Cables/Connecting

What makes good connector design?

15th October 2020
Alex Lynn

Whether they’re used in a commercial aircraft, on a military vehicle or a medical ventilator, very few components are subjected to extreme conditions in the same way as connectors. Here, Ammar Lokhandwalla, customer application engineer at connector specialist PEI-Genesis, has explained what makes a good connector design.

According to a 1986 essay by Dr Fred E.H. Schroeder, ‘The electrical appliance plug is something that Thomas Edison did not invent. It was a curious oversight, because Edison anticipated almost everything that might relate to the incandescent light bulb and its applications’.

You see, the concept of an ‘appliance’ or something that could be readily plugged into a wall outlet didn't come around until the twentieth century, around two decades after the popularisation of the light bulb. The initial popularity of electric lighting was based on the assumption that these devices would be permanently wired into the house.

Today, engineers across the world consider the complete lifecycle of every component from cradle-to-grave. A modern A380 aircraft, for example, is made up of some four million individual parts produced by 1,500 companies from 30 countries around the world.

Connectors, and more specifically those used in aviation and military applications, must endure extreme conditions. Whether it’s the rapid temperature fluctuations and changes in humidity, to persistent vibration, impacts and signal interference, these cables must operate reliably to ensure their users can get the job done.

Take the industry-standard D38999, for example. This is a military-specification connector that was originally designed in the 1970s and is now on its third-generation design.

Like other connectors of its kind, it’s made up of a few basic components: a hard outer shell, a neoprene rubber insert with holes to house the pins, or contacts, and sometimes a backshell on the outer housing that provides additional shielding and durability.

Material choice

When selecting a connector, engineers must consider a wide variety of properties depending on their application. One of the primary considerations is the choice of materials, for both the electrical terminations and shell housing. For example, although copper offers better electrical and thermal conductivity, aluminium is cheaper and easier to form and plate.

So, while copper may be chosen for high-voltage industrial applications where heat dissipation and conductivity are vital, aluminium may better serve aerospace and military applications where weight and corrosion-resistance are more important.

Water barrier

Ingress protection is another consideration. Connectors designed for industrial food and beverage manufacturing must be sealed against water jets to allow equipment and machinery to be washed down between shifts.

This protection extends to marine applications, such as those in the oil and gas sector where equipment may need to be fully submersible for prolonged periods of time. In these applications, it may be necessary to select a polycarbonate connector, with the right o-rings and grommets to provide a moisture seal.

Plating

While aluminium is the preferred choice of connector material for many construction, rail, industrial and military applications, it may still need to undergo plating to improve its corrosion-resistance, to provide further electromagnetic shielding, and to meet camouflage and colour needs.

For example, some military applications use olive-drab green, a colour that was historically achieved with a toxic cadmium coating. In recent years, this has been replaced with a black zinc nickel plating that meets RoHS and REACH regulations. If engineered correctly, this black plating can deliver the same performance as cadmium coatings and withstand over 500 hours of salt-spray.

Soldering vs crimping

Many people may not be aware, but not all connector contacts can be solder-terminated. Under certain extreme conditions, the operating temperature of the application can exceed the melting point of the solder, causing connection failure. For applications where this is a risk, engineers may prefer to specify a crimped connector.

With crimping, contacts are joined to the wire by mechanically squeezing them together to ensure that they remain in contact no matter the temperature. Instead of a soldered connection where the wire is fed through an eyelet or hook and then soldered, crimping involves material being deformed to lock the termination together using a special crimping tool.

Design early, design once

One of the biggest mistakes I see manufacturers make is considering connector design too late in the design process of their product. This often means that a product’s time to market is delayed while the design is reworked.

It’s important to remember that your connector may have physical design constraints like a minimum wire-gauge or number of contacts, so it’s vital to consult with your connector supplier early in the process. At PEI-Genesis, we’ve made this problem easy for our customers by offering customers a 3D wire model of the connector that customers can drop into their design to see if it fits.

If it doesn’t, our engineering team can help refine or redesign the existing design, or propose a different connector entirely, that meets the specification. This includes changing features like threaded, bayonet and friction fittings, or accessories like backshells, or something simple like a dust cap.

So, while Edison might have made a curious oversight by not inventing the electrical plug, luckily manufacturers don’t have to make the same oversight today by considering the factors that make good connector design.

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