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Old 27-07-2016, 00:01   #166
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Re: Unveiling Bullseye Strops (eg for low friction rings)

Quote:
Originally Posted by Alan Mighty View Post
Being a slow thinker, I took a while to remember my experience with what Evans and SWL are calling 'throat angle' and referring to in ratios.

It's an angle for which I cannot remember a dedicated name in wire rope rigging, but is effectively the exit angle of wire from a thimble or sheave (in the odd case when a wire is made temporarily around a sheave).

And the rule of thumb I was taught for the thimble exit angle is nothing smaller than 60 degrees. Since tan 30 is 0.57 then if my understanding is correct you should aim for a 'throat angle' never less than 1.14:1.
No, I am no mathmetician, but I think a 60 exit angle for a ring is equivalent to a throat ratio of 1.5 : 1 (see my earlier rough sketches for what this ratio refers to).

The 60 is actually what we are calling the throat angle.
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Old 27-07-2016, 00:31   #167
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Re: Unveiling Bullseye Strops (eg for low friction rings)

Obviously from all these discussions physics is not my forte and I am an absolute novice in the area of load testing (just progressed from crawling to toddler stage, a really dangerous time ).

Given Evans's wealth of knowledge and experience in this field, I bet dozens of people reading this think I should keep quiet and not disagree with anything he says or raise new issues.

That is not how advances are made though. I know he is not working much in the load testing area any longer, but that does does not mean he will never return to this (although his load testing document covers a massive amount) and something I say may spark new ideas for him, or for someone else. In turn, I am learning at an exponential rate from all the errors and frequent stupid comments I am making. I think it is very beneficial to have discussions like this. I thank Evans for the time he is spending sharing his knowledge and expertise.
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Old 27-07-2016, 01:09   #168
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Re: Unveiling Bullseye Strops (eg for low friction rings)

EDITED TO ADD: in a Bullseye the throat angle can actually be less to restrain the ring, as the method of restraint is via the combination of the two loops, not their actual throats, so ignore this. I think throat ratio can actually be a bit greater than 2:1.

A 2:1 throat on its own will not actually perfectly retain a ring with the type of flanges the Wichard LF rings have. The ring can be forced out, although not easily if only the throat is held. It needs to be less than 2:1, with greater associated system strength losses.

What throat angle is needed to retain the ring with flogging?
Well it depends on the size of the flange relative to the bit the line sits on.

The size the little loop at the end needs to be before the ring comes off is complex to calculate mathematically, as it is not the difference between the diameter of the bit the little loop clenches and the total diameter of the LF ring (the line moves obliquely in the flange as the little loop is being forced off).

By the way, if you are using fingers to do this, it is less, as two fingers can spread the throat. A 2:1 throat on the Wichard LF ring comes off pretty easily doing this.

I have just had a bit of a rough play by using a constrictor knot on some 6mm dyneema with a Wichard ring: 25mm inner diam, 35mm outer diam of the central bit, 60mm overall diameter, ie overall diameter).

I looped the dyneema around the ring, tied a tight constrictor knot in the dyneema using slightly slippery waxed twine, then forced the ring off by gripping the ring and the base of the throat. The constrictor knot slid down, enabling me to do this.

I put it back on the ring, applied a little load to the legs to tighten the system up, photographed it and took measurements from the photos:
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Old 27-07-2016, 01:36   #169
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Re: Unveiling Bullseye Strops (eg for low friction rings)

EDITED TO ADD: throat angle can actually be less to restrain the ring, so ignore this.
I think throat ratio can actually be a bit greater than 2:1.

Using the above photograph where the diameter of the open bit in the middle in the ring is 25 mm, I calculated the following:

The throat ratio is 1.8 : 1.
The throat angle is 45

Could someone please check my calculations?
It is easy to make a simple mistake.

SWL
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Old 27-07-2016, 02:23   #170
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Re: Unveiling Bullseye Strops (eg for low friction rings)

Quote:
Originally Posted by Seaworthy Lass View Post
No, I am no mathmetician, but I think a 60 exit angle for a ring is equivalent to a throat ratio of 1.5 : 1 (see my earlier rough sketches for what this ratio refers to).

The 60 is actually what we are calling the throat angle.
You are correct SWL. From Alan's link:



This is a ratio of 1.5:1 with a throat angle of 60

A ratio of 2:1 gives a throat angle of 39

Edit: And 1.8:1 is 45 +/- 0.5
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Old 27-07-2016, 02:58   #171
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Re: Unveiling Bullseye Strops (eg for low friction rings)

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Originally Posted by StuM View Post
You are correct SWL. From Alan's link:



This is a ratio of 1.5:1 with a throat angle of 60

A ratio of 2:1 gives a throat angle of 39

Edit: And 1.8:1 is 45 +/- 0.5
Thanks, Stu

The bit you added at the end helps a lot.
45 is what I calculated above (45.24 is the precise amount for a 1.8:1 ratio, but my ratio was actually 1.8057, not that I could measure to this degree so accurately from the photo)

I can progress from there and post the next bits now.

SWL
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Old 27-07-2016, 09:25   #172
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Re: Unveiling Bullseye Strops (eg for low friction rings)

Guys, I am struggling here with the basics. Some of it is thankfully becoming clearer.
It is slowly dawning on me that I have written some absolute rubbish regarding throat angle issues, but that really helped clarify things for me.

I hope the brief explanation I have added to the calculation of system strength loss in post #163 helps others struggling.
Feedback regarding this from someone with an understanding of the principles involved would be very appreciated .

None of this effects the Bullseye design, it is just me trying to understand the the load issues.

Final conclusion:
• The Bullseye loop strop will most likely break at the connection at the base. If two legs, not the base are pulled to eliminate bending losses, then I think it will break at the splice on a leg.
• The Bullseye soft shackle strop will most likely break at the stopper or noose depending on the soft shackle design used.

The strength of the Bullseye depends very much on equalising the load evenly. I have already posted tips on how to do this for the Bullseye loop strop (added edited notes in bold). I will do this tomorrow for the Bullseye soft shackle strop. The principle is the same - the length of the two lines between the locks either side of the weave needs to be identical.

SWL
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Old 06-08-2016, 02:22   #173
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Re: Unveiling Bullseye Strops (eg for low friction rings)

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Originally Posted by Seaworthy Lass View Post
I was looking at a few shots of low friction rings in use and found a pair lashed together in a photo Dockhead took on a racing Swan recently.

This is a close up:




It must be very difficult to distribute the load evenly with lashing like this, weakening the system.

The Bullseye weave can be used in this application as well.

Make a loop and twirl it around two rings to join them very snuggly together. Here I used 4 mm dyneema end to end spliced with a 72x line diameter bury tapered over the last quarter of the bury.
Predicted breaking load of this is a whopping 8000 kg! if the line strength is 2000 kg for the 4mm dyneema used.

The rings can still be removed, but only with a fair bit of difficulty (bring the rings together, tip one towards the other, pull on one of the loops going around one ring and eventually one loop can be forced over):

BULLSEYE TWIN RING STROP:



I have a better grasp now of how system strength can reduce in situations like this, so the original estimate of 8000kg I made above is incorrect. Losses in strength will occur for the reasons listed below. I think these losses are not cumulative, so the biggest one simply overrides the rest. I have added the following edit to the above post:

1. Although the throat angle here does not create any "peeling" force (no side load at the crossing), the angle of deviation of the line from the primary angle of pull will reduce strength.
The formula I have come up with for system strength efficiency due to this
= cos (throat angle / 2) x 100%.

The throat angle you would use is the greatest one in the system, as this is what will be limiting things.
I put a protractor on the photo and estimated the greatest throat angle. This seems to be around 60.
So estimated system strength efficiency = cos (60/2) x 100 % = 87%

2. Bending losses will also occur. The diameter of the dyneema (d) compared to the diameter of the portion of the LF ring it is going around (D) need to be taken into account. In the example here the diameter of the line is 4mm and the ring 35mm therefore D/d = 8.75.
From the graph given in Evans's load testing document, the system efficiency is therefore around 83% due to the bend around the ring.

3. Losses in strength will also occur due to compression and bending of the dyneema at the crossings. Evans Starzinger has pointed this out a few times in both this thread and the one on Antal whipping. Also Brion Toss has said:
"Although fantastically strong in tension HM fibers are quite weak in compression. Since knots invariably compress the rope under load, it is clear why knots typically weaken HM rope by at least 70%."
The losses here due to compression are not known, but I think are unlikely to be anything similar to actually knotting the dyneema.

So, if the above is correct (any structural engineers here that could confirm this?), then the system efficiency is being reduced primarily by the bending of the dyneema going around the ring, therefore it would be around 83%.

So system strength here would roughly = 4x 2000 x 83% kg = 6640 kg
Still a whopping amount .
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Old 06-08-2016, 07:21   #174
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Re: Unveiling Bullseye Strops (eg for low friction rings)

SWL,

For the 2 loop strop 17% reduction in the 4X strength sounds about right. It might be off a little but not enough to affect the safe working load. That's why safe working ratios were invented. Sometimes it's too hard to accurately compute the actual strength so a safety factor papers over the estimated margin for error (and the unknown).
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