Chocolate fireguard
Active member
langcliffe said:
It's nice to know one's meager efforts to entertain are appreciated. By some.
This looks like a nice abseil
- Thread starter Pitlamp
- Start date
It's nice to know one's meager efforts to entertain are appreciated. By some.
Chocolate fireguard
Active member
Sorry. I typed meagre and changed it without checking when I saw the red underline.Fulk said:Is there such a word as meager?
Re the abseil: only two of those bars are hollow, the top two. The top one is thin-walled but No 2 is thick-walled and has the holes for the rack sides drilled such that they do not go through on a diameter - causing the rope to be diverted more, presumably to increase the friction if necessary. 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.
All of the other 6 bars are thin-walled and U-shaped. Somebody must have worked out that this gets heat away from the friction surface and into the moving air faster than conducting it away through a solid aluminium alloy bar.
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.
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.
Hollow, as in tubular, bars are not a good idea. Perhaps they have to do it with the top bar with its extension and pin, but they have made sure it doesn't divert the rope much so heat generation will be less than in No 2. They made that one thick-walled.
Decreasing the mass to increase the surface area, without making sure that forced convection will always get to that area, will just cause the bar to get hotter for a given heat input.
Chocolate fireguard
Active member
Yes the spell check tried it on again, but I was ready for it this time!Fulk said:
Kenilworth
New member
Chocolate fireguard said:
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.
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
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
kay
Well-known member
mat said:
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.
kay
Well-known member
mikem said:
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.
andybrooks
Member
I was thinking arched/vaulted = humped
Chocolate fireguard
Active member
Kenilworth said:
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.
Kenilworth
New member
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.
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.
Chocolate fireguard
Active member
Thank you.
Chocolate fireguard
Active member
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.
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.
Kenilworth
New member
Also, research glass transition, which best describes glazing (not melting). Heat and pressure are both involved.
