This paper introduces the screw fastener assembly method and assembly grading technology in detail. When tightening, according to the requirements of the actual joint structure, select the most suitable method and process, and illustrate the application of advanced assembly technology, while improving reliability. The bolt size is reduced, the coupling structure is compact, and the purpose of weight reduction and cost reduction is realized, which is of great significance in the weight reduction of automobiles. 1 Introduction Threaded connection is one of the four commonly used coupling methods for automobile parts. Because of its simple structure, reliable connection, easy assembly, easy disassembly and good reusability, it is the most standardized mechanical part in automobile manufacturing technology. One of the more economical manufacturing methods in manufacturing. On the passenger car, the average number of thread fasteners per car is about 4,000 and the weight is 50kg. On commercial vehicles, the average number of threaded fasteners per car is about 7000, weighing 88kg, of which high-strength bolts account for 1 /3. The cost of threaded fasteners on a car is about 2.5% of the cost of the entire vehicle, and the amount of threaded fasteners on the assembly line is 70%, which is important. 2. Assembly method The purpose of the threaded joint is to reliably couple the jointed parts by tightening the bolts/nuts. The essence of the assembly is to control the axial preload force during the tightening process. The requirements of the axial preload force are the product design and material process level. The comprehensive embodiment. When tightening the threaded fastener, it is necessary to overcome the friction between the thread and the friction of the supporting surface. For example, in the case of a hexagonal bolt, only about 10% of the energy is used to provide the axial preload, and the remaining 90% of the energy is used. Overcome friction. It is very difficult to directly measure and monitor the preload in the actual assembly process, and it can only be controlled indirectly. Tightening torque, angle of rotation and elongation have a certain relationship with the axial preload. Controlling these parameters to achieve indirect control of the preload is the basic principle of the thread tightening technology. There are four methods for assembling typical threaded fasteners: torque method, torque-turn angle method, yield point method, and elongation method. 2.1 Torque method The torque method is the most widely used control method. According to the basic relationship between the pre-tightening force of the bolt and the tightening torque, the pre-tightening force is controlled by controlling the tightening torque, which is generally used in the elastic zone, as shown in Figure 1. Shown. Figure 1 Torque method The relationship between the tightening torque T and the axial preload force F is: Where: T: tightening torque, F: axial preload, P: pitch, α: flank angle, d 2 : thread diameter, d W : equivalent friction diameter of the support surface, μ S : thread friction coefficient, μ W : friction coefficient of the bearing surface. When the torque method is assembled, due to the influence of the friction coefficient, the axial preload force fluctuates greatly, and the material potential is not fully utilized, the bolt strength utilization rate is low, but because of its simple operation, low cost, and for most threaded joints Still effective, it is still the most common assembly method. 2.2 Torque-turn angle method The torque-turning method is to achieve a specified starting torque (ie, the fitting torque) when tightening, and then rotate the screw to a prescribed angle. This method produces a certain axial elongation of the bolt based on a certain angular displacement and is compressed by the coupling member, resulting in a certain preload. The angle of rotation needs to be obtained by calculation or experiment in advance. There are two commonly used torque-turning methods. One is to tighten the threaded fastener to the elastic zone. As shown in Figure 2a, the relationship between the angle of rotation and the axial preload is as follows: Where: θ: corner, F: axial preload, P: pitch, C 1 : bolt stiffness, C 2 : stiffness of the joint. The axial preload of the bolt is related to the stiffness of the system. The other is that the threaded coupling is tightened above the yield point, ie the plastic zone range, as shown in Figure 2b, where the axial preload is related to the strength of the bolt. Figure 2 Relationship between rotation angle and preload During the tightening process, the friction coefficient has a small influence on the tightening quality (only affecting the stage when the fitting torque is reached, and has no effect on the angle control phase), and a relatively high pre-tightening force can be obtained, and the pre-tightening force is obtained. The dispersion is small; when tightening to the plastic zone, the bearing capacity of the bolt can be fully utilized, and the bolt with the quality problem can be picked out. However, this kind of tightening method is complicated in operation and high in cost, and is not suitable for short bolts with small corners; due to the large pre-tightening force (especially tightening to the plastic zone), it is necessary to consider the application of the bolts with poor plasticity and repeated use. Sex. 2.3 Yield point method The yield point method, also known as the torque slope method, is a method of tightening the threaded member to the yield point by monitoring the slope of the torque versus angle curve during tightening. During the tightening process, the tightening curve changes from the elastic zone to the plastic zone, and the linear relationship between torque and angle changes, and the slope also changes. When the change in slope reaches a certain range, it is considered to reach the yield point, as shown in Figure 3. Figure 3 yield point method The tightening quality of the yield point method (preload tension dispersion) is only related to the yield strength of the bolt, and is not affected by the friction coefficient and the starting point of the corner, which can improve the assembly accuracy; the bolt can be tightened to its yield point to maximize the The ability to play with bolts. The disadvantage is that an automatic tightening machine with an arithmetic function is needed, the control system is complicated, the price is high, the material, structure and heat treatment of the bolt are very high, and it is generally applied to a mounting part requiring relatively high. 2.4 elongation method The elongation method is to measure the elongation length of the bolt during the tightening process or after the tightening by means of a micrometer or ultrasonic wave, and to control the axial preload by using the relationship between the preload force and the bolt length variation. method. In the range of elastic deformation, the axial preload force is related to the force cross-sectional area, elongation and strength of the bolt, namely: Where: δ b : elongation, F: axial preload, C b : bolt stiffness, l e : bolt effective length, E b : bolt elastic modulus, A b : bolt cross-sectional area. After the strength grade and size of the bolt are determined, the pre-tightening force is only related to the bolt elongation, which can eliminate the influence of variable factors such as friction coefficient, contact deformation, deformation by the coupling member, etc., so that the highest control precision can be obtained and used as Preload control method for important bolted joints. However, measuring devices (such as micrometers, ultrasonic waves, etc.) are inconvenient to implement on a specific joint structure and affect the production cycle, so they have not been widely used in the automotive industry. Under laboratory conditions, the elongation method is an indispensable means for calibration, calibration, and experimental development. The four methods of assembly of threaded joints have their own advantages and disadvantages. When tightening, it should be determined according to the actual joint structure. It is necessary to clarify the requirements of the joints, the accuracy requirements of the axial preload and the application of the control method. Through experiments and analysis. Choose the most appropriate method. 3. Assembly grading At present, domestic and foreign automobile manufacturers classify the assembly of threaded fasteners. In the domestic automotive industry standard Q/T 518, the tightening accuracy of fasteners is divided into three grades, which are different; Volkswagen's assembly below the yield point is divided into four grades; DEUTZ is based on the clamping force after tightening. The range of fluctuations is divided into 3 levels; for torque method assembly, Mercedes-Benz is divided into 3 levels, and Toyota is divided into 5 levels. FAW Group divides the torque method assembly into 4 levels. When the bolt assembly level is increased, the fluctuation range of the tightening torque is reduced, the axial pre-tightening force is increased, the reliability of the threaded connection is improved, and when the clamping force requirement is constant, the bolt size can be reduced to achieve weight reduction, and Make the coupling structure compact. 4. Application examples in car lightweighting 4.1 Torque-turn angle method The position of the sub-frame and the body of a passenger car is bolted to M14×1.5×85, 10.9, friction coefficient μ=0.18-0.14, assembled by torque method, tightening torque (110±20)Nm, minimum axial preload The force is 34kN. When assembling by the torque-angle method, the bolt of M12 can be used, the tightening process is 70Nm+180°, and the yield is tightened to the yield. The minimum axial pre-tightening force is 44kN, which meets the requirements of use. The comparison results of the screw parameters under different assembly methods are shown in Table 1. Table 1 Comparison of bolt parameters under different assembly methods After tightening by the torque-angle method, the usage rate of the bolt is improved, the stability of the assembly quality is also improved, and the bolt specifications (M14→M12) can be reduced accordingly, the weight reduction (36g) can be realized, and the cost can be reduced accordingly. . 4.2 Assembly grading Taking a coupling structure of a commercial vehicle as an example, a hexagonal flange bolt with a 10.9 grade, a friction coefficient of μ=0.18-0.14, and an M14×1.5×70 is used. When the FN4 assembly is used, the axial preload force is ( 31-77) kN, the pre-tightening force has a large dispersion. When the assembly level is increased to FN1, the axial preload is (73-87) kN, the dispersion is significantly reduced, and the minimum axial preload is 73kN. It is 2.35 times of FN4 assembly (Fig. 4a), which can greatly improve bolt utilization and greatly improve the stability of assembly quality. The comparison results of bolt parameters under different assembly fans are shown in Table 2. Figure 4 Tightening torque and axial preload force at different assembly levels Table 2 Comparison of bolt parameters under different assembly levels If the minimum axial preload (31kN) of the FN4 assembly meets the requirements of the coupling structure, the FN1 assembly can reduce the bolt size to M10 and the minimum axial preload force 33kN, as shown in Figure 4b. . In this case, due to the reduced bolt size, a single bolt can achieve a weight reduction of 66g. The coupling structure is equipped with 10 bolts, and the bicycle can achieve a weight reduction of 660g, and the cost can be reduced. 5 Conclusion This paper introduces the common assembly methods for automotive threaded fasteners, and introduces the technology of assembly grading. In actual tightening, the requirements of the actual joint structure can choose the most suitable method and process. The torque-turn angle method and assembly grading technology are adopted to improve the assembly reliability, improve the bolt utilization rate, reduce the bolt specifications under the condition of the pre-tightening force requirement, and make the joint structure compact, achieving weight reduction and lowering. Cost is of great importance in terms of vehicle weight reduction. Brass Fittings,Brass Valves,Other Brass Valves & Fittings Slion Industrial Co., Ltd. , http://www.cnbathtub.com
