sensors & systems - Industrial Technology Magazine

MECHANICAL JOINING
Fasteners & Adhesives
Adhesives gain
vibrations victory
Bob Orme, a Senior Technology Specialist at Henkel, maker
of Loctite brand products, dramatically illustrates how
adhesives help fasteners stand up to high levels of vibration
Bolts that become loose as a result of vibration
have presented problems ever since threaded
joints have been around. And many ingenuous
methods of providing a solution have been
tried. But far and way the most successful has been the
use of adhesives. Indeed, the very first job undertaken by
Dr Vernon Krieble, the pioneer of Loctite adhesives, was
to lock and seal a screw in a car’s carburettor – a part
that is subjected to harsh vibration. In fact, the result
was so outstanding that anaerobic adhesives soon
became a trusted method of guarding against the
potentially devastating effects of vibration in a whole
range of industries.
But let’s get back to basics. Just what is the function
of a bolt? Essentially, it is threefold: it provides a means
of locating the components of an assembly; it creates a
clamp force; and (unlike technologies such as welding
and riveting) it allows a joint to be readily disassembled
and reused. As far as establishing a vibration-proof
fastening system is concerned, we need only concern
ourselves with the last two points.
Building the tension
Threaded fasteners provide a clamping force. Once the
tightening operation of a threaded assembly has been
completed, the clamp load is maintained by the pre-load
of the bolt. However, without some locking ‘mechanism’,
shock, alternating loads, thermal changes and vibration
can lead to a loss of this tension and allow loosening of
the fasteners to take place.
Once initial loosening occurs, the tension of the
fastener and the clamping load are lost. And when that
(Right) Threaded fasteners are held securely in
place on the floor of this minibus through the use
of a threadlocking adhesive. (Below) An anaerobic
threadlocker will provide secure fastening, even
under extremes of vibration.
happens, there is a strong likelihood of
machinery failure – with all the associated
aggravation.
Mechanical methods such as Nylon
inserts, spring washers, locknuts, split pins
and tab washers have been the traditional means of
combating this loosening effect. But, as we shall see,
their effectiveness can be limited. A far more reliable
solution lies in the use of engineering adhesives. That
may sound like a bold claim, but it can be supported
with hard facts.
Shocking results
An independent assessor built a transverse shock test
machine to compare anaerobic threadlockers, spring
washers, patch bolts, distorted lock nuts and nylon ring
nuts. The trial involved assembling a 3/8in, 16-grade
bolt in the machine and then tightening it to a controlled
tension. Air hammers were then activated and the
resulting bolt tension was plotted against time.
In each case the adhesive provided better protection
against the bolt working loose than any other method.
Indeed, the results showed that every tested mechanical
fastener failed while the adhesive continued to work. For
instance, where a spring washer was used as the locking
method the assembly became loose after just ten
seconds. More significantly,
this technique didn’t perform
any better than the assembly
with no locking mechanism.
Also, within a short period of
time, the tension in the bolt
was lost when secured by the
nylon ring nut. However, the
adhesive-treated assembly
proved permanently secure
and the tension in the bolt
remained fairly constant.
Those results are
dramatic, and pretty
conclusive. The use of
engineering adhesives – in
particular, anaerobic products
– provides a much better
vibration-proof system than
mechanical methods. These
single part adhesives cure on metal surfaces in the
absence of air. The air is generally excluded from the
joint as two mating surfaces – whether threaded or not –
are brought together. What’s more, anaerobic adhesives
fill all the spaces between the threads of a bolt and a
nut, thus simultaneously sealing the assembly. Further,
because the locking action does not depend heavily on
the shear strength of the adhesive, a relatively low
strength product can be used to provide a vibration proof
assembly.
This matter of product strength brings us back to the
third point mentioned earlier. Even though a joint treated
with an adhesive is secure against vibration, the bolt can
be readily disassembled, if necessary. Therefore
operations such as servicing and maintenance are not a
problem. Where a bolt is unlikely to be removed
regularly, a high strength adhesive can be employed.
As well as different strengths, anaerobics are
available in a selection of viscosities to suit various
purposes. For example, where the application involves
fine threads, a low viscosity product should be utilised.
On the other hand, high viscosity alternatives are more
suited to coarse threads.
The benefits
So, there can be no doubt that adhesives offer a superior
vibration-proof system for locking joints. But there are
other benefits. As previously explained, using different
product grades can control the strength of the joint.
Next, as well as providing a secure joint, the adhesive
allows the components to be sealed against the ingress
of moisture or the leaking of internal fluids. Finally, there
is a cost-saving implication, too. Through the use of
adhesives, any fastener can become a locking fastener.
All in all, it’s good news for those who want to ensure
they have a vibration-proof system.
MORE INFORMATION:
Enter D405 on the enquiry card or visit ‘latest issue stories’ at
www.industrialtechnology.co.uk for further details. You can also find
more stories from Henkel Loctite and more news on adhesives
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INDUSTRIAL TECHNOLOGY • May 2008