Pull Test--Ice Tail Splice

[pdqdl]

Here is an idea: put your load cell on the chained side of the hydraulic cylinder (instead of the spliced side). Pre-tension the whole rig to a measured amount of tension, then subtract the initial tension from the breaking tension. When you break the test rope, you may wish to compare the load on the pre-tension line just to compare for changes.

This might produce some strange results, or it might stabilize the rig for repeated tests.

Does your load cell just produce a single number that you must read, or could you plot a graph of its numbers as the rope breaks? It would be really interesting to see a chart of force vs. time on the rope breaking.

 

[moray]

OK, for the last set of tests I did move the load cell. The components are in this order: chain, load cell, cylinder, experiment, chain. The load cell is now out of harm's way.

The load cell has an LED readout that I watch as I pump up the pressure. It also has a peak hold feature so that it continues to show the highest reading until you reset it. This is very handy for my purposes.

If I were to try to add some sort of data logging feature, I would want to see force vs. distance, but then I would need to simultaneously accurately measure distance. Why force vs. time?

 

[pdqdl]

1. If you were plugged into a computer, it would be easy. The computer would already be logging the time.

2. It would reveal how long it takes for the rope to tear in half. "POP!", "RIP!" or "T E A R R R !" ?

I imagine that the rope (on such a non-dynamic load) would begin to tear, and the rope would consequently lengthen as part of the fibers begin stretch out of alignment, having been broken. This "lengthen during failure" of the rope might be great enough to match the stretch on your pulling system. If that occurred, I would hope to see a down-sloping graph as the rope lengthens and the load applied by the cylinder is reduced.

I guess you can just tell us, too. Which is the right word for how the rope breaks: "POP!", "RIP!" or "T E A R" ?

 

[pdqdl]

It occurred to me that distance is pretty easy too. Graduate (mark with calibrations) your cylinder with some indelible marker. Record distance vs force, and you will probably get a parabola, essentially plotting elongation of the rope.
(this assumes no air bubbles in your hydraulic system that will expand after the rope breaks)

That would be cool too.

 

[moray]

I think there is no question that the force vs. time curve, or for that matter, the force vs. distance curve will be a sharp sawtooth pattern. Each time the pump handle reaches bottom the rope will begin to relax and the tension will begin to drop. If I turn off the peak hold feature I will be able to watch the rope relax after each pressure pulse. I'll report back what I see.

I must say, even though it reflects poorly on my capacity for curiosity, this doesn't interest me a whole lot. The overall tension vs. distance curve (the stress/strain curve) DOES interest me, but I have to accurately measure distance or I can't get at it. Using a video camera and some sort of sliding tape measure might handle it if the camera could also see the tension readout. It wouldn't be hard to hand-plot the values and get a pretty good curve...

 

[moray]

Snubbing

Here's a picture from a test I did today.

The snubber rope, 3/8 in. Stable Braid, is wrapped around the test rope about 3 times. All the slack is in the other leg of the snubber, near the ground in the background. When the test piece breaks, the snubber quickly goes taut and the recoiling test piece is forced into a tight spiral as it flies toward the anchoring clevis off to the right. This has proved to be very effective: the recoiling rope is totally captured by the spiral snubber, and the snubber stops all the hardware from moving more than a couple of inches.

Right now I have to guess how much slack to leave in the snubber. I need a convenient way to feed slack to the snubber right up to the moment of breakage.

 

[pdqdl]

If your force is backing off between pump strokes, you have an internal leak in either your cylinder or your pump. The check valves in most pumps are pretty reliable; I would bet on the seals inside the cylinder.

Does the pressure bleed off completely (given enough time), or just drop some and then stop ?


Pretty good idea wrapping the rope around the test rope. Better visibility than a blanket.

I would just keep the snubber rope twisted around the test rope, go around the base of the tree and secure the tail with a rubber bungee. If you really wanted to control the length, you could tie off the tail to limit the total travel.

Kinda like a skilled groundman letting the rope run before stopping it.

So do those splices pop!, rip!, or t_e_a__r ?

 

[pdqdl]

I think you should take more pride in your handiwork. Show us some pics of the whole setup. Stretched between two trees is fine with me. If anybody gets smart about it, we can suggest they show us a better way, with testing their own splices and establishing their own test procedures included.


Homespun methods are fine by me: they used to call that Yankee Engineering. (there are other less auspicious terms in use also)

 

[moray]

If your force is backing off between pump strokes, you have an internal leak in either your cylinder or your pump.

No, the rope is relaxing. I could try pulling just the chain and show that the effect is greatly reduced.

So do those splices pop!, rip!, or t_e_a__r ?

None of the above. They BANG!

 

[moray]

...I pumped steadily on the hand pump, adding about 1000 lbs. every 5 seconds. After the gauge reached about 9800 lbs., the next pump stroke took the read-out out of limits, and the stroke after that broke the rope. Quite a maiden voyage for my new rig!

I believed all this when I reported it, but after nearly two months and many more tests, I am sure some of this is wrong. Here's the correction.

The actual maximum force is what the gauge actually said at the end--8000 lbs. I was reading the LED gauge in bright sunlight, and it was very hard to see. The 8000 lbs, after the break, was read by shading the gauge with a shirt. The runup to the final break is the part I should have left out. When I thought the gauge was going through 9800 lbs, it was certainly actually 7800 lbs. There was no out-of-limit condition, just an out-of-limit imagination.

Having since broken a few items in the 6000- to 8000-lb range, I am quite certain that even with my full weight on the pump handle I could not have achieved 9800 lbs. I would have had to change gears on the pump.

The actual point of the experiment remains entirely valid--the short splice held and the rope broke. I regret posting inaccurate information without also mentioning, at the time, that I had my own doubts about it. The doubt is now gone--it was wrong.

 

[pdqdl]

Nothing makes me like a fellow better than when he owns up to a mistake. Once again, you demonstrate that you are in the upper echelon of folks to admire.

Besides, I don't really care too much for someone that is perfect, and folks that try hide their faults are only fooling themselves.


Regarding Icetail: how hard is it to cut?

The Beeline I have been using is really p###ing me off. I can't cut it smoothly with ANYTHING. Freshly sharpened knife: it dulls in one cut. Box knife with new blade: Same story. Scissors: by the time you gnaw through a few strands, you have created a giant frayed mess. Sears razor & anvil cutters: nope - it won't squeeze hard enough.

I think a good sharp chisel and hammer pounding on some really firm material might do a good job of cutting this stuff.

 

[moray]

Thanks for being easy on me, pdqdl. Your original misgivings were entirely justified.

Ice Tail is a ##### to cut. My best scissors can't deal with it. I either hold a strand or two in one hand and use my freshly sharpened pocket knife in the other, or I hold the rope down against a cutting board and use the knife against that. Amsteel Blue behaves the same way.

 

[046]

how's about a picture of entire setup... would mind being able to duplicate your set-up. being able to pull to destruction produces very useful info indeed...

 

[outofmytree]

Sharp knife on a log is fine for beeline guys. Icetail I have no idea. Frankly I want my prussik cord to be hard to cut!

 

[Ghillie]

Electrical tape to keep it from fraying and a fresh utility knife on an old cutting board. I don't remember it being all that bad. Diferent but not that bad.

 

[pdqdl]

Sure, fresh utility knife will do it. To bad you have to get a new knife blade after two or three cuts. It's not that I can't cut it at all, I'm just tired of replacing all the blades.

 

[pdqdl]

Sharp knife on a log is fine for beeline guys. Icetail I have no idea. Frankly I want my prussik cord to be hard to cut!

As hard as it is to cut with a knife, it won't stand up to a chainsaw. I took my first splicing attempts with the Beeline and made a saw lanyard with it. I got careless one day, and it exploded into a fluff of "no longer attached" where the chainsaw connected.

That was the first lanyard I ever cut off, so I can't compare to more conventional ropes in that size bracket. I guess it put up as much fight as my 1/2 stable braid rope did on a different day in much different conditions.

 

[outofmytree]

I agree. Beeline is in no way saw proof even hand saws for that matter. Just hard to cut unless your knife is sharp and you cut onto something softer like wood.

 

[Slvrmple72]

Mallet, 1" carpentry chisel, electrical tape, and a big ol' chunk of wood. Press the chisel down hard and give it a good smack with the mallet. I get a nice clean cut everytime. Now when it comes to splicing.... I may have to make a trip to Ghillie cuz I am just not doing it right!

Moray, great posts! What hitch do you use when climbing?

 

[Ghillie]

Mallet, 1" carpentry chisel, electrical tape, and a big ol' chunk of wood. Press the chisel down hard and give it a good smack with the mallet. I get a nice clean cut everytime. Now when it comes to splicing.... I may have to make a trip to Ghillie cuz I am just not doing it right!

Moray, great posts! What hitch do you use when climbing?

I will have to try that. I need to splice some icetail for my girlfriends son. He's really interested in climbing and so far he seems to be catching on quickly.

Be glad to share any knowledge I have (or think I have) SilverMaple.