Chain Plates

[doug748]

Evening all - wow, lots of ideas - all of which are gratefully received. Many thanks to everyone who helped to keep me shipshape.

Good show. I have recently replaced some lower U bolts. Yours are relatively young and should have some life yet but do have a good look. On inspection mine did seem to have very tiny horizontal lines at the root, they could well have been inclusions but, as they were over 30 years old I replaced them.

It was impossible to find replacements off the shelf, either the diameters were wrong or the spacing or the depth. I thought I had cracked it, only to find, on delivery, that the U bolt had rolled threads and the stock diameter was below the 10mm that I wanted.

I could have gone to the original manufacturer at almost £100 a pop but had four made up in a local fabrication shop.



I had the dia upped from 3/8in to 10mm and also had them cranked to match the rigging angle - the advantage of having them custom made.

In the end I paid c £170 for four. Less than 1/2 the price of OEM.

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[Rappey]

the U bolt had rolled threads

Isn't rolled threads a lot stronger ? Less likely to fracture across the thread?

I found a web site that sells the u bolts in different designs off the shelf and had a page to enter your own dimensions and angles, but can't find it anymore.

 

[penberth3]

Isn't rolled threads a lot stronger ? Less likely to fracture across the thread?......

Yes.

 

[vyv_cox]

Isn't rolled threads a lot stronger ? Less likely to fracture across the thread?

Yes, rolled threads compress the surface, reducing the likelihood of fatigue. Also, unlike cut threads, the roots are rounded, further improving fatigue properties.

 

[Rappey]

SOme years back was chatting with a guy, can't remember what about but he handed me a bolt, around m6 x45. It was amazingly light. He asked how much I thought it would be.
Had a crazy expensive guess and was not even close.
Turned out it was a titanium bolt used on satellites so it was rolled, inspected and certified pushing the cost to a ludicrous amount.
A tube of silicon was hundreds ?

 

[doug748]

Isn't rolled threads a lot stronger ? Less likely to fracture across the thread?

Very true.

Though I mentioned it because, in relief at finding a 8mm thread form (for example), you might deduce you have a component made up from 8mm rod. In fact it is rolled up to 8mm.

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[penberth3]

Very true.

Though I mentioned it because, in relief at finding a 8mm thread form (for example), you might deduce you have a component made up from 8mm rod. In fact it is rolled up to 8mm.

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Is the difference in diameter worth worrying about?

 

[TernVI]

SOme years back was chatting with a guy, can't remember what about but he handed me a bolt, around m6 x45. It was amazingly light. He asked how much I thought it would be.
Had a crazy expensive guess and was not even close.
Turned out it was a titanium bolt used on satellites so it was rolled, inspected and certified pushing the cost to a ludicrous amount.
A tube of silicon was hundreds ?

I've done a little work with satellites, everything is comically expensive.
Most of the electronics in them has to have such a long quality control history that half the components are obsolete.
Satellites are very reliable. Either that's because of the quality control, or, a popular theory in the industry, 'because there are no people up there fiddling with them'.

 

[TernVI]

Very true.

Though I mentioned it because, in relief at finding a 8mm thread form (for example), you might deduce you have a component made up from 8mm rod. In fact it is rolled up to 8mm.

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With an 8mm thread, most of the tensile strength is in the core, which is roughly the tapping diameter 6.8mm.
An 8mm thread would be rolled from stock of about 7.4mm.
So in pure tension it's just as strong.
The problem may be the curved bits of the U are not as strong.
Or the stress is higher at deck level where the force is not exactly aligned with the metal.

 

[TernVI]

Is the difference in diameter worth worrying about?

In my view, chainplates need to be irreproachably a lot stronger than the rig, by a factor of 'several', so that fatigue is not an issue.
If quibbling about a small change is significant, then the safety factor was probably way too small initially.
These types of chainplates have been a source of concern for as long as I've been involved in yachts.

 

[doug748]

With an 8mm thread, most of the tensile strength is in the core, which is roughly the tapping diameter 6.8mm.
An 8mm thread would be rolled from stock of about 7.4mm.
So in pure tension it's just as strong.
The problem may be the curved bits of the U are not as strong.
Or the stress is higher at deck level where the force is not exactly aligned with the metal.

Just so, exactly, indeed and as you say.

The new U bolt would have ended up, probably, weaker than the old one.

I wanted it to be stronger and to point out the trap to the OP, in case he should go down the same route.

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[penberth3]

Just so, exactly, indeed and as you say.

The new U bolt would have ended up, probably, weaker than the old one.

I wanted it to be stronger and to point out the trap to the OP, in case he should go down the same route.

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That's not really what Tern is saying. Possibly a tiny bit weaker in theory, but no practical difference. I think you're worrying too much.

 

[doug748]

That's not really what Tern is saying. Possibly a tiny bit weaker in theory, but no practical difference. I think you're worrying too much.

I thought I was getting it and then it drifted away again.

I might have it in my grasp, now........

...........I should not worry about fitting a weaker component than the original because it does not matter much. Unless I make it loads and loads stronger, I might just as well make it weaker.

That's it.

An unusual engineering philosophy, I feel the OP should reserve judgement on it.

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[penberth3]

I thought I was getting it and then it drifted away again.

I might have it in my grasp, now........

...........I should not worry about fitting a weaker component than the original because it does not matter much. Unless I make it loads and loads stronger, I might just as well make it weaker.

That's it.

An unusual engineering philosophy, I feel the OP should reserve judgement on it.

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But you wouldn't be making it weaker, the minimum diameter is still the core of the M8 thread.

 

[doug748]

But you wouldn't be making it weaker, the minimum diameter is still the core of the M8 thread.

The thread is not the issue, the part that matters would be adversely affected.
Tern, saw the point I was making:

"The problem may be the curved bits of the U are not as strong. "

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[TernVI]

The thread is not the issue, the part that matters would be adversely affected.
Tern, saw the point I was making:

"The problem may be the curved bits of the U are not as strong. "

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Athough to be fair, it may be the threaded bits which are most likely to fail as they are a crevice corrosion stress raiser fatigue inducing generally nasty thing that I really don't like very much in case you haven't got that.
Just my opinion.
But when did you last meet anyone having trouble with nice wide chainplates made out of 6mm plate?

 

[Rappey]

But when did you last meet anyone having trouble with nice wide chainplates made out of 6mm plate?

They can suffer a different type of fate, galvanic corrosion due to leaks and the water being held in the core rather than dripping inside giving a visual something is wrong.

At 37 yrs old I am considering replacing mine and would like to go up to 10mm but having to increase the hole sizes from 8 to 10 will be hell on earth to do especially as it would involve 16 holes.

 

[doug748]

Athough to be fair, it may be the threaded bits which are most likely to fail as they are a crevice corrosion stress raiser fatigue inducing generally nasty thing that I really don't like very much in case you haven't got that.
Just my opinion.
But when did you last meet anyone having trouble with nice wide chainplates made out of 6mm plate?

It's all if's but's and maybe.

The threads won't fail on a custom made article because you only thread the first inch, well below deck. The critical area is just above and below deck level.

On the matter of marginal increases in stock diameter, it's common practice when replacing an imperial item with metric. In this case you would have to be very cavalier, or marginal bonkers, to go 20 thou under rather than 20 thou over. That is a lot in engineering terms.

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