Crimping machines and hand crimp tools all close the crimping dies to a
set position; that position on all bench machines and most hand crimp
tools is adjustable and is set to qualify the crimp. The cross-section
area of the conductor strand bundle is the dominant factor.
Optimally, the crimp conditions are correct for just one gage wire and
one strand count; most non-automotive cables are either 7 or 19 strand
because those counts fill the outer layer of the bundle fully--it is
geometry driven. Use a circle templet to draw a bundle: one strand in
the center, six strands around it fill the next layer and the next layer
brings the total to 19 strands. Seven and nineteen strand counts are not
arbitrary--it is geometry.
Connector manufacturers try to qualify a crimp terminal size for a range
of wire gages for economy and accept variation in how gas tight the
crimp is. But in optimal production, the crimp height is set using pull
force testing with the actual wire gage used. Connector manufacturers
supply recommended crimp heights for each terminal and wire gage (or
gage range). This height is measured with a special micrometer made for
the purpose.
Designing a crimp terminal is an art as well as science. The shape of
the crimp wings in a typical "B" crimp (in which the tips of the crimp
wings are folded around and down into the strand bundle) as well as the
exact shape of the crimp tool dies are critical. Worn tools produce poor
crimps. Even the exact shape of how the crimped crimp wings meet in the
strand bundle is important to temperature cycle performance of the
crimp; that shape can be inspected only by cross-sectioning the crimp zone.
Another qualification test of a crimp terminal and crimp dieset is
resistance change following high temperature soak. Typically, numerous
test crimps are made and voltage-measuring fine wires are spot welded
near the crimp zone. The initial resistance within the crimp zone is
measured with a high current, low resistance milliohm meter, the
terminal is thermal soaked at 125 or 140°C for 240 hours and the
resistance measured again. Only a few milliohms (sometimes less!)
*change* is acceptable for a good design.
Automotive wires are different only, I believe, because of
history--originally the individual wire gage was picked for volume cost
or convenient availability and the number of strands adjusted to provide
the required current carrying capacity. Unfortunately this means
automotive wire bundles usually don't have geometrically full bundles.
Geometrically full (i.e. 7 and 19 strand) bundles crimp more uniformly.
Automotive wires are not tin coated, another cost savings. The usage
volume is so high the bare copper strands don't corrode before assembly
and once crimped properly air does not get into the crimp zone. Many
automotive connectors are "sealed" with seals at the connector shell
interface and also around each insulated wire where it enters the shell.
This sealing is surprisingly effective and these mated connectors pass
240 hour salt fog testing; you routinely see these sealed connectors in
underhood applications.
Ok, there is a lot of science and engineering in making a good crimp.
But end users don't do this themselves, it is done by the connector
manufacturers. In production environments where reliability is important
(automotive in this case) the pull force testing I described previously
is routinely used, often at the start of each production shift in a good
production house.
In the case of Power-Pole connectors, as someone else described, the
exact placement and alignment of the crimp, and how the terminal deforms
during the crimp, is important to successful insertion of the crimped
terminal into the plastic shell where the contact-force leaf spring
retains the terminal and actually supplies the contact force. That's why
Power-Pole crimp tools position and align the contact end of the
terminal for crimping.
Yes, lots of details to consider. Nevertheless, crimped terminals are
more reliable when done correctly than soldered terminals.
One aspect of soldered terminals that is often overlooked is that solder
wicks down the strand bundle under the wire insulation, creating a solid
where the stranded wire enters the soldered terminal. That is a
stress-riser and a likely source of flexure failure.
Larry McDavid
On 10/6/2019 7:25 AM, jimlux wrote:
On 10/5/19 8:16 PM, Larry McDavid wrote:
I've used Power-Pole connectors for many years successfully and I've
always crimped them with appropriate Power-Pole crimp tools. I never,
never solder crimped connections! Heating a crimped connection to
soldering temperature will relax the crimp force in the crimp zone
and, if properly crimped, there is no gap among the wire strands for
solder to flow into. The result is always a loss of connection quality.
Stranded wire can be tinned or coated with solder by the wire
manufacturer and crimped successfully so long as the wire is
"non-fused-tin-coated." But, much stranded tinned wire *is* fused to
keep the strands together after removing the insulation; this type of
stranded wire should not be crimped. Much MIL Spec wire is silver
coated, inherently non-fused and crimps well.
Professionally (in both aerospace and high-rel automotive air bag
applications), I've had the "crimp zone" of very many crimped
connector contacts metallurgically mounted, cross-sectioned and
examined microscopically after polishing and etching to reveal the
individual strands even in the crimp zone. This is the ultimate method
to "qualify" a crimped connection. A "gas-tight" crimp shows under
microscopic examination no air gaps within the crimp zone--the crimped
wire bundle has gone solid and is "gas tight."
"Crimp pull force" is another, production level, crimp quality control
method but the proper method requires making numerous crimps at
various "crimp heights" (how reduced in dimension is the height of the
crimp zone) and pull force testing the resultant crimps. The requires
crimping by a machine or qualified hand crimp tool that is adjustable.
The pull force values are plotted against crimp height and the shape
of the curve examined. A crimp height resulting in a pull force just
as the pull force begins to *decrease* after reaching a peak value is
selected. A "looser" crimp is not "gas tight" and a "tighter" crimp
reduces the cross-section area of the wire bundle enough to weaken the
crimped connection. Crimped connections have to be crimped within a
narrow zone of compression and only the appropriate crimp tool,
appropriately calibrated, can provide this. Forget about all types of
"crimp pliers;" these are worthless tools.
interesting...
And I assume, then, that the degree of compression (set by the dies and
their position in the crimper) is wire gauge dependent - that is, the
crimper doesn't crimp to a specific force, it crimps to a particular
mechanical dimension, so if the number and size of strands is different,
then the degree of crush is different.
That sort of makes the "crimping a tiny wire by folding it back on
itself" or "crimping a tinywire by putting it with a big wire" a tricky
operation.
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Best wishes,
Larry McDavid W6FUB
Anaheim, California (SE of Los Angeles, near Disneyland)
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