Preload decay law of rail bolts and long-term torque retention technology
What are the main causes of track bolt preload decay, and what are their respective proportions?
Preload decay stems from three primary factors: first, "stress relaxation" in the thread pair, accounting for approximately 40% of total decay, an inherent characteristic of steel under long-term stress; second, "creep deformation" of contact surfaces, accounting for 35%-micro-plastic deformation of contact surfaces (fish plates, washers) under pressure reduces bolt elongation; third, "fretting wear," accounting for 25%-micro-displacement between threads during vibration wears the tooth surfaces, causing preload loss. These three factors interact synergistically to drive continuous preload decline.
To what extent does preload decay endanger track operation safety?
Industry specifications clearly stipulate that when the actual preload of track bolts drops below 70% of the design value, operational safety is endangered. At this point, the longitudinal and lateral resistance of the fastening system decreases drastically, failing to effectively constrain rail displacement. On conventional lines, this causes gauge widening and rail creep; on high-speed lines, even a 30% preload reduction exacerbates dynamic wheel-rail interaction, triggering abnormal rail vibration and potentially switch conversion failure. Thus, 70% preload is regarded as the "safety red line."
What are the main aspects of long-term torque retention technology, and what are their respective roles?
It includes three core aspects: material optimization-using high-strength alloy steel and refining heat treatment to enhance stress relaxation resistance; structural design-adopting wedge lock washers and thread locking adhesives to structurally prevent preload loss; process control-implementing precise torque control and preload testing to ensure initial preload meets standards and is uniform. Material optimization addresses "internal causes," structural design blocks "external causes," and process control guarantees the "initial state," collectively achieving long-term torque retention.
What is the inhibitory effect of combining thread locking adhesive with lock washers on preload decay?
Thread locking adhesives (e.g., anaerobic adhesives) fill thread gaps and form a rigid bond after curing, completely eliminating fretting wear between threads and inhibiting stress relaxation. Combined with wedge lock washers, this achieves "dual protection": washers compensate for preload loss via mechanical action, while adhesives lock the thread pair via chemical action. Test data shows this combination reduces bolt preload decay rate by over 80%; after 5 years of service, preload remains above 90% of the design value-far superior to single anti-loosening measures.
How to monitor bolt preload decay on-site, and what is the most accurate detection method?
Routine inspections use torque wrenches for "retightening tests," indirectly judging preload decay by comparing retightening torque with initial torque-this method has large errors. The most accurate method uses an "ultrasonic preload tester," which calculates actual bolt elongation by measuring ultrasonic propagation time, directly deriving preload values with an error of ≤3%. For critical sections of high-speed and heavy-haul lines, comprehensive inspections using ultrasonic testing should be conducted regularly to promptly replace bolts with insufficient preload.