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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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Problem #7: Belt speed too fast and/or material not on load scale for long enough

Materials can begin to “bounce” off the conveyer if it’s moving too quickly. If measuring takes place during the period of time when material is in the air, that weight will not be measured. In addition, when the material lands, it will create turbulence, which can also affect the accuracy at the time that the measurement is made.

Belt speeds greater than six meters per second (1180 ft per minute) can create inaccuracy of one to five percent, which can be an issue if high accuracy is required.

Consistent material profiling can help mitigate the impact of this issue if the bouncing effect is repeatable. However, other factors would also need to be kept consistent at the same time, including moisture content, bulk density and particle size.

Similarly, whether it’s from bouncing or another reason, it’s important that the material being weighed is on the scale itself for a sufficiently long period of time so that its weight can be accurately recorded. Load cells are essentially springs. There needs to be adequate “retention time” for changes in forces to settle on that spring, and the weight to be captured and transferred, before the material is moved from the scale area. This multi-step process cannot take place with the appropriate precision if the material moves too quickly over scale.

As a general rule, the specified product data can be achieved if the retention time is greater than 0.4 seconds. If it is 0.25 to 0.4 seconds, there can be a 0.5 to two percent impact on accuracy and repeatability. The impact can be between one and five percent if the retention time is less than 0.25 seconds.

Problem #8: Dynamic frequency ratio causing resonance effects

In 1940, one of the most famous engineering failures in history was captured on film when the Tacoma Narrows Bridge collapsed, twisting itself and ripping apart spectacularly because of a resonance effect where its natural frequency was excited in heavy wind conditions.

For a belt scale, a similar effect occurs with the device’s “dynamic frequency.” Scale oscillation creates a resonance effect, and this must be taken into consideration. In the weighing system, there is a natural frequency associated with the force that the belt exerts on the weigh idler, as everything is supported on two load sensors. Loading, spacing between idlers and the speed of the conveyer can potentially work together to induce enough oscillation to match the natural frequency of the load cells. If this happens, the whole arrangement could become unstable. At that point, it becomes not just a matter of reliability but safety as well.

To ensure this does not occur, you need belt scale experts to perform some complex mathematical analysis and determine your scale’s “dynamic frequency ratio.” The idler, belt loading and belt speed are on one side of the ratio, while load cell detection is on the other side.

If this ratio is one, the load cells will fail, just like the bridge collapsing. In fact, any value above 0.3 could represent a problem and impact accuracy by up to 10 percent. When the Siemens weighing team encounters anything above 0.3 they will engineer a solution to reduce the number down to 0.2 or lower, where the impact is minimal.

Problem #9: Load cell signal too low

If we use our cell phone without any interference, we tend to enjoy great signal quality, and the little icon on our device will verify this with five bars in the top corner of the screen. If there is lots of interference, the signal can be really weak – with just one bar on the signal strength indicator – and we might strain to have a decent conversation.

There’s an analogous situation for belt scales involving the signal from their load cell to the integrator. Each load cell needs a clear “resolution” in which detailed weighing information about the material being conveyed is captured, while any non-material weight (such as the weight of the belt scale itself or the idler) is able to be dismissed as being irrelevant. Consider an aluminum can recycling operation. Each can is very light so that the weighing system must have a high resolution, because an influence such as a belt tension change could have an impact greater than the actual weight of some cans.

The guideline is to use at least 30 percent of the entire load cell capacity to prevent other influencing factors from having a disproportionately large impact on the scale’s performance. Accordingly, a live load signal indicates the output range that the load cell can generate based on material loading. If that signal is less than 2 mV – reflecting a low resolution – there could be a one to five percent change on accuracy and repeatability. If it’s 2-3 mV, the impact can be 0.5 to two percent. Product resolution specifications can be attained at greater than 3 mV (using a 2mV/V-style load cell).

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