Bolt Strength Grade and Installation Specifications
- What are the common strength grades of bolts? What is the bearing capacity of each grade?
Common strength grades include 8.8, 10.9, 12.9, etc. The tensile strength of 8.8 - grade bolts is not less than 800MPa, and the yield strength is not less than 640MPa, suitable for connections in ordinary lines, such as fishplate connections. The tensile strength of 10.9 - grade bolts reaches more than 1000MPa, and the yield strength is not less than 900MPa, with strong bearing capacity, used in heavy - haul railways or important connection parts. 12.9 - grade bolts are of high strength grade, with tensile strength ≥1200MPa and yield strength ≥1080MPa, suitable for occasions with extremely high requirements for connection strength, such as high - speed railway turnout connections.
- What are the differences in materials and heat treatment between bolts of different strength grades?
8.8 - grade bolts mostly use medium carbon steel such as 45 steel, which is quenched and tempered to obtain good comprehensive mechanical properties. 10.9 - grade bolts often use alloy steel such as 35CrMo, and through quenching and tempering (quenching + high - temperature tempering), the strength and toughness are improved to meet higher bearing requirements. 12.9 - grade bolts use high - strength alloy steel such as 40CrNiMo, and the heat treatment process is more stringent. The quenching temperature and tempering time must be accurately controlled to achieve extremely high strength and hardness.
- What are the preload requirements for bolt installation?
The preload must be determined according to the bolt strength grade and diameter. The preload of 8.8 - grade M20 bolts is generally 150 - 180kN; the preload of 10.9 - grade M24 bolts is 280 - 320kN; the preload of 12.9 - grade M27 bolts can reach 400 - 450kN. If the preload is too small, the bolt is easy to loosen and the connection is unreliable; if it is too large, it may cause plastic deformation or fracture of the bolt. A torque wrench must be used during installation, and appropriate torque must be applied according to the conversion relationship between preload and torque.
- What are the specifications for the order and method of bolt installation?
The installation sequence should follow the principle of symmetry and uniformity. For example, fishplate bolts should be tightened symmetrically from the middle to both ends to avoid fishplate deformation due to uneven stress. For connection parts with multiple bolts, they must be tightened in stages, first pre - tightened to 50% of the specified torque, then tightened to 80% in turn, and finally reach the specified torque to prevent excessive stress from a single tightening.
During installation, it is necessary to ensure that the bolt is perpendicular to the connecting surface to avoid deflection, which affects the transmission of preload.
- How to judge whether the bolt installation meets the specifications?
First, check whether the torque reaches the specified value. Use a calibrated torque wrench to retighten. If the torque does not decrease significantly, it indicates that the preload is qualified. Observe whether the bolt has plastic deformation, such as bolt shank bending and thread damage. If so, the installation is unqualified. Check whether the connection part is tight, and measure the gap with a feeler gauge. A gap not exceeding 0.1mm is qualified. In addition, after installation, an appearance inspection must be carried out to ensure that the bolt has no defects such as rust and oil stains that affect performance.