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FEATURE FASTENINGS & ADHESIVES


UP TO THE CHALLENGE of fastener specification


Kevin Moorcroft, MD, Challenge (Europe), examines the issues of supply and correct specification of threaded fasteners in a CAD age


T


he business of fastener manufacture and distribution has always been


to provide the fullest of support to customers in all aspects of specification and supply. This has often involved anticipating changing demand patterns during periods of industrial upheaval – such as the one we are all presently experiencing. During these difficult times many


suppliers and customers will look to reduce stock holding, whereas specialists like ourselves will increase stocks, recognising that in the ‘little and often’ scenario created by recessions, pandemics and the like, customers need an instantaneous response to their needs. This ability to meet difficult demand situations spills over into being able to provide and support Kanban, VMI and other supply to line systems which can be infinitely adapted to suit individual customer needs. It is these flexible protocols which enable survival during recession and rapid regrowth when opportunities arise. A specialist service therefore does not


stop at supplying goods, but includes a total partnership support package, as illustrated below.


BEWARE OF THE DANGERS In recent years, the complex nature of fasteners has tended to be overlooked or misunderstood. Due to the wide range of specifications it often proves difficult for design and production engineers to decide upon the most suitable fastener for specific applications. The increased use of CAD systems can compound the problem of selection by identifying the correct dimensional requirements, but not necessarily the appropriate material or tensile strength. With all the benefits of the internet


it has become apparent that there are dangers – who of us has not come across the life-critical disease we have ‘self-diagnosed’ online, or the false news/conspiracy theory which can reinforce our worst fears? At a professional industrial level,


Challenge Europe often finds design engineers can overlook the sophistication


18 SEPTEMBER 2020 | DESIGN SOLUTIONS


of threaded fasteners and fall into the trap of ‘internet specification’ or ‘specification by CAD system’. This practice seems to derive from pressure of time, coupled with the perception that fasteners are a low-end product and that sourcing can be done over the internet, relying on urgent sample supply in case of doubt. Again, with the help of Challenge’s


partnership support package, assistance can be provided in not only the choice of suitable specifications but also those specifications most readily available and hence economic in use. Samples can be made available for testing and proving purposes as an additional aid to correct selection. In some cases, it is not always possible


to avoid the use of a customised fastener and Challenge is also able to advise and support such requirements. For maximum benefit, consultation at the concept/design stage is recommended. Ultimately, one can


never know completely how a fastener will function in-situ until you try it – which is why accelerated ageing trials and extended excess load tests are frequently performed to ascertain a realistic safety margin. Guidelines from


hard-won experience can normally be accessed, but sometimes an unusual situation needs to be evaluated or perhaps a replacement of the exact


Design engineers can overlook the sophistication of threaded fasteners and fall into the trap of ‘internet specification’ or ‘specification by CAD system’


same type is simply not available and a suitable replacement is required. It is easy and tempting to over-specify, but Challenge Europe can help keep such a project on track. Aerospace, automotive and rail


industries all follow this design for a specific lifetime philosophy – an approach appropriate where safety critical assemblies are required – but if adopted for more general engineering it sometimes leads to over-specification in products which do not need it. This over-specification is often a comfort issue, as a sort of safety margin which often makes the custom spec prohibitively expensive if not actually impractical. Over-specification is a significant potential problem in the fastener industry.


COLLABORATION Applications where there are issues of weight and space can lead the designer to adopt a correct ‘cost is secondary’ attitude when there are safety critical elements involved. Often, however, given a minimum lifetime expectancy even in non-critical situations, there can be a tendency to ‘play it safe’ and add a safety margin on top of a safety margin, leading to gross over-specification. For example, going up one or two


sizes with consequent weight and space implications; or calling for an exotic and expensive alloy where a lower-cost high-grade treated steel would be more appropriate. Over-specifying one fastener may not be a problem, but over specifying thousands can make a big difference overall. Design for a specific lifetime is


Collaboration with a specialist can help ensure the right fastener is selected for the application


often a highly skilled art of combining knowledge from different areas such as specific performance of the fastener, effect of varying material, options for head types and the stress they involve, stresses inbuilt into the fastener from its manufacturing process or treatment. The solution can be achieved in collaboration with a fastener specialist


such as Challenge Europe.


Challenge (Europe) www.challenge-europe.co.uk


/ DESIGNSOLUTIONS


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