Materials: A Buyer's Guide
Having a commitment to defense and aerospace projects extends to the choice of materials we use to build our solutions. We use rugged, durable components that can take a beating and maintain high performance for mission after mission, while remaining affordable for your budget. The following overview will help you determine the best materials for your needs.
Wire
The following wire specifications are ideal for low cost and availability:
• MIL-W-22759/18 for low cost and high durability under 150 degrees C
• MIL-W-22759/32 for low weight and diameter under 150 degrees C
• MIL-W-22759/11 for high temperature up to 200 degrees C
• MIL-C-27500 for cable such as shielded and jacketed twisted pairs
• MIL-C-17 for radio frequency cable
Braided metal shields are usually specified to meet the requirements of QQ-B-575. Tinned copper is the normal material for temperatures below 150 degrees C. For 150-200 degrees C, specify silver-coated copper. Drawings specifying braided shields should call out a minimum percent coverage requirement. Typically 85% minimum coverage is at the low end and 95% minimum coverage is at the high end. Multiple shields may be specified to provide more shielding coverage. Nortech Aerospace Systems purchases the individual braid wire and winds our own bobbins to provide a braided shield to meet any coverage or braid angle.
When building a round cable it's helpful to bind the wires using a tape wrapped directly over them. This binder holds the cable together until it can be further processed. The material most often used in cables is a clear polyester (Mylar) tape IAW MIL-I-631 Type G, Form Tf, Class 1, .001 inch nominal thickness. For a more flexible cable, Nortech Aerospace Systems can use Teflon tape as a binder. If EMI/RFI shielding is required, a polyester tape with aluminum coating may be applied. The aluminum coating side is applied in contact with the braided shield or a drain wire: The coating improves shielding effectiveness. A common requirement for the aluminized Mylar tape is a minimum 50% overlap. Tape may be applied over and under a braided shield.
Jacket
A jacket (sheath) is the outer layer in a round cable. It provides physical protection and moisture resistance. Jackets commonly specified include the following:
Polyurethane jacket is specified where excellent toughness, tear strength and abrasion resistance are required. This material is a thermoplastic elastomer of the ether type. Its temperature range is -45 to 100 degrees C. It's economical, and it's the material of choice when used for straight cables. It's more difficult and expensive to apply polyurethane jacket to cables with breakouts (legs). The standard polyurethane Nortech Systems Aerospace uses and extrudes onto cable meets the requirements of MIL-3930/21.
Polychloroprene (Neoprene) jacket has excellent flexibility, toughness and tear strength. This material is a true thermoset elastomer. Polychloroprene is excellent for use in cables with breakouts (legs). It is best for low temperatures down to -55 degrees C. Its upper operating temperature limit is 100 degrees C. Polychloroprene is not as economical as polyurethane for straight cable uses. The standard material Nortech Systems Aerospace uses meets the requirements of SAE-R-6855, Class 2, Type A, Grade 60. A typical wall thickness for this material is .062 inch nominal.
Fluorocarbon Elastomer (Viton) jacket has excellent high temperature resistance. Its temperature range is -20 to 200 degrees C. This material is a true thermoset elastomer. Fluorocarbon elastomer has low strength and tear resistance and should only be specified where its temperature or resistance to chemicals, fuels and oils are required. The standard material Nortech Systems Aerospace uses meets the requirements of ASTM-D-2000.