Threaded fastener connections are one of the commonly used methods of automotive assembly, along with welding, riveting, and adhesive bonding. The purpose of threaded fastener connections is to tightly join the connected parts and withstand external forces. They offer advantages such as high precision, ease of assembly, and convenient disassembly of components.

Threaded connections experience four types of external forces: tensile stress, shear force, tensile stress + bending stress, and tensile stress + bending stress + shear stress. To ensure that threaded fastener connections can overcome various static and dynamic external forces acting on the connected components, they must generate an axial clamping force on the connected parts, which is provided by the applied torque. Bolts overcome friction and achieve elastic deformation based on the relationship between the bolt and the connected part's rigidity, thereby creating clamping force between the bolt's two contact surfaces. The correct tightening process involves controlling this clamping force, so the essence of assembly tightening is to control the axial clamping force of the bolt within the appropriate range.

During the assembly of threaded fastener connections, the variability of clamping force is influenced by the chosen assembly method and other external conditions. During assembly, the clamping force must be sufficient to ensure reliable mating of the connected components during operation, preventing relative movement. On the other hand, the applied clamping force must ensure that the bolt and connected parts do not experience damage or plastic deformation during assembly and service. Market research shows that the majority of problems in bolted connections, such as loosening, breakage, component indentation, and damage, are caused by inaccurate or incorrect clamping forces. Studies suggest that the greater the axial clamping force of the bolt, the better its resistance to loosening and fatigue, with the best results typically achieved when the bolt is tightened to yield.

Currently, in automotive assembly, due to production constraints and practicality, it's often not possible to accurately measure the clamping force for each bolted connection pair. Therefore, torque is used as an intermediate value to assess the assembly quality of bolted connections. For torque control, the precision of tightening tools and tightening methods are critical factors. In general, pneumatic tools have torque control precision ranging from 10% to 40%; manual torque wrenches have precision around 10%; and electric torque wrenches can achieve precision of 3% to 5%. The choice of tightening tools and torque control precision directly determines the assembly grade.

In the domestic automotive industry standard QC/T518-2013 for automotive threaded fasteners, the tightening accuracy is divided into three grades: Grade I, Grade II, and Grade III, each corresponding to specific torque tolerances and torque ratios as shown in Table 1.

Table 1: Torque Ratios Corresponding to Different Tightening Accuracy Grades

Taking an example of an M20X1.5X100 hex bolt with a friction coefficient of 0.12 for Grade 10.9, when using Grade III assembly, the target tightening torque is low, but the torque fluctuation range is high, resulting in the minimum clamping force on the threads. Increasing the assembly grade to Grade I results in a higher target tightening torque and a smaller torque fluctuation range, increasing the clamping force. Grade I assembly increases the target torque by 0.087% compared to Grade III assembly, resulting in a 32.5% increase in clamping force on the threads. According to foreign threaded fastener assembly standards, when the threaded connection grade is Grade III, using an M16 bolt meets the requirements, while choosing Grade I assembly allows for a reduction in bolt size to M12 when the clamping force requirement for the connection structure remains unchanged.

It is essential to classify the assembly of automotive threaded fasteners. When accepting orders, fastener suppliers should first consider the requirements of technical agreements. Compared to Grade III assembly, using Grade I assembly increases the bolt's target tightening torque, reduces torque fluctuation, and enhances clamping force. When the connection structure requires the same clamping force for the bolt, using Grade I assembly allows for a reduction in bolt size, achieving weight reduction and compact structural design.

Peak Fasten Technologies is a professional automotive fasteners supplier in China since 2010. We are dedicated into designing, producing and delivering high quality automotive bolts, nuts and rivets to global market with professional engineers in all processes including design, structure, production processes, quality control etc. Please feel free to contact us if there is any query or bottleneck problems for automotive fasteners at your side.