Causes and preventive measures of bolt looseness

1. Fastener loose reason

When the fastener is loose, it is because the fastening force (bolt pre-tightening force) is reduced, which has two reasons: non-rotating loosening and rotating loosening.

1.1 Fasteners are not loose due to rotation.

Initial wear

Cause: uneven wear of connectors leads to fatigue.

Main countermeasures: conical spring washer with spring reaction force.

Micro-embedding of contact surface

Cause: Local plastic deformation of all contact surfaces (nut surface and bolt surface).

Main countermeasures: a firm and rigid flat surface will not sink.

Fretting wear and looseness caused by external force (such as over-tightening)
Cause: Wear is accompanied by horizontal displacement of contact parts.
The main countermeasures are: combining the design of high strength materials, surface treatment (such as carburizing or nitriding) and the ultimate pressure of lubricant materials.

Loose due to heat
Cause: Relaxation caused by thermal deformation difference.
Main countermeasures: Consider the design of linear expansion coefficient of materials.

1.2 Fasteners rotate loosely
Cause: Cyclic load in the axial direction, transverse direction and axial rotation direction of bolts.
Main countermeasures: prevent loose parts.

2. Danger of decreasing bolt pre-tightening force

When the pre-tightening force of the bolt is low or decreased, the number of cycles until the bolt fails is reduced, resulting in a dangerous situation.

2.1 Insufficient pre-tightening force
The bolt bears most of the external force, which leads to a high risk of bolt fracture or damage.
2.2 Sufficient pre-tightening force
Because the fastening assembly as a whole bears most of the external force, the bolt will not be impacted or weakened.
Combined with the measures to prevent non-rotating loosening, the hard lock nut adopts wedge principle, which has strong anti-loosening effect. Therefore, the hard lock nut can be used safely without worrying about the decrease of tightening force.

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