Double crotching with a butterfly 2

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I've clipped a 3rd biner directly to the suspension bridge, but that made me a little nervous as the angle between my two lines approached 180 degrees. Without doing any calculations,it just seems that the first line (acting on the alum ring) and the second line (acting through biner
#3) were really pulling the harness apart. It seems like it would be much easier on the harness if I could manage to clip both lines to one really strong ring. That way the lateral forces would not be acting on the bridge at all.

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I see the forces on the harness as being very real. I'm betting a climber won't be able to get enough tension in the lines to pull the harness apart.

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I'm sure you're right for most scenarios. Where I get concerned is when the angle gets close to 180 degrees. If I remember my vector math correctly, the tension actually goes to infinity as that angle gets closer and closer to 180. So if a climber were to get into a position where both lines are almost parallel to the ground, and then sit back into his harness, the lateral loads on the harness could be huge. It really depends on how close to 180 the two lines remain as the climber's weight is applied.

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Just to prove, once and for all, that I have too much time on my hands, see the attached....I know it's not exact,just meant to be an estimate of what might happen.
 

Attachments

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I'm sure you're right for most scenarios. Where I get concerned is when the angle gets close to 180 degrees. If I remember my vector math correctly, the tension actually goes to infinity as that angle gets closer and closer to 180. So if a climber were to get into a position where both lines are almost parallel to the ground, and then sit back into his harness, the lateral loads on the harness could be huge. It really depends on how close to 180 the two lines remain as the climber's weight is applied.

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You are correct. But, I still don't believe a climber will create an angle close enough to 180 degrees for this to be a concern in relation to the safety gear. I get nervous about breaking my anchor points, not tearing my equipment apart.

Joe
 
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Many, not all, of the climbers that I've talked to about using the Petzl Swivel don't like it.

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hate to be contrary, but i LOVE my swivel. have used it for 2+ yrs and find it annoying to climb without it any more. to me the only negative is it adds like 4 inches of "flop" to your rig. way outweighed for me by benefits.

avoids the whole (rather contentious) issue of the screw link.
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Many, not all, of the climbers that I've talked to about using the Petzl Swivel don't like it. The rotation lets the climbing hitch or whatever is attached, to rotate 'out of phase'.

The bow shackles are LARGE and really CHEAP too. A dab of Locktite or a snug turn on the allen wrench will keep them closed.

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For some reason Kathy's last two posts aren't showing up for everyone. Here they are...

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Many, not all, of the climbers that I've talked to about using the Petzl Swivel don't like it.



hate to be contrary, but i LOVE my swivel. have used it for 2+ yrs and find it annoying to climb without it any more. to me the only negative is it adds like 4 inches of "flop" to your rig. way outweighed for me by benefits.

avoids the whole (rather contentious) issue of the screw link.


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Kathy Holzer
Out on a Limb Tree co
Seattle WA

Quote:

Many, not all, of the climbers that I've talked to about using the Petzl Swivel don't like it. The rotation lets the climbing hitch or whatever is attached, to rotate 'out of phase'.

The bow shackles are LARGE and really CHEAP too. A dab of Locktite or a snug turn on the allen wrench will keep them closed.



--------------------
Kathy Holzer
Out on a Limb Tree co
Seattle WA
 
One more point about shackle pins- I don't know about Tom's but mine are "spring locked"-the pin compresses the opening in the bow slightly before the pin "bottoms out" Consequently the assembly is highly resistant to vibrational loosening...or any other loosening that isn't done deliberately with a tool. Even if the allen wrench,pliers or spud used to tighten a bow shackle is only 3 inches long it generates an enormous ammount of torque compared to friction on a smooth ,rounded surface a fraction of an inch displaced from the longitudal center of mass/center of rotation. Dan's trepidation is natural but a little bit of experimentation with bow shackles reveals that their designers know something about physics and working rigging (remember these stainless steel bow shackles were/are designed for rigging on sailing crafts-uses that are critical and involve lots of movement and repetitive loading.).
 
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I doubt you get a lot of torque with a standard allen wrench.

At any rate I bet you don't know how much torque you put on that screw and that you don't know the amount of friction generated when it rolls along the bridge.

I trust you that you are safe Tom. I still fine it counter intuitive and I would imagine a lot of people do.

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i tested this with a torque wrench fitted with a allen key, on a allen boltstyle D Shackle (stainelss steel, 1/2" 7000lb WLL)

i was able to torque the fixing bolt to almost 40footlbs of torque. (Snap On torque wrench 1/2" drive) Couple that with the drop of red Loctite and that bolt IS SIMPLY NOT COMING LOOSE EVER (without chemical seperation of the loctite and a bullybar (cheater bar)

your milage might vary, depending on the make and model of shackle. seriously though...a drop of red loctite and you will never have to worry about it.
 
Thanks SRTTech. That was interesting. I didn't want to have to buy a little torque wrench just to try that, but i think I will just to set the shackles on TreeFlex. 40lbs/ft of torque is ALOT. Thats just about the same as the bolts on some 6" chipper blades!
 
I'm sure you're right for most scenarios. Where I get concerned is when the angle gets close to 180 degrees. If I remember my vector math correctly, the tension actually goes to infinity as that angle gets closer and closer to 180. So if a climber were to get into a position where both lines are almost parallel to the ground, and then sit back into his harness, the lateral loads on the harness could be huge. It really depends on how close to 180 the two lines remain as the climber's weight is applied.

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In the book "On Rope" Padgett and Smith discuss that 'Due to the elasticity of rope this event (180 degrees) will never happen.'
 
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