Rigging to Chainplate Termination Problem Query

savageseadog

savageseadog

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I've seen this problem on many boats and wondered whar the solution is. The rigging wire terminates onto a bottle screw (turnbuckle) the lower part of the bottle screw is usually some kind of fork through which a cotter pin passes through both the fork and the chainplate.
The problem comes about when, as is often the case the thickness of the tang of the chainplate is a fair bit less than opening of the fork. If the tang is centered with spacers or washers then the force bears on the middle of the pin thereby applying maximum bending force. Alternatively the tang can be set to one side or other of the fork which then tends to splay the fork open.
In an ideal world the tang would be a snug fit in the fork but this is often not the case.
Suggestions?
 
Provided it is a nice fat pin it just does not matter. On my boat the pins are quite a bit thicker than the wire diameter, and it would take more force to shear one than it would to break the wire.

As far as I know the wire is the weakest link in good rigging design.

EDIT: On my boat the rigging screws actually end with a tee terminal, with a slotted stainless plate wrapped over it, and a pin through that to the chainplate, but in all of them there would be about enough room to fit a chainplate twice as thick, but I don't think that matters at all.
 
My screws have a toggle where they go on the chain plate to deal with any misalignment. Don't think having the pin off centre makes any difference. Failure at pin would only come after wear which would be easy to see.
 
Yes I agree and the original post is quite correct. The fork should be as close a fit to the chainplate as possible. If it isn't the bending moment on the pin is considerable especially when its bearing on a round shackle rather than a flat plate The answer is to fit a bearing spreader which is a SS tube the width of the fork with a hole through the dia of the pin. this then has a central groove cut to match the shackle or chainplate centralising it and adding strength to the pin. You rarely see them these days though and unless you own a lathe and make them yourself they have dissapeared from chandlers shelves. The other answer is to fit a smaller fork. The problem being that this may well have a smaller pin!
 
Quite right. For example, for a rig with 4mm wire the fork would be 10mm wide and have an 8mm diameter pin. The wire would have a breaking strain of 1340kg and would be tensioned to a maximum of 335kg (25%). The 8mm pin, even over its full useful length of 20mm, has a breaking strain of 3200kg.
So, the fork, pin, chainplate connection is typically far stronger than the breaking strain of the wire.
The biggest danger is the chainplate sawing through the pin, or the pin sawing through the chainplate but this would only happen if unsuitable materials were used.
 
This is what I'm talking about.


101_0059.jpg
 
Easy put the plate toggle in a press and press the lower part closed a little.

Using a press which supports the whole of one side could kink both sides so to keep the hole parallel to prevent sharp edged biting into the pin.
 
Whatever you do, the bottlescrew fork should not fit direct onto the chainplate. There should be a toggle to relieve any directional misalignment. The type in Savageseadog' photo work well.
 
If you look at the photos it's the type of screw with a built in toggle. What I refer to as a fork is in fact a piece of stainless plate bent round 180 degrees to form one part of the toggle, the other degree of freedom is provided by the pin and fork. The opening out of the fork is clearly visible on the photo. Wouldn't dream of using anything but a fully articulated toggle.
 
Re: Rigging to Chainplate Termination Problem Query Re savageseadog

Yes, as soon as I had posted, I realised that the photo was yours. Your original post made it sound as if the forks might not have had articulation. I grovel.
 
I had that problem.

I ended up by welding a flat plate on the side of both the shroud plate tangs to give the correct total thickness to fill the fork width.

I had drilled three holes in both the plates to match the three holes in my shroud plate tangs and through bolted it in place before welding.

In your case, unless you could get a slightly shorter cotter pin, I would add a washer under the split pin to help prevent the fork from splaying wider.
Iain
 
[ QUOTE ]
I had that problem.

I ended up by welding a flat plate on the side of both the shroud plate tangs to give the correct total thickness to fill the fork width.

I had drilled three holes in both the plates to match the three holes in my shroud plate tangs and through bolted it in place before welding.

In your case, unless you could get a slightly shorter cotter pin, I would add a washer under the split pin to help prevent the fork from splaying wider.
Iain

[/ QUOTE ]How did you weld it? In place or what? and what method?
 
As at that time I was still building the boat I was able to remove both shroud plate assemblies .
Then I TIG welded the plates on.

It would have been possible to weld them in situ but I would have put an old fire blanket around each shroud plate to avoid damaging the deck.

Cheers

Iain
 
Actually having seen your photo its not exessive. I thought you were talking about a forged toggle. These are fabricated out of sheet. What seems to have happened is the toggles have splayed a bit due to the fact that the pin is a bit too long. You don't need a press. Just take them off and pull the distortion out of them by putting a bolt through and tightening. Then fit the washers on the outside rather than inside. You should ideally have a washer under the split pin anyway.
 
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