Frequently Asked Questions – PEM® Fasteners

The RoHS compliant status of all PEM®, SI® and ATLAS® cataloged fasteners can be found in the RoHS Lookup Tool.

No, a self-clinching fastener must be installed using a squeezing action.  A quick impact installation will not allow sufficient time for the sheet material to cold flow.

Generally, you must have access to both sides of the sheet to properly install self-clinching fasteners.  However, there are some 1/4’” / M6 or larger nuts which can be drawn in from one side using an impact-torque wrench.  For information on blind threaded inserts for one sided access installation go to ATLAS® Fasteners Product Information.

This is a question that comes up on a regular basis because passive 300 series stainless steel and aluminum are some distance apart in a galvanic series and there is the potential for galvanic corrosion if an electrolyte is present.  Whether or not using our 300 series fasteners in aluminum panel is acceptable depends on the specifics of the application.  The two important factors of the application are as follows:

1. What type of electrolyte, if any, is present, including the type of flow and amount of oxygen present.

2. How critical to the function of the product is the fastener to panel interface?  Or stated another way, will a small amount of corrosion at the interface be a concern?  Typically the mechanical holding power is not the critical concern.  Factors such as electrical conductivity or cosmetics are often the more critical issues.

The customer’s technical staff must ultimately make the decision on the acceptability of using our passive 300 series stainless steel in aluminum panels.  If they decide it is not acceptable, we have the following three options, presented in decreasing order of effectiveness.

Our first recommendation is to consider the use of aluminum self-clinching fasteners.  Aluminum nuts are our type CLA, made from 2024-T4 aluminum alloy.  Aluminum studs are our type FHA, also made from 2024-T4 aluminum alloy.  Aluminum standoffs are our type BSOA/SOA, made from 7075-T6 aluminum alloy.  All of these aluminum fasteners will clinch into panels with a hardness of HRB 50 or less.  This option is preferred whenever the aluminum panel hardness is below HRB 50.  Non-heat treatable aluminum alloys will almost always be well below this hardness.  Some heat treatable alloys may also be below HRB 50.

If this option is used, it must be verified that the aluminum fastener strength is adequate for the application.  Externally threaded fasteners will have the greatest strength reduction when the fastener material is switched from stainless steel to aluminum.

A second option we have used in the past for critical military applications is to create special parts of 300 series stainless steel with cadmium plating.  This is an effective solution because cadmium and aluminum are very close on the galvanic series.  We can still offer this solution, but the parts will need to be YC3 specials to produce the required thread size allowance to accommodate the cadmium plating.  An additional point on a cadmium plated part is that we can currently offer cadmium with trivalent clear chromate, in case there is an objection to the use of hexavalent chromate typically used with cadmium plating.  This option will work in panels up to a hardness of HRB 70, which will cover almost all heat treated aluminum alloys.  Also, there is no reduction in fastener strength with this option.

The third option is to apply zinc plating to special stainless steel fasteners if the customer determines that zinc is close enough to aluminum (depending on the alloy) in the appropriate galvanic series.  Many published galvanic series with voltage values are for flowing seawater which may not apply to this application.  This option should only be considered if cadmium is not acceptable.

Stripping and re-plating is not recommended. Stripping a panel after plated fasteners are installed can damage the fastener. The recommended alternatives to this are:
First choice – Plate the panel and then install the plated fasteners.
Second choice – Install “X” finish (un-plated) fasteners into an un-plated sheet and then plate all together.

The only self-clinching fasteners that will provide a complete water tight seal are the CH line of concealed head studs and standoffs. These fasteners are clinched (pressed) into a blind hole, thereby keeping the reverse side of the sheet undisturbed and free of any voids that might allow any liquid to seep through. Please see PEM® bulletin CH for specifications.

No. Self-clinching fasteners are installed using any type of parallel acting press which will squeeze the fastener in place.

A complete list of fastener installation forces are available on our website at:
https://stage.pemnet.com/files/design_info/InstallationForceUnified.pdf
https://stage.pemnet.com/files/design_info/InstallationForceMetric.pdf

No, the fastener does not deform in any way. There is no flaring, crimping, swaging, peening or riveting necessary.

When determining the distance between two or more fasteners, you can calculate the distance by the formula, C/L to edge + 1/2 the diameter of the second mounting hole.

No. All self-clinching fasteners are installed into a round punched or drilled hole. A hexagonal head will cause the sheet material to cold flow around the head to provide high torque-out resistance. The hex head will be flush in the sheet when installed.

Generally, there is no specified maximum thickness for sheets. However, because of their special design and function, a few fastener types do specify a thickness range which includes a maximum.

These lists are available on our website:
MIL Cross Reference PEM Brand Fasteners
MS/NAS Part Numbers for aeroPEM™ Wrenchable Nuts

They can be installed into thicker sheets, but you run the risk of drawing the part through the panel with the tightening of the screw. This can cause the cracking between the knurl and barrel portion of the shank. If going into a thicker panel, you should pay particular attention to the tightening torque so that you do not cause this type of failure.

Violating the CL-Edge distance specified in our catalog may cause the board to crack or exhibit some sort of other damage like delamination. Due to the broaching nature of the K style product, there may also be deformation of the holes in the area. The question will be whether the deformation will cause a problem for attaching other components. It is suggested that you first install the fasteners in some test material to determine the potential risk of board damage.

By violating the centerline-to-edge feature, you will likely experience a bulging on the edge of your panel. Once bulging occurs, one can expect some reduction in performance of the fastener due to the material not flowing into the undercut of the fastener. What performance to expect is difficult to predict and will depend on the installation and the application. In order to prevent some of the bulge, a tool can be made that braces the edge of the panel. You have to take great care doing this to ensure the product does not stick on the tool during install. Also, the performance will still be reduced as the hoop stress of the material is compromised by the thinner section of material. If you can live with the bulge and a reduced performance value, it is likely you can still use the parts in your application.

The squeezing force on the fastener causes the sheet material beneath the head to cold flow into the back-tapered shank or undercut of the fastener securely locking it in place.

The original CLS and S (in the #10-32, #10-24, and M5 thread sizes) were created for a smaller (.234″) mounting hole. Due to many performance issues with these sizes, (because of the thin section between the thread and the undercut portion of the clinch profile) a beefier version was developed for a larger mounting hole. The original design (S and CLS) was eventually made obsolete and we were left with the SS and CLSS for these thread sizes.

Type FH studs are not made to a property class, but the minimum final hardness exceeds the minimum of both the 8.8 and 9.8 Property Classes. However, the type FH studs are neither a “true” Property Class 8.8 or 9.8 for the following reasons:
1. The 10B16 material we use may not be allowed (subject to customer approval).
2. Our maximum hardness exceeds the maximum hardness of Property Class 8.8.
3. Our maximum hardness may exceed the maximum hardness of Property Class 9.8 (depending on the standard used, ISO or SAE).
4. Parts are not marked with a property class designation.
5. We do not do all of the testing required on each lot by ISO and SAE standards.
6. Larger sizes (about M5 and up) of type FH studs fail in the head when tested in tension and therefore will not meet the tensile strength requirement of the standards which are based on the tensile stress area of the thread.

The fastener should be installed into the punch side of the hole. The die side of the hole has a blow-out effect. This increases the diameter and the size is less predictable.

Generally, there are two basic requirements. First, the panel must be a ductile material softer than the fastener which is going into it. Second, the panel must meet the minimum sheet thickness required by the particular fastener. Some self-clinching fasteners can be installed into sheets as thin as.020″ / 0.51 mm but generally .030″ / 0.76 mm or .040″ / 1mm is the minimum sheet thickness necessary.

Certain PEM fasteners are designed specifically to install into stainless steel sheet. These include types SP, SO4, BSO4, FH4, FHP, and PFC4. You can consult our Fasteners for Use in Stainless Steel Sheets bulletin for details on each type.

The self-clinching feature of a self-clinching fastener makes welding an unnecessary step. Self-clinching fasteners, when properly installed, have adequate holding power in the sheet without welding. In fact, self-clinching technology was originally conceived, and is, a less costly, cleaner and more dependable alternative to weld fasteners.

The negatives of welding include the following. Welding a steel fastener will melt the zinc plating, which would speed up corrosion in the welded area. Welding an austenitic stainless steel fastener increases the risk for sensitization (sensitization can happen to austenitic (300 series) stainless steels from uncontrolled cooling which can allow carbides to precipitate at grain boundaries making the material susceptible to intergranular corrosion). If the fastener is made from 303 stainless steel, welding could lead to cracking due to out-gassing of the low melting point free machining additives. A hardened type 400 series stainless could be re-tempered and weakened by welding.

No. Typical torque-out values are generally quite high compared to the rotational force that will be put on them. In fact, for most quality self-clinching nuts, the screw will fail before the nut rotates in the material.