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Locking Mechanisms — Complete Skill

Vibration Loosening of Threaded Fasteners

Mechanism of Self-Loosening

Junker's theory: transverse vibration causes relative micro-slip at thread flanks and bearing surface → bolt unwinds incrementally → loss of clamp load Axial vibration: not as effective as transverse in causing loosening (thread flank contact maintained) Transverse slip threshold: slip occurs when F_transverse > μ × F_clamp [μ = friction coefficient; F_clamp = bolt clamp load]

Loosening sequence:

1. Applied transverse load exceeds friction → relative motion at thread flanks
2. Nut back-rotates partial amount (small helical motion in unscrewing direction)
3. Clamped surfaces spring back → small net rotation persists → clamp reduced
4. Repeat each vibration cycle → progressive clamp loss

Stripping vs. loosening: Loosening: backspin of nut/bolt → clamp loss without thread damage Stripping: thread shear failure → permanent; occurs at very high torque application or overtightening

Vibration Testing

NAS 3350 (Junker Vibration Test): Standard vibration loosening test; transverse load applied cyclically Specimen: bolt + nut in plate with slotted holes; transverse motion ±0.5 mm at 12.5 Hz Pass criterion: clamp load retention ≥ 50% after specified cycles

NASA-STD-5020 (Threaded Fastening Systems): Defines anti-rotation requirements for spaceflight fasteners Positive locking required for all fasteners subject to vibration in critical applications

Friction-Based Locking

Prevailing Torque Nuts

Mechanism: nut interference with bolt thread prevents free spinning; torque required even without clamp load Types: All-metal prevailing torque: distorted thread (flange nut); eccentric or non-circular thread section Nylon insert (Nyloc, Nylstop): soft nylon ring at top of nut grips bolt thread; re-usable ≤ 5 times per AS/NAS specs Combination: metal + polymer

Prevailing torque range: Nylon insert M10 (Grade 8): prevailing torque = 5–16 N·m (per ISO 7042) All-metal distorted M10 (Grade 10): prevailing torque = 3–10 N·m (ISO 2320)

Temperature limit: Nylon insert: -40°C to +120°C (nylon Tg ~90°C); not suitable for elevated temperature All-metal: up to 250°C+; required for engine/exhaust zone fasteners

Standards: Nyloc: ISO 7042; ASME B18.16.6; NAS 1022 (aerospace) All-metal: ISO 7042 style B; NAS 1291 (aviation)

Thread Friction Enhancement

Preload efficiency: F_clamp = T_applied / (K × d) [K = nut factor; d = nominal diameter; K = 0.12–0.20 typically] Higher friction (μ) → higher K → more torque absorbed in friction → less clamp for given torque Target: clean, lubricated threads for consistent K → verify K by torque-tension testing

Thread form friction: Standard UN/ISO thread (60° flank angle): K = 0.15–0.20 (lubricated) Torque-tension test (Skidmore-Wilhelm or equivalent): measure actual K before assembly

Mechanical Locking

Cotter Pins and Castle Nuts

Castle nut (castellated nut) + cotter pin: Nut has slots (typically 6); cotter pin passes through hole in bolt shank + slot in nut → prevents any rotation Absolute positive locking; zero clamp loss from loosening; standard for critical control rod ends, turnbuckles, steering linkages

Cotter pin requirements (NAS 380, MS24665): Material: cadmium-plated steel or stainless steel (corrosion resistant) Minimum diameter: 1/6 × bolt diameter (structural); tabulated in MIL-HDBK-5 Pin ends spread 180° outward; do NOT return pin once installed (single-use)

Castellated nut alignment: Drill through bolt after assembly with nut torqued to minimum spec; align slot; insert pin If slot not aligned: add washers (shims) or advance to next slot (advance = tighten only, never loosen) Drill-through timing: use drill jig; perpendicular to bolt axis; QA inspection of pin security

Tab washers (tab locks): Folded metal tab locks nut flat against adjacent surface feature Used for: wheel bearing nuts, bearing ring caps; aerospace stud nuts One-time use; re-installed fresh each time bearing is disturbed

Safety Wire (Lockwire)

Material: 0.032" (0.8 mm) or 0.040" (1.0 mm) annealed stainless steel wire (MS20995C32, C40); monel or annealed carbon steel per MIL-DTL-83519

Pattern: run wire through bolt head holes → tension in wire OPPOSES loosening rotation (wire goes from bolt to adjacent bolt in tightening direction) Double-twist style (MIL-A-8625): 6–8 twists per inch (25 mm); max 6 bolt heads per wire Pigtail: twisted tail ≥ 3 wraps; bent away from safety hazard

Torque-after-lockwire: wire does not contribute to prevailing torque; purely position-locking

MS21042 (aerospace lockwire usage): standard specifications for lockwire installation in aircraft

Retaining Rings (Snap Rings)

Circlip/snap ring (DIN 471/472, ASME B18.27.1): External (shaft): groove on shaft; ring expands into groove; withstands axial load Internal (bore): groove in bore; ring contracts into groove

Groove geometry: Groove width = ring wire diameter ± 0.05 mm; groove depth = ring radial height × 0.95 (ring sits flush) Corner radius: < 0.2 mm (sharp) to ensure ring seats fully

Axial load capacity: F_axial = σ_y_ring × A_contact × n_groove_contact [approximate] Typical: 7,000–60,000 N for carbon steel E-clip on 20–100 mm shaft

Spiral retaining rings (Smalley, Rotor Clip): multi-turn; self-locking in groove; no ears; removable from either side

Chemical Locking

Anaerobic Adhesive (Loctite/Henkel)

Mechanism: anaerobic (cures in absence of oxygen with metal ions as catalyst); liquid fills thread gaps → cures to thermoset → bonds threads

Loctite grades: 222 (Purple): low strength; 1–10 N·m; for small screws; easily disassembled with hand tools 243 (Blue): medium strength; 8–25 N·m; most common; removable with tools 263 (Red): high strength; 24–60 N·m; typically requires heat (250°C) for removal 290 (Green): wicking grade; penetrates pre-assembled threads; medium strength

Performance: Thread friction increased; acts as lubricant during assembly (lower K) → then cures → locked Temperature resistance: 243 (Blue): continuous −55 to 150°C; 263 (Red): up to 220°C

Gap fill: 0 mm (metal-to-metal); 243 cures in gaps up to 0.15 mm Larger gaps: use thread-locking compound (Loctite 567 — PTFE paste type; up to 0.5 mm gap for pipe threads)

Application procedure: Clean threads: degrease (acetone/MEK); primer (7471) for passive metals (Al, zinc plated, stainless) Apply two drops at thread start or full thread coat for blind holes Cure time: 24 hours at 22°C (full strength); 1 hour functional cure; heat accelerate to 150°C for 30 min

Spiralock Thread Form (Self-Locking)

Wedge ramp thread form: 30° wedge ramp on bolt thread flanks → nut threads cam down ramp under vibration → increases friction → prevents back-rotation No friction between bolt flanks and nut flanks at rest → normal assembly torque; under vibration → self-energizing

Performance: passes NAS 3350 Junker test at significantly lower clamp loss than standard thread Application: aerospace, defense; thread insert (Spiralock insert in aluminum) for high-cycle assemblies Thread dimensions: compatible with standard metric/UNC bolt; only nut thread form modified

Spring-Loaded Locking (Quick-Release and Latches)

Spring Pins (Roll Pins, Spiral Pins)

DIN 1481 / ASME B18.8.5: Split (slotted) tubular spring pin; drives into hole; spring tension grips hole wall; transmits shear Hole tolerance: H11 (loose: for easy assembly); actual fit controlled by pin OD spring-back Replacement: punch out → discard; cheap enough to replace each time

Spiral pins (Spirol): multi-turn; lower radial stress → used in softer materials (Al, plastics)

Over-Center Latches (Toggle Latches)

Mechanism: linkage passes over center (straight line geometry) → any load tries to drive it FURTHER over center → self-locking Used for: access panels, tooling fixtures, packaging

Geometry: Locked state: crank arm + link aligned (θ_over = 3–10° past center) → over-center force required to unlock Load capacity: F_latch = F_applied × mechanical advantage at toggle point Draw latches (Southco, Kipp, Elesa): standardized; MS24648 (military over-center latch)

Spring-biased latch: spring returns to locked position after actuation; fail-safe if spring intact

Aerospace Locking Requirements

MIL-HDBK-516 (Airworthiness Certification Criteria): All fasteners in primary structure: positive locking required if vibration load expected Locking type: cotter pin, lockwire, or prevailing torque nut; Loctite alone not approved without additional mechanical lock on primary structure

NAS standards: NAS 1022: nylon insert prevailing torque nuts (aerospace grade) NAS 3350: vibration testing of threaded fasteners NAS 1291: all-metal prevailing torque nut NAS 677: castellated nut series

AS (SAE Aerospace Standard): AS3219: specification for self-locking nuts AS8879: screw thread series for aerospace (UN and UNJ series)

Standards

Output

Provide: application type (structural/control/engine/closure), vibration level (Junker transverse displacement [mm] and frequency [Hz]), required clamp retention [%] after test cycles, locking method selected (prevailing torque/cotter pin/lockwire/chemical/Spiralock) with justification, nut type (ISO grade, NAS or AS number), prevailing torque range [N·m], temperature range compatibility [°C], reuse allowance (single-use or N cycles), Loctite grade (if chemical lock; strength [N·m]; temperature limit; cure time), cotter pin size (diameter, material, NAS number), safety wire specification (diameter, material, MS number), retaining ring (DIN 471 number, axial load capacity [N]), aerospace locking compliance (MIL-HDBK-516, NASA-STD-5020 as applicable), and applicable standard (ISO 7042, NAS 3350, MS24665).