Connector Backshells: Glenair, Electro Adapter, and Sunbank Backshells;
Also Military and Commercial Backshells by Spacecraft
Our company offers connector accessories, such as backshells from connector manufacturers like Glenair, Electro Adapter and Sunbank. We
also offer our own line of connector backshells, for both military and commercial use. Please contact us for more info or to buy.
Backshells are by far the most complex and widely used connector accessories. Please see below a classification by function:
NON-ENVIRONMENTAL BACKSHELLS
Non-Environmental Backshells are sometimes referred to as backshell extenders. They are used when there is a requirement for additional
space between the rear of the connector and the strain relief to allow for cable service loops, jumping, filter networks, etc.
ENVIRONMENTAL BACKSHELLS
Backshells categorized as Environmental are those which protect the wiring side of the connector from dust, dirt, moisture and fluids.
Almost all recent Military Specification cylindrical connectors are designed so that when the wires are inserted through the grommet, and assuming all holes in the grommet contain either a wire or a
sealing plug, the connector will be enviromentally sealed once the backshell or strain relief is secured, The environmental seal will remain funtional as long as the wires entering the grommet are
not bent more than 13 degrees from the axis of the connector. The exact number of degrees which the wires can be bent depends upon the type of wire and insulation materials used. If any bending or
flexing of the wire bundle is anticipated, some type of strain relief or cable clamp and backshell must be utilized to maintain the integrity of the environmental seal.
EMI/RFI BACKSHELLS
Most cabling design for transmission of electrical power or high voltage signals is relatively unaffected by electrical interference.
However, the introduction of foreign electronic energy into cables carrying low voltage data signals can have disasterous results. The lower the system signal voltage levels, the greater the
susceptibility to interference. Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) can originate from a number of sources. Notorious generators of interference include
components such as coils, electromagnets, electric motors, transformers and a variety of radiating electrical equipment like fluorescent lighting, high power radar, broadcast stations, teletype and
other communications circuits. When strong external noise fields exist, the conductors of a cable act as antennas and pick up the radiated signals. The reception of this interference results in
inaccurate or interrupted data transmission. Today, with cabling carrying more low voltage data signals than ever before, proper shielding is of prime importance if the integrity of the transmitted
data is to be maintained. An effective shield should be capable of preventing the reception of undesired signals as well as confining extraneous signals generated within the cable itself. The most
common method of shielding against EMI/RFI is to enclose the cable in a tightly woven metal braid comprised of wire strands which is then terminated at one or both ends of the cable to ground
potential. The termination and grounding of the shield is accomplished by using a suitable connector backshell. Many military programs require Electromagnetic Pulse (EMP) protection. Designing
against EMP is basically the same as designing against EMI/RFI. The best cable shields for EMP are solid materials such as rigid conduit or pipe. However, where light weight is a consideration, metal
braided shields can be used effectively. The mechanical assembly of the shield must have clean metal-to-metal contact with the backshell. Continuity is extremely important in EMP protection. There
must be a 360° circumferential mechanical bond and electrical continuity between the shield and the connector backshell. Shielding voids or pigtail terminations do not provide acceptable EMP
protection. A variety of backshell configurations are available for termination of cable shielding. The designer can select from cone and ring, light bulb thread, compression spring, magnetic
forming, crimp ring or stainless steel strap configurations.
CONE AND RING BACKSHELLS
The Cone and Ring method has been used for over 60 years, originating with coaxial (UG type) connectors. Several variations of this
concept have evolved and are widely used as they provide excellent shielding as well as strain relief capabilities. All variations are based on the basic Cone and Ring design. The cable braid is
placed between the inner and outer ground rings at the rear of the backshell. As the strain relief or nut is tightened, the braid is clamped between the two rings. Other variations of this method
included shield tie-off, inverted cone and 3-ring shield grounding. For optimum performance, the Cone and Ring shield termination requires a great deal of care by the assembler. To assure 360°
continuity, the braid must be .combed out. so that there are no high spots when the Cone and Ring come together. Care must also be taken to prevent cutting the soft copper wires of the braid on sharp
edges or by overtightening the strain relief. The fact that this method is fully field repairable accounts for its popularity.
LIGHT BULB THREAD BACKSHELLS
As the name implies, the outside rear of the backshell has coarse round threads similar to an ordinary household light bulb. A nut with
corresponding threads is slipped over the braided shield.
The shield is then distributed evenly over the backshell threads and the nut is screwed in place, securing the shield to the backshell.
This method is field repairable but requires care to be sure the braid does not tear as the nut is tightened or removed. These backshells have no strain relief provisions for the cable or conductors.
If a jacketed cable is used, environmental sealing can be achieved with the use of a heat-shrinkable boot over the backshell and jacketed cable.
COMPRESSION SPRING BACKSHELLS
The Compression Spring is sometimes called a Ground Spring) Backshell. A ring-shaped coil spring is placed over the braided shield and
positioned in the rear of the backshell. A .backup. ferrule is sometimes used between the conductors of the cable and the braided shield to eliminate the possibility of cutting into or through the
shield if the cable clamp or compression nut is tightened excessively. This method of shield grounding is simple, field repairable and requires no special assembly skills. Compression Spring
Backshells are best used in applications with little or no vibration. It has been reported that under certain vibration conditions when a backup ferrule is not used, the spring will bounce and
shielding integrity could be compromised. EMI/RFI Backshells are available in both Environmental and Non-Environmental configurations.
SHIELD TERMINATING BACKSHELLS
Shield Termination Backshells are used to terminate the outer braid shield of a cable by means of a magnetically formed ring, a crimp ring
or a stainless steel strap. Their configuration is distinct from EMI/RFI backshells in that they rely entirely on the ring or band for strain relief rather than on a mechanical clamping device.
MAGNETIC FORMING BACKSHELLS
Magnetic Forming is based on the principle that a pulsed magnetic field generated near a metal conductor can produce pressures up to
50,000 psi capable of reshaping the metal conductor (in this case, a metal ring) without physical contact. After the braid is placed over the backshell end and the metal ring (which has a zero
temper) is in position, the assembly is placed in a magnetic forming machine which induces a very intense magnetic field by the discharge of high voltage capacitors. This field sets up an opposing
field in the metal ring and the net Magnetic Force compresses the ring onto the backshell, securely clamping the braid. Magnetic Forming is currently the preferred EMP termination method as a 360°
bond is assured. Magnetically formed assemblies are not field repairable and the cost of the forming equipment is in the $50,000 range. Magnetic Forming Backshells have no provision for strain relief
of the cable or conductors, however, the ring is crimped so tightly that the shield material will break before it will pull free of the backshell. There is a tendency for the finish of the aluminum
ring to crack and expose base metal when the ring is formed to the backshell. Under certain conditions, these cracks promote corrosion which affects the conductivity path. In extreme instances, the
rings have corroded so badly that they have fallen off. Environmental sealing can be obtained by using jacketed cable and a heatshrinkable boot over the assembly.
CRIMP RING BACKSHELLS
Crimp Ring shield terminations are similar to Magnetic Forming except that the ring is crimped to the backshell with a hydraulic tool.
Some users do not like this concept in that there can be areas of little or no contact when using a hex-crimp die. A new method which eliminates any voids has been developed. This method produces a
circumferential crimp using a stainless steel ring. Use of the circumferential crimp and stainless steel ring eliminates any possibility of the ring corrosion problems associated with magnetic
forming. Crimp Ring terminations have the same strain relief, environmental and field repair parameters as those of Magnetic Forming.
STAINLESS STEEL STRAP BACKSHELLS
As noted above, Magnetic Forming and Crimp Ring terminations are not easily field repairable. The Cone and Ring, Light Bulb Thread and
Compression Spring methods are field repairable but require some level of skill and care in assembly and disassembly to avoid damage to the braided shield. The Stainless Steel Strap was developed in
an effort to circumvent these shortcomings. The Stainless Steel Strap resembles a metal cable tie and is tensioned by a tool. The strap is held in place by the tension applied to the strap locking
mechanism by the hand tool. Easily field repairable, the procedure requires no special assembly skill. The backshells have no provision for strain relief, but environmental sealing is obtained by
using jacketed cable and a heat-shrinkable boot over the assembly. As can be seen, the designer has a wide variety of terminations and features to choose from. The characteristics and features of
each termination type are summarized in the following table.
ENVIRONMENTAL EMI/RFI BACKSHELLS
Environmental EMI/RFI Backshells resist both physical and electrical contamination. Environmental EMI/RFI Backshells are available with
all termination types. Due to the severe operating environments connectors are subjected to today, they have become the most widely used
backshell for military applications.
GROMMET NUTS
Grommet Nuts can be described as metal rings with inside threads. They were originally designed for use with solder contact connectors to
provide environmental sealing. Since solder contacts are non-removable and must be wired in place, the rear sealing grommet is supplied as a loose piece. The wires are threaded through their
respective holes in the grommet, and the grommet is postioned over the wired contacts when soldering is completed. As the Grommet Nut is tightened, it holds the grommet in place and compresses it
around the individual wires to effect an environmental seal. All new military specification connectors released since the early 1970.s have crimp contacts.Grommets of crimp contact connectors are
self-sealing and are a non-removable component. These features make the use of Grommet Nuts redundant for sealing purposes. Their only function on a crimp contact connector would be to protect the
rear accessory threads and teeth of the connector or to protect the outside of the grommet if it extends beyond the rear connector shell. Since Grommet Nuts provide no cable support, it is our
recommendation that they should not be used.
POTTING BOOTS AND RINGS
In the 1950.s, encapsulation potting of electronic components and assemblies was introduced as a method of environmental sealing. Potting
became a popular means of sealing connectors since it produced a more positive seal than the rubber grommets in use at the time. Weight reduction was also achieved because potting eliminated the need
for grommets, ferrules, backshells and strain reliefs.Potting Boots act as molds or forms to contain the liquid potting compound until it sets or cures. Potting Rings are used to attach the Potting
Boot to the rear of a connector, however, some Potting Boots are designed to fit the connector without the use of a Potting Ring. Two characteristics of potting led to its decline in popularity. One
is that potted assemblies are not field repairable. The second, and more serious, surfaced in the early days of the Vietnam conflict. The Air Force began losing an inordinate number of F84 aircraft
due to catastrophic electrical failures. The problem was traced to components which had been potted. Due to the climate and operating conditions, the potting compound began to revert. It became fluid
again and, in the case of connectors, ran out the back of the potting boot, leaving the wire bundle unsealed and vulnerable to shorting. Although new compounds were formulated to eliminate the
problem, the military opted for the use of improved, redesigned connectors using synthetic rubber sealing grommets and removable crimp contacts. In commercial applications, potting is an acceptable
method of termination.
SHRINK BOOTS AND ADAPTERS
Shrink Boots are used to environmentally seal a jacketed cable to a connector. The Shrink Boot Adapter is a specially designed backshell
which attaches to the connector"s accessory threads and provides the necessary gripping area for the Shrink Boot.
The Shrink Boot and Adapter are slipped over the cable and after wiring is
completed, the Adapter is attached to the connector. The Shrink Boot is positioned over the Adapter and the jacketed cable. As heat is applied to the Shrink Boot (usually with an electric hot air
gun), it shrinks tightly around the Adapter and cable forming an environmental seal.
In addition to the basic configurations defined in MIL-C-85049, manufacturers offer a number of Shrink Boot
termination variations including Crimp Ring, Cone and Ring, Integral Woven Braid, Stainless Steel Band and Light Bulb Thread.
STRAIN RELIEFS AND CABLE CLAMPS
Movement of a cable can produce stress which, as it is transmitted toward the connector, can result in wire breakage, wires coming loose
from the contacts, contacts being pulled out of the insert or any number of disastrous consequences. The function of a strain relief device is to hold or grip the cable fast at some point anterior to
the location where any damage can potentially occur. Any stress or strain on the cable is then transfered through the strain relief to the connector shell and the area of potential damage is thereby
protected. One point that has always presented some confusion is the interchangeable use of the terms Cable Clamp and Strain Relief. We feel that we have resolved this problem by virtue of the
following definitions:
CABLE CLAMP
Any cable support or clamping device which requires the use of an intermediate component for attachment to the back (wiring side) or the
connector.STRAIN RELIEF - Any cable support or clamping device that attaches directly to the body of the connector.
PROPER ASSEMBLY OF STRAIN RELIEFS & CABLE CLAMPS
Cable Clamps and Strain Reliefs are usually shipped unassembled. The reason for this is that unless a cable of
minimum diameter is used, the clamp must be disassembled prior to use. On all Cable Clamps and Strain Reliefs which utilize two straps (or saddle bars), care must be exercised to assure proper
assembly. Although no M85049, MS or NAS drawing mandates screw orientation or that the clamp be assembled when shipped, there is a proper and preferred method of assembly. The clamp screw heads are
oriented so that any torquing force applied with a screw driver is in a direction that will tighten rather than loosen the attachment between the Strain Relief and the connector or backshell to which
it is fastened.
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