Although electrical connectors are passive, dumb devices without any power and intelligence, they have a critical role in electrical engineering because they convey signal and power between pieces of equipment. To ensure continuity and reliability of connections it is of paramount importance that the two mating parts do not come apart accidentally. So, why should we use connectors without locking device? The main reasons are simplicity of manufacture, hence cost, and simpler connect and disconnect operations.Connectors without locking device must provide for a sufficient retention force- where retention force is the force which holds the mated pieces together must be overcome to disconnect the two parts.Retention force will come from two sources. Firstly, it is the friction between the metallic pins and their corresponding sockets. Thus, pin- and socket-selection affects both the insertion force and the retention force. The second source of retention force is the friction between the mating plastic parts of the connectors, ie, the insulator holding the pins, and the receptacle.When we are designing connectors without locking device pair for use where frequent connect and disconnect operations are expected, it is a good idea to generate the major part of the retention due to friction between the metallic pins and their sockets. That is because plastic parts are more likely to wear out fast and lose their retention force.It is important to note that where the number of pins is small, total friction between pins and sockets will be low and a greater reliance will have to be placed on the friction between the plastic parts. When the number of pins is large, the designer of connectors without locking device may not give too much importance to friction between the plastic parts: the insulator and the receptacle shell.Because of manufacturing variations, not all pins have the same exact friction force with their corresponding sockets. This aspect has been dealt with by Robert S. Mroczkowski, Sc.D, in his paper entitled “The Mating Game” which appeared in the magazine “Connector Specifier”, December, 2001. The conclusion is that the total friction force will be greater than the individual pin force multiplied by the number of pins.Reducing Chances of UnlockThe force required to unlock a connect may be enhanced by indirect means. Consider a connector where an accidental axial tug on the cable will apply directly to the connector. The entire force will be transferred to the pins. If that force magnitude is greater than the retention force, the connector parts will slip apart. Now consider an angle connector-one where the cable axis is at an angle with the axis of the pins. One popular angle is the 90 degree and the resultant connector may be called an L-connector. A tug on the cable will transfer very little force to the pins and the connector will most likely not come off. Continuous vibration can, of course, gradually unlock such plugs also.Retention Force SpecificationSince the retention force of key significance in a connectors without locking device, and it depends on the number of pins also, decisions are made at an early stage in the design as to:
Number of pins
Connector Life in terms of mate-unmate operations
AdjustmentsEven a careful design may not provide the exactly desired retention force. Therefore, it may be useful to design for a slightly higher retention force and then trim material to reduce retention force to the desired level.Testing For Retention ForceOnce production parts have been manufactured, verification testing will confirm that all specifications, including connector retention force, are met. For connectors without locking device, Design Verification Testing will typically include measuring retention force at pre-established intervals during mate and un-mate cycle testing. This will confirm that retention force is maintained over the design life of the connector.