Self-clinching fasteners provide strong, quality threads in metal and P.C. boards too thin to be tapped. Broadly defined, a self-clinching fastener is any device, usually threaded, that, when pressed into ductile metal, displaces the host material around the mounting hole, causing it to cold flow into a specially designed annular recess in the shank or pilot of the fastener. A serrated clinching ring, knurl, ribs or hex head prevents the fastener from rotating in the host material once it has been properly inserted (see Figure 1). Thus, self-clinching fasteners become a permanent part of the panel, chassis, bracket, or other item into which they are installed. The main brand of self-clinching fasteners used at Hansen Industries is PEM® fasteners, which are manufactured by the PEM® Fastening Systems Division of Penn Engineering & Manufacturing Corp.
Self-clinching fasteners:
- Provide good pullout and torque loads in sheet metal that is too thin to provide secure fastening by any other method.
- Are often more economical to use than tapping the metal sheet when guageable threads are required.
- Require less space and less handling than when using caged or anchor nuts.
- Require fewer assembly steps as hardware is installed during fabrication resulting in less total assembly time.
- Provide for a neat appearance through compact design and low profile.
In order to provide corrosion protection or surface preparation for powder coating, it is common to anodize or chromate conversion (Alodine®) aluminum parts. Due to the chemical reactions of different materials during the finishing processes, designers should be aware the following potential problems when using self-clinching fasteners on parts and assemblies:
- Cannot insert any self-clinching fasteners before anodizing. Under normal anodizing conditions, most metals such as steel or copper that are attached to the aluminum part will plate off and effectively disappear.
- Cannot insert any plated self-clinching fasteners before chromate conversion coating as the acid will take off the plated material during cleaning Can insert only stainless steel self-clinching fasteners before chromate conversion coating, if the color of the fasteners is to be the same as the color of the parts.
Designing fasteners too close to the edge or the bend of a part is a common source of product failure and can make the part expensive to manufacture. When installing self-clinching fastener, we have to pay attention to the minimum distance required between the centerline of the mounting hole to the edge or the bend of the part. As mentioned above, a self-clinching fastener is pressed and displaces the host material around the mounting hole. When working close to the edge, the sheet metal bulges allowing critical material to move away from the clinch area, reducing the pullout and torque loads of the fasteners (check the minimum distance requirement in the PEM® fastener catalog). When working near a bend, even though the material will not move away from the mounting hole, the hole will deform to an oval shape if too close to the bend. Also, we have to take into consideration of the accessibility of installation equipment and tools.
There are many different types of PEM® fasteners available for different applications. The most commonly used are self-clinching nuts, self-clinching studs, self-clinching standoffs, fasteners for PC boards, and panel fasteners. The threads are available in both unified and metric. Different shank codes are also available for different thickness of the sheet metal. Typically materials are steel, carbon steel, stainless steel, and aluminum with different finishes, such as, zinc or nickel-plated. Each PEM® fastener is identified by its specific part number, for example, figure 2 below.
Other design tips are:
- Don’t design in a fastener that is impossible to buy. Become aware of the most commonly used fasteners. This can be done by phoning Hansen or the local representative of PEM® fasteners (1-800-DIAL-PEM®) and finding out the availability of PEM® fasteners. Some PEM® fasteners have 6 – 8 week lead times with minimum purchase of 5,000 fasteners.
- Try to use common fasteners across product lines.
- PEM® fasteners can easily double the cost of a part. Consider using slots or rivets as an alternative
Insertion Equipment
We insert the PEM fasteners with 3 PEMserter Series 2000 fastener installation presses. These machines have bowl feeders that automatically feed stud, nut and standoff fasteners to the work piece.
The PEMserter Series 2000 Press is an 8-ton (71.kN) fully automatic machine with programmable logic controller, menu driven touch scree, built in safety device and 24" throat depth. The presses can install fasteners at up to 1.2 seconds per fastener.
