Thread Fatigue?

[NSboatman]

Originally Posted by billknny

There is a good reason you never heard of this before...



For metal to "fatigue" it has to move, either stretch or bend. Threads do neither, and do not fatigue.



Now... I’ll happily entertain arguments about how it might be a good practice to pull screws at some interval to reapply your favorite corrosion inhibitor, but fatigue? No.

Sorry, but this is incorrect. When metal “moves” (stretches or bends) this is called yielding. The load on the metal has reached a point where the stress (which is load or force divided by the area of the metal under load) exceeds the ‘yield stress’ of that material. This point on the stress-strain curve is known as the proportional limit; and any (linear elastic) material pushed beyond this limit will exhibit permanent deformation- stretching or bending.

Fatigue can most definitely occur in metals without ANY yielding or stretching. Fatigue is a property that is determined by “cycles at load”.

This is graphically displayed on an S-n curve (the S is stress, the n is number of cycles); and is usually a logarithmic scale. Every material has one, and they are all quite different. Many Steels for example, have what is known as an “infinite life” value- a distinct asymptote on the graph- or a stress under which a designer can essentially consider their life spans to be infinite (or at least in the tens of millions of cycles)

Aluminum, interestingly, does not have this value- there is no asymptote, and no value of stress under which it has “infinite” life.

While I do agree that rigging threads are subject to stress cycles and can certainly fatigue; I’m very suspicious of that rigger’s claims. Show me the numbers please...

[s/v Moondancer]

I had one break on the forestay coming out of Campbeltown in Scotland, in 30 knots we had the rollerfurler flying around our heads attached to a 400sq ft jib. Kept the mast vertical but it was a close-run thing. The boat was an old UK charter boat.

[Mr B]

A bit of knowledge is dangerous,
Sounds like the rigger sells bolts on the side for extra income,
I wonder how many he has sucked in with that one, Good money spinner,

As for pressure vessels which are constantly moving, Plus corosian factors,
They do have a predetermined life cycle from manufacture and are then scrapped,



Then diesel engines can run for millions of miles with the original bolts, And have never been touched, Piston rings wearing out usually kill them, Not the bolts holding it together,
Which are also constantly moving,

[JPA Cate]

Dockhead,

We have never had a rigging screw fail. We have had a cracked swage (the kind done in a machine, not a Sta-Lok). Have had a forestay break from cycled loads, but no loss of rig. When we lost the mast, it was a clevis pin that came out, so the guess is that the cotter pin failed.... sorta like the loss of the horseshoe nail.

Ann

[deluxe68]

Originally Posted by NSboatman

Sorry, but this is incorrect. When metal “moves” (stretches or bends) this is called yielding. The load on the metal has reached a point where the stress (which is load or force divided by the area of the metal under load) exceeds the ‘yield stress’ of that material. This point on the stress-strain curve is known as the proportional limit; and any (linear elastic) material pushed beyond this limit will exhibit permanent deformation- stretching or bending.

Fatigue can most definitely occur in metals without ANY yielding or stretching. Fatigue is a property that is determined by “cycles at load”.

This is graphically displayed on an S-n curve (the S is stress, the n is number of cycles); and is usually a logarithmic scale. Every material has one, and they are all quite different. Many Steels for example, have what is known as an “infinite life” value- a distinct asymptote on the graph- or a stress under which a designer can essentially consider their life spans to be infinite (or at least in the tens of millions of cycles)

Aluminum, interestingly, does not have this value- there is no asymptote, and no value of stress under which it has “infinite” life.

While I do agree that rigging threads are subject to stress cycles and can certainly fatigue; I’m very suspicious of that rigger’s claims. Show me the numbers please...

I was told by my boatyard PM that the 10 year recommendation is worst case scenario and is stated for liability purposes. There are boats out there that have 30 year old rigs and crossing oceans. But there is absolutely no way to tell exactly what type of load cycles the rig has been thru short of installing test equipment when the boat was new. Some people have their rig torn down every year and inspected, I think that is a bit too cautious.

[Dockhead]

Originally Posted by deluxe68

I was told by my boatyard PM that the 10 year recommendation is worst case scenario and is stated for liability purposes. There are boats out there that have 30 year old rigs and crossing oceans. But there is absolutely no way to tell exactly what type of load cycles the rig has been thru short of installing test equipment when the boat was new. Some people have their rig torn down every year and inspected, I think that is a bit too cautious.

I wasn't asking about the rig as a whole -- it was replaced but for the rigging screws and lower terminals, just a few years ago. My question related specifically to the threads -- studs and rigging screws. None of the other riggers mentioned this being an area of concern. Yet this guy is highly respected (he rigs all the new Oysters), and has no interest in telling me fairy tales (he has far bigger orders than some little job on one boat, which he doesn't have time for in any case).

[double u]

read somewhere the other day about replacement of rodrigging, & there it too said (to my surprise) that the first thing to go were the turnbuckles...

[double u]

found it:
"At the same time the rod is re-headed, Navtec recommends replacing the turnbuckle
screws. The screws could last for many additional years, but it is less expensive to replace a few rigging screws than to replace the mast and all of the rigging."

[deluxe68]

Originally Posted by double u

read somewhere the other day about replacement of rodrigging, & there it too said (to my surprise) that the first thing to go were the turnbuckles...

Yup. They told me that the rod body never failed, it was always the modified attachment points and hardware. Mine has been re-headed one time and will likely need to be replaced in 10 years. Hopefully there will be a source for the rod then. I read a story one time talking about stress failure in masts due to excessive pumping. THAT would be scary.

[Exile]

I imagine that one of more significant variables that could affect durability would be proper tension. Lots of chatter about loose leeward shrouds moving around when heeled, for e.g., but not sure there's a consensus about whether all this movement contributes to excessive wear on stud threads & turnbuckles, etc.

[Dave_S]

It will be dependent on the sizing of the parts. If the rigging hardware was designed for lightweight performance then theroretically they will not last as long as an over specified part.

Stainless steel in 316 grade like many as grades is prone to work harden were wear and especially continuous deformation or cycling through elastic movement hardens the material to the point it may fail.

When I dropped my reacher last I found the D shackles bent. Lucky to find it now and not end up like Moondancer will a furler flying around my head.

[MikeFergie]

If you go to all the trouble of replacing the stainless wire it seems a bit silly not to replace the turnbuckles. The cheapest parts. I would hate for my mast to come down due to turnbuckle breaking

[cal40john]

I once was puzzled about a bronze turnbuckle on a club boat, the cotter pin was tight up against the body. I finally realized the threads were worn and it had walked up until cotter pin was mostly holding the shroud. I showed it to a friend who took a wrench to it. It promptly failed. The people about to take the boat out yelled at us for ruining their outing.

[Adelie]

Originally Posted by cal40john

I once was puzzled about a bronze turnbuckle on a club boat, the cotter pin was tight up against the body. I finally realized the threads were worn and it had walked up until cotter pin was mostly holding the shroud. I showed it to a friend who took a wrench to it. It promptly failed. The people about to take the boat out yelled at us for ruining their outing.

Who was that, Dan or Norm?

[jtsailjt]

It sounds to me like he's being ultra, ultra, conservative. I've never heard of anyone doing that but then, it's not something that would necessarily come up in conversation unless I happened to be present the day it was being done.

I've kept my boats at various yards and have occasionally been amazed by the "recommendations" made to "gold plater" type customers. Things like, "your depth sounder is 5 years old now and out of warranty and you wouldn't want it suddenly failing to interfere with next sailing season so we recommend replacing it with this new, improved model to eliminate any chance of that happening." Never mind that the "old" depth sounder is working perfectly and has only been actually used for 2 or 3 weeks per year. The other thing that amazes me is that customers are willing to actually sign off on things like this. Apparently it's a useful tool that some boatyards employ to ensure they can keep their workers busy over the winter months. I know that the removed parts often find a new home on boatyard workers own boats.

Though your situation is a little different because there are no real expensive parts involved, if your rigger is planning to charge you the hours to replace all these bolts I think I'd be a little bit suspicious about his motivation for being so very conservative about your rigging bolts. I think I'd call another rigger in a different area (could be a local tradition) and ask him the question about what he recommends for a boat that's used the way you use yours.