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20/02/2004
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A high strength hexagonal insert has been developed that is ideal for use in high volume hydro-formed parts, including plastics and composites. Dean Palmer reports
In the drive to reduce weight, many design engineers are turning to lighter materials such as aluminium, magnesium and composites combined with relatively new manufacturing techniques such as hydro-forming. The end result is thin-walled extruded, tubular or boxed components. These new materials often require mechanical fasteners, since they cannot be welded together with the installation of load bearing threads. Traditional fasteners for this include weld nuts, clinch nuts, J-nuts and threaded inserts.
Weld nuts account for 80% of the market but the investment in robots, fixtures and welding systems can be very high. Other problems such as weld spatter, positional accuracy and the fact that they are not always compatible with aluminium alloys and stainless steel has led to manufacturers specifying clinch nuts.
Clinch nuts are slightly more expensive than weld nuts but are pressed rather than welded into the hole. But both fasteners require both sides of a workpiece to be accessible during installation and final part assembly. But clinch nuts cannot be used with composites or plastics.
So, for one-sided fastener solutions, the two main options today for a design engineer are J-nuts and threaded inserts. But J-nuts can be difficult to automate for high volume production and need a large access hole that takes up valuable space on the component that can also weaken it.
Threaded inserts, on the other hand, are easy to automate and offer the ideal solution as they are capable of providing a securely anchored female thread in any shape of material including composites. The body of the insert is allowed to bulb in a controlled, ductile fashion to create a large 'footprint' on the blindside of the parent material that resists pull-out loads. But, if the application demands that the clamp load be maximised, there is a risk that the insert's thread will strip as the female thread is weaker than the male mating thread of the screw or bolt.
This was the problem faced by motor car manufacturer Volkswagen recently. The company is starting to produce high volumes of hydro-formed aluminium and steel structural components. On its latest T5 utility van, VW used a hydro-formed steel crossbeam. The 'kinked' tubular shape of this beam meant that there was no access to the blindside to apply conventional fasteners. VW also wanted a fastener that could deliver a high joint performance and was robust against damage during assembly, so as to avoid expensive re-work or scrapping of parts.
Initially, VW considered using a standard threaded insert with an extended nut thread for increased resistance to the possibility of thread stripping caused by over-tightening a metric property class 10.9 screw. This proved unsatisfactory, so Textron Fastening Systems began work developing a high strength hexagonal, Hexsert.
The high torque capability of Hexsert ensured that the shank of the installed metric property class 10.9 screw would break before any thread damage occurred. Therefore, should excess torque be applied during assembly or during subsequent repair or servicing, the broken screw could simply be removed from the insert and replaced without any risk of damage to the beam.
Hexsert works by combining the relatively soft and ductile portion of a standard threaded insert to anchor onto the blindside of the parent material, with a much harder female thread. The result is an insert that complements high tensile metric screws up to property class 12.9.
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Author
Tom Shelley
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