Bolt-Related Knowledge
Date:2026-08-19View:4Tags:Pipe fitting,Alloy steel pipe,Epoxy pipe
Bolts are among the most widely used fasteners. They typically consist of a head and a shank (a cylinder with external threads), and are used with a nut to fasten two parts with through holes, allowing for detachable connections.
1. Performance Grade: Understanding the Numerical Code on Bolts
The bolt head or shank is usually marked with numbers in the form of "X.Y". This indicates its performance grade and is an internationally recognized standard for measuring the mechanical properties of bolts. It directly tells you how much force the bolt can withstand.
2. Numerical Meaning: The grade marking consists of two parts. The number before the dot (X) represents 1/100 of the nominal tensile strength, and the number after the dot (Y) represents the yield strength ratio (yield strength/tensile strength) of 10 times.
3. Calculation Example: Taking the most common 4.8 grade bolt as an example:
Nominal Tensile Strength: 400 MPa.
Nominal Yield Strength: 400 × 0.8 = 320 MPa.
The meaning of "8.8 grade": This indicates that the nominal tensile strength of the bolt material reaches 800 MPa, the yield strength ratio is 0.8, and the yield strength reaches 640 MPa.
High-strength bolts and ordinary bolts: Bolts with a performance grade of 8.8 and above (including 8.8 grade), made of medium carbon steel or low carbon alloy steel and heat-treated (quenching + tempering), are generally referred to as high-strength bolts; those below 8.8 grade are called ordinary bolts.
4. Classification and Application Scenarios
Bolts can be classified in various ways, and their application scenarios also differ:
According to the force-bearing method and head shape: mainly divided into ordinary bolts and bolts for reamed holes; according to head shape, there are hexagonal heads, countersunk heads, round heads, and square heads, among which hexagonal heads are the most commonly used.
According to manufacturing precision: divided into three grades: A, B, and C. Grades A and B are precision bolts, machined on a lathe, offering high accuracy but at a high price. Grade C bolts are coarse bolts, made directly from round steel, with lower accuracy but easier installation and lower cost, making them the most widely used in steel structure connections.
Special types: For example, U-bolts, shaped like a "U," are commonly used to secure water pipes or leaf springs in automobiles.
⚖️ Design and Verification: Key Calculation Points for Bolted Connections
In engineering design, bolted connections require rigorous verification. According to China's *Steel Structure Design Standard* (GB 50017), the following stress conditions are mainly considered:
Tension bolts: It is necessary to verify whether their tensile force exceeds the design tensile bearing capacity N_t^b = A_s · f_t^b (where A_s is the effective cross-sectional area, and f_t^b is the design tensile strength).
Shear bolts: It is necessary to check whether the shear force exceeds the design shear capacity N_v^b = A_g · f_v^b (where A_g is the gross cross-sectional area and f_v^b is the design shear strength). Simultaneously, the bearing capacity of the connecting plates must be checked to prevent the hole walls from being crushed.
Tension-shear combined stress: When the bolt is simultaneously subjected to shear and tension forces, the combined formula √[(N_v/N_v^b)² + (N_t/N_t^b)²] ≤ 1.0 must be satisfied.
High-strength bolt friction connection: Its bearing capacity depends on the friction between the plates. The shear capacity formula is N_v^b = k · n_f · μ · P, where P is the bolt preload and μ is the anti-slip coefficient of the friction surface, which is the key difference between it and ordinary bolts.
