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 #4: Scale not installed in the proper location – especially if it’s too close to inclines or a vibration source

Choosing a sub-optimal location for the belt scale represents another leading source of reduced accuracy.

For example, both convex and concave style conveyers can present significant challenges (Fig. 3). The belt scale should be placed at least 12 meters before or 12 meters after the tangent point of a curve. Consider an agricultural facility where apples are moved on a conveyer belt. If the scale is too close to the incline, some apples could pass over the scale, and then roll back down the incline and be measured again, making the reading inaccurate. The impact on accuracy and repeatability in this circumstance could be as great as 30 percent.

Fig. 3: Conveyor design and scale location all influence how a belt scale system performs.                                                   

A variable-incline conveyer, going from flat up to an inclination of 16° will negatively impact accuracy by four percent. In general, inclines of just 20° can create roll back, and if the incline is 40° or more, the impact on accuracy can be so significant that it cannot even be predicted.

To address conveyer inclines and prevent roll back, some belts have integrated ridges to provide additional material traction. But those parts of the belt with these ridges will not behave in the same manner as the rest of the belt (in particular, not enabling a proper “trough” position that is needed for measuring), which can also limit accuracy.

Vibration is another key issue to consider when selecting where to locate the scale. For instance, in the aggregate business, mobile crushers that can generate a lot of vibration are used. If the proper scale design is not used, they can cause the load cells to move up and down, creating erroneous outputs of up to two percent. Generally, there is no need for concern if vibration is not actually visible. However, if it is visible, accuracy and repeatability can be affected. In fact, in rare, extreme cases of vibration, a scale could fail. Mounting a scale close to a support structure can be a good way of combating the effects of vibration.

Other installation locations to avoid include those that are:

  • too near transition idlers that change the profile of the belt, leading to a change of as much as 15 percent in accuracy and repeatability;
  • under skirt boards or seal strips where material can “pinch” into the belt at the scale idler, which might mean a change of as high as 10 percent; 
  • near a winged pulley, as the pulley can wrap around the conveyer belt, “whipping” it off the scale, which can have an impact as high as 10 percent; and 
  • close to metal detectors, which can negatively influence accuracy by up to five percent.

To avoid these problems, installation guidelines should be carefully followed.

Problem #5: Poor conveyer quality or incompatible conveyer design

The quality of conveyers and their components can significantly influence accuracy.

A frame that is improperly designed or has deteriorated over time may not adequately support the weight of the material, thus causing the conveyer to twist. This could negatively impact accuracy and repeatability by up to 20 percent. Another issue is that idlers could be worn out or do not rotate smoothly. These effects can create friction, leading to vibration, which in turn can mean measurement inaccuracies of up to 10 percent. Faulty tensioners are another common source of concern, because they are incapable of serving their intended purpose of compensating for loads on the conveyor belt or temperature changes. And of course, the belt itself is also a key factor. It could have poor splicing, holes or tears, resulting in lost material and therefore inaccurate measurements.

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