Why is the tightened bolt loose and how to prevent it from loosening?

                        點擊數:2962021-05-13 09:06:43 

                        Hexagon bolt connection is a kind of fastener which is widely used in industrial production and life. It is used in transportation tools, airplanes, automobiles, ships, machines used in factories, electronic products used in life and so on. The main reason why the hexagon bolt is selected is that it is easy to disassemble. However, this feature will lead to the automatic loosening of the hexagon bolt in some cases, resulting in the loss of preloading capacity.

                        In the rapid work of parts, the loosening of hexagon bolt will lead to very serious consequences. If the hexagon bolt of the machine in operation suddenly loosens, the light will cause the whole factory to come to a standstill, and the economy will suffer losses, and the heavy will cause the workers to have potential safety hazards and safety accidents. For another example, in the running aircraft, cars, etc., the loose hexagon bolts will threaten the safety of people's lives.

                        In our development history, safety accidents caused by a small screw can be said to be everywhere.

                        Case 1

                        On June 10, 1990, during the flight of British Airways flight 5390, a windshield in the cockpit suddenly flew off, and the captain's upper body was sucked out of the plane. Fortunately, with the efforts of the vice captain, he landed safely at Southampton airport. The cause of the accident was that the windshield had just been replaced in the cockpit, and 84 of the 90 fixing screws were less than or shorter than the standard.

                        Case 2

                        At 18:35 on November 6, 2017, a female worker in a factory in Shenzhen was knocked down by another punch which suddenly collapsed nearby when she was operating the punch. After rescue, she died. The reason was that there was no anchor hexagon bolt installed on the base of the collapsed punch, but only one hexagon bolt was installed on the side, and it was loose during long-term operation. Finally, the center of gravity imbalance led to the collapse.

                        When the hexagon bolt is impacted or collided, why the tighter hexagon bolt will also loosen? Let's take a look at the working principle of the hexagon bolt first. It mainly uses the inclined plane circular rotation and friction of the object to enter the fastener parts step by step, and the tightness of the thread between the two objects is particularly important.

                        When the hexagon bolt is vibrated violently and for a long time, under the frequent vibration, the originally tightly occluded thread will produce tiny gap or even deformation, and the working state of the machine is mostly long-time and repetitive movement, when accumulated to a certain extent, it will lead to the loosening of the hexagon bolt. In addition to the loosening caused by severe vibration, the hexagon bolt will be loosened by embedding, gasket creep and thermal expansion, strong impact and so on.

                        Since the hexagon bolt will be loose for so many reasons, how can we effectively prevent the hexagon bolt from loosening? Here are some relaxation methods.

                        1. Add flat washer - increase the contact area, reduce the pressure, prevent loosening, but this method of disassembly is frequent, and is only limited to prevent relatively weak screws from loosening, and can not play a strong effect of preventing screws from loosening.

                        2. Self locking nut is self-locking by friction, which can prevent the relative rotation of nut and hexagon bolt, and play the role of preventing screw from loosening. But it is a high-end product with high cost.

                        3. Split pin is used to fix the screw, but in the process of mechanical movement, in this way, the pin may be damaged, the screw nut is easy to wear, the screw teeth are broken, the screw and the screw teeth are easy to slide, and the screw anti loosening performance is unstable.

                        4. Double nut - use tightening force to make double nut achieve anti loosening effect and increase weight. It is only limited to relatively weak looseness and can not play a strong role in preventing looseness. Improper use in practice can not play the role of anti loosening, and the anti loosening performance of screw is unstable.

                        5. Lockwire is one of the more traditional methods of anti loosing, which is widely used in aircraft related fields. However, in this method, the pin may be damaged, and the scope of use is limited.

                        6. U-shaped nut changes the shape of the nut, directly destroys the screw body of the nut, indirectly destroys the screw teeth of the hexagon bolt, resulting in the falling off of the hexagon bolt and the instability of the screw anti loosening performance.

                        7. Lock nut - at present, Aozhan mainly has two kinds: nylon lock nut and all metal lock nut. Nylon lock nut mainly hoops the hexagon bolt through the tension of nylon ring. The essence of metal lock nut is that the embedded steel plate is perpendicular to the nut axis and parallel to the nut end face, and the profile angle and pitch of the last thread are changed by the embedded steel plate, The elasticity of the steel sheet is used to prevent looseness.

                        The loosening of the nut is very important. In the case of no absolute guarantee that the nut will not loosen, regular maintenance and inspection is the best way to eliminate the hidden danger of safety in time.
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