1. Torque tightening method of fasteners
In automobile assembly, threaded connections account for 31% of the work of automobile assembly. The quality of threaded fastener assembly will directly affect the assembly quality and driving reliability of the vehicle. To this end, before applying external load, it is necessary to tighten threaded fasteners to tighten the connected parts. Tightening threaded fasteners is pre-tightening, and this force is called axial pre-tightening force. To ensure the reliable service of the bolts, it is necessary to ensure proper axial clamping force during assembly. In the process of part assembly, even if the same bolts and nuts are used and assembled by the same assembly process, the preload force will still be different. At present, the most common and economical tightening method for threaded parts is still torque tightening. The most economical and feasible method in assembly process is to indirectly control the axial clamping force by controlling the torque.
Torque is a force that produces rotational motion. The torque acts on the threaded fastener, and it rotates, and the rotation can cause the joint parts to be subjected to the clamping force through the thread. This initial clamping force is the required pre-tightening force.
Since the fastener assembly is mainly in the elastic strain stage of the material, the fastener and the connected parts can be considered as elastic bodies in the force model of the fastener assembly. Before the fastener is tightened, all the "elastic bodies" are in a free state. After tightening, the tension of the bolt makes the spring stretch, and the connected parts are compressed due to the pressure, so that the entire connection system can be tightened by torque to the minimum required. preload.
2. Classification of fastener connections
1) Classification principle of fastener connection:
According to the degree of influence of the importance of the safety, regulation and function of the automobile, the danger that may occur when the automobile fastener connection fails is defined as 3 risk levels. Different risk levels have different requirements for the assembly and documentation of fasteners and installation tools. The specific risk levels are classified as follows:
A. High risk level: When the fastener fails, it will cause direct or indirect physical and life danger or operational danger;
B. Medium risk level: When the fastener fails, it will cause the vehicle to malfunction;
C. Low risk level: When the fastener fails, the driver and passenger feel uncomfortable.
Among them, high-risk level A is mainly used in important connection applications such as braking system, safety system, powertrain, steering, and fuel system; medium-risk level B is mainly used in automobile chassis, lower body, opening and closing parts, electrical appliances, etc. Important occasions; low risk level C is usually used in interior and exterior trims, body accessories, etc.
2) Fastener connection pair division
There are 3 kinds of fastener connection pair states: hard connection, soft connection and joint connection. The ISO5393 standard specifies the following for hard connection and soft connection:
A. Hard connection: refers to the connection with relatively high hardness or rigid connection. Generally, the torque attenuation of this connection is very small after assembly, and even the static torque will be greater than the dynamic torque of the assembly. After the fastener reaches the fitting point during assembly, it is generally rotated 30 °The target torque is reached.
B. Soft connection: Refers to the connection piece itself is relatively soft or there is elastic material between the connection pieces. After this connection is assembled, the torque is attenuated greatly. After this connection and assembly 2 reaches the fitting point, it is generally rotated more than 720° to achieve the goal. torque.
C. In daily use, most fasteners are connected between hard connections and soft connections, also known as joint connections.
3) Fastener assembly space requirements
Fastener assembly is driven by the assembly tool through a special sleeve or gun head to achieve the designed torque value. In the design process of fastener products, it is necessary to consider that there should be a certain distance from the surrounding parts during the assembly process, and there should be no interference with other parts, and it is easy to be operated by the assembly workers on the production site.
4) Fastener assembly boundary design requirements
When designing fasteners, the tool installation space should be considered, and the following situations should not occur:
1. The location of the fastening point is in a closed space, and the operator cannot see it. In this case, it is difficult to locate tools and fasteners, and it is difficult to remove the fasteners after they are dropped.
2. There are other parts above the fastening point (interference in the axial direction), there may be a small space in the final installation position, but the length of the fastener needs to be considered, and the tool in the initial state interferes with other parts above.
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