Thursday, May 10, 2012

Aircraft vs Commercial Wire Terminals

UL486A commercial terminal on top and MS25036 aircraft terminal on bottom

Stronger: Aircraft quality solderless terminals conform to MS25036 and MIL-T 7928 aircraft specifications, whereas other solderless terminals meet weaker UL 486A. One difference is in the force required to break or separate the terminal from the conductor. Notice that the aircraft terminal has more metal at the crimp. For example, a 18 gauge (red) terminal manufactured to MIL-T-7928 Class 2 has a minimum tensile strength of 38 lbs. whereas UL are only required to meet 20 lbs. 

Vibration Resistant: 
Aircraft vibrate so it's important to support the wire close to but separate from the wire crimp to avoid failure. The longer metal sleeve is designed to be crimped to the wire insulation. This is called an 'insulation crimp' and provides strain relief. The insulation crimp on the wire connector prevents vibration of the wires where they leave the crimp. Any flexing of the copper wires, hardeness the wires and results in breakage and a failed connection. The wire insulation needs to be cut back the proper length to achieve this crimping of the wire and insulation. Also, the right crimping tool (MIL-DTL-22520) is required for aircraft specification crimps.

Two crimps
; one to hold the aircraft wire and one to hold the insulation. Shown below the yellow dot is the wire crimp and the red dot shows the insulation crimp. Each crimp is formed to its own size and shape. Notice the insulation crimp on the right is formed around the insulation so it supports and grabs the insulation without crushing it. The insulation crimp prevents vibration and wire movement from stressing the crimped wire and improves safety. 

Better Electrical Contact: MIL wire connectors also have multiple 'V' grooves inside the barrel to help grip the aircraft wire. These grooves also help break up the tin plating upon crimping. This exposes clean copper to the wires and assures a low resistance connection. Making a more reliable electrical connection.
MS25036 terminals are double crimped. Yellow dot is the wire crimp. Red dot shows the insulation crimps.

Safer: No solder required or desired. Boeing doesn't allow you to crawl throughout their aircraft manufacturing line with a hot soldering iron. No electrical danger, no burn danger.

Less Corrosion: 
No solder flux means less corrosion potential.

Fewer crimp errors:
 Crimp connector tool forms both crimps at the same time. The ratchet handle will not open and release the terminal until it is fully squeezed. This prevents undercrimps. MIL-DTL-22520 tools come with a calibrating GO-NO-GO tool that verifies the proper crimp diameter. No operator guessing as how much to crimp the terminal. 
Ratchet style crimper does both crimps at onceMIL-DTL-22520 crimp connector tool 

"The crimp on terminal lugs and splices must be installed using a high quality ratchet-type crimping tool" Quote from AC43.13-1B Chg. 1 11-69. 

These tools are qualified to MIL-DTL-22520 are to be used for crimping terminal lugs. These tools crimp the barrel to the conductor, and simultaneously form the insulation support to the wire insulation. Most hobby shop tools do not perform this function. All MIL-DTL-22520 hand crimping tools have a self-locking ratchet, which prevents the tool from opening until the crimp is complete.
UL on top and AS7928 (MIL-T-7928) on bottom. This is called a 'window butt splice'. The window guarantees proper wire insertion and crimp tool alignment. This military qualified (MIL-T-7928/5) butt splice also provides an insulation grip for superior strain relief.

AS7928 (MIL-T-7928) terminals use two crimps - one to the wire and the other to the wire insulation. This prevents the wires at the crimp from vibrating and breaking.
The colors on the insulation barrel (red, blue, yellow) are used to indicate the wire diamater range. You may notice different shades of the same color. PVC and Nylon take the same color differently. For example, the PVC barrels will be dark yellow. The nylon barrels will be lighter, translucent yellow.

Notice that the colors repeat themselves. Red is used for insulated terminals using 18-22 AWG and for 8 AWG.

Vinyl is used in lower-grade terminals. Nylon is used more or MIL terminals as Nylon stands up better against hydrocarbons like gasonline , hydraulic fluid, and oils.

MIL-DTL-22520 establishes a single specification which set forth performance requirements for all crimp tools to be used on military standard connectors. It is a 3rd generation design that establishes good electrical and mechanical characteristics.  The Mechanic's Toolbox, software for the mechanic, has more information on electrical wire. 

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