Author Topic: This looks like a nice abseil  (Read 2420 times)

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #25 on: March 09, 2019, 11:46:14 pm »
Quote
It all must work well as the heat generation runs to well over a kilowatt if the rate of descent is anything like has been said.

I worked it out as just over a kilowatt – still a lot of heat to get rid of. (And are you sure you didn't mean aluminum bars?)
Yes the spell check tried it on again, but I was ready for it this time!

Offline Kenilworth

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Re: This looks like a nice abseil
« Reply #26 on: March 10, 2019, 05:02:47 am »

 No 2 has a groove in it, presumably due to wear, as I don't see why they would want to decrease the above effect.

Another feature is that short tubes have been fitted to one of the sides to stop the top 3 bars getting too close together. And a collar under the top bar means that the gap between 1&2 is a bit bigger than that between 2&3. These tubes slide off at the end of the clip when the rope is removed and all the bars except No 1 hang open. They had fitted a pair of tubes (rather than just one) between each pair of bars, so this seems to be a judgement they make beforehand.
I assume it's to share out the heat generation, stopping the ones at the top, mainly Nos 2&3 I think, doing all the work.

The groove in the second bar is manufactured, and is intended to keep the rope running on center. Otherwise it's possible for the rope to creep to the end of the bar and run on the frame.

Bars 2 and 3 will do most of the work with or without spacers. The spacers are there to keep the bars from pinching too tightly together and slowing the descent.

Online mikem

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Re: This looks like a nice abseil
« Reply #27 on: March 10, 2019, 08:37:16 am »
Useful info on orientation of rack:
http://onrope1.com/descending-equipment/smc-14-stainless-steel-rack-kit/

Seems CMI use open bars to make it obvious which way around to thread it rather than heat dissipation, the pin on top of some is to increase friction & easy lock offs in working situations. The golden offset bars provide more friction than cylinders:
https://www.cmi-gear.com/collections/rappel-racks

Although SMC claim their U shaped steel bars are to increase heat dissipation & reduce weight. Aluminium improves friction, whilst steel copes better when more heat developed:
https://www.adrenalindreams.com/mobile/gallery4.htm

Offline kay

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Re: This looks like a nice abseil
« Reply #28 on: March 10, 2019, 08:56:31 am »

Ultracrepidarianism is the habit of giving opinions and advice on matters outside of one's knowledge. The term ultracrepidarian was first publicly recorded in 1819 by the essayist William Hazlitt in an open Letter to William Gifford

I wonder if it's the root for Crepidula fornicata, the slipper limpet, which grows in piles with older females at the bottom and younger males at the top. As the oldest females die off, some of the males become female, thus could be regarded as stepping outside their knowledge.

Online mikem

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Re: This looks like a nice abseil
« Reply #29 on: March 10, 2019, 09:24:26 am »
Unfortunately, in Latin, crepidula just means small boot / sandal (therefore slipper) & fornicata means arched / vaulted.

Mike

Offline kay

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Re: This looks like a nice abseil
« Reply #30 on: March 10, 2019, 11:45:03 am »
Unfortunately, in Latin, crepidula just means small boot / sandal (therefore slipper) & fornicata means arched / vaulted.


Ah, now you mention it. I think I did know that. According to my Little Gem Latin Dictionary, small sandal, as opposed to caligula = small boot, with the emporor being so-called because as a baby he was small enough to fit into a soldier's boot, allegedly. Didn't know fornicata though, assumed it was more related to the modern meaning.

Online mikem

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Re: This looks like a nice abseil
« Reply #31 on: March 10, 2019, 12:24:42 pm »
Theory is it gained that meaning from prostitutes hanging out in archways.

Offline andybrooks

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Re: This looks like a nice abseil
« Reply #32 on: March 10, 2019, 03:44:04 pm »
I was thinking arched/vaulted = humped

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #33 on: March 11, 2019, 08:41:16 am »

 No 2 has a groove in it, presumably due to wear, as I don't see why they would want to decrease the above effect.

Another feature is that short tubes have been fitted to one of the sides to stop the top 3 bars getting too close together. And a collar under the top bar means that the gap between 1&2 is a bit bigger than that between 2&3. These tubes slide off at the end of the clip when the rope is removed and all the bars except No 1 hang open. They had fitted a pair of tubes (rather than just one) between each pair of bars, so this seems to be a judgement they make beforehand.
I assume it's to share out the heat generation, stopping the ones at the top, mainly Nos 2&3 I think, doing all the work.

The groove in the second bar is manufactured, and is intended to keep the rope running on center. Otherwise it's possible for the rope to creep to the end of the bar and run on the frame.

Bars 2 and 3 will do most of the work with or without spacers. The spacers are there to keep the bars from pinching too tightly together and slowing the descent.

The first para is confirmed by some of the references provided by mikem.

The second one interests me, as it suggests you know stuff that I would also want to know.
If you have any references that deal with heat input to abseiling devices I would be grateful for them.

Two reasons:
many years ago a couple of us badly glazed a nearly new 9mm rope on a 50m pitch in Derbyshire - Elizabeth shaft in Nettle - without doing anything out of the ordinary. We were both steady middle-aged cavers who had done this pitch several times, and have done it several times since. All without incident apart from this one time.
My friend finished up with a bad burn on his left hand, through a (thin) glove. He was using a STOP.
I was using a rack, which became too hot to touch part way down and instantly evaporated the water drops that hit it.
Thereafter the 60m rope took up a whole tackle bag and was so stiff I was glad to retire it.

Since then I have wanted to find out why that happened, on one occasion fitting a couple of thermocouples to a STOP, alas without learning anything useful.

The second reason is that I have just done a crude study of what the temperature rise might be in bars 2 & 3 of the el capitan abseil, and it comes out at about 7 degrees Celsius per second initially, which seems to me to be dangerously high. However, there is obviously something going on that I don't understand, because nothing unpleasant happened.
I am happy to PM you the details, or to put them on here (although I am aware that I do go over the top on this sort of thing and don't want to bore people too much!).

Again, if you have any references you think would be helpful to me please consider posting on here, or PMing me.

 

Offline Kenilworth

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Re: This looks like a nice abseil
« Reply #34 on: March 12, 2019, 01:18:59 am »
I haven't got any references on hand, only speaking from experience and TAG tradition, both of which may be wrong. The spacer debate has a long history in the part of the country with lots of deep pits, but I don't think the heat aspect was ever part of it. Some people think that spacers are unnecessary or dangerous. In a pit with lots of rope weight below, and a long way to ride, some light cavers need to feed rope or try to spread the top bars. Spacers solve this problem, but also reduce the maximum friction available. Like many gear-related arguments on this forum, I think the anti-spacer fears were almost exclusively hypothetical.

That the second and third bars get the hottest has been scientifically established by someone, forget who, but informally understood for a long time.

No idea what to say about your glazing incident. I've done plenty of fast rappels (have never wetted the rope or descender) with every type of device and have never glazed a rope or seen it done.

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #35 on: March 12, 2019, 08:43:16 am »
Thank you.

Online mikem

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Re: This looks like a nice abseil
« Reply #36 on: March 12, 2019, 09:19:01 am »
It could have been the construction of the rope that caused the glazing, rather than anything you did.

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #37 on: March 12, 2019, 12:04:29 pm »
The rope was cut from a 200m reel and no other ropes did that throughout their lives. One of them survived, in short lengths, until quite recently.
It wasn't so much the glazing that surprises me, it was how hot the descenders got - the glazing was well nigh inevitable after that.

The only explanation I could ever come up with (and I'm not really convinced) is if the relative humidity at that time was very low, and the rope was much drier than normal. It wouldn't pick up much moisture in the short time it took us to get down the entrance pitches. Unfortunately I can't even remember what year it was, let alone the date, otherwise meteorological records might help.

It seems likely that somewhere in the USA there are big surface pitches in areas that have much drier air than we ever get here.


Offline Fulk

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Re: This looks like a nice abseil
« Reply #38 on: March 12, 2019, 02:40:42 pm »
I've known two of our ropes get badly glazed; one was 10/10.5 mm Marlow, which suffered on the big pitch in Bar Pot (~30 m) from, I think, only one descent, the other was 9 mm Edelrid, which got glazed by a single descent of Alum Pot Main Shaft (SE end – so 50 m?

Offline Kenilworth

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Re: This looks like a nice abseil
« Reply #39 on: March 12, 2019, 05:20:12 pm »
Also, research glass transition, which best describes glazing (not melting). Heat and pressure are both involved.

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #40 on: March 12, 2019, 05:35:37 pm »
I've known two of our ropes get badly glazed; one was 10/10.5 mm Marlow, which suffered on the big pitch in Bar Pot (~30 m) from, I think, only one descent, the other was 9 mm Edelrid, which got glazed by a single descent of Alum Pot Main Shaft (SE end – so 50 m?

Both dry pitches, usually.
I always used to keep ropes dry if possible, having heard that they were stronger like that.
Since that incident I make sure they are wet.
I think even when wet 9mm caving rope is plenty strong enough!

Offline langcliffe

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Re: This looks like a nice abseil
« Reply #41 on: March 12, 2019, 06:46:43 pm »
I had a 10.5mm rope glazed on the Lancaster Hole entrance pitch by a student club. I suspect that they abseiled too fast, and then braked and hung about on the same bit of rope. In general, I wet my (I now use 9mm) ropes the day before. This allows them to drain, but also retain enough dampness to keep any heat down.

Offline Ian

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Re: This looks like a nice abseil
« Reply #42 on: March 13, 2019, 07:25:04 am »
I would suggest that it is our design of rack that does it? Having a solid bar means that on a short pitch it doesn't get as hot (which is good) but on a longer pitch the bar will retain a lot more heat resulting in the potential for glazing of the rope. Having a bar which is thinner (or hollow) allows the heat to dissipate quicker (more surface area) and also limits the amount of heat energy stored in the bar.

As a result the hollow / curved section bars might get hot quicker but will not retain the heat / will dissipate the heat quicker resulting in less chance of glazing the rope.

I like the solidity of our rack and would not want to switch to bars shown in the video but there are significant downsides to the Petzl type rack. And as for Stops etc ..........

Online mikem

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Re: This looks like a nice abseil
« Reply #43 on: March 13, 2019, 08:18:57 am »
Following Kenilworth's suggestion, this paper states that water causes a decrease in the glass transition temperature of nylon, so my assumption is that a wet rope will dissipate the heat throughout the whole structure, whilst a damp patch on the sheath is more likely to glaze (as the heat is kept within that part of the rope, as well as reducing its ability to cope):
http://personal.strath.ac.uk/andrew.mclaren/Journal%2520of%2520Materials%2520Design%2520and%2520Applications/AJ%2520McLaren.pdf

Elsewhere I found that nylon 6 has a glass transition temperature of only 48'c, compared to melting at 214'c.

I like the sound of Langcliffe's soaking the previous night & then draining somewhere cool enough that it won't dry out. (The only rope I've heard of totally failing a drop test, that wasn't damaged or affected by chemicals, was one that was left hanging on a heater, so the temperature built up within).

Mike
« Last Edit: March 13, 2019, 08:46:32 am by mikem »

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #44 on: March 13, 2019, 09:20:33 am »
Following Kenilworth's suggestion, this paper states that water causes a decrease in the glass transition temperature of nylon, so my assumption is that a wet rope will dissipate the heat throughout the whole structure, whilst a damp patch on the sheath is more likely to glaze (as the heat is kept within that part of the rope, as well as reducing its ability to cope):
http://personal.strath.ac.uk/andrew.mclaren/Journal%2520of%2520Materials%2520Design%2520and%2520Applications/AJ%2520McLaren.pdf

Elsewhere I found that nylon 6 has a glass transition temperature of only 48'c, compared to melting at 214'c.

I like the sound of Langcliffe's soaking the previous night & then draining somewhere cool enough that it won't dry out. (The only rope I've heard of totally failing a drop test, that wasn't damaged or affected by chemicals, was one that was left hanging on a heater, so the temperature built up within).

Mike
I'm afraid the link wouldn't work for me.

That's an interesting idea (about a damp patch on an otherwise dry rope being more likely to cause glazing).

If so it would follow that a completely dry rope (presumably like the el capitan one) may not suffer damage below (say) 100C, whereas one that had been kept for a long time in dry conditions but had then picked up some surface moisture in the cave could glaze completely at lower temperatures.

I have requested a recent publication on the subject of glass transition temperature.
I suspect, from what little I have just read on the (entirely new to me) subject, that I shall struggle to understand much of it.


Online mikem

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Re: This looks like a nice abseil
« Reply #45 on: March 13, 2019, 09:38:05 am »
If you Google, glass transition of rope, it came up top for me.

It would explain why the rope at Lancaster Hole glazed, where drips would fall onto & run down the sheath, if they took a long time before they all abseiled - quite likely with a student group. (Putting the rope down momentarily in puddles, or a shower of rain, could have the same effect).
« Last Edit: March 13, 2019, 09:52:59 am by mikem »

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #46 on: March 13, 2019, 09:49:30 am »
If you Google, glass transition of rope, it came up top for me.

It would explain why Langcliffe didn't glaze his rope at Lancaster Hole, where drips would fall onto & run down the sheath before the next group arrived & used his rope... (putting the rope down momentarily in puddles or a shower of rain could have the same effect).

OK, I shall have a look.

But I have just realised that it wouldn't fully explain what happened to us in Nettle - those descenders got REALLY hot.

Offline ALEXW

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Re: This looks like a nice abseil
« Reply #47 on: March 13, 2019, 10:05:47 am »
Would the loose mantle on a new dry rope not act as an insulator and prevent the heat from being transferred into the more dense core of the rope.  This may only be an issue the first time the rope is used as the mantle very quickly tightens. I always wash new ropes and repeatedly pull them through a descender, it is surprising how quickly the mantle tightens and the rope shrinks.

Online mikem

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Re: This looks like a nice abseil
« Reply #48 on: March 13, 2019, 10:18:21 am »
The overheating in Nettle is likely to be a combination of factors, but ALEXW has a good point... (a hot day / low humidity would exacerbate the effect).

Nylon 66 has a melting point of 268'c, but dry glass transition of 60'c can reduce to 10'c when wet!

Climbing rope is normally made of 66:
http://www.chemistryislife.com/the-chemistry-of-2
« Last Edit: March 13, 2019, 10:47:23 am by mikem »

Offline Kenilworth

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Re: This looks like a nice abseil
« Reply #49 on: March 13, 2019, 11:40:17 am »
I like the solidity of our rack and would not want to switch to bars shown in the video but there are significant downsides to the Petzl type rack. And as for Stops etc ..........

Having used racks with solid, tubular, and u-shaped bars, I prefer the stainless u. They wear well, ride well, and I've never experienced any heat problems. My SS rack was stolen from my car, so now I stick to a micro with stainless tubular bars. Little people like me often struggle to move with sticky  Al rack bars, and I think that is the only significant downside.

I once made a set of oak bars, with the intention of trying to scorch them (to be honest I wanted the rack to burst into flames). The bars glazed/polished after a few fast rappels but never became dangerously fast or scorched or damaged the rope.