The accessory manufacturers are divided into two basic groups:
- Connector manufacturers that offer accessories for their products such as tools to facilitate contacts, crimp tools, insert/extraction tools, sealing plugs, plastic cap plugs, and basic strain-relief clamps.
- Backshell manufacturers that offer accessories to facilitate wire interconnect systems, metal protective covers, and torque tools to tighten backshells.
Glenair, Inc. manufactures the second group--Backshells and interconnect cables or harnesses.
Backshells are a difficult product to select. This is due to several factors: design proliferation, complex assembly procedures, and simple nut/bolt appearance. Application education, design simplification and standardization are keys to selecting the proper backshell product for its intended use.
Before we describe what a connector backshell accessory is and how to select the proper accessory for its intended use, we must understand what a connector is and its basic use.
Links to chapters below
The Connector - General Description
The multi-contact electrical connector is a sub-assembly within the interconnect wiring system. The connector is made up of two (2) separate component assemblies known as "plug and receptacle" which intermate to connect wires with pin and socket contacts. The intermating or coupling (holding plugs and receptacles together), the shape of the connectors--circular (cylindrical) or rectangular (rack and panel), and the method of mating individual multi-strand wires together so they make positive electrical contact, determines the basic connector families.
The method each connector family utilizes to make a connection divides each connector family into contact types--Crimp contact, insulation displacement contact, solder contacts, and fiber-optic termini. Each contact termination method describes the contact, crimp, solder, insulation displacement contacts (IDC), or fiber-optic termini.
To terminate electrical contacts, wire insulation is removed from the end of the wire exposing the conductor. The conductor is placed into the contact and the contact is either swaged or crimped around the conductor for "crimp" type contacts, or soldered onto the conductor for "solder" type contacts.
IDC's do not require the removal of insulation to expose the conductor--The contact penetrates the insulation when crimped or pressed onto the wire. Fiber-optic termini require extensive optical polishing and close tolerance alignment.
Both solder and IDC are permanently fixed to the connector insulator. Crimp contacts and fiber-optic termini are separate components inserted into the insulator after the contact is terminated. Crimp contact type connectors are divided into two groups determined by the method the contact is removed from the connector insulator or insert.
Front release connectors are those where the contact extraction tool enters the connector from the interface, opens the contact retention clip, allowing the tool to push the contact out the rear end of the connector.
Rear release connectors are those where the contact extraction tool is installed from the rear of the connector (the wire end), opening the retention clip and pulling out the contact back toward the wire. Both front and rear release connectors have the crimp contact inserted from the rear or wire end of the connector.
To recap basic connector families:
- Circular or cylindrical connectors
- Bayonet coupling
- Threaded coupling
- Ball detent coupling (push/pull)
- Breech lock coupling
- Spring rack/panel coupling
- (All of the above are available with crimp or solder contacts)
- Rectangular or rack and panel connectors:
- Flange mounting/jackscrew
- Center drive jackscrew coupling
- Guide pin/flange mounting--No mechanical coupling
- Flange mating--No mechanical coupling
- Cam lock, center engagement device
- Spring loaded/flange mounting
- (All of the above are available with crimp or solder contacts; insulation
displacement contacts are available on some rectangular connectors.)
Military Connector Design Features
The military connector finds many diversified applications due to severe environments, mobility of equipment and field repairability. All design disciplines have been developed as a result of service experiences, and as a result of these experiences, the state of the art continues to develop in order to meet new design demands. Military connectors are primarily the circular family, with rectangular connectors being the secondary family. Circular connectors are selected by the military because of their compact, rugged design. Rectangular connectors are selected for a maximum number of contacts in a restricted space with limited environmental constraints. Both connector families utilize several contact configurations: power or high-voltage contacts, signal or circuit contacts, or RF: coaxial and triaxial contacts, and fiber-optics termini.
In some connector designs, all contact types can, and do, occur in the same connector insert arrangement! Military contacts are usually gold plated, copper alloy material. Large diameter power contacts and solder type contacts may be silver plated copper alloy. Fiber-optic termination may also be present in some insert arrangements.
Some of the standard environments to which military connectors are subject include temperatures of -65° to 200°C (or special high temperature classes up to 1500°C), severe vibration, severe shock, severe corrosion, fuels, hydraulic fluids, sand, dust, ice, bending moments, moisture, water emersion, high altitude, humidity, vacuum, space, dissimilar metal electrolysis, lightning, electro-magnetic interference (EMI), radio frequency interference (RFI), electro magnetic pulse (EMP), and many more. Most of the above environments can be applied to the same connector. Unique or special environments require special connector classes within the same connector type, thus creating connector service classes.
The Military Circular Connector
With the exception of the protective cover which mates to the connector coupling device and front-end interface, the accessory manufacturer is concerned with the rear end or wire end of the connector.
The main connector housing is called the "shell". The shell of a circular connector is a cylinder available in incremental sizes starting as small as .375 diameter up to 3.25 diameter and larger. The most common shell sizes are available in .0625 increments starting at shell size 8 (.50) to shell size 36 (2.25), i.e.: 24 shell size is determined by 24 x .0625 = 1.50 outside diameter of the connector shell. This nomenclature becomes significant, as backshells (accessories which attach onto the connector shell) must intermate with the connector shell rear end geometry. Both the connector and accessory manufacturers use the term "shell size" to designate the size of their respective products.
Connectors which utilize threads to attach backshells, are usually equal to or within .062 of the connector shell size. Some common names used for connector rear accessories are: endbells, strain-relief clamps, cable clamps, and backshells.
Caution must be used when selecting receptacle connector accessories due to the fact that some box mounting receptacles do not have accessory threads or the threads are of a different size from the plug and may be internal versus external threads.
The single most important design feature of the circular connector is the connector shell rear accessory threaded interface geometry. Early, post 1950, connectors did not always have accessory threads. Some shell designs had tapped holes, grooves, and notches. Power connectors were usually designed with externally threaded shells to accommodate cable sealing and straight strain-relief backshells. But most connectors were not designed to accommodate accessories. The shells were threaded with any size thread or modified thread that would allow attaching simple strain-relief clamps or wire seal nuts.
Not until the mid 1960's were connectors designed to accommodate backshells. Even today, connectors are manufactured with modified threads and may have less than three (3) threads, making backshell interface designs a compromise in many applications.
Early design connectors were smooth ended, tubular cylinders with threads added. There were no provisions for positive interlocking of the backshell independent of the threads. The smooth cylinder surface was modified to add tooth-like protrusions to interlock the accessory. This allowed angular backshells to be oriented or positioned and prevented cable torque from loosening the backshell.
The most positive interlocking design to date is the splined connector and accessory shells. The interlock provision allows the accessory manufacturer to design backshells with captive, rotatable coupling nuts; a feature which promoted a new generation of accessories. Rotatable coupling accessories became difficult to perform o-ring environmental sealing because of the uncontrolled sealing surfaces incorporated at the connector shell, compounded by the addition of interlocking teeth.
Prior to 1970, military connector documentation controlled the interface mateability of plug to receptacle. The accessory rear interface on the majority of military specifications was uncontrolled and, in many connector families, the accessory interface geometry varied greatly between individual manufacturers. This made it necessary for the accessory manufacturer to designate the shell size and connector manufacturer when ordering accessories to mate properly to post 1970 connectors.
New design connectors control the rear accessory interface and post 1970 connector manufacturers have stabilized their designs and rarely make design changes and new manufacturers of the older connectors design around existing configurations. This development makes it possible for accessory manufacturers to eliminate the need to designate the connector manufacturer's name when selecting accessories. The controlled rear accessory interface was an important development toward connector standardization.
Crimp type connectors are the most popular military connectors. Solder types are usually selected when cost is the primary consideration and repairability secondary, or when using a hermetic connector. The crimp contact can be automated for high speed contact to wire installation, and the gold plating makes a high reliability termination. Crimp contacts can be removed from the connector for servicing or replacing a bad contact. Solder contacts are permanently fixed in the connector. Both crimp and solder type contact connectors require unobstructed working room behind the connector rear end to install the contacts. Rear release crimp contacts require additional working room to install the extraction tool to remove the contact, thus connector contact type is an important factor when selecting connector accessories because of the required working room necessary to terminate the contact and repair or service the connector.
Another important design feature of crimp type contact connectors is the connector insert wire sealing grommet. The grommet is permanently fixed to the connector shell, and provides moisture sealing around each individual wire. The grommet configuration is basically uncontrolled or controlled within maximum envelope dimensions. The grommet geometry is important and proper compression of some connector wire seal grommets is essential to insure connector design goals. The grommet extension must also be considered in the working room, and when low profile angular accessories are selected.
The following pages recap the circular military connector design features including illustrations of each popular military connector, showing individual design characteristics important to the accessory manufacturer when selecting or designing backshells.