The pins are the key parts for industrial plug and socket. It directly decides the product quality.
The reasonable design and reliable manufacture are critically important for the pins. All the main industrial socket and plug suppliers have been devoting to do research and development of this technology.
The common types of pins
- insert type pins
The pin is inserted into the elastic sleeve. The pins are made of yellow brass. The yellow brass has better mechanical performance than red copper, but it has higher specific resistance than red copper and silver. Besides, the yellow brass is easier to be oxidized. The oxidation will produce a larger contact resistance.
In order to prevent temperature rise which is caused by large contact resistance, the pin itself must have sufficient pressure. However, too much pressure will make the plug or socket difficult to insert and extract.
The insert type pins have obvious advantage of low material cost and easy manufacture. The quality can be easily controlled. It is IEC (CEEform) approved technology.
- contact type pins
This type features in greater tolerance to the short-circuit current. But the pins material must be silver alloy. The cost is high. These pins are used widely, especially in the electric locomotive filed.
The parameters and material
- contact resistance:
the additional resistance in the contact area of two metal conductors is called contact resistance. In microscope, the contact happens only on some contact points. The electric streamline is shrinking on the contact surface, producing shrinking resistance Rs. In addition, the contact surface that exposed in the air may form a thin film promptly (oxidation film, for example), and a film resistance Rb as well. Therefore, the contact resistance is:
Rj = Rs + Rb
- pin pressure F
This pressure acts on both pins to make them contact. To increase the pin pressure can reduce Rj, because increased pressure will make the contact surface larger. Moreover, the enlarged pressure will also destroy the film so that the Rb is reduced.
- temperature rising
With long time electrified, the temperature of the pin will rise. This change is related with not only the current, the material and size of the pin, but also the pin pressure.
- The material
The physical property, such as the specific resistance, the mechanical strength, the chemical characteristics, will directly influence the contact resistance. When choosing the material, you need to consider the combination of reducing both the shrinking resistance Rs and film resistance Rb.
For example, material like brass, nickel, wolfram are easy to form the oxidation film. Only if the film is removed by mechanical or electrical method, you can choose them. The oxidized silver still has good conductivity, but it may produce sulfide. Material like gold is hard to oxidize, but it is too soft with the risk of plastic deformation.
The design requirements
There are some criteria for quality industrial plug pins:
- The temperature of the pin won’t rise too high under normal current.
- Short-circuit current tolerance.
- Medium insert and extract effort.
- Easy operation.
One thing mentionable is that in the actual design process, the contact resistance is hard to calculate. A large amount of experiments and good experience are needed.