18 May 2018

How important is belt scale calibration?

After a belt scale has been installed and calibrated in an application, as with any other piece of equipment, you have to consider what type of periodic maintenance should be performed. Routine calibration is one of those considerations. There can be many reasons why recalibration should be performed.

It can be as simple as:

To understand the importance of proper belt scale calibration, consider what proper belt scale calibration should do. Proper belt scale calibration should:

Let’s look at each of these items individually.

Early identification of a scale error

Changes in a conveyor belt system can occur slowly over time. These changes can cause a small error in the scale. As the changes in the conveyor accumulate over time, so do the errors in the scale. In many industrial environments, belt scale calibration is required when quality control reports a problem. At that point, the scale has already developed enough error to cause product quality issues. Regular periodic maintenance and calibration of the equipment assures that it operates within acceptable tolerances.

Scale and conveyor inspection

Conveyors are designed for continuous operation over long periods of time. For this reason, conveyors often receive little maintenance unless a problem develops that threatens to prevent the conveyor’s operation. When a belt scale is installed in a conveyor, it becomes an important part of the weighing system and should be inspected periodically for changes that may affect the scale’s accuracy. For example, an idler that is not aligned properly may have very little effect on the operation of the conveyor; however, if the idler is in the area of the scale, it could cause errors in the scale. When a periodic calibration is performed on a conveyor belt scale, an inspection of the scale and conveyor should be done to verify that changes that have occurred on the conveyor, will not affect the performance of the scale.

Test of the scales repeatability

When regular scale calibrations are performed, the scale should be tested for repeatability. The scale’s repeatability can easily be tested when doing a routine calibration by performing two consecutive calibrations in a row without making any changes to the scale or conveyor. If the error repeats, a correction can be made to the scale’s calibration. Consider a scale that has the desired accuracy of ± 0.50%. If a single calibration is performed and the scale is adjusted for that error but the error is not repeatable, the scale could be inaccurate.

For example, let’s say a scale indicates an error of 1.25% after the first calibration. To ensure this error is repeatable, a second calibration is performed without any changes to the conveyor or scale and this calibration shows an error of 0.10% (a difference of 1.15%); there is no adjustment that can be made to the scale to achieve the desired ±0.50%. In this case, a thorough inspection of the scale and conveyor should be done to determine the source of the error. However, if the second test shows an error of 0.95% (a difference of 0.30%), the scale’s calibration can be adjusted by 1.10%. This would leave the scale with an error of ±0.15%, well within the desired accuracy of ±0. 50%.

Adjust the scales calibration

Repeatability tests and an inspection of the scale and conveyor should indicate any issues that may be developing that will impact the scale’s accuracy. After preforming these checks, it may be found that the scale’s calibration is still within the acceptable limits. In these cases, no adjustment to the scale’s calibration is required. It is important to keep in mind that sometimes changes in the conveyor, e.g. conveyor maintenance or weather changes can cause errors in the belt scale. After it is determined that the scale is performing correctly, the calibration of the scale can be adjusted to keep the scale reading accurately.

Importance of calibration

To understand the importance of calibration, let’s think about the costs associated with an error in a belt scale since it can vary depending on the application.

Direct loss of finished product

If errors develop in a belt scale used for billing, it could result in the direct loss of material. For example, consider a scale that is loading a railcar with 95 tons of material; if the loading scale has a 1% error, the railcar could have as much as 95.95 tons. That may not seem significant, but on a 50 car train that could be as much as 47 tons. Now consider 50 cars per week over a year. That would be 47 tons over X 52 weeks = 2444 tons per year from an error of only 1%.

Excessive equipment wear and increased energy costs

A scale out of calibration can produce excessive wear on equipment in a couple of ways. If the scale is used to control the feed rate into a processing machine, the machines are usually designed to be fed at a specific rate. If material is fed into the machine below the designated rate, the machine will need to run longer to process the same amount of material. On the other hand, if the material is fed into the machine too fast, the material may not be properly processed so the material has to be transferred through the machine a second time. In either case, the machine will need to run longer to produce the same amount of product; thus, increasing wear on the machine, labor and energy costs.

Excessive use of raw material additives

In applications where the scale is used to measure multiple components in a blending application, the material is blended in the correct proportions based on the output of the scale. In these applications, if an error develops, it can result in an increased use of raw materials. Additives typically cost more than the base product. If the scales are not reading correctly, more additive may be added than is needed, which in turn increases the production cost. In cases where the error has grown large enough, it may produce material that is out of specification, increasing the amount of scrap product.

Conclusion

Properly calibrated and maintained belt scales are capable of reducing production costs in a variety of ways. However, if the scales are not properly calibrated and maintained, the investment in the scale is only achieving a portion of the savings it was intended.

What periodic maintenance program do you have for your belt scales?

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