Böllhoff’s RIVNUT range of blind rivet nuts and studs have become something of a standard for 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, a new range has been created for joining composite materials. The RIVNUT SFC (Special For Composites) operates in an entirely different manner and brings about a number of distinct advantages, says the company.
“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,” states Andy Witts, managing director of Böllhoff.
“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,” he adds.
Other significant benefits are also highlighted. “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 says it can supply.
What about installation? 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).
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. It 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.
According to Böllhoff, at 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.”
