stunthanger.com
General control line discussion => Open Forum => Topic started by: Colin McRae on September 04, 2024, 06:41:37 PM
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I've heard brass crimps seen on older models on the lead outs and bell crank are not good. Now I know!!
The pic here is from a Flite Streak I acquired already built. It is an older Top Flite model. I've flown it maybe 30 times.
I was flying the model this past weekend with no obvious issues. Then on the 4th flight prior to takeoff I noticed some controls binding. I did not take the model airborne - thankfully!
I cut into the wing, removed the parts and this is what I found. All the brass crimps were splitting. One completely failed.
But what is so confusing is the brass actually splitting longitudinally. I would not have thought this to be the failure mechanism. It's almost like the brass crimp had a seam that was splitting, but it looks to be seamless. Very strange.
If the experts out there know, I would be interested on the actual failure mechanism.
Thanks in advance.
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Brass can come in many different hardness ratings. They all can be annealed, but if this was an ARF you have not choice in the matter other than changing them out right away with wrapped ends. The tubing is extruded so it has like a grain that runs the length of the tube, so that is the way it will split. I have seen brass tube in tanks split longitudinally also, after it gets hard from the soaking in methanol and nitro. They may have used defective material for the crimps, of even just over did it on the pressure. If it is an ARF I'm surprised to see the bell crank was bushed. If it was built by the previous owner, than he over did the pressure or used the wrong type of material. I am just not a crimp fan on light cable!!
Type at you later,
Dan McEntee
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Pretty simple, really. The tubing has too large an ID and has to be severely formed to bite down on the cable. The brass is coldworked when drawn and has not been annealed so when the diameter is collapsed a longitudinal crack occurs in the tightest radius. The ductility is insufficient. The cold-forming during crimping is severe. The tools used are wrong.
Crimping can work well if ductile copper tubing is used when sized so that three passes of your cable will just fit thru. You need the heavy wall tubing. It cannot be hard material. It must be ductile to cold-form around your stranded cable rather than damaging individual strands. You cannot crimp with sidecutters. Properly done, crimps can carry the full-rated load of the wire itself.
The crimp in the picture came from a Vector 40 ARF. Every design rule broken. The other leadout had a cracked crimp in the same place. It pulled thru during a "fingers only" test. Don't be fooled by the color of the crimp. It was nickel-plated brass. Same problem....
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I suggest using ONLY annealed copper tubing.
I use a swaging tool purchased from SAVA Industries (part number T185) and it gives excellent results for crimping/swaging all sorts of cable.
Bob Z.
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i have quit using tube crimping years ago, totally sold to needle bush and copper wired ends