It is important to consider both the mechanical and electrical requirements when selecting a connector. They are the type of tongue, i.e. slotted ring, hook, spade, etc., and the mechanical size, i.e. tongue size and thickness, hole size etc., and the electrical considerations such as the current carrying capacity, that may also determine some of the mechanical dimensions. This will determine minimum tongue size and barrel size required by the cable diameter.
The plating on the connector, and whether to use insulated or uninsulated barrels, should be determined by consulting the manufacturers’ information. The base materials should be selected, with plating finishes, by considering the environmental conditions, the material to which the connector will be bolted, the voltage and the temperature range the connector will be expected to endure.
Automatic machine crimping connectors may be crimped by hand, but it is not always practical to use hand-crimp connectors for machine crimping. Solder connectors should be chosen with similar care.
The base materials are those used for the construction of the terminal. The most commonly used base materials are copper and brass. Nickel, aluminium and steel are also used, but less frequently.
Copper is usually plated; it has good compatibility with copper conductors. It is physically suited to closed-barrel terminals.
Brass may be used plated or unplated; it has a higher connection resistance than copper but is better than both steel and nickel. It is a good material for open-barrel connectors.
Nickel has a very good corrosion resistance; conductivity of the terminal contact may be improved by gold plating. Nickel has good compatibility with iron, nichrome and manganese-nickel. This material may be used to temperatures of 650°C.
Steel is normally only used in a plated state, often with nickel for high temperature applications, or tin on top of copper for normal applications. Stainless steel alloys are sometimes used, but steel is more commonly used for high-strength applications and usually for open-barrel terminals.
Aluminum has satisfactory physical strength when used in specially designed fittings. There are some corrosion problems, but it gives good high current, light-weight service. It is compatible with aluminum conductors and suitable for low-temperature applications.
Terminal surface materials
The surface or the plating of the terminals must be considered and be appropriate for the application. Plating improves contact conductivity and corrosion resistance of the terminal. Surface contact is improved by increasing the surface contact area. Common plating materials used are: tin, nickel, silver and gold.
It is probably the most common plating material for corrosion resistance on brass and copper. Steel terminals are also plated with tin after first having been copper plated or copper flashed. Tin reduces galvanic corrosion when in contact with aluminum. Other advantages are: good conductivity, low cost, and being a softer material, it gives a large cont area due to the compression action of the screw.
Nickel has very good corrosion resistance. It is used to plate copper and steel, but can cause corrosion on brass. Nickel is a hard metal that requires a good wiping action to remove oxides and provide metal-to-metal contact. Nickel may be used where temperatures can cause other plating materials to melt.
Silver is used on a good conductivity base metal, such as copper or brass. Silver has good conductivity and gives a large contact area, but suffers from oxidisation, even in very low-sulphur atmospheres.
Gold has a very good corrosion resistance and conductivity. It is excellent for low-voltage application, its disadvantage is its high cost.
If the terminal has too deep a crimp, the strength of the joint is, reduced. With too shallow a crimp, the electrical contact has a high resistance. Selection of the correct crimping tool is essential.
The only time that the insulation of the wire should enter the crimp barrel, is where special provision has been made for it, such as in the terminal using an insulation support sleeve. Care must be taken to avoid insulation entering the wire contact area.