Four Qualities to Look for When Selecting D-Sub Backshells for Transportation Applications
It’s easy to overlook backshells, but they play a critical role in protecting connectors, cables, and overall system reliability.
Mike Kahn, Director of Marketing, NorComp
Connector reliability is essential to the performance of electronic systems in transportation applications. Off-highway equipment, long-haul freighters, bus and rail systems, and all other modes depend on critical systems including telematics, lighting, doors, brakes, refrigeration, and more. These applications demand rugged connections designed to withstand harsh environmental conditions such as extreme temperature, water, oil, antifreeze, shock, vibration, and mechanical stress while providing clean, protected, and reliable signals. As such, specifying robust connector systems is common practice.
Backshells, on the other hand, often don’t receive the same consideration, despite the critical role they play in protecting connectors and cables and determining overall system reliability. All systems are limited by their weakest link, and in today’s electronic infrastructure, mechanical cabling failures occur far more often than board level components.
D-Sub backshells are available in a variety of options designed to ideally suit a wide range of applications. To ensure that the backshell you choose is one that will best maintain the overall integrity of a given design, it’s important to select the right combination of characteristics for your application.
D-Sub backshells are offered in several different materials, each with different performance properties.
Die-cast metal backshells are usually made of zinc or lightweight zinc alloys and are plated with high-quality nickel or black oxide over a copper undercoat to provide excellent EMI/RFI shielding. These backshells offer high-reliability performance that is ideal for use in rugged, harsh-environment applications. They are available in several sizes, accommodating both standard and high-density connector types. A large variety of cable exit and mounting hardware mounting options are also widely available.
Metalized plastic backshells offer the benefits of both lower weight and cost while providing the abrasion resistance and shield efficacy of metal shells, which can often satisfy the requirements of transportation applications with less demanding operating environments. These backshells are generally available in both screw- and snap-together options and also offer a wide variety of cable exit and mounting hardware options.
Plastic backshells offer the most cost-effective solution, but aren’t anywhere near as durable or resistant to harsh-environments as die-cast metal or metalized plastic versions, and they offer no EMI/RFI shielding. They are generally better suited to less demanding applications where shock, vibration, and signal interference are not design considerations. These too come in various sizes to accommodate standard and high-density connectors, offer a variety of cable exits and mounting hardware options, and are typically available in a wide range of colors to make cabling installations easier.
Figure 1: NorComp’s 970 Series 180° top-entry D-Sub backshells are offered in die-cast and metalized plastic versions for EMI/RFI shielding. A variety of compression insert options provide maximize cable strain relief and prevent damage to wires and contacts in rugged environments.
Cable Exit Angles
Selecting the proper cable exit for a given application ensures that cables are never bent at excessive angles, which helps ensure mechanical longevity of the panel connection and prevents degradation in both shield effectiveness and signal integrity. Key considerations in exit selection include cable flexibility, clearance behind the panel, and the direction the cable is required to travel after leaving the panel.
Common options for cable exit angles include 180° (straight exit), 90° (right-angle exit), 30°, 35°, 40°, 45°, and 60° exits, with 30–60°-exit options typically based off of a 0° straight exit and a 90° right-angle exit.
Regardless of backshell material type or cable exit, there are typically four options for securing the external cable to the panel: pan-head screws (e.g., slotted, Phillips, torx, etc.), thumbscrews, spring clips, and slide latches.
The choice of mounting hardware is also application-driven and depends on factors including how often the connection will be cycled (plugged/unplugged), how accessible the connection is (clearance), and how much shock and vibration is expected. Screw attachment is by far the most reliable, but requires more time and space with regard to panel accessibility. Spring clip systems are faster to cycle and require less space, but provide less reliable retention than screw attachments. Slide latch systems offer a good compromise between the two, combining a faster cycle time than screw attachments with improved retention over spring clips.
Once you’ve evaluated your application requirements and have defined a list of desired characteristics, it’s often easy to identify the backshell that best suits your application using a supplier or distributor selection guide like the one pictured in Figure 2.
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