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Introduction
 You've
made it through all the connector catalogs, found
the connector that meets all your design criteria
and is just right for your application. With the
right current rating, voltage rating, circuit size,
engagement force, wire AWG capabilities, configurations,
termination method and safety features, e.g. positive
locks, fully-isolated contacts, polarization and
agency certifications, it is, in short, the perfect
connector.
But
don't let out a huge sigh of relief quite yet
--especially if the connector you've chosen uses
a crimp termination system. While this can be
one of the fastest, most reliable and rugged termination
methods, if the terminal isn't crimped onto the
wire correctly you can forget all about the hard
work you put into selecting the right connector.
And, although there are 13 common crimping problems
that can reduce the reliability of your product,
these problems are easy for you to avoid with
a little knowledge and advance planning.
To
begin with, it helps to understand that a terminal
has three major sections: Mating, Transition and
Crimping (Illustration
A). The Mating section, as the name
implies, is the section of the terminal that mates,
or becomes the interface, with the other half
of the connection. This section was designed to
mate with a terminal of the opposite gender and
to perform in a certain manner by the connector
design engineer. Anything that you do that deforms
the Mating Section, especially during the crimping
process, will only reduce the connector's performance.
The Transition Section also is designed so that
it would not be affected by the crimping process.
Here again, anything you do that changes the position
of the Locking Tangs or Terminal Stop affects
the connector's performance.
The Crimp Section is the only section that the
crimping process is designed to affect. Using
termination equipment recommended by the connector
manufacturer, the crimp section is deformed so
it can be securely attached to a wire. Ideally,
all the work that you do to crimp a terminal onto
a wire occurs only in the Crimp Section.
An example of a properly performed crimp is seen
in
Illustration B. Here, the insulation crimp
compresses the insulation without piercing. The
wire strands (or brush) protrude through the front
of the conductor crimp section by at least the
diameter of the wire's conductor. For example,
an 18 AWG wire would protrude at least .040".
Both the insulation and conductor are visible
in the area between the insulation and the conductor
Crimp Section. The conductor Crimp Section shows
a bellmouth shape in the leading and trailing
ends, while the Transition and Mating Sections
remain exactly the same as they were before the
crimping process.
If your crimped terminal does not look like the
terminal in Illustration
B - the problem may have been caused
by something that went wrong during the crimping
process. Here are 13 of the most common problems
that may occur during the crimping process and
what you can do to avoid them.
- The
crimp height, which is the cross sectional height
of the conductor Crimp Section after it has been
crimped, is the most important characteristic of
a good crimp. The connector manufacturer provides
the crimp height for each wire size for which the
terminal was designed. The correct crimp height
range or tolerance for a given wire may be as small
as 0.002". With a specification this tight, verifying
that the press is setup correctly is very important
for achieving a good crimp.
A crimp height that is either too small (Figure
I) or too large (Figure
II) will not provide the specified
crimp strength (terminal retention to the wire),
will reduce the wire pull out force and current
rating, and may generally cause the crimp to under
perform in otherwise normal operating conditions.
A crimp height that is too small also may cut
strands of the wire or fracture the metal of the
conductor crimp section.
- A
crimp height that is too large will not compress
the wire strands properly, causing excessive voids
in the Crimp Section because there is not enough
metal-to-metal contact between the wire strands
and the metal of the terminal.
The solution to problems #1 & #2 is very simple:
adjust the conductor crimp height on the crimp
press. Using a caliper or micrometer as shown
in Illustration
B, verify that the crimp height is
within specification when the press is first used
for a production run and recheck it as frequently
as necessary during the run to maintain the proper
crimp height.
- Connector
manufacturers do not typically supply a crimp height
for the insulation due to the variety of insulation
types and thicknesses. The insulation crimp provides
a strain relief for the conductor Crimp Section
so that as the wire flexes, the wire strands do
not break. An insulation crimp section that is too
small may overstress the metal in the insulation
Crimp Section, weakening the strain relief function.
Most types of crimp tooling allow the insulation
crimp height to be adjusted independently of the
conductor crimp height. The correct adjustment
allows the terminal to grip the insulation for
at least 180 degrees without piercing the insulation.
An insulation displacement, or compression where
the OD of the terminal's insulation crimp and
the OD of the insulation are approximately the
same, is ideal.
Loose
wire strands (Figure
V) are another common cause of crimping
problems. If all the wire strands are not fully enclosed
in the conductor Crimp Section, both the strength of
the crimp and the current carrying capability may be
greatly reduced. To get a good crimp you need to meet
the crimp height the connector manufacturer specifies.
If all the strands are not contributing to that crimp
height and therefore, crimp strength, then the crimp
will not perform to specifications. Generally, the problem
of loose wire strands is very easy to solve by simply
gathering the wires back into a bunch before inserting
them into the terminal to be crimped. The strands may
have been inadvertently separated during the handling
or bundling process if stripping the insulation from
the wire is done as a separate operation. Using a "strip
and retain" process for insulaton removal, where the
insulation slug is not completely removed from the wire
until it is ready to have a terminal crimped onto the
wire, helps minimize the problem.
- If
the strip length is too short or if a wire is not
fully inserted into the conductor Crimp Section,
the termination may not meet the specified pull
force because the metal-to-metal contact between
the wire and the terminal is reduced. As shown in
Figure
VI, the strip length of the wire is
too short (note that the insulation is in its proper
position), not allowing the required one wire OD
extension in front of the conductor Crimp Section.
The solution is simple: increase the strip length
of the wire stripping equipment to that specified
for that specific terminal.
-
7.
Wire Inserted Too Far
- Another
crimping problem that relates to a too short strip
length occurs when the wire is inserted too far
into the crimp sections. As Figure
VII shows, the insulation is too far
forward of the insulation Crimp Section and the
conductors protrude into the Transition Section.
This may cause as many as three failure modes in
the actual application. Two relate to a reduced
current rating/wire pull out force due to a reduction
of the metal-to- metal contact in the conductor
Crimp Section. A metal-to-plastic contact isn't
as strong, nor does it conduct electricity, as well
as metal-to-metal.
The third failure mode may occur when the connectors
are mated. If the wire protrudes so far into the
Transition Section that the tip of the male terminal
hits against the wire, it may prevent the connectors
from fully seating or it may bend the male or
female terminals. This condition is known as "terminal
butting".
Under extreme cases, the terminal may be pushed
out the back of the housing even though it was
fully seated in the housing. To solve this problem,
make certain the wire is not inserted into the
press with so much force that it overcomes the
wire stop on the press, or adjust the position
of the wire stop so that it correctly axially
positions the stripped wire.
-
8.
"Banana" (Excessive Bending) Terminal
- One
of the most descriptive crimping problems is known
as a "banana" crimp (Figure
VIII), because the crimped terminal
takes on a banana shape. This makes it difficult
to insert the terminal into the housing and may
cause terminal butting. This problem is easy to
solve by adjusting the position of the hold down
pin on the crimp press. This small pin is located
in the crimp press and contacts the terminal in
the mating section while the crimp sections are
being crimped onto the wire. During crimping, a
significant amount of metal on one end of the terminal
(in the crimp section) moves. These high forces
tend to force the front of the terminal upwards,
unless it is held down by the aptly-named "holddown
pin".
- One
of the more obvious crimping problems is when part
of the Transition Section is damaged, as shown in
Figure
IX. In the terminal shown, the tab
sticking up is a design feature called a "terminal
stop". Its function is to prevent the terminal from
being inserted too deeply into the housing. If the
stop is extremely damaged, the terminal can actually
be pushed all the way through the housing.
The solution is relatively simple. What causes
the problem is that the terminal and carrier strip
(the band or strip of metal the terminals are
attached to when you receive them from the manufacturer)
is not properly located with respect to the press.
To solve it, simply loosen the base plate of the
interchangeable tooling and realign it to the
press.
- The
correct size for a bellmouth (Figure
X) is approximately 2X the thickness
of the terminal material. For example if the terminal
is made from material that is .008" thick, the bellmouth
should be approximately .016". While a few thousands
of an inch either way will not materially affect
the terminal's performance, if the bellmouth is
missing or if it is less than one material thickness,
there is a risk of cutting the wire strands. The
fewer strands that remain, the lower the termination
strength. To correct the problem, make sure the
punch and anvil on the crimping equipment are properly
aligned.
- There
is also a problem if the bellmouth is oversized
(Figure
Xl), because this reduces the total
area that the crimp section of the terminal has
in contact with the wire. The less the wire-to-terminal
interface, the lower the wire pull out force. If
the crimp height is correct, then it is likely the
problem is caused by worn tooling, which should
be replaced.
- The
carrier strip is cut off of the terminal during
the crimping process. If the remaining cutoff is
too long (Figure
XII), problems can occur. The extra
metal may protrude out the rear of the connector
when the terminal is inserted into the housing,
causing the connector to arc between adjacent contacts
when higher voltages are applied. If the carrier
cutoff at the front of the terminal is too long,
the extra length may interfere with connector mating
and result in "terminal butting".
The solution is fairly simple. Adjust the baseplate
on the press so the terminal is centered properly
in the crimp press. Another indication that the
terminal is not centered correctly is that the
bellmouth isn't properly former either. This occours
because the tooling for the bellmouth and the
carrier cutoff are spatially related.
- Although
bent lock tangs are not necessarily the result of
a poor crimping process, the connector can fail
just the same. Lock tangs (Figure
XIII may be bent either in or out too
far, which impacts the terminal's ability to completely
lock into the shelf in the housing that was designed
for this purpose. The tangs may be damaged as the
terminals are unwound from the reel if the friction
wheel on the reel holder of the crimp press is too
tight or it can be caused by handling after the
terminals are crimped onto the wires. Typically,
terminated wires are gathered into a bundle and
inventoried or transported to another location in
the plant. During the bundling, or as each terminated
wire is removed from the bundle, the locking tangs
may be bent.
If the damage is occurring at the crimping press,
then the friction wheel needs to be adjusted so
it is only tight enough to keep the reel of terminals
from being unwound by their own weight. If the
problem is occurring during the bundling process,
smaller bundles or improved handling procedures
need to be implemented.
- While
there are 13 problems that may be caused during
the crimping process, there are just four simple
rules that will help ensure a successful connector
application:
1.Choose the right connector for your application
requirements.
2. Use
the crimp tooling specified by the terminal
manufacturer.
3. Properly
adjust and maintain the crimp tooling in good
working order.
4. Periodically
replace the parts that displace metal (e.g.
conductor and insulation punches, anvils and
terminal cutters).
Since most of the problems that are reported
to connector manufacturers relate to one of
these thirteen crimping problems, Molex offers
an easy-to-use guide to help you avoid problems
or recognize them quickly enough so that you
make only good crimps. To order this guide contact
Molex Incorporated, 2222 Wellington Court, Lisle,
Illinois 60532, Attention: Good Crimp Drawings.
*The parts of the
crimp tooling that significantly displace the
metal of the terminal - the conductor and insulation
punch, anvils and the terminal cutoff tooling
will need to be periodically replaced.
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