Why do Screws Work Loose?

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.

Classification of Loosening

Loosening not due to return rotation	Loosening 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.	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.

Loosening not due to return rotation

Loosening 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.

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 Test

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.

Looseness Prevention Countermeasures

Effect	Function	Adaptation example
Initial loosening countermeasure	Spring pressure	Disk spring washers, spring washers
Collapse loosening countermeasure	Reducing the surface pressure	Highly rigid flat washers
Return rotation resistance	Stopping mechanical rotation	Slotted nuts, split pin-attached bolts, tongued washers, claw washers
	Increasing the screw part’s degree of adhesion	Sheet metal screws, coiled inserts
	Increasing the return torque	Non-metallic insert-attached detent nuts (e.g. nylon, nuts, nylock), all-metallic detent nuts (e.g. U nuts), flange-attached bolts (e.g. flange nuts, skirt nuts), pre-belling torque form nuts (e.g. tough lock nuts, space lock nuts)
Return rotation prevention	Forced locking by eliminating the fitting gap	Double nuts HARDLOCK Nuts
Return rotation prevention	Solidification and adhesion within the fitting gap	Anaerobic glue (e.g. Lock Tight), glue-containing capsule-attached bolts (e.g. chemical anchor bolts)

A. The return rotation resistance-use group are countermeasures that in almost all cases can prevent any reduction in the pretension force (axial force), which means that their effectiveness against looseness due to return rotation is considered high. However, in the case of double nuts, unless they are completely locked (pinioned), their performance is likely to decline remarkably, as has been shown in variety of tests. Also, anaerobic glue does not exhibit its effectiveness unless it is used in line with the using instructions. Compared with these measures, at the present time HARDLOCK Nuts are considered to be the most effective connection method available.
B. The return rotation resistance-use group includes methods that do not allow the pretension force to decline by more than a limited extent and others that merely slow down the rate at which the pretension resistance declines. With such methods, the level of reliability varies, with some methods serving only as a means of screw disjointing prevention or dropping prevention.
C. It is considered that initial loosening and collapse loosening countermeasures are not usually suitable for preventing loosening due to return rotation. This is because their seating surface prevention functions are poor.

Test Data on HARDLOCK Nuts

HARDLOCK Nuts are classified as return rotation prevention-use countermeasures. They are evaluated as looseness stopping nuts that exhibit a performance superior to that of ordinary double nuts. A variety of tests have yielded data on the performance of Hard Luck Nuts, as follows.

1）Axis perpendicular vibration tests: These tests, of which the Junker vibration test developed in Germany is a famous example, are a representative method of evaluating screw loosening. Junker test data on the HARDLOCK Nut’s performance gathered by the Shonan Institute of Technology is available.
2) Axial rotation vibration tests: No test data is available at present.
3) Axial direction load increase/decrease tests: These tests form a method of measuring the axial force reduction due to bolt fatigue with repeated loading. No HARDLOCK Nut-related axial direction load increase/decrease test data is available at present. However, test data is available on friction joint-use high-tensile bolts employed in structures such as pylons.
Application example: HARDLOCK Nuts are employed in pylons and other structures (NTT pylons, etc.) as a replacement for high-tensile bolts.
4) Impact tests: a) The US NAS 3350 and 3354 standard vibration tests are well known internationally, HARDLOCK Industry has adopted these test standards as in-house standards as well.
b) Regarding dropping tests, in-house comparison test data from a certain ironworks is available.
c) Regarding hammer tests, no test data is available but there are extensive actual application records.
5）The HARDLOCK Nut’s major features apart from its loosening stopping function: a) Sufficient fastening is possible regardless of the axial force (pre-set axial force control is possible).
b) Effectiveness is maintained even in the case of repeated use (repeated re-use is possible until nut metal fatigue occurs).
c) Compatible with a wide range of environmental conditions (flexible combinations of shape, material and surface processing).

*Reference: extract from Neji Teiketsu Guidebook (Screw Fastening Guidebook), section on "Fastening".

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