Wire strain relief

Hi all,
Recently, I had an altimeter failure. When I examined the examined the installation, I found the (+) lead from the battery broken about 1/32" from the terminal screw on the altimeter. The screw was tight and the wire simply fatigued and finally broke during launch as shown on the post flight data graph. I'm contemplating ways to prevent movement of the wire at that point during R & R of the altimeter and alunching. Is there any "potting" compound that would work that isn't conductive? Thanks for any ideas.

Was it solid wire? Stranded is a better choice. And a decent set of wires strippers to avoid nicking the wires.

Needs stranded for sure. I have used the liquid electrical tape to make a pseudo cord boot so to speak. If needed it is removeable.

Thank you both, yes, it's stranded and very flexible(from my RC stuff). Yes, I am extremely careful when stripping. One thing that might have contributed, was that I soldered a small droplet of solder on the end, for the terminal screw to grab onto and not a bunch of separate wires. It broke right past that. I suspect I created a fatigue point. I'm going to eliminate that and maybe try the liquid electrical tape you mentioned.

Again, thank you both for your suggestions, this really helps!

WillMarchant said:

Was it solid wire? Stranded is a better choice. And a decent set of wires strippers to avoid nicking the wires.

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What he said. I'd guess the failure was related to one of those issues, not strain relief. But if you really want to add strain relief, a couple pieces of heatshrink tubing, with staggered lengths, placed one over the other, isn't a bad way to go.

I have used the shrink tube method also. The only reason I use the liquid tape now is it starts at and includes the "joint" where the wire connects and it has a adhesive quality to hold everything in place.

sunderll said:

Thank you both, yes, it's stranded and very flexible(from my RC stuff). Yes, I am extremely careful when stripping. One thing that might have contributed, was that I soldered a small droplet of solder on the end, for the terminal screw to grab onto and not a bunch of separate wires. It broke right past that. I suspect I created a fatigue point. I'm going to eliminate that and maybe try the liquid electrical tape you mentioned.

Again, thank you both for your suggestions, this really helps!

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Ah, we had a thread about soldering stranded wire, or not, a while ago. If memory serves, the consensus was that, in the aerospace industry, stranded wire is not soldered for exactly the reason you summarize.

The down side is that that then forces hobbyists to be extremely careful to not let strands get loose to wander into other connectors. And it places a premium on precise insulation stripping as it is easier to create wire debris by nicking stranded wire.

Look inside the connector and see if the end of the screw is mashing directly into the wire. And then take a look at something likes the connectors that doghouse rocketry sells that have a metal plate that is pushed by the screw head down onto the stranded wires. You want the latter, IMO.

I recommend zip-tying the wires to the sled right after they exit the terminal strips (or solder pads...). This prevents any strain of them, so as long as your screw is tight there aren't going to be any G/vibration induced issues.

cerving said:

I recommend zip-tying the wires to the sled right after they exit the terminal strips (or solder pads...). This prevents any strain of them, so as long as your screw is tight there aren't going to be any G/vibration induced issues.

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Excellent advice. The better goal is not to make it so it tolerates flexure at the failure point; it's to make it so there IS NO flexure at the failure point.

sunderll said:

Is there any "potting" compound that would work that isn't conductive? Thanks for any ideas.

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For semi- to permanent use, hot-melt glue (glue gun) works well.

While I agree with the idea of eliminating the flex of the wire, I also would think in terms of assuming some will occur anyway. (Unless you are ready to commit the whole assembly to sled and not be able to change any parts out.) Now if your ready to do that, I think Silicon RTV would be what I would look at. Then you could pot the whole assembly. But it would then be impractical to change anything short of making a new sled.

These are some great ideas and I am incorporating some. On my later installation with another Stratologger, I've used black thread to tie the wire to the sled about 1/2" away from the terminals. I tie it off, THEN tighten the screw to make sure there is no pre-load or tension. I also have loosely tied the remaining runs of wire every inch or so to prevent a large loop of it having too much pull under high G load. Lastly, I am conducting a very thorough visual inspection prior to each launch. I usually do, and of course didn't, which resulted in a failure. This would likely have been very easy to see.

zanfar said:

For semi- to permanent use, hot-melt glue (glue gun) works well.

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I do everything to each connection. I use pigtails to move the connection away from the board. Hot glue at the board/wire joint and again to fix the wire mid-length to the sled. RC connectors (Deans) between board and switches so board can be removed. Shrink wraps at these connectors. Radio Shack blue screw/blade connectors epoxied to bulkheads. Strain relief loops in solid wires. The only stranded wires I tin and stick in a screw connector are my "single use disposable" ejection charge wires because tinned stranded wire always fails unless you also hot glue the joint, and that kind of defeats the point of a screw connector. Got it all connected and working? Hot glue the snot out of everything. Except the barometer port. (One of the few things I don't know from experience.)