introduction
As a component for threaded joints, bolts are the standard components used in the most common mechanical joints and are widely used in various mechanical assemblies. Assembly parts assembled by bolts are generally required to be tightened before they can work. As the most important bolt specification and pre-tightening force in the assembly process, we rarely carry out the standard design. Reasonable matching can maximize the effect of bolts in the assembly and maximize the service life of the assembly. Whether it is a bolt or a sub-part that needs to be assembled, there is a certain yield limit. During the assembly process, if the pre-tightening force is too large, the deformation of the part exceeds the yield limit of the part, and the part will be damaged. Therefore, the assembly must be stable and effective for a long time, and the designer must design the bolt preload force.
1. Selection of bolt pre-tightening force
Bolts are important couplings. They must be tightened when the assembly is installed. Before the connection is subjected to the working load, the force is pre-stressed. The pre-stress is the pre-tightening force. The purpose of the pre-tightening is to enhance the reliability of the connection. And tightness, to prevent the assembly from being subjected to force when working, and there is a gap or relative slip between the joints. Therefore, in the design of the assembly, the size of the preload must be standardized.
1.1 Reasonable selection of preload
In the professional bolt and fastener assembly, there are generally standard wrenches. Different diameter bolts use different lengths of wrenches. The length of the wrench is about 15 times the bolt diameter. On this basis, professional mechanical tools can be used. Reflecting the exact tightening torque and achieving a quantifiable preload is especially important for some key components and important components. Once a small size bolt is used to tighten a small size bolt, it tends to cause over-tightening, which destroys the part itself and disables the entire joint structure.
When the nut is tightened, two or more parts are pressed and the part itself is compressed, just like the compression deformation of the spring. The contact surface part between the nut and the bolt and the fitting itself generates a large force. The force causes the bolt to undergo tensile deformation, which is calculated to be 1.3 times the simple axial tensile force.
When the tensile stress generated by the bolt exceeds the strength limit of the material, the bolt is broken. The tightening of the bolts only by the operator's experience is very unscientific for mass-produced products. When tightening small bolts with long wrenches, you should pay more attention to the preload force to avoid excessive preloading. When using a standard wrench, the strength can refer to the following table:
1.2 Torque values ​​of common specification bolts
Table 2 lists the tightening torque values ​​for the different performance grades of some common specification bolts. [2]
For the designer, how much pre-tightening force is required at the joint to achieve the working requirements of the part and no more than the safety stress of the bolt, which requires calculating the minimum stress required at the point. Use the appropriate bolt fasteners. The preload force applied to the bolt fastener depends on the yield limit of the bolt fastener, and the lower limit value depends on the minimum preload force required to meet the work requirements [3].
2. Reasonable selection and design calculation of bolt specifications [4]
2.1 The importance of selection
Assembly is an important part of the realization of assembly parts, especially for the production of large assembly parts. The connection between parts is usually realized by bolts, especially the bolts at key parts. The connection quality determines the reliability of the assembly. . Since the value of the pre-tightening force applied by the bolt affects the quality of the threaded connection, it is necessary to select the appropriate bolt specification on the premise of ensuring the assembly with the appropriate pre-tightening force.
For fasteners such as bolts and nuts, their performance parameters are varied within a certain range, so there is basically a reference value, so it is calculated whether the bolt specifications of the fastening joints are working in performance. The strength requirements must be considered by the designer.
2.2 Calculation method
In the following, only the connection of the frame of a group of cylinders in the CNG supply system on the frame is taken as an example, and a simple calculation method of whether the bolts are selected is briefly introduced. As a supply system, the CNG system should include a gas cylinder and a fixed frame. For the N3 series of vehicles, according to the ECE R110 regulations, the entire fixture must ensure that the direction of motion is 6.6 g 6.6 times the acceleration and the direction of motion is perpendicular to the horizontal direction of 5 g. The fixture does not allow destructive displacement. In order to select the appropriate bolt fastener specifications, the following steps can be used to calculate the design: device parameters: single cylinder 145L, single weight 125kg, full weight 155kg, cylinder bracket weight 315 kg, a total of eight cylinders, with The 24*M14 bolts are fixed to the frame.
2.2.1 Calculate the weight of the entire frame (including gas cylinders filled with gas)
G=155×8+315=1555 Kg
2.2.2 Calculating the external load
Т=KG/F (Equation 1)=20×1555/0.2=155500 Kg
Where: K is the acceleration value; up to 20.
F is the friction coefficient ("connecting plate - frame") taking the conventional value F = 0.2
2.2.3 Calculating the load of a threaded fastening force
Р=1.2T/h (Equation 2)=1.2×155500/24=7775 Kg
Where: 1.2 is the efficiency of the pre-tightening force
h is the number of threaded connections (bolts), h=24
2.2.4 Calculating the cross-sectional area of ​​the bolt rod
Fc=π(d/2)2 (Equation 3)=3.14×(0.0125/2)2=0.00012265625 m2
Where d is the cross-sectional diameter of the M14×1.5 screw, d=14-1.5=12.5mm=0.0125m
2.2.5 Calculating the tensile stress of the bolt rod
Σc= P/(FC) (Formula 4)=7775/0.00012265625=63.39×106 Kg/m2
2.2.6 Calculating the tightening force of the bolt
Mk≈Pd0k1 (Equation 5)≈7775×0.014×0.12≈13.062 Kg·m
D0 is the outer diameter of the bolt, d0 = 12mm, and K1 is the efficiency (including the angle of the spiral and the friction of the thread). Generally K1=0.12
2.2.7 Calculating the stress tangent of the bolt rod
The stress of the 10.9 class M14 bolt is 900 Mpa, which is equal to 91.83×106 (Kg/m3).
According to the above calculation procedure, the connection safety factor at this point can be calculated to be 1.21. If the coefficient is less than 1, it is necessary to select a bolt with a higher strength and a higher yield strength for matching.
3, the conclusion
In industrial production, the use of fasteners such as bolts is becoming more and more extensive. How to ensure the safety and reliability of batch industrial products is very important. In the design process, the exact bolt size and pre-tightening force can be used to improve the assembly quality of the bolted joint assembly and reduce the loss caused by product quality problems. For the calculation and control method of bolt pre-tightening force, it requires rich experience, but also needs careful study and careful analysis. According to the calculation of tightening torque, the collection of experimental data, simulation analysis and other methods, choose the most suitable bolt. Specifications and corresponding preload. In the actual production process, in addition to the necessary design calculations, there are more and more complex factors to be considered, which is mainly determined according to the characteristics of the products in various industries and the actual conditions of the process.
references
[1] Fastener Standard Implementation Guide. China Standard Press.
[2] GB/T3098.1-2000 fastener mechanical properties bolts, screws and studs.
[3] GB/T16823.2-1997 General rules for fastening threaded fasteners.
[4] Xu Wei. New Mechanical Design Handbook [M]. Beijing: Mechanical Design Press, 1995.
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