Differing stainless hardness levels and degrees of corrosion resistance can complicate fastener selection.
Designers often turn to self-clinching fasteners when they need a practical method to provide threads in thin metal sheets. The fasteners install permanently, reduce hardware, and promote thinner and lighter designs. In stainless applications, though, designers may run into some particularly hard choices. A prevalent misconception is that all stainless self-clinching fasteners will perform as intended in all stainless sheets. But, the relative hardness of the fastener and sheet looms as an overriding influence, because self-clinching requires that the fastener always be harder than its host sheet. In general, installation of self-clinching fasteners is accomplished by pressing the fastener into place in a properly sized drilled or punched hole. This process causes displaced sheet material (softer than the fastener) to cold-flow into a specially designed annular recess in the shank or pilot of the fastener, permanently locking the fastener in place.
For effective installation into stainless, however, users should realize that work hardening can occur around the mounting hole during the process. Every effort must be made to enable the displaced sheet material to flow as intended. Proper installation can best be accomplished by using a special anvil with a raised ring, which acts as a second displacer of the stainless sheet material, thereby ensuring that the annular groove of the fastener is filled.
With differing stainless hardness levels available and widely varying degrees of desired corrosion resistance, decision-making can become complicated. Following are some guidelines and hardware profiles to help make the job easier when selecting stainless fasteners for 300 Series stainless applications (the most-often used stainless panel material):
- Fastener Material. Standard stainless self-clinching fasteners made from 300 Series cannot be expected to perform reliably in 300 Series stainless sheets, due to the hardness relativity issue. These fasteners can, however, serve ideally when installed in other types of metal sheets (steel or aluminum) whose hardness is 70 or less on the Rockwell “B” scale (also designated HRB 70).
- The proper fastener solutions for installation into 300 Series stainless sheets include types manufactured from 400 Series stainless or those made from special alloy (precipitation hardened) stainless. Depending on type, these fasteners can be used effectively in sheets with hardness ranges from HRB 88 to HRB 92.
- Corrosion Resistance. Precipitation-hardened stainless clinch fasteners provide extremely high corrosion resistance for use in challenging environments, including medical, food service, fluid handling, and marine applications, among others. The 400 Series stainless types usually will provide corrosion resistance comparable to zinc-plated steel.
- Fastener Types. Significant strides have been done over the years in developing “stainless for stainless” clinch fasteners to meet various application requirements, whether for basic component attachment, stacking or spacing, or enabling subsequent access to assemblies. Today’s marketplace offers designers precipitation-hardened stainless self-clinching nuts appropriate for stainless sheets with hardness up to HRB 90. After installation the reverse side of the sheet remains flush and smooth, and a mating screw completes the final component attachment.
Stainless self-clinching thru-hole threaded standoffs and blind threaded standoffs made from 400 Series can be used in stainless sheets with hardness up to HRB 88. In varying lengths, these fasteners enable components to be stacked or spaced in an assembly. (Unthreaded types have been introduced for spacing multi-panel assemblies.) These fasteners are installed with heads flush with one surface of the mounting sheet and, using blind types, the outer sheet surface is both flush and closed. Traditional stainless self-clinching flush-head studs are intended for use in stainless sheets of HRB 92 or less. Newly designed specialty stainless self-clinching flush head studs for use in stainless sheets with hardness up to HRB 92 demonstrate very high hardness and corrosion resistance and mount flush in stainless sheets. Stainless self-clinching panel fasteners (400 Series) represent a new generation of “access hardware” for stainless applications. These complete, spring-loaded assemblies meet UL 1950 service area access requirements and can be installed in stainless sheets up to HRB 88 (and as thin as .060"/1.53mm). A captive-screw design keeps hardware to a minimum and eliminates risk of loose parts. This article was written by Jay McKenna, Special Projects Manager, at PennEngineering®. For more information, visit http://info.ims.ca/5659-141.