Matching Bolt Strength Grade and Torque
- What is the basis for classifying bolt strength grades?
Bolt strength grades are mainly classified according to tensile strength and yield strength, expressed by two digits, such as 8.8 grade and 10.9 grade. The first digit represents 1/100 of the tensile strength, and the second digit represents 10 times the ratio of yield strength to tensile strength. For example, 8.8 grade bolts have a tensile strength of ≥800MPa and a yield strength of ≥800×0.8=640MPa. Bolts of different strength grades have different materials and heat treatment processes to meet different strength requirements.
- What are the different application scenarios of bolts with different strength grades?
4.8 grade bolts have low strength and are suitable for connections with small stress, such as auxiliary fixing bolts for sleepers and ballast. 8.8 grade bolts have moderate strength and are widely used in fishplate connections and elastic clip fixing of ordinary railways, which can meet general load requirements. 10.9 grade and 12.9 grade high - strength bolts are suitable for key connection parts of high - speed and heavy - haul railways, such as turnout connections and large component fixing, and can bear huge impact forces and loads.
- How is the torque of the bolt determined? What is the relationship with the strength grade?
The torque of the bolt is determined according to its strength grade, specification and thread type, usually through calculation formulas or industry standards. Bolts with higher strength grades can bear greater torque under the same specification. For example, M24 8.8 grade bolts have a recommended torque of generally 600 - 800N·m, while M24 10.9 grade bolts have a recommended torque of up to 800 - 1000N·m. Too small torque makes the bolt unable to be fully tightened; too large torque may cause the bolt to break.
- What impact does insufficient or excessive torque have on bolt connections?
Insufficient torque will loosen the bolt connection, create gaps under train vibration, cause component displacement, abnormal noise, and even connection failure, affecting the stability of the track structure. Excessive torque will make the bolt bear excessive preload, which may exceed its yield strength, leading to plastic deformation or fracture of the bolt. At the same time, it may also damage the thread or the connected parts, causing irreparable damage.
- How to ensure the correct matching between bolt strength grade and torque?
First, select bolts of appropriate strength grade according to the stress of the connection part, then refer to relevant standards to determine the recommended torque according to the bolt specification and strength grade. Use a calibrated torque wrench to apply torque to ensure accurate torque. Check the strength grade mark of the bolt before installation to avoid mixing bolts of different grades. Regularly calibrate the torque wrench to ensure its measurement accuracy and accurate torque application.