Rivets mechanically join two or more sheets of thin material by being mechanically deformed using a tool. Quick and easy to insert (when the user is provided with a powered tool), they remain hugely popular. One great advantage in tightly-packed production lines is that the user needs access from only one, rather than both, sides.
Given the fastener’s popularity, it’s no wonder that the design has spread into other purposes; the rivet nut is one of these. Although it may well join materials together adequately, its main job is in the latter half of the name: to provide a secure mounting for a threaded bolt on materials – such as metal sheet – that are normally too thin to be tapped.
Rivet nuts can also be inserted and set entirely from one side. They leave an internal thread which is strong enough to create a load-bearing thread that provides a secure mounting for a threaded bolt to be used. “The installation tool squeezes the rivet nut, which forces it to crush and, once set, leaves a bulge on the back end, clamping the material between it,” explains Ian Roberts, sales manager of rivet and rivet nut distributor Scell-It UK.
Just like rivets, rivet nuts come in a variety of materials, surface finishes and range of metric thread sizes, which makes them suitable for use in a variety of materials, including aluminium, stainless steel and galvanised steel.
Their sheer variety has been turned into a selling point for supplier Gesipa, 60% of whose offering is customised. Observes technical manager Stuart Booth: “There’s so many permutations: it can have a different size head, it can have countersunk head, it can have a KK or a very small head. It could be a hexagon body. There are so many variants. Then there’s rivet nuts for carbon fibre; that’s quite a special thing. A rivet nut in its basic form is quite a dull animal.”
Rivet nuts have increased in volumes over the past decade because, once set, costs are lower than other types of fasteners such as weld nuts, even if they are more expensive seen in component-by-component terms, according to Xavier Sutz at Bollhoff, product manager for the Rivnut brand.
PRECISE BY DESIGN
Booth from Gesipa explains that the rivet nut’s design helps ensure a secure installation. The part’s deformation zone, which is classed by the company, technically speaking, as a counterbore, is its thinnest part, so it is going to always deform there first, and will always fully collapse before the installation tool reaches its target torque. (He adds that a semi-automatic setting tool offers operator feedback – in the form of a green light – once it has done so).
Both Gesipa and Bollhoff provide and integrate automatic tooling such as rivet feeding systems and robotic installation tools to speed up production.
Rivet nuts (like rivets) can very easily be integrated into production. Sutz continues: “For more than 20 years now, Bollhoff has been pushing to set Rivnuts using force and not stroke, which is the historical way. Stroke means you anticipate a certain distance, and you ask your tool to repeat this distance. Force is that you target a certain force, and as soon as that force is achieved, [it] starts unscrewing.”
On the plus side, pull-to-force offers the potential to achieve production conformity. He adds: “We can measure many production parameters before or during setting to validate conformity of the setting. That’s different to a welding nut, for which the only way to make really sure is to do a destructive test.”
Compared to a weld nut, rivet nut installations pose fewer hazards to workers: there are no fumes, bright light or heat hazards to workers that must be mitigated by PPE. While welding is a skilled task, setting a rivet nut is a simple mechanical process.
Richard Avery, sales and marketing director of metal fastening equipment distributor Zygology observes: “Rivet nuts are great for a jobbing subcontractor that doesn’t know what work is coming in next. You can give it to any operator to use. You can set up people walking in off the street and they will do a satisfactory job. Low-volume work, or large applications where getting that material under the press is difficult, are applications where rivet nuts come into their own.”
The process reliability is what makes rivet nuts stand out from clinch/press nuts (see box), where it can be more difficult to distinguish between a good setting and a bad setting, according to Sutz. The latter are also mechanically cold-formed, but use a press rather than a gun, which means they need access to both sides of a panel. AUTOMOTIVE USES
Rivet nuts are popular for body-in-white and chassis automotive uses; Sutz estimates there are an average of 50 in any passenger car (as a ratio of cars sold to the number of Bollhoff rivet nuts, in the same period). Uses include for example on the dashboard, in the tailgate, roof bars and in wheel arches. The latter is a tricky example of a multi-material joint (aluminium and steel) that could lead to galvanic corrosion, if the fasteners hadn’t been specifically designed to feature an oversized plastic edge ensuring no metal-to-metal contact.
The M6 size of Rivnut is Bollhoff’s biggest seller, and comes in two thicknesses: 0.5-3mm covers most automotive applications. Other industrial uses are catered for 3-5mm depths which feature a larger root branch, chamfer and deformation zone.
Scell-it offers rivnuts in sizes from M3-M12 in flanged or reduced-head styles, as well as pneumatic and battery-powered setting tools. New from the company is the E008 battery-powered tool.
Rivet nuts are measured by among other things their grip range; first grip is 0.5-3mm, and second grip is 3-6mm for Gesipa. Booth points out that this dimension is vital to its performance: “If you use a rivet not out of its grip range, and if it’s if the grip range is too small, then the rivet the rivet nut has got to deform more than what it should do, so there’s more material trying to clamp that application part and you could end up with a loose joint.
“Obviously, if you go the opposite way and your material’s too thick on the set side, there’s not enough material to fold around your application. So you wouldn’t get a realistic footprint on the rear. And the consequence is rivet nut could pull out of the hole.” Materials used include stainless steel, low carbon steel, 5052 aluminium and monel nickel copper alloy.
A rivet nut will resist pull-out up to its design strength. Pointing out design standard ISO 898, Sutz at Bollhoff adds: “We place our part to be comparable to a standard nut, but we are not looking to make over-quality.” Standard models will be equivalent to a medium-strength Class 8 nut. Says Sutz: “A screw must be the fuse in an assembly. If you overtorque the joint, the screw will break and the thread must be reusable. We designed our blind Rivnut according to these rules.”
OTHR VARIANTS
In addition, rivet nut producers have also developed high-strength versions. Gesipa offers a higher-strength range of Class 10 nuts with a different design and different (unspecified) material. They cater for structural automotive applications.
Drawing on different materials, a different production process and setting force, Bollhoff’s HRT, high resistance thread, achieves pull-out performance of a Class 10 or 12 nut in steel, or Class 8 in aluminium, also for structural applications.
It is being used in the crash box of a German brand of passenger car, for example. While an M6 Rivnut fastener offers 16.7kN of pull-out resistance, a standard Class 10 nut offers 20.9kN.
BOX: CLINCH NUTS AND PRESS NUTS
These are alternatives to rivet nuts. Bollhoff’s version is branded a Kapti Nut; Zygology distributes the PEM range. Richard Avery describes how they work: “The self-clinch nut has to be harder than the panel it is installed into. As it is installed and pressed in, special rims and recesses on the underside of the fastener flow the panel material into those recesses. You get a very solid bond into the panel. That means that installation forces are relatively high: several tonnes as a rule” – 6t for a small fastening, up to 8-10t for a larger one.
Because of their material properties, self-clinch fasteners are more robust in terms of pull-out forces, and can be smaller and shallower than rivet nuts, providing a neat appearance, says Avery, though they tend to be installed on the rear of panels, to not detract from their aesthetics. They are especially favoured at small diameters, including M2 and M2.5, below the smallest rivet nut (M3).
A recent addition to the PEM self-clinch nut range is a geometrical thread-locking feature. The Spiralock ramped-thread technology has been designed into the latter to prevent the need to add threadlocking compound or a spring washer to make the joint self-locking. It is available for M3-M8 nuts at no extra cost.
Given the high installation forces, press nuts cannot be installed everywhere, he cautions. “With self-clinch, you have to be aware not to get too close to the edge of the panel. For all self-clinch fasteners there is a centreline to edge-of-panel measurement we ask users to be conscious of, because you can actually create a bulge at the edge of the panel because of the radial forces.”
For example, on an M5 fastener, the centreline-to-edge distance would measure less than 10mm. Provided the part can be squeezed from both sides, self-clinch fasteners can work on any material thicknesses, for example 15-20mm-thick two-part castings where a long screw is required to hold them together. “The main engineering requirement of self-clinch fasteners is that the material is ductile.”
BOX: SPECIAL TYPES OF RIVET NUTS
Composites. In response to the automotive industry’s lightweighting push, Bollhoff has introduced the SFC rivet nut, which stands for ‘smart for composites.’ The standard rivet nut design clamps down hard on the substrates, imposing some axial and radial forces, which don’t harm ductile materials such as sheet steel, but which tend to crack the more brittle composite sheet. Bollhoff’s SFC rivet has been designed to deform in a particular way that preserves substrates. Internal and external bulges don’t clamp the material immediately surrounding the drilled hole, but a wider flange imposes exclusively axial loads over a larger contact area.
Sealed rivet nuts. The automotive industry requires boxes containing batteries to be sealed to IP7 according to ISO 20653. As a result, rivet nuts from Bollhoff and Gesipa have been developed that include a polymeric seal underneath the flange to stop any fluid transfer. ”Now the battery box market is taking off phenomenally. And every component that you see now is coming in with a request for an under-head seal,” says Booth at Gesipa. He also points out such fasteners can be used in wet applications such as beneath the wade line in off-road vehicles.
