Measuring device and application of pretightening

2022-09-19
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The measuring device and application of the pretightening force of injector bolt group

bolt connection mode is widely used. With the continuous improvement of people's requirements for product assembly, strict requirements and testing requirements are also put forward for the pretightening force of bolt assembly in key parts. This paper introduces a method to measure the pretightening force of bolt group by using an ordinary axial force meter and a special measuring device, and its application in the pretightening force measurement of engine injector pressing plate bolt assembly

connection and application of threaded fasteners

the assembly and connection mode of threaded fasteners has the advantages of simple structure, low cost, low requirements for assembly tools, convenient installation and disassembly, easy maintenance and replacement, so it has been widely used in machinery, automobile and other industries. For example, only an ordinary car needs to be equipped with thousands of threaded fasteners at the 2013 titanium dioxide industry annual conference held not long ago

in the assembly and connection of threaded fasteners, bolts (screws) account for the vast majority, and bolts are mostly connected in groups. Although these bolts are small, their role cannot be underestimated. Once they fail, the whole mechanical device will often fail to work, and even lead to the consequences of machine destruction and human death. This is why many automobile companies recall all models because of the quality problem of a small screw

measurement demand of bolt assembly pre tightening force

with the continuous improvement of social requirements for product quality, in the design and assembly connection of many products, the requirements for assembly quality such as assembly torque and assembly pre tightening force of bolt groups at key connection parts are also higher and higher. For example, in the newly developed diesel engine high-pressure fuel injector pressing plate bolt group assembly device that meets the national III emission standard, strict requirements are put forward for the assembly preload of the bolt group, and the demand for measuring the bolt assembly preload is also put forward

in order to develop the assembly process of bolt group and verify the rationality of the assembly process of bolt group for new product research and development, many users put forward the requirement of measuring the assembly preload of bolt group, but limited to the lack of special measuring devices and methods, there is no way to measure the assembly preload of bolt group at present

fuel injector is a key component of diesel engine, which requires accurate assembly preload, because its assembly quality plays a very important role in whether the engine can work reliably

the fuel injector is usually fixedly assembled on the engine cylinder head through the pressure plate and its fastening bolt group (see Figure 1). In order to ensure the reliable operation of the engine fuel injector, it is necessary to ensure the correct installation of the fuel injector on the engine cylinder head and strictly control the assembly preload of the bolt group

Figure 1 engine injector and fixed pressing plate

since there is no effective method to directly measure the axial preload of the injector pressing plate bolt group on the engine, we can only simulate the assembly state of the injector on the engine in the laboratory to realize the measurement of the axial preload of the injector, and then formulate a reasonable pressing plate bolt tightening process according to the measurement results to realize the indirect control of the axial preload of the injector

measurement method of pre tightening force of single bolt (screw) in the laboratory

for the pre tightening force test of single threaded fastener, at present, ordinary axial force sensor (commonly known as axial force meter) is used in the laboratory to test the pre tightening force of bolt (screw) assembly and the tightening performance of bolt

the ordinary axial force meter is a hollow structure (see Figure 2). During the test, the test fixture is installed first, then the accompanying nut and the accompanying base plate are placed on both sides of the axial force meter respectively, and then the test bolt is inserted and gradually tightened. The accompanying nut and the accompanying base plate will exert force on the axial force meter from both sides of the axial force meter, so that the axial preload of the bolt in the tightening process can be measured by the axial force meter

Figure 2 structure of ordinary axial force meter and schematic diagram of preload test of single bolt

development of preload measurement device for bolt assembly

in order to meet product research and development and user's testing requirements for bolt assembly preload, and meet the design and verification requirements of bolt assembly process, we began to study the technical scheme for bolt assembly preload test

1. determination of test device

bolt preload test requires corresponding test device. Due to the numerous connection forms of bolt groups, there is no special measuring device at present. Therefore, in order to realize the test of bolt assembly and preload, only the test method and device of bolt assembly and preload can be developed by ourselves. To develop a special test device, its cost will be very high, and its applicability is very narrow

in order to achieve the purpose of testing the pre tightening force of bolt assembly in a more, faster, better and less expensive way, we have determined to make full use of the existing testing equipment and carry out the feasibility study of using ordinary axial force meter to test the pre tightening force of bolt assembly

to realize the pre tightening force test of bolt assembly, it is necessary to test the sum of the pre tightening force of each bolt assembly. For the test of the pre tightening force of the bolt group of the engine injector pressure plate, the axial pre tightening force of the injector assembly can be obtained by measuring the sum of the pre tightening force of the bolt group around the injector pressure plate after tightening

to measure the preload of the engine injector pressure plate, only connect the fixing bolt groups around the injector pressure plate to the axial force meter at the same time, and then tighten the fastening bolt groups around the pressure plate one by one according to the specified assembly torque to measure the assembly preload of the injector pressure plate

because the common axial force meter is mainly designed to measure a single bolt, the inner hole of the axial force meter is small. For example, the diameter of the inner hole of the existing axial force gauge is smaller than the center distance of the diagonal bolts of the flange type fuel injector pressure plate bolt group, so it is impossible to directly place multiple bolt groups of the fuel injector pressure plate in the axial force gauge hole at the same time, and it is also impossible to measure the preload of the fuel injector pressure plate bolt group according to the traditional method

how to measure the preload of bolt group on the existing axial force meter with small aperture has become a research topic

2. determination of test plan

for the existing axial force meter, the author has designed several test plans, which have been improved for many times. With the help of the innovative design of the test fixture, a test plan for the preload of the bolt group has been gradually formed

due to the small inner hole of the ordinary axial force meter, it is impossible to pass all the bolts of the bolt group. Therefore, in order to test the pre tightening force of the bolt assembly on the ordinary axial force meter, we must start from changing the bolt connection mode of the bolt group

to realize the connection of the pre tightening force of the bolt group on the axial force meter, we can only try to realize the assembly connection of the bolt group bolts outside the inner hole of the axial force meter. Therefore, by moving the position of the accompanying nut forward and improving the form of the front pressing plate and the accompanying gasket, we can avoid the limitation that the inner hole of the axial force meter is small and the bolt group cannot be assembled, so as to solve the problem that the ordinary axial force meter cannot pass through multiple bolt groups because of the small inner hole, Then, a special rear pressure block is designed to be connected with the test nut, and finally a scheme for measuring the preload of the fuel injector with the flange pressure plate fixed by the existing axial force meter is formed (see Figure 3)

Figure 3 bolt assembly and preload measurement device on the ordinary axial force meter

measurement of the preload of the injector assembly

the flange pressing plate of the injector is directly sleeved on the injector (see Figure 1). The pressing plate presses the injector through the flange boss on the injector, and the flange pressing plate is assembled and connected with the cylinder head through the surrounding fastening bolt group. Therefore, The sum of the preload of the bolt group around the pressing plate is the axial preload finally obtained by the fuel injector. Since the fuel injector is fixed on the engine cylinder head through the pressing plate, and the fuel injector pressing plate is pre tightened through the pressing plate bolt group, as long as the pre tightening force of the bolt group around the fuel injector pressing plate is measured, the assembly pre tightening force of the fuel injector can be obtained

using the principle of the bolt group measuring device invented by us, according to the structural characteristics of the tested injector and the size of the pressing plate and fastening bolt, combined with the connection size of our axial force meter, we have designed a special test fixture, a special test backing plate and a test nut, which are connected according to the scheme shown in Figure 4. The measurement scheme simulates the connection mode of fuel injector, pressure plate (accompanying gasket) and fixed bolt (test bolt) on the engine, and is equipped with specially designed accompanying nut, rear pressure block and front pressure plate

the fuel injector, pressure plate and its fixing bolts used in the measurement are the components used on the engine. They are connected according to the structure shown in Figure 4. The pre tightening force generated by tightening the fastening bolts of the pressure plate is transmitted to the test pressure plate through the pressure plate of the fuel injector, and then to the axial force meter through the pressure plate. The pressure plate is connected with the companion nut through the surrounding fixing bolts, and the companion nut is connected with the rear pressure block at the other end through a group of hexagon socket screws, The force borne by the companion nut through the rear pressing block shall have sufficient capacity and ensure that the temperature rise is quickly transmitted to the axial force meter. In this way, tightening the fixing bolts around the pressure plate of the fuel injector can realize the measurement of the preload of the pressure plate of the fuel injector

Figure 4 pre tightening force measuring device of injector pressure plate bolt group

popularization value of pre tightening force measuring device of bolt group

the innovative design of pre tightening force measuring device of bolt group introduced in this paper realizes the measurement of pre tightening force of engine injector pressure plate bolt group, and has been successfully applied in the research and development of two engines, meeting the needs of engine research and development and assembly review, It also determines the appropriate assembly process for the pressure plate bolts of the fuel injector to ensure the assembly quality of the fuel injector. At the same time, in 2007, the device has applied for the national utility invention patent

the length of each crack should be measured in the assembly and connection of threaded fasteners. The structure is widely used. Most threaded fasteners are connected in groups, and many product assembly and connection have strict requirements on the pre tightening force of bolt groups, so it is necessary to measure the pre tightening force after bolt assembly. This innovative design device also provides the possibility for the measurement of the preload of similar bolt groups, and has wide application value. (end)

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