Pull Testing, measuring the Tensile Test of a wire to terminal crimp has been a measurement of quality since the advent of pressing a wire to a terminal for electrical assembly.
Pull Test Methods have varied from weights, fish scales, portable and benchtop manual and motorized digital. I have even heard stories of an acceptable pull test being performed by pulling the terminal by the teeth. A dentist’s nightmare.
Given today’s demands for higher reliability and failure rate approaching zero, how does the Pull Test process fit in with the other testing methods being deployed? Are you even performing pull tests properly? This article will answer these questions and more as we put Pull Testing Into Perspective.
Pull Testing Defined.
Pull Test is a destructive test designed to determine the mechanical strength of a terminal crimp. A good mechanical crimp assures the crimp can withstand the normal handling and installation process.
Considering a cross section of quality standards, typical process parameters for pull test include:
- Disengaging the insulation support so the pull force reading is based on the wire crimp alone.
- Pull at a constant rate of 50 to 250mm/minute.
- Wire should be taut prior to applying pull force. Remove slack from the wire.
But Pull Test is not the whole picture.
Pull testing is not a measurement of electrical performance. A quality crimp includes a secure mechanical crimp with low electrical resistance. Low electrical resistance comes from a crimp with a wire under compression. Terminal suppliers validate crimps by optimizing the crimp barrel size to match the wire. Crimp tool profile is also a critical factor. Using the proper crimp tool profile, the wire and terminal are compressed together. Pull and electrical resistance reading are made and the recommended conductor crimp height is established and published. It is the responsibility of the end user to follow the crimp guidelines in order to assure an optimum performing crimp. Which includes conductor crimp height as a primary measurement with mechanical pull test as a secondary standard.
A few facts to consider:
- Pull test and electrical resistance measurements rarely follow in tandem. Typically pull strength peaks before electrical resistance. Therefore it is possible to meet a minimal pull test while not optimizing electrical resistance. There will always be a compromise between Electrical Resistance and Pull Test.
- Pay careful attention to how the wire separated from the terminal. This is an indication of the wire compression. Strands completely broken at the wire crimp indicate over compression. Conversely strands which completely pull out of the wire crimp still in a round shape indicate severe under-compression. Crimps with good compression should primarily break outside of the wire crimp.
Depending on Mechanical Strength as your primary crimp quality measurement leaves you open for premature crimp failure. And the consequences can be wide ranging. Even a single failure can be costly. Large scale recalls (not uncommon today) can cripple even a large company.