r 18, 1999
Bob Nuckolls
6936 Bainbridge Road
Wichita, KS 67226-1006
Voice/Fax: 316-685-8617
E-mail: nuckolls@aeroelectric.com
Anatomy of a Good Solderless Terminal Connection
One of the people who looked this article over asked why
I didn't title it "Anatomy of an Aircraft Quality Termi-
nal." I would probably have done that twenty years ago;
now I'll suggest that the term "aircraft quality" has no
quantifiable meaning. But that's a topic for another
article. For now, take a few minutes to understand how
a really GOOD terminal is made, how it works and what
techniques are required to utilize its capabilities.
The first figure illustrates major components of a good
terminal. Item 1 is the electrically conductive portion of
a solderless terminal. It's made from copper and
generally plated with tin to forestall corrosion.
Terminals are fabricated in a variety of sizes for both
stud size (hole) and wire size (barrel i.d.). In some
products, the bare, uninsulated terminal is
sufficient for bringing a wire up to a stud.
Bare terminals can be used where there are no
vibration concerns. They're commonly seen in
a variety of non-vehicular applications.
When any amount of vibration is anticipated,
it's important to support the wire close to but
separate from the wire grip on the conductors.
Flexing of the conducting strands where they
leave the back of the terminal barrel must be
controlled. Like aluminum, copper is one of
those non-ferrous metals that readily stress
cracks. When any flexing occurs in the strand-
ing between terminal's wire-grip and end of
the wire's insulation, hardening of the wire
will occur, precipitating cracked strands and
a failed connection.
The standard technique for adding support is
to slip a malleable plastic sleeve over the terminal barrel
and leave it long enough to permit a second, insulation
supporting crimp. The plastic sleeve is generally made
from nylon but other plastics are suitable. The vast
majority of off-the-shelf solderless terminals offered by
electrical and automotive parts stores are two piece
devices consisting of terminal (1) and insulation support
sleeve (2). These terminals are not recommended for use
in airplanes. The plastic wire-grip sleeve has a memory.
Temperature cycles encourage a pure plastic support
sleeve to recover its original round shape! You loose wire
support leading to probable failure of the connection.
About 50 years ago, a third component was added to solder-
less terminals to prevent this problem. Item 3 is a copper
sleeve slightly shorter than the plastic sleeve; a metal liner
inside the plastic. When the insulation grip crimp is made,
the copper liner becomes a permanently formed wire
support, effectively overriding any memory characteristics
of the plastic insulator. A more obscure feature of a good
terminal is a "funnel" shaped entry (item 4) at the back of
the wire grip barrel. Not every terminal with a metal lined
insulation grip will also have funnel shaped wire guides but
they are really helpful. Square entry to the wire grip barrel
will more likely snag single strands of wire causing them
to fold back under the insulation where they are difficult to
detect. The best terminals are readily identified by inspection:
Just look to see if there's a thin copper liner inside the
plastic. These terminals are manufactured by many
manufacturers including AMP, Incorporated as their
"Pre-insulated Diamond Grip" (PIDG) brand and
Waldom-Molex in their "Avi-Crimp" brand.
There's a range of styles in crimping tools. MOST will
properly install a terminal. The trick in terminal appli-
cation is to achieve the proper HEIGHT of finished
crimp.
The figure above illustrates the relationship between
crimp height and two important terminal parameters:
electrical resistance and mechanical (tensile) strength.
As crimp height decreases, electrical resistance goes
down while tensile strength goes up. At some point, both
parameters level off showing that further crimping
doesn't make the joint any better. Continued reduction in
height will eventually upset the strands too much.
Strength goes down and electrical resistance starts to go
up.
The best tools have ratchet handles. The mechanism
prevents under-crimping by forcing the user to operate
the tool through its total stroke. When the tool is fully
closed, sculptured dies enclosing the terminal barrel
work against hard stops thus insuring uniform crimp
height. Ratchet handled, hard-die tools from AMP and
Molex can start at $100 and go up from there. One of my
favorite tools from AMP cost me about $150 in 1965; it
costs over $500 now! There are some excellent
alternatives in imported ratchet-handled tools offered on
our website for $40. Ratchet handled tools will also do a
complete terminal installation with a single stroke.
The Low Cost Alternative
Just about everyone has a stamped, sheet metal,
rivet-jointed crimping tool in their toolbox. These tools
are commonly supplied with a kit of terminals and splic-
es by automotive parts stores in a compartmented
container. These tools are capable of producing
satisfactory crimps but they require some practice. The
terminals that are sold with them need to be saved for use
on your washing machine or stereo speaker system.
First, strip outer insulation from the wire so that when fully
seated in the terminal, the ends of wire stranding should
just protrude from the stud side of the crimp barrel. Center
the appropriate die on the tool over the wire grip section,
approx 1/3 of way from stud end to entry end of sleeve.
Apply firm grip pressure but it won't take a great effort.
When you think you've done it about right, tug on the wire
to the tune of 5-10 pounds for 22AWG wire and up to 20
pounds for 10-12AWG wire. What?? you don't know what
an 8 pound pull feels like?
Heres an accurate, poor-mans pull test for terminal
application: (1) Drive a finish nail into the front of your
workbench. (2) Hang the terminal lug on the nail and tie a
plastic gallon milk jug of water onto a 22AWG wire (red
terminals). For 18AWG (blue) use two jugs. For 12AWG
(yellow) use three jugs. The task is to do is "calibrate" your
grip. The range of pressures required to produce an
adequate crimp are quite large and for the most part, you
have to really work at over-crimping with low-cost hand
tools.
The second crimp is 2/3 of the way along the barrel closed
just enough to grip the wire's insulation . . . generally
MUCH less pressure than the amount required to grip the
wire. There's some ol' mechanic's tales wandering around
out there suggesting that wire grip and insulation grips
should be put on 90 degrees displaced from each other.
This is not a helpful technique and, in my not so humble
opinion, makes for a crummy looking terminal.
The plastic insulator of a finished crimp should have a
smoothly sculptured appearance; no sharp indentations or
cracked insulation. Some low cost tools punch an
indentation into the side of the terminal barrel. These
tools are for uninsulated terminals on solid wire. The
shapes of proper crimping dies range from oval to rounded-
rectangular but in no case is the resulting crimp anything
but smoothly molded around the terminal barrel and wire.
Dispelling a Myth
Some folks recommend a combination of soldering in
addition to crimped joints for reliability. Keep in mind that
the Boeings, Beeches, Pipers and even the lowly Cessnas
havent soldered a terminal on a wire in over 30 years.
People like AMP and Molex have carved an honorable
place for themselves in the aviation marketplace selling
termination systems that do not require solder to achieve
the highest levels of reliability. Please forget the solder.