Tightening torque and breaking torque are two distinct concepts. Tightening torque refers to the recommended value for screwing the bolt into the workpiece, while breaking torque (or breaking moment) is the minimum value at which the screw breaks. Clearly, the tightening torque is less than the breaking torque.
Standard for Metric Bolt Tightening Torque: Q/STB 12.521.5-2000Scope: This standard is applicable to bolts with a mechanical performance of grade 10.9 and specifications ranging from M6 to M39. It does not apply to bolts using nylon washers, sealing washers, or other non-metallic washers.
Note: For designs with specific torque requirements on drawings, the torque should be executed according to the drawing.
High-quality bolts, as connecting components, are widely used in environments with high strength and stress, such as locomotives, aerospace, and wind power units. In passenger vehicles, most of the main components operate in low-stress cycles, but significant risks still exist. From a safety perspective, the components connected by bolts are often expensive. Therefore, when bolts fail, it damages not only the bolts themselves but the entire product.
Whether threaded fasteners serve as connectors or sealants or need to be assembled with sub-parts, they have a certain yield limit. During assembly, if the preload force is too large, causing the deformation of the parts to exceed the yield strength of the parts, the parts will be damaged. To ensure that the assembled components work stably and effectively for a long time, designers must make standardized designs for bolt preload forces.
In professional bolt fastening assemblies, standard wrenches are generally provided, with different diameter bolts requiring wrenches of different lengths. The length of the wrench is about 15 times the diameter of the bolt. Using professional mechanical tools on this basis can reflect accurate tightening torque, achieving quantified preload forces, which is particularly important for critical and important components. Using a large-sized long wrench to tighten small-sized bolts often leads to over-tightening, damaging the parts themselves and causing the entire connected component to fail.
When tightening the nut, two or more parts are compressed, and the parts themselves are compressed, similar to the compression deformation of a spring. The contact surface force between the nut and the high-quality bolt and the assembled part will generate a considerable force. This force will cause the bolt to undergo tensile deformation. Through calculation, this stress is 1.3 times the simple axial tension, and when the tensile stress generated by the bolt exceeds the material's ultimate strength, the bolt will be pulled apart. Simply relying on the operator's experience for bolt fastening is not scientific for mass-produced products. When tightening small bolts with a large-sized long wrench, attention should be paid to the magnitude of the preload force to avoid excessive pre-tightening.
The assembly of locomotives, machinery, and automobiles is a crucial step in producing assemblies, especially large assembly components. The connection between components is usually achieved through bolts, especially high-quality bolts in critical positions. The quality of the connection determines the reliability of the entire assembly. Because the numerical value of the preload force applied to the bolt affects the connection quality of the thread, it is crucial to use appropriate bolt specifications while ensuring the use of suitable preload forces. For threaded fasteners, their performance parameters vary within a certain range, and there are reference values, so calculating whether the bolt specifications at the fastening connection meet the strength requirements is something that designers must consider.
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