Fall factors

[RescueMan]

Re: Rope Stretch

Burnham,

Rope Stretch statistics are another area which could greatly benefit from some industry-wide consistency - as is evident from the various reporting methods you've shown.

The Cordage Institute, a national trade association, has set these standards for rope stretch:

static: <6% @ 10% MBS (since 10% is usually the SWL)
low-stretch: 6-10% @ 10% MBS
dynamic: >10% @ 10% MBS

On that standard, True Blue and ArborPlexis are aprox. 2.2% @ 10% MBS, which makes them very static ropes.

Compare those numbers to these kernmantle ropes:
Sterling nylon 1/2" SuperStatic - 8.5% @ 10% MBS
Sterling 1/2" high tenacity polyester - 2% @ 10% MBS

I can't make too much sense of Yale's numbers. I suspect the 50# at permanent elongation is a typo. But what those numbers do show is that while a new rope will stretch 2.7% with a 200 lb load, after being permanently elongated due to overloading it will stretch only 0.3% with the same 200 lb load. Which is why we don't use life-lines for bull ropes and then hang on them again.

- Robert

 

[ptar]

</font><blockquote><font class="small">In reply to:</font><hr />
Actually, the fall factor wouldn't be halved.

[/ QUOTE ]

Robert, reread my post. I said nothing about the fall factor.

 

[RescueMan]

ptar,

Reread the thread. I was merely contradicting my earlier statement based on your posting, which was nothing more than the obvious, but sometimes the obvious is what gets us thinking.

So, since this is a thread on fall factors, what is your conclusion from the info you posted? How does that effect the fall factor and the impact load?

- Robert

 

[knudeNoggin]

Re: Rope Stretch

RE:
------------------- by RescueMan -------------
&gt;
&gt;The Cordage Institute, a national trade association, has set &gt;these standards for rope stretch:
&gt;
&gt;static: &lt;6% @ 10% MBS (since 10% is usually the SWL)
&gt;low-stretch: 6-10% @ 10% MBS
&gt;dynamic: &gt;10% @ 10% MBS
&gt;
&gt;On that standard, True Blue and ArborPlexis are aprox. 2.2%
&gt;@ 10% MBS, which makes them very static ropes.
&gt;
&gt;Compare those numbers to these kernmantle ropes:
&gt;Sterling nylon 1/2" SuperStatic - 8.5% @ 10% MBS
&gt;Sterling 1/2" high tenacity polyester - 2% @ 10% MBS
------------------------------------------------------

There is a problem to these definitions of "static", etc.,
in that the relevant ropes can differ significantly in MBS
(e.g., Arbor-Plex, True Blue, &amp; Sterling's two are listed as:
--resp.-- 6_000, 7_300, 9_285, 10_180 pounds),
so the "stretch at 10% of MBS" figure becomes dissimilar to
the forces one is concerned about. I.e., given some force
of expected use, it will be maybe near the 10% of the
weaker rope, but well less in the stronger. (A thick and
definitionally "dynamic" rope might be "static" in effect.)
Within the narrow ranges of diameters often considered,
the scatter only goes so far. But it does show that the
definitions are a little beside the point (which is how much
will this rope stretch with THIS given weight (regardless of
what % that happens to be of the rope's MBS)?).

(-;

 

[RescueMan]

Re: Rope Stretch

</font><blockquote><font class="small">In reply to:</font><hr />
There is a problem to these definitions of "static", etc.

[/ QUOTE ]

There's no problem if you're using the ropes at or near their recommended SWL. These industry standards describe the "stretchiness" of ropes at their design load.

But, you're right that if you need a 1/2" rope for "gripability" but are going to load it at much less than the SWL, then you need to consider the stretch at that lower load.

- Robert

 

[NickfromWI]

Re: Rope Stretch

If you are quoting stretch of a line at 10% of MBS....My climbing line is like 7000lbs MBS. I am way less than two hundred pounds. So does the stretch only come in to play when I fall hard enough on the rope to put like 4 or 5 times my body weight on the climbing line?

love
nick

 

[RescueMan]

Re: Rope Stretch

</font><blockquote><font class="small">In reply to:</font><hr />
So does the stretch only come in to play when I fall hard enough on the rope to put like 4 or 5 times my body weight on the climbing line?

[/ QUOTE ]

YUP. In choosing a climbing line, you're more concerned with strength, abrasion-resistance, and enough stiffness to make it climbable. As Rocky and others have pointed out, taking big falls is not what you generally plan on.

Rock climbers use rope with much less MBS because they are concerned with having enough bounce to survive a long fall and limiting the stress on anchor points.

You could probably get by with a much weaker rope than what you use but it might not have the other characteristics (like enough diameter to grab onto) that you need.

I know of no cases of a climbing or life support rope that has broken from being overloaded. They get abraded or cut or shredded by sharp edges or high-frictional surfaces. So the ultimate breaking strength is probably the least important characteristic of a good rope.

- Robert

 

[knudeNoggin]

Re: Rope Stretch

&gt; There's no problem if you're using the ropes at or near
&gt; their recommended SWL. These industry standards describe
&gt; the "stretchiness" of ropes at their design load.

Hmmm, CI points out that design load / SWL are typically the
characteristic of the application, not the rope--rope vendors
always shy from giving such recommendations. And you will
likely choose a rope for a variety of reasons. My point was
that if there were some spec. for "static", among other specs
such as a dia for presumed resistance to abrasion, one might
find that a high-strength rope really would fit the bill,
except that IT wasn't "static" because at 10% of its (high)
MBS it has too much stretch--even though at the typical loads
of the intended application, it might fall within range.
(And, again, one might have to somewhat contrive a case of
this in current ropes, within the usual reasonable ranges;
but there's some effect to be found--just maybe usually not
so much to make a big diff.)

SO, the general remark of this thread that the data on
rope elasticity is too sparse shows that a user can have
some trouble making such choices wisely. (Sometimes the
elasiticity is graphed, up to 30% or more.)

--knudeNoggin

 

[RescueMan]

Re: Rope Stretch

</font><blockquote><font class="small">In reply to:</font><hr />
CI points out that design load / SWL are typically the characteristic of the application, not the rope--rope vendors always shy from giving such recommendations.

[/ QUOTE ]

But manufacturers design and sell specific ropes for specific applications. Both the Sampson and Yale catalogs list SWL for each of their ropes, and its always 10% of the MBS. This seems to be an industry standard. And they're not shy about publishing it.

I think we agree that the issue of "stretchiness" is a confusing one. Just what % of stretch is right for a particular application? Most people, even pros, won't know this. That's why it's more useful to look at the general classification of ropes into static, low-stretch, and dynamic.

At the same time, it's important for end users to understand that buying too strong a rope can be as dangerous as buying too weak a rope, depending on the itended use.

- Robert

 

[Tom Dunlap]

Re: Rope Stretch

Seveal years ago when I was beginning to learn about rope construction I went to TCI Expo with a goal of learning what "stretch" really meant. When I wandered into the Yale booth I had the opportunity to spend close to an hour talking with Dick Hildebrand about the topic. I got a good education in that time. Since there is no industry standard about how rope stretch is tested, the numbers really can't be compared between companies. They can be used as a generalization only.

Rope stretch in a climbing rope isn't one of the main criteria for choosing one over another. From talking with arbos it seems like there are other more important issues. This is an interesting topic though.

Tom