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20/02/2004
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A new connect system allows plastics and ductile light alloys to be joined quickly, firmly and permanently. Tom Shelley reports
An inexpensive press-in fastener has been designed to stay firmly attached to a range of substrates, and not to rotate, with a range of versions available for different tasks.
First major application is automotive, attaching an electric cable to an aluminium alloy component, but the devices can be used to attach almost anything to most things.
The TriPress from the German company Arnold builds on the firm's ability to roll a trilobular shape onto a fasteners - a profile that is almost round but at the same time is slightly triangular. The effect is to provide a high resistance to rotation.
Retention is achieved by using circumferential locking grooves. For additional resistance to rotation, the shank can be made partly with knurled longitudinal grooves and partly with locking grooves. A threaded section for the attachment of nuts can be provided either beyond the attachment section or on top of the head, according to requirement.
Product Manager Thomas Jakob told Eureka that it is suitable for thermoplastics and light metals but that, "It is a bit of a problem in steels, although it is sometimes possible." Cost saving as opposed to use of conventional fasteners is, "Significant", because it can be inserted tight so much more quickly. Assembly time is generally considered to be reduced by 75%.
In an electrical automotive application for a major European manufacturer whom we have been asked not to name, the fastener has been used to replace a hexagonal headed screw, previously used to make an electric cable connection to an aluminium part. The previous design involved use of a lock nut plus a second nut to secure the cable. In the redesign, the TriPress fastener is pressed in, and the cable attached using a single nut.
Nominal diameters are from 3.3mm to 10.6mm". Threaded sections, where applicable are from M3 to M10. The guide includes recommendations for core hole diameters in various plastics and ductile light metals. The guide, however, advises, "The above dimensions for core holes …. are based predominately on theoretical calculations. It is important to determine the accurate parameters (press-in and ejection force, core hole diameters, torsion stress) in lab tests with production parts.
Arnold
Pointers
* Fasteners are press in and forget
* They work with thermoplastics and ductile light alloys and may sometimes work with some steels
* Trilobular shafts prevent rotation which may be further enhanced by longitudinal grooves if required
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Author
Tom Shelley
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