Thread Fatigue?

[jtsailjt]

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.

Now that's frightening!

[Dockhead]

Originally Posted by 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.

It's definitely not to drive business, at least not in any direct way -- he's fully booked and can't work on my boat.

So I'm still puzzled. But anyway I've now agreed with a third eminent rigger (former chief rigger of Moody) who will actually do the setup and tuning next week and go through the report from this guy. We shall see.

[cal40john]

Originally Posted by Adelie

Who was that, Dan or Norm?

It was you that took a wrench to it.

[Adelie]

I don’t remember that. Guess I should be proud of myself then.

[a64pilot]

My guess is
A. you’re replacing the rigging, best to replace pin to pin and go all new. I did as mine was of unknown age, pretty sure original to the Boat, which made it 30 yrs old. The plating on my turn buckles was beginning to fail and their appearance wasn’t pretty.
B. Turn buckles etc. should age out due to corrosion and work hardening etc. I assume if correctly sized that fatigue is not an issue. Sort of like fatigue on a chainplate isn’t the reason they get replaced, or we would be NDI ing the holes frequently, and I have never heard of that.

The statement of all metal fatigues isn’t always true, if I give you a coat hanger and tell you to bend it back and forth by hand, then of course it will fatigue and fail, if I give you a rail road spike and tell you to do the same, it will never fatigue and fail as the forces are outside of the fatigue cycle. I would assume the forces on a rigging turn buckle are outside of the fatigue cycle as well.

High strength engine bolts are replaced based on number of times they have been torqued, some that use the wrench arc torque method are required to be replaced after one use, but I have never heard of a mileage or age they needed to be replaced at. The stresses they are under from a running engine is outside of their fatigue cycle.

[jtsailjt]

Originally Posted by Dockhead

It's definitely not to drive business, at least not in any direct way -- he's fully booked and can't work on my boat.

So I'm still puzzled. But anyway I've now agreed with a third eminent rigger (former chief rigger of Moody) who will actually do the setup and tuning next week and go through the report from this guy. We shall see.

It’ll be interesting to hear what the Moody rigger has to say about it and It sounds like he’d be the ideal person to know of any failures of these bolts and fittings that may have occurred over the long term.

[double u]

true or not I can't say:
an aircraft mechanic (Austrian airforce, no less) once told me, that all aircraft-spares have an expiry-date when they have to be discarded, even if they've been sitting on the shelf. hm?
could be a result of marginal sizing due to weight being the main consideration?

[Dockhead]

Originally Posted by jtsailjt

It’ll be interesting to hear what the Moody rigger has to say about it and It sounds like he’d be the ideal person to know of any failures of these bolts and fittings that may have occurred over the long term.

Well, he won't say much. He's an easy going guy, the opposite of the other, tight-ass, passionate about rigging, guy. This guy rigged my boat when she was new and remembered her when I first went to him years ago. He inspected the rig year before last and thought it was fine. No mast has ever fallen off any Moody that I've ever heard of, but still, he's a little TOO easy going for my taste. I think the truth is somewhere in between these two extremes, so I guess it is right to have gone to both of them.

[danielamartindm]

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.

Agreed. I really think there needs to be movement for metal to fatigue. A screw is a device to resist movement.

[Dockhead]

Originally Posted by danielamartindm

Agreed. I really think there needs to be movement for metal to fatigue. A screw is a device to resist movement.

Well, I beg to differ. Fatigue of threads is a well known phenomenon.

https://www.asminternational.org/doc...mp17208p18.pdf


Like with all metal fatigue, the metal in the threads needs to be stressed to a certain percentage of its yield strength. But it certainly happens and it's certainly worth considering whether it might not be happening in our rigs.

Thread fatigue is of sufficient concern to designers that rigging screws are made with rolled threads, which are less subject to fatigue than normal ones.

[a64pilot]

Rolled threads are also much cheaper in mass production, that is why even cheap hardware store bolts are rolled as opposed to being cut.
Cut threads have stress risers, especially right at the end of thread.
To explain stress risers think of the serrated edges of a candy bar wrapper

[Dockhead]

Originally Posted by danielamartindm

. . . A screw is a device to resist movement.

That's an odd way to screw!

[wyb2]

Does this rigger check the number of threads on each customer's turnbuckles before he makes this recommendation?

Seems to me there are 2 questions here. 1 - are turnbuckles cycled enough for fatigue to be a consideration? and 2 - where will a turnbuckle fail?

If the answer to 1) is "yes", and 2) is "the threads", than thread fatigue should be a concern.

Seems this rigger feels the answer to 1) is yes, but most here disagree. I don't believe bronze has a fatigue limit like steel, but neither does aluminum, and we make airplanes (and boats) out of that stuff. It's all about the S-N curve.

For 2), the whole turnbuckle is under load, it will fail at the weakest point, whether due to ultimate load or fatigue. In a bolted connection, the bolt body is almost always the weakest link, not the threads. I would hope turnbuckles are designed the same way, but if not, it wouldn't be the first thing in the sailing world that seems backwards to me. cal40john's story would suggest the threads may in fact be the weak link (post 28).

[RaymondR]

Originally Posted by wyb2

Seems this rigger feels the answer to 1) is yes, but most here disagree. I don't believe bronze has a fatigue limit like steel, but neither does aluminum, and we make airplanes (and boats) out of that stuff. It's all about the S-N curve. (post 28).

Comet airliner, built of alloy with square corners on the windows. Fatigue and stress risers.

[james247]

Originally Posted by 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,

Agreed the only failure I have ever seen is the bottle but not
To the thread. Usually caused by excessive tightening or trying to adjust a frozen device.
Doesn't necessarily fail there and then but cracks and metal fatigue over time causes the failure.