Introduction
Belt scales are an integral part of many production plants for determining flow rates and conveying quantities and are at home wherever bulk materials are conveyed.
In addition to requirements such as reliable measurement and low maintenance, the accuracy of the scales is of central importance.
What does the accuracy of a belt weigher have to do with a sports car? A sports car is - in most cases - a fast and agile vehicle.
However, it makes a significant difference whether it is driven on a highway, a gravel road or even "off-road". In poor ground conditions or bad weather, it will not be able to deliver its full performance.
The situation is very similar with belt weighers. These are also heavily dependent on the conditions under which they operate.
Unlike road conditions or the weather, the boundary conditions for weighbridges can be influenced, sometimes with little effort.
The nine most important measures that can be tested and adapted on existing belt scales to improve the accuracy of the measurement are ...
Alignment of the roller chairs
The alignment of the roller chairs is of central importance. At least two roller chairs - one before and one after the belt weigher - must be aligned with the roller chairs on the weigher.
In practice, it has been shown that with troughed belts, the horizontal (middle) rollers in the roller frame in particular must be aligned.
In the case of calibratable belt scales of class 1 (maximum inaccuracy +/-1.0 %), for example, the roller chairs should be within a tolerance of +/-0.5 mm (starting from the top of the horizontal roller).
The outside of the lateral rollers may differ by a maximum of +/-1 mm. Uniform leveling of the roller chairs reduces the influence of fluctuating belt tension, among other things.
In general, the force transmission into the belt scale is better overall (more even).
Avoiding misalignment
Belt vibrations or misalignment lead to strong zero point fluctuations. Eliminating the cause helps to achieve much more reproducible measurement results. A misalignment control can also help here.
However, the lateral guide rollers must not be installed in the area of the scales.
Misaligned, wandering and vibrating belts lead to an uneven application of force to the belt scale, resulting in different measurement results with the same conveying capacity.
Removing the side wipers
Side scrapers and guide rollers must be removed in the area of the scale (i.e. at least one roller frame before and after the scale).
Tip: To prevent the material from being pushed off the belt as soon as the side guides are used again after the scale, the conveyor flow can be constricted a little more tightly in front of the scale.
Side scrapers that press on the conveyor belt in the area of the belt weigher distort the measurement result.
The side scrapers represent a force shunt. They can load the scale unevenly due to the wedge effect, even if no material is being transported, thus suggesting a - non-constant - flow rate.
2,000 tons per hour run over this built-in twin-roller belt weigher.
Focus on the conveyor belt
The conveyor belt should be in good condition, i.e. without patches or holes. An old belt that is no longer intact affects the zero point and can result in an unstable measurement.
One of the most effective and sometimes easiest ways to improve accuracy at low conveying capacities is to reduce the conveying speed (at low conveying capacities below 30 t/h).
A lower speed causes the material to compress more on the conveyor belt and a greater product weight is placed on the scales. This reduces the relative influence of errors and increases measuring accuracy.
Zeroing the belt scale
The belt scale should be zeroed at regular intervals. Depending on the conditions, this can be done before the start of a shift or before loading, for example.
Before zeroing the scale, the belt should have already completed several belt cycles so that the belt temperature is stable. Zeroing the scale should take at least one belt cycle.
Tip: With modern evaluation electronics, this can be done automatically by the electronics, manually or specifically by a connected control unit.
A shift in the zero point due to caking or a change in belt tension (due to ageing of the belt, sunlight or after the belt has started up) leads to an offset of the flow measurement.
This error sometimes leads to considerable measurement inaccuracies, which can, however, be easily eliminated by regular zeroing/taring. The measuring error of a belt scale can be checked relatively easily.
After "correct zeroing", the belt is allowed to continue running without a load and the fluctuations in the zero point are observed. It is important that the so-called "zero point window" of the evaluation electronics is switched off.
The zero point window suppresses the fluctuations of the belt and thus suggests a stable zero point. This window is usually set to one to two percent of the measuring range.
If the zero point window is too large, it "swallows up" part of the conveying capacity - especially if the conveying capacity fluctuates greatly.
Uniform belt tension
The belt tension should be as constant as possible and not too strong. Lifting of the belt in the area of the scale should always be avoided. Gravimetric tensioning stations are ideal and therefore mandatory for legal-for-trade scales.
Spring-loaded tensioning stations can also generate a relatively even belt tension. Changing belt tension has a major influence on measuring accuracy, especially with non-aligned roller chairs.
Due to the changing tension, the belt can load or unload the "weighing roller chair(s)".
Gravel is weighed in this twin-roller conveyor belt scale.
Functional idlers
The rollers in the area of the scales must not be stationary but must run smoothly. Rollers that have too much run-out (guide value: concentricity tolerance <2 mm) distort the measurement result and should be replaced.
Rollers that do not run true can cause pulsating forces that affect the measuring accuracy. Stationary rollers can cause horizontal forces, which - depending on the design of the belt scale - can have a strong influence on it.
A clean, please!
The mechanics of the scales should be checked for force shunts and these should be eliminated. The same applies to the speed measurement (if available).
Check that the measuring wheel runs smoothly on the lower belt and does not lift off. Force shunts can be caused by heavy soiling or foreign objects that have become lodged between the weighing frame and the base frame.
As with static scales, this means that the weight cannot be measured accurately.
Caking, which is only present on the weighing frame but does not represent a force shunt, has no influence and can be compensated for by regular zeroing of the scales.
The belt scale can also be installed in loading conveyors for trucks.
Setting the electronics
The settings in the electronics must be checked for plausibility and recorded if possible. Important parameters are, for example, the incoming load cell signal, the amplification factor, the length of the weighing path, etc.
A specialist company or the manufacturer of the electronics can help here with a corresponding calibration certificate. Incorrect settings in the belt scale electronics or a lack of calibration can sometimes lead to very large measurement inaccuracies.
If conveyor belts are being planned, attention can be paid in advance to optimum installation conditions for a belt scale.
Further points here are, for example
Installation location (e.g. not in the area of angular changes)
Belt troughing as low as possible (maximum 30°, troughing over three - not two - rollers)
Gravimetric tensioning station for constant belt tension
In difficult cases, it is advisable to have a specialist company carry out an assessment. In general, the boundary conditions are decisive as to whether a belt scale provides good measurement results.
It makes sense to design the effort for an accurate measurement in such a way that the required measuring accuracy is actually achieved.
A belt scale that collects process data does not necessarily have to meet the requirements of a legal-for-trade class 0.5 belt scale.
