The connection between the lower joint and the crank pin box 1 failure analysis can be seen from the failure of the bolt (a, b). In this group of bolts, the double nut anti-loose configuration is the thin nut on the outside of the thick nut, so the bolt The load (including the impact load) is mainly borne by the thin nut, so the probability of failure is much greater than that of the thick nut. The thin nut fails. When the thick nut is loose, the connecting rod force will be fully borne by the bolt. When the thick nut continues to loosen, the bolt not only has to withstand all the connecting rod tension, but also bears a certain bending moment. The result is pulling Under the action of shearing external force, the bolt fails to break. After the side of the lower joint of the connecting rod fails (the connecting bolt is broken), the other connecting rod is bent. Bolt failure indicates that bolts and nuts are loosened and the pre-tightening force is controlled by bolted parts. In order to ensure reliable connection and prevent looseness, anti-loose devices such as spring washers, split pins and double nuts are installed. The most widely used anti-loose structure on large equipment is the double nut. In this anti-loose structure, only the nut has a thin thickness, and the most common installation sequence is a thick nut on the inner side and a thin nut on the outer side. The order of assembly often results in an excessive preload, which makes the anti-loose performance insufficient. This is because during the installation process, when the thin nut is tightened with the spanner wrench, it is actually rotated together with the thick nut, but the pre-tightening force is increased, and the anti-loose effect is not achieved. More important 1D connector; D bolt; 3D nut 1; 4D nut 3 double nut anti-loose structure, from the force analysis point of view, this assembly sequence is also inadequate. Tighten the nut. When the lower end plane of the nut is in contact with the coupling member, the bolt and the nut generate an interaction force. When the nut is tightened to a certain extent according to the pre-tightening force, the spiral of the bolt thread faces the corresponding helicoid of the nut. Apply force F1. The nut 1 is tightened, and when the lower end plane of the nut 1 is in contact with the upper surface of the nut, the bolt and the nut 1 generate an interaction force. When the nut 1 is tightened to a certain extent according to the requirement of the back tightening force, the screw thread has a force F corresponding to the corresponding spiral surface of the nut. If F1=F, the force acting on the upper end surface and the lower end surface of the nut is equal. In the opposite direction, the nut only acts as a washer. At this time, the stress on the bolt is concentrated on the section below the lower end face of the nut, that is, the section where the bolt fails to break. If F>F1, the stress concentration of the bolt will be transferred to the plane between the nuts as the difference of F to F1 increases, which is also the most dangerous section of bolt failure. At this time, the nut 1 bears the external load of the connecting member, and the nut 1 is a thin nut with limited strength. If the nut 1 fails, the loosening effect is lost. Therefore, installing the thin nut on the outer side of the thick nut in the assembly is Not proper. This is also the cause of the above bolt failure. In conclusion, in the oil mine machinery, for equipment that is subjected to alternating load, if it is encountered to prevent loosening of the nut, it is recommended to lengthen the bolt by a small part when the equipment space permits. The thick nut anti-loose structure is changed to double thick nut, which is a safer and more reliable anti-loose solution.
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