Stainless steel for clevis pins and through mast bolts

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TradewindSailor

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My cap shrouds are hung off tangs at the top of the mast that are attached to the mast by a stainless steel bolt passing through the mast.

I've had to modify the design of the tangs after one of the originals broke due to fatigue.

I am intending to use a standard 316 thick walled tube through the tangs, and passing a standard 316 bolt through the centre of the tube (i.e. with the head of the bolt at one end of the tube, and the nut at the other end) to hold the tube in position.

I have been told that the through mast tube should be case hardened, and that using anything else would be madness.

I can find no information to support this. Any engineers / riggers out there with comments please?
 
TradewindSailor said:
I have been told that the through mast tube should be case hardened, and that using anything else would be madness.

I can find no information to support this. Any engineers / riggers out there with comments please?
Click to expand...

The obvious question would be why case harden it? The usual reason for case hardening is to resist wear, but this is a static tube that has nothing rubbing on it, so what function does the case hardening have? The case hardening process usually adds carbon to the surface layer but it doesn't strengthen it, the strength is provided by the core material. It could even be argued that its mechanical properties would be reduced, as it will be more brittle and may well contain cracks.

The second point is that I'm not at all sure that it is technically possible to case harden an austenitic stainless steel. Some info here supports that. There is a new process called Kolsterising that seems to work but I doubt if there are many places that do it.

Summary: I can't see the slightest benefit in doing it and it is technically/economically problematic. Every masthead tube like this that I have seen was ordinary austenitic stainless steel
 
There was a short article in PBO a while back on making a set of tangs to replace swaged t-bar ends. If I remember correctly, the tube was supplemented with top-hat section ends sitting in plates rivetted to the spar.

If you PM me with your email address I can send you a copy.

Rob.
 
In the good old days when all masts were supplied with tangs they were fitted with a alluminium compression tube and then a tang plate over the bolt hole. This was a very strong way of fitting tangs. The loads went on the compresion tube and not on the mast wall. All stainless parts need to be kept apart from the alloy parts to stop corrosion. We use Zinc Cromate paste.
 
Case hardened? Stainless is much harder and stronger than the aluminum it is attached to. Use Tef-Jel on contacting surfaces to allow removal at a later date and prevent corrosion.
 
Tangs

Claerly nuts to the case hardening.
The Tangs (ie a plate of SS with a bolt hole one end and an attachment for wire) is attached by a through bolt from one side (tang) to the other. The loads of the wire are pulling downward in the hole in the mast. Worst case scenario is that the hole in the Al mast wall will be elongated downward and possibly cracking or splitting the mast.
You can use a larger bolt to increase the contact area of the ss on the Al or as you suggested fit a tube into the hole. The tube has the advantage that you can then tighten the bolt up very tight compressing the tube and not squashing the mast sideways.
If you consider that under heeling tension the windward stay has the tension and the leeward is slack then the load will only be on the windward wall of the mast the leeward side tending to cock upward. So there will be no load sharing done by the tube. Only a small part of the load vector would be pulling the bolt outwards mostly downwards.

However it seems to me that if you were to rivet a ss plate over the hole but with a hole in it the load of the bolt could be effectively taken on the edge of the stainless steel plate and transmitted into the mast via the rivets(plus the edge of the Al mast. Use of lock nuts on the bolts will ensure enough squashing tension but not too much.
Obviously the hardware needs to be strong and sized commensurate with the rigging wire size ie bolts about twice the rigging wire diameter on small boats. good luck olewill
 
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Thanks all.

Just for clarification:

The tangs are connected to the mast by 6 rivets and a 5mm bolt. The 15mm bolt passes through the port tang, through the mast, and then through the starboard tang. A 21mm od, 16mm id 316 stainless steel tube passes through both tangs, with the 15mm bolt passing through it's centre.

The first photo shows a similar tang arrangement, but for the bottom of the lower diamond set .... rather that the cap shrouds.

The second photo shows the new shroud tang plate on the left and the thin fatigued original on the right. When the original failed, the through mast bolt and tube were left to bear only on the mast wall ....... which of course elongated the hole a little.

I used Lanicote for galvanic protection. Never used it before ..... I usually cut a strip out of a plastic bottle to isolate the mast and stainless.
 
The design of the through bolting looks fine but in my opinion the weak point is where the stainless tang is welded. The side that juts out has to handle the same load as the straight side and ultimately breaks at the weld. A better way might be doubled tangs that curve out to accomodate the pin - each equally strong.
 
Yes .... if you use two plates for the tang you need them equal length and similarly bent otherwise one will fatigue and not do much work.

The original was just a single plate with a cold formed bend, which is why it fatigued.
 
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