Fastener Testing

Here J LANFRANCO & Cie outlines the three types of tests – Junker, vibration and impact, as well as helicopter – that it is able to perform on all its metal nuts to ensure they are a reliable solution.

The guarantee of the resistance to spontaneous unscrewing of assemblies in industrial environments is based on three distinct test methods. To evaluate this resistance, most current standardised tests focus on tests that induce transverse mechanical stresses, the most severe in the face of spontaneous unscrewing phenomena. The approach to evaluating these tests involves a comparative experimental study integrating three distinct types of stresses and tests, covering various configurations of assemblies and security solutions, which are: Junker test, vibration and impact test, and the helicopter test.

Junker test

A Junker test looks at the transverse loads generated by alternating movements, according to ISO 16130, DIN 65151 or DIN 25201-4 B. The test consists of tightening and tensioning a screwed or bolted assembly, then subjecting it to a succession of alternating transverse movements. The screw and nut or thread are positioned in two different parts of the test fixture, one in a fixed part and the other in a moving part.

Vibration and impact test

Vibration and impact testing uses transverse stresses generated by alternating shocks. The test consists of tightening and tensioning a screwed or bolted assembly, then subjecting it to vibrations – generating a succession of alternating transverse shocks. A test device consisting of a vibratory pot supporting a test fixture shall consist of a part attached to the vibrator having an oblong hole.

The screw and nut and a back-up washer are tightened on a shouldered spacer, this assembly is mounted freely, with play, in the slotted hole of the first part of the assembly. After tightening to achieve the target tension, the joint is subjected to alternating transverse displacements. Tests are performed on a test bench defined according to ISO 7481, ISO 8642, NAS 3350 or NASM 1312-7.

Helicopter test

Helicopter testing involves testing eccentric axial loads by alternating bending. The test consists of tightening and tensioning a bolted joint, then subjecting it to a succession of alternating bending stresses thanks to the eccentricity of the beam’s centre of gravity. 

The clamping force is measured and monitored during tightening and testing using an ultrasonic sensor positioned at the end or on the head of the screw. The screw and nut are positioned on a mobile support that allows a martyr beam to be tightened in offset.

After tightening to the target tension, the assembly is subjected to alternating displacements that generate the alternating bending of the martyr beam according to the parameters defined XPE 25-046-2.

Mechanical tests in Junker or impact laboratories are often defective in real conditions and represent a minority of assemblies subject to dynamic stresses. It is crucial to choose the test method closest to the final assembly to avoid failures.