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Partner cracklover


Feb 18, 2011, 9:30 AM
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DIY gear force test
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The subject of recreational climbers wanting to do a simple test of gear strength has come up many times over the years, so I thought I'd share a little on the subject here in The Lab forum.

Why test your gear? For me, the issue was around CCH Aliens, and their well documented issues with QC. I'm not interested in rehashing all that here, but suffice it to say that it made me question my assumption that the published strength rating could always be trusted.

For a few years, it didn't get much further than that - just kind a general unease, with me mulling over various possible means of testing the gear's strength. What I wanted to do was to test my Aliens to a point that simulated a small but significant fall, say 4-5kN. Not enough to compromise the integrity of the piece, but enough to push them beyond the point where the ones with faulty brazes failed in the field.

I considered seeing if I had any friends who could sneak me into a lab at MIT (I lived in Boston at the time) with a force meter. I couldn't think of anyone, so cross that off the list.

Next idea was to "bounce test the crap out of them". Okay, but what does that mean? How much force am I putting on my gear if I stand in my aiders and jump up and down on the piece. I needed a way to calibrate the maximum force that I could generate. I realized that there is one piece of gear I own that is in fact a finely calibrated force-meter - my Screamers. The Screamer is designed and produced so that, as long as there are still stitches left to blow, the force will never exceed around 2kN. I figured I'd start by bouncing on one, and if the stitches blew too easily, I'd put two in parallel. I was amazed by the result.

Turns out that, on an overhang, bouncing as hard as I could in my nylon aiders, I could not blow a single stitch in a Screamer. Now I'm only 150lbs, so YMMV, but this went against a lot of "internet wisdom" about peak force from a bounce test. Anyway, it was absolutely definitive, so I had to scratch that off the list.

So that raised the question: how do I generate more force? As climbers, we know the answer, of course - all the things we're taught *not* to do if we don't want to rip gear! Fall farther, use "static" material, and have a high fall factor. Well I'm sure as hell not going to subject my body to that, but I could use a backpack or a big rock and factor-two it using a spectra sling. That should generate more force for sure.

This raised a different problem though. Jumping up and down in an aider I figured I could train myself to do with around the same level of force each time by the screamer test. But dropping a big rock is a whole different deal. Calculating the force for each drop is impossible, and calibrating it using screamers would go through a lot of screamers, and that could get expensive. So how to get the exact same force for each drop, and how to make sure it didn't go too high and destroy my gear?

What I needed was a fuse. I found one, and after a little time to calibrate everything, my testing worked.

In my next post, I'll give the specific details of my testing.

GO


JimTitt


Feb 18, 2011, 10:29 AM
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Re: [cracklover] DIY gear force test [In reply to]
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It´s easier and very accurate to use a lever, either add weight (water is handy) or move the weight outwards.


wwalt822


Feb 18, 2011, 10:29 AM
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Wont these tests only be valid if your fuse has the same elasticity of whatever you plan to test? Look at the drop tests on spectra and nylon slings. It was the same test with very different peak forces.


Partner cracklover


Feb 19, 2011, 6:43 AM
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The test:

Put a spectra sling around a big rock (~40 lbs). Haul big rock above the cam with ~ 2 feet of static material plus fuse connecting them. Make sure the Alien is in a good placement that doesn't stress the stem. Drop the rock.

So the attachment is: rock -> spectra sling -> biner -> nylon fuse -> biner -> alien.

Then factor 2 the rock onto the alien. When dropped, the nylon fuse breaks, unless the placement was poor, in which case, the alien rips out. Or, of course, if there's a manufacturing defect, the cam itself would fail, but I haven't seen that yet.

The fuse I used in my first set of tests was: PMI 3mm Tech Cord. They rate it with a minimum breaking strength of 1.7kN. I made a loop of it tied with a double fishermans. If the MBS is exact, it should fail at just about 3kN. Though if there's a safety margin on the cord, It might put as much as 4kN of force on the cam before failing. It usually failed at the knot.

For my second set of tests I found a 3mm tech cord from Sterling, rated with an MBS of 2.6kN. A loop of that should get me a force of around 4.5-5.5kN. This is exactly the range I'm aiming for.

By the way, the biner on the cam takes very little abuse, but all the other gear takes plenty (because it hits the ground with the rock). After a few test rounds, with a big rock falling on it, the spectra sling and the biner attaching it to the fuse (which fall to the ground each time) were pretty beat up. After about 10 tests, the spectra sling finally gave up the ghost completely. So if you feel like playing along these lines, don't use your favorite gear. Oh, and the cams look great after the test. Certainly some fairly deep gouges in the lobes if they're set up against a crystal but nothing troubling.

Here's a video of one test session to see what it looks like in action (in slow-mo): http://www.youtube.com/watch?v=jMeGtWjmS54

GO


Partner cracklover


Feb 19, 2011, 6:45 AM
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wwalt822 wrote:
Wont these tests only be valid if your fuse has the same elasticity of whatever you plan to test? Look at the drop tests on spectra and nylon slings. It was the same test with very different peak forces.

No. The fuse pops, limiting the force on the piece of gear.

GO


Partner cracklover


Feb 19, 2011, 6:49 AM
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JimTitt wrote:
It´s easier and very accurate to use a lever, either add weight (water is handy) or move the weight outwards.

That's a cool idea, but I don't know how I'd rig up a lever like that in the field. I'd need a long steel bar, and the rock face would have to have a notch to fit the end of the bar in just the right spot under each Alien sized crack.

GO


JimTitt


Feb 19, 2011, 10:49 AM
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I used this to test some old in-situ gear, you dont need a notch you drill a hole in the end of the lever and fix a piece of gear below to take the upward pull. We were re-bolting at the time and could use a bolt below us and extended up with some slings and draws until the lever was horizontal.
The movable weight was me, a convenient way to find a self-transporting 90kg object!
You want a restraint on the lever doing it this way as when the gear lets go the bar hits you on the head!!

In the workshop we have a similar set-up with a large girder and some calibrated weights for checking the tester strain guages.

To get things like your fuses calibrated go to a lifting gear company, they´ll usually bust them for a beer or two in my experience and they have calibrated pullers, slightly less accurate but still good is get a friendly crane driver to bust them while watching his load indicator.

Jim


Partner cracklover


Feb 19, 2011, 5:45 PM
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JimTitt wrote:
To get things like your fuses calibrated go to a lifting gear company, they´ll usually bust them for a beer or two in my experience and they have calibrated pullers, slightly less accurate but still good is get a friendly crane driver to bust them while watching his load indicator.

Jim

Oh cool, thanks for the suggestion! Now I just need to find a lifting company.

GWink


jt512


Feb 19, 2011, 10:38 PM
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Re: [cracklover] DIY gear force test [In reply to]
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cracklover wrote:
JimTitt wrote:
To get things like your fuses calibrated go to a lifting gear company, they´ll usually bust them for a beer or two in my experience and they have calibrated pullers, slightly less accurate but still good is get a friendly crane driver to bust them while watching his load indicator.

Jim

Oh cool, thanks for the suggestion! Now I just need to find a lifting company.

GWink

Yeah, I get the impression from Jim's posts that you can walk down any street in Munich and hail down a crane and have them do your bidding.

Jay


(This post was edited by jt512 on Feb 19, 2011, 10:39 PM)


USnavy


Feb 20, 2011, 12:51 AM
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cracklover wrote:
If the MBS is exact, it should fail at just about 3kN.

A loop of that should get me a force of around 4.5-5.5kN.

I dont know that that statement is correct. Awhile back I did some testing of 1" webbing tied in a loop with a water knot. I tested it in a hydrulic pullback ram press with steel carabiners attached between the equipment and the webbing. The 1" webbing was rated for 19 kN. With the webbing tied in a loop as I described above, the loops of webbing failed at 22 - 23 kN, which is no where near double its rated strength. Interestingly enough the webbing never failed at the knot, it always failed at the carabiner. In once case the webbing started to tear at the knot about 15% of the way through, and in the end the webbing still failed at the carabiner.

I looked at the biner and there were no sharp edges on it. I swapped them out with others but still got the same result. I came up with a theory as to why this is but I have never been able to really research it so I am just guessing here. But I theorised that the webbing is failing,

a. only slightly above its rated strength vursus double its rating.

b. at the carabiner

because although the load is being supported by two pieces of webbing between the carabiners, at the dead center of the carabiners there is only one piece of webbing. Think of it this way, if you took a picture of a loop of webbing and drew a line straight through the middle in MS Paint, each side of the line would have a piece of webbing (two pieces total) which would share the load. But right where the line intersects the webbing, there is only one piece of webbing, there is nothing else to share the load, thus the loop is limited to the breaking strength of a single piece of the material your using.

It seems like this could be a viable theory as if you look at 1" sewed slings, they are normally rated to 22 kN. Well half that reason is because UIAA requires them to be hold 22 kN, but if creating a loop of webbing would effectively double its strength, those slings would hold a lot more than 22 kN, but they don't. I have tested them, they fail around their rating.

It would be nice if someone who understood the mechanics of whats going on in my scenario chimed in. Smile


(This post was edited by USnavy on Feb 20, 2011, 12:58 AM)


vegastradguy


Feb 20, 2011, 12:57 AM
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USnavy wrote:
cracklover wrote:
If the MBS is exact, it should fail at just about 3kN.

A loop of that should get me a force of around 4.5-5.5kN.

I dont know that that statement is correct. Awhile back I did some testing of 1" webbing tied in a loop with a water knot. I tested it in a hydrulic pullback ram press with steel carabiners attached between the equipment and the webbing. The 1" webbing was rated for 19 kN. With the webbing tied in a loop as I described above, the loops of webbing failed at 22 - 23 kN, which is no where near double its rated strength. Interestingly enough the webbing never failed at the knot, it always failed at the carabiner. In once case the webbing started to tear at the knot about 15% of the way through, and in the end the webbing still failed at the carabiner.

I looked at the biner and there were no sharp edges on it. I swapped them out with others but still got the same result. I came up with a theory as to why this is but I have never been able to really research it so I am just guessing here. But I theorised that the webbing is failing,

a. only slightly above its rated strength vursus double its rating.

b. at the carabiner

because although the load is being supported by two pieces of webbing between the carabiners, at the dead center of the carabiners there is only one piece of webbing. Think of it this way, if you took a picture of a loop of webbing and drew a line straight through the middle in MS Paint, each side of the line would have a piece of webbing (two pieces total) which would share the load. But right where the line intersects the webbing, there is only one piece of webbing, there is nothing else to share the load, thus the loop is limited to the breaking strength of a single piece of the material your using.

It seems like this could be a viable theory as if you look at 1" sown slings, they are normally rated to 22 kN. Well half that reason is because UIAA requires them to be rated to 22 kN, but if creating a loop of webbing would effectively double its strength, those slings would hold a lot more than 22 kN, but they don't. I have tested them, they fail around their rating.

It would be nice if someone who understood the mechanics of whats going on in my scenario chimed in. Smile

gabe's using cord, not webbing. completely different animal.


JimTitt


Feb 20, 2011, 1:08 AM
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Hey, I live in the middle of the high-tech boom area of Germany!
The nearest crane company is 1/2mile from here and lifting gear manufacturers that can test to 600tons are 1/2hrs drive. Within an hour there is everything possible on the face of the earth I guess including the biggest CE testing lab in the world.
For difficult stuff like electron microscopy and metal analysis I see a buddy and he whips it into the Lufthansa technical centre just down the road.
This isn´t like Minnesota!

Jim


JimTitt


Feb 20, 2011, 1:35 AM
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The tensile strength of tape is tested using large diameter clamping rollers, from memory something like minimum 50mm. From testing done for slacklines it is clear that with diameters under 10 times the thickness of the tape the strength drops off considerably. I´ve got a table somewhere from when I worked on this and normal nylon tube tape needs to be de-rated to about 2/3 of the original for a 10mm dia which you commonly find on karabiners and is the pin diameter used for sling testing.

Rope and cord strengths are tested in the same way but the pin diameters vary depending on the standard (ASTM D4268, EN919 etc) and other factors such as construction. Termination strength reduction is then applied for finished products.

It is also our experience that tapes fail at the karabiner not the knot, rope seem more prone to fail at the knot.

Jim


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Feb 20, 2011, 3:39 AM
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JimTitt wrote:
The tensile strength of tape is tested using large diameter clamping rollers, from memory something like minimum 50mm. From testing done for slacklines it is clear that with diameters under 10 times the thickness of the tape the strength drops off considerably. I´ve got a table somewhere from when I worked on this and normal nylon tube tape needs to be de-rated to about 2/3 of the original for a 10mm dia which you commonly find on karabiners and is the pin diameter used for sling testing.

Rope and cord strengths are tested in the same way but the pin diameters vary depending on the standard (ASTM D4268, EN919 etc) and other factors such as construction. Termination strength reduction is then applied for finished products.

It is also our experience that tapes fail at the karabiner not the knot, rope seem more prone to fail at the knot.

Jim

A pressure effect, then? I had a quick visit to Google U. - can only get abstracts from home, but it seems that Nylon-11 starts undergoing a phase transition to a lower melting (and hence presumably lower strength) crystal form at pressures of a few hundred MPa. Coincidentally (?) these are the sort of compressive pressures you'd see in a sling over a carabiner at 20kn.


Partner cracklover


Feb 20, 2011, 1:32 PM
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vegastradguy wrote:
USnavy wrote:
cracklover wrote:
If the MBS is exact, it should fail at just about 3kN.

A loop of that should get me a force of around 4.5-5.5kN.

I dont know that that statement is correct. Awhile back I did some testing of 1" webbing tied in a loop with a water knot. I tested it in a hydrulic pullback ram press with steel carabiners attached between the equipment and the webbing. The 1" webbing was rated for 19 kN. With the webbing tied in a loop as I described above, the loops of webbing failed at 22 - 23 kN, which is no where near double its rated strength. Interestingly enough the webbing never failed at the knot, it always failed at the carabiner. In once case the webbing started to tear at the knot about 15% of the way through, and in the end the webbing still failed at the carabiner.

I looked at the biner and there were no sharp edges on it. I swapped them out with others but still got the same result. I came up with a theory as to why this is but I have never been able to really research it so I am just guessing here. But I theorised that the webbing is failing,

a. only slightly above its rated strength vursus double its rating.

b. at the carabiner

because although the load is being supported by two pieces of webbing between the carabiners, at the dead center of the carabiners there is only one piece of webbing. Think of it this way, if you took a picture of a loop of webbing and drew a line straight through the middle in MS Paint, each side of the line would have a piece of webbing (two pieces total) which would share the load. But right where the line intersects the webbing, there is only one piece of webbing, there is nothing else to share the load, thus the loop is limited to the breaking strength of a single piece of the material your using.

It seems like this could be a viable theory as if you look at 1" sown slings, they are normally rated to 22 kN. Well half that reason is because UIAA requires them to be rated to 22 kN, but if creating a loop of webbing would effectively double its strength, those slings would hold a lot more than 22 kN, but they don't. I have tested them, they fail around their rating.

It would be nice if someone who understood the mechanics of whats going on in my scenario chimed in. Smile

gabe's using cord, not webbing. completely different animal.

And in my scenario (described in detail above) the cord was failing at the knot. This means the knot was the weak point. This means your assertion has little bearing on my test rig.

Another possibility that hasn't been mentioned is that the shape of your biner is causing the webbing to get pulled more on the outside(s), causing it to rip.

But, as VTG pointed out, it's a mostly unrelated discussion.

GO


(This post was edited by cracklover on Feb 20, 2011, 1:54 PM)


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