When two parts are fastened with a bolt, tension and compression forces are in a balance where the bolt is elongated and the fastened part material is compressed. If an external forces such as shock loads and vibrations apply to this fastening force balanced condition of the two parts, the bolt may become loose. This bolt loosening may result in equipment damages, defective product production, and critical defects that may lead to accidents. Here, typical mechanisms of screw loosening and countermeasures will be explained.

(1) Screw loosening

• A bolt fastening two parts may become loose when the following a) and b) simultaneously occur (see [Fig.1]).
  • There are contacting surfaces 1~4 of the two parts being fastened, and external forces are applied...
  • There is a clearance between the male and female threads at nut contact 1, and a relative slippage occurs.
• The external forces that affect the bolt loosening are: 4 types shown in [Fig.1] a)~d). These external forces cause "Reverse rotation motion" on the bolt resulting in loosening (see lower rows of [Table 1]).
  • Axial direction external force
  • External force perpendicular to axis
  • Axis rotational moment
  • Bending moment

• Other than the above, loosening not associated with "Reverse rotation motion" such as temperature variations (expansion and contraction), mechanical properties of materials, and wear (see upper rows of [Table 1]).
• Screw loosening fundamental patterns are organized in [Table 1].

[Table 1] Screw loosening fundamental patterns

(2) Typical bolt loosening prevention components

Typical bolt loosening countermeasures and prevention components are summarized in the following table.

[Table 2] Bolt loosening countermeasures and prevention components