A car manufacturer found two problems on the rear suspension bracket bolts of a model: 1. On some vehicles (not all) of this type of vehicle, the phenomenon that the bracket is worn due to the displacement is found. This relative displacement occurs on the bolt pair of the connecting bracket, which causes a gap in the bolt mounting hole on the bracket, and the screw portion of the bolt is necked. This happens when the bolt is applied with thread glue before tightening. A close look at the failed bolt reveals a definite fact: it is not loose due to the rotation of the bolt. 2. In the assembly, in some batches of bolts, although the tightening applied by the torque wrench with guaranteed torque accuracy, it was found that some of the bolt failures were when the torque method was first tightened. This failure bolt is like this. Background of the problem In the initial test and analysis of the assembly design, the total pre-tightening force of the bolt set was 105kN, and the bracket was not prevented from being displaced due to the load caused by the suspension. Four 8.8 grade M12 zinc galvanized bolts were used in the bracket and suspension connection design. In the design phase, the purpose of using thread glue is to increase the screw friction. The coefficient of friction of the thread is 0.2, and the coefficient of friction at the bottom of the screw head is 0.14. Calculated according to the above coefficient of friction, the pre-tightening force of the single bolt 34kN requires a torque of 90 Nm to tighten. After the four bolts are installed, the total preload of the bolt set should be 136kN, which meets the design requirements. However, because the bolt fails, it is obvious that the design of the assembly meets the design requirements. A re-evaluation is needed on this issue. After a rigorous and meticulous investigation, I found the following: 1. The coefficient of friction of the thread is between 0.14 and 0.25. 2. The friction coefficient of the bottom end of the screw head is between 0.10 and 0.18. 3, because the use of thread glue, will increase the coefficient of friction, resulting in additional torque of 7Nm. Root cause of the problem Finally, the engineers realized that the pain they experienced was caused by the discreteness of the coefficient of friction, which was negligently neglected during the design phase. The torque wrench for tightening this bolt set has a torque accuracy of 5%, and the torque of 85.5 Nm is the minimum torque required to tighten the four bolts. If this tightening torque is used, the maximum friction coefficient is brought in at the same time, and the additional torque due to the thread glue is taken in 7Nm. In the worst case, the bolt pre-tightening force is only 23.9kN, which is much smaller than the designed 34kN. . solution In order to solve the above problems, the engineers re-evaluated the relationship between the tightening torque and the bolt preload. In short, the engineers finally came to the conclusion that raising the bolt level would solve the problem. Moreover, the solution proposed by everyone is: 1) replacing the bolt of class 8.8 with the class of 10.9; 2) changing the flange structure partially to provide greater resistance to vibration and solving the problem of vibration induced relaxation, and Another benefit of the improvements is the ease of disassembly and reassembly during maintenance work. Engineers have done a lot of tests on the torque process and the final preload of this bolt set, and finally determined: 1. Use the minimum value in the friction coefficient interval. Taking into account the composite action of bolt tension and torsion stress, the torque is set at 110.5 Nm, and the bolt preload is guaranteed to be at the 90% bolt yield strength point. 2. Later, the torque value was reduced by 5% (105 Nm) to prevent the torque error of the torque wrench from bringing the bolt into the yielding section. This torque value is iteratively confirmed to ensure that under extreme conditions, the bolt will not fail after the initial tightening torque is tightened. 3. Using the torque value of 105Nm and the highest value of the friction coefficient interval, the bolt pre-tightening force is 30.4kN. After repeated designing, it is considered as the minimum bolt pre-tightening force under the worst assembly conditions. Distribution of 105Nm moment This bolt improvement scheme, which was redesigned and changed the fastening process, passed smoothly in the next series of tests and was adopted in the recalled vehicles. Since then, there have been no complaints about such problems.... The world of bolt engineers is too flat... Air Conditioner Copper Pipe,Seamless Copper Pipes,Flexible Copper Pipe,Air Conditioner Connecting Pipe Suzhou Green New Material Technology Co., Ltd. , https://www.glooptube.com
