Tag Archives: Tension

More force testing on 5:1 systems

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This post follows up on some initial testing done on 5:1 mechanical advantage systems used to tension tyrolean crossings done a few months ago. I suggest anyone who has not read that report catch up with it here before reading on as I don’t explain everything again here.

For this batch of testing I used the same site but rigged things using metal strops instead of rope loops. This would act more like the solid bolt anchors used underground and would nearly eliminate false readings from knots tightening.

I used 2 types of readily available Type A rope

  • 11mm Mammut Performance semi-static
  • 10mm Beal Antipodes / Industrie

The tests were repeated with 3 different progress capture devices

  • Brand new Petzl Stop (rigged both fully and half threaded)
  • 10 year old worn Petzl Stop fully rigged
  • Brand new Petzl RIG

I created a 5:1 system on 10m section of rope using a Petzl Ascension jammer, Petzl Tandem pulley and a Petzl Partner pulley. These are all items that would likely be used by leaders underground or of similar type. No big rescue pulleys or prussics.
I pulled all of the tests on my own with un-gloved hands. I weight approx. 90kg and pulled as hard as I could using just hand grip.
The final tension in the line was estimated by hanging off it and the force on the jammer ascertained using a Rock Exotica Enforcer load cell measuring in kN.

11mm rope

New Petzl Stop – fully rigged
2.06kN
2.10kN
New Petzl Stop – half rigged
1.96kN
2.04kN
Old Petzl Stop – fully rigged
2.04kN
2.04kN
Petzl RIG – belay mode
1.94kN
2.08kN

10mm Rope

New Petzl Stop – fully rigged
1.88kN
1.88kN
New Petzl Stop – half rigged
1.70kN
1.82kN
Old Petzl Stop – fully rigged
1.98kN
1.92kN
Petzl RIG – belay mode
1.78kN
1.80kN

There clearly was a drop off in force required to tension a 10mm system over the 11mm system, although only small. The fully rigged Petzl Stops required the highest force to tension although the old Stop in the 10mm test oddly required more than the new one (*see foot note).

I took the highest force generating configuration and added some more people to the pulling end.

11mm rope with a fully threaded brand new Petzl Stop

2 smaller adults pulling
2.00kN
2.22kN
2.34kN

2 small adults & myself pulling
3.56kN
3.24kN
3.54kN

I think it is entirely possible to exceed the 4kN figure if 3 large and/or strong adults were to be pulling on a 5:1 tensioning system. Both ropes used were clean and supple, with a dusty rope friction would again increase and coupled with some less efficient pulleys might tip the force higher still. I think that it is still appropriate to give out the advice that no more than 2 people are used to tension 5:1 systems, perhaps 3 if using youths or very small adults but certainly no more. The force required to damage a rope at the teeth of the jammer is rather large, especially on 11mm rope, but repeated tensioning on the same spot in the rope may, over time, lead to degredation of the rope.

The best advice I can give is to echo what is already taught at LCMLA and CIC:

  • Keep your pulling ratios at 5:1 or lower and don’t exceed 10 men equivalent pulling power. i.e. 3:1 with 3 pulling or 5:1 with 2 pulling.
  • Keep ropes clean and supple.
  • Use only Type A ropes compatible with your choice of progress capture device.
  • Thick ropes are stronger and stretch less but require more force to initially tension.
  • Thinner ropes are strong enough but stretch a little more and require less force to initially tension.
  • Where very high tension systems are required consider doubling up on ropes and using a non-toothed rope clamp like a prussic or Petzl Shunt / Rescuecender.
A final thought. It is only a short period that the tension is applied to the rope via the teeth of a jammer in these set ups. It is the resultant tension and forces in use that are just as, if not more important to keep an eye on. Tensions in tyroleans can easily exceed 2.00kN, the maximum load Petzl advise for a Stop descender. Consider all components carefully and practice safely before using for real.
* Having given this some thought I believe that I can explain the added friction for this configuration. Over its life, the older Stop has been used for many miles of 10mm rope, wearing the alloy bobbin into a matching profile. Now there is a larger contact area between the alloy and the rope when compared to the brand new Stop. The larger contact area requires more friction to overcome and hence the greater force required to pull the rope through.
2 Stops

Loads on a 5:1 Tensioning System

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Tyroleans have been a bit of hot topic with me recently. I’ve developed some sites to use in my woodland near Whaley Bridge and been involved in some testing with BCA Trainer Assessors for the LCMLA scheme. We’ve measured the actual forces held by the anchors in a number of tyroleans but a really interesting questions was yet to be answered definitively:

In using a high mechanical advantage tensioning system, how much force is being applied to the rope via the teeth of the jammer and could we be at risk of damaging the rope?

To explain, when using a 3:1 or 5:1 system as is common with tyrolean set-ups, a toothed jammer is most commonly used to create the attachment point on the rope to build the mechanical advantage system. The force applied by whomever is hauling in is multiplied in a mechanical advantage system, which is kind of the point, and all this force is transmitted to the rope via the toothed jammer. The picture below shows a 5:1 set up with a Rock Exotica Enforcer load cell.

5to1 load test (1)

If you omit the load cell from this set up you have a standard 5:1. As you can see it is the toothed cam on the Petzl Ascension device that is the contact point with the rope. This device, like many of the Petzl rope clamps, is approved for use with 8 to 13mm ropes but comes with the warning that the toothed cam can damage or cut the rope at forces around 4kN for smaller diameters and 6.5kN for the largest. As it is hard to compare one rope to another, even of the same diameter, most rope professionals simply take the 4kN figure as that which must never be achieved in use.

5to1 load test (3)

Using 2 people to tension the 5:1 system, the Enforcer gave a max force of 2.88kN through the jammer. Had we been on more solid ground (and my partner not been a positively tiny 5’2″ & 50kg) I think we could have gone higher.

Inspecting the rope (Gleistein 9mm Type A) after moving the jammer showed a flat spot and gaps in the sheath where the teeth had opened up the weave. There was some furring but it was impossible to say if this is new or was already present on this rope.

5to1 load test (5) 5to1 load test (6) 5to1 load test (7)

A repeat test on a different section of rope produced a force of 2.66kN and a similar flat spot and sheath opening.

We then set up a standard 3:1 ‘z-rig’ and repeated the test.

3to1 load test (1)3to1 load test (3)3to1 load test (2)

This test gave us a force of only 1.84kN using the same 2 person team with a less pronounced, but still visible, opening of the rope sheath bundles and overall flattening.

I think these observations uphold the understanding that the tensioning in tyrolean systems must be done with great care and by using the least amount of tensioning required for the crossing. I will conduct a further observational test at a real underground site with 10mm or above diametre rope for a comparison but the force figures will not be too dissimilar. It would be interesting to find the 2 heaviest/strongest volunteers I can and use them on a 5:1 system to see if it is possible to creep further toward the 4kN limit.

In conclusion, you can get close to, or potentially exceed, the 4kN safe load on a toothed-cam jammer when using tensioning systems in tyroleans. Tyroleans really are an element of verticality that you need to understand well and get training for to know how to be safe. Go and do a CIC/MIA/UKMR or other course or get in touch with me for a chat.

I’ll be investigating this further at some point but it might be worth looking at employing the use of a non-toothed rope grab like the Petzl Shunt or even an appropriate prussic knot as a way of limiting damage to ropes in high mechanical advantage systems.

NB – The current Petzl literature for the current Croll and Basic do not show a load at which the ascenders may damage the rope. These devices are sold as personal ascenders and are only labelled to take up to 140kg of user weight.