By Leon M. Attarian
Leon M. Attarian is Director of Marketing at Penn Engineering
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A closer look at self-clinching fastener product families:
• Nuts. Standard types have load-bearing thread strengths greater than mild steel screws. Variations focus on nut size, locking-thread properties, and special alloy materials for manufacture. All clinching during installation occurs on the fastener side of the sheet; the reverse side remains flush and smooth. A mating screw finishes the job. • Studs. These externally threaded self-clinching fasteners are generally selected for applications where a component must be positioned in advance of final attachment. Flush-head studs are standard, but variations can specifically satisfy high torque, thin sheet, or electrical applications. Studs without threads can double as permanently mounted guide pins or pivots. • Spacers and Standoffs. They primarily stack or space components. All install with their heads flush within the host sheet and, using blind-threaded types, outer panel surfaces of an assembly are smooth and closed. • Access Hardware. Self-clinching panel fastener assemblies incorporate captive screws to keep loose parts to a minimum and eliminate risks associated with hardware that can loosen, fall out, and damage internal components. Ideally, they attach metal panels or other thin material components in applications where subsequent access will be necessary. • Cable Tie Mounts and Hooks. These provide permanent attachment points for mounting wires and cables to electronic chassis or enclosures. Ties slide easily through the eye in mounts, and hooks can be used to attach, remove, and return tie-bundled wires at their mounting points when components need to be accessed for service or when wires or cables must be replaced. |
Self-clinching fasteners provide excellent torque resistance in applications where sheet metal is too thin to be securely fastened by other methods.
Marketplace directives for smaller and lighter packages produced quickly and at low cost- have driven many of the recent innovations in fastening technology. With lighter packages made of thinner materials, hardware can get in the way. This has created a demand for more durable, rugged, flexible methods of fastening and joining, such as self-clinching fasteners.
Thin sheet metal is a popular choice for use in packaging because it is lightweight and takes up less space. However, finding a suitable fastening approach has not been easy. Adhesives, for example, have been known to fail, especially when heat or vibration are present; welding can be dirty and time consuming; and sheet-metal screws or loose hardware do not have the necessary holding power and often cannot be reused.
Self-clinching fasteners: form and function
Dozens of types and thousands of variations of self-clinching fasteners have been developed over the years, including free-running, self-locking, floating, and blind-hole types meeting unified, ISO, and MIL standards. Unlike other methods, self-clinching fasteners give permanent, reliable, and reusable load-bearing threads in thin metal sheets. Since they become integral parts of the assembly during fabrication, they do not loosen or fall out and never have to be handled again.
The self-clinching nut is a threaded fastener with an annular recess in its shank. During installation, the fastener is pressed into ductile metal. It displaces the host metal, causing it to cold flow into undercuts in the recess. Its job is to prevent the fastener from rotating once the installation completes.
With self-clinching fasteners, components can be removed and re-attached for access or service.Additionally, the fasteners can dramatically reduce or eliminate the amount of attachment hardware, such as loose washers, lock washers, and nuts; and usually require only mating hardware to complete final component attachment. Fewer parts promote lighter designs and less hardware in an assembly yields more savings in production time and costs.
Traditional product families are nuts, studs, spacers and standoffs, access hardware, and cable tie mounts and hooks. Specialized types have been engineered to satisfy application specific purposes. Self-clinching hardware typically is made from steel, stainless steel, or aluminum, and many types can install permanently in metal sheets as thin as 0.020 in. (0.51 mm).
Regardless of type, self-clinching fasteners install permanently in thin ductile metal sheets by being pressed into place in a properly sized drilled or punched hole and by applying sufficient squeezing force. This process causes displaced sheet material to cold flow into a specially designed annular recess in the shank or pilot of the fastener, permanently locking the fastener in place. A serrated clinching ring, knurl, ribs, or hex head prevents the fastener from rotating in the metal when applying tightening torque to mating hardware. Fasteners can be installed in small quantities with a tool as simple as an arbor press or in high volumes using automated or in-die equipment.
Marketplace demands for smaller, lighter packages are generating innovative developments in the fastening industry.
Regardless of type, self-clinching fasteners install permanently in thin ductile metal sheets by being pressed into place in a properly sized drilled or punched hole and by applying sufficient squeezing force. This process causes displaced sheet material to cold flow into a specially designed annular recess in the shank or pilot of the fastener, permanently locking the fastener in place. A serrated clinching ring, knurl, ribs, or hex head prevents the fastener from rotating in the metal when applying tightening torque to mating hardware. Fasteners can be installed in small quantities with a tool as simple as an arbor press or in high volumes using automated or in-die equipment.
Innovations
Compact assemblies have given rise to a new generation of self-clinching miniature threaded fasteners.All provide strong, permanent, and reusable threads in especially thin metal sheets. For example, compared with standard clinch nuts, self-clinching threaded nuts for extremely thin sheets (0.025 in. (0.64 mm)) exhibit a lower profile and can be mounted up to 50% closer to the edge of ultra-thin steel sheets. Their small diameters and low height contribute to a low profile and free valuable space in an assembly. Components made from the thinnest metal sheets can be stacked or spaced securely using smaller types of threaded standoffs. Self-clinching threaded studs with low-dis-placement heads can also be mounted close to the edge. Depending on thread size, they can be installed 25% to 50% closer than standard self-clinching studs without causing the edge of the metal sheet to bulge. Similarly, non-flush threaded studs for thinner sheets will provide close-to-edge installation in metal assemblies half as thick compared with conventional flush-head studs.
Variations of self-clinching studs can specifically satisfy high torque, thin sheet, or electrical applications.
More and more mechanical fastener attachment applications demand corrosion resistance and hardness. Self-clinching fasteners manufactured specially for use in stainless steel have been developed for medical, food service, and other industrial applications.
Standard steel, stainless, and aluminum self-clinching fasteners have been joined by a growing number of types made from materials offering capabilities beyond those provided by all-metal fasteners. Hybrid fasteners incorporate a combination of metal and injection-molded plastic elements and, depending on type, can be less expensive, lighter, and easier to manipulate and install than standard mechanical fasteners. They have color coated plastic caps that flag components, designate restricted or limited access areas, or correlate with equipment instructions. They can also be specified to match the color of a panel or other component or to simply enhance overall cosmetics.
The benefits derived from unconventional materials have extended even to fastener manufacture. Powdered metal processes produce fastener shapes that cannot be formed any other way. Examples of self-clinching fasteners made from such processes include right-angle types and cable tie mounting hardware.
Design tips
Here are a few basic guidelines to keep in mind when specifying self-clinching fasteners:
•Accommodate the specific application. Every self-clinching fastener type has an advantage. If an assembly requires stacking of PCBs or components, standoffs are the best choice. Panel fasteners work well if UL requirements for subsequent access to an assembly are an issue.
• Evaluate secondary benefits.Many self-clinching fasteners demonstrate unique performance capabilities — often more than one — that contribute to end-product assembly. For example, a self-clinching fastener that mates two panels at a right angle offers an added benefit by enhancing EMI/RFI shielding because it eliminates the need for cutouts in the middle of panels.
• Ensure integrity of fastener design. The production of quality self-clinching fasteners begins with good research, design, development, and testing. Precision is necessary in all facets of fastener production. Dimensional accuracy and consistency are crucial, and, if these are lacking, the costs are rejected panels, chassis, or boards upon fastener installation. Even minute size variations among parts can cause automated equipment to jam. Equivalents rarely, if ever, are a viable substitute.
• Factor installed cost into selection. Costs will rise if fasteners are time-consuming to install, fail upon installation and need to be replaced, necessitate additional hardware, or are difficult to feed into the established production process.
Working with the manufacturer
These basic tips will help you get started. Once the process has begun, however, it helps to consult with a hardware manufacturer. They know their products well and can advise about particular factors unknown to you.
For example, a prevalent misconception is that any stainless self-clinching fastener will perform without fail when installed in any stainless sheet. Standard stainless self-clinching fasteners made from 300 Series cannot be expected to perform reliably in 300 Series stainless sheets because of the relative hardness issue.
The proper stainless self-clinching fasteners for installation into 300 Series stainless sheets include types manufactured from 400 Series stainless. Types made from special alloy (precipitation hardened) stainless will offer even higher corrosion resistance. Depending on type, these fasteners can be used in sheets with hardness ranges from HRB 88 to HRB 92.
Additionally, when working with stainless, work hardening can occur around the mounting hole during the process. Every effort must be made to allow the displaced sheet material to flow as intended.
Self-clinching fastening technology has made significant strides and can be an ideal mechanical fastening solution. But partnering with a hardware manufacturer early in the design proccess is essential to get the tightest fit from your fastener.
PennEngineering
www.pemnet.com
Filed Under: Fastening + joining, Screws • nuts • bolts • rivets
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