Towards better testing for self-loosening

Article originally published in Fastener + Fixing Magazine, March 2011.

The issue of fastener self-loosening has always been of major concern to the fastener and fixing industry for a number of reasons, some obvious right away, others less so.

For users of fasteners, the question is how to make the right choice. There are a bewildering number of options available today. So how do you know the fastener you’re being offered is right for the purpose and won’t self-loosen?

If you’re a manufacturer of fasteners, you know that quality assurance is key to inspiring customer confidence but how do you go about guaranteeing this? Also, how do you know you really are complying with tests like the German DIN 65151 standard?

The arrival of Danish company Vibrationmaster with its range of Junker Test Benches is already shaking up the fastener and fixing industry. Vibrationmaster offers high quality, lower price machines specifically designed to offer the most demanding standard of vibration testing for self-loosening possible.

Why does self-loosening occur?

Dr Bill Eccles of Bolt Science, a Consultant Engineer specialising in the technical aspects of bolting and an expert well-known to the readers of Fastener + Fixing has written:

’Nuts coming loose and detached from bolts is not a new phenomenon. Patents started to appear in the middle of the nineteenth century proposing improvements in bolt and nut design to prevent them coming loose unintentionally. It has been known for forty years that transverse joint movement can completely loosen non-locked nuts.

Once relative motion occurs between the threaded surfaces and other contact surfaces of the clamped parts the bolted connection would become almost completely free of friction in a circumferential direction. The bolt preload acting on the thread, which is sloped, creates a torque in the circumferential direction that results in self-loosening of the nut.’

Put simply, self-loosening reduces the clamp force acting on a joint. Joint slip occurs and this, in turn, leads to a bolt being subject to bending loads and eventually failing as a result of fatigue.

Is there a solution to self-loosening?

There is. The easiest way to stop nuts becoming loose is to design the joint so there’s sufficient preload that joint movement won’t occur. This is done by tightening the nuts.

All well and good in theory but, it’s extremely difficult to know what the forces acting on a joint actually are or will be and certainly impossible to anticipate every condition.

One of the most common methods of providing resistance to self-loosening is by using prevailing torque, or stiff, nuts. The higher the torque, the greater the resistance. There are a number of different torque nut options available on the market, divided into all-metal nuts and those with a non-metallic insert.

But, for the customer, the question becomes which of the multitude of thread locking products – free spinning, friction lock or chemical locking – to choose from.

For the manufacturer, the challenge is demonstrating to the customer that his products are the right choice.

As Bill Eccles says, ’self-loosening tests are important in allowing the comparative performance of locking mechanisms used on fasteners to be determined’.

The Junker Test

The Junker Vibration Test (DIN 65151) is the standard transverse vibration test. It involves inducing transverse movement into a joint while simultaneously measuring the fastener preload.

This creates the most severe conditions possible to bring on self-loosening and test a product’s likelihood of giving way to the problem.

Complying with DIN testing?

Testing your products on a Junker Test machine is the only way to comply with DIN 65151 and to test for self-loosening characteristics in general.

Up until now there has been an extremely limited choice of machines available and, in any case, these have not actually met the demands of DIN 65151. Which is where Vibrationmaster comes in.

Located at Ribe in Denmark, Vibrationmaster currently employs 15 people and is already making waves in vibration testing. Because, as company CEO Morten Schiff explains, ‘Vibrationmaster Junker Test Benches really do comply with the DIN standard and even go beyond it. They’re also around 30% cheaper than the competition with more advanced technology and, of course, additional benefits.’

Vibrationmaster Junker Test Benches

Formed in 2010, Vibrationmaster is ‘a company born out of inspiration and a powerful desire to offer peace of mind and add value by lifting mechanical vibration testing standards to new levels’.

Vibrationmaster’s Junker Test Benches are currently the only machines capable of testing from 5mm (0.2in) to 24mm (1.0in) in all fastener classes including Class 12.9.

The company currently offers three Junker Test Benches, the portable 100, compact 400 and the 600. Of the three machines on offer, the Junker 600 is perhaps of most interest to Fastener +Fixing readers and boasts some eye-catching features:

• 100% accurate amplitude – the Closed Loop System continuously measures the actual amplitude and adjusts it to what you’ve specified during operation.
• No need to change load cells or calibration – one load cell tests all fastener sizes from 5mm to 24mm.
• Severe quality testing of bolt and nut material – using rapid 0 to 4mm peak-to-peak adjustable amplitude during operation.

No wonder Vibrationmaster’s Junker Benches are generating so much interest throughout the industry.

Vibrationmaster and the Bill Eccles connection

At the heart of Vibrationmaster’s innovative technology is the revolutionary Variator. The Variator was invented by the brilliant Danish engineer Jesper Boesen Nielsen in 1988.

After he’d invented the Variator and was excitedly searching around for potential applications, Jesper got in touch with Bill Eccles. Bill explains what happened next:

‘After Jesper told me what the Variator was capable of, we saw that it could be applied to vibration testing and to creating a truly effective Junker Test machine because there was a gap in the market for this type of machine.

But, although I would definitely say anyone needing to test their fasteners for self-loosening to DIN standards or for their own peace of mind should find out more about Vibrationmaster’s Junker Test benches, my own recent research suggests that development of the machine is needed so it can impart axial as well transverse loading to allow the truly effective testing of certain locking mechanisms.

Knowing what I do about Vibrationmaster, I’m sure the company is looking at how to make this a reality.’

It certainly is.

The author thanks Dr Bill Eccles of Bolt Science, a Consultant Engineer specialising in the technical aspects of bolting. I’m especially grateful for permission to refer to Bill’s article ‘The Loosening Of Prevailing Torque Nuts’, published in Fastener + Fixing Issue 60 November 2009.

Editor’s note, 2026: this article describes Vibrationmaster and its Junker Test Benches as they were in 2011. The Junker 100 has since been succeeded by the Desktop Series (J123, J161 and JD10-P) for standardised Junker testing; the Junker 400 and 600 by the R-Series for real-joint validation. The axial loading Dr Eccles refers to is addressed today by the RDX-Series. The company is now based in Luxembourg.

Read the original article: Fastener + Fixing Magazine, March 2011