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Hex Bolt Failure Analysis: Common Causes & Fixes
In numerous fields such as mechanical manufacturing, construction engineering, and the automotive industry, hexagonal bolts are undoubtedly the "connection heroes". It may seem insignificant, but it plays a crucial role in fixing components and transmitting loads. Once it fails, the consequences could range from equipment shutdown to safety accidents, with unimaginable consequences.
I. Review of the "Culprits" Behind the Failure of Hexagonal Bolts
The failure of hexagonal bolts is not accidental but the result of the combined effect of multiple factors. From design, material selection to installation and use, every link may have hidden dangers.
(1) Design Flaws: The "root cause" of Inherent deficiencies
Omissions in the design process are an important source of bolt failure. If the actual working load is not fully considered during the design process, for instance, ordinary bolts are selected on equipment with frequent vibrations without the installation of anti-loosening devices, the bolts are prone to gradually loosen under alternating stress and eventually break. Unreasonable thread design can also cause problems. Both too large and too small pitches will affect the connection strength. A pitch that is too large is prone to cause the thread to slip, while a pitch that is too small will increase the installation difficulty and the stress concentration phenomenon will be more obvious. In addition, if the stress concentration area of the bolt is improperly designed, such as the transition fillet radius between the bolt head and the bolt being too small (usually R≥0.15d, where d is the bolt diameter), this part will become a "hotspot" for stress concentration. Under the combined stress of tensile and bending, it is very easy for cracks to initiate and propagate.
Taking the connecting rod bolt of a certain diesel engine as an example, its design did not fully consider the impact load in the high-strength working environment of the diesel engine. After the connecting rod bolt was reused, it was stretched due to excessive force, resulting in insufficient tightening force of the bearing bush and eventually causing the bearing bush rolling and burning accident. This is a typical failure caused by design flaws. If optimization for impact loads is carried out during the design stage and high-strength and disposable connecting rod bolts are selected, such problems can be effectively avoided.
(2) Material issues: The hidden danger of "weak constitution"
The quality of the bolt material directly determines its service life. If there are cracks, inclusions and other defects inside the material, it will significantly reduce the strength and durability of the bolt. When subjected to load, these defect areas are prone to become crack sources, causing fracture. In addition, the mismatch between the strength grade of the material and the application scenario is also a common problem. For instance, if low-strength grade bolts are used in parts that require high-strength connections, the bolts may deform or break due to their inability to withstand excessive loads.
The 12.9 grade hex socket cylindrical bolt used in a certain medical device, made of 42CrMo, was supposed to have high strength performance. However, due to contact with acidic media during the production process, hydrogen seeped into the metal interior, causing hydrogen embrittlement fracture. Hydrogen embrittlement fracture is a common failure form of fasteners, especially for 12.9 grade high-strength bolts. Its hydrogen embrittlement sensitivity is much higher than that of 10.9 grade bolts. Under static stress lower than the material's yield strength, it may suddenly break, which is extremely destructive. This case warns us that the selection of materials should not only focus on the strength grade but also take into account their tolerance in specific environments.
(3) Improper installation: "Mistakes" in operation after birth
Improper operation during the installation process is a common cause of the failure of hexagonal bolts. Problems will arise when the tightening torque is too large or too small during installation. If the tightening torque is too large and exceeds the yield limit of the bolt material, the bolt will undergo yield deformation and break due to excessive elongation under impact loads. If the tightening torque is too small, it will lead to insufficient preload, loose connection, and the bolt is prone to fall off under vibration and other working conditions.
All the 10.9 grade hexagonal head bolts broke at the thread during assembly. After analysis, it was found that the excessive torsional force during assembly was the cause. The heat treatment process and material of this bolt both met the requirements. However, a force-limiting wrench was not used during installation, and the operator tightened it merely based on experience. As a result, the torsional stress the bolt was subjected to exceeded its limit, eventually leading to ductile torsional fracture. In addition, improper fit between bolts and nuts during installation, such as forcibly screwing in misaligned threads, can cause thread damage and lead to stripped threads and failure. If the bolts are not tightened in a symmetrical cross sequence, the components will be subjected to uneven force, and some bolts may fail due to excessive load.
(4) Usage environment: The test of "external erosion"
The usage environment of bolts has a huge impact on their service life. In damp and corrosive environments, bolts are prone to corrosion. For instance, in chemical workshops and equipment in coastal areas, bolts that are in long-term contact with water vapor, corrosive gases or liquids will gradually rust, their mechanical properties will decline, and they may even break. The S32750 duplex stainless steel hexagonal head bolts used in the water pumps of a certain enterprise suffered corrosion failure after two years of operation. After analysis, it was found that the medium in contact with the bolts contained dissolved oxygen and salts such as sulfides and chlorides, and there were harmful σ phases in the microstructure of the bolts, which led to the depletion of chromium elements in the adjacent matrix, reduced corrosion resistance, and ultimately pitting failure.
In addition to corrosion, high and low temperature environments can also affect the performance of bolts. At high temperatures, the strength of bolt materials will decline, and oxidation and creep may also occur. At low temperatures, materials become brittle, their toughness decreases, and they are more prone to breakage when subjected to loads. In a vibrating environment, bolts are constantly subjected to alternating stress, which can easily lead to fatigue fracture. For instance, in a certain stage truss, some M20 steel structure large hexagon head bolts experienced hydrogen-induced brittle cracking under prolonged vibration, causing the nuts to fall off. Fortunately, timely maintenance was carried out and no serious accident occurred.
Ii. "Diagnosis and Treatment Plan for Hexagonal Bolt Failure"
When hexagonal bolts fail, we should adopt targeted repair methods based on different failure forms, and at the same time, do a good job in prevention to avoid the recurrence of the problem.
(1) Repair methods for different failure forms
Bolt fracture: Precise disassembly and replacement. Bolt fracture is the most serious form of failure. When repairing, the first step is to remove the broken bolt. If the head of the bolt is still exposed, you can try to unscrew it with a suitable wrench. If the head is worn or deformed, a hacksaw can be used to cut off the head, and then the threads can be cleaned with a tap. For bolts that are broken inside, a bolt remover can be used. Insert it into the bolt break and rotate it to remove the broken part. If none of the above methods work, electrical discharge machining can be adopted to drill holes at the bolt fracture and then remove them with a tap.
When replacing new bolts, ensure that the material, specification and strength of the new bolts are consistent with those of the original bolts. Tighten them in accordance with the specified torque and symmetrical cross sequence to ensure uniform force distribution on the components. For instance, when replacing the connecting rod bolts of a diesel engine, disposable bolts of the same grade must be used and tightened strictly in accordance with the standard torque to prevent further breakage accidents.
2. Thread slippage: When there are minor scratches or wear on the thread, taps and dies can be used for repair. The damaged thread can be re-tapped to restore its connection function. If the thread is severely stripped and cannot be repaired, it is advisable to consider increasing the thread specification, for instance, changing the original M10 thread to M12, re-tapping and then installing the corresponding bolts. It is also possible to install threaded bushings in the bolt holes and then install bolts of the original specification. This method can effectively restore the connection strength.
The connection structure between the bolts of a certain aluminum alloy subframe and the aluminum internal thread has a problem of thread slippage and disengagement. After analysis, it was found that the meshing length did not meet the requirements, and the strength of the external thread was greater than that of the internal thread, resulting in shear failure at the root of the internal thread.By optimizing the meshing length of the internal threads of the subframe and adjusting it to 2d - 2.5d (where d is the diameter of the bolt), the connection reliability was effectively enhanced and the problem of thread slippage was solved.
3. Bolt Loosening: When bolts become loose, first check if the preload is sufficient and use a torque wrench to retighten them according to the specified torque. To prevent loosening again, anti-loosening measures can be taken, such as installing spring washers, stop washers, using anti-loosening nuts, or applying anti-loosening glue on the threads. On equipment with frequent vibration, a combination of mechanical and chemical anti-loosening methods can also be adopted to ensure a firm bolt connection.
The torque of the connecting bolts between the subframe and the body of a certain model exceeded the tolerance. After investigation, it was found that the large dispersion of the friction coefficient of the parts led to inconsistent clamping forces. By adding flat washers, the contact area is increased, the friction coefficient is stabilized, and the torque attenuation is controlled within ±15%, effectively solving the problem of bolt loosening.
(2) Preventive Strategies for the failure of Hexagonal Bolts
1. Optimized design: Laying a solid foundation. During the design stage, factors such as the working load of the bolts, ambient temperature, and vibration should be fully considered, and appropriate bolt specifications and types should be selected. For parts subjected to alternating loads and vibrations, high-strength bolts should be selected and anti-loosening devices should be installed. Rationally design the thread parameters and stress concentration areas, increase the radius of the transition fillet between the bolt head and the screw, and reduce the stress concentration phenomenon. Meanwhile, conduct simulation and analysis to predict the force conditions of the bolts under actual working conditions and optimize the design plan in advance.
2. Strict material selection and inspection: Control material quality and choose reputable suppliers to ensure the reliable quality of bolt materials. Conduct comprehensive quality inspections on incoming materials, including chemical composition analysis, mechanical property testing, non-destructive testing, etc., to identify internal cracks, inclusions and other defects in the materials. Select appropriate materials based on the usage environment. In corrosive environments, choose stainless steel bolts with good corrosion resistance or bolts that have undergone anti-corrosion treatment. High-temperature resistant alloy bolts should be selected in high-temperature environments.
3. Standardize installation operations: Ensure that professional training for installers is correctly strengthened in the future to enable them to master the correct installation methods and torque requirements. Use professional tightening tools, such as torque wrenches and force-limiting wrenches, to ensure the accuracy and consistency of the tightening torque. When installing, tighten the bolts in a symmetrical and cross sequence to ensure uniform force distribution on the components. Avoid forcibly screwing in misaligned threads. Clean and lubricate the threads to reduce damage during installation.
4. Regular maintenance and monitoring: Promptly identify potential hazards and establish a regular maintenance system. Inspect the bolted connection parts, including visual inspection, torque testing, and loosening checks, etc. In corrosive environments, increase the inspection frequency, promptly clean the corrosion products on the surface of the bolts, and carry out anti-corrosion treatment if necessary. By using advanced monitoring technologies such as vibration monitoring and stress monitoring, the force state of bolts can be grasped in real time, potential failure hazards can be detected in advance, and preventive measures can be taken.
Iii. Conclusion
Although hexagonal bolts are small, they are crucial to the stable operation and safety of equipment. Understanding the common causes of its failure and mastering scientific repair and prevention methods can help us quickly solve problems in actual work and avoid losses caused by bolt failure. Only by controlling quality from the design source, standardizing operation during installation, and strengthening maintenance during use, can hexagonal bolts play their due role and ensure the smooth operation of production activities in various fields.