Special Products Manager
Edited by Stephen J. Mraz
Self-clinching fasteners were designed almost 70 years ago as way to attach components to thin sheet metal and ensure the connection would withstand hard tugs and torque loads. They are also useful for components that will be replaced and where loose nuts and hardware would be inaccessible. They create permanent and reusable load-bearing threads and once installed, will not loosen or fall out, and use less hardware. These advantages have helped make them popular for a host of applications since they were first introduced.
But engineers must have a basic understanding of the technology and how best to use the fasteners to get the most out of them.
Purposes and Profiles
There are dozens of types and thousands of variations of self-clinching fasteners, including free-running, self-locking, floating, and blind-hole types meeting unified, ISO, and MIL standards. They are usually made from steel, stainless steel, or aluminum.
Regardless of type, self-clinching fasteners install permanently in thin ductile metal sheets by pressing them into a properly sized hole and squeezing them. This forces displaced sheet material to cold flow into an annular recess in the shank or pilot of the fastener, locking it in place. A serrated clinching ring, knurl, ribs, or hex head prevents the fastener from rotating in the metal when technicians apply tightening torque to the mating hardware. The fasteners can be installed during fabrication or final assembly.
Standard product families include nuts, studs, spacers and standoffs, access hardware, “face-on-face” panel-mounting hardware, and cable-tie mounts and hooks.
Nuts. Standard types are designed with load-bearing threads stronger than those of mild-steel screws. They can vary in size, locking-thread properties, and the alloys they are made of. Clinching during installation takes place on the fastener side of the sheet; the reverse side remains flush and smooth. A mating screw finishes the job.
Studs and pins. These externally threaded fasteners are used when components must be positioned in advance of final attachment. Flush-head studs are standard, but variations can meet high-torque, thin-sheet, or electrical requirements. Studs without threads can be used as permanently mounted guide pins and pivots.
Spacers and standoffs. The most common types of these include thru-threaded and blindthreaded versions, and they are used primarily to stack or space components. All have their heads flush within the host sheet after installation. Using blind-threaded versions leaves outer panel surfaces smooth and closed. Standoffs with concealed heads let circuit boards snap into place for easier board assembly and disassembly.
Access hardware. Self-clinching panel fasteners have captive screws that keep loose parts to a minimum and eliminate the risks associated with hardware that can loosen, fall out, and damage internal components. Panel fasteners are well suited for attaching metal panels and other thin-material components if subsequent access is necessary.
“Face-on-face” panel mounting hardware. These include fasteners that can join two metal sheets into flush-attachment connection without protrusions on either side. Others can join two sheets flat against each other and let them be easily disconnected by sliding the top sheet sideways and lifting it from the “panel-on-panel” assembly. Because single fasteners rotate, they can also serve as hardened pivot points.
Cable tie mounts and hooks. These provide permanent attachment points for attaching wires and cables to electronic chassis and enclosures without screws or adhesives. Ties slide easily through the mounts’ eyes. And hooks can be used to attach tie-bundled wires at mounting points for components that need to be serviced or wires or cables that will need to be replaced.
Here are a few rules of thumb for using self-clinching fasteners:
The panel must be ductile and softer than the fastener, even if both panel and fastener are made of steel. This is what lets the softer material clod flow into fastener and lock it in place. The panel must also have the minimum thickness for a particular fastener. For example, some self-clinching fasteners work ins heets as thin as .020 in. (0.51 mm), but the minimum thickness is generally 0.030 in. (0.76 mm). There’s usually no maximum sheet thickness.
Self-clinching fasteners do not need special installation equipment. They are installed using any type of parallel-acting press that can be adjusted to alter the force it delivers. Quick-impact installation methods, such as a hammer blow or similar force, will not work because it doesn’t allow time for the sheet material to flow into the fastener.
Testing self-clinching fasteners
A self-clinching fastener’s reliability depends on many factors, beginning with a properly sized hole, the thickness and hardness of the host panel, proper installation, and the application. Traditionally, three tests determine reliability.
Torque-out test: Determines a fastener’s ability to resist rotation within the panel. This test is often made on the fastener’s head with values usually exceeding the ultimate torsional strength of the mating screw or nut.
Pushout test: Indicates a fastener’s axial resistance to being pulled out of the sheet. It should be roughly 5 to 10% of the force used to install the fastener.
Pull-through test: Pinpoints a fastener’s resistance to being pulled through the metal sheet when applying a clamping torque.
With a press, installation is relatively simple. The fastener’s shank or pilot slides into a properly sized mounting hole in the sheet. Holes should not be chamfered or have broken edges in excess of 0.005”/0.127mm, and hole tolerances must be held. Force from the press pushes the fastener until the head touches the sheet metal. (Some fasteners are fully installed when the head is flush within the sheet.) Mating hardware can then be installed from the side opposite the fastener’s head.
The fastener manufacturer’s “minimum center line (C/L) of hole to edge of sheet” distance should also be maintained. If not, the edge of the panel likely will bulge. And once the panel bulges, performance degrades because material cannot flow into the fastener’s undercut.
The distance between two or more self-clinching fasteners can be calculated using this formula:
DBH = DCL + 0.5 (DSH)
where DBH = distance between the two holes, DCL = distance from the center line of the first hole to the edge, and DSH = diameter of the second hole.
Designers also need to consider galvanic corrosion when choosing fasteners Although passive 300 Series stainless and aluminum are some distance apart in a galvanic series, the possibility exists for galvanic corrosion if an electrolyte is also present. So determine what electrolytes, if any, are present, and in what amounts. Also determine how much oxygen is present because it can also lead to galvanic corrosion Then determine if the fastener-to-panel interface is critical. Will a little bit of corrosion at the interface be a concern?
If passive 300 Series stainless steel is unacceptable in an aluminum panel, there are several other options. In descending order of effectiveness, these include:
Using an aluminum fastener verified as strong enough for the application.
Specifying 300 Series parts with cadmium plating.
Zinc-plated versions or special stainless-steel fasteners if zinc is too close to aluminum (depending on the alloy) in the appropriate galvanic series.
And here’s an important note about installation in stainless: It can work-harden the area around the mounting hole. This must be avoided because it prevents displaced sheet material from flowing as intended.
Two more recommendations can help in choosing fasteners:
Evaluate potential secondary benefits. Many self-clinching fasteners have one or more capabilities that can benefit assembly. For example, a self-clinching fastener that mates two panels at a right angle also enhances EMI/RFI shielding. Right-angle fasteners in an enclosure means internal components can be mounted without using bent tabs. These tabs need cutouts to accommodate brackets and other hardware. By not having cutouts, the enclosure truly encloses the components, protecting them from EMI/RFI.
Ensure integrity of fastener design. Making quality self-clinching fasteners takes research, design, development, and testing. Precision is also necessary, with dimensional accuracy and consistency being crucial. If any of these are lacking, the result is rejected panels, chassis, and boards. Even small variations among parts can cause equipment to jam, increasing downtime.
Fasteners should be evaluated in terms of proper thread fit, exacting heat treatment to the proper Rockwell hardness, and plating, among other relevant specs. Substituting lesser-quality hardware will, in turn, degrade fastener performance.
And a final tip: Enlist a fastener supplier’s support and inherent resources in the early stages of a design. Then, after a fastener is chosen and manufacturing problems arise, the partnership established with the supplier can open the door to invaluable experience and assistance.
PennEngineering, (800) 237-4736