Wire-to-turnbuckle size

[benz]

Can anyone tell me where to find a table or a guide to determine what size of turnbuckle is suitable for a certain size of wire? If you look at a 1/4" turnbuckle, it is ridiculously small for 1/4" wire--it seems they always use several sizes up. And also, does anyone know why the 5/8" shackle I just bought has a 3/4" pin? I'm trying to get my chainplates drilled prior to installation, and rather than rely on measurements, I've simply bought some turnbuckles and shackles so I can make sure all the pieces will fit together with proper clearances before they're bedded and bolted on. But I keep finding that shackles stamped "5/8" have bigger pins than that, and that turnbuckles labelled "5/8" have far skinnier material around the eyes than that, and don't even get me started on propane fittings that say "3/8".
While I'm here, a guy at the hardware store told me the safe working load of the aforementioned 5/8 shackle was 4,000 lbs, and breaking strain was 8,000. Can something this big really be so weak? If anyone knows where a table of specs for shackles can be found, I'd be keen to know. By the way, I'm dealing with all galvanized hardware here.
Thanks in advance,
Ben

[cwa]

Well my guess is that the pin is in shear loading and the rest of the shackle is under tension. that would account for the difference in size. As for the load rating, my understanding is that in the "industrial rigging" area a safety factor of 5 to 1 is used. So with that the 5/8 shackle should have a breaking load of 5 times the rated WLL (working load limit). If the WLL is not stamped into the shackle (there is lots of stuff that only says "CHINA" on it) then its anybody's guess as to what it will take.

Carl
S/V Moonflower of Moab

[Brion Toss]

Oy,
First, I think it is safe to disregard the hardware store clerk as a source of reliable engineering data. Next, as noted in the previous post, the standard SF for industrial gear is 5:1, though who knows what standards these shackles will meet; the catalog for this specific brand should provide that info. Next, galvanized shackles always have a pin one size bigger than the bail. The diameter is measured at the narrowest portion of the bail, just above the eyes. The way that the loads are distributed this makes sense (stainless yacht shackles often have bail and pin of the same size, but this has to do with shackle design and use).
Next, partly because of loads needing to turn corners, and partly to assure that turnbuckles and shackles are appropriately stronger than the wire they hold, wire will always be smaller in diameter than the other hardware.
All of this has been worked out by centuries of people breaking stuff, figuring out why it broke, and finally coming up with appropriate standards. If you want to understand the reasoning behind those standards (and I recommend it) the resource materials are widely available. But it might be enough for you, and far simpler, simply to adhere to the standards. I can recommend my book as one source of sizing information. Also Skene's, Larsson and Eliasson's, and others.
Can't help you with the propane fitting anomaly, but it seems a poor bet to assume there's no hard-won logic there, just because it isn't obvious to you.
Fair leads,
Brion Toss

[benz]

I had pegged Hardware Store Guy as unreliable--having been one myself once, and knowing that we just made up whatever we didn't know.....
Brion, I found the table to answer my question in your book, just like you said--perhaps I should read the rest of it. My copy of Skene's doesn't have a lot of the information people keep saying it should, but I guess there's differences between various editions. Bother. There is a handy chart, though, with strengths of various sorts of wire, which leads me to ask: what differentiates aircraft wire from others, and: is it suitable for boat rigging?
If only there were a GasTalk forum for all my propane-related questions....
Thanks!
Ben

[Bw1]

A related question:

Swage terminals for 1/4" wire are available in 3/8" and 1/2" pin (or thread). If I do my math right a 1/2" pin (or thread) is about 78% stronger than 3/8". That's a huge difference, and the choice propogates to turnbuckle size, toggle size, etc. So which is more appropriate for 1/4" wire? Is 3/8" risky or 1/2" grossly over sized.

Thanks,
Bryan

[Brion Toss]

Hello,
The available pin sizes are there to accommodate different wire constructions. The pins should always be at least a third stronger than the wire (as should turnbuckles, tangs, chainplates), and 3/8" is right for 1/4" 7x19 and 7x7. But it is barely stronger than1/4" 1x19, which is why the 1/2" pin is the correct size.
What too often happens is that builders will simply decide to save money, putting swages (and turnbuckles, tangs, chainplates) meant for 7x19 onto 1x19, with predictable consequences for safety factors. Similarly, they might choose aircraft eyes and forks instead of the spendier marine ends. Make sure of your wire size -- perhaps you don't really need 1/4", for instrance -- and then choose components to suit.
Fair leads,
Brion Toss

[Bw1]

Thanks. That clears it up.

What's the reasoning behind making the pins 30+% stronger than the wire?

-Bryan

[Brion Toss]

Originally Posted by Bw1

Thanks. That clears it up.

What's the reasoning behind making the pins 30+% stronger than the wire?

-Bryan

Hi again,
A couple of things. One is that we expect pins (and chainplates and tangs) to last longer than wire, so we make them stronger than the wire. And because these structures are more vulnerable to fatigue, and often harder to inspect, we make them stronger still. Which brings up one of the great mysteries of my life: Skene's chainplate specs are supposed to result in 4/3 the wire strength, but if you run the numbers they are quite a bit higher than that. Does antyone know why?
Fair leads,
Brion Toss

[P38]

Hi Bryon,

Just a guess, but holes in a part increase the stress in the area around the holes by 300% to 400%. Perhaps Skeene was allowing extra to give some fatigue protection?

Regards, Paul