Crimping is a method of attaching a terminal lug to an electrical wire. This gives us the advantage of being able to connect and disconnect wires from units without the possibility of damaging heat sensitive circuitry by hot soldering irons.
This crimping operation sounds complicated but is not. Depending on the material of the wire being either aluminium or copper, dictates which of the following crimping methods should be employed.
Aluminium wire crimping is carried out in the following manner:-
- Refer to the relevant wiring diagram and maintenance manuals and IPC to find out the correct crimp terminal and paste ( Normally petrolatum and Zinc based ) to use ( the barrel of the crimp lug is normally provided half full of crimp paste).
- Strip the insulation on the wire back sufficiently to be able to insert bare wire into the terminal lug sufficiently so that the bare wire reaches to the end of the barrel ( there is an inspection hole at the end oof the barrel to confirm this ) and so that the insulation butts up against that back or entry point to the barrel.
- Cover the inspection hole or the end of the barrel with your thumb or finger to prevent the paste from escaping and insert the wire into the barrel of the lug.
- Now using the approved crimping tool for the terminal ( as designated by the terminal manufacturer ) Which is normally in the form of a set of crimping pliers which may cover a range of crimp sizes, Select the appropriate jaw for your terminal and squeeze the handles together tightly ( modern crimp pliers normally have a ratcheting mechanism which will not release until the crimp is satisfactory to give you a perfect crimp joint between the terminal lug and the wire).
- If the terminal is not normally pre-insulated so then you must use an approved insulation sleeve which is normally transparent and tied to the wire with a lacing cord to prevent it from moving. This insulation sleeve must cover the barrel and about 3/4 to 1 inch of insulated wire behind the terminal.
Copper wire crimping is carried out in the same manner as aluminium wire but there is no need for a crimping paste to be employed. The reason why aluminium wire crimp joints must employ a crimp paste is because of the inherent tendency for aluminium to form an oxide layer on its surface. The crimp paste breaks down this oxide layer and prevents it from reforming by excluding any air or moisture form the joint.
Copper wire terminals may also come pre-insulated but if not then they must have an insulated sleeve tied to them in the same manner as the aluminium wire.
Pre-insulated copper wire terminals may be identified by the colour of the insulation on the barrel with reference to the relevant data provided by the terminal manufacturer.
Testing of Crimp Joints
The inspection and testing of crimped joints is carried out in accordance with British standard G178 and the cable manufacturers recommendations or Aircraft maintenance manual.
In general though you are looking for a secure crimp fitting with sufficient cable purchase, not exposing un-insulated wire and with a very low resistance from one end of the cable to the other end of the crimped joint.