Reflection on The Application of Belt Scales

0. Overview

In each factory, there are more or less various types of instruments, such as temperature, pressure, flow, level, component analysis, weighing and so on. In addition to the difficulty in the application of level and component analysis instruments, weighing instruments may be the most difficult instrument category to apply.In the application of weighing instrument there are often more problems is the belt scale.If according to the instrument operation rate, good and good rate of operation and other indicators for statistics, the last ranking may be belt scale.The reasons for the unsatisfactory running state of belt scale are as follows.Because the belt scale belongs to the dynamic weighing instrument, this is undoubtedly one of the main reasons, the belt conveyor while conveying material weighing, the interference brought by the belt is inevitable, such as the belt tension change, thickness change, deviation and so on will affect the weighing process;The other reason is brought by the material itself, the material flow is uneven, the material block is too large, the change of the material section shape on the belt section will affect the weighing process;The third reason is that the belt scale installation position selection is improper and the installation does not meet the requirements, such as the belt conveyor is not allowed to install the belt scale or the improper position is installed on the belt scale;Of course, belt scale manufacturers of poor product quality, improper selection of belt scale is also one of the important reasons.In view of many belt scales in the application of the problems, the author according to many years of experience to answer some of the problems.

1.Selection of belt scales

To choose the most suitable electronic belt scale for application, the following factors need to be considered: the intended use of belt scale, the accuracy of belt scale requirements, the requirements of each component of the belt scale.The intended use of electronic belt scales can be roughly divided into process monitoring scales, batching scales, measuring scales and trade settlement scales.The process monitoring scale is mainly used for the monitoring of the process parameters, and it does not require high measurement accuracy.The batching scale is mainly used for quantitative feeding or proportioning feeding, the accuracy of the material and the measurement accuracy have certain requirements;The measurement scale is used for the measurement of important raw materials or the internal economic accounting of enterprises, which requires higher accuracy of measurement.Trade settlement scale is used to measure the materials exchanged between enterprises, which requires the highest accuracy of measurement.Thus it can be seen that the intended use of belt scales has generally determined the accuracy of their requirements, but before the selection, the exact accuracy requirements still need to be put forward.Belt scale is mainly composed of scale frame, sensor and accumulator.

The electronic belt scale used for process monitoring can be used for horizontal bar steelyard and single roller directly bearing suspension scale.For the batching of electronic belt scale, can choose horizontal bar steelyard, single roller directly bearing suspension scale, cantilever scale or whole machine scale.If the site has a ready-made belt conveyor, and the belt length is greater than 5m, generally optional horizontal bar steelyard or single roller directly bearing suspension scale;If it is a newly designed batching system, the center distance between the feed port and the discharge port is 2 ~ 5 m, the whole machine scale is optional;If the field space is small, the center distance between the feed port and the discharge port is 0.6 ~ 2.0m, the cantilever scale is optional.For measuring the electronic belt scale, you can choose multi-roller suspension scale, parallel bar steelyard or designed to improve the accuracy of weighing cantilever scale, the whole machine scale.For trade settlement of the electronic belt scale, you can choose the roller set more roller suspension scale, parallel bar steelyard.In terms of the structure form and the number of weighing rollers, the accuracy of the single lever scale frame is lower than that of the double lever scale frame, and the accuracy of the scale frame with fewer weighing rollers is lower than that with more weighing rollers.In terms of test interval periods, the accuracy of long test interval periods is lower.After the scale frame structure form is determined, the specific manufacturer and product model can be determined according to the technical data of various electronic belt scales.When choosing, we should comprehensively consider the performance, accuracy, shape size, accumulator function, price and other factors of various types of electronic belt scales.For example, the belt scale accumulator used in the batching regulation system, its output signal (such as 0 ~ 10 mA, 4 ~ 20 mA, 1 ~ 5 V) should meet the system selection of the regulator or motor controller and other equipment on the input signal requirements;Similar to the accumulator for loading and loading weighing scales, when the accumulator reaches the set value, the contact signal is automatically given to stop the feeding equipment;Some of the latest designs require the accumulator to have a fieldbus, such as a PROFIBUS-DP bus, in order to transmit more process information to the upbit control system.The condition of the belt conveyor and the verification and test conditions of the belt scale should also be taken into account when selecting the belt scale.The installation of belt scales has a great relationship with the condition of the belt conveyor. If the condition of the belt conveyor is not good, it is not accurate measurement at all. Therefore, the condition of the belt conveyor should be fully understood before the design.The verification and test conditions of belt scales are the key to ensure the normal use of electronic belt scales. The verification and test contents should be determined in advance so as to provide necessary facilities for the verification and test work.

2.Key technologies of belt scale application

A world famous brand electronic belt scale manufacturer said that to use the belt scale, the key technology "depends to a large extent on the installation environment, installation location and installation quality".For an electronic belt scale with higher accuracy requirements, the installation environment, installation position and installation quality account for approximately 80%, while the quality of the product itself accounts for only 20%.In terms of the installation environment, refers to the belt conveyor belt, roller, auxiliary equipment and material conditions to meet the external conditions of the belt scale weighing;In terms of the installation position, usually includes two aspects: the selection of the installation of electronic belt scale belt conveyor and the selection of the belt conveyor to determine the installation position;The installation quality mainly refers to the fine degree of installation, which focuses on the alignment calibration of the scale frame and the calibration of the roller spacing.The above points can be called the field installation technology of belt scales. From the perspective of the author's experience and the actual operation of the field, these are the key technologies of the application of belt scales.

2.1 Installation Environment

Installation environment requirements of the belt conveyor belt itself uniform quality, thickness and unit length weight deviation is small, the roller is a pin-roller or groove roller (groove Angle is not greater than 35°), and flexible operation, eccentricity is small, the belt is equipped with automatic tensification device, the belt is not installed on the unloading plough, unloading trolley and other unloading equipment.This installation environment can meet the external conditions of the belt scale weighing requirements, to ensure that the belt scale can reach a certain weighing accuracy when used.In the actual production site, the purpose of using the belt conveyor is to transport materials, the overall design of the conveyor is also to ensure that the material transportation is given priority to, rarely take into account whether it is suitable for the installation of belt scales, so the installation environment is often not ideal, sometimes even can be said that the installation environment is very poor.The previous practice is not installed belt scale, give up measurement;Or forced installation, cannot guarantee the accuracy of measurement.The current approach is to take a positive attitude, if the site installation conditions are not ideal, technicians should try to improve the installation conditions.If the original feeding uniformity of a factory is poor, the material quantity is large or small, sometimes without material, sometimes beyond the range, they try to change the diameter of the blanking mouth, control the material flow, so that the material flow is as uniform as possible;If a factory belt use time is longer, the number of belt cementing sections, the quality of the cementing is not uniform, in the overhaul will be replaced with a new belt;If the roller eccentricity on the belt conveyor is large and the operation is not flexible, you can use the roller provided by the belt scale manufacturer as the weighing roller, and replace the roller that does not meet the requirements originally used on the scale frame. Some manufacturers will even replace all the rollers with several groups adjacent to the scale frame (for example, before and after the scale frame, each 2 ~ 5 groups).Thus, a relatively ideal local installation environment is created to meet the needs of high precision measurement under poor installation environment conditions.

2.2 Installation location

When through several belt conveyor transport transport of the same material, usually according to the basic conditions of the belt conveyor for comprehensive comparison, and then choose the most suitable belt conveyor belt scale installation.Select the basic conditions of the belt conveyor mainly include the following: feeding uniformity, belt speed, belt conveyor length, belt conveyor section shape, belt conveyor inclination, roller groove Angle, straight line length, feeding point, balance frame support status.

[1] The principle of determining the installation position on the selected belt conveyor is as follows: select the place where the belt tension value is small and the belt tension change value is small;The material shall be stabilized on the belt before entering the weighing length, and shall not slide;Installed in the straight line section of the belt conveyor part;Away from the discharge plough;There should be a certain distance from the unloading point.[2]

2.3 Installation quality

The installation quality will focus on the calibration of the alignment of the scale frame and the calibration of the roller spacing.The alignment calibration of the scale frame refers to: in the installation process of the belt scale frame, all the weighing rollers on the scale frame and the corresponding parts of the adjacent 2 ~ 3 groups of rollers on the scale frame should be calibrated into a straight line.Assuming that the collimation of the weighing roller on the scale is not well calibrated with its adjacent roller, and there is A height difference of h, then at the inflection point A and B, the vertical component V of the tension S acts on the weighing roller, and the vertical downward component V can be obtained through the force decomposition.

The additional force V interferes with the measurement of material gravity. Therefore, the collimation calibration should be carried out during the installation of the scale frame to eliminate or reduce the influence of the additional force V.Of course, several groups of weighing rollers on the scale frame should also be calibrated well, otherwise the force of several groups of weighing rollers is uneven, and the influence of additional force V is greater.The specific requirements of collimation calibration are as follows: there is no height difference between the weighing roller and its adjacent rollers on the scale frame, and it is required that the height of 2 ~ 3 groups of adjacent rollers (this area can be called the weighing influence area) is equal;The height of the roller in the weighing affected area is higher than that outside the weighing affected area, and the height difference can be selected within the range of 3 ~ 5 mm;Allowable error of collimation for high accuracy belt scales can be required to 0.5 mm, usually 1 mm.In the process of adjustment, the error value of the weighing roller should be slightly positive, that is, slightly higher than that of the adjacent rollers (such as 0.5mm)[3].Roller spacing calibration refers to: if possible, try to adjust the spacing of all the rollers on the scale and 2 ~ 3 groups of adjacent rollers (i.e. the weighing affected area) in the same way, and the adjusted spacing error of the rollers should be less than 1 mm.The advantage of roller spacing calibration is to make the force of all rollers on the scale and 2 ~ 3 groups of adjacent rollers uniform and consistent. At the same time, it is convenient to carry out simulation calibration, such as rolling chain calibration, weight calibration and electrical signal calibration, because the theoretical value calculation of these simulation calibration methods is closely related to the roller spacing.Do a good job of the above basic work, for the field application of belt scales to create good conditions, many examples show that most of the production site use is not very good belt scales are in these key links out of error.Such as a field belt scale stability has been very poor, zero and full value frequently change, to the site after inspection found that the scale frame is installed in the belt conveyor section shape is only 1 m from the end of the curve section, the scale frame moved to 6 m before the beginning of the curve section, zero and full value is basically stable;Several 10 belt scales produced by a factory were not installed according to the collimation calibration requirements due to the time limit, and the verification results were not up to the requirements. After recalibrating the scale frame according to the collimation calibration requirements, the verification results basically met the requirements.It is a great pity thatI saw in many production site are randomly determine scale installation location, not to scale quasi laymen calibration (such as weighing roller intentionally raise to higher than the location of the adjacent rollers 30 ~ 50 mm), without supporting roller spacing calibration such as some common problems, of which a considerable part of belt scale is manufacturer to send someone to the on-site installation.Therefore, it is very important to publicize the installation points of electronic belt scales and standardize the on-site installation technology.

3. Selection of belt scale manufacturers

The above mentioned belt scale product quality only accounts for 20%, this sentence is a premise, that is, the "installation position and installation quality" included by the user to complete their own work.If the work included in "installation position and installation quality" is completed by the manufacturer, and the use effect of belt scales is still not ideal, then more than half of the responsibility should be borne by the manufacturer.At present, the reality is indeed such, many manufacturers on-site service personnel have some do not know the belt scales "installation position and installation quality" involved in the problem and how to do, some are know how to do and did not seriously do.There are currently hundreds of manufacturers of belt scales in the country, and it is clear from the product samples that most of them are modeled after the 14, 17, 20, 30 scale shelves of the Ramsay Group, THER moFisherScientific.Do not own things, even the product photos are also used by others, this kind of manufacturers just rely on low prices to attract customers, such manufacturers must not choose.Should choose a larger scale, higher visibility, longer history, have their own unique product manufacturers.It is very important not to choose a vendor that does not have engineering capabilities.Engineering ability refers to the ability of the manufacturer's technical personnel to answer all kinds of questions about the belt scale raised by users, to correctly install or guide the installation of the scale frame, and to carry out the verification and test of the belt scale.Another meaning of engineering capacity is that the manufacturer provides the user with a complete product instruction manual, which should include detailed installation procedures of belt scales, steps of scale rack installation and adjustment, verification and test methods.The importance and seriousness of the manufacturer to the on-site installation and commissioning is also a factor that cannot be ignored.The author participated in the on-site installation of belt scales by Thermos FisherScientific Group Ramsay, and a box of gaskets of various thicknesses was brought to the site along with the scale bracket. These gaskets could be supported under the roller bracket during site installation to facilitate alignment of the scale bracket.Some manufacturers make gaskets on site, which is a waste of time, processing and not standard, resulting in the thickness adjustment of the appropriate, resulting in the quality of the alignment calibration of the scale frame is difficult to ensure.

4. Accuracy of belt scale

According to the "continuous accumulative automatic weighing device (belt balance) national metrological verification regulations", the belt balance precision grade is divided into three levels, namely 0.5, 1.0, 2.0.The actual accuracy of the vast majority of users' belt scales is within this range, but this does not mean that the belt scales can only achieve the accuracy of 0.5 level. In the belt conveyor designed for the purpose of measurement, the special structure of the high precision scale frame, the careful installation and debugging process, the perfection of the calibration means,The accuracy of many foreign belt weighing devices has reached 0.1% ~ 0.25%, and some belt weighing devices have also passed the field test and verification of authoritative organizations (such as the United States NTEP national type test evaluation program), meeting the requirements of "legal trade" measurement.The "electronic belt scale measurement and calibration integrated system" recently launched by Tongling San Aisi Electronics Co., Ltd. in China can achieve the actual accuracy of 0.2% ~ 0.3% in general application occasions.The system installed in the same belt conveyor, two independent suspension frame scale in two platform scale frame weighing roller and adjacent area, a total of 17 roller adopts elaborate set of roller, can use in the actual process of conveying material for verification of the material weight stack calibration method to balance the quasi laymen calibration and roller spacing.

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