The Mechanism of Vibration Loosening
Vibration causes periodic changes in the normal pressure between thread pairs and repeated attenuation of friction. After the bolt is tightened, a preload is generated between the thread teeth, and the friction resists loosening. But the high-frequency vibration of train operation causes slight relative displacement between the thread teeth of bolt and nut, which leads to uneven contact stress distribution. In each vibration cycle, the stress point changes, the normal pressure in some areas decreases instantaneously, and the friction decreases correspondingly. When the friction is less than the external loosening torque, micro-loosening occurs, and with increasing vibration cycles it accumulates into complete loosening. Vibration also causes fatigue deformation, making the preload decay further, which is why loosening is more prominent on heavy-haul and high-speed lines.
Microcapsule Anti-Loosening Coating
The microcapsule coating wraps the curing agent in microcapsules applied to the thread surface. When the bolt is tightened, the microcapsules are crushed and the curing agent reacts with the coating matrix to form a high-strength bonding layer that firmly bonds the thread teeth, with an anti-loosening service life exceeding 15 years. The application process has three critical links: surface pretreatment by sandblasting to the specified derusting grade and roughness, coating application by electrostatic spraying within hours of sandblasting, and a two-stage curing process. Quality control includes online thickness detection, adhesion testing by the cross-cut method, and anti-loosening verification by vibration testing.
Nylon Locking Coating
The nylon locking coating melts a layer of nylon on the thread surface. Nylon has excellent elasticity and wear resistance: after tightening, the nylon layer deforms elastically to fill the thread gap and increase friction, and its self-lubricity avoids thread galling damage. This type is suitable for bolts that need repeated disassembly and assembly, because the nylon coating maintains a good anti-loosening effect after multiple disassembly cycles at a low cost. It is the preferred choice for ordinary-speed railways with frequent maintenance.
Anaerobic Anti-Loosening Coating
The anaerobic coating is a one-component coating that cures automatically in an oxygen-free environment. After tightening, the oxygen between the thread pairs is discharged and the coating cures rapidly to form a hard adhesive layer with stable anti-loosening effect, oil resistance and water resistance. It suits humid and oily environments and large load impact, such as heavy-haul railways, where the hardened layer resists thread deformation. The coating thickness for all three types is controlled in the 5 to 20 micrometer range so the thread assembly accuracy is not affected.
Selection by Line Conditions
High-speed railways, with vibration frequencies up to 50 to 100 Hz and long maintenance cycles, require long-acting coatings: microcapsule type is preferred for its long service life and high bonding strength. Heavy-haul railways, with large axle loads, prefer the anaerobic type for its hardness and oil resistance. Ordinary-speed railways, with frequent disassembly, use the nylon type. Underground urban rail sections, with high humidity, use a nylon plus galvanized composite coating where the zinc layer provides anti-corrosion and the nylon layer provides anti-loosening.
Detection and Acceptance
Detection includes appearance inspection, physical and chemical performance inspection and anti-loosening performance inspection, with acceptance per the applicable railway bolt standard. Thickness is measured by eddy current gauge, adhesion by cross-cut test and corrosion resistance by neutral salt spray. The anti-loosening performance is the core indicator: with the bolt preload set to 80 percent of rated value and a vibration load applied at a defined frequency, after one million vibrations the preload loss rate must be within 5 percent with no loosening or coating peeling. Batch sampling is applied with double sampling on failure.
Frequently Asked Questions
Why do bolts loosen under vibration? Vibration causes micro-movement between thread teeth, friction decay and preload loss, which accumulate into complete loosening.
What are the three anti-loosening coating types? Microcapsule, nylon locking and anaerobic coatings, each suited to different line conditions.
Which coating suits high-speed railways? The microcapsule type, for its long service life and high bonding strength under high-frequency vibration.
What thickness should the coating have? 5 to 20 micrometers, so the thread assembly accuracy is not affected.
How is anti-loosening performance verified? By vibration testing: after one million cycles, preload loss must be within 5 percent with no loosening.