Shear strength of stainless bolts

[silverseal]

Yesterday, nearly lost the mast in a Force 5, when a phosphor bronze casting failed- a fin to which the forestay is attached sheared away. Am currently making two mild steel plates to be bolted each side of the undamaged portions of the fin, and am proposing to use 4 off 6 mm stainless bolts to spread the load over an 8 inch length of plate. Anybody any idea of the force required to shear a 6 mm stainless bolt. If this works OK will get stainless plates made over the winter...

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[Guest]

and waht size pins are in shackles etc.

Look at rest of forestay - what size pins etc. in shackles / toggles / fastenings etc. particularly at top end. You may be surprised to find that you are exceeding those comfortably with your bottom end ?

To have a fin break away - indicates fatigued fitting - possibly cracked over time.

I replaced my stem fitting with self designed stainless steel on previous boat that also had a tang down the stem to give better vertical hold. But it was not cheap.


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Rohorn]

Re: and waht size pins are in shackles etc.

Hi...rule of thumb take half the tensile stress for shear. Check carefully which stainless the bolts are made of. Get the tensile strength. Take cross sectional area of bolt, try not to have the thread under stress, but if you have to, take root diametre eg bottom of thread. Allow half of tensile for shearstress. This is at any one point of shear. A single plate more than,say, one bolt diametre thick held tight in a fork (your situation with the fin between two ms plates counts as two diametres in shear.
Cheers.....R

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[nordic]

Re: and waht size pins are in shackles etc.

As previous - very important that shear is not applied across threads. Measure plain length (part of bolt not threaded) required allowing for fitting and washers and buy accordingly. Inevitably they will be too long in the thread but can be cut and dressed carefully with a file (especially important with "Nyloc" nuts as ragged thread will cut plastic and impair locking) so that nuts fit with approx 2 threads showing.

Its amazing how many proprietory fittings come with machine screws rather than bolts and on a wooden boat the threads just mill their way into the wood and the fastening comes loose!

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[silverseal]

Re: and waht size pins are in shackles etc.

The area of the fin which broke away indicates from the remains that there was indeed a prolongued period of stress cracking, and that there were horizontal cracks about 10mm long both sides of what was a 12mm (?) hole, which shows signs of stretching. Looking at the semicirle of metal round the hole, and 2/3rds of metal was cracked, so possibly it was better that it went in a Force 5 and not a 7, from which it would not have been recoverable. The boat is 32 years old, and I bought her last year

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[Birdseye]

most austenitic stainless steels have a tensile strength in the annealed state round the 35tt mark. any processing to make a bolt therefater will increase that a bit, but it is still very soft. if you assume the above figure you wont be far out. then use a safety factor of at least 2. ie twice the area you reckon is needed.

personally, if I went this way (which I wouldnt) I would use high tensile carbon steels and protect them from corrosion either through buying galvanised or through painting.

really you should replace the bronze fitting. if it has failed, you dont know how far back the damage goes in terms of metallurgical structure, or how much further you are weakening by drilling the holes in it. and in any case the fewer dissimilar metals the better.

what you propose sounds like a bit of a bodge. your insurance company might not like it if a claim resulted

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[silverseal]

With regard to your comment on the bronze fittings, the bronze "shoe" is moulded into the bow of the boat, it is 13 inches long, and 9 inches wide, and has three fins, which create two hause channels for port and starboard morrings. The central fin is about 8 inches long and has several holes drilled into it over the last 32 years.
I bought the boat last year, and it looks like the failed portion of the fin was very close to the edge o fthe fin to accomodate the bottle screw clevis pin. I am proposing two metal plates about 8 inches long, with multiple attachments to spread the load over at last 4 points so that such failures do not happen again.
From an engineering point of view, I dont think it is a bodge. Removal of the complete assembly would require rebuilding the bow of the boat, which in grp would not be easy!

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[richardandtracy]

Shank area of M6 bolt is 26.6mm^2 (BS 3692) - don't shear the thread as the minimum area is only 16.6 mm^2!

If bolt material is A2-70 or A4-70 the Ultimate Tensile Strength (UTS) is 700 N/mm^2 and yield strength is 450 N/mm^2 (BS 6105). The shear strength is 65% of this, ie 455 N/mm^2 ultimate or 292 N/mm^2 yield, giving a single shear breaking strength of 12100 N (1230 kg force). The single shear onset of permanent distortion load is 7767 N (790 kg force). Try to use the bolt in double shear (like in a fork end), as the breaking load doubles.

It would be wise to have a reserve factor of at least 2 on yield (ie max load of 395 kg in single shear).

For grade A2-80 or A4-80 the UTS increases to 800 N/mm^2, and for A2-50 or A4-50 the UTS is down to 500 N/mm^2

Regards

Richard.


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