Technical

Problem Solving

Fasteners:- When a threaded joint is assembled the bolt is subjected to a pulling force (tension) along its length. This tension in the bolt pulls the nut toward the head of the bolt thus keeping the joint tight and together.

Self Loosening of fasteners - Loosening by exposure to vibration

If the torque applied to tighten the bolt is removed the bolt naturally tries to undo itself to relieve this tension. The friction between the threads of the mating parts of the joint, and the friction under the head and nut of the fastener is all that is working to retain that tension in the joint.

Technical

In order for a threaded joint to be assembled there must be a small amount of room between the mating parts of the threaded fastener (tolerance). By applying loads to the clamped parts (in any direction) it is possible to overcome the frictional forces in the joint allowing the nut to move on the mating threads (due to the tolerance of the parts) and relieve tension in the bolt. The clamped parts are now loose. This effect is worse when the load on the clamped part is due to transverse vibration rather than longitudinal vibration; however either mode of vibration or a combination of both will cause loosening if the vibrational forces involved are great enough.
Loosening by relaxation

The term ‘relaxation’ is applied to a threaded combination that has elongated in use. Two examples of relaxation are settling and creeping. Settling occurs when the uneven mating faces of an assembled joint are made smooth by the action of tightening the fastener and putting tension in the joint. Creep is induced when the surface pressure on the bearing surfaces of the joint exceeds the compressive strength of the material employed in the joint. Creep and settling may occur over time such that a fastener that originally appears tight loosens with time.

Whilst good fastener design or the use of mechanical systems such as washers or collars can be employed to reduce the risk of failure of a joint none of these methods are failsafe. Other methods of preventing loosening (self loosening or after relaxation) are: -

a) Physical connections – i.e. welding or interference fasteners.

b) Increasing friction between mating parts.

c) Removing the gap (tolerance) between mating parts.

Physical connections often require additional processes during manufacture and are often single use (i.e. spot welding the completed joint) and therefore not desirable in a production scenario. Friction between mating parts can be influenced by use of different materials for the mating/bearing surfaces of the threaded combination.

The simplest and often most effective means of preventing loosening is to remove the tolerance between mating parts of a threaded combination. This can be done by applying a compound to the threads prior to assembly. This may be achieved by the use of ‘active’ adhesives that both take up any tolerance and increase friction through adhering to the surfaces of the fasteners or take the form of a simple ‘passive’ patch that takes up the free space and increases the friction in the threaded joint.
Active Threadlocking

Active threadlocking is the use of chemicals to ‘glue’ the mating parts of a fastener together. Traditionally this has been achieved through use of bottled anaerobic adhesives; however this is messy, expensive and not usually suited to mass production values. Developments in chemistry led to the invention of micro-encapsulated adhesives. These adhesives can be ‘pre-applied’ to fasteners in bulk and only when the fastener is used does the chemistry within the ‘patch’ become effective. Produced mainly in acrylic or epoxy forms these active patches rely on shear forces produced during assembly to break tiny capsules (micro-encapsulates) in order to bring about a chemical reaction within the joint. A resin is formed that: a) fills any gaps in the joint and b) adheres to the surfaces of the fasteners.
Passive Threadlocking

Passive Threadlocking is generally achieved by the application of an inert substance to the threads of a male fastener. During assembly the patch takes up any tolerance between mating parts of the threaded combination thus preventing movement under vibration and therefore stops loosening of the joint.
Typically such a patch is available in 2 forms.

a) Nylon – nylon powder is cured on to the fastener using heat (approx. 200o C). As the fastener is assembled the nylon patch is cleaved such that the nylon left on the screw exactly fits the tolerance gap between the mating parts. Any excess nylon falls away from the joint as residue. This type of patch is re-useable some 4 to 5 times before the effect of the patch is negated

b) Acrylic resin – Applied cold, an acrylic resin patch retains cold flow properties

which improve its anti-vibrational properties over time. When the threaded parts are assembled the thin acrylic coating is effectively squeezed from those areas under higher tension to the voids created by the joint tolerance. This action effectively takes up any tolerance between mating parts preventing loosening by vibration. This type of product is ideally suited to small fasteners (M4 and below) as it has a very low installation torque when compared with nylon, helping to prevent stripping and galling of threads. Acrylic resin patching greatly reduces the amount of residue created when compared to a nylon patch. Due to its cold flow properties, this type of patch can be re-used as many as 20 to 30 times without serious degradation of the patch effectiveness

Alston’s product ‘VIBRA-STOP’ is the ideal material for use in these circumstances.