The Correlation between Bolt Strength Grade and Tightening Effectiveness
- What are the classification standards for the strength grades of common bolts?
Bolt strength grades are usually expressed by numbers, such as 4.8, 8.8, 10.9, etc. The number before the decimal point indicates 1/100 of the tensile strength, and the number after the decimal point indicates the yield ratio (the ratio of yield strength to tensile strength). The tensile strength of 4.8 grade bolts is 400MPa, the yield ratio is 0.8, and the yield strength is 320MPa; the tensile strength of 8.8 grade bolts is 800MPa, the yield ratio is 0.8, and the yield strength is 640MPa, with the strength increasing step by step.
- What are the differences in fastening effects between bolts of different strength grades?
High - strength grade bolts (such as 10.9 grade) can provide greater preload. When bearing the same load, the fastening effect is more stable and not easy to loosen, which is suitable for scenarios with high fastening requirements such as heavy load and high speed. Low - strength grade bolts (such as 4.8 grade) have small preload, are easy to loosen under vibration and impact loads, and are only suitable for connection parts with small stress.
- How to select the bolt strength grade according to the stress situation of track components?
Bolts for connecting rails and fishplates need to be high - strength bolts of 8.8 grade or above because they bear large shear force and tension generated by train load. Bolts for connecting sleepers and fasteners, which are relatively less stressed, can be 4.8 or 6.8 grade bolts. Bolts for key parts such as turnouts, which are complex in stress and of high importance, should be 10.9 grade high - strength bolts.
- What is the relationship between bolt strength grade and service life?
High - strength grade bolts are made of better materials and have stronger fatigue resistance. Under alternating loads, their fatigue life is longer than that of low - strength grade bolts. For example, 10.9 grade bolts can have a service life of 5 - 8 years in heavy - haul railway scenarios, while 4.8 grade bolts may need to be replaced only after 2 - 3 years under the same conditions, and frequent replacement will increase maintenance costs.
- In the process of use, how to judge whether the bolt strength grade meets the requirements?
Regularly check whether the bolts are loose, deformed, broken, etc. If low - strength bolts frequently loosen or deform in specific parts, it indicates that their strength grade is insufficient, and bolts with higher strength grades need to be replaced. Through torque detection, if the bolt slips under the standard torque, it also indicates that its strength is insufficient, and the bolt strength grade should be upgraded in time.