Why do bolts and nuts have lower strength with increasing use?
The primary task before using bolts and nuts is to design their strength and material properties based on the specific usage environment. Normally, bolts and nuts can be reused after installation. However, in reality, when a bolt is repeatedly used, its strength will decrease, a phenomenon known as bolt torque decay. This gradual decrease in strength is particularly common in the use of automotive bolts and nuts. We have been tracking the usage of automotive bolts and nuts for a long time and have conducted detailed analysis and recording of the causes and processes of strength attenuation.
So, can we solve the problem of bolt strength attenuation? Through experiments, we have found that we can indeed further optimize the material properties of bolts and nuts through heat treatment. Accurately calculating the maximum friction coefficient and thread engagement length and controlling the heat treatment time and temperature within a certain range can effectively solve the problem of bolt torque attenuation.
Generally speaking, after two to three years of use, we will find that the attenuation amplitude of car bolts and nuts can reach 20%. For the strength design of bolts and nuts, if the fluctuation range of torque parameters exceeds 25%, the bolt is considered a defective product and cannot be put into use. The main reasons for the decrease in bolt strength are as follows.
Firstly, the thread locking length is unreasonable, either too long or too short.
Secondly, the diameter of the bolt head is too small, and the contact area has not reached a reasonable range, resulting in an unstable friction coefficient.
Thirdly, the material properties of the bolt itself have changed beyond the estimated range.
The fourth issue is the unreasonable design of accessories.
We can find corresponding corrective measures to address these issues. If it is due to the length of the thread, the solution is relatively simple, just replace the thread with a suitable length. But sometimes, the standard range of bolts and nuts is difficult to fully match the design parameters.
To save procurement time and costs, people may overlook the issue of thread length, thinking that being longer or shorter is not a big deal. In fact, we should fully consider the importance of quality. We would rather increase the cost than strictly produce according to the design drawings, and cannot use standard parts to replace them at will.
If the friction strength is insufficient due to the small diameter of the bolt head, a redesign of the drawing is required. Moreover, it must undergo extensive experimental verification to ensure accuracy before it can be used with confidence. The diameter of the bolt head is closely related to the installation environment. If the standard bolt cannot be inserted into the corresponding hole, the relevant dimensions of the bolt can only be redesigned. Once the size and performance change, it is necessary to file a record, detailing the reasons for the size change and the time of first use. For such bolts and nuts, relevant personnel should have a clear understanding, track and observe their changes at any time, and adjust the replacement time of the bolts and nuts accordingly, which is usually shorter than the replacement time in conventional environments.
In addition, the variation of friction coefficient is not only related to the size of the contact area, but also to the force of thread locking and the type and standard of nut fit. To compensate for the low friction coefficient of bolts and nuts, we can improve the nut fit, such as using locking nuts to match with these types of bolts and nuts.
If the material properties of bolts and nuts are unstable, the solution is relatively clear, but it requires an increase in cost. Of course, every enterprise has its own approach, and we generally choose to increase costs rather than face these unstable factors in the later stage. Why can increasing costs solve the problem of unstable bolt material performance? We can choose bolts and nuts with higher material grades. If conditions permit, if the original designed bolt is grade 6.8 and it is found that the performance of this grade bolt is unstable, then it can be replaced with a grade 8.8 bolt. Although the price of grade 8.8 bolt material is slightly higher than grade 6.8, the cost of use and maintenance will be significantly reduced.
After comprehensive calculation, the final application cost actually does not increase much.
The last reason, which is the unreasonable design of the accessories, is actually not directly related to the bolts and nuts themselves. As professionals in the bolt industry, it is difficult to provide specific recommendations in this regard. This involves the improvement of the basic level. Only when the accuracy and design level of each link are improved as a whole, can the basic industry of the entire society make significant progress. So, whether it's a small bolt or a large product like a car, we need to work together to constantly improve its quality to a higher level.
In the world of mechanics, although bolts and nuts are small, they play an indispensable role. From towering bridge buildings to everyday household appliances that are within reach, bolts and nuts are ubiquitous. But have you ever thought about how many times this small bolt can be used? The answer to this question is much more complex than it appears on the surface.
Main reasons: High strength bolts and nuts are prone to thread damage and surface treatment wear during use, which affects the torque coefficient (pre tension) and coefficient of variation of the bolt, thereby affecting the axial tension of the bolt from reaching the standard range. Additionally, repeated high torque operations may break the bolt or cause severe torsional shear deformation, posing a high risk when reused.
The following are torsional shear type high-strength bolts and nuts and high-strength large hexagon head bolts and nuts. Regarding whether high-strength bolts and nuts are used for steel structures, please refer to previous articles as follows. For high-strength bolt applications, they can be divided into shear connections and tensile connections. Generally, in shear connections, the bolt only bears the initial pre tension force, and the connected parts have high pressure between the contact surfaces under the action of the initial pre tension force, which can generate considerable frictional force when subjected to external forces.
When high-strength bolts and nuts are used for tensile connections, they generally have to withstand external working loads or dynamic fatigue loads in addition to the initial preload force. In order to ensure relatively high fatigue strength and high load-bearing capacity, high-strength bolts and nuts generally require a large pre tightening force, which is usually achieved by tightening with this pre-tightening torque method.
The torque method requires a large amount of torque to be applied for tightening, and the accuracy of the pretension highly depends on the stability of the friction coefficient. High-strength bolt connection pairs generally require use in the same batch and cannot be mixed. The bolts and nuts and washers are all matched and cannot be used randomly.
Due to the high tightening torque during a single use, both the thread profile and the nut contact surface on the threaded pair experience high-intensity friction, resulting in significant changes in the surface roughness of the thread contact surface and nut contact surface. Therefore, the friction coefficient also undergoes significant changes. When tightened with the same torque, it may generate greater tensile force and potentially break the bolt.
High strength bolts and nuts that have been used for a long time may have rusting at the threaded connection, so a large torque may be required to loosen the nut during disassembly. High torque may break the bolt or cause severe torsional shear deformation, which poses a high risk when reused. High strength bolts and nuts are generally used in important connection positions, so risks should be avoided.
Therefore, it cannot be reused.
Domestic norms and standards
The "Code for Acceptance of Construction Quality of Steel Structures" (GB 50205-2020) explicitly states that high-strength bolt connections should not be reused. This standard has strict requirements for the use of high-strength bolts and nuts in the construction field, stipulating that the pre tightening force of high-strength bolt connections will decrease due to plastic deformation after installation, and therefore should not be reused. GB/T 1228-2006 "High Strength Hexagonal Head Bolt Connections for Steel Structures": This standard also states that high-strength bolt connections should not be used again after installation, mainly because the bolts and nuts will undergo certain plastic deformation during their first use, and their fastening performance may not be guaranteed when used again.
The characteristic of using large hexagonal high-strength bolts and nuts is that they can only be used once. Generally, the design of large hexagonal high-strength bolts and nuts requires the pretension of the bolts and nuts, and most of them are constructed using the torque method. Therefore, a torque coefficient test is required.
Based on the results of the torque coefficient test and the pretension force, the torque of the construction is calculated. The torque coefficient test adopts the method of sampling bolts and nuts from the same batch, which requires a small difference in bolt torque coefficient, so that the torque coefficient of the sampling test is meaningful.
At present, phosphating and saponification processes are commonly used for surface treatment of large hexagonal bolts and nuts in China. This process can ensure stable torque coefficient and small dispersion of bolts and nuts. After using the torque method for construction, the pretension dispersion of bolts and nuts is also small.
The surface of bolts and nuts treated with phosphating and saponification will form a large number of micropores containing saponification solution. After the first tightening, the microporous structure will be destroyed, and the saponification solution will precipitate to lubricate the contact surface, making the torque coefficient relatively stable and with small dispersion.
If the second time is used, the torque coefficient dispersion will significantly increase, and tightening with torque method cannot guarantee the accuracy of pretension.
In addition, I would like to mention the requirement for a threaded buckle on the outside of the bolt. High strength bolts and nuts should have 2-3 exposed threads, with 10% of the bolt threads allowed to have 1 or 4 exposed threads.
Above, we are talking about high-strength bolts and nuts used in steel structures in the construction industry. How are they regulated in other industries?
JG/T5057.40-1995 "Technical Conditions for High Strength Fasteners of Construction Machinery and Equipment" A7 stipulates about repeated use: "High strength bolts and nuts and nuts should be disassembled and reused no more than twice after use. The removed bolts and nuts and nuts must be free of any damage, deformation, slipping, missing teeth, rust, significant changes in thread roughness, etc. Otherwise, they should be prohibited from being used again for connecting load-bearing components.
