Edited by Stephen J. Mraz
Engineers routinely turn to self-clinching (SC) fasteners when they need a practical way to put threads in thin-metal sheets. The permanently installed fasteners reduce hardware and promote thinner and lighter designs. But when working with stainless steel sheets and parts, engineers have to make some difficult choices. Knowing more about SC fasteners can simplify those decisions.
In general, SC fasteners are forcefully pressed into properly sized holes during installation. This forces some of the sheet material to cold flow into a specially designed annular recess in the shank or pilot of the fastener, permanently locking it to the sheet. (Cold flow is when a metal is deformed by force rather than temperature.) After installation, the reverse side of the sheet remains flush and smooth, and a mating screw completes final component attachment.
As a rule, SC fasteners should be used whenever components must be readily replaced and where loose nuts and hardware can’t be reached, such as on an inaccessible side of a metal sheet. SC fasteners can be installed during metal fabrication and can simplify and expedite subsequent component mounting and assembly, even when done in the field.
A prevalent misconception is that all stainless SC fasteners will work in all stainless-steel sheets. But the relative hardnesses of the SC fasteners and sheets are critical because the fasteners must be harder than the sheets.
The metal around the mounting hole can undergo work hardening when the hole is created if the punch tool is dull. This raises the hardness in the area around the hole and will lead to improper installation of self-clinching fasteners because the metal will not flow as predicted.
One solution is to use a special anvil with a raised ring during installation. It helps displace the stainless-steel sheet material and ensures the fastener’s annular groove gets filled.
The array of different hardness levels in stainless steels and widely varying degrees of corrosion resistance can complicate the issue of choosing the right SC fastener. So here are some guidelines and hardware profiles to help select the right stainless SC fasteners that will be installed in 300-Series stainless, the most-often-used stainless panel material).
Fastener material. Standard, stainless SC fasteners made from 300-Series will not perform reliably in 300-Series stainless sheets because they are not harder than the sheet. These fasteners can, however, work well when installed in metal sheets — usually steel or aluminum — that have hardnesses of 70 or less on the Rockwell “B” scale (HRB 70).
Proper SC fasteners for 300-Series stainless sheets include those made from 400-Series stainless or from special alloy (precipitation-hardened or PH) stainless. Many types of SC fasteners work effectively in sheets with hardnesses from HRB 88 to 92.
Corrosion resistance. Precipitation-hardened stainless SC fasteners provide extremely high corrosion resistance for use in challenging environments, including medical, food-service, fluid-handling, and marine applications. And 400-Series stainless fasteners resist corrosion as well as zinc-plated steel.
Fastener types. Fastener designers have developed “stainless for stainless” clinch fasteners that permit basic component attachment, stacking and spacing, and access to assemblies.
SC through-hole threaded standoffs and blind threaded standoffs made from 400-Series stainless work in stainless sheets with hardnesses up to HRB 88. These fasteners let engineers stack or space components in assemblies. Unthreaded versions can provide spacing on multipanel assemblies. All of these fasteners are installed with heads flush on one side of the mounting sheet. With blind types, the surface on the outside of the sheet is also closed.
Traditional stainless SC flush-head studs are for use in metal sheets softer than HRB 70. Specialty-stainless SC flush-head studs work in stainless sheets with hardness up to HRB 92. They have high hardness and corrosion resistance and mount flush in stainless sheets.
400-Series stainless SC panel fasteners represent a new generation of access hardware for stainless applications. They consist of spring-loaded assemblies which meet UL 1950 service-area-access requirements and can be installed in stainless sheets as hard as HRB 88 and as thin as 0.060 in. (1.53 mm). Captive-screw versions keep hardware to a minimum and eliminate loose parts.
|The benefits of self-clinching fasteners|
• Provide strong threads in metal as thin as 0.016 in.
• May be installed using any parallel-acting squeezing force.
• Provide high pushout and torque-out resistance.
• Do not require special hole preparation, such as chamfering or deburring.
• Reverse side of metal sheet remains flush.
• No retapping necessary after application.
• Low installation costs.
Testing SC fasteners
A second reliability measure is pushout. Pushout values indicate a fastener’s axial resistance to removal from the sheet with force opposite in direction to the installation force. It should be roughly 5 to 10% of the installation force.
A final test is pull-through. Pull-through is the fastener’s resistance to being pulled through the metal sheet when a clamping torque is applied. It generally applies only to self-clinching studs and standoffs.
Selection criteria for self-clinching fasteners
Dimensional tolerances: Self-clinching fasteners work best when they have with tight tolerances. For example, if an SC fastener specified with a dimension 0.01 in., and that dimension varies by only 0.002 in., it can degrade the fastener’s performance by 20%.
Thread fit: One reason to specify an SC fastener is to ensure the design meets one or more government specifications for thread tolerances. When considering equivalents, be sure they meet the same specifications.
Prevailing torque: Be sure the SC fastener meets required locking-torque specifications.
Heat treatment: Improper heat treatment can cause SC fasteners to fail during or after installation. Improper tempering can lead to brittleness and cause fasteners to crack. And inadequate heat treatment can make SC fasteners so soft they are literally crushed during installation.
Plating: Plating standards set limits for preparing the metal, plating thickness, adhesion, corrosion protection, salt-spray testing, and other operations. Poorly plated SC fasteners will diminish the appearance and performance of the final product.
Performance: SC fasteners should be tested to be sure they meet manufacturer’s published performance data. In addition to basic performance testing, be sure the fasteners also meet requirements for vibration resistance, thread locking, heat resistance, and electrical characteristics.
Quality control: Ensure the SC fastener manufacturer complies with ISO 9001 and QS 9000. This usually means they meet all the above criteria.