[time-nuts] DC distribution

[Larry McDavid]

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)