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How do you prevent nuts and bolts from vibrating loose?

How do you prevent nuts and bolts from vibrating loose?
Colin Chapman of Henkel, makers of Loctite brand products, says that adhesives are the key to removing an age-old problem.

Vibration is a fact of life. It has caused bolts to work loose since threaded joints first came into existence, and many and varied - often ingenuous - methods of resolving the problem have been tried. Yet of all the processes that have been trialled, one of the most successful has centred on the application of adhesives. Indeed, it was the very problem of vibration that led Dr Vernon Krieble - the pioneer of Loctite adhesives - to set about finding a satisfactory way to lock and seal a threaded component.

That was back in 1953, and the professor was keen to provide a solution to a problem that had dogged the automotive industry for some time - that of securing a screw in a car's carburettor in such a way that it could withstand severe vibration. Dr Krieble was extraordinarily successful in his labours - and his development of anaerobic adhesives was promptly recognised across many industries as the dependable technique of defending parts against the potentially devastating effects of vibration.

To understand how adhesives help prevent nuts and bolts (and other components) working loose it is helpful to ask a very basic question: what is the function of a bolt? There are three central purposes: first, it creates a clamp force; second, it allows a joint to be readily disassembled and reused; and third, a bolt gives a means of locating components within an assembly. When it comes to producing a vibration-proof fastening assembly, points one and two are the relevant factors.

As already stated, threaded fasteners create a clamping force. Once a threaded assembly has been tightened, the clamp load is retained by the pre-load of the bolt. However, where some form of locking mechanism is missing, loosening of the fastener can occur when subjected to conditions such as shock, alternating loads, thermal changes and, of course, vibration. This loosening leads to the clamping load being compromised which, in turn, allows the fastener to become loose.

Limited effectiveness
Mechanical solutions such as spring washers, split pins, locknuts, nylon inserts, and tab washers are seen by some as the way to overcome this loosening action.  And while some success can be achieved, their effectiveness is limited. A far better solution is the use of engineering adhesives. And that, in a nutshell, is the answer to the question posed in the title of this article.

Of course, we would say that, wouldn't we? Yet there are some pretty conclusive test results to back up that claim. A transverse shock test machine was used by an independent assessor to test how spring washers, patch bolts, distorted lock nuts, nylon ring nuts and an anaerobic adhesive responded under matching vibrating conditions. The test centred on assembling a 3/8in, 16-grade bolt in the machine and then tightening it to a controlled tension. Powerful air hammers were subsequently activated and the resulting bolt tension plotted against time.

The results to this severe test proved conclusively that the adhesive provided better protection against the bolt working loose than any of the other methods.  Furthermore, the results show that every tested mechanical system failed while the adhesive continued to remain effective against loosening. To look at just one example; where the spring washer was utilised as the locking method the assembly worked loose after just ten seconds. Perhaps more significantly - even worrying - is the fact that this technique didn't perform any better than the assembly without any locking mechanism!  

This test - along with thousands of practical applications across half a century - proves that anaerobic adhesives offer a better vibration-proof method than mechanical threadlocking methods. These single part adhesives cure in the absence of air when in contact with a metal. These adhesives fill all the spaces between the threads of a bolt and a nut - thereby sealing the assembly at the same time. Such sealing also stops the ingress of moisture - preventing corrosion and making disassembly easier.  

It's also worth noting that because the locking action does not depend heavily on the shear strength of the adhesive, a relatively low strength product can be confidently chosen to provide a vibration-proof assembly. And talking of strengths, anaerobics are available in low, medium or high strengths depending on how much future disassembly is required. Yet even a joint that has been locked with high strength product can be disassembled, if necessary, using the appropriate tools. 

There is more than sufficient proof that adhesives offer an advanced vibration-proof system when it comes to locking joints. Test after test and application upon application prove the fact beyond doubt. As well as providing a locked joint, the adhesive allows the assembly to be sealed against the ingress of moisture or the leaking of internal fluids. So there are numerous benefits to be gained from this way of preventing vibration. But, to answer our initial question, 'how do you prevent nuts and bolts from vibrating loose?' just one word is needed - adhesives.
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