About Pete

Outdoor professional and enthusiast.

Holmebank Chert Mine

Holmebank Chert Mine
Fan Entrance lock installation 9th January 2018

Today I was up at Holmebank Chert Mine near Bakewell on behalf of the Derbyshire Caving Association and the Peak Instructed Caving Affiliation. One of the access points to the mine required a new locking mechanism which allowed explorers to access the locking bolt from inside and out.

Using some high strength steel components bought by the DCA, I fabricated up a simple rotating hatch, allowing a person to reach through and access the sliding bolt on the inside of the gate. The hatch is locked with a combination padlock and cavers can obtain the code from the usual source. Currently it is set to the same combination as the other lock on the top entrance.

Many thanks to Ewan Cameron from Evolution Outdoors for the company and assistance installing the hatch today and of course to Joe at the architects firm for his continued support of access to the mine for outdoor centres and instructor training.

Testing the strength of heavy duty caving belts

A Method of Testing the Strength of Heavy Duty Caving Belts

The aim of this little test was to establish a method to test the strength of heavy duty caving belts that did not rely on having access to a load cell. I hoped to produce a simple system that needed very little equipment and that would deliver a test load to a belt that exceeded the minimum strength requirement for its use.

What strength does a belt need to be?

Well, this one is a potential can of worms…. Let’s be clear, the manufacturers do not condone the use of their heavy duty belts for taking any load at all. There is a historical use in cave and mine exploration that involves using the belt for the purpose of slip prevention and security on steep ground. If you were intending to use it for this purpose, you’d need to be sure that the belt was strong enough for that, something that the manufacturers will not say.
We take the figure for a heavy person with kit that is used for some load ratings on PPE equipment: 120kg.
If a caver has a short lanyard which they climb above, ignoring all stretch or slack in a system, we will assume a possible fall of 1 metre onto the belt. The person will then be travelling 9.81 m/s (acceleration due to gravity).

velocity = √ (distance x gravity x 2)

v = √ (1 x 9.81 x 2)
v = 4.43 m/s

Kinetic energy = (v²m)/2

Ke =  (4.43² x 120)/2
Ke = 117.72 J

Impact force = Kinetic energy / Impact distance

iF = 117.72 / 0.1 (arbitrary minimum for complete stop)
iF = 1177.2 N

Impact Force = 1.177 kN

Confirmation:

Force = Mass x Acceleration

F = 120 x 9.81
F = 1177.2 N / 1.177kN

So a 1m drop of a 120kg caver onto a belt, not taking into account any stretch or bounce, produces a force of nearly 1.2kN.

Apply to this any safety factor you wish. Worse case? Twice the force falling onto half the strength of kit due to wear and age: 2 x 1.2kN = 2.4kN force will need to be held for a worst case fall and we halve the strength for old, worn or wet kit so 2 x 2.4kN = 4.8kN.

So as long as we can apply a test force of 4.8kN or more to the belt, we can be assured that the item can hold the greatest possible force we can apply to it even if not in new condition. The only remaining factor of concern is that would applying this force in test render the belt unsafe to use again, in essence, are these tests destructive? Only 1 way find out…..

You could easily argue that this figure is seriously overkill for a belt so you should undertake your own discussion as to the correct safety factor to apply and then make sure you test to that, e.g. 2.4kN using 2 or 3 people on the 3:1 MA system.

Testing

Using 1 very large Corsican Pine and a good sized Birch tree, we set up a pull testing rig with a simple 3:1 theoretical configuration. I used a Rock Exotica load cell to get live feedback on the testing here but if you copy the method, you would not need to use one.

For the estimation of test force we regarded each person capable of pulling 50kg (see Gethin Thomas’ work on tyroleans). Through a theoretical 3:1 MA system that would be 150kg per person. With 5 undertaking the pull reaching 750kg and 6 equalling 900kg or approximately 7.5kn and 9kN respectively.

Kit used (minus load cell): Petzl rescue pulley, Petzl Basic jammer, Petzl Partner pulley, Lyon wire sling for tree, assorted karabiners, 20m rope.

Due to the force expected to be placed on the rope, I did not anticipate that I would be able to untie the end knot (fig 8 loop). This was accurate and the knot had to be cut from the rope end. Bare this in mind with your own rope!

We also used a Petzl Rollclip to redirect the angle of pull to make it easier to stand on the tarmac of the road alongside the trees.

Initially we had 5 people pulling the first test on a Lyon roller-buckle belt (brand new).
This produced a force of 5.9kN with no damage or slippage. This is lower than expected but there was a lot of tightening in the knot and stretch in the rope coupled with a general timidness of the pulling team.

The remaining tests used 6 people to pull. This one was conducted on my 10 year old Caving Supplies square buckle belt (already retired). This belt has nicks, fluff and rust  and comfortably took a force of 7.74kN showing no damage or slippage. Next came my current AV belt, with it’s central maillon removed and directly attached to the pull line. This belt held 7.7kN without failure or slippage.Finally, the pulling team seemed at their most confident that nothing was going to break and send shards of metal and wood at them so they really gave the last belt some pain. This Warmbac square buckle belt was subjected to 8.64kN with no damage or slippage noted at the time.It is not surprising that the force exerted by the pulling team was less than the theoretical 3:1 system implied. In practice with the loss of friction due to bearings and turns in the rope a 2.5:1 is a more real world figure and so our 5 x 50kg pulling average adults could be expected to make 500kg/5kN using this system. Add a 6th person if the ground is poor or your team are small!

Conclusions

Using a system like the one shown here, with 5 people pulling at average strengths, you can apply a force greater than 4.8kN to your test belt.

Once the test is complete you should thoroughly examine the belt to see if any damage or slippage has occurred. Any that do show signs of damage should be retired. Any slippage may be down to the buckle tightening down so consider testing again. If a belt has taken the test load and shows no damage or deformity then you can be comfortably sure that the belt will be fit for its intended use.

Final inspection of belts:
Lyon roller buckle                                5.9kN            No damage, continuing to use.
Caving Supplies square buckle           7.74kN          No damage, already retired.
AV maillon closed harness buckle       7.7kN            No damage, continuing to use.
Warmbac square buckle                      8.64kN          No damage, slight curvature to webbing now when hung vertically which indicates over stretching or broken fibres down one side. Retired from service.

As a side note, I maintain that the Caving Supplies belts are the tanks of the heavy duty caving belt world and, if kept very clean, will ultimately outperform every other type or brand available. I think this test shows that well as the CS belt had at least 5 more years of abuse over the other belts. I will dispose of the Warmbac belt just in case but don’t tend to use these anyway, but that’s another blog post!

Worn Connectors – Pull Testing 11-6-2017

Over the last few months I’ve been collecting a few bits of retired equipment from stores checks and ‘isolation’ bins with a view to looking at loss of strength due to wear. Nothing here constitutes a scientific test and this is purely for my own satisfaction, but I’m writing it up anyway. I used my home made breaking rig with the Hilti HAT-28 anchor tester to provide the pull force. Each item was pulled up to the maximum possible load of the Hilti, 20kN, and the results were recorded.

Petzl Omni SL

Rated to 20kN main axis. Worn inside arc in 2 places after use with large steel pulley for 12 months. Failed PPE inspection due to wear depth being felt by fingernail and visually obvious.

Pulled to 20kN – No breakage, gate / lock working correctly.

I suspect that the wear on this item had not yet reached a sufficient depth to form a significant weak point. The connector was certainly retired at an appropriate time, i.e. with visible wear but before strength loss occurred.

Petzl M33 OK Oval SL

Rated to 24kN main axis. Wear visible at both ends of the connector. Large radius wear from twin cheeks of a steel pulley and small / deep radius wear from a long term connection to a steel 7mm Maillon Rapide.

Pulled to 20kN – No breakage, deformed beyond elastic recovery. Gate no longer closes and shape is visibly distorted. This item deformed at 15% below its rated strength. This shows that wear had already reduced the strength of this connector and it should really have been retired before reaching this level of wear.

Petzl Vertigo Twist Lock

Rated to 25kN on main axis. Worn on inner surface due to repeated contact with steel cable zip wires whist in use as a cowstail. Retired during routine PPE inspection.

Pulled to 20kN – No breakage, deformed heavily under test but recovered almost completely after. Connector permanently deformed and the gate locking mechanism does not function correctly. This item deformed 20% below its rated strength. This shows that wear had already reduced the strength of this connector and it should really have been retired before reaching this level of wear.

Do not take this test as advice to use kit beyond it’s manufacturer stated working life. Get a quailed person to advise you if unsure or go and do a PPE/FPE inspector course. If you have any retired gear you want to send to me to test in this manner then please get in touch.

Removing Sleeve Anchors (SPITS etc…)

I guess this post is a bit of a continuation from the blog post I did about pull testing SPIT type anchors in 2015. Sorry it has taken me so long to get round to doing this!
The original post can be seen here: http://www.peakinstruction.com/blog/pulling-spit-anchors-back-garden-test/.

One of the points of that testing was to ascertain if the sleeve anchors could be removed from the rock in a cave or mine to either de-clutter the wall or allow a resin anchor to be placed in the same location. This is important from a conservation point of view, these sleeve anchors are a bolt rash on the walls of our caves and once stripped of threads, are there forever…. or so I thought.

Jump ahead to now. Simon Wilson has developed the IC Resin Anchor in the Dales and his website has expanded to become a good resource of information relating to the installation and removal of anchors. Most relevant here is the method that he uses to remove old sleeve anchors, one which I am shamelessly copying here in an effort to spread the knowledge and encourage the tidying up of pitch heads. Simon’s site is here: http://www.resinanchor.co.uk/5.html.

I started with the original block of Stoney Dale limestone used for the original testing in 2015.


 

 

 

 

 

Method used:

  1. If required, dress the rock near your anchor sleeve with a chisel to create a flat area for drilling.
  2. Drill a 6 or 7mm hole immediately next to the anchor sleeve.
  3. Drill a second hole parallel to and as close to the first as possible.
  4. Bore out into a slot using an old drill bit and some wiggling.
  5. Tap the anchor sleeve into the slot using a cold chisel or old screwdriver.
  6. Remove the anchor from the hole. This may well need some jiggling about or a bit of extra chiselling. If possible, screw a bolt into the sleeve to aid extraction.
  7. Fill hole with resin or drill out for the installation of a new resin anchor.

Shown with a SPIT 12mm self drilling anchor:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SPIT 12mm self drilling anchor that had sheared off in a previous test:


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Another SPIT 12mm self drilling anchor that had sheared off in a previous test:


 

 

 

 

 

 

 

 

 

 

 

This removal method works well once you’ve had a little practice and was far less destructive or time consuming than pulling the anchors out with the HAT-28. I also used the same method successfully to remove 10mm lipped sleeve anchors as well (HKD / drop-ins etc…) although have no photos.
As I was drilling so close to the sleeves, occasionally hitting them, this was very hard on drill bits. I melted the head off my cheap 7mm half way through and swapped to the 6mm bit. I’ve just ordered some quad tipped bits from Hilti in the hope that they are tougher. I’ve only ever destroyed one of them in the tough welsh rhyolite of Parc mine.

The remaining triangular hole can then be cleaned out and filled in using an expired resin cartridge with some limestone dust thrown on. It won’t disappear 100% but will be a huge improvement over the old rusty sleeve.
If you plan on re-using the hole for a new anchor, try to position your slot to the intended orientation of the head of a ‘P’ style resin anchor. Once the anchor is placed, make sure the whole hole is filled with resin leaving no voids. You can’t use the SPIT resin vials for this job as they have a set quantity of contents, you will need to have a resin cartridge gun to fill the irregular hole properly.

I hope to begin removing some of the decades of old sleeve anchors in sites now resin bolted and potentially earn some karma points back for anchors of this type that I myself have placed in the past prior to my ‘enlightenment’.

Thanks for reading.

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.

Coiling Caving Ladders

Caving ladders are an integral part of the LCMLA Level 2 award. Being practiced with a ladder not only saves time but lots of faff. It can be hard to pack loosely coiled ladders into tackle bags, meaning they get dragged and thrown about the cave, something that no kit really deserves. Practice coiling your ladders and look well polished on your assessment and in front of your clients.

Struggling with ladder coiling? Collar me for a face to face demo or contact me.

Review – Petzl Club, semi-static rope

I had an email about a month ago from Shaun at Hitch n Hike. He’d been sent a 70m sample rope from Petzl to evaluate and once it came out of the box and he saw the colour, he knew who to call.

20151215-0000 Club 10mm pt1This rope was a 70m coil, sold in an un-shrunk condition. Rather uniquely I think, Petzl will be supplying this rope with an additional 10% of length over the advertised sale length. This is to ensure that the shrunk length is not shorter than the labelled length. So if you buy 70m, you receive 77m.
20151215-0000 Club 10mm pt2There are no sold prices advertised as yet but a conversation with Shaun indicates that this rope will come in a little below the similarly spec’d Beal Antipodes 10mm which is so popular as a caving rope.
We opted to reduce the 70m length to a 25m and 45m, both of which were of course 10% longer pre-shrink due to the generous measuring of the manufacturer.
I washed and shrank the rope before allowing it to dry naturally as per usual preparation methods.Petzl Type A rope

The rope is a fierce shade of orange and in the hand feels supple and easily knotable. This characteristic made for pleasant use and did not deteriorate with a month’s heavy use. It was used in SRT rigging and group management with Italian Hitches and was easy to use in all knotting applications.
The feel is quite like a floating rope as seen in throw lines, sort of hollow. Although a normal kern-mantle construction, the rope would compress down flatter as it moved over karabiners and more importantly, through a Petzl Stop. I don’t have a brand new Stop but it could not be described as heavily worn. The rope crept through the Stop in most applications, including haul and belay. I did have the opportunity to abseil using a Petzl Pirana canyoning descender and found it was a really nice abseil. I suspect this rope has been designed more with those type of descenders in mind.
The rope gets pretty heavy when wet but the water does not adversely affect the handling, certainly no more than any other. It’s been dragged through mud, slate dust and water and still cleans up well.
The rope packs into tackle bags well due to its suppleness but it this does cause a problem for SRT. It does not push well through ascenders and requires some manual feeding at times when a stiffer rope would be pulling through on it’s own.
It does feel quite heavy for it’s size. I’m sure on paper it won’t be much different from Beal or Mammut equivalents but to me it felt heavier than most of my other ropes of similar lengths.

Over the last month the rope has been used as an SRT line, Level 1 handline and belay rope, a ladder lifeline and as a hauling line for rescue. It has been used by me, friends, other instructors and course students.

I like this rope. It is nice to handle, bright and has a good feel. I won’t be purchasing this line for SRT, it is not the right rope for that. I think this is a rope designed for canyoning, where descents are done on figure eights and not generally assisted breaking descenders like Stops. I’d own a few lengths for group work through. It’s nice handling and tough feel would make it a pleasant Level 1 rope. In summary, this is nice stuff but perhaps not as an SRT caver’s rope.

LCMLA Level 2 – Dec 2015

The North Wales panel of the Local Cave & Mine Leader Award scheme has just run a block of courses based out of Oaklands OEC near Llanwrst. I am a member of the N Wales panel and I will be working alongside other T/A’s as I move towards completing my assessor apprenticeship. The centre was host to 2 full LCMLA Level 1 Mine training courses and a full Level 2. There were also a number of people on mine to cave transfers and 1 person on a 3rd day of cave training.
I worked alongside Nige Atkins CIC, one of the UK’s most experienced trainers and it was a pleasure to absorb his knowledge. We ran day 1 from Oaklands OEC, in their annex building, where there was a small but amply equipped training wall. This venue is a perfect base for LCMLA courses and technical training. Day 2 started with an introduction to tyrolean traverse rigging and then it was off to a local mine to put it all into practice.

LCMLA L2T Dec15 (1)

We set a 40m floating SRT line for longer spells of practice on the rope.

LCMLA L2T Dec15 (2)

Course members and Nige taking a break.

LCMLA L2T Dec15 (4)

Alun contemplating knots.

LCMLA L2T Dec15 (6)

The first day was a relaxed, candidate-led workshop environment where we managed to cover a huge amount without needing to follow a rigid plan. The candidates were a great bunch and it was nice to have a mix of other outdoor qualifications in the room.

If you are interested in becoming a BCA cave leader award holder then the first stop for you should be the BCA Training web pages. You need to register for the LCMLA scheme before Level 1 training and this registration will take you through the whole LCMLA scheme.
BCA Training Website

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.