MECHANICAL JOINING Fasteners & Adhesives Adhesives gain vibrations victory Bob Orme, a Senior <strong>Technology</strong> 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 40 INDUSTRIAL TECHNOLOGY • May 2008
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