More force testing on 5:1 systems

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

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.

Woodland Tyrolean Development

Recently I headed out to our private woodland site to have a play with my new Rock Exotica Enforcer. We have recently developed a tyrolean crossing here along with calculations of anticipated loads and safety factors. Using the Enforcer on this tyrolean would give a real world check of my calculated figures as well as giving me a relatively safe and controlled location to experiment.

The tyrolean spans a 20m gully and is rigged using large trees slung with Lyon steel strops and the tensioning is done on the lower end using a 3:1 system through a Petzl RIG clutch. In this testing I purposely tried to over tighten things to see how much tension, and hence force at the anchors, it was possible for 1 person to produce.
I used a 5:1 pulley system and installed the Enforcer between the anchor and the RIG so it gave a reading on the total tension force being held by the lower anchor strop.

The calculations I had done previously were based on an average weight of participant of 100Kg. The span was measured and the sag was estimated at 10% as in practice we’ve found it impossible to achieve less than that with semi-static rope (usually more like 15-20%). The load on each anchor (so x2 for the rope itself) was calculated using a number of methods, some involving scary trigonometry, but the simplest equation was:

Tension = (Load × Span) / (4 × Sag)
Tension = (100kg × 20m) / (4 × 2m)
Tension = 250kg (roughly translated to 2.5kN)

The WLL (working load limit, or safe working load) of each component was calculated at a fairly standard ratio of 5:1, that is a fifth of its MBS (minimum breaking strength). Using this ratio the lowest figure was 4.8kN for a Petzl OK Oval karabiner. Technically the Petzl RIG is weaker but as it will slip before it’s WLL is reached then it can be discounted*.

*providing the RIG is not locked off and the rope is dogged back into the rigging so a running slip could not result in a complete slackening of the system.

So the maths with a 10% sag gave me 2.5kN tension force on each end of the tyrolean.

The tension force graph is downloaded off the Enforcer to iPhone and then edited in Microsoft Excel looks like this, with Time on the x and Force in kN on the y axis. Click to expand:

Little T 5to1 Graph

The graph starts with me applying tension to the system and having a few test bounces. The main force peak near 2m20s is me hanging suspended and pulling myself to the centre of the crossing and bouncing. I then pull up to the higher end, take a breather, and run back off, giving the last spike.

Little T 5to1 Graph top end

The second graph was me installing the Enforcer at the high end of the tyrolean crossing and tensioning it back up again with the 5:1 system. The tension at the top anchor was a little less than before on the lower anchor but the peak tension (me doing a running jump crossing) was similar to the previous graph.

Some interesting observations from the day:

  • At no point was it possible to install more than 1.5kN of pre tension in the rope prior to crossing. This was tested up to 9:1 and on 2 different tyroleans.
  • Tension in the line always dropped after the first crossing and remained at or below 1kN. Probably after the knots tightened up.
  • Tension could then be raised back up with additional hauling but the force remained below 1.5kN. Do not keep re-tensioning in real use as the increase in tension for each loading may cumulate to break a rope. See BMC technical reports.
  • The peak force was close to our 2.5kN calculation. This is predicated on having at least 10% sag in the system. A set of specifications for mechanical advantage systems and number of people pulling should be set by a company to limit over tensioning.
  • Whatever the tensioning method, we could not achieve less than 10% sag on a tyrolean in use with a correctly installed clutch.
  • Even on a 9:1 tensioning system the peak force created was only 2.8kN, probably because the Petzl RIG slipped at that point.

The 2.8kN high figure was achieved on a 50m tyrolean set up later that day. Here I was using a 10mm Beal rope today with a MBS of 24kN and 5:1 WLL of 4.8kN. The 5:1 safety factor is acheived with this rope. The rope I have on order for this when it is done with the public is a Mammut 11mm, the same as we use on the 20m line. It has a MBS of 35kN and hence a 5:1 WLL of 7kN, far above the expected loads in use.

Why all this effort? I like to know that the real world forces are actually near to how we calculate things, especially in tyrolean systems. Vector forces are scary and you just need to watch YouTube clips of slackliners breaking rated kit to see that. I can sleep well knowing all our research and calculations are backed up by real world testing and I can be confident that we are delivering as safe a service as we can.

Thanks for reading and remember, I can be hired to come to your site and test your rigging with the Enforcer too. Contact me direct for a quote and a chat.

Pull Testing Ground Anchor Pins

It is quite common that where you need to anchor ropes to things, there are no natural things there to anchor to (like boulders). Climbers and cavers get round this by installing bolts and/or ground anchors. A ground anchor is a (hopefully) long length of angle, pole or bar that is driven in deep and (hopefully) can hold a load due to it’s friction with the ground. These are most commonly seen at the top of moorland cliff faces, installed by rock climbers as a top anchor to abseil from or belay a second. As part of the process of looking into developing a mobile zip wire / team building exercise we had decided that some form of ground anchor would be needed. There are a number of options, from home made to high cost but we opted for the tried and tested ‘mooring pin’ approach as used by mountain and cave rescue teams.

Now, just before people get a bit worried, this is not for life preserving! These pins are intended to act as guy lines for a tripod structure in poor, rocky soil.

Myself and Beth hammered the 50cm long 16mm galvanised steel pins in, one to 1/2 way and one to 3/4. Both of these pins came loose with very little force, about twice average body weight. We then drove the pins in up to their necks and set up a 10:1 pulley system from the back of my van, Brick.
The pins held well but at about 300kg they bent and pulled out. Even rigging a pair in a V made little difference, in fact I was dismayed to see my £20 pins bent after about 5 minutes of testing.

The final test involved setting the pins up as done by MR and CRO teams. We needed a 3rd pin so I used a section of what looked like 14mm rebar fencing spike. The 3 pins were hammered in full length in a direct line to the load. A rope was then used to tie each in line, stacking the pins with clove hitches seemed easiest. The force was applied to the front pin which, as it moved forward to take the strain, came tight on the 2nd pin which in turn came tight on the 3rd. The 3 pins worked as one and we were able to pull a force of 4.84kN with no failure of the anchor.
We stopped at that as we were now in danger of breaking the smaller pulleys used in the MA system and my van was beginning to move.

In conclusion:
Good news – the 3 pin method is very strong and would make a suitable ground anchor for guy lines but also, if backed up to a second system, would be okay for rigging safety lines from. Also, fencing spikes are seriously strong!
Bad news – £20 worth of 12 hour old pins are now bent like beech twigs. These just were not up to the job, poor steel. I’ll be picking up some fencing spikes and using them in the future.

Testing Ground Anchors

Testing Ground Anchors