Thread connection is widely used in machinery, structure and equipment assembly, its core is the synergy of threaded holes and threaded fasteners. As an important connection bearing part, once the failure occurs, it often leads to the functional failure of the whole connection system and even equipment damage. This paper summarizes the common failure modes of threaded holes, analyzes the failure causes and proposes targeted improvement measures.

1. Common threaded hole failure mode

1.1 Failure of thread stripping

Thread stripping refers to the phenomenon of shear failure in the thread engagement site due to excessive force.

Failure mechanism The bolt preload force or excessive external load exceeds the shear strength of the threaded hole material, leading to the destruction of the thread root.

· Characteristic and obvious metal tear marks appear on the surface of the screw holes.

1.2 Thof thread failure

·Thread overload refers to the phenomenon of deformation or fracture due to excessive force.

·Failure mechanism The insufficient strength of the thread hole material, or improper design, leads to local stress concentration, exceeding the yield strength or limit strength of the material.

Typical characteristics of the thread deformation is obvious, appear sliding wire or hole wall cracking phenomenon.

1.3 Fatigue failure of the screw hole

·Fatigue failure is the failure of threaded holes under long-term cyclic load due to fatigue crack expansion.

·Failure mechanism cyclic load causes fatigue cracks on the surface or inside of the thread hole, and gradually extends to failure.

Typical features of thread hole rupture surface presents shell pattern or fatigue fracture characteristics.

1.4 Corrosion failure of the screw hole

·Corrosion failure is the material damage caused by the chemical or electrochemical action of the threaded holes.

·Failure mechanism thread holes are exposed to corrosive environment (such as high humidity, acid and alkali atmosphere), resulting in corrosion thinning or pitting of the material surface.

Typical features of the screw hole surface rust spots, corrosion pits or serious surface peeling phenomenon.

1.5 Heat F failure

·HEF failure usually occurs in high temperature operation or improper heat treatment, resulting in degradation of material properties.

·Failure mechanism Material softening in high temperature or welding heat affected area reduces the strength and rigidity of threaded holes.

Typical characteristic screw hole surface discoloration or microstructural changes in metal tissue.

2. Improvement measures for thread hole failure

For the above failure modes, the reliability and longevity of the screw increased.

2.1 Improve the material strength of the threaded holes

1. Optimize the material selection

O Use high strength alloy steel, stainless steel or nickel-based alloy adapted to high load or corrosive environment.

O Enhance the strength and toughness of the material by heat treatment (e. g., quenching and tempering).

2. Surface reinforcement technology

O Rolling and peening techniques are applied to strengthen the surface of thread holes to improve their fatigue strength.

2.2 Improve the geometric design of the thread holes

Increasing the thread engagement length and increasing the engagement length of the screw hole can significantly improve the bearing capacity of the thread.

1.Optimize the thread parameters and choose the reasonable thread tooth type and tolerance grade to reduce the stress concentration.

2.3 Control the machining accuracy and quality

1. High-precision machining equipment uses CNC machining center and high-precision cutting tools to reduce the deviation of threaded holes.

2. Strict detection and verification

The size and geometric accuracy of the screw holes are examined using a coordinate measuring machine (CMM) and optical measuring equipment

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Increase the frequency of quality sampling at key parts to ensure processing consistency.

2.4 Environmental protection and coating protection

1. Use an anticorrosive coating

O Apply a coating (such as zinc nickel alloy, chrome plating or ceramic coating) on the threaded hole surface to improve corrosion resistance.

2. Reasonable lubrication and protection

O Use a suitable lubricant to reduce friction and wear when assembling the threaded holes.

2.5 Optimize the assembly process

Control tightening torque Use precision torque tools (e. g. torque wrench or hydraulic tension) to ensure the same pretension force.

3. Pretension monitoring technology Use the stress sensor or digital tightening tools to monitor the pretension force in real time to avoid overload failure.

There are various modes of thread hole failure, involving mechanical properties, processing quality, environmental factors and assembly process. By improving material performance, optimizing geometric design, improving machining accuracy and adopting scientific protection measures, it can effectively avoid thread hole failure and prolong the life of thread connection.

The reliability research and improvement of threaded holes is an important part to ensure the normal operation of mechanical equipment. Only by strictly controlling the whole process from design to assembly, can we ensure that every threaded hole can stand the test of time and load.