Self-loosening of Nuts under Dynamic Axial load

In this test, we will investigate the self-loosening properties of bolted joints when exposed to dynamic loads in the axial direction. Unlike Dynamic loads in the transverse direction which quite easily loosen bolted joints, dynamic loads in the axial direction require certain conditions for the nut to come loose. One of these conditions is that the repeated load is at least twice the preload.

Test frequency

Cycle load: 1 Hz

Test photographs

Test conditions

(1) Two combinations of the initial axial force F0 and 20%/70% of the bolt yield axial force
Fy = 340 × 84.3 = 28662 N (1) 20% of Fy = 5732 N (2) 70% of Fy = 20063 N

load W and 50/80% of the bolt yield axial force Fy (3) 50% of Fy = 14331 N (2) 80% of Fy = 22930 N

(3) Combinations of the tensile load and initial axial force

Conditions/ combination	Initial axial force F0	Repeated load W	Comments
②ー④	20063 N	22930 N	No loosening
①ー③	5732 N	14331 N	The possibility of loosening
①ー④	5732 N	22930 N	Loose

Test nut

HLN: Hardlock Nut
Standard rim WN: Hex nut (one type) + hex low nut (three types)
SN + SW: Hex nut (one type) + spring washer
SN: Hex nut (one type)

As it is understood that rotational looseness does not easily occur if an adequate initial preload is set, it can also be seen that even with a low initial preload, rotational looseness does not easily occur with the Hardlock Nut if the concave nut is tightened adequately.