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Technology

Mechanical and Electrical Stability   Minimum Terminal Spacing
Distribution of Stress   Terminal Diagonal Chart
Easy Removal   Producing a Standard Wire-Wrapped Connection
Types of Wrap   Producing a CSW Wire-Wrapped Connection
Selection of Wire   Quality Assurance
Selection of Terminals   Four Common Problems of Wire Wrap
    Strip Force Chart

 

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

altBy 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.   altA 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

Standard Wrap(S)Standard Wrap - Only the bare wire is wrapped around the terminal.   Modified Wrap(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 g
16 0.051 1.30 4 15 6800
18 0.0403 0.02 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:
alt



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.
   alt

 

Terminal Diagonal Chart

Terminal Diagonal Chart for Wire Wrapping

Producing a Standard Wire-Wrapped Connection

Step 1
altInsert the pre-stripped wire into the wire slot of the wrapping bit.
  Step 2
altAnchor the wire in the notch of the wrapping sleeve.
     
Step 3
altInsert the terminal into the center hole of the wrapping bit.
  Step 4
altActivate the wire wrapping tool. This rotates the wrapping bit and wraps the wire around the terminal.



Producing a CSW Wire-Wrapped Connection

Step 1
altInsert insulated wire into bit and sleeve.

 

Step 2
altPlace tool over terminal to be wrapped.

 

 

 

Step 3
altActivate the wire wrapping tool. Excess wire is cut off as tool starts to wrap

 

Step 4
altInsulation 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
altDon'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
altPush 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
altDon'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
altSelect 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.

 

Strip Force Chart

A distinct advantage of wire wrapping is the ease with which a wire may be removed from a terminal to correct errors or modify wiring. An unwrap tool is slipped over teh terminal pin, engaging the first turn of the connection. Rotating the tool, the connection is removed in seconds without damage to the terminal pin.

There is a constant surveillance of manufacturing dimensions. Each wrapping bit is subject to a series of "Qualification Tests". These consist of wrapping groups of wire on various types of terminals. The wrapped wire is then subjected to a "Strip Test" to determine adequate tightness. Additionally, "Unwrap Tests" are performed to ensure against an "Over Tight" wrap.

Solid wire is used for wire wrapped connections. Copper is the most commonly used wire. Minimum elongation of 15% is required for 24 through 32 AWG, while 20% is required for larger wire sizes.


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