Pulley jammer warning

[MarkS]

[...]  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

 

[Bob Mehew]

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.

 

[Chocolate fireguard]

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.

 

[mikem]

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...

 

[andrewmcleod]

Climbers routinely belay from the top of a pitch as well as from the bottom.

 

[Bob Mehew]

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. 

 

[mikem]

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).

 

[Bob Mehew]

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. 

 

[georgenorth]

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!

 

[mikem]

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.

 

[georgenorth]

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.

On multi-pitch sport climbs (the only common situation in which you would use a direct belay with a Grigri) the belay (anchor) is almost always positioned on the line of the climb. It really is no different from belaying on a pitch underground (except using a dynamic rather than semi-static rope).

 

[glyders]

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.

I was simply looking at the link upthread to the Petzl tests for the RIG. There is no mention of a dynamic lanyard in the tests of top-belaying, so the loading and slippage quoted was relevant to the use in caving.

 

[glyders]

Yes, but they usually keep themselves in the system, which provides additional shock absorption

When guiding it is standard to belay clients directly. This leaves the guide readily able to render assistance/rescue, which would take more steps and effort if they were in the system like with a normal climbing pair.
When I'm doing it, I use an ATC in guide mode. I've considered using it in caves but haven't because a) I don't want gritty ropes carving it up and b) I am less worried about weight and so bring a RIG.

 

[mikem]

Interestingly the technical notice for the RIG shows it as a rope access or rescue descender & a type 6 belay-rappel device (picture shows belaying from below), but for usage on an anchor, it only shows lowering & giving slack (not belaying a climber up from above). Additionally, "The RIG is not suitable for use in a fall arrest sytem.":
https://www.petzl.com/sfc/servlet.shepherd/version/download/0681r0000078gStAAI

 

[georgenorth]

See here for information on using the RIG as a belay device: https://m.petzl.com/INT/en/Professional/Primary-uses-of-the-RIG?ProductName=RIG&Familly=Descenders#CT-177-4

?Fall arrest system? is industrial terminology and refers to personal fall arrest (using via ferrata lanyards would be the ?sport? equivalent). It does not mean that the RIG can?t be used for belaying.

 

[mikem]

The test situation on the original document is a fall arrest.

From that Primary Uses document: "The RIG on the anchor allows for belaying a person who is using another progression system". It only shows belaying a climber from below.

 

[georgenorth]

From that Primary Uses document: "The RIG on the anchor allows for belaying a person who is using another progression system". It only shows belaying a climber from below.

No. It quite clearly also shows belaying a climber from above (using a direct belay). It shows giving out slack, taking in slack and holding a fall.

 

[mikem]

Yes, giving out slack when LOWERING, taking in slack ON DESCENT, the only statement on belaying upwards from above is that climber is attached to something else for ascent...

 

[andrewmcleod]

"Belaying with RIG attached to an anchor
The RIG on the anchor allows for belaying a person who is using another progression system.

The user must never let go of the brake-side rope. The user must always make sure to keep slack out of the rope between the RIG and the anchor, to reduce the risk of a fall."

'another progression system' means climbing a ladder or something... it just means that the RIG is not the progression system and that it is truly belaying and not hauling.

 

[andrewmcleod]

'Stopping a fall'

This is incontrovertibly belaying. Stopping a fall of a weight or person on a rope descending directly down from the Rig which is attached to an anchor.

There are also:
Giving slack on descent

and
Taking up slack on ascent