Bolt Loosening Prevention Measures and Failure Analysis
- What are the common anti-loosening measures for bolts? What are their characteristics?
The double nut anti-loosening structure is simple, and it prevents loosening through the friction between two nuts, suitable for low-speed and low-vibration lines, but the weight increases and the cost is slightly higher; the spring washer anti-loosening relies on the pressure generated by the elastic deformation of the washer to prevent loosening, which is easy to install and suitable for ordinary bolt connections, but the anti-loosening effect will decrease under high-frequency vibration; thread glue anti-loosening (such as Loctite 243 glue) locks the thread through glue layer curing, with long-lasting anti-loosening effect, suitable for high-speed and heavy-haul railways, but the glue layer needs to be heated and softened during disassembly.
- What are the main reasons for bolt loosening?
Insufficient preload is the main reason. The specified torque is not reached during installation, and relative sliding occurs between threads under vibration load; thread wear leads to increased fit clearance, gradually losing preload, especially in dusty environments, the wear speed is accelerated; temperature changes cause differences in thermal expansion and contraction between bolts and connected parts, generating additional stress, which leads to loosening after repeated action.
- What are the common forms of bolt failure? How to judge?
Common forms include thread slipping, which is manifested as the bolt cannot be tightened or cannot be fixed after loosening, which can be judged by observing whether the thread is deformed or slipped; bolt fracture mostly occurs at the rod or the transition between the head and the rod. The fracture surface with fatigue striations is fatigue fracture, and the one with obvious plastic deformation is overload fracture; head deformation or cracking is mostly due to excessive torque during installation or material defects, which can be found through appearance inspection.
- How to select anti-loosening measures for bolts in different environments?
In humid environments, thread glue plus galvanized bolts are preferred, which are both anti-loosening and anti-corrosion; for mining railways with more dust, double nuts plus dust covers should be used to reduce dust entering the thread gap; in high-temperature environments (such as near the locomotive exhaust port), high-temperature resistant anti-loosening washers (such as copper washers) are needed to avoid ordinary spring washers losing elasticity due to high temperature; for high-speed railways with high-frequency vibration, Spiralock threads (self-locking threads) are recommended, combined with pre-applied glue, the anti-loosening effect is more stable.
- How to improve the anti-loosening design of bolts through failure analysis?
Metallographic analysis of broken bolts, if excessive inclusions in the material are found, the smelting process must be improved; if the proportion of fatigue fractures is high, it indicates that the anti-loosening measures are insufficient, and the preload can be increased or self-locking threads can be used; count the bolt failure data of different lines, use higher-strength bolts (such as 12.9 grade) in heavy-haul lines, and encrypt the anti-loosening structure, such as adding a stop washer outside the nut.