Summary:
In order to investigate the torque attenuation mechanism of high-strength galvanized bolts, torque attenuation control analysis was performed. Taking the high-strength galvanized bolt as the research object, the force analysis of the bolt is carried out to establish a mathematical model for determining the torque attenuation failure. For the soft connection assembly, the torque attenuation control method is used to reduce the tightening speed or step-by-step tightening, and the ATLAS electric tightening system is utilized. The bolts are tightened and tested by different assembly methods. The endurance test results show that the torque failure criterion can effectively judge the reliability of the assembly connection.
Therefore, in the actual assembly process of the soft connection, changing the tightening strategy is of great significance to ensure the reliability of the bolt connection.
Threaded fasteners are one of the most widely used connectors in the assembly process of the whole vehicle. The connection forms mainly include hard and soft connections. The connection method seems to be simple, but the tightening process is required to ensure the tightening quality.
Hard connection generally means that the hardness of the connected part is large, and the tightening state can be achieved by rotating the thread pair after reaching the bonding point and rotating within 30°. The hard connection generally has no torque attenuation; the soft connection means that the connected part itself is soft. Or there are elastic components such as shock-proof cotton, RTV sealant and sealing rubber ring. The threaded pair can reach the target point and rotate more than 270° to reach the target torque. The soft connection has severe torque attenuation.
This paper combines theoretical analysis and experimental research, based on the ATLAS electric tightening system, analyzes the obvious torque attenuation during the tightening process of assembly positions with special requirements (such as anti-shock cotton), and verifies the improvement measures of torque attenuation.
Therefore, the mathematical model of torque attenuation is established and tested according to the improvement measures. The test results have guiding significance for ensuring the reliable connection of bolts in the whole vehicle production process.
1. Establishment of torque attenuation model
1.1 Determination of axial preload
During the assembly process of the vehicle, an axial pre-tightening force is generated after the tightening torque is applied to the threaded fastener, and the axial pre-tightening force is required to ensure that the connected member is reliably fitted under the external load, and the thread is ensured. The fastener does not plastically deform, break, and is not crushed by the connector.
According to GBT16823.3-2010 fastener torque-clamping force test requirements, on the basis of considering the tightening accuracy m%, the tightening torque value is required to achieve the best pre-tightening effect, and the axial force is set as the guaranteed load. 75%, there are:
Where: SP—guaranteed stress (SP=0.9σs), MPa; β—thread elevation angle, β= 2°∙30; ds—thread equivalent diameter, mm; α—thread flank angle; μs—thread pair friction coefficient , 0.15; d2 - thread diameter, mm; P - pitch.
ψ It is related to thread friction coefficient and bolt related parameters. Table 1 shows the ψ values ​​corresponding to different friction coefficients.
1.2 Checking the preload force upper limit
For the torque control method, the tightening action is achieved by applying torque. The axial preload force generated by the upper limit of the torque is less than the axial force of the bolt and the joint is not collapsed; according to the importance of the assembly part, the precision of the tightening tool is controlled within 10%, ie m%=10% , the preload force upper limit is less than the yield preload:
1.3 Checking the preload force lower limit
The lower torque limit is to ensure that the axial preload of the transformation is greater than the total tensile force of the variable load transformation under the operating conditions and can be effectively and reliably fitted.
Threaded fasteners can be expressed as only lateral loads and axial loads under the action of alternating loads. The force deformation diagram is shown in Figure 1.
Figure 1 bolt deformation diagram
From the above, the formula for the total tensile force of the bolt is:
among them:
F—the total tensile force of the bolted joint; FPV—the axial force to which the bolt is subjected; FPH—the lateral force to which the bolt is subjected; Kf—reliability coefficient; m—the number of joint faces; f—the friction coefficient of the joint surface; C1— Stiffness of the bolt; C2—the stiffness of the coupled part.
For ordinary bolted joints, the strength conditions of the tight bolt joints that are only subjected to the preload force are:
In order to determine that the minimum value of the preload force lower limit is greater than the total tension force, take the tightening accuracy m%=10%, then:
For high-strength bolts used in important occasions, the safety factor is taken to a larger value; to meet the check, the minimum value is checked and calculated, then S=1.5. The minimum safety factor is considered, and the ratio results are shown in Table 2.
Table 2 Preload force lower limit value and total tension ratio
According to Table 2, the lower limit of the preload is greater than the total tension:
Within a certain range of friction coefficient, through the establishment of the mathematical model and the check analysis, it is known that it is feasible to set the pre-tightening force to 75% of the guaranteed load, and it is possible to maximize the bolt use efficiency.
1.4 Determination of torque failure criteria
Starting from the efficiency of bolt use, the bolt performance is greatly enhanced. It can be seen from Table 2 that the preload force lower limit value and the total tension ratio are approximately 1.2, then:
It can be seen from (14) that the total pulling force of the bolt under the alternating load is 75% of the target torque conversion preload, that is, the applied torque is converted into the pre-tightening force reduced to 75%, which cannot guarantee the effective connection.
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