Towards better testing for self-loosening
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.
or 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 provide 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
“ Self-loosening tests are impor tant in allowing the comparative performance of locking mechanisms used on fasteners to be determined”. Dr Bill Eccles, Bolt Science
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 is sufficient preload that joint movement will not 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. 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 its products are the right choice.
108 Fastener + Fixing Magazine • Issue 68 March 2011