Loopie pull tests--importance of orientation
- Thread starter moray
- Start date
I have some new results which I report in a new thread .
In the other thread I presented experimental evidence supporting the idea that there is a right way and a wrong way to hook up a loopie in an open pull. To finish this off, I decided to test more or less the same arrangement that Brion Toss uses. As a close substitute for the Spectra he uses, I chose some 5/16-inch Amsteel Blue I had on hand. Like Spectra, this is made from high molecular weight polyethylene (HMWPE). I could find no instructions for making a loopie from this material, so I used the whoopie instructions instead. The adjustable eye in the whoopie calls for a splice length of 3.5 fids, or about 24 inches.
All tests were performed with the loopie horizontal, in the open loop configuration. Only one bearing point was on the spliced section, and it was always placed at either the throat end (appendix) or tail end of the splice. Both bearings were 3/4-inch steel shackle pins. In every test but the last there was lots of slack in the splice cover--the whole idea was to see what happened to the slack as tension was applied.
The picture shows the general layout of the sling, this one hooked up "right", prior to applying tension.
All tests were performed with the loopie horizontal, in the open loop configuration. Only one bearing point was on the spliced section, and it was always placed at either the throat end (appendix) or tail end of the splice. Both bearings were 3/4-inch steel shackle pins. In every test but the last there was lots of slack in the splice cover--the whole idea was to see what happened to the slack as tension was applied.
The picture shows the general layout of the sling, this one hooked up "right", prior to applying tension.
The tests of the "wrong" setup, the one Brion uses, were the most straightforward. As tension was applied, the sling immediately began slipping. The tail (red arrow in picture) steadily crawled into the splice, and there was at least some sling motion at both pins. The total amount of slippage was probably in the neighborhood of an inch and was much faster at low load. The test was stopped when the picture was taken at 2132 lbs. of tension. The black arrow points to the splice cover, which is completely slack.
In this second photo we see the rest of the splice. The sling is tensioned to over 2000 lbs., but the splice is slack and doing nothing.
In this second photo we see the rest of the splice. The sling is tensioned to over 2000 lbs., but the splice is slack and doing nothing.
The "right" setup for the open sling has the bearing at the appendix, as shown in the photo. As tension was applied, there was motion of the sling at both bearings, and the tail rapidly crawled into the splice. All motion slowed dramatically as pressure grew until, by about 2200 lbs., any motion of the tail into the splice had become imperceptible. The picture was taken at 2300 lbs. at which point the experiment was stopped. The red arrow points to the splice cover, which is extremely taut. On the other side of the pin the short bit of cover is completely slack.
This is the place to mention one final experiment using the "right" setup in which all slack had been milked out of the splice. As tension was applied, all parts of the system appeared motionless until the tension reached about 1500 lbs., at which point the free-hanging tail started to crawl slowly into the splice. As in the other experiments with the "right" setup, this motion could no longer be detected after about 2200 lbs.
This is the place to mention one final experiment using the "right" setup in which all slack had been milked out of the splice. As tension was applied, all parts of the system appeared motionless until the tension reached about 1500 lbs., at which point the free-hanging tail started to crawl slowly into the splice. As in the other experiments with the "right" setup, this motion could no longer be detected after about 2200 lbs.
What prevents the "wrong" setup from pulling apart? Not the splice. It is hard to imagine that the splice, even if it started out with no slack, would ever produce much friction between cover and core.
With a non-functional splice, there remain 3 sources of friction to resist the sling pulling apart. The picture shows two of them--the actual bearing point (pink arrow) and the core exit point (red arrow). The bearing at the other end of the sling, not shown, also creates significant friction. For slippage to occur, core has to enter the splice at the red arrow and pass through the cover at the pink arrow. In addition, since slippage means the sling is lengthening, there must be slippage around one or both of the bearing pins. There is virtually no friction to resist core movement through the slack splice cover (black arrow).
With a non-functional splice, there remain 3 sources of friction to resist the sling pulling apart. The picture shows two of them--the actual bearing point (pink arrow) and the core exit point (red arrow). The bearing at the other end of the sling, not shown, also creates significant friction. For slippage to occur, core has to enter the splice at the red arrow and pass through the cover at the pink arrow. In addition, since slippage means the sling is lengthening, there must be slippage around one or both of the bearing pins. There is virtually no friction to resist core movement through the slack splice cover (black arrow).
I have not devised an experiment to determine where the major source of friction lies, but the pictures may give a clue. In the first picture we see the bearing point after a pull of a little over a ton. Note that the splice has been severely flattened to the point it is about the same size as the undisturbed piece of rope to the left. One imagines it would be exceedingly difficult to pull the core through such a constricted space.
The next picture shows what happens to the tail as it is pulled into the splice. In the photo the tail has been pulled out after test in which the tension reached about 2500 lbs. Inside the splice the core (red arrow) is highly compressed. But as the tail crawls into the splice, the rope builds up a significant shoulder (green arrow) that appears to offer significant resistance to further crawling. When I try to pull the tail into the splice by hand after an experiment, when everything is slack, the resistance is still quite noticeable. It has to be very much greater when the splice cover is under great tension.
The next picture shows what happens to the tail as it is pulled into the splice. In the photo the tail has been pulled out after test in which the tension reached about 2500 lbs. Inside the splice the core (red arrow) is highly compressed. But as the tail crawls into the splice, the rope builds up a significant shoulder (green arrow) that appears to offer significant resistance to further crawling. When I try to pull the tail into the splice by hand after an experiment, when everything is slack, the resistance is still quite noticeable. It has to be very much greater when the splice cover is under great tension.
What do we conclude from all of this? There is only one significant difference between the "right" and the "wrong" configurations of the open loop loopie. In the "right" arrangement the splice cover is taut and performs its job of gripping the core and transferring load from cover to core over the length of the splice.
In the "wrong" configuration, the splice does nothing or next to nothing. All transfer of load from the cover to the core must take place at the bearing point or at the tail exit point. Evidently, in HMWPE, these supply a substantial amount of friction. These same exact sources of friction are present in the "right" hookup, but in addition we have a nice long working splice.
Even though I think using a loopie in the open configuration is dicey in the first place, for anyone who wants to do it AND feels they need to hook it up "wrong" I have the following question: would you still use the sling if the spliced section was only 3 inches long?
In the "wrong" configuration, the splice does nothing or next to nothing. All transfer of load from the cover to the core must take place at the bearing point or at the tail exit point. Evidently, in HMWPE, these supply a substantial amount of friction. These same exact sources of friction are present in the "right" hookup, but in addition we have a nice long working splice.
Even though I think using a loopie in the open configuration is dicey in the first place, for anyone who wants to do it AND feels they need to hook it up "wrong" I have the following question: would you still use the sling if the spliced section was only 3 inches long?
- Location
- Port Townsend, Washington
Hi again,
I tried the "right" way, but had to call it off and repick the "wrong" way. Bear in mind, I utterly agree with your conclusions as far as they go, but the weight of the coil of unsused running part kept pulling slack into the splice -- remember, it is up in the air in our use. That, plus at least with the Spectra we use, I've found that slack does not get into the splice if it is carefully massaged first. So I'm left thinking that I can't keep using the Yippie the way I have been, but can't use it the other way either. Am considering making up a series of fixed-length pendants, suitable for different masts.
Also, will be working with New England Ropes in the near future, on this and other matters.
Fair leads,
Brion Toss
I tried the "right" way, but had to call it off and repick the "wrong" way. Bear in mind, I utterly agree with your conclusions as far as they go, but the weight of the coil of unsused running part kept pulling slack into the splice -- remember, it is up in the air in our use. That, plus at least with the Spectra we use, I've found that slack does not get into the splice if it is carefully massaged first. So I'm left thinking that I can't keep using the Yippie the way I have been, but can't use it the other way either. Am considering making up a series of fixed-length pendants, suitable for different masts.
Also, will be working with New England Ropes in the near future, on this and other matters.
Fair leads,
Brion Toss
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