In industrial manufacturing, fasteners are typically classified as standard or generic components. Common fasteners include bolts, nuts, screws, washers, pins, and rivets, which may seem to require only procurement according to specifications and installation based on design.
However, within modern industrial engineering systems, fasteners are far more than simple connecting elements. They are subject to strict standards, directly impact equipment safety, and serve as fundamental structural units built upon well-defined engineering assumptions.
Numerous operational and failure cases demonstrate that fastener connection problems rarely stem from “insufficient material strength.” More commonly, the root cause lies in inadequate understanding or improper implementation of engineering standards, leading to systemic risks. To truly understand fasteners, attention must shift from focusing solely on individual parts to adopting a system- and standards-oriented perspective.
1. The Nature of Fastener Connections
From an engineering mechanics perspective, bolted connections in industrial equipment do not rely on the bolts themselves directly resisting shear forces to maintain structural stability. Instead, they depend on the axial preload generated during tightening, which creates a stable frictional system between the connected components.
Under ideal conditions, external loads are primarily carried by the friction force between the mating surfaces, while the bolt body mainly experiences steady tensile stress.
This operational state is the default engineering assumption in most industrial standards. Therefore, the reliability of fastener connections does not depend on how tightly the bolts are tightened, but rather on whether the preload remains within a reasonable range and can be maintained stably throughout the service life.
In both domestic and international standard systems, fasteners are defined as components used to securely join two or more parts, which can be disassembled when necessary. Common foundational standards include:
• GB/T 3098 series: Mechanical properties of fasteners
• GB/T 5780 / 5782: Hex bolts
• GB/T 6170 / 6175: Hex nuts
• GB/T 93 / 97: Spring washers and flat washers
Corresponding international standards include ISO 898, ISO 4014, ISO 4032, among others.
The core objective of these standards is not to standardize appearance, but to regulate material properties, mechanical performance, and dimensional tolerances—ensuring that fasteners exhibit predictability and verifiability in engineering calculations.
2. Preload of Fasteners
Preload is not a parameter that can be arbitrarily set; it is closely related to load conditions, friction characteristics, and safety factors.
In engineering design, an appropriate preload must meet three fundamental requirements:
1. The joint surfaces must not slip under maximum working load.
2. The bolt must always operate within the elastic deformation range.
3. The connection system must have sufficient fatigue life.
Whether in the GB standard system or the ISO standard system, the basic engineering assumptions are consistent:
• Load transfer in fastener connections relies on friction generated by preload.
• Bolts should not serve as primary shear load-bearing elements.
• Most failures result from preload degradation rather than instantaneous overload.
Insufficient preload may cause slight slippage at the joint interface, leading the bolt to gradually experience alternating stress. Excessive preload, on the other hand, may result in bolt yielding, thread damage, or crushing of the connected components. Therefore, the “safety” of fasteners is not simply a function of material strength, but rather a combined effect of preload, friction conditions, and load spectra.
3. Strength Grades of Fasteners
The strength grading system for fasteners provides clear mechanical performance boundaries for engineering design. Common bolt grades include 8.8, 10.9, and 12.9 (GB/T 3098.1 / ISO 898-1):
• Grade 8.8: Tensile strength ≥ 800 MPa, yield strength ≥ 640 MPa
• Grade 10.9: Tensile strength ≥ 1000 MPa, yield strength ≥ 900 MPa
• Grade 12.9: Tensile strength ≥ 1200 MPa, yield strength ≥ 1080 MPa
Higher strength grades allow for greater working stresses but also result in reduced ductility reserves. The selection of fastener grade must consider tightening processes, friction conditions, and stress concentration control. For equipment subjected to vibration, impact, or cyclic loading, blindly choosing a higher strength grade may actually accelerate fatigue failure. A rational choice should be based on actual load characteristics and engineering calculations, rather than simply pursuing the “highest grade.”
4. Fatigue Failure
According to ISO, DIN, and high-reliability industry standards such as those in aviation and wind power, most fastener failures are not caused by static load failure but rather by fatigue failure.
The typical progression of fatigue failure is as follows:
1. Insufficient initial preload or gradual preload decay
2. Minor slip occurs at the joint interface
3. The bolt transitions from a stable tensile state to an alternating stress state
4. Stress concentration develops at the thread root, leading to microcracks
5. Cracks propagate until fatigue fracture occurs
Therefore, preventing fatigue failure is not simply about increasing material “hardness,” but rather maintaining a consistent preload condition. This is why, in critical equipment:
• Bolt disassembly and reassembly cycles are limited
• Reusing critical bolts is prohibited
• Tightening sequences and methods are strictly specified
5. Material Selection
In the fastener standard system, material selection emphasizes compatibility with service environments.
Carbon steel and alloy steel fasteners: offer high strength and excellent fatigue performance; corrosion resistance can be enhanced through surface treatments, making them suitable for dry or mildly corrosive environments.
Stainless steel fasteners (GB/T 3098): provide superior corrosion resistance, although their strength is generally lower than that of high-strength alloy steels.
Neglecting material mechanical properties may compromise connection reliability, especially under high-load conditions. Material selection must comprehensively consider mechanical properties, environmental conditions, and service life.
6. Surface Treatment
The surface treatment of fasteners not only determines their corrosion resistance but also significantly affects the friction coefficient of threads and bearing surfaces. Since most of the tightening torque is consumed by friction, even minor variations in the friction coefficient can lead to noticeable deviations in preload. Therefore, in standard practice, high-strength bolts typically require specific surface treatments, along with corresponding tightening procedures and surface conditions.
For example, Dacromet coating is widely used for high-strength bolts—not only because it offers “better corrosion resistance,” but more importantly due to its stable and repeatable friction coefficient, which helps maintain consistent preload control.
7. Anti-Loosening Design
The engineering objective of anti-loosening design is not merely to prevent nut rotation, but more importantly, to delay or suppress the decay of preload.
In environments with continuous vibration or impact, traditional spring washers are increasingly unable to meet reliability requirements and are being gradually replaced by more systematic solutions, such as:
• Structural designs that prevent relative rotation
• Maintaining preload through material or geometric means
• Reducing the rate of preload decay via manufacturing processes or maintenance strategies
Anti-loosening design is not simply about adding “extra components,” but rather an integral part of the overall connection system design.
8. Fastener Lifecycle Management
In high-reliability industries such as wind power, rail transit, aviation, and nuclear energy, fasteners have long been integrated into equipment reliability engineering systems.
Design Phase
Perform load and fatigue calculations according to standards
Manufacturing Phase
Control materials, heat treatment, and surface treatments
Assembly Phase
Strictly follow tightening procedures
Operation Phase
Regularly inspect critical connections
Maintenance Phase
Standardize replacement and ensure traceable management
Fasteners are engineered components with a defined service life, not mere consumables.
9. Conclusion
The performance ceiling of industrial equipment depends on design standards and manufacturing capabilities, while its reliability floor is almost entirely determined by whether the most fundamental connection systems are properly understood and implemented.
Fasteners may seem ordinary, yet they bear the most real and enduring engineering challenges within equipment. Respecting fasteners is, in essence, respecting engineering principles and industrial expertise.
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