28 September 2016
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Changing radial to axial force creates new breed of composite fasteners

Böllhoff’s RIVNUT® range of blind rivet nuts and studs have become something of a standard form joining thin and often disparate materials. In fact, for joining hollow components they are often the only solution.

While the RIVNUT® is ideal for joining thin walled materials including all metals and plastics, for joining composite materials, a new range has been created. The RIVNUT® SFC (Special For Composites) operates in an entirely different manner and brings about a number of distinct advantages.

The first thing to understand is that with a standard RIVNUT® the forces applied to form the distortion that secures the nut are radial in nature. With composite materials, such as carbon fibre, these forces can cause cracking and delamination around the hole. The RIVNUT® SFC exerts axial forces that preserve the material’s integrity.

This is a product development to overcome difficulties experienced when installing blind rivet nuts into composite panels and so is of significant interest to the automotive industry. There are other significant benefits.

For example it is possible to site the fixing holes closer to the edge of the composite material than was previously possible. It is also the case that the hole tolerance has been expanded substantially to 0,6mm, while at the same time it is possible to have off axis setting without compromise.

RIVNUT® SFC is available in alternative configurations upon request (stud, under head seal and so forth). The grip range can also be increased in certain specific conditions when associated with certain substrate materials albeit in these cases

a prototype validation will be necessary - that Böllhoff can supply.

To install the RIVNUT® SFC the fastener is inserted with a threaded mandrel that is then retracted drawing the body of the nut up against the underside of the material being fastened, whereupon the body distorts to form a tight fit and the mandrel is fully retracted.

The recommended setting force is dependent upon a combination of information coming from RIVNUT® SFC parameters (force to obtain a good installation) and screwing parameters (tensile strength after assembly and during service).

Intriguingly, when an assembly is in use, external influences generally increase the tensile strength in the screw. With correct installation, RIVNUT® SFC exhibits the same behaviour as a standard nut.

RIVNUT® SFC is installed using a single-sided setting technique. Often the only fastening solution for hollow sections, housings or where access is limited to only one side of a component. This simplifies the design avoiding any need for a dedicated access hole in the rear panel.

The fasteners can be installed without operator training and several levels of installation process control are available to enhance quality management. It is possible to easily and non-destructively test the installed RIVNUT® SFC.

In installation, there are no fumes (no exhaust needed) and no heat (no protection needed) and hence, there is no heat impact on application (surface treatment, deformation, material resistance and so forth). There is also no pollution or risk to the operator. The assembly is a fast, reliable, repeatable and cost competitive process with optimised installation possible in under three seconds. There is a quick tooling exchange and adjustment operation – for example to switch from M6 to M8.

Finally, there is a full range of setting equipment, from manual to full automatic

RIVNUT® SFC blind rivet nuts and studs can be fitted at any stage so bringing great flexibility to the production process. RIVNUT® SFC is a captive system.

Böllhoff offers the services of its own certified laboratory to assess and report on the performance of its products when installed into the customer’s components.

Bollhoff Fastenings Ltd

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