Dienstag, 26. Februar 2019

Knots in industrial slings

In a blogpost somebody asked:
"Roundsling question: The datasheets specifically forbid knots of any type (i.e. to strop two slings, or shorten a sling), under any circumstances, regardless of the load, don't even de-rate it, just no.... why?"

My answer:


Here is a video I took.

The slings have a WLL of 2 tons. 
 
By the way ... here is a video where I tested industrial slings in a girth hitch. It were 2 destroyed slings from a previous test, each knotted together.
Again ... the knot had no influence.


Rules (according to my tests, not statistically approved):

1. If you can open a knot after a sling being loaded ... relax, it was safe.
2. If the force in the sling is divided in two branches, a knot is no problem, because it has about 50% reduction.

Only if the knot is directly loaded, it reduces the breaking load.


Freitag, 1. Februar 2019

Is Friction in a knot dangerous? No. Its a modern urban legend


Recently there was a discussion on facebook about this knot:

One reason was mentioned why to put a thimble where you could just do this:

"Rope on rope equals friction. That friction generates heat and that can be devastating to the rope integrity."

If you load this knot then the rope may cut/melt through the loop. 

Ok, My approach is: Test it.

I built a Cyclic load  machine:


It looks very complicated but it loads the knot with 1 kN.

After 4.000 times loading I did the breaking load test.
 

 It broke at 20,58 kN.

As a comparison I tested an unused rope. It broke at 22,24 kN:

I decided to do the ultimate "fatigue" test. I cyclic loaded the knot between10 kN and 1 kN.

I checked the rope after 1200 cycles:


They look damaged, but the core is not visible.

At 1500 cycles I stopped. 




Although the core was not visible I think everybody would have discarded the rope.

Then I tested the breaking load:
 21 kN.

So there was no difference between a new rope and this rope.

It is a modern urban legend that the rope will become damaged in this application only by loading it in rope access.
 











Fatigue test with cable clamps axial loaded

In a previous post I tested cable clamps (EN 13411-5-1) in axial load.
Now I wanted to find out what happens when cyclic loaded. 
I choose a load of 15 kN because it is 2,5 times the allowed force that may occur in a safety system of a ropes course when a climber falls. 
In fact, there are 3 kN maximum, so we have a safety factor of 10.

Very roughly speaking:
The fatigue resistance of steel is about 50% of the breaking load, that means, if a clamp breaks at 30 kN it should be resistant to fatige if loaded with 15 kN.

Here is my assembly. I tested 3 clamps. As a side effect I also tested some steel wire terminations and aluminum carabiners.

The two carabiners failed after 8.200 and 8.700 cycles. The MBL is 35 kN. After

 
The cable clamps did not react.

I did the breaking load test:

Breaking  load was
32,84 kN, 37,38 kN, and 44 kN.

This is like uncycled clamps.



Mittwoch, 30. Januar 2019

cable clamps - in other than intended use

Is it ok to use a cable clamp this way?
It seems to be a good solution but it is definitely "not intended use".

Clamps should be used THIS way:

As a matter of fact we have many clamps which are used this way since many years. Now I wanted to find out: What is the breaking load of a 10 mm clamp in axial load?

First a rough calculation: The steel quality should be minimum property class 5.8.

A clamp for a 10 mm cable has a bail with two M 8 screws which should be able to hold almost 2 tons each side (M8 has 19.000 N breaking load). This means: The breaking load should be minimum 3.8 tons.
Considering that the clamp will be wedged, crooked etc., still we should be in a range of more than 3 tons. Thats the theory.
That sounds a lot, considering the size of the clamp ... its so small.
I tested in this application:


And indeed, the breaking load was in all cases more than 30 kN. This was the weakest clamp (33 kN).



I tested 10 clamps:
All of them more than 3 tons.


The axial loaded applications have to hold less than 6 kN (worst case, normally max. 3 kN) In this case they would have a safety factor of 5 minimum.
Seems enough to me.

Dienstag, 25. Dezember 2018

Merry Xmas with ... cable clamps




Cable clamps are a very common cable termination used in ropes courses, adventure parks etc.
Here is a typical application, an eye, clamped, with a thimble.

But there is a very special application which is not mentioned in the Eurostandard for CLAMPS (EN 13411-5) but only in the standard for Ropes Courses (EN 15567):

In case you have to "tie" two ends together, make a cable longer etc., you should do like this:


Here is a video about the tests.

Interesting is to apply different torque. It says 9 Nm in the standard, but I could not find out why we should not apply a higher torque to the clamps. I only hear assumptions, but no facts.

Here is the slipping force of 1 clamp, different torque.



 It is obvious that the higher the torque the higher the force they can hold before slipping.

I asked the DIN, the German standardization institute, but they never answer.

So I make a bold statement here:
9 Nm are a modern urban legend. It is better to apply about 18 Nm, maybe even 20 Nm.
I could not find a reason why not.
The discussion os now open.

Freitag, 16. November 2018

Again webbing ... Be careful!

I received some webbing ... Some runners from climbing walls and some material retrieved from the mountains.
First lets look at the runners not exposed to the outdoors for a long time:



The weakest had signs of abrasion.








But still they held more than 10 kN which is enough for all loads in climbing halls.

The conclusion: As long as you replace runners with obvious signs of abrasion you are on the safe side.

Now comes the bad news.

In the material were some slings exposed to the outdoors. Retrieved from frozen waterfalls, fixed slings in walls etc.
For example this sling, exposed for 10-15 years on a crag. It was yellow once:






 6,3 kN are not very much. We are getting closer to the loads that are possible in climbing.

The next sling: On a belay ...


Breaking load only 4,02 kN. Here its getting dangerous, for rappelling it may be enough. But consider that Polyamide gets weaker when it is wet.

Now my negative record:


Breaking load only 3,8 kN.

We have a couple of documented accidents, when climbers used such slings for rappelling or lowering down the partner, and the slings broke under body weight without impact of a fall.


I could not find any rope which was too weak.
An example:
A rope in a frozen waterfall on a belay:

Inside static ropes you find sometimes a stripe where you can identify the rope.
This one is a Teufelberger semistatic rope
It was produced 1998, that means: 20 years old.


 It held 19,48 kN.

My conclusion:

I strongly recommend:

Do not trust slings exposed to the outdoors. They may fail under body weight.

I would even say: Just do not buy them. 

There is better stuff available, core/sheath constructed slings, or take your oldest rope and cut off a piece.

















Sonntag, 21. Oktober 2018

Comparison of 6 drops until fail

Here are the curves of 6 drops of an old semistatic rope.
Drop test according to the standard. 2 meters with 100 kg. Repeated until fail.
The rope failed in the 6th drop.

I tried to scale it and adapt it so it is a bit comparable.

The energy is always the same (in the first 5 drops). The peak is increasing as the elasticity plus friction in the knot goes down.

In the 6th drop the peak reached the breaking load of the rope. Some of the energy "landed on the ground"

Here is another rope: 5 falls, no fail. Similar pattern.