|Frequently Asked Questions
- Can I install a self-clinching fastener with a hammer blow or similar force?
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.
- Can I install these blind, from one side, if I dont have access to both sides of the sheet?
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 www.pemnet.com.
- Can PEM fasteners made from 300 Series stainless steel be used in aluminum panels without having to consider contact corrosion from galvanic reactions?
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 (http://www.pemnet.com/design_info/galvanic.html) 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.
- Can we strip and re-plate a panel after the plated fasteners are installed?
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 (unplated) fasteners into an unplated sheet and then plate all together.
- Do any of the self-clinching fasteners offer a water tight seal?
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.
- Do I need special equipment to install self-clinching fasteners?
No. Self-clinching fasteners are installed using any type of parallel acting press which will squeeze the fastener in place.
- Do you have a complete list of fastener installation forces?
- Does the shape of the self-clinching fastener change during installation?
No, the fastener does not deform in any way. There is no flaring, crimping, swaging, peening or riveting necessary.
- How far apart from each other can I reliably install PEM fasteners?
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 secound mounting hole.
- I notice some of the fasteners have a hexagonal shaped head. Do I have to punch a hexagonal mounting hole to install these?
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.
- Is there a maximum sheet thickness I should be concerned with before specifying self-clinching fasteners?
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.
- Is there a Military Specification / National Aerospace Standards cross reference available for PEM® part numbers?
- The FE and PL series calls out a sheet thickness with a range as opposed to "min sheet thickness". Can they be used and installed properly in sheet thicknesses thicker than the high end shown in the range(s)?
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.
- What happens if I install a broaching fastener (in P.C. board) closer than the recommended minimum edge distance?
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.
- What happens if I mount a self-clinching fastener closer to the edge of a panel than recommended in the catalog?
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 peerformance value, it is likely you can still use the parts in your application.
- What holds the fastener in the sheet?
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 fastaener securely locking it in place.
- What is the difference between an S and SS part number and a CLS and CLSS part number? Why is there an additional S in the part number with the same thread size?
The original CLS and S (in the 10-32 and 10-24 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.
- What is the property class of a type FH fastener?
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 fall 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.
- When installing a SI® type insert for plastic, what is the recommended wall thickness for the boss?
The boss thickness for an insert is very dependent on the material being used as well as the mold configuration creating the hole. As as rule of thumb, 2X the part diameter is usually suffcient, however, if there is a potential for knit lines along the boss due to the mold configuration, this diameter should be increased.
- When using self-clinching fasteners, what are my panel requirements?
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.
- Which PEM® fasteners can I successfully install (clinch) into stainless steel sheets?
- Won't these fasteners fall out, twist out, or spin in their mounting holes if I tighten down too hard?
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.