Author Topic: testing long-splices  (Read 8599 times)

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testing long-splices
« on: March 28, 2009, 01:02:38 PM »
Hi all

Long-splices - testing them.  For fibre laid 3-strand rope.  Can't
work out how to do it.  Static test is readily done - lift a known
mass in gravity.  Happy with that for short-splices.  Done it for a
long-splice.  But not enough for long-splices because you do a
long-splice when the rope must pass over a pulley.  So it follows that
a realistic test includes durability of the splice as it passes around
pulleys.  A "dynamic" test.

Have access to overhead cranes with capacity to 5Tonnes SWL.  But
can't get away with using for a long time.  Usually practice with 10mm
3-strand cheap polypropylene (pp) cut-film rope.  Not British
Standard, but if it was, minimum strength for pp. would be guaranteed
1.5Tonnes-force (15kN).

Was trying to think of a rig with a beam and two pulleys - essentially
one at each end.  The long-splices you are testing would need to form
one piece of rope into a strop - a loop of rope, as in a drive-belt.
Obviously, you are testing the finer variants of long-splicing
techniques for their general properties, not a specific splice between
two ropes.  But couldn't arrive at a design where I knew I'd "got it",
by reason of a design of beautiful simplicity emerging.  Beam and two
pulleys - simple with fixed (though adjustable) distance between
pulleys useful for a start.  How to simulate jolting and rough
service?  Making one of the pulleys with its shaft off-centre in
relation to the rim is good enough?

Struggle was to work out how to maintain tension for prolonged test.
Could stop the test to re-tension frequently.  Would want to
frequently visual inspect condition anyway.

The point???  Realistically, how durable and strong are these
long-splices?  How many lays of the rope should you splice, in
reality?  And which of the several variants of tucking the 3 pairs of
strand-ends - outboard of the overhand knots - works best in a given
situation?  And does the optimum vary with pulley size?

You want a splicing specification with proven characteristics
including satisfactory inspection intervals.

"don't re-invent the wheel" - has this sort of testing already been
done?

Is there a good testing method or testing rig?

Thanks in advance

Dan_Lehman

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Re: testing long-splices
« Reply #1 on: March 28, 2009, 09:19:24 PM »
The test might be to do TWO things:  condition the splice through
accelerated normal or higher-tension-than-normal usage; and then test
the splice for strength in the usual way.

--dl*
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Re: testing long-splices
« Reply #2 on: March 29, 2009, 08:34:14 AM »
Thanks Dan
This conditioning...  Would you tell me about this?
Help with another question?  Do you need more tucks "outboard" of the 3 overhand knots for (slippery) synthetic fibre rope?


Fairlead

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Re: testing long-splices
« Reply #3 on: March 29, 2009, 06:11:43 PM »
A couple of points that might help:
When first tying the three overhand knots in the strand ends, do not pull them up with too much force - then secure one side and attach the other side to a ratchet winch (comealong?) and put the splice under just enough tension to pull the rope taut.  Now check that each strand is bearing an equal weight by adjusting the overhand knots one at a time. Release the tension and make your splicing tucks with the ends (I like the halved ends method myself).  I feel the most critical part of your long splice under test, is going to be getting this balance right with each strand under exactly the same tension which is going to be virtually impossible - so you will always have a variant in each splice, not matter how small.
For testing with pully wheels - Remember the sheave diameter should be not less than 10 times the rope diameter.  If you have access to cranes, you may well find a few 'Cargo' or 'Coaling' Gins on site, as builders tend to use them for hoisting buckets etc on scaffolding.
Good luck - I will be following this test with interest

Gordon

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Re: testing long-splices
« Reply #4 on: March 29, 2009, 09:43:02 PM »
Thanks for the detailed ideas and concepts, Fairlead.  You've made me feel a challenge to make these tests happen!

Dan_Lehman

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Re: testing long-splices
« Reply #5 on: April 02, 2009, 01:56:57 AM »
Thanks Dan
This conditioning...  Would you tell me about this?
Help with another question?  Do you need more tucks "outboard" of the 3 overhand knots for (slippery) synthetic fibre rope?
Well, the "conditioning" was put in italics to suggest a somewhat offhand use of
the term, but what I mean is simply giving it (as best you can figure) some bit
of concentrated usage--not radically different from what's expected, but of
some degree to try to draw out any weaknesses of the structure (such as
incremental slippage-adjustment of the wedded strands.

FYI, though I've not put such a thing to much practical usage,
I've taken the short-splice mechanism into the long splice start:
i.e., once the initial staggering and matching of strands of the
long splice was done, I then separated the yarns of the mated
strands into triplets and short-spliced them into the whole rope
(as opposed to trying to fatten one strand with its opposite's yarns,
however that might be done!).
Now, perhaps this sort of thinning of a strand will not have much
durability in a pulley situation; but does look neath & thin whe straight.
(Hmmm, there might be some compromise methods in which
the mated strands became two part each vs. three, giving more
meat to each, and then sailor-tucked.

 :)

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Re: testing long-splices
« Reply #6 on: April 07, 2009, 04:59:41 AM »
Thanks for guidance, suggestions and help.  Won't promise I can do
these tests.  Have "static" tensile tested some rope and splices -
didn't expect that to come along!  For 10mm polypropylene 3-strand
cut-film "builder's merchant rope", found eye-splices (4 tucks) gave
full-strength end terminations around the 12.7mm pins which would
normally lock the jaws of the testing machine into place.  The rope
broke in the middle at 13kN (1.3Tonnes lifted in gravity).  So seen
for myself what is commonly claimed: eye-splicing gives full-strength
terminations.  Tried testing a short-splice.  Wasn't a good one - bit
manky and (only(?)) 5 tucks - and the eye-splices were no better.
Something started slipping at 8.7kN (870kg in gravity), holding that
load until the testing machine ran out of moving head travel.  After
this test the rope was something like 10% longer and thinner.  And
felt very "hard".  Which presents a problem for testing.  Need to
pre-stretch samples made deliberately short with rope in "normal"
condition.  Implies same problem would be with me if ever did putative
"dynamic" testing of long-splices around a pulley arrangement.
Already know in rough tests using crane that my long-splices will
static-lift some hundreds of kilos - so into the "stretch" zone?  And
also implies need to investigate if there is a "recovery" towards
previous length and if so how long it takes - before re-applying load
simulating service, then storage, then service, then ...  The
short-splice loaded to 8.7kN then released to zero load was quite
altered in shape afterwards, with a very tightened-up self-gripping
short lay in the middle transitioning to loose-locking extended lay at
the two ends of the splice.  So would have to see what became of the
long-splice.

[Inkanyezi] gone

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Re: testing long-splices
« Reply #7 on: April 07, 2009, 10:17:41 PM »
There is a factor in PP that many tugboat skippers know of, which decreases tensile strength uncontrollably.

When you apply load to it, it takes up energy, and its heat dispersion is very slow because it is a good thermic isolator, so the rope heats up in the middle, in practice, if it is a large rope, it may even melt the film fibres, also if it is water-soaked. The reason of course is slow heat transfer through the thickness of the rope.

Therefore, when testing PP, with repeated jerks (as when towing with too much force in high seas), you get far less tensile strength than if you apply the load slowly, slow enough for the heat that is generated by the energy that the rope takes up to dissipate to the environment. By applying load slowly and only once you arrive at a higher tensile strength than if you do it faster and repeatedly at short intervals.

Of course small stuff does not suffer as much as large stuff, due to its relatively larger cooling area for a given cross-section area as well as its shorter distance to the outer environment, which probably is the reason that the effect has been discovered by tugboat people.

The same of course may apply also to other fibres, but PA for example does not take up the energy in the same way, it is loaded elastically in the rope to a large degree and released when the load is eased. PA also withstands higher temperatures.

The different way of taking up energy makes PP preferrable over PA, because when a PA line breaks, the energy is elastically loaded in the rope and it lashes back as a rubber band, cutting through anything that's in the way, while a PP line will fall dead when it breaks.
« Last Edit: April 07, 2009, 10:30:07 PM by Inkanyezi »
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Dan_Lehman

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Re: testing long-splices
« Reply #8 on: April 08, 2009, 05:53:33 PM »
Many commercial-marine ropes are of a combination of PP & PES
(or of some CoEx (PP/PE) + PES).  Maybe 8-plait braids offer some
reduction in core-to-surface distance?
 :)


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Re: testing long-splices
« Reply #9 on: April 10, 2009, 05:17:36 PM »
Inkanyezi, Dan

That's amazing information!  So for towing PP is preferred over PA...
Wouldn't have guessed that given for lifting PA is generally referred
to as a far superior material to PP.  Doesn't knowledge prove to have
layers upon layers of depth...?!