Why do nuts come loose?

Causes of fastener looseness

When a fastener becomes loose, it is due to a reduction in the tightening force (the bolt preload), which has two causes: non-rotational loosening (slackening) and rotational looseness (spontaneous loosening).

cause of looseness

1Fastener non-rotational looseness

  • • Initial wear
    Cause: Fatigue due to uneven wear in the connecting partsMain countermeasure: A conical spring washer having a spring counterforce

    Initial wear
  • • Micro-embedding on seating surface
    Cause: Localized Plastic deformation across all seating surfaces (nut face and bolt face).
    Main countermeasure: A strong, rigid flat seating that does not sink

    See more

    Sinking looseness
  • • Looseness from micro-movement wear and external forces such as excessive tightening
    Cause: Wear accompanying horizontal displacement of the contact parts
    Main countermeasure: A design incorporating high-strength materials, surface treatment such as carburization or nitriding, and the limiting pressure of the lubricant material

    external forces
  • • Looseness due to heat
    Cause: Relaxation caused by a difference in thermal deformation
    Main countermeasure: A design taking into consideration the linear coefficient of expansion of the materials


2Fastener rotational looseness

Cause: Cyclic loads in the bolt axial direction, bolt transverse direction and bolt axial rotation direction
Main countermeasure: Parts that prevent looseness

Fastener rotational looseness

The danger of a reduced tightening force (bolt preload)

When the bolt preload is low or drops, the number of cycles until bolt failure is reduced, creating a dangerous situation.

reduced tightening force

1. Reduced preload
The bolt bears most external forces, leading to a higher risk of bolt breakage or damage

loosening danger

Loosening danger

2. Adequate preload
Because the fastened assembly as a whole bears most external forces, the bolt is not impacted and does not weaken

tight and safe

Tight and safe

Incorporating countermeasures against non-rotational loosening, the Hardlock Nut operates on the wedge principle for a powerful anti-loosening effect.

The Hardlock Nut can therefore be used safely without concern about the tightening force dropping.

To learn more about the anti-loosening effects of the Hardlock Nut, read “Why the Hardlock Nut Does Not Loosen.”