Railway sleeper bolt strength grade and selection compatibility
What is the meaning of the strength grade of sleeper bolts?
The strength grade of sleeper bolts is represented by two digits, such as grade 8.8. The first digit represents the minimum tensile strength of the bolt, in units of 100MPa, and the second digit represents the yield ratio (the ratio of yield strength to tensile strength). Taking an 8.8-grade bolt as an example, its minimum tensile strength is 800MPa, and the yield ratio is 0.8, so the minimum yield strength is 640MPa. The minimum tensile strength of a 10.9-grade bolt is 1000MPa, the yield ratio is 0.9, and the minimum yield strength is 900MPa. The minimum tensile strength of a 12.9-grade bolt is 1200MPa, the yield ratio is 0.9, and the minimum yield strength is 1080MPa. The higher the value of the strength grade, the stronger the bearing capacity of the bolt, and the greater the tightening torque and load it can bear.
Which strength grade should be selected for sleeper bolts in heavy-haul lines?
Heavy-haul lines have large train axle loads and strong load impacts. Sleeper bolts need to use high-strength bolts of grade 10.9 or 12.9. These two grades of bolts can provide sufficient preload and fatigue resistance. The yield strength of 10.9-grade bolts is as high as 900MPa, which can withstand the huge transverse and longitudinal loads brought by heavy-haul trains and avoid plastic deformation of the bolts. The performance of 12.9-grade bolts is better, with a yield strength of more than 1000MPa, which is suitable for extra-heavy-haul lines with an axle load of more than 30 tons. If 8.8-grade bolts are used in heavy-haul lines, under long-term load, the bolts are prone to tensile deformation or fracture, leading to rail loosening and potential safety hazards. When selecting high-strength bolts, high-strength nuts should also be matched to ensure the strength matching of the connection system.
What is the application scenario of 8.8-grade sleeper bolts?
The 8.8-grade sleeper bolt has a tensile strength of 800MPa and a yield strength of 640MPa. It is a medium-high strength bolt, suitable for medium and low load lines such as ordinary railways, urban light rails and factory special lines. In these lines, the train axle load is small and the running speed is moderate. The bearing capacity of the 8.8-grade bolt can fully meet the track connection requirements. This grade of bolt is made of medium carbon steel through quenching and tempering heat treatment process, which has high cost performance and can effectively control the line construction cost. In the secondary sections of ballastless tracks or temporary track projects, 8.8-grade bolts can also play a good fixing role. However, under harsh working conditions such as high speed and heavy load, the strength of 8.8-grade bolts is insufficient to ensure the long-term stability of track connections.
What is the relationship between the strength grade of sleeper bolts and the tightening torque?
There is a positive correlation between the strength grade of sleeper bolts and the tightening torque. The higher the strength grade, the larger the maximum allowable tightening torque. The role of tightening torque is to make the bolt generate preload, compress the connected parts, and prevent loosening during train operation. The maximum tightening torque of 8.8-grade bolts generally does not exceed 350N·m. Exceeding this torque will cause yield deformation of the bolts. The maximum tightening torque of 10.9-grade bolts can reach 500N·m, and 12.9-grade bolts can withstand a tightening torque of more than 600N·m. In actual construction, the reasonable tightening torque should be determined according to the strength grade of the bolt. Too small torque cannot provide sufficient preload, and too large torque will damage the bolt. At the same time, the tightening torque should be applied evenly to avoid uneven stress.
How to identify the strength grade of sleeper bolts through appearance and marking?
The primary method to identify the strength grade of sleeper bolts is to check the marking on the bolt head. National standards stipulate that the bolt head must be marked with the strength grade code, such as 8.8, 10.9, 12.9, etc., with clear markings and uniform engraving depth. The second is to observe the appearance of the bolt. High-strength bolts (grade 10.9 and 12.9) are usually treated with blackening or phosphating on the surface, showing a uniform black or gray color, while 8.8-grade bolts are mostly treated with hot-dip galvanizing or electro-galvanizing, with a silver-white surface. In addition, the head size of high-strength bolts is relatively large, and the thickness and opposite side width are slightly larger than those of 8.8-grade bolts of the same specification. During procurement and acceptance, the strength grade of the bolt can be accurately judged by checking the marking, observing the appearance and combining with sampling inspection to avoid misuse.