Why Threaded Joints Loosen: Two Unrelated Mechanisms

A plain thread is actually "self-locking" under static, unidirectional load—the thread lead angle (about 1.5°–3.5° for metric coarse) is far smaller than the friction angle of a steel-on-steel thread pair (about 8°–10°), so in theory it will not unscrew on its own. But in real service, bolted joints almost always see transverse vibration, alternating loads and temperature cycling, so loosening takes two completely different paths.

Path one: transverse slip self-loosening (the Junker mechanism). When the connected parts repeatedly slip relative to each other perpendicular to the bolt axis, the friction at the thread contact and bearing face is momentarily broken, the relative rotation between bolt and nut is no longer locked by friction, and the nut unscrews itself after a few to dozens of vibration cycles. German engineer Gerhard Junker's 1969 transverse-vibration test (the method now corresponding to GB/T 10431 and ISO 16130) proved that under transverse vibration, an ordinary bolted joint with no extra anti-loosening measure decays to near-zero preload in a very short time. This is the most common failure starting point on high-speed trains, engine chassis and battery-pack vibration rigs.

Path two: self-rotation loosening. When a joint sees axial alternating load or strong bending, the bolt axis develops slight elastic bending, the friction torque between thread flanks drops, and the nut slowly spins itself off under the component of the thread lead angle. It is slower than transverse slip, but under sustained alternating bending it likewise drives preload steadily down.

Preload Decay: A Failure More Hidden Than "Loosening"

Many engineers simplify "the joint is loose" to "the nut screwed out", but the more common field case is: the nut did not move at all, yet preload quietly dropped by half after a few months. This decay comes from four superimposed factors:

• Extrusion and micro-slip: microscopic asperities on connected-part surfaces are flattened under preload, the joint "shrinks", bolt elongation retracts, and preload drops. A new steel-on-steel joint typically decays 5%–15% within 24 hours after tightening.
• Material creep and stress relaxation: under high temperature or sustained high stress, metals (especially superalloys, stainless) and engineering plastics creep; the bolt's elastic elongation is eaten by plastic deformation, and preload trends down long-term. Joints on plastic housings and aluminum-alloy die castings show this especially clearly.
• Temperature cycling: steel bolts and aluminum/magnesium connected parts have different coefficients of thermal expansion, so thermal cycling repeatedly changes clamp force, forming periodic "loosen-tighten" cycles.
• Contact-face wear under vibration: repeated fretting oxidizes and wears bearing and thread faces, lowering the friction coefficient.

The engineering countermeasure given by GB/T 16823.2 "General rules for tightening of threaded fasteners" is: for important joints, re-tighten once during the "initial relaxation period" after tightening; for high-temperature or soft-material joints, choose materials per the -200°C to +700°C service covered by GB/T 3098.8, and leave decay margin by setting target preload at a reasonable fraction of yield strength (usually 60%–75% of bolt yield).

Friction Anti-Loosening: The Boundary of Spring Washers and Nylon Inserts

Friction anti-loosening is the most direct idea: deliberately create an "added resistance torque" between thread pair or bearing face, so the nut cannot turn even if it wants to. It does not change the thread itself but generates friction through extra normal pressure.

The spring lock washer is the cheapest scheme, but note: Junker transverse-vibration rig tests show that an ordinary spring washer has almost no anti-loosening effect in high-strength bolted joints; its open ends' cutting edges also press pits into the bearing face and scratch the coating. In GB/T 16823.2 practice, spring washers suit light-load, room-temperature, non-critical ordinary joints rather than critical load-bearing spots on vibration rigs.

The nylon-insert lock nut is the workhorse of friction anti-loosening, corresponding to DIN 985/982 and the domestic prevailing-torque nut system. It embeds a non-metallic nylon ring at the nut top; when the bolt is screwed in, the nylon ring is forced over the thread, producing a sustained "prevailing torque". Lock-nut catalogs from suppliers such as DSF cover M2–M20, property classes 8/10/12 and brass material; the nylon ring can be repeatedly assembled/removed about 5 times at room temperature, and softens and fails above about 120°C.

The all-metal prevailing-torque nut (corresponding to GB/T 3098.9, GB/T 6185.1 type-2 all-metal hex lock nut) replaces nylon with a deformed thread, elliptical collapse or reduced thread; it withstands temperatures up to 300°C or more, suiting engines, high-temperature piping and other service nylon cannot handle. GB/T 3098.9 sets clear limits on proof load, prevailing torque and screw-in/screw-out torque for these nuts.

Mechanical Anti-Loosening: Cotter Pins and Tab Washers

Mechanical anti-loosening does not rely on "adding friction" but uses an independent part to geometrically "hard-lock" nut and bolt (or connected part), leaving the nut no freedom to rotate. It is the most reliable, the "last line of insurance".

• Cotter pin + slotted nut: the cotter pin passes through the slotted nut's slots and the bolt's tail hole, joining the two into one—the classic anti-loosening method on aircraft, tractors and steering gear, fully reversible and re-checkable.
• Tab washer (external/internal tab washer): the washer's inner or outer tab is bent down against the nut side and the connected part, mechanically locking it.
• Double nut (jam nut): two nuts press against each other, using the axial reaction between them to add friction in the thread pair; simple but heavy and sensitive to tightening sequence.
• Safety wire: stainless wire passes through holes in bolt heads, linking and pulling each other; common on aero engines and hydraulic systems—essentially a mechanical stop.

Chemical Anti-Loosening: Pre-Applied Adhesive "Locks" the Thread

Chemical anti-loosening (threadlocker / pre-applied adhesive) pre-coats the thread face with an anaerobic or microcapsule glue layer; after the bolt is screwed in, the glue cures under oxygen deprivation and contact with metal ions, bonding the thread pair into one. Miniature-fastener catalogs call it Nylon Patch / Thread Locking; its advantages are no added parts, no added weight, vibration resistance and locking at any position.

Engineering grades by strength: low strength (removable, for small screws and covers), medium strength (removable, most used), high strength (permanent lock, for spots that must not be disassembled again). Pre-applied adhesive also doubles as a sealant, often paired with sealing structures on liquid-cooling modules and sealed housings. Its limits: it does not compensate preload decay, and cure strength drops on contaminated (oiled, plated) threads; for high-temperature service choose a heat-resistant type.

Cross-Comparison of the Three Families and Selection Advice