Wire breaking strength and pin diameter

[phorvati]

I'm going over turnbuckle pin diameter vs. breaking strength of wire for relevant turnbuckle.

For instance for 8m wire, standard turnbuckle options are those with pin/screw diameters of 5/8" and 1/2".
Breaking strengths for various flavors of 8mm wire are in the range of 11.8k to 13.5kpsi or so.

if you look at 1/2" pin, breaking strength would be considered a shear strength which is usually 60% of tensile strength of SS (75-80)kPSI. Usually number used for shear strength is 45kpsi.

so shear area of a pin is Pi*r^2.
So for pin of 1/2" diameter, area would be 3.14*.25*.25 = 0.19sq-in.
So its shear strength would be only 8.8kpsi. Somehow West marine lists Hayn 1/2" clevis pin at 11kpsi breaking strength. Not sure how they got that number since hayn does not list it in their catalog.

It appears that 1/2" pin would break before 8mm wire. But they are a standard option for 8mm wire.
Idk what i'm missing but if anyone could chime in that would be great.
Petar

[a64pilot]

Just a swag, but my guess is that the breaking strength is several orders of magnitude of what a rig ever sees forces wise, it’s after the wire becomes corroded and fatigued that failure is possible.
My guess is also that the pin doesn’t see near the fatigue that wire does, so that at replacement time, since it’s less susceptible to fatigue, it may be stronger than the wire?

Now I’m not really a sailor, and certainly no rigger, but believe that all of this “stuff”has been figured out decades ago and am in fact sort of betting on that.

On edit, cruising boats and production boats, not highly stressed racing boats. They I assume don’t have nearly the safety factor of a cruising Boat.

[Jim Cate]

Originally Posted by phorvati

I'm going over turnbuckle pin diameter vs. breaking strength of wire for relevant turnbuckle.

For instance for 8m wire, standard turnbuckle options are those with pin/screw diameters of 5/8" and 1/2".
Breaking strengths for various flavors of 8mm wire are in the range of 11.8k to 13.5kpsi or so.

if you look at 1/2" pin, breaking strength would be considered a shear strength which is usually 60% of tensile strength of SS (75-80)kPSI. Usually number used for shear strength is 45kpsi.

so shear area of a pin is Pi*r^2.
So for pin of 1/2" diameter, area would be 3.14*.25*.25 = 0.19sq-in.
So its shear strength would be only 8.8kpsi. Somehow West marine lists Hayn 1/2" clevis pin at 11kpsi breaking strength. Not sure how they got that number since hayn does not list it in their catalog.

It appears that 1/2" pin would break before 8mm wire. But they are a standard option for 8mm wire.
Idk what i'm missing but if anyone could chime in that would be great.
Petar

Seems to me that for the clevis pin to shear in a rigging screw it must shear in two places, not just one. That is, on either side of the chainplate or on both sides of the fork. Would this not double the shearing load?

Jim

[UNCIVILIZED]

It is a curious question. Perhaps you might post it over on Brion Toss's rigging forum & report back if/when you recieve answers. SparTalk - Powered by vBulletin

[phorvati]

There is definitely a factor of two. Somewhere. Usually pins are in a fork or a jaw. So perhaps that might be x2. Usually there's a safety factor for wire. It's between 2x and 3x. I'm using 2.5 in my re-rig. It might be that pins are not subject to same safety factor.

[a64pilot]

Wire rope safety factor will change by use of course, lifting people it will be higher, but I think the average safety factor is 5, I’d be surprised if it’s not higher for our rigs.

[UNCIVILIZED]

Something about these numbers seems strange/off. As the pin has a cross sectional area of 2.56x that of the wire. And it's a solid cylinder of metal, unlike the wire, who's true material cross sectional area is significantly less than that of a 5/16" solid cylinder. Yet the pin is "weaker"? This one don't make sense to me, at least not at the moment. Thoughts ???

[phorvati]

I'm referrencing the safety factor from riggers apprentice when using the RM30 method. Righting moment at 30deg of heel. It's only one part of the multiplier. Overall safety factor is probably higher. Wouldn't be suprised if it's more than 5.

Im not sure how they rate breaking tensile strenght of wire rope. 80kpsi has to come into equation but idk how. Pin is in shear stress and wire is in tensile. Safe number for Ss shear breaking strength is 60% of tensile. That's why pin area has to be larger than wire. But even with 2.5x the area it appears to be inadequate. Unless the 2 sides of the fork or jaw let you multiply by 2.

[UNCIVILIZED]

Originally Posted by phorvati

I'm referrencing the safety factor from riggers apprentice when using the RM30 method. Righting moment at 30deg of heel. It's only one part of the multiplier. Overall safety factor is probably higher. Wouldn't be suprised if it's more than 5.

Im not sure how they rate breaking tensile strenght of wire rope. 80kpsi has to come into equation but idk how. Pin is in shear stress and wire is in tensile. Safe number for Ss shear breaking strength is 60% of tensile. That's why pin area has to be larger than wire. But even with 2.5x the area it appears to be inadequate. Unless the 2 sides of the fork or jaw let you multiply by 2.

Earlier I looked up the numbers on wire's breaking strength, & you've pretty much got them correct. But I haven't pulled out any reference texts to look up all of the formulas for sizing things as yet.
Rigging Only's 1x19 stainless wire is type 316 for standing rigging and can be used with swage fittings and mechanical wire terminals

And there's an irony to this, in that some people will "upgrade" their wire from 1x19 to Dyeform/Compacted Strand when replacing their standing rigging. Wanting to have more of a safety factor in the strength of their rigs via the stronger wire type (stronger per diameter). But this then reduces the safety factor of some of the other components in the system. Particularly given that when tuning it, in order to achieve the same level of rig tune so that the mast remains properly in column, the loads on all of the standing rigging (including the mast tube) are a good bit higher. So it's a bit of a catch-22.

[bstruebing]

I agree with jim, if the pin has to shear in 2 places it is in double shear which means the load you came up with x2. You formulas for breaking strength are sound, though I didn't check the math. I am a mechanical engineer and calc pin and bolt strength on the reg.

Cheers
Brian

[Uncle G]

If you assume that the 8mm wire is solid (which it isn’t, obviously) then the cross sectional area is 0.076 sq in. If you take the break strain given for the 8mm wire (11,800 psi), then the tensile strength of the material in the wire is 11,800/0.076 = 155,263. Since the wire isn’t solid, the tensile strength of the material in the wire must actually be higher than this.


If you now multiply the 155,263 by your 60% to get sheer strength, you get 93,157. Multiply that by your 0.19 area of the half-inch clevis pin, you get 17,699. Which is higher than the 11,800 for the wire.


Given that the wire and the pin are made of the same material and that the wire isn’t actually solid then the pin has a much higher break load in sheer than the wire.


The actual tensile strength isn’t relevant, is it? You only need to know that the wire is weaker than the pin.


Maybe I’ve made an error in the maths? I’m not accustomed to working in imperial units...

[barnakiel]

The turnbuckle has holes in the fittings that accept only one size of a pin.

You are not over-drilling the turnbuckle fitting, are you.

The hole matches a pin that is apt for the job.

E.g. our screws that hold 5mm wire have 10mm pins, screws that hold 6mm wire have 12mm pins, etc.

barnakiel

[a64pilot]

If your really worried about pin strength, replace the SS pins with 6/4 Titanium

[InTheDish]

Horizontal verses perpendicular loading?

[Thumbs Up]

I just went through this and am replacing all of my 1/2" turnbuckles and pins with new 5/8" ones. This is what brian toss says about it:
If you run pin strength (pi x r squared x strength of metal), you will see that a 1/2" pin is stronger than 5/16" 1x19. The problem is that it is not stronger enough. Pins -- and chainplates and tangs -- need to be at least a third stronger than the wire, in order to deal with vulnerability to fatigue and corrosion, as well as the weakening effects of threading rod for studs, and shaping barrels, for instance. Taking all this into account, it is not impossible to end up with a turnbuckle that is weaker than the wire. That is why 5/8" is the standard pin size for 5/16" 1x19.
The reason that you can nevertheless get 1/2" terminals for that diameter is that those terminals are meant for 7x19 and similar constructions, which are much weaker than 1x19. 1/2" is the appropriate pin size for those constructions. If people don't realize this, or if perhaps they are just being cheap, they will put a too-small pin size on 1x19.