The Manufacturing Process Mechanism and Development Trend of Load Cell

I:Overview

At the beginning of human civilization and society, they recognized the status and role of manufacturing technology and manufacturing technology. Labor creates the world, manufacturing technology, and manufacturing process have always been the connotation of active productivity, the most active factor in the materialization of engineering technology, and the bridge for the transformation of scientific research results into commodities. The technology and products of strain gauge load cells (hereinafter referred to as load cells) are no exception. Since their introduction in the early 1940s, they have been developed entirely on the basis of manufacturing processes and relying on continuous improvements in manufacturing processes. The structural principle and production process of the load cell determine that there are more manual operation procedures, and human factors have a greater impact on product quality, which restricts the development of load cell technology.With the advancement of science and technology and the improvement of industrial production automation, digitization and intelligence, the general consensus of foreign weighing sensor manufacturers in the position of leader in the electronic weighing instrument market is: the manufacturing process of weighing sensors is definitely not what some people think. "Workshop craftsmanship", manufacturing process is not only a common basic technology in the development and production process, but also a professional technology that plays a key role. It is a comprehensive technology integrating machinery, electronics, information, materials and management. The concept and connotation of manufacturing process continue to expand, from narrow to broad, from part to whole, from single item to system, combining traditional load cell manufacturing technology with modern manufacturing technology, and deep integration of information technology and manufacturing technology to achieve digitalization. Networked and intelligent design and manufacturing form a new modern design and manufacturing process system. These concepts are more and more accepted by domestic weighing sensor manufacturers, and the development and innovation of manufacturing processes are increasingly concerned and valued by domestic technicians. The competition of weighing sensors in the international electronic weighing market is mainly the competition of manufacturing processes, the application of high-tech research and development of new products and the competition of independent intellectual property rights products, which is finally reflected in the competition of accuracy, stability and reliability of the weighing sensor.The common pursuit of load cell manufacturers in a leading position in the international market is: the structure of the load cell is more reasonable; the material of the elastic element is more sophisticated; the environmental stress screening of the components is more stringent; the support process, basic process, core process, special The application and integration of the process are more scientific, reasonable and repeatable; the circuit compensation and adjustment equipment and compensation process are more complete; the application of high-accuracy load cells is more extensive to enhance the core competitiveness of the enterprise. This should also become the common pursuit of my country's load cell manufacturing enterprises, and special attention should be paid to improving and enhancing the manufacturing processes and process equipment that are far from foreign companies; understanding the supporting processes, basic processes, core processes, and special processes that make up the manufacturing process Mechanism and innovation development trend.

II.Position and role of manufacturing process in the development and production of load cells. Analysis of the structural principle and manufacturing process of load cells shows that there are four key issues in the development and production process, namely, elastic element structure and elastic element metal material. , The whole process manufacturing process and testing equipment and methods. Among them, the scientificity, rationality and repeatability of the manufacturing process are the fundamental guarantee to ensure that every load cell produced meets the requirements of national standards. Therefore, the manufacturing process is the core professional technology that plays a key role in the development and production process. The manufacturing process and the load cell technology are complementary to each other to promote and drive the relationship. The rationality, stability and repeatability of the manufacturing process have promoted the development of the load cell technology. In turn, the increase in the measurement level of the load cell requires the manufacturing process. Towards automation, intelligence, and high stability. It can be said that the manufacturing process is an important means for the load cell manufacturing enterprises to strengthen their competitiveness and improve economic benefits. Which company attaches importance to the research and application of manufacturing technology, which company will benefit more and make rapid progress. In order to survive, develop and obtain economic benefits in today's fierce and almost cruel market competition, load cell manufacturers must rely on diverse product varieties, superior quality, low cost and timely delivery to occupy the market.The main means to obtain these comprehensive advantages is to rely on product development, scientific and reasonable selection of manufacturing processes, and continuous improvement and innovation. The German weighing instrument industry is well-known in the world weighing instrument market for manufacturing high, precise and cutting-edge technology products, and has a high level of expertise in the field of weighing technology. Especially in the research, manufacture and application of load cells, there are a group of manufacturing experts in load cells such as HBM Electronic Measurement Technology Co., Ltd., Balluff Sensor Co., Ltd., etc., which can produce high stability and reliability. Accuracy products are based on advanced manufacturing technology. Manfred Manfred, senior consultant of the German Institute of Engineering Physics Dr. Kochschko believes that this is mainly due to the growing technological and technological innovation capabilities and insatiable spirit of innovation in the German weighing instrument manufacturing industry. The load cell is a knowledge-intensive, technology-intensive and skill-intensive high-tech product, which requires the coordination and integration of multiple disciplines, multiple technologies and multiple manufacturing processes. The so-called high-tech products, "high" is high in terms of the technical content and the difficulty of the manufacturing process, especially in the middle and large-scale production, it is strict to the scientificity, rationality and stability of the manufacturing process and its process flow. test.Just as the evaluation of international manufacturing experts: "If product development takes ten times more effort, then mass production technology development will take a hundred times more effort." The load cell manufacturing companies in some industrially developed countries in the United States and Europe are theirs. Hundreds of efforts have been used in the development of mass production technology to become the leader in the international load cell technology and market competition, and it has remained so. The important role of the load cell manufacturing process in the development and production is often not recognized by people and has not been given enough attention. Some companies in our country adapt to the manufacturing process during the design stage of the load cell product, and adapt to the process equipment during the manufacturing process design. This is the root cause of the long-term lag in the weighing sensor technology and manufacturing process, and it is also the consequence of not paying attention to technological transformation. The reason is that the research work of the manufacturing process itself is relatively difficult, and certain test equipment is required, and the practice of the manufacturing process needs to be repeated. It needs to accumulate many years of practical experience. After several years of hard research and testing, certain technologies and Process difficulties. Therefore, it is difficult to produce results, talents, and benefits in the short period of 2020.09 of the 19th Weighing Technology Symposium.However, the quality and cost of the load cell are closely related to the manufacturing process, and companies must pay attention to it. The development practice of load cell products has proved that optimizing the structure design and manufacturing process is the main link to ensure that each product in mass production meets the national standards. As far as load cell products are concerned, to underestimate the manufacturing process is to relax the product quality. Only by paying attention to and grasping the manufacturing process can a large number of hidden dangers in the production process be eliminated in the bud. There is nothing wrong with focusing on product quality, and there will be no peace if you do not grasp the manufacturing process. This also reflects the status and importance of the manufacturing process in the development and production of load cells from another side. How to implement the "Made in China 2025" development strategy in the current load cell industry and how to deeply integrate information technology and manufacturing technology to achieve digital, networked, and intelligent design and manufacturing are issues that larger companies in the industry must face.

In recent years, three-dimensional digital design and manufacturing capabilities have become the core of competition between domestic and foreign companies, and are the fulcrum and breakthrough point for the transformation of corporate research and development capabilities. This change with the nature of a weather vane should cause load cell companies to attach great importance to it. Although the principle and manufacturing process of the load cell determine that it cannot be mass-produced on an automated, intelligent, seamless production line like machine parts, the structural design and manufacturing process of the load cell should also be upgraded with Internet thinking. It is as close as possible to digital, intelligent or partially digitalized, intelligent production, to keep up with the pace of development of international weighing sensor manufacturers.

III. Key manufacturing process and mechanism analysis of load cell

The structure and working principle of the load cell determine that its production process is a technical product based on the manufacturing process. The scientificity, rationality and repeatability of the manufacturing process, the advanced nature of the manufacturing process equipment and the level of automation and intelligence are the fundamental guarantee for producing products that meet the requirements of national standards. According to the structural principle of the load cell and the characteristics of the manufacturing process, the manufacturing process can be divided into support process, basic process, core process and special process.

1:Support process and mechanism analysis

The supporting processes of the load cell production process are mainly forging, machining, heat treatment, and surface treatment of the elastic element blanks. Among them, the quality of forging processing, machining size and shape tolerance, comprehensive performance of heat treatment, and surface treatment quality directly affect the accuracy, stability, reliability and appearance quality of the load cell.

(1) The forging process should make the elastic element have the best mechanical properties. The purpose of forging of the elastic element blank is to smash the coarse grains to make them uniform and compact. At the same time, the direction of the flow line of the metal material and the load cell are required. The force direction is the same, and no crossover, overlap, etc. are allowed. Therefore, only the elastic element blanks are allowed to stretch during the forging process, and the piers are not allowed to be thick, so that the elastic element has the best mechanical properties, which is very important to improve the measurement performance of the load cell, especially the long-term stability.For 40 CrNiMoA medium-carbon alloy steel materials commonly used in China, the forging heating temperature is 1180℃, the heating time is 3 to 4 hours, the holding time is≥30 minutes, the opening forging temperature is 1140～1160℃, the final forging temperature is≥850℃, air cooling. The annealing process is: heating temperature 860℃, heat preservation for 1.5 hours, furnace cooling. If it is slowly cooled after forging, annealing treatment is not necessary, and the elastic element can be processed directly. After forging, the metallographic structure of the elastic element blank belongs to sorbite and tempered α phase (10% HNO3 alcohol solution corrosion), and the hardness of the 19th Weighing Technology Conference Proceedings 2020.9 Nanjing 4HRC30. In recent years, large-scale load cells ranging from several hundred tons to thousands of tons have been applied. The size of the elastic element blanks is relatively large, and the slow cooling should be paid attention to after forging.

(2)The hot and cold processing technology should make the elastic components produce as little residual stress as possible

Since the most important mechanical part of the load cell is the elastic element, its function is to counteract the applied load and at the same time concentrate the effect of the load in a uniform strain field. Therefore, it is required that the blank of the elastic element can only be stretched during the forging process. The direction of the metal flow line should be consistent with the direction of the elastic element's force; in the process of machining, heat treatment, and surface treatment, ensure that the manufacturing process is as small as possible Of the resulting residual stress. The main sources of residual stress in the elastic element are: the residual stress generated during the forming process of the material in rolling or drawing; the residual stress generated by the uneven cooling temperature and phase change during the heat treatment process; in the machining process In, the residual stress generated by the cutting force. The latter forms a metamorphic layer on the surface of the elastic element, which makes the tissue in an unstable state, and the internal stress relaxes over time, resulting in dimensional changes. Machining such as planing, milling, turning, and grinding makes the surface of the elastic element deform unevenly, resulting in greater residual stress. The greater the amount of cutting, the greater the residual stress on the surface.

During turning, the axial and circumferential residual stresses are different for different feed rates. The maximum residual tensile stress is on the surface of the elastic element, and the maximum residual compressive stress is 40-80μm from the surface. During grinding, the residual stress generated is the largest. The greater the grinding depth, the greater the residual stress generated. The maximum residual stress is located at a distance of 20 to 40μm from the surface. The residual stresses generated by the four mechanical processing methods of planing, milling, turning and grinding can be summarized as follows:

① The maximum residual stress is between the surface of the elastic element and the depth of 100μm, the value is relatively large;

② The residual stress decays quickly, and it is very small at a depth of 200μm;

③ The greater the cutting amount, the greater the residual stress;

④ When the elastic element is finished by grinding, the residual stress is the largest, and its value can reach 900N/mm2. Therefore, it is required that the finish machining of the elastic element after heat treatment does not use grinding as much as possible, and the cutting amount is as small as possible when the size of the strain zone is close.

(3) Stability treatment process to eliminate residual stress in cold and hot processing The stability treatment to eliminate residual stress in cold and hot processing of elastic elements mainly includes cold and heat cycle process, secondary vacuum tempering process, and high temperature oil boiling process.

Heat and cold cycle process-the elastic element after heat treatment, the core part is tensile stress, and the surface layer is compressive stress. In the heat and cold cycle treatment, due to the difference of cold and heat temperatures, the thermal stress and residual stress in the opposite direction are offset each other. The process specification is:-196℃×4h /+190℃×4h, cycle three times. Secondary vacuum tempering process-to transform the retained austenite in the elastic element into martensite. For medium-carbon alloy steel 40CrNiMoA elastic elements, after 480℃ tempering, vacuum tempering is performed again to eliminate the residual stress of cold and hot processing. The process specification is: 460℃×2h, vacuum degree 10-5 Support. High-temperature oil boiling process-for steel elastic elements that are qualified for cold and hot processing and have been surface treated, in order to eliminate their cold and hot processing, the residual stress of the 19th Weighing Technology Symposium 2020.9 Nanjing 5 can also be implemented in High temperature oil boiling process for 2-4 hours in a high temperature oil tank of 250℃.

2. Basic technology and mechanism analysis

The core component of the load cell, the resistance strain gauge and its strain adhesive, the raw materials and components used in the circuit compensation and adjustment process, and the various protective and sealing materials used in the package all belong to the basic process category. They are the basis of the load cell manufacturing process. The preconditions for producing products that consistently meet the accuracy and stability requirements of the national standards for load cells. In order to save space, this article only analyzes the core component of the load cell resistance strain gauge manufacturing process and its strain transmission principle.

Resistance strain gauges for load cells should have the best working characteristics and compatibility with elastic components. The magnitude of the strain transmitted by the resistance strain gauge is very small. Take the resistance strain gauge with a sensitive grid length of L=5mm and a deformation of 1000με at full scale as an example. If the resolution is 10-5, then △L=5×10-8mm. That is to say, the elastic element must reflect the distance of one atom in the length of 5mm. This requirement is indeed very high. The working characteristics, stability and reliability of the resistance strain gauge determine the accuracy, stability and reliability of the load cell to a large extent. Since the strain gauge cannot be used twice after the test, it can only measure the working characteristics and fatigue life under laboratory environmental conditions, which brings great difficulties to research and selection. At present, resistance strain gauges are mostly selected around sensitive gate materials and structural forms, substrate and cover layer materials, manufacturing processes and process equipment levels.