Anti-loosening Quick Reference
Vibration is the main cause of fastener loosening. Each locking method works differently — choose the wrong one and it may fail to lock, damage the joint or make servicing hard.
| Method | Principle | Best for | Watch out |
|---|---|---|---|
| Spring washer | Elastic rebound keeps axial load | General machinery, light-medium vibration | Limited effect on high-strength bolts; flattens out |
| Flat washer | Spreads load, protects surface | Soft materials, slotted holes | Not a locking device by itself |
| Nylon-insert lock nut (DIN 985 / GB/T 889) | Nylon ring friction locking | Vibrating equipment, outdoors, solar racking | Locking drops after 3–5 reuses |
| Flange lock nut (DIN 6923 / GB/T 6177) | Serrated base bites, larger bearing face | Automotive, brackets, power towers | Needs higher torque; wrench to remove |
| All-metal lock nut (DIN 980 / GB/T 6184) | Thread deformation locking | High temperature (>120°C) | Nylon softens — use all-metal for heat |
| Double nut | Two nuts locked against each other | Temporary fixing, maintenance | Bulky, moderate reliability |
| Threadlocker (anaerobic) | Fills thread gap and cures | Precision micro screws, instruments | Clean and degrease first; heat to remove |
| Cotter pin + slotted nut | Positive mechanical stop | Safety-critical, clevis pins | Most reliable; needs drilling, not quick-release |
Selection Rules
- Vibration-driven: nylon-insert or flange lock nuts — nylon for joints rarely removed, flange for frequent servicing.
- High temperature: above 120°C nylon softens — switch to all-metal lock nuts (DIN 980).
- Precision micro screws: threadlocker saves space versus lock nuts below M3.
- Safety-critical: use cotter pins, slotted nuts or mechanical stops — never rely on friction alone.
- Preload is the root: most loosening comes from insufficient preload — check torque first.
Common Mistakes
- Trusting a spring washer alone: limited locking effect on 10.9 / 12.9 grade bolts.
- Reusing locking hardware: nylon-insert and self-locking nuts lose holding force after one removal — replace them.
- Ignoring torque: with insufficient preload no locking device can stop micro-movement.
- Mixing metals: a stainless bolt with a carbon steel nut invites galvanic corrosion in wet conditions.
For torque values see the Torque & Preload Guide; for washer differences read spring washer, flat washer or lock nut?; for high-vibration applications such as rail see how to select railway fasteners.