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August 2020 Newsletter

Vibrational Detachment of Threaded Fasteners

A question was recently asked if it was possible for a loose nut to act against gravity and rotate off a bolt. Such instances can happen, but only under dynamic load conditions, i.e. the bolt being subjected to some degree of vibration.

The process of previously tightened nuts coming loose is now reasonably well understood. Relaxation effects can occur (such as creep, stress relaxation and embedding) in which the preload created by the tightening process is lost, or partially lost, without the nut rotating. Self-loosening is when the nut rotation results in the preload being lost.

The usual mechanism for self-loosening is transverse joint movement. Once the preload is lost, the question is at what point will the nut become detached from the bolt? Accidents can occur once the nut is detached and the joint falls apart.

Loaded Joint

Researchers have looked at the conditions that can cause nut rotation on bolts once the preload has been lost. Researchers in 1996 experimentally investigated the response of threaded fasteners to axial harmonic vibration. They found that the direction of rotation of a fastener, either in the loosening or tightening direction, depended upon the frequency and amplitude of the vibratory input. The tests were performed on ΒΌ - 28 UNF threads that were loose, i.e. no preload present. Screws in tapped holes were used rather than a nut on a bolt but there are obvious similarities. Work was also completed to establish a theoretical framework by completing a kinematic analysis of the twisting of threaded components loaded by gravity and subjected to axial harmonic vibration. Essentially, the rotation is due to the clearance that is present between the threads. It was shown that at high vibration amplitudes, the screw would rotate upwards against gravity. At low amplitudes, the screw would rotate downwards. This work presents a mechanism by which a nut can become detached from a bolt once it is completely loose.

Nut Falling Off Bolt

It can be somewhat surprising to see a nut rotating against gravity to fall off a bolt thread. The video above shows such a situation. The bolt contains a small motor that induces vibration, the motor is activated when it is near to a magnetic field. Such bolts are sold as a magic trick but illustrates the principle involved. Normally, when the bolt either held by its head, or is seated on its head, the nut will rotate towards the end of the bolt thread irrespective of the direction of gravity. The direction of rotation of the nut can be altered by changing the vibration amplitude that it experiences. This can be achieved by holding the bolt at the end of the thread rather than at the head. Like the previous research showed, reducing the vibration amplitude can reverse the direction of rotation. < Continue reading ... >

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  Did you know? Anti-friction coatings
AF coatings' are dry lubricants consisting of suspensions of solid lubricants, such as graphite, PTFE or molydbenum disulphide of small particle size in a binder. Such coatings can be applied to fastener threads to replace metallic coatings such as zinc and cadmium and offer maintenance free permanent lubrication. By careful selection of the lubricants, AF coatings can be designed to meet specific applications. The coatings are permanently bonded to the metal surface and provide a lubricating film preventing direct metal to metal contact.

 

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About the Author
Bill Eccles formed Bolt Science in 1992.  The company is a provider of independent technical expertise in bolted joint technology.  Bill has extensive experience in the design, analysis and installation of bolted joints and has published several technical papers on the subject.  He has a Doctorate in Engineering on The Self-Loosening of Threaded Fasteners and has delivered training courses around the world on the analysis of bolted joints and bolting technology.