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Key Points for Selecting High-Strength Bolts2

[Abstract]:This article covers performance grades, materials, matching principles, and selection tips for high-strength bolts in critical connections.

In critical fields such as modern steel structures, heavy machinery, bridges, wind turbine towers, and aerospace, high-strength bolt assemblies have become core components for transmitting primary loads and ensuring structural integrity. The level of their application directly relates to the safety, reliability, and economic viability of engineering projects. However, "high-strength" is not a universal panacea. Incorrect selection, mismatched components, or improper installation can prevent high-strength bolts from realizing their full potential and may even lead to catastrophic failures. As a professional fastener manufacturer, Shenzhen Toprecision Technology Co., Ltd. aims to systematically outline the key points for selecting high-strength bolts, guiding users to comprehensively grasp performance grades, material systems, matching principles, and application details to achieve precise and safe connection design.

I. Profound Understanding of Performance Grades and Material Systems

The core identifier of a high-strength bolt is its performance grade, which encodes the mechanical property boundaries of the bolt through a two-digit code.

Interpretation of Performance Grades: Taking the common "Grade 10.9" as an example, "10" represents 1/100 of the nominal tensile strength (σb), meaning the tensile strength is 1000 MPa; ".9" represents the yield strength ratio (yield ratio), meaning the ratio of nominal yield strength (σs) to tensile strength (σb) is 0.9, resulting in a yield strength of 900 MPa. Common grades include 8.8, 10.9, and 12.9. The higher the grade, the stronger the load-bearing capacity of the bolt, but the more stringent the requirements for materials, manufacturing processes, and installation control.
Material Requirements: Different performance grades correspond to specific materials and heat treatment processes.

Grade 8.8: Typically uses medium carbon steel (e.g., grades 35 or 45) or low-alloy steel (e.g., ML35), treated with quenching and tempering.
Grade 10.9: Typically uses low-alloy structural steel (e.g., 20MnTiB, 35VB, 40Cr), treated with quenching and tempering, with higher requirements for material hardenability and purity.
Grade 12.9: Uses alloy structural steel (e.g., 35CrMo, 42CrMo), subjected to stricter heat and surface treatments, requiring special attention to hydrogen embrittlement sensitivity.

Selection Principles: Higher grades are not always better. Selection should be comprehensively determined based on the design load of the connection, safety factors, stiffness of connected parts, fatigue requirements, and costs. Excessive strength may reduce bolt toughness, increase sensitivity to stress concentration, and raise costs and installation difficulty.

II. Adherence to the Matching Principle of Connection Assemblies

A high-strength bolt connection is a system comprising the bolt, nut, and washer. Achieving its high-strength performance relies on the precise matching and synergistic operation of the entire assembly.

Bolt and Nut Matching:

Equal-Strength Matching: The performance grade of the nut should match or be slightly higher than that of the bolt to ensure that the failure mode of the threaded connection assembly is bolt shank fracture rather than thread stripping. Typically, Grade 10.9 bolts are paired with Grade 10 nuts, and Grade 8.8 bolts are paired with Grade 8 nuts.
Proof Load: The proof load value of the nut must be greater than the minimum tensile load of the bolt, which is the foundation for preventing thread shear failure.

Function and Selection of Washers:

Bearing and Anti-loosening: High-strength bolt connections typically use high-hardness flat washers. Their functions are twofold: to distribute the pressure from the bolt head or nut on the connected plate surface to prevent crushing, and to provide a relatively stable friction surface in certain cases (such as torque-controlled installation) to reduce torque variation. For slip-critical high-strength bolt connections, washers are indispensable.
Special Requirements: When used on inclined surfaces, beveled washers must be employed to make the bearing surface perpendicular to the bolt axis. Where anti-loosening is required, serrated washers or self-locking nuts can be selected.

Procurement and Use of Complete Sets: It is strongly recommended to procure bolts, nuts, and washers of the same batch and performance grade as a complete set from reputable suppliers. Mixing components from different manufacturers or batches may lead to inaccurate preload control due to dimensional tolerances, hardness, and friction coefficient variations.

III. Focus on Key Application Points

Preload Control is the Soul of High-Strength Bolt Connections: High-strength bolts rely on applying extremely high preload in the shank to generate massive frictional resistance between connected parts to transmit shear forces (slip-critical type), or to bear pressure and tension directly. Therefore, accurately applying and maintaining the design preload is the key to ensuring connection performance.

Torque Method: The most common method, indirectly controlling preload (F) by controlling applied torque (T), with the relationship T = K * F * d, where K is the torque coefficient and d is the nominal diameter. Since K is greatly affected by the friction coefficient, matching lubricants or treatment processes must be used, and K values must be periodically verified.
Turn-of-Nut Method: Initial tightening is performed to bring all plate layers into tight contact, followed by final tightening through a specified angle. This method is insensitive to variations in plate stack thickness but requires consistent initial torque.
Torque-Turn Method: Combines both by applying a certain torque first and then turning a specific angle, offering greater precision.
Direct Tension Method: Uses hydraulic tensioners or directly measures bolt elongation. This is the most direct and accurate method, used for particularly critical connections.

Friction Surface Treatment: For slip-critical high-strength bolt connections, the anti-slip coefficient (μ) of the contact surface of the connected plates is the core design parameter. Friction surfaces must be specially treated (e.g., sandblasting, shot blasting, grinding wheel grinding, pickling) to meet specified roughness and cleanliness. During installation, friction surfaces must be strictly protected against contamination and rust.
Installation Process and Inspection:

Installation Sequence: Tightening should proceed sequentially from areas of high joint stiffness toward the free end, or symmetrically outward from the center, to prevent loss of preload in previously tightened bolts.
Initial and Final Tightening: For large hexagon bolts, two processes are usually required: initial tightening (reaching 50%-70% of final tightening torque) and final tightening.
Torque Inspection: After installation, calibrated torque wrenches should be used for spot checks to verify if torque values meet requirements. For torque-shear bolts, the breaking of the splined end serves as the indicator.

Anti-Corrosion and Fatigue Considerations:

Anti-Corrosion: In corrosive environments, corrosion-resistant materials (e.g., weathering steel bolts) or long-lasting anti-corrosion coatings (e.g., hot-dip galvanizing, Dacromet) must be selected. Attention should be paid to the impact of coatings on friction coefficients and hydrogen embrittlement risks.
Anti-Fatigue: For connections subjected to dynamic loads, bearing-type bolts or other measures to reduce stress amplitude should be prioritized. Details such as the transition fillet between the bolt head and shank, and thread rolling processes, significantly affect fatigue life.

Conclusion
The selection of high-strength bolts is a rigorous engineering discipline that intersects materials science, mechanics, tribology, and construction management. Shenzhen Toprecision Technology Co., Ltd. emphasizes that successful connections begin with correct selection and matching. Users should abandon the "strength-only" mindset and systematically consider performance grades, assembly matching, preload control methods, friction surface conditions, and anti-corrosion requirements based on specific working conditions. By collaborating closely with professional manufacturers to obtain technical data support and qualified products, and by strictly executing standardized installation and inspection procedures, every high-strength bolt connection point can become a solid and reliable cornerstone in the structure, fulfilling the safety mission entrusted by the design.

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Key Points for Selecting High-Strength Bolts2

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