Bolt loosening falls into two categories:

• Rotational loosening – relative rotation between internal and external threads causes the preload to drop.

• Non-rotational loosening – preload is lost without relative rotation (e.g., local surface collapse or embedding after tightening). The amount of clamp-force loss depends on bolt and joint stiffness, the number of contact surfaces, surface roughness, and the bearing stress applied.

Friction-Lock Methods

Common solutions include double nuts, spring washers, and self-locking nuts.

1.1 Plain Washer

A flat metal ring placed between the joint and the nut. Protects the joint surface from nut damage and spreads the clamp load.

1.2 Plain Washer + Spring Washer

Spring washers are widely used in both load-bearing and non-load-bearing structures.

• Pros: inexpensive, easy to install, good for frequent disassembly.

• Cons: low anti-loosening capability.

1.3 Self-Locking Nut

Designed primarily for anti-loosening and vibration resistance in special applications. They generate prevailing torque via friction.

Types: nylon-insert, deformed thread (top-lock), metal-locking inserts.
Caution: harder to run down because of prevailing torque.

1.4 Double-Nut Method

Two nuts of the same hand are first tightened normally, then the top nut is backed off slightly against the bottom nut. The mutual “jamming” creates an axial preload between the two nuts and extra friction on the washer, resisting loosening.

1.5 Elastic-Insert Nut

Fiber or nylon is embedded at the thread entrance to increase friction and act as a seal against fluid leakage.

Mechanical-Lock Methods

These are more reliable and should be used for critical joints.

2.1 Cotter Pin + Slotted Hex Nut

A cotter pin is inserted through a drilled bolt and the slots of a castellated nut after tightening, preventing rotation.

2.2 Round Nut + Tab Washer

The tab washer’s inner tongue fits into a shaft keyway; after tightening, one outer tongue is bent into a nut slot.

2.3 Tab Washers (Single-Ear / Double-Ear)

After the nut is tightened, the tabs are bent up against both the nut and the joint surface, locking the assembly.

For dual-bolt locking, double-tab washers are available.

2.4 Safety Wire

Low-carbon steel wire is threaded through holes in the bolt heads and twisted so that any loosening tendency is countered by the adjacent bolt. Direction of winding must be correct.

Permanent-Lock Methods

These render the fastener non-reusable because removal destroys the joint.

• Spot welding

• Riveting

• Structural adhesive bonding

Other Supplementary Methods

4.1 Staking / Punching

After tightening, the thread crest at the nut end is deformed with a center punch to jam the threads.

4.2 Thread-Locking Adhesive (Anaerobic)

A liquid anaerobic adhesive is applied to the threads before assembly. Once the nut is tightened, the adhesive cures in the absence of air, locking the threads securely.

Summary

• Friction-lock and mechanical-lock methods allow disassembly and are therefore “re-usable anti-loosening.”

• Permanent methods are “non-re-usable.”
  Select the appropriate method based on joint criticality, maintenance requirements, and operating environment.