Join Date: Oct 2015
Location: Igloo, Yukon Territory
Thanked 3 Times in 3 Posts
Rep Power: 0
Re: Service life of a Sta-Lok fitting?
Swaging and compression fittings (stalock,etc.) are two completely different animals. The swage is deformed metal, which will have areas of compression and work hardening and areas of shear stresses where the metal is stretched. There are substantial differences in the structure of the metal within the fitting. More deformed metal= more potential for cycle failure, corrosion, fatigue. This sets the stage for failure of the swage or wire/swage junction. This fixture is also highly dependent upon the application of the force to the swage. If the wire is swaged quickly, the wire/fitting can be worked hardened and become more brittle. If you apply too much pressure to the swage, it can become worked hardened. Does the person fitting the swage have a torque/pressure limited tool? Do they have training in compressing these fittings? Yeah, I thought so.
If the fitting can be inspected, passed, and subsequentially prematurely fail, then the fitting design has to be changed so that its inspection and passing can lead to a predictable safe service life.
In high reliability electrical/mechanical connections (aerospace), swaged type connections are increasingly out of fashion, there are too many variables to obtain a 100% reliable connection. The aerospace field is moving towards compression type fittings. The compression fitting is not deformed, the wire is not gouged, stressed,etc. Compression is better. The service life of a compression fitting is very very long; It depends on the application.
Coming from aviation, I am stunned at the lack of safety engineering/failure mode analysis in nautical engineering. Standing rigging and keel attachments would be to me, primary structure- absolutely critical for nautical safety. If primary structure fails (eg: wing in airplanes or keel / less so the mast in boat) then the trip is over. There are no TBO (time before overhaul) rules? No inspection procedures after grounding? The letters NDT are unknown in sailing? Nobody (USCG, operator, manufacturer?) can agree upon standing rigging inspection or replacement? If your standing rigging and mast fail because of a poorly applied 5 dollar part, is this acceptable to you? Remember the chances of the system failing are directly proportional to its need to be functional at that moment.
What we know is that wire will fail. More corrosive environment, will lead to earlier failure. More stress/strain cycles, with greater loads- earlier failure. Stainless wire is corrosion resistant (good) less ductile and more brittle (bad). Compression fittings, if properly designed, applied, and maintained have a very very long service life. If properly designed, the chance of wire failure at the fitting is very very low as well. So on the next ocean boat I own, it will have compression type fittings and the wire will be fresh. Current boat is a Macgregor on an interior lake, with a huge overdesign factor. Swaged, but because of the application, I feel comfortable.
Disclaimer: I do not work in any way,shape or form for any nautical-related business.
At the end of this rant, please forgive me. I find it completely unforgivable that a captain and their crew is screwed because of the mast/standing rigging failing them. Or the keel falling off. If the boat is rated Cat A, it's rated A. and that should include a wide latitude for substandard maintenance and driving skills. I also firmly believe in the superiority of a compression type fitting on rigging.
Rant off, cheers, Leo.