Why do screws loosen?

Definition of Screw Loosening

Materials fastened using screws are held together by the force of tension generated by the elongation of the bolt shaft (the bolt axis force) and by the force of compression generated in the objects being tightened (the tightening force). These two forces remain in balance as long as no external forces are applied to the objects being fastened by the screws. The general term for the forces involved in pulling or fastening the two materials together is the pretension force.

In some situations, such as in the course of using machinery, the pretension force applied at the time that the materials forming the machinery were originally fastened may decrease for a variety of reasons. This spontaneous decrease in the pretension force is what is described in general terms as screw loosening.

Classifications of Loosening

Loosening not due to return rotation:

1) Initial loosening: In the case of fine irregularities such as surface roughness, external forces acting on the bolt and the parts in contact following initial tightening cause the screw to loosen with the passing of time.
2) Collapse loosening: When the surface pressure on the parts in contact is too high, plastic deformation of the surfaces can occur.
3) Loosening due to minute tremor-induced abrasion: Abrasion can occur between the parts in contact, particularly at the joint surfaces of parts tightened parts as they slide against each other due to external forces. This abrasion can cause the bolt to loosen.
4) Loosening due to permanent deformation of sealing material: In the case that a different type of sealing material such as a gasket is used, deformation of this material can cause the bolt to loosen.
5) Loosening due to excessive external force: Apart from surface collapse caused by the surface pressure on the parts in contact, excessive external pressure can lead to plastic elongation of the bolt, causing it to loosen.
6) Loosening due to thermal causes: Changes occur in the bolt’s axial force induced by changes in temperature. In the case that a bolt elongates when exposed to high temperature, the bolt axis force is reduced, causing the bolt to loosen.

Loosening due to return rotation:

1) Loosening due to repeated external force applied in the axial rotation direction: The moment of the bolt’s axial line rotation works on the materials being tightened, causing the parts in contact to slide against each other and also against the nut or bolt head parts that are above the point of contact. In such a case, the nut or bolt can undergo return rotation, causing it to loosen.
2) Loosening due to repeated external force applied perpendicular to the axis: If an external force is repeatedly applied in a direction perpendicular to the bolt’s axis, the parts in contact may slide against each other forcing the nut or bolt to undergo return rotation and causing it to loosen.
3) Loosening due to repeated external force applied in the axial direction: If an external force is repeatedly applied in the direction of the bolt’s axis, regardless of whether the force is quasi-static or shocking, it can cause the bolt to loosen.

Screw Looseness Tests

Type of test		Explanation
Axis perpendicular vibration		Connect a fixed plate and a diaphragm by means of test bolt(s) and nuts(s), then apply an external vibration force in the direction perpendicular to the diaphragm axis and generate vibrational displacement. Make the displacement parallel without including a rotational component.
Axial rotation vibration	Torque	Apply torque to the diaphragm against the fixed plate and generate rotational displacement in the direction of the bolt’s axis. Make the displacement rotational without including a parallel component.
Axial rotation vibration	Vibration	Set an arm against the diaphragm and set a weight on its end. Generate rotational displacement by placing the fixed plate onto the diaphragm and applying vibration.
Increased/decreased axial direction load		Apply clamps to the bolt head and nut seat, respectively, then repeatedly apply a load in the direction of the bolt’s axis by means of a tension tester.
Impact	Vibration (NAS)	Vertically place a screw-fastened body tightened by means of a test bolt and nut into a slotted hole, and vibrate the hole itself up and down on a vibration table. Then apply impacts in a direction perpendicular to the bolt’s axis to the lower and upper ends of the slotted hole.
	Dropping	Drop screw fastened body consisting of two cylinders tightened by means of a test bolt and nut from a certain height, and apply impacts aimed at separating the cylinders in the direction of the bolt’s axis.
	Hammer	Connect a fixed body and an impact receiving plate by means of a test bolt and nut, and apply impacts in a direction perpendicular to the bolt’s axis direction by hammering on the impact receiving plate.