Introduction to Steel Structural Bolts & Anti-Loosening Solutions

Once a bolted connection loosens, the bolt may fall off and trigger major safety hazards. Even if the bolt does not come off, the reduced preload will drastically shorten the fatigue life of the connection. Therefore, appropriate anti-loosening measures shall be adopted in design to prevent bolt loosening during service.

Common anti-loosening measures adopted in design are listed as follows:

1.Double nut 

Double nut anti-loosening, also known as jam nut locking. After two nuts are tightened against each other, continuous contact pressure exists between them. Rotation of either nut has to overcome the friction between meshed threads. The pressure between the jam nuts remains constant even with varying external loads, thus delivering reliable anti-loosening performance.

2. Self-locking Nut

This type of nut mainly relies on friction for anti-loosening. For clinch nuts with knurled teeth, the teeth are pressed into preformed holes on sheet metal, where the hole diameter is slightly smaller than the nut body. The nut is integrated with a locking mechanism, which clamps the bolt threads when the nut is tightened. 

Nylon Insert Lock Nut

Distorted Thread Lock Nut

Hard Lock Nut

Wedge-action Self-locking Nut

3. Thread Locker

Thread locker is a type of adhesive formulated with (meth)acrylates, initiators, accelerators, stabilizers (polymerization inhibitors), dyes and fillers in specific proportions.

4. Cotter Pin

After tightening the nut, insert the cotter pin through the nut slot and the hole at the bolt end, then bend the ends of the cotter pin apart to prevent relative rotation between the nut and the bolt.

5.Slotted Nut

Slotted nuts are used in combination with drilled bolts and cotter pins to prevent relative rotation between the bolt and the nut.

6.Safety Wire Locking

For safety wire locking, steel wire is threaded through holes in bolt heads to connect all bolts together for mutual restraint. This method provides highly reliable anti-loosening performance, though disassembly is relatively cumbersome.

7.Preload

Generally, high-strength bolted connections require no additional anti-loosening measures. A large preload is applied to high-strength bolts, which creates substantial compressive force between the nut and connected components. This force generates friction torque to resist nut rotation and prevent loosening.

8.Lock Washer / Tab Washer

After tightening the nut, bend the single-tab or double-tab lock washer tightly against the side faces of the nut and connected part to lock the nut in place. For dual locking of two bolts, use a double tab washer to interlock the two nuts mutually.

9.Spring Washer

Split Spring Washer

Conical Spring Washer

10. Double Lock Washer