Mechanical
and Electrical Stability
Distribution of Stress
Easy Removal
Types of Wrap
Selection of Wire
Selection of Terminals |
Minimum
Terminal Spacing
Producing a Standard Wire-Wrapped Connection
Producing a CSW Wire-Wrapped Connection
Quality Assurance
Four Common Problems of Wire Wrap |
| The
Bell Laboratories in 1952 proved that a wire tightly wrapped around
a terminal with two sharp edges could form a consistent gas-tight
electrical connection without soldering. Since then, the electronic,
telecommunication, and aviation industries have demanded more reliable
and inexpensive means for solderless connections. The problem was
resolved by wire wrapping. Wire wrapping is now a common method to
connect signal and power terminals in today's advanced industries.
|
 |
| Mechanical
and Electrical Stability |
| Solderless
wrapped connections have excellent mechanical and electrical stability
due to their "gas-tight" contact area. They remain stable through
exposure to severe temperature changes, humidity, corrosive atmospheres,
and vibration. The high shearing force of the wire at the corner of
the terminal and high contact pressure remove surface contamination
from the wire and the terminal, producing an intimate, clean, oxidation-free,
metal-to-metal contact with a large contact area. Initial pressure
at the center of the contact area may go as high as 100,000 psi. After
wrapping is complete, cold flow causes pressure to drop to approximately
30,000 psi, at which point the metal stabilizes and the pressure remains
constant. |
|
| Distribution
of Stress |
| By bending the
wire around the sharp corner of the terminal, the oxide layer on both
wire and terminal is crushed or sheared, and a clean, oxide-free,
metal-to-metal contact is obtained. |
 |
A cross section
through the terminal edge shows stress distribution produced in the
wire wrapped with high tension around a terminal. |
 |
|
| Easy
Removal |
| Although
wire wrapped connections are quick and permanent with superior mechanical
and electrical qualities, they may be easily removed if necessary
without damage to the terminal. To remove the connection, an unwrapping
tool is placed over the terminal, engaging the first turn of the connection.
Rotation of the tool removes the connection in seconds. |
|
| Types
of Wrap |
 |
(S)Standard
Wrap
- Only the bare wire is wrapped around the terminal. |
 |
(M) Modified
Wrap
- The first one-half to two turns are made with insulation wrapped
around the terminal. These turns are in addition to the recommended
turns of bare wire. This process increases the ability of the connection
to withstand vibration and reduces wire breakage. |
|
| Selection
of Wire |
| Any
good grade solid conductor with normal ductility can be used as the
wrapping wire, but the most common choice is tin-plated copper wire.
The tinning makes no difference in the quality of the connection,
but it is helpful if the connection is to be soldered later. Minimum
elongation of 15% is required for 24 through 32 AWG, while 20% is
necessary for larger wire sizes. |
|
AWG
|
Diameter
|
Minimum # of Turns
Bare Wire
|
Minimum Strip Force
|
|
in
|
mm
|
lb
|
gms
|
|
Wrapping Specifications
for Wire
|
|
16
|
0.051
|
1.30
|
4
|
15
|
6800
|
|
18
|
0.0403
|
1.02
|
4
|
15
|
6800
|
|
20
|
0.032
|
0.81
|
5
|
8
|
3600
|
|
22
|
0.0253
|
0.64
|
5
|
8
|
3600
|
|
24
|
0.0201
|
0.51
|
6
|
7
|
3200
|
|
26
|
0.0159
|
0.40
|
7
|
6
|
2700
|
|
28
|
0.0126
|
0.32
|
8
|
5
|
2200
|
|
30
|
0.010
|
0.25
|
8
|
3
|
1400
|
|
32
|
0.008
|
0.20
|
8
|
3
|
1400
|
|
|
| Selection
of Terminals |
| A
terminal must have at least two sharp edges. The following terminals
are suitable for wire wrapping connections: |
 |
|
| Minimum
Terminal Spacing |
To
assure adequate clearance for the selection of a bit and sleeve, use
the formula at right to calculate the minimum terminal spacing.
To find M (Minimum terminal spacing), the operator must know the following:
B - Bit radius, listed in the wire wrapping bit specification
chart.
W - Wire diameter, for a standard wrap, or the Insulation diameter,
for a modified wrap.
S - Sleeve wall thickness, listed in the wire wrapping sleeve
specifications.
T - Terminal width. Measure the widest part.
If the minimum terminal spacing as found by this formula is less than
or equal to the terminal spacing of the desired application, the bit
and sleeve combination selected has adequate clearance. If the spacing
found by the formula is greater than the spacing in the desired application,
another Standard Pneumatic bit and sleeve combination should be selected. |
 |
|
Producing
a Standard Wire-Wrapped Connection
There
are four steps involved in making mechanically and electrically sound,
solderless, wrapped connections. |
Step
1
Insert the pre-stripped wire into the wire slot of the wrapping bit. |
 |
Step
2
Anchor the wire in the notch of the wrapping sleeve. |
 |
Step 3
Insert the terminal into the center hole of the wrapping bit. |
 |
Step 4
Activate the wire wrapping tool. This rotates the wrapping bit and
wraps the wire around the terminal. |
 |
|
Producing
a CSW Wire-Wrapped Connection
There
are also four steps involved in making Cut, Strip and Wrap wire-wrapped
connections. |
Step
1
Insert insulated wire into bit and sleeve. |
 |
Step
2
Place tool over terminal to be wrapped. |
 |
Step 3
Activate the wire wrapping tool. Excess wire is cut off as tool starts
to wrap. |
 |
Step 4
Insulation stripping and wire wrapping are simultaneous. |
 |
|
| Quality
Assurance |
There are two tests to verify tightness of the wire wrap:
1. The Pull-Test is a stripping or removal test that is performed
to verify that the wire is being wound tightly enough around the terminal.
2. The Unwrapping Test determines whether or not the wire is being
wound too tightly around the terminal. This is done by unwrapping
the wire from the terminal. If the wire breaks during the unwrapping
test, it has been wound too tightly, and the wire wrapping bit should
be rejected. |
|
| Four
Common Problems of Wire Wrap |
Overwrap
Don't press too hard. Pressing down too hard on the tool during
the wire wrapping operation results in "overwrapping," in which one
or more turns of wire can slip over the preceding turns. An anti-backforce
device is helpful in preventing overwrap. |
 |
Insufficient
Turns
Push wire all the way into wire slot. Improper feeding of wire
into the wire slot of the bit results in insufficient turns of wire
for regular wraps or insufficient insulation turns for modified wraps.
|
 |
Spiral
Wrap or Open Wrap
Don't remove the tool too quickly. Removal of the wrapping
tool before the wrap is completed can result in "spiral" or "open"
wraps, where one turn of wire is more than 0.005" from another turn.
"Pigtails," where the final turn of wire is not completely wrapped,
are also caused by too rapid a removal of the wrapping tool. An anti-backforce
device will help reduce this problem. |
 |
Pigtails
Select the proper bit and sleeve. The particular wire wrapping
bit and sleeve depend upon the size (terminal diagonal) of the terminal
to be wrapped. If the terminal hole diameter of the bit is improperly
matched to the terminal diagonal, defects ranging from loose turns
to "pigtails" can result. |
 |
