4. Wire-wrap Joints

Figure 13 - Wire-wrap joints - 1 wrap pin, 2 wire to be connected

Wire-wrap joints serve to connect solid copper wires of 0.3 mm to 1.0 mm diameter with the respective connecting elements. The wire-wrap joint restricts the electric connection to the two mechanical elements absolutely necessary:

- the wire to be connected
- the wrap pin as connecting element.

Using technological properties of the wire and wrap pin, it is possible to do without solder, flux and head. Disadvantages of soldered joints are excluded. A maximum of 7 windings is required to achieve a correct joint mechanically and electrically. The joint is generated by plastic flow in the wire at the edges of the wrap pin. The sharp edges of the wrap pin are penetrating into the wire with the oxide layers on the wire and/or wrap pin being removed by scraping and squeezing. Since 7 windings are made, several metallic, clean and gas-proof contact faces are generated. Corrosion on the contact faces is not possible.

Steps of work

· Cutting to length and stripping of conductors

The stripping length results from the number of turns multiplied by the circumference of the wrap pin. As a general rule, a stripping length of approximately 25 mm can be considered for any wire diameter.

Why must stripped wires not be notched?
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________

· Feeding the wire into the wrapping mandrel

The wire is to be fed into the smaller off-centre hole of the wrapping mandrel, to be bent over and held in place.

· Making the wire-wrap joint

The wrapping mandrel is slipped on the wrap pin with the central hole. The wrapping process may start at any point in the wrapping area of the pin. By rotating the wrapping mandrel the wire end is drawn off the hole and firmly wrapped around the wrap pin. In order to ensure that the individual windings are closely fitting, the wrapping mandrel is to be pressed towards the base during wrapping. If several wires are to be connected to one wrap pin, the wires are to be wrapped one after another.

Figure 14 - Making the wire-wrap joint - 1 windig direction, 2 wrapping mandrel, 3 wrapping mandrel part for socket, 4 socket for wrap pin, 5 wrap pin, 6 wire fed into off-centre hole, 7 wire to be connected

· Inspection of the wire-wrap joint

A correct wire-wrap joint features:

- the correct number of turns (windings),
- closely fitting windings,
- no overlapping windings.

The inspection also includes strength testing.

The strength is tested by means of a drawing-off claw and a spring balance.

The joint has the correct strength is not achieved, the joint is to be re-made.

Figure 15 - Strength testing - 1 tensile force direction of the drawing-off claw

· Opening the wire-wrap joint

Any wire-wrap joint can be detached by means of the wrapping needle and then be re-made. The corkscrew type wrapping needle is put on the wrap pin with the hole. By rotation opposite to the wrapping direction of the wire, the wire is lifted and thus the joint is opened. Once used for wrapping, the piece of wire must not be used again for a wire-wrap joint since because of notches there would be a risk of breakage. The wire must be cut off and stripped again.