problems often occur at the same time. This makes estimating an exact impact extremely difficult. So in addressing the 10 problems outlined in this article, note that each estimated impact is based on an individual issue being looked at in isolation. In reality, it’s likely that multiple issues are involved.

In addition, all the estimates provided are not based on empirical data. They come from the real-world experience of engineers and technicians of the author’s company. Even though that experience is extensive, any numbers provided should be viewed only as an approximate guide.

Problem #1: Inaccurate Input when setting up means inaccurate Output when measuring starts

There’s an old adage in computer science – garbage in, garbage out. It means the quality of an output is determined by the quality of an input. The integrator is your belt scale’s computer. If the information entered into it is not accurate, every measurement it produces will not be accurate.

As a result, careful attention must be paid to critical parameters that are keyed into the integrator before any measurement takes place. This includes the spacing between idlers, belt length, the speed sensor’s pulley diameter and the mass of the weight being used for calibration. For example, if the space between idlers is off by five percent when that data point is entered into the integrator, accuracy will also be off by five percent. Or if the pulley diameter is off by five percent, then the accuracy will also be off by the same amount. The correlation is direct no matter what the parameter.

Problem #2: Idlers placed too high or too low – the most common and significant source of reduced accuracy

The single biggest problem with scale accuracy, repeatability and linearity is determining where the idler for the weighing device is to be located. It cannot be too high above the conveyer belt or too low beneath it. There is an optimum height adjustment for every conveyor belt. And it can involve a painstaking process to get that alignment just right, because if it is off from the ideal by as little as half a millimeter, it can result in a sizable negative impact.

On the one hand, if a weigh idler is placed too low relative to the other idlers, a tight conveyer belt could “bridge” over the scale. Simply stated, it could pass over the scale completely undetected. The weight of the material on the conveyer at one point in time might not push it down far enough to even obtain a recording. More material might be added later, which will then push the belt down so that it comes into contact with the scale, but the results will be inconsistent, with the weight being measured only some of the time.

On the other hand, if the weigh idler is too high, the opposite effect will take place. It can significantly increase belt tension, much like a rubber band being dramatically different when stretched compared to not stretched. As a result, a sizeable downward gravitational force could be exerted on the scale that exaggerates the weight, making it inaccurate on the heavy side.

Belt scales from the author’s company are designed to be accurate within plus or minus 0.5 percent, with some very high accuracy systems guaranteed to have an accuracy of 0.25 percent. For those levels of accuracy to be obtained, weigh idlers for the regular scales must be within 0.8 millimeters of the optimum location, and within 0.4 millimeters for high-accuracy devices. An alignment out by as little as 1.5 millimeters can throw accuracy and repeatability off by as much as three percent. If it’s off by three millimeters, the impact could be as high as 10 percent. And if it’s six millimeters, it’s possible for inaccuracy to be as extreme as 30 percent.

To put that into financial perspective, consider the case of a gravel producer operating a production facility with a capacity of 200 tons per hour. If the gravel costs $80 per ton, an inaccuracy of just 0.25 percent would cost that company $83 200 in lost material over the course of a year.

Problem #3: Belt tension too high or too low – a close second as a source of reduced accuracy

Right behind idler alignment – and closely related to it – is belt tension. If the belt tension is too high, bridging over the scale, as described in Problem #2, can become an issue. If the tension is too low, the material will shift as it rides over the “hump” of an idler. This can cause repeatability errors and material spillage.

Even if the tension is set up correctly at the outset, the ongoing operation of a conveyer can lead to tightening or loosening through a range of factors – including temperature fluctuations, ineffective gravity tensioners, changes to the nature of the material being fed onto the conveyor, inclines that may vary along the belt path and erratic belt tracking.

There needs to be an ideal belt tension, resulting in a “sag” of two percent between the idlers. The further away from two percent, the more inaccurate the weighing and the greater the number of non-repeatable results. Belt sag of less than one percent can alter accuracy and repeatability by as much as 10 percent. Belt sag of more than 2.5 percent has an impact of up to two percent. If the sag is greater than five percent, then there could be major issues that must be addressed.

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