In modern construction and machinery manufacturing, high-strength bolts play an indispensable role. They are key components that ensure structural stability and safety. High-strength bolt connections have the advantages of simple structure, excellent mechanical performance, removability, fatigue resistance, and resistance to loosening under dynamic loads. These advantages make high-strength bolts a promising connection method. They perform well under heavy loads and effectively prevent structural slippage and damage. Today, let's take an in-depth look at high-strength bolts, including their classification, performance, application scenarios, and differences from ordinary bolts.
The full name of high-strength bolts in production is high-strength bolt assembly, and they are generally not abbreviated. These bolts are widely used in important structural connections due to their high strength and reliable connection performance. They are typically used in situations that require bearing large loads, such as bridges, high-rise buildings, and heavy machinery. Next, let's explore the classification of high-strength bolts in detail. Only by understanding the different types of high-strength bolts and their characteristics can engineers make reasonable choices based on actual engineering requirements.
High-strength bolts can be divided into two major categories based on installation: large hex head bolts and twist-off type bolts. Large hex head bolts are the most common type and suitable for a variety of installation environments. Twist-off type bolts are only used in grade 10.9 and achieve the required preload through the twist-off end during installation.
High-strength bolts are mainly divided into grades 8.8 and 10.9. Grade 8.8 includes only large hex head bolts. In the marking system, the number before the decimal point represents the tensile strength after heat treatment, and the number after the decimal point represents the yield ratio, which is the ratio of measured yield strength to measured ultimate tensile strength. Specifically, grade 8.8 means the tensile strength of the bolt rod is not less than 800 MPa with a yield ratio of 0.8; grade 10.9 means the tensile strength of the bolt rod is not less than 1000 MPa with a yield ratio of 0.9. This classification allows engineers to select bolts according to different design requirements.
In structural design, common diameters of high-strength bolts include M16, M20, M22, M24, M27, and M30. Among these, M22 and M27 are secondary options, while M16, M20, M24, and M30 are mainly selected under normal conditions. The selection of these sizes is based on load analysis and installation convenience in actual engineering.
High-strength bolts are designed in terms of shear resistance as either bearing type or friction type.
The load-bearing capacity of friction-type high-strength bolts depends on the slip resistance coefficient of the friction surfaces and the number of friction surfaces. Shot-blasted (or shot-peened) bolts with red rust have the highest friction coefficient, but in practice, it is greatly influenced by construction quality. Supervisors often consider whether standards can be reduced to ensure project quality. The design principle of friction-type bolts is to transmit load through friction. As long as the axial force is smaller than the friction force, the components will not slide, and the connection will not be damaged.
The load-bearing capacity of bearing-type high-strength bolts depends on the minimum value of the bolt's shear capacity and the rod's bearing capacity. For a single connection surface, M16 friction-type bolts have a shear capacity of 21.6–45.0 kN, while M16 bearing-type bolts have a shear capacity of 39.2–48.6 kN, performing better than friction-type bolts. Bearing-type bolts are relatively simple to process; the connection surface only needs to be cleared of oil and rust.
For axial tensile capacity along the rod direction, structural steel codes express design values differently for friction-type and bearing-type bolts. The design value for friction-type bolts equals 0.8 times the preload, while for bearing-type bolts, it equals the effective area of the rod multiplied by the material's tensile strength design value. Although the expression differs, the two values are essentially the same.
When high-strength bolts simultaneously bear shear and axial loads, the design requirements differ. For friction-type bolts, the ratio of bolt shear to shear capacity plus the ratio of axial force to tensile capacity must be less than 1.0. For bearing-type bolts, the square of the shear ratio plus the square of the axial ratio must be less than 1.0. This means that under the same load combination, bearing-type high-strength bolts of the same diameter have a higher safety reserve than friction-type bolts.
Under repeated strong seismic action, friction surfaces may fail, and shear capacity depends mainly on the bolt's shear resistance and the plate's bearing capacity. Therefore, seismic codes provide formulas for calculating the ultimate shear capacity of high-strength bolts. Although bearing-type bolts have advantages in design values, they belong to shear-compression failure types, and the bolt holes are similar to ordinary bolts. Their deformation under load is much greater than that of friction-type bolts. Therefore, bearing-type high-strength bolts are mainly used for non-seismic component connections, non-dynamic load components, or components not subject to repeated action.
The principle of high-strength bolt connections is to generate preload on the connected members through nuts and washers. Under preload, the friction between connection surfaces increases. As long as the axial force is smaller than this friction, the components will not slide, and the connection will not be damaged. To ensure sufficient friction at the contact surface, clamping force must be increased, and the friction coefficient of the contact surface enhanced. Preload is achieved by applying tensile force to the bolt, so high-strength steel must be used to manufacture the bolts.
High-strength bolts are widely used in construction structures and machinery manufacturing. In construction, bolts connecting main structural members generally adopt high-strength bolt connections. For example, in bridges and high-rise buildings, high-strength bolts ensure structural stability and safety. In machinery, high-strength bolts connect critical mechanical components, ensuring they do not loosen under dynamic loads.
Before exploring the wide applications and design points of high-strength bolts, it is necessary to clarify the differences between high-strength bolts and ordinary bolts. Understanding these differences helps select the most suitable bolt type for different engineering requirements and scenarios.
Ordinary bolts are generally made of common steel (e.g., Q235) and mainly used for temporary or removable connections. High-strength bolts are generally made of high-strength steel, such as 45# steel (grade 8.8) and 20MnTiB (grade 10.9), and are usually used for permanent connections. Ordinary bolts are generally grade 4.4, 4.8, 5.6, and 8.8, while high-strength bolts are generally grade 8.8 and 10.9, with grade 10.9 being more common.
Ordinary bolts can be reused, but high-strength bolts cannot. This is because high-strength bolts experience preloading during use, and repeated use may reduce their performance.
The bolt hole of ordinary bolts is not necessarily larger than that of high-strength bolts. In fact, ordinary bolt holes are smaller. For A- and B-grade ordinary bolts, the hole is generally 0.3–0.5 mm larger than the bolt; for C-grade, 1.0–1.5 mm larger. Friction-type high-strength bolts rely on friction to transmit load, and the gap between rod and hole can reach 1.5–2.0 mm. Bearing-type high-strength bolts have a slightly smaller gap, about 1.0–1.5 mm.
As an important connection component, high-strength bolts play an irreplaceable role in modern engineering. They are made of high-strength steel and can generate large preload, ensuring the stability and safety of connections. Whether in construction or machinery, high-strength bolts provide reliable connection solutions. Understanding their classification, performance, and design requirements helps engineers make correct choices in practice, ensuring structural safety and reliability.
When selecting high-strength bolts, the choice between friction-type and bearing-type must be based on specific engineering requirements and design specifications. At the same time, attention should be paid to differences from ordinary bolts, choosing materials and bolt hole sizes reasonably to achieve optimal connection performance. In summary, high-strength bolts are an indispensable “powerful link” in modern engineering, and their proper use provides solid assurance for safety and stability.