Author Topic: Pulley jammer warning  (Read 2618 times)

Offline Chocolate fireguard

  • obsessive maniac
  • ***
  • Posts: 372
Re: Pulley jammer warning
« Reply #25 on: June 19, 2019, 09:34:53 am »
The only question I have is where was the load cell located? As force on climber will not be the same as force on pulley (the easiest place to locate the cell).

If the belayer is still pulling on the rope then that force will register on the load cell at the top as well as the force on the climber, so the load cell will register a bigger force than the force on the climber - by the few 100N a person is capable of in such a situation.

If there is no such pulling force then the force on the climber will be the same as the force on the load cell (to within a fraction of a Newton - I will explain this caveat to anybody who is interested, but not now).

Neglecting the weight of the rope:
The upwards force on the climber is the same size as, but opposite in direction to, the downwards force on the rope (Newton's 3rd law).
The downwards force on the load cell is the same size as, but opposite in direction to, the upwards force on the rope (ditto).
If the force on the climber was less than the load cell reading there would be a net upwards force on the rope.
Bearing in mind that the mass of a few metres of rope will be less than 1kg then a difference of even 10N would result in the rope accelerating upwards at more than 1g.
This means that in the course of arresting the fall (taking typically a few 100ms) the rope (represented by its centre of mass) would travel UPWARDS a distance of a few 10s of cm.

Someone would have noticed this by now.

Online Oceanrower

  • obsessive maniac
  • ***
  • Posts: 300
Re: Pulley jammer warning
« Reply #26 on: June 19, 2019, 09:49:43 am »
GB ladder dig has a thoroughly bizarre and not entirely safe method of access, but anchors well back from the pitch once you are up there.

It is, however, vastly amusing when you get there with someone who's not done it before and watch them trying to work it out...

Offline andrewmc

  • forum star
  • ****
  • Posts: 722
  • EUSS, BEC, YSS, prov. SWCC...
Re: Pulley jammer warning
« Reply #27 on: June 19, 2019, 10:00:01 am »
In a class bottom-roping situation, i.e. a karabiner at the top, a person on the ground belaying, and a person falling off who hasn't hit the deck, the forces on the anchor, the climber and the belayer are all different.

The force on the anchor is largest as it has to hold the downwards force from both the climber AND the belayer.
The force on the climber is the next largest.
The force on the belayer is the smallest, as friction reduces the peak force from the climber (which is why belaying over a pulley is silly and you will never see a climber doing it).

In the steady-state case (i.e. someone hanging on the rope), the force on the climber and belayer will be equal and the force on the anchor will be twice this.
In the dynamic case, the peak forces can be different - but how different will heavily depend on the situation e.g. a pulley will reduce friction and thus the difference in force.

When directly belaying off an anchor, there is not a belayer hanging down from the anchor, so forces on the climber and anchor will be more similar.

All of this has bugger all to do with toothed ascenders which should be nowhere near any belaying situation.
It also has bugger all to do with the testing carried out, which shows damage to the pulley, the ascender and the rope in this situation. The actual forces reached are irrelevant.

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #28 on: June 19, 2019, 04:41:42 pm »
Sludge Pit does have traverse bolts before the pitch head. SRT ain't friendly there though as lots of rough edges close to the line.

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #29 on: June 20, 2019, 07:03:21 am »
The numbers are important as, if we understand what is going on in the system, we can use the results to model what will happen with other belay devices.

If others are correct about no force being taken by the pulley jammer, and if Grigri, I'D or Rig won't slip until 6 to 8kN, then using them to belay from fixed anchors at the top will also result in similar (high) forces being experienced by the climber if they fall off with slack on a short rope - thus suggesting they are probably not the safest items to be using in that position...

Obviously in the real situation the knots in the rigging & belay rope will provide most of the shock absorption, along with the way the body of the climber flexes (the only one of these that is in the test is the knot attached to the climber). The energy absorbed by the core & sheath of the rope (due to strectching of short lengths & squashing around device) & deformation of bolts / belay device will be miniscule.
« Last Edit: June 20, 2019, 07:14:38 am by mikem »

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #30 on: June 20, 2019, 07:56:45 am »
Actually I've worked out where my error was - the load cell isn't measuring the force on the pulley, but the force on the whole system (which will be what the climber experiences). The pulley has been subjected to a much greater force, but this hasn't been recorded as it resulted in the pulley deforming, however, this did reduce the force on the climber as for a 1.2 fall factor onto 0.44m of rope attached to just a jammer the peak force was 6.6kN, whilst a 1.4 FF onto 0.45m rope running through a pulley jammer was only 5.4kN (82% - but higher FF, so probably even bigger reduction).

Other considerations are that somewhere like Swildons 20 isn't suitable for a handled belay device as the climber falling off will pull the device against the rocks, potentially either releasing the handle or obstructing it so can't be used.

Looks like Bob needs to do the same tests on a Grigri (or similar)...
« Last Edit: June 20, 2019, 08:20:32 am by mikem »

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #31 on: June 20, 2019, 08:29:36 am »
There are a couple of discrepancies between the notice (weight given as 100kg):
Quote
that the rope suffered varying degrees of serious sheath damage.
& the main report (weight given as 85kg):
Quote
There was no visible damage to the rope in these three tests.

Offline andrewmc

  • forum star
  • ****
  • Posts: 722
  • EUSS, BEC, YSS, prov. SWCC...
Re: Pulley jammer warning
« Reply #32 on: June 20, 2019, 08:41:59 am »
Numbers!

https://www.petzl.com/INT/en/Professional/Static-and-dynamic-tests-on-the-RIG?ProductName=RIG

My figures for the Rig are probably a bit high for likely situations.

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #33 on: June 20, 2019, 04:20:45 pm »
So, yes a fall factor 1 drop of 1m on 10-11mm rope with a Rig does give 6-7kN force.

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1211
  • breaking knots is fun
Re: Pulley jammer warning
« Reply #34 on: June 20, 2019, 04:40:03 pm »
There are a couple of discrepancies between the notice (weight given as 100kg):
Quote
that the rope suffered varying degrees of serious sheath damage.
& the main report (weight given as 85kg):
Quote
There was no visible damage to the rope in these three tests.
Apologies for the discrepancies in the Note.  The correct weight for the test mass was 85kg, though this is considered to be equivalent to a 100kg human body when allowance is made for the energy absorption properties of the human body.  I can confirm that there was no visual damage to the rope in the three drops of the pulley / jammer set up.  I suspect the quote was lifted from the jammer alone set up where rope damage was observed, as has been by other persons.

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1211
  • breaking knots is fun
Re: Pulley jammer warning
« Reply #35 on: June 20, 2019, 05:26:25 pm »
Actually I've worked out where my error was - the load cell isn't measuring the force on the pulley, but the force on the whole system (which will be what the climber experiences). The pulley has been subjected to a much greater force, but this hasn't been recorded as it resulted in the pulley deforming, however, this did reduce the force on the climber as for a 1.2 fall factor onto 0.44m of rope attached to just a jammer the peak force was 6.6kN, whilst a 1.4 FF onto 0.45m rope running through a pulley jammer was only 5.4kN (82% - but higher FF, so probably even bigger reduction).

<SNIP>

Looks like Bob needs to do the same tests on a Grigri (or similar)...
It appears as if you refuse to accept the load cell measured the peak force in the whole system but I can't conclusively demonstrate that by simple argument.  I note that the pulley's safe working load was only 4kN so high forces are not needed to damage it.  I guess we will have to do some experiments to try and convince you.

Using Petzl data based on a different system to compare with our work is somewhat doggy. 

You suggest testing a Grigri which is built to meet EN 15151 which covers braking devices.  Unfortunately that standard does not place a limit on peak forces for a given drop and also is only valid when used with dynamic ropes - a most significant point I suggest.  I also note the Grigri instructions include a climber and a belayer, so you have two human bodies to absorb the load as well as being used with a dynamic rope. 

I hesitate to suggest an alternative as the EN standards are rather complex.  But EN12841 covers rope adjustment devices which are designed to “… link a seat harness to a working line … to allow access … to work positions, to give support and protection against falls” and includes a test for peak loads as well as permitting the use of low stretch (semi static / SRT) ropes.  andrewmc's link to the Rig is a device which meets EN12841. But as the data indicates, if you switch from dynamic to low stretch  (semi static / SRT) rope, peak forces go up.  (Note Petzl state they use a lanyard of dynamic rope to meet EN 12841.  One has to read the fine print!)

I can only suggest one gets the instructions and work out what device fits the set up you wish to use it for.  And take great care to work out what the symbols mean (Petzl appear to use a weight symbol as a 'dead' load as opposed to a person or a harness as a 'live' load.)  I freely admit I remain uncertain about what can be used for what.

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #36 on: June 20, 2019, 05:37:16 pm »
It appears as if you refuse to accept the load cell measured the peak force in the whole system but I can't conclusively demonstrate that by simple argument.  I note that the pulley's safe working load was only 4kN so high forces are not needed to damage it.  I guess we will have to do some experiments to try and convince you.

Using Petzl data based on a different system to compare with our work is somewhat doggy. 
The pulley should see at least the same, if not more force than a single jammer, but the load cell recorded less...

Quoting from the report (page 3 of 13, although the last is 16 of 15!):
"A simple theoretical analysis indicates that the pulley sees near double the force that is recorded on the load cell due to the mechanical advantage created by this particular set up, see Annex 1."

Yes, I know the Petzl data isn't compatible, but it is ball park of the expected figure. Some of the ropes give much lower values (mostly smaller diameter, but not all).
« Last Edit: June 20, 2019, 05:47:01 pm by mikem »

Offline glyders

  • stalker
  • ***
  • Posts: 254
    • ACS Adventure
Re: Pulley jammer warning
« Reply #37 on: June 20, 2019, 06:43:07 pm »
(Note Petzl state they use a lanyard of dynamic rope to meet EN 12841.  One has to read the fine print!)
Indeed is does, but that is a rope positioning standard. The ones more relevant to the situation in discussion is EN341 (and to a lesser extent EN15151).These are tests where the device is directly attached to an anchor in belay mode and a load is dropped. EN241 is tested on 10.5mm and 11mm semi-static rope. When a 120kg mass was dropped 60cm onto 4m of paid out rope, the RIG gave an impact force of 5600-5800N and a slippage of 2cm.

Offline andrewmc

  • forum star
  • ****
  • Posts: 722
  • EUSS, BEC, YSS, prov. SWCC...
Re: Pulley jammer warning
« Reply #38 on: June 20, 2019, 07:19:37 pm »
It doesn't really matter what peak force you reach in a drop test (as long as it is below 6kN anyway).

What matters is that if you reach a peak force of 2/3 kN or more on an ascender, I will get very nervy. When you get a peak force of 4kN or more on an ascender, you are into rope damage and failure territory.

You might get a higher impact force on a non-toothed device (be it a Rig or a Grigri or a standard belay plate or whatever) but what you _won't_ get in normal circumstances is rope damage - you will get slippage instead. Any device used for belaying should fail by slipping, not stripping.

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1211
  • breaking knots is fun
Re: Pulley jammer warning
« Reply #39 on: June 20, 2019, 07:59:57 pm »
"A simple theoretical analysis indicates that the pulley sees near double the force that is recorded on the load cell due to the mechanical advantage created by this particular set up, see Annex 1."
Apologies, the penny has finally dropped.  But the reason for the pulley seeing double the force is due to the jammer which in arresting the rope generates a force within the system, a highly unusual set up.  My prediction is that the force seen at the bottom end of the rope (and hence by the person) will be the same as that seen by the load cell.

The ones more relevant to the situation in discussion is EN341 (and to a lesser extent EN15151).
EN341 like EN15151, does not specify a peak force limit (except for once only use devices).  (I presume 241 is a typo since EN241 is about petrol.)  Could you specify which clause you are citing from which the 120kg mass comes?  My take is EN341 and EN15151 are only concerned that the device does not break apart under reasonable shock loads and not about limiting the load which could be incurred by the person using the device.

Offline MarkS

  • Global Moderator
  • forum star
  • *****
  • Posts: 672
  • BBPC, YCC
Re: Pulley jammer warning
« Reply #40 on: June 20, 2019, 08:39:31 pm »
[...]  I also note the Grigri instructions include a climber and a belayer, so you have two human bodies to absorb the load as well as being used with a dynamic rope.  [...]

Maybe just worth noting that Petzl do not specify that a grigri should only be used from a harness: https://www.petzl.com/US/en/Sport/Belaying-a-second-directly-off-a-fixed-anchor-at-the-belay-station

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1211
  • breaking knots is fun
Re: Pulley jammer warning
« Reply #41 on: June 20, 2019, 09:13:08 pm »
Maybe just worth noting that Petzl do not specify that a grigri should only be used from a harness: https://www.petzl.com/US/en/Sport/Belaying-a-second-directly-off-a-fixed-anchor-at-the-belay-station
But the technical note for the Grigri Plus 1 at https://www.petzl.com/GB/en/Sport/Belay-devices-and-descenders/GRIGRI-PLUS and for the Grigri 3 at https://www.petzl.com/GB/en/Sport/Belay-devices-and-descenders/GRIGRI do not replicate that drawing.  All of the drawings in their technical notes clearly include a person at either end.  Don't know where that leaves us.  Apart from crabbing a grigri to an anchor will clearly give rise to a much higher peak force across the system in a fall situation compared to another person.

Offline Chocolate fireguard

  • obsessive maniac
  • ***
  • Posts: 372
Re: Pulley jammer warning
« Reply #42 on: June 20, 2019, 09:50:09 pm »
In the setup described, with the ironmongery attached directly to the load cell above, and via a length of rope to the object whose fall is being arrested below (and with no hand on the control rope) the only difference between the upwards force experienced by the object and the reading of the load cell, for the whole of the arresting process, is the (trivial) weight of the ironmongery and rope.
Any other difference between those 2 forces, even only a few 10s of Newtons, would result in the rope accelerating vertically at many m/s^2. I think that's ludicrous, as I suggested before.

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #43 on: June 20, 2019, 11:55:36 pm »
Nobody is recommending using a pulley jammer. However, we are trying to clarify what is actually happening in the system, because it may be applicable to other situations.

It would appear that the specifications of the test are such that several devices that are believed to be safe might fail to remain below the 6KN recommendation because of the way we use them compared to climbers. The chances of those factors all coming together in a real situation are remote but possible (the length of rope is so short that the climber is likely to be on a ledge rather than subject to freefall etc).

So what actually happens - the weight drops on one side of the pulley, which is unable to balance the forces equally due to the low stretch rope being firmly held on the other side by the jammer, so it acts more like a fixed curve, creating friction with the rope (sorry, the report about the force being doubled is almost certainly wrong as well). On the other side the rope is in tension, whilst the pulley / linking karabiner / jammer are in compression, which may cancel each other out in the great scheme of things, but does result in the pulley deforming due to it being forced around the inside of the karabiner. Meanwhile, much less force will be acting on the jammer (there was no damage to the rope, & presumably the cam was releaseable as it is not mentioned, but does jam in the next scenario - jammer in front of pulley) - so maybe the maths isn't as complicated as we thought...

Offline andrewmc

  • forum star
  • ****
  • Posts: 722
  • EUSS, BEC, YSS, prov. SWCC...
Re: Pulley jammer warning
« Reply #44 on: June 21, 2019, 07:28:43 am »
Climbers routinely belay from the top of a pitch as well as from the bottom.

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1211
  • breaking knots is fun
Re: Pulley jammer warning
« Reply #45 on: June 21, 2019, 07:53:20 am »
The analysis for the statement "A simple theoretical analysis indicates that this is the set up places a force on the pulley near double that seen in the rope and the load cell" is contained in Annex 1 of the report and, I hope, clearly states the approximations made. 

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #46 on: June 21, 2019, 08:44:19 am »
Climbers routinely belay from the top of a pitch as well as from the bottom.
Yes, but they usually keep themselves in the system, which provides additional shock absorption & the climber is in contact with the rock, so much less likely to freefall (as a caver might off a ladder).

The analysis for the statement "A simple theoretical analysis indicates that this is the set up places a force on the pulley near double that seen in the rope and the load cell" is contained in Annex 1 of the report and, I hope, clearly states the approximations made. 
Indeed, but the pulley isn't able to self balance as it is not free to rotate, so force likely to be less on the jammer side (although I don't think this is worth investigating, for one thing it will vary massively depending on the equipment tested). We can agree that the force on the pulley will be greater than that recorded on the load cell, even if we never know the actual value.

So, the forces are the faller acting downwards, the anchor holding it up (this is the load cell reading) & the tension in the rope on jammer side versus the compression in the metalwork (there might be slight movement in this system, but almost certainly not significant).

Offline Bob Mehew

  • forum hero
  • *****
  • Posts: 1211
  • breaking knots is fun
Re: Pulley jammer warning
« Reply #47 on: June 21, 2019, 09:01:57 am »
Indeed, but the pulley isn't able to self balance as it is not free to rotate, so force likely to be less on the jammer side ...
I accept the pulley ability to rotate may be in question and friction between rope and the pulley surface will stop movement to some extent.  But it is clear from Petzl's work that the rope will slide over non rotating metal surfaces as presented by the quick links in the climbing set up.  From memory, one petzl drop recorded 2.5kN on the climber, 1.5kN on the belayer and 4kN on the anchor.  That suggests static friction between metal and rope is easily overcome by forces above say 1kN.  So I suggest the system is not as 'off balance' as you imply. 

Offline Cripplecreeker

  • menacing presence
  • **
  • Posts: 161
Re: Pulley jammer warning
« Reply #48 on: June 21, 2019, 09:36:43 am »
Climbers routinely belay from the top of a pitch as well as from the bottom.
Yes, but they usually keep themselves in the system, which provides additional shock absorption & the climber is in contact with the rock, so much less likely to freefall (as a caver might off a ladder).

Outside of the UK, it is standard practice to bring up a second on a direct belay. The second part of your statement is nonsense! Falling off a rock climb is no different from falling off a ladder.

It’s worth noting that Petzl specifically don’t advise using the Grigri to direct belay a second (when climbing). This is essentially the same scenario you would have when belaying somebody climbing a caving ladder: https://m.petzl.com/GB/en/Sport/Belaying-a-second-directly-off-a-fixed-anchor-at-the-belay-station?ProductName=GRIGRI-PLUS&Familly=Belay-devices-and-descenders

That said, I know that most climbers use the direct belay method, rather than Petzl’s recommended technique!

Offline mikem

  • forum hero
  • *****
  • Posts: 2514
  • Mendip Caving Group
Re: Pulley jammer warning
« Reply #49 on: June 21, 2019, 06:23:25 pm »
All I meant was that the anchors on ladder pitches are more likely to be directly above the drop than they are on climbs (where someone might use a direct belay), so when you get to the point of having only 0.5 to 1 metre of rope left, freefall is a more likely consequence underground than above.

With the petzl example given, the force on the belayer is only 60% of that on the climber - I'd say that was a significant difference.