Near the end of part one, our crimpologist, John D. Butler, PE., had presented his “B” Crimp Tolerances and Their Effect on Crimp Compression paper at the 1998 UL 1659 Meeting in Fort Lauderdale, Florida. Certainly, at least to many in the electrical industry, the paper or “manifesto” is still the mother of all white papers on crimping; and perhaps somewhat cringeworthy since one of Butler’s prime examples of “bad crimping” includes an erroneous UL 1659 crimp tolerance. Butler’s exposé revealed how tolerances, published by manufacturers and renowned safety agencies worldwide, often contain inaccuracies which can cost manufacturers plenty on rework and agency certification resubmittals.
One example of inaccurate tolerances concerned a U.S. wire harness manufacturer supplying terminals to a major automaker. The automaker’s crimps were literally falling off the end of the wire. The manufacturer of the terminals had supplied the automaker with recommended crimp heights for multiple gauge sizes, such as 14-18 American wire gauge (AWG) sizes; also, a conductor crimp height tolerance of =/- 0.003 inches was given. According to Butler, obvious possibilities were examined and dismissed by the automaker’s engineers. What specifications should have worked simply did not.
Since the supplier had the right terminal, gauge of wire, and tooling setup, engineers on both sides were baffled. The automaker’s production rapidly decreased, and the company’s office and production personnel found themselves gripped in panic. Then, engineers discovered the problem and worked quickly to replace its 18 gauge wire. Unlike standard 18 gauge, this wire was a new, compacted-strand Japanese wire, which greatly diminished mechanical pull strength—the specifications were written for standard 18 gauge. The takeaway for Butler became one of his maxims: measuring crimp height does not determine a good or “proper” crimp.
Butler’s second example cited an appliance manufacturer that confronted their terminal supplier over failed electrical tests. Also, the appliance manufacturer questioned the life of their crimping tools as they were breaking them at a rapid rate. Samples were sent to the supplier for inspection. What the supplier first observed was the length of the terminals—they measured 0.560 inches in length. This was longer than the original 0.500 terminals they’d sent. Also, the crimps looked extremely flattened. The response from the appliance manufacturer was that better pull test results came from lower crimp heights, which explained the flattened crimps. However, by making the B crimps flatter (and longer), the manufacturer’s crimps often failed electrical tests—the reduced terminal cross-section had been significantly altered leading to electrical failure.
The tool breakage issue? A result of the additional force of over-crimping. Butler’s next maxim? That a pull test alone cannot decide a “proper” crimp.
Lastly, the UL 1659 standards error. All suppliers to automotive, electrical, and appliance industries receive recommended specifications from terminal and connector manufacturers for guidance on recommended crimp specifications. That’s the good news. The bad news is that suppliers look for manufacturer’s guidance on crimp specifications. The reason for the catch-22 is that recommended specifications and tolerances are often incorrect, according to Butler. Part of the incorrections are tolerances so lenient that testing “fails” occur.
An example of incorrect tolerances was found in section 18.1 of UL 1659 (Attachment Plug Blades). A tolerance of +/- 0.003 is given. In some cases, the tooling allows tolerances of +/- 0.005, which suddenly becomes intolerant, so to speak. Other dimensions also appear to be incorrect in UL 1659. The assumption of manufacturers is that such publications, especially from renowned global safety agencies such as UL, CSA, or VDE, are infallible sources. They are not. However, this should not detract from the overwhelming majority of standards the safety agencies get right.
What is the Optimal crimp? Butler states that one common thread is woven through all crimp performance characteristics, including mechanical strength, maximum electrical performance, heat, corrosion resistance, and proportional size. His one common thread is the cross-sectional area, the area where the wire strands compact inside the “B wings” of a B crimp. The more compacted (as though one solid wire) the greater the crimp. The crimping process compacts the cross sectional area to produce optimal B crimp compression and conductivity along the surface of the wire strands. That is, if your tooling is set up correctly to achieve such performance.
Specifically, the degree of compression should be between 15-20%. Moreover, the crimp ratio (crimp height divided by crimp width) should lay between 50-70%. This can be seen in Butler’s graph in Fig. 1.
Interpower’s testing lab in Ames, Iowa, performs several tests on crimps.
A trained Interpower production worker measures crimp height and width with a micrometer to see if the size of the crimp meets the spec range which the production team has already established.
At Interpower, batches of crimped cords are routinely sent to Interpower’s testing facility in Ames, Iowa, for detailed inspection and testing. Crimps are evaluated in four basic areas: Crimp height, crimp width, conductor wire compaction within the terminal crimp, and conductor wire pullout force.
At the lab’s Crimp Cross Section Station, crimp barrels (the two rounded protrusions of a crimp) are cut in half. They are then washed in acid before ground smooth at the micrograph station. Next, the crimps are photographed under a microscope to check conductor compression, and to check for any “voids” or gaps in the crimp. Optimal crimp quality results in uniform strand compression similar to a honeycomb appearance. A micrometer is used once again to measure the crimp.
Now comes the pull-to-fail or pullout test—how much force is required to pull conductors out of the crimp? Pull force is typically measured in Newtons. While there are no guidelines for visual inspections, there are guidelines supplied by the manufacturers of terminals. There are conductor wire pullout guidelines published by Underwriters Laboratories (UL) and the International Electrotechnical Commission (IEC). The test parameters are specified in UL 486A-486B section 9.3.4.1 (see Table 27), and in IEC 60352-2 clause 5.2.2.1.
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