On the Durability of Belt Scale

On the Durability of Belt Scale

[Abstract] This article mainly points out that in order to determine the durability of belt scales, it is necessary to clarify its definition, test items and purposes, specific methods that can be operated, and who will perform it.

【Key words】 Belt scale durability

1. Raising the problem

In recent years, the product quality of domestic belt scale manufacturers has improved significantly. Some manufacturers in order to enable their products to have a recognized or legally recognized technology and measurement indicators. As a result, a proposal to modify the highest level of the original belt scale to level 0.2 was proposed, and the connotation of the durability of the belt scale and how to test it were also discussed extensively. Next, I would like to talk about the durability of belt scales for your reference.

2. About durability

The concept of durability of weighing instruments is learned in the text of the international recommendation OIML R76 (non-automatic weighing instruments). Now 1992

The relevant content in the annual edition is copied as follows:

"Terms: T.5.5.7 Durability error—the difference between the inherent error of a weighing instrument during the entire service life and its initial inherent error.

T.5.5.8 Significant durability error-durability error greater than e.

Note 1: The durability error may come from mechanical wear, or from drift and aging of electronic components. The significant endurance error only applies to electronic devices.

2: For multi-division weighing instruments, the value of e should correspond to its partial weighing range.

The following conditions are not considered to be significant durability errors, even if they exceed e;

The error produced after the weighing instrument has been used for one cycle is obviously due to the failure of the device and (or) components or due to interference.

Therefore, its indication: as a measurement result, it cannot be explained, stored or converted, or it means that any measurement is impossible, or it is obviously wrong, so that it must be noticed by all people who pay attention to the measurement result. "

"Text: 3.9.4.3 The durability error caused by wear shall not be greater than the absolute value of the maximum allowable error.

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The adoption of this requirement assumes that the weighing instrument has passed the durability test specified in Article A6 of Appendix A. This test is limited to

Weighing apparatus with Max≤100kg. "Appendix A (mandatory) A6 Durability test (only applicable to Class II, III, and IV weighing instruments with Max≤100) Durability test should be carried out after all other tests have been done.

Under normal conditions of use, the instrument is subjected to repeated loading and unloading of approximately 50% of the Max load. The loading and unloading should reach one hundred thousand times, and the speed of its frequency should make the weighing instrument reach a balance when loading and unloading. The force of loading shall not exceed the force achieved under normal loading conditions. Before the endurance test starts, a weighing test shall be carried out in accordance with the conditions described in A.4.4.1 to obtain the inherent error of the weighing instrument. After the above loading is completed, a weighing test is performed to determine the durability error caused by abrasion.

If the weighing instrument has an automatic zero-setting or zero-point tracking device, it can be in working condition during the test. At this time, the zero-point error shall be determined according to the method described in A.4.2.3.2. "

The following is a description of the durability in my country's belt scale verification regulations.

"Terms: T.7.5 durability test durability test

A test to check whether the belt scale under test (EUT) can maintain its performance characteristics after the use period. "

It should be pointed out that apart from this, there is no longer any language expressing durability in the entire procedure. Here are some expressions about durability copied from the term (standard):

"Significant durability error refers to the durability error greater than e.

The following situations even if they exceed the above-defined values, they are not considered as significant durability errors referred to in this recommendation; the errors caused by the use of a weighing instrument after a period of use are obviously due to damage to the device/component, or interference or durability If the protection feature fails, the indication value is as follows:

-Cannot be interpreted, memorized or transmitted as a measurement result;

-It is impossible to perform any measurement; or-can draw attention to all obvious errors of such measurement results.

Finally, there are some terms and definitions related to durability.

Endurance test:

A destructive, or faulty, sometimes (in a weighing instrument) test that increases inaccuracy due to wear. It is usually a test in which the same load is repeatedly applied at the point where the expected result occurs, and the number of times the load is calculated.

An endurance test aimed at testing the number or time that the weighing instrument and its components can be used.

Breakdown test

An endurance test for the purpose of testing the mechanical strength of the weighing instrument and its components or the endurance of permanent damage.

According to OMLR60’s ‘durability requirements’ for sensors:"

Durability (durability, durability)

In summary, the terms and definitions of the durability of weighing instruments cited above. So far, OIML International recommends that only non-automatic weighing instruments, sensors and belt scales below 100kg have durability requirements. And only the test error of the durability of non-automatic weighing instruments weighing below 100kg puts forward specific operational requirements, while the durability of sensors and belt scales only puts forward a conceptually inoperable “definition”. Only the OIMLR 76 international recommendation proposes that the durability test is mandatory and should be understood to be implemented by the legal department.

Three, the characteristics of belt scale

There are three kinds of weighing instruments in actual use, which have a lot to do with its conditions of use. The measurement results of dynamic truck scale and dynamic railway scale are closely related to the "approach (track)" at both ends. Therefore, in foreign countries, except for the fixed dynamic truck scale, the portable truck scale cannot be used for trade measurement, but can only be used for monitoring. Because it is placed in different locations, there may be significant differences in the measurement results of the same car. The third type of weighing instrument is a belt scale. Its weighing result is not only related to the installation conditions but also very obviously affected by the condition of the belt conveyor. Almost all articles on belt scales mention the influence of the "belt effect" of the transmission belt when discussing measurement errors. Generally speaking, the error caused by this influence is much larger than the influence of the "approach rail" of the dynamic truck scale and the dynamic railway scale. This is why the measurement results in actual use are often different from the promised indicators of the belt scale manufacturer and the results expected by the user. When designing a system for material transportation, when considering the state of the transmission belt as an afterthought, the result is a transmission system that can be weighed by the way and has unreliable accuracy. The real belt scale should be a conveying weighing system with a certain degree of reliability and accuracy with transmission function.

From a physical point of view, the weighing mechanism of the weighing carrier of the belt scale is not complicated, and it is similar to the carrier of other weighing instruments.

However, due to the "belt effect" of the belt conveyor, the influence of the interference force in the weighing process is very complicated. It can neither be effectively removed by "filtering", nor can the analytical digital expression of the influence of the interference factor be written. In the advanced stage, some manufacturers have introduced the "belt type correction device" technology to improve the measurement accuracy of belt scales. The leading idea of this method is to try to replace the conventional "segment zero point" with a series of "segmental zero points" when weighing the belt scale. Average zero method". This method is to correct the belt zero point through software "synchronization".

Another difficulty in belt scale measurement is that the weight of the belt is often heavier than the object being weighed. The interference force of the "belt effect" also has an obvious effect on the weight. Moreover, the weighing time of the belt scale is very long. It often lasts for several hours. In this way, the influence of the drift (temperature drift and time drift) of the measuring instrument is higher than that of other automatic weighing instruments, and other weighing instruments except belt scales can be set to zero in time between two separate weighings. Although, there are clear test methods and error requirements for the long-term stability of its zero point in the measurement performance test of the belt scale. However, the maximum deviation test of zero load in the field test cannot reflect the long-term stability of the actual belt scale zero point. In particular, the belt lengths of belt conveyors today are very long, often far exceeding the belt length limit in Handbook 44 of the United States. Not only that, today's large-scale belt scales have reached the level of 10,000 tons per hour. In this way, the width of the belt is getting wider and the speed is getting faster and faster. With the development of the conveyor to the direction of large-scale. The traditional understanding and "static design" of belt conveyors means that the conveying mass points are accelerated or decelerated at the same time, and the assumption that the conveyor belt is regarded as a rigid body can no longer meet the actual requirements. The current understanding is that the conveyor belt is a viscoelastic body with viscoelastic properties, and the transmission of speed, acceleration and tension in the conveyor belt takes a certain amount of time. Therefore, the speed, acceleration and tension of each point of the conveyor belt over the entire length of the conveyor have dynamic characteristics and are a function of time. This dynamic characteristic has little effect on low-speed and short-distance conveyors, but for long-distance and high-speed belt conveyors, this dynamic characteristic will be apparent. Coupled with the influence of the site environment temperature and humidity

This makes it difficult for the belt scale to buckle the actual influence of the “zero point” of the belt during actual weighing operation, which seriously affects the measurement results. Especially in the case of long-distance transportation, when the belt needs to be driven by dual drives and multiple drives, the operation of the belt is more complicated.

The third feature of belt scales is that there are too many factors that affect belt tension. From the perspective of physics (mechanics), many scholars have tried unsuccessfully to give a mathematical analytical expression for belt tension. To some extent, the basic point of the design and calculation of the belt conveyor is the calculation of the tension of the conveyor belt. In order to standardize the design and simplify the design. The International Organization for Standardization and some countries have promulgated standards for belt conveyors. Such as the international standard ISO5048, the American standard CEMA, the German standard DIN22101, and the national standard GB/T171. These standards give the engineering mathematical expression of belt conveyor tension. When designing belt scales, engineers from some foreign belt scale manufacturers who I have contacted must use these engineering expressions for calculating tension in advance according to the actual belt conveyor situation to estimate the magnitude of its tension. However, when installing belt scales in China, many people hardly consider the size and changes of belt tension. With the development of my country’s economy, long-distance, large-capacity, and high-speed belt conveyors have been used more and more. For example, in the coal long-profile belt conveyor in Tianjin Port, there is a single belt conveyor. The conveyor belt is 8980m long.

The fourth feature is that because of the large scale of belt scales, belt scales weighing tens of thousands of tons per hour are already a common measuring device in many ports. So far, it is still recognized internationally that on-site material testing of belt scales is the only traceable verification method. On-site material testing is a very time-consuming, labor-intensive, and expensive test, especially for large-scale belt scales. In order to make the material test convenient and reliable, it is often necessary to carefully test the calibration device before setting up the belt scale. Such as bucket scales, railroad scales, truck scales and the coordination of the relative position and error to ensure the reliability of calibration. Otherwise, it will cause a lot of inconvenience during the calibration, and the calibration error caused by the throwing of materials caused by the too long distance between the calibration place and the belt scale should be avoided.

As we all know, a newly installed belt scale can become an approved weighing instrument after the initial verification (Initial Verification). Except for special circumstances, the first verification of a belt scale must be a material test. In order to ensure the use of belt scales, in addition to the first verification, follow-up verifications and in-use inspections performed by the legal metrology agency are also specified in the belt scale manufacturer.

4. My understanding of durability

From your articles on the durability of belt scales, I found that everyone has different understanding of the definition of belt scale durability. Moreover, the definition of the durability of the belt scale is rarely given in the article, and it is not obvious from the durability test of the belt scale what kind of test needs to be passed to test the durability of the belt scale. From the related durability statement that I drew in the second part, except for the non-automatic weighing durability of Max ≤ 100kg, it is more clear and operable, that is, "the durability test should be done after all other tests are completed. Later” (from the Chinese translation of OIML R76. 1992), and gives specific operational test methods and error requirements. The purpose of the test is also stated, "to determine the durability error caused by wear." It clearly stipulates that "the durability error caused by wear should not be greater than the absolute value of the maximum allowable error."

The second question: The relationship between durability test and subsequent verification and in-use inspection is to be discussed. Even when users are concerned about the accuracy of the belt scale used in doubt, they can test the measurement error of the belt scale by themselves. The latter is an inspection in use. The follow-up verification generally refers to "compulsory periodic verification." According to the verification procedures of belt scales, the durability test is different from it only in that the durability test is "after a prescribed period of use. Whether to maintain its performance characteristics." is a clause that cannot be specifically operated. First of all, it is not a periodic verification. In addition, it is carried out on the basis of "... and whether its indication error exceeds the maximum allowable error in use) as specified in the in-use test. "One of the inspections" is also not in compliance. This kind of inspection operation is the simulated load test that the user often uses in order to ensure the normal use of the belt scale during the verification cycle or after the periodic verification, so the relationship between these tests is also to be explained.

Third, the durability test of a non-automatic weighing instrument can inform the user a clear message that the error of the weighing instrument can be guaranteed to be within the "absolute value of the maximum allowable error" after one hundred thousand tests. However, in many articles discussing the durability of belt scales, almost failed to inform users of useful information about the belt scales it uses.

Fourth, how to treat products that have undergone durability testing, even if they are non-automatic weighing instruments, still require subsequent verification and in-use verification in their subsequent measurement management. Subsequent checks are often what we refer to as periodic checks. In practice, whether in the subsequent verification and in-use verification, it is found that if the indication of the weighing instrument is out of tolerance, and the use of the weighing instrument has not reached the 100,000 times required by the endurance test. But after recalibration, the error is within the tolerance, the weighing instrument can still meet the statutory use. In this actual operation, the information displayed is inconsistent with the information of the durability test. How to define durability for a belt scale that can be used after adjustment and recalibration.

Finally, who will perform the durability test, is it the statutory measurement department or the user? when? In what way.

I think the five aspects mentioned above are the issues that should be considered in the verification procedures that should be considered in the definition of durability and durability tests that are actually operable and provide useful information.