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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There are also conveyers that may be brand new, but they can be even more problematic than older, poor quality ones. This is because their design may not be suited to housing a belt scale. If they are equipped with flat or troughed idlers, they should work well. But v-roll, catenary and wire rope idlers will simply not be compatible. In the case of the wire ropes, even though the idlers may be rigid, as soon as the conveyer begins to move, all of the measuring equipment will also move due to the inherent rope design (Fig. 4).

Fig. 4: These positions are ideal.                  
         *Acceptable in some applications.

The type of belt used is also a factor. Some customers choose belts that are over engineered for their needs, most typically a belt that is too thick or with a core of steel. In these cases, the belt might not flex enough to give the sag needed to establish contact with the scale (Fig. 5)

Fig. 5: Proper belt flexure is required for belt scale accuracy.                   

Problem #6: Thinking you don’t need a speed sensor

In a dynamic weighing environment, speed monitoring is just as important as load sensing. It demands the same care and attention, and utilizing a speed sensor gives you the peace of mind you need to ensure that part of the weighing equation is covered (Fig. 6).

Fig. 6: As material enters the weigh span, the load is measured by the deflection of the load                 
cells while the belt speed is monitored using a speed sensor.                                               

It’s common to hear some operating companies state that their conveyers run at a constant speed, so there’s no need for a speed sensor. Unfortunately, no conveyer actually runs at one uniform speed. There is always some variation. For example, at a rock quarry, a site manager might have a conveyer running at 100 feet per minute. The assumption is that it’s always running at that speed. But when the conveyer is tested, it generally is off a little, running at a speed, for example, closer to 102 feet per minute. That two percent difference directly translates into an accuracy that is also off by two percent. 

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