Bolt loosening prevention measures and failure analysis
- What are the common anti-loosening measures for bolts and their characteristics?
Spring washer anti-loosening is simple and easy to implement. It generates pre-tightening force through the elastic deformation of the washer, with low cost but general anti-loosening effect, suitable for parts with small vibration. Double nut anti-loosening produces friction through the mutual extrusion of two nuts, with reliable anti-loosening effect but increased weight and cost, often used in important connection parts such as turnouts. Spiralock thread anti-loosening increases friction by using a special tooth profile, without additional parts, with long-lasting anti-loosening effect, suitable for high-speed and heavy-haul railway bolts. Spot welding anti-loosening fixes the nut and bolt by welding, with thorough anti-loosening effect but difficult disassembly, only used in parts that are not frequently disassembled. Adhesive anti-loosening applies anaerobic adhesive on the thread, which forms a glue film after curing to prevent loosening, detachable and with good anti-loosening effect, widely used in track bolts.
- What are the common forms of bolt failure and how to identify them?
Fatigue fracture is the most common form, with obvious fatigue lines and instantaneous fracture areas on the fracture surface, mostly occurring at the transition between the bolt head and the shank, which can be detected by flaw detection. Tensile fracture is characterized by a flat fracture surface without fatigue lines, often caused by excessive pre-tightening force or insufficient material strength, which can be identified by measuring the fracture diameter. Thread slippage is manifested as wear or deformation of the thread profile, unable to be tightened, which can be judged by a thread gauge when the profile deviation exceeds 0.2mm. Corrosion failure is shown as rust on the bolt surface, and in severe cases, rust pits and reduced cross-section, which can be identified by visual inspection and thickness measurement, and bolts need to be replaced when the rust depth exceeds 5% of the original diameter.
- What are the main causes of bolt failure?
Improper pre-tightening force control is an important reason. Excessive pre-tightening force exceeding the bolt yield strength will lead to bolt tensile fracture; insufficient pre-tightening force cannot effectively prevent loosening, and the bolt is easy to loosen under vibration, leading to failure. Unqualified materials will make the bolt performance substandard, such as strength lower than the design requirement, inclusions and other defects, which are easy to break under stress, so strict material inspection is necessary. Thread damage during installation, such as collision and scratch, will cause stress concentration, and cracks are easy to generate at the damaged part under alternating load, eventually leading to failure. Environmental corrosion will damage the bolt surface, especially in humid and salt-fog environments. Rust will weaken the bolt cross-section and reduce the bearing capacity, and long-term corrosion will lead to bolt failure. Fatigue load is a long-term factor. The alternating load generated by train operation makes the bolt bear tension and shear force repeatedly. When it exceeds the fatigue limit, fatigue cracks will be generated and gradually expand to fracture.
- How to improve bolt design and use through failure analysis?
Analyze the fracture surface of the failed bolt to determine the fracture type (fatigue, overload, etc.). If it is fatigue fracture, the bolt diameter can be increased or high-strength materials can be used to improve its fatigue strength. Adjust the pre-tightening force standard according to the failure cause. If the fracture is caused by excessive pre-tightening force, reduce the pre-tightening force to a reasonable range; if the loosening is caused by insufficient pre-tightening force, increase the pre-tightening force and optimize the anti-loosening measures. Improve the bolt structure design, such as increasing the fillet radius at the transition between the head and the shank to reduce stress concentration, or using hollow bolts to reduce weight and improve toughness, reducing the failure probability. For corrosion failure, improve the anti-corrosion process, such as galvanizing, chrome plating or applying anti-corrosion coating, to improve the corrosion resistance of bolts and extend their service life. Adjust the use scenario according to the failure data, replace the easily failed bolts from parts with severe vibration or serious corrosion, and use more suitable bolt types, such as Spiralock thread bolts in parts with large vibration.
- How to select appropriate bolt anti-loosening measures in different scenarios?
For the straight section of ordinary railways with small vibration, spring washer anti-loosening can be used, which is low in cost and can meet basic needs, and regular inspection can ensure reliability. High-speed and heavy-haul railways have severe vibration, so Spiralock thread or glue anti-loosening should be used. Spiralock thread has no additional parts and long-lasting anti-loosening effect, while glue anti-loosening can adapt to frequent vibration and ensure that bolts do not loosen for a long time. Important parts such as turnouts need double nut anti-loosening. The double protection makes the connection more reliable. Even if one nut loosens, the other can still play a fixing role, suitable for scenarios with high safety requirements. In humid or corrosive environments, a composite anti-loosening measure of glue coating and galvanizing should be adopted. While the glue coating prevents loosening, the galvanized layer can resist corrosion and extend the service life of bolts. For parts that are not frequently disassembled, such as track fixing devices, spot welding anti-loosening can be used. Once installed, there is no need to disassemble, and the anti-loosening effect is thorough, avoiding loosening hazards.