The loose connection of threaded fasteners is one of the common problems. The most commonly used fasteners are loose due to vibration, high and low load changes and impact during the use of the parts, resulting in the reduction of equipment accuracy and safety and reliability and affecting the tire. quality and productivity. In order to make better use of threaded fastener connections, the following standard parts network shares the reasons for the loosening of threaded fastener connections and related structural measures commonly used to prevent loosening.
1. Reasons for loose connection of threaded fasteners
The essence of threaded fastener connection is to keep the connected parts together through axial force. When the bolt is tightened, the axial force attenuation is called loose threaded connection. The main reasons for its loosening are as follows.
1.1 Design flaws
(1) Improper selection of bolts In the process of bolt tightening, the clamping force between the connecting parts increases rapidly with the increase of the pre-tightening force, and plastic deformation begins to occur when the yield point is reached. At this time, the clamping force increases with the With the increase of the bolt pre-tightening force, the increase is small or even unchanged, and when the pre-tightening force is increased again, the clamping force gradually decreases until it breaks. Therefore, designers need to accurately analyze and calculate the tightening torque, and fully consider the weight of components, load bearing, safety standards and other factors, and select the appropriate bolt connection.
(2) The anti-loosening design or the poor anti-loosening method are not considered. During the use of the bolted connection, the connection is loosened or the bolts fall off due to vibration, high and low load changes and shocks. Therefore, effective anti-loosening measures must be implemented in product design to avoid loosening of bolts or nuts due to harsh working conditions.
1.2 Insufficient preload
The pre-tightening force of bolt tightening directly determines the clamping force between the two connecting parts. Insufficient pre-tightening force will inevitably lead to the loosening of the connecting bolts and the loosening of the connecting parts. The bolt preload should approach or reach the yield strength in the bolt material. However, in the actual assembly process, due to the limited arm force of the operator or the mismatch of the selected tool type, the output tightening torque is insufficient, and the bolt cannot reach the required preload force.
The design and process do not put forward specific requirements on the torque of tightening bolts, and there is no corresponding torque wrench at the assembly station. When tightening, the operator often judges whether to tighten by feeling and experience, resulting in insufficient pre-tightening force of the bolts in some parts with large vibrations and loosening. .
1.3 Deformation and looseness of the bearing surface
When the bearing surface of the nut or bolt is under great pressure, the bearing surface of the nut or bolt and the contact surface of the connected parts will be indented and deformed, resulting in the reduction or even loss of the pre-tightening force of the threaded fastener, resulting in loose connection.
1.4 Improper assembly process
For the regularly distributed tightening of multiple bolts, the assembly process does not formulate a reasonable tightening process document, and the operator performs the tightening operation completely based on personal experience, resulting in some bolts due to the wrong tightening sequence and unbalanced force. loose. For example, for the installation bolts with square distribution in the assembly process, the method of diagonally tightening is generally adopted to ensure that the force of the bolts is balanced as much as possible, otherwise the bolts will be loosened, and even the connecting parts will be deformed due to uneven force. .
1.5 Defects in processing quality
The dimensional accuracy of threaded holes or bolt holes is particularly important when parts are connected, and the size of the thread directly affects the size of the preload obtained by the bolt. If the size of the bolt mounting hole is too small, it will be difficult to assemble, and if the size is too large, the contact between the surface of the part and the bearing surface of the bolt or nut will cause indentation deformation, resulting in loosening of the bolt or nut.
zj-junyue.com