How do they survive? Backshells. Backshells protect connectors and cables from the many forms of destructive treatment Mother Nature has to offer. Backshells also protect connectors and cables from the occasional human tendency to treat an expensive electrical assembly worse than a backyard garden hose.

The business end of the connector plug or socket is right up front, where signal and power contacts mate together to interconnect avionic systems and other electronic devices. But at Glenair, we would argue that the single most important design feature of the modern high-density connector is the set of threads found on the back. In the 1950's, connectors did not always have threads on their backsides. While some connector shell designs had tapped holes, grooves and notches, most were not equipped for the attachment of back-end accessories. At best, connector shells were outfitted with random, unpredictable thread geometries designed to accommodate simple strain-relief clamps or cable sealing nuts supplied by the connector manufacturer.

Not until the mid 1960's were most connectors consistently equipped with threads to accommodate backshells. Even today, some connectors are manufactured with modified thread geometries, and in some cases have less than three threads for the backshell to attach to - making backshell interface design a compromise in many applications.

Early connectors also lacked any provision for positive interlocking of the backshell independent of the threads. But the smooth cylindrical surface on the rear end of the connector was eventually modified to include tooth-like protrusions to interlock with an accessory. Hand in hand with the newly added threads, the toothed interface allowed angular backshells to be locked down in a particular orientation, and also prevented cable torque from loosening the backshell. The most positive interlocking design to date is the splined connector and accessory shell which accommodates accessories with captive, rotatable coupling nuts.

The introduction of threads and interlocking teeth made it possible for accessory engineers to design new backshells which addressed a broad range of environmental, electrical and mechanical requirements: Environmental sealing backshells for improved immersion protection, shield termination devices to combat electromagnetic interference, shrink boot adapters for harsh environmental resistance, extenders to provide working room for contact termination and repair, strain-reliefs to prevent damage to contact terminations, angled fittings to improve cable routing and so on.

Over the years, Glenair has been at the forefront of these and many other accessory design innovations. Glenair connector accessories have contributed to the durability and performance of hundreds of thousands of mission-critical interconnect assemblies - thanks in large part to historical improvements in the connector-to-accessory interface, and the design expertise of a generation of interconnect engineers.

Back to the Future

The design and development of today's broad range of connector accessories took over 30 years to complete, and the number of types and styles is truly extraordinary. Glenair's Circular Connector Accessories catalog, for example, presents 24 different categories of backshells and other accessories. The D-Subminiature portion of our new Rectangular Accessories catalog (available in January 2000) features over 30 different styles of backshells for use with popular rack and panel connectors. Glenair also produces dozens of different composite and fiber-optic backshell designs, not to mention thousands of Mil-Spec configurations. Perhaps the most revealing figure is the 25,000 part numbers - mostly connector accessories - now available in our Same Day Inventory. The staggering number of backshell types available today makes it all the more important for interconnect engineers, and others tasked with the responsibility of specifying connector accessories, to become expert at the art of backshell selection. Here are the basics:

Step One: Determine Accessory Functionality and Performance Requirements

The first step in successful backshell selection is proper product application: In other words, what exactly is the accessory supposed to do, and what are the conditions in which it must accomplish its job? It's not enough, for example, to know that the accessory must provide strain-relief. It's critical to also understand the working environment - the high heat of an engine compartment, for example - before moving forward in the selection process. And the more complex the application, the more detailed the questions need to become, lest some important functionality or performance requirement be forgotten.

The following questions will reveal the most critical functional and performance requirements for the accessory, and help a knowledgeable engineer make the best design, material and plating selections:

(1) What is the intended working environment of the interconnect system - shipboard, space, airframe, ground support and so on?

(2) What level of environmental protection is required - full water immersion, moisture resistance, chemical/caustic fluid resistance, extreme corrosion resistance, mud?

(3) What level of electromagnetic shielding is required - overall cable shielding, individual conductor or both? Is any particular style of shield termination preferred by the user?

(4) What level or amount of strain-relief, from light duty to gorilla proof, is required to protect shield and conductor terminations from damage?

(5) What is the temperature range of the application environment? Is it so hot that stainless steel is called for, or is it so cold that an elastomeric seal might potentially fail?

(6) Is repairability a design requirement? Some split shell designs make field repair a snap, while some solid shell models make repair and maintenance extremely difficult.

(7) Are there size or shape constraints which need to be considered? Is working room so important, for example, that a long extender should be designed in? Or is available space so tight that a low profile design is called for? Does cable routing dictate 45o, 90o or straight cable entries?

(8) Is a metal shell required or is composite thermoplastic an option? Is saving weight so critical, in other words, that only composite backshells need apply? Or is metal preferred due to cost, strength or design approvals?

(9) And speaking of approvals, does the application require performance per MIL-C-85049 or another commercial, military or industrial performance specification?

Step Two: Determine Connector Part Number and Interface Designator

By 1970, Mil-Spec connector documentation controlled the interface mateability of plug to receptacle for all the major circular connector families - MIL-DTL-5015, MIL-DTL-28840, MIL-C-38999 and so on. Which meant that no matter who manufactured the connectors they would, at least in theory, intermate with the same family of parts from any other manufacturer.

Shortly thereafter, rear connector-to-accessory interface geometries were also standardized and came under the control of a number of military specifications, principally MIL-C-85049. This meant it was no longer necessary for accessory makers to consider both the connector type, shell size and manufacturer in order to correctly mate their accessories to a given family of connectors.

Today, interconnect system designers simply need to identify the correct part number for the chosen connector and match it to an alphabetic interface designator to select accessories which correctly fit the part. Tables listing all the major connector-to-accessory interface designators used by Glenair can be found in the Backshell Interface Standards table in the general information section of the Circular Connector Accessories catalog.

While the interface designators take much of the guess work out of the process, it is still important for everyone on the accessory side of the business to add value to the process by helping to evaluate the chosen connector in terms of application performance requirements. And if necessary, to work with Glenair engineering to suggest an alternative connector for the application.

Step Three: Determine Cable Make-up

Several key questions in backshell selection cannot be answered without a solid understanding of the overall make-up of the cable and harness on which the accessory will be used. Basic dimensional elements, for example the diameter of the backshell cable entry, cannot be specified without accurate descriptions and measurements of the cable or wire bundle.

Sometimes it is necessary to perform a complete analysis before sound decisions can be made regarding the make-up of the cable and how it will affect the selection of strain-relief, environmental protection or shield termination technologies. A basic analysis should include:

• Wire Numbers and Types - twisted shielded pairs, coaxial power, signal, fiber optic, etc.
• Shield Material Gauge, Number and Type - tin, nickel, silver plated copper wire, and so on.
• Jacket Material and Thickness

This is your opportunity to match up the functional/performance requirements you discovered in Step One, with the physical characteristics of the wire and cable. If you finish this step without a solid grasp of the needs of the application (strain-relief, EMI shield termination, working room, repairabilty, and so on) and how these needs will be addressed on this particular cable, then you need to go back and re-do your homework.

Step Four: Develop Accessory Part Number

Glenair follows a standardized format for the development of accessory part numbers, so mastering the process once will enable you to build valid part numbers from any accessory product family - circular, rectangular, composite, shipboard, fiber optic, and so on. Some differences are of course to be found from one product series to the next because of the range of available options. But the following steps should serve as general enough guide to the process for most accessories:

(1) Select the Product Series: do you want a dust cap, a banding adapter, an environmental backshell or a Qwik-Ty? Use the graphical product selection guide at the beginning of each catalog for help in getting to the right section.

(2) Select the Connector Designator: as described above, this alphabetic designator is used to match each connector family with the correct backshell interface dimensions.

(3) Select the Angular Function: choose 45°, 90° or straight. For some accessories this step will select some other physical attribute, such as flange vs. jam nut mounting on a bulkhead feedthru accessory, or cable entry.

(4) Select the Basic Part Code: this code fine-tunes your selection within the product series. For example, for Series 39 EMI/RFI Cable Sealing Backshells, your Basic Part Code selection could tell the factory what style of shield termination technology you prefer.

(5) Select the Finish Symbol: this symbol, selected from tables right there on the catalog page, or in some cases from a larger table elsewhere in the book, tells the factory what surface finish or plating should be applied to the product.

(6) Select the Shell Size: This of course is simply the matching of the accessory shell size to that of the chosen connector. Use Table 1 - Backshell Interface Dimensions - to find the part number code for your choice.

(7) Select the Cable Diameter: Referred to most commonly as just the "dash number" this piece of the part number specifies the minimum and maximum size of cable the rear end of the accessory can accept. Dash number selection tables are generally found right there on the page or on an immediately adjacent page.

(8) Select the Strain Relief Style: Most accessories that are able to accept cable clamps or saddle bars offer a range of style choices. The selection is usually based on the level or duty of strain-relief that is required, and design drawings of applicable options are featured right there on the catalog page.

(9) Select the Environmental Sealing: When different levels of environmental sealing are available, the choices are presented right there on the page. Full immersion protection is accomplished by capturing the cable grommet and follower within a cylindrical socket. Moisture proof designs merely force the cable grommet seal against a conical ferrule.

(10) Select Unique Options: Other available options, such as drain holes, wire attachment lengths, special material designators, and so on, are tacked on to the end of the part number. These options are generally explained right there on the page.

Step Five: Accessorize the Accessory

Because we are more inclined to offer our customers complete solutions to their problems, rather than just sell them backshells, may we suggest that a thorough backshell selection process includes the step of considering other Glenair products which could contribute to the protection of the connector and cable? If appropriate, some or all of these products should be considered as accessories to the accessory:

• shrink boots
• band-it tools and bands
• connector accessory holding tools
• cable braid and shielding
• dust caps and stowage receptacles
• support rings and shield socks

Step Six: Quote Price and Delivery

If necessary, work with Glenair engineering to clarify any outstanding part number or component design issues, then submit the RFQ to the factory - complete with the business case for special pricing or delivery.

Expect Same Day Shipment for 25,000+ commercial and Mil-Spec accessories, and 2 to 3 weeks turnaround for non-stock items. And speaking of our Same Day Inventory, by all means build the habit of searching stock for close or exact matches on your most important design variables (accessory function, termination style, shell size, connector designator and so on). You may find that, especially for prototype purposes, a suitable accessory is in stock, in quantity and ready for immediate shipment.

Turning to Plastic

Every connector accessory, no matter what it is made of, still needs to thread onto the back of a connector. It must also intermate with other accessories, including both composite and metal versions. Other dimensional standards, such as the number of teeth on an interlock or the location and shape of polarizing keyways on a connector must be observed - again, regardless of what material is used to build the part.

Shield Termination Devices Selecting the most appropriate shield terminating backshell for a particular application requires a detailed analysis of the cable and the application environment in which the assembly will be used. There is no single shield termination technology or methodology that will meet every customer's requirements. For this reason, Glenair supports every popular shield termination method with the full range of shell sizes, materials, platings and tooling, including: Single and Multiple Conical Rings Crimp Ring and Banding Terminations Castellated or Splined Rings Lampbase Thread Rings Radial Compression Springs Magnaformed or Crimp Ring with Shield Socks and Boots Tinel™ Lock-Rings Customer selection depends on many factors, including cost, repairability, shield type and construction, cable diameter and type, cable jacket thickness, weight, shock and vibration, strain-relief, corrosion resistance and so on. The primary factor is cable construction: what type of shield is being terminated, where is the shield or combination of shields located within the cable or wire bundle, and how hard is the outer jacket to work with. Customer preference, established methods and practices, tradition, manual skill levels and inspection must also be considered. ™Tinel is a trademark of Raychem Coporation Cable Entry Styles The art of successful backshell selection also includes specifying the most appropriate shape and cable entry for each connector accessory. Unlike shield termination devices, this selection has little to do with tradition and everything to do with working room, repairability, anti-decoupling requirements, cable routing, strain-relief and a host of other mechanical considerations. The range of configuration and shape options offered by Glenair include: Straight 45° Elbow - Standard Profile 45° Elbow - Low Profile 90° Elbow - Standard Profile 90° Elbow - Low Profile 45° Split Elbow - Clam Shell 90° Split Elbow - Clam Shell Ultra Low Profile 90° Elbow Split Cobra Strain-Reliefs Typical Mil-Aero cable assemblies often have over a hundred wires terminated to a single connector. Preventing the wires from pulling on the contacts and damaging the termination is critical, and is usually accomplished with a strain-relief backshell that employs a mechanical clamping device to isolate the strain applied to the cable. Strain relief on electrical connectors can be accomplished in other ways, such as with a wire service loop that allows the wire to move between the clamping device and the contact without over-stressing the termination. However the basic method of clamping the wire bundle or cable jacket with saddle bars has historically been the most common method of protecting contact terminations. The saddle clamp method, by the way, dates back over 60 years to the original Cannon K Series and Army/Navy/5015/3057 type connectors and clamps still in use today.

It's a simple matter to compare thermoplastic materials against steel or aluminum and point out the advantages plastics bring to the table: They're lighter, they don't rust, they don't rattle loose and they can hide from radar. It's much more difficult to actually design composite components which take advantage of these properties while still meeting the requirements of the application for electromagnetic compatibility, environmental protection and physical strength. At Glenair, we haven't just pioneered a few experimental designs, we've made a complete range of composite accessories available as commercial-off-the-shelf products.

Glenair composite thermoplastic backshells are qualified to MIL-C-38999 and MIL-C-85049, and have proven performance track records in commercial and military applications including the G4, F-22, RAH 66, JSF, EH 101, F/A 18, LAV and 717 programs. Glenair's new Composite Thermoplastic Connector Accessories and RFI/EMI Enclosures catalog (available in January 2000) introduces dozens of new composite accessory part numbers, including junction boxes, through-box and panel fittings, protective covers, rectangular and circular backshells, fiber optic backshells, Qwik-Tys and more. Part number development follows the standard format, and many designs are in stock in Glenair's Same Day Inventory.

Fill In the Box

D-Subminiature MIL-C-24308: Glenair's Series 55 line of connector accessories for MIL-C-24308 connectors feature total enclosure of the mated plug and receptacle connectors for optimum EMI Shielding. A variety of cable/harness shield terminations are offered for overall shields and/or individual shielded complexes. Optional conductive EMI gaskets are available for installation under the receptacle connector flange to further assure EMI shielding effectiveness. Three cable entry locations are offered. A straight "top" entry and two 90° "end" and "side" entries to satisfy almost any installation requirement. Split backshell designs provide easy assembly and convenient access for future connector service or maintenance.

MIL-C-81659: Glenair manufactures a full-spectrum line of rectangular backshells for MIL-C-81659 connectors. Built from rugged aluminum alloy and available in a wide range of finishes, our MIL-C-81659 backshells provide robust EMI and environmental protection. Solid and split-shell designs are available for cable strain-relief and EMI shield termination. Top and side cable entry configurations provide flexibility in contact termination. Selected 81659 backshells, including Qwik-Ty, G-Spring and Tag Ring part numbers are available for immediate shipment from Glenair's Same-Day Delivery Program.

Micro-D MIL-C-83513: Glenair's extensive Micro-D backshell offering includes EMI banding shells, switching shells, potting shells and protective covers. EMI bands and installation tools are sold separately. Most designs are available in up to six standard finishes. Backshells are available for all standard M83513 metal shell connectors.

MIL-C-83527: The Glenair 527-002 series backshell is designed for use with MIL-C-83527/1 connectors. Eight cable entry points are available on the backshell's side, ends and top (a maximum of three entry combinations may be selected). Made from rugged aluminum alloy, the backshell is available in a full range of standard finishes. Please consult factory for custom finish options. Backshell hardware is passivated stainless steel. Each unit ships with an RFI gasket.

MIL-C-83733: Glenair's split backshells for MIL-C-83733 connectors are available in three designs which offer either a standard cable clamp, saddle-bar style strain-relief, or an EMI shield banding platform. Tongue and grove joints provide optimal EMC. Customers may choose from up to six cable entry locations. Backshell bodies are made from aluminum alloy with passivated stainless steel hardware. A full range of standard finishes are available, including custom options.

ARINC Series 600: Glenair has a full line of RFI/EMI backshells for ARINC Series 600 connectors. We also have special backshells for various connectors such as Cannon DPKA, DPKB, DPJM, DPXA, DPX3, DPX4 and DL2-96. Every functional requirement - from overall and individual shield termination to basic saddle-bar style strain relief is included in Glenair's backshell series for ARINC type connectors. The backshells are built from rugged aluminum alloy and are available in all common surface finishes and cable entry configurations.

Hypertronics: Glenair backshells and protective covers for Hypertronics connectors are built from rugged aluminum alloy. A full range of plating options are available as are custom finishes. Strain-relief backshells feature potting holes and top and 45o cable entry styles.

In addition to the above categories of rectangular accessory product information, the new catalog has a wealth of technical design and support information intended to assist users in building reliable and trouble-free interconnect assemblies. Be sure to take advantage of the ability to check stock on these accessories by accessing our Same Day Inventory Search Service at www.glenair.com., or by requesting a printed copy of the inventory from the factory.

The Tunnel at the End of the Light

Glenair's fiber optic product line grew up in the military-defense environment. Our backshells are designed to work with military standard connectors, such as the MIL-C-28876 and the MIL-C-38999, which have been modified to accept optical fiber. The design requirements for a functional fiber optic connector accessory used in this demanding application environment fall into six categories:

Weight Reduction: One of the principal benefits of deploying fiber optics in the first place is weight savings. In fact, next to expansion of bandwidth, weight savings are the primary reason for the transition to fiber in many aerospace applications. It is essential that the accessory components in the fiber optic interconnect system also contribute to overall weight savings. For this reason, composite thermoplastics have become the material of choice for fiber optic connector accessories.

Crush Resistance: The same amount of clamping force which would be required to adequately protect the wire terminations in an electrical connector could crack the glass core of a fiber cable. For this reason, all forms of strain relief which apply severe compression forces directly to the cable are not appropriate for use in fiber optic assemblies. QwikClamp style strain-reliefs, elastomeric grommets, and heat shrinkable boots are preferred over saddle bar clamps to avoid applying severe compression to the cable, while still maintaining optimal fiber position and alignment.

Cable Bending: Cable bend radius, or the amount a fiber optic cable can bend before breaking or suffering increased attenuation, is an equally critical design requirement. Accessory elbows, conduit transitions, and other fittings which subject fiber optic cables to abrupt changes in direction beyond the acceptable bend radius of the fiber are therefore extremely risky. Again, the dangers are either breakage of the fiber optic core or attenuation of the optical signal. Angular accessories must be designed with smooth 45° or 90° bends to insure the non-abrupt routing of the cable. Low profile backshells, such as the popular Cobra design, are not appropriate for use with fiber.

Micro-Bending: Fiber optic terminations differ from electrical in one critical way: during connector mating the fiber optic spring-loaded socket or pin retracts from .040 to .080 inches. It is critical that the backshell design accommodates this movement within the shell cavity to prevent data loss due to micro bending which leads to localized light refraction. Glenair has developed a unique solution to micro-bending called the Glenair's "Fiber-Con" backshell. For both single fiber leads as well as multi-strand applications, "Fiber-Cons" provide full support and vibration dampening while allowing the fiber to "float" as required to eliminate micro-bending caused by termini retraction.

Axial Alignment: Axial alignment of connector termini is critical to prevent loss or decrease in power from one point to another in a fiber optic cable, and needs to figure into every fiber backshell design. The fiber optic contact or terminus is the primary alignment mechanism for connecting two optic fibers. The Glenair "FiberCon" design, which features a unique grommet which accurately aligns each individual fiber, insures the connector accessory will not adversely impact the alignment of the fiber optic termini.

Field Service and Repair: Field termination of fiber optic interconnect systems presents one of the most difficult design challenges to the accessory engineer. Given the difficulty of polishing and terminating fiber, as opposed to copper conductors, a completely unique approach to designing the "business end" of the fiber optic interconnect assembly is required, and special conduit transitions and breakouts offer the best solution. By transitioning into a predetermined length of conduit at the end of the cable (approximately 2 feet) the technician can strip back the outer cable to allow for ample working room for the individual polishing and termination of the fibers. Light weight conduit and composite conduit fittings make the entire assembly easy to manipulate and handle.

Backshells Are My Life