Application of bolt automatic tightening technology in valve product assembly

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This paper introduces the operation process of manual tightening of valve bolt assembly and the current situation of product connection reliability and production efficiency. The limitations of manual bolt tightening technology and the development trend of automatic bolt tightening technology are analyzed. The idea of introducing automatic bolt tightening technology into the valve bolt tightening process is put forward. On this basis, the process test is carried out with a single-shaft automatic tightening equipment, and the feasibility of the application of multi-axis synchronous tightening technology of bolt group in valve product assembly is discussed.

1. Overview

Bolt connection is the most important connection method in valve products. In natural gas, nuclear power and other fields, valve products mostly exist in the form of ring bolt sets, and their connection reliability has a direct impact on the sealing and design performance of products. Once the connection fails, it will cause serious consequences. For example, the bolt connection failure of LNG valve is likely to cause natural gas leakage, which may cause fire and explosion. In the process of valve product design, the tightening process is usually constrained by simple torque method or torque+angle method to ensure the connection reliability. In the manufacturing process, vacuum technology network（ http://www.chvacuum.com/ ）It is found that manual tightening is commonly used at present, that is, manual assembly is carried out by means of dial wrench, constant torque wrench or angle meter, and the connection reliability and production efficiency are low. This paper introduces the bolt automatic tightening technology into the valve bolt tightening process, analyzes the limitations of the traditional manual tightening technology, summarizes the development of the bolt automatic tightening technology, and on this basis, applies a single-shaft automatic tightening equipment to carry out the process test, and discusses the feasibility of the multi-axis synchronous tightening technology of the bolt group.

2. Process analysis of manual tightening of valve bolts

2.1 Characteristics of valve bolt connection

The basic purpose of bolted connection is to use threaded fasteners to reliably connect the connectors together, while the essence of assembly and tightening is to control the axial preload of bolts in an appropriate range. The bolted connection in valve products has the characteristics of large number of bolts, high reliability requirements and more than 90% of bolts in the form of ring bolt group. Take a certain valve product as an example, each valve includes five flanges, namely, inlet flange, outlet flange, opening actuator, closing actuator and top flange. Each flange is assembled by 6~20 double-head bolts arranged in a ring, and the total number of bolts of both is more than 60. Reasonable and uniform preload plays an important role in the internal stress at the flange connection. Under the working condition of high frequency vibration, excessive internal stress may become the hidden danger of cracks in the connecting parts, and too small preload may easily cause the risk of sliding or even tripping under the vibration working condition, directly affecting the sealing effect. Therefore, the applied torque and rotation angle must meet the design requirements during the assembly process.

2.2 Manual valve bolt tightening process and limitations

At present, domestic valve manufacturers generally use manual tightening. For ring bolt group, in addition to the tightening quality of a single bolt, the tightening sequence between bolts has an important impact on the uniform distribution of axial preload. Different tightening sequences will cause the sequence of internal stress generation of bolts to change, resulting in the phenomenon that the same torque produces different tightening forces. In the manual tightening mode, the diagonal assembly mode as shown in Figure 2 is currently widely used.

It should be noted that even in the diagonal assembly mode, due to the mutual influence of the tightening process between the bolts, in order to ensure that the torque applied to the bolts is uniform, the three-step tightening strategy is usually adopted, that is, the first step is pre-tightening (applying very small torque), the second step is applying the torque required by the design, and the third step is repeatedly applying the torque value required by the design. Affected by many factors, the manual tightening process has certain limitations.

(1) The torque application accuracy is low. On the one hand, when the dial wrench is used, it mainly depends on visual reading, which may cause some errors. On the other hand, the tightening speed cannot be kept uniform, which will cause differences in dynamic friction torque and affect the assembly accuracy.

(2) Low production efficiency and high labor intensity. The whole tightening process depends entirely on manual tools. Due to the interference in the position of each bolt, the operation of torque wrench requires constant conversion of work positions, which leads to low production efficiency and high labor intensity. This problem is particularly prominent in the process of assembling large valves (high torque requirements).

(3) It is easy to produce flange warpage, which affects the sealing performance. In the diagonal assembly mode, due to the tightening force applied successively at both ends of the diagonal, it is easy to cause the warping phenomenon of high flange ends and low flange middle, resulting in valve leakage.

(4) When there are many torque points for applying force, it is easy to repeatedly apply torque or omit to apply torque.

(5) The degree of digitization is poor, and automatic recording is not supported. The operator needs to record the completion status and data to the paper quality control card when completing a work, which leads to low production efficiency and opaque working process.

3. Automatic bolt tightening technology

Automatic bolt tightening technology is an electromechanical integration technology that integrates mechanical transmission, electrical transmission, electronic technology and automatic detection. The final product of its technical development is automatic bolt tightening machine. At present, automatic bolt tightening technology has been widely used in automobile assembly, electronic product assembly and other industries. At present, the automatic tightening technology of bolts has started earlier in foreign countries, and the theoretical technology is becoming mature. Companies with high market share include ATLAS in Sweden, BOSCH in Germany, STANLY in the United States, INGERSOLL-RAND, INSIGHT in Ingersoll Rand, DDK in Japan, ESTIC and SETECH in South Korea. At present, the electric tightening machine has been widely used in the automobile industry at home and abroad. Figure 2 and Figure 3 are application examples of single-shaft and multi-axle tightening technology respectively.

The domestic research on bolt tightening technology is still at the research stage. In recent years, some domestic companies have initially mastered the key technology of internationally advanced tightening equipment manufacturing through technical exchange and cooperation. For example, the Dalian Institute of Modular Machine Tools has cooperated with the Swedish ATLAS company to develop an automatic tightening machine for the cylinder head, main bearing cap, connecting rod and flywheel for Jiangxi Isuzu Automobile Factory. Shandong Assembly Technology Co., Ltd. has successfully developed a series of multi-axle thread assembly tools. Its products include 3 series and 14 specifications, with a rated torque of 5~3500N · m, and can control up to 40 tightening shafts. Wang Chunhua of Dalian University of Technology and others designed the control system of multi-station electric bolt tightening machine based on industrial computer, which was successfully applied in the gearbox factory of Changchun FAW Group. The measured working beat is less than 30s, the torque control accuracy is less than 3%, and the angle control accuracy is less than 0.1 °.

4. Valve bolt assembly process based on automatic bolt tightening technology

4.1 Bolt tightening process test of single-shaft automatic tightening equipment

Select an automatic bolt tightening machine, and take the bolt group of the inlet flange of a valve as the research object to conduct the tightening process test. The bolt group contains 10 bolts, the design torque requirement is 151N · m, and the applied torque range of the automatic tightening machine is 7~23N · m. Adopt the same diagonal assembly mode and three-step tightening strategy as the manual tightening process. The first step is to tighten to 2N · m, the second step is to tighten to 15N · m, and the third step is to tighten again to 15N · m. During the three operations, maintain the rotation speed of the automatic tightening machine at 600r/m. The tightening operation process and parameters are preset by the management system at one time. During operation, it is only necessary to tighten in sequence. After tightening, use the digital torque wrench to check the effect of the applied torque.

(1) The torque application accuracy is high. In the manual tightening mode, there are 2 bolts exceeding the torque tolerance range, and the tightening accuracy is only 11.2%. The main reason for this problem is the impact of human factors. In the automatic tightening mode, the ATC characteristics of the automatic tightening machine play a good role, excluding the possible impact of other factors, the tightening accuracy reaches 3%, and all bolt torques meet the design requirements. In addition, the comparison of standard deviation between the two groups of test data is S automatic=0.09

(2) The production efficiency has been significantly improved. It takes about 22min to tighten 10 bolts manually, but it is shortened to 1min under automatic tightening mode, and the production efficiency is increased by more than 20 times. When the design torque value of bolts increases, the advantage of automatic tightening process in production efficiency will be more obvious.

(3) It has better automatic error prevention function and digital processing function. The whole assembly process is carried out according to the preset procedure flow. In case of missing or repeated tightening, the alarm will be given in time to avoid man-made low-grade accidents. In addition, all actual tightening torque values can be automatically recorded and stored. At the same time, the analysis software provided by the system can carry out statistical analysis of the whole tightening process, and all data can be imported into the computer system connected with the industrial computer. The whole operation process is reliable and transparent.

4.2 Multi-axis synchronous automatic tightening technology of valve bolt group

According to the analysis results, the application of single-shaft automatic tightening technology can greatly improve the assembly accuracy and efficiency of bolts. Multiple single-axis automatic tightening tools are connected in parallel to form a multi-axis synchronous tightening device. Compared with single-shaft tightening technology, multi-axis synchronous tightening technology has more technical advantages.

(1) The flange bolts are tightened synchronously to better avoid flange warping.

(2) The productivity can be multiplied by the number of tightening tools included.

(3) It can further reduce the mutual influence of the tightening process between bolts and improve the assembly accuracy.

Multi-shaft synchronous tightening technology has been well applied in the field of automobile engine and wheel hub assembly. Compared with other industries such as automobiles, valve products have their own characteristics. If there are many flange specifications, the diameter of each bolt group varies from 30 to 500mm, and the number of bolts varies, mainly including 4, 6, 8, 10, 12, 16, 20, 24, etc. In addition, the volume of valves is usually small, and most of the diameters are within 120mm.

Therefore, the synchronous tightening technology of valve products needs to meet some special requirements.

(1) It has high flexibility (variable operating diameter) to meet the requirements of bolt groups with different diameters.

(2) Considering the large number and specifications of bolts in the bolt group and the high cost of automatic tightening machine, it is generally impossible to achieve complete synchronous tightening in the actual production process. It is recommended to replace it with partial synchronous tightening. For example, the combination of a 3-axis and a 4-axis tightening device can meet the assembly process of most valve bolt groups.

(3) On the basis of meeting the torque application range, try to select the automatic tightening tool with smaller diameter to avoid interference when facing the bolt group with smaller diameter.

At present, the bolt automatic tightening machines of most enterprises support the multi-axis parallel working mode, that is, the electric control box group is connected in parallel with one of the electric control boxes as the main machine to realize the synchronous control and operation of multiple tightening machines. Therefore, the valve bolt multi-axis synchronous tightening device should be designed to achieve flexible tightening machine fastening tooling. One feasible way is to design the position adjustment screw to adjust the diameter of the tightening unit to ensure that the diameter of the tightening unit can be adjusted freely within a certain range.