The first step in using bolts is to design their strength grade and material performance based on the service environment. After installation, bolts can be reused in some scenarios; however, in practice, after repeated use, a bolt's preload or torque will decrease-a process known as "torque decay" or "preload loss" of bolts. This decline in strength performance is most common in automotive bolts. We have conducted long-term tracking of the usage of automotive bolts, and carried out detailed analysis and documentation of the causes and processes of strength degradation.
So, can the issue of strength degradation be resolved? Through experimental verification, we can effectively mitigate or resolve torque decay by further optimizing the bolt's heat treatment process, accurately matching the friction coefficient required for the target preload, determining a reasonable thread engagement length, and strictly controlling the precision of heat treatment temperature and time. Typically, after 2–3 years of use, the torque decay of automotive bolts may reach 20%. In the strength design of bolts, if the deviation between the actual preload torque and the design value exceeds 25%, the bolt is deemed unqualified and cannot be used safely. The key causes of the decline in bolt strength performance mainly include: first, improper thread engagement length (either too long or too short); second, insufficient bolt head size or unreasonable contact form, resulting in insufficient contact area with the connected parts and fluctuations in the friction coefficient during preloading; third, material performance stability of the bolts exceeding the expected range (such as uneven heat treatment or fluctuations in material composition); fourth, unreasonable design of supporting components (such as nuts and holes in connected parts).
Targeted rectification measures can be taken for the above problems. If degradation is caused by improper thread engagement length, the solution is relatively simple-just replace the bolt with one that has a thread engagement length meeting the design requirements. However, in actual procurement, the thread length of standard bolts sometimes does not fully match the design requirements. To save procurement time and costs, some enterprises ignore the issue of thread length deviation, assuming that "a slight excess or shortage will not affect use." In fact, quality should be prioritized: it is better to increase costs slightly and produce strictly in accordance with design drawings, rather than randomly replacing custom requirements with standard parts.
If insufficient bolt head size leads to inadequate contact area and thus unstable preloading, the bolt head size needs to be redesigned and verified through sufficient performance tests before it can be safely put into use. The size of the bolt head must be compatible with the installation environment: if a standard bolt cannot fit into the mounting hole due to its head size, the bolt's dimensional parameters need to be readjusted. After adjustments to size or performance, proper documentation should be made, recording the reasons for the adjustment and the first use time. This ensures that relevant personnel are aware of the bolt's status, and the inspection and replacement cycle should be shortened compared with standard bolts to enhance tracking and maintenance. In addition, fluctuations in the friction coefficient during preloading are not only related to the contact area, but also to factors such as the precision of preload torque control, nut type (e.g., whether it has a coating), and thread surface condition. To improve the stability of the friction coefficient, measures such as optimizing nut selection (e.g., using nuts with phosphated or coated surfaces) and controlling thread processing precision can be adopted, rather than solely relying on lock nuts (the core function of lock nuts is anti-loosening, not stabilizing the friction coefficient).
If strength degradation is caused by unstable material performance of the bolts, the issue can be resolved by upgrading the material, though this will increase costs slightly. Practitioners at Wuxi Fan'ao generally believe that it is better to increase upfront costs to avoid failures caused by material issues later. Specifically, bolts with higher performance grades can be used as replacements (ensuring compatibility in thread specifications, head size, connection form, etc., with the original design). For example, if performance fluctuations occur in the originally designed 6.8-grade bolts, 8.8-grade bolts can be used as replacements, provided that the installation space and load matching allow. Although the material cost of 8.8-grade bolts is slightly higher than that of 6.8-grade bolts, the frequency of later maintenance is reduced, and the increase in comprehensive use cost is limited.
The last cause (unreasonable design of supporting components) has nothing to do with the bolts themselves, so it is difficult for practitioners in the bolt industry to directly provide rectification suggestions. This requires the entire industrial chain to improve basic design precision and manufacturing standards-only when the design and processing precision of all links are improved overall can my country's basic industry achieve a qualitative leap.
Therefore, whether it is a small bolt or a complex product like an automobile, the entire industry needs to work together to continuously improve quality.
