Oxford University Cave Club
Proceedings 10 : "Pozu del Xitu"
|OUCC Proc 10 Contents|
As those that have read OUCC Proc. 9 may suspect, there have been interminable arguments in the club about rope walkers and rope walking versus sit-standers and Petzl jammers. To summarize for those lucky enough to have missed out on this prolonged infighting, I have drawn up this table:
|Rope walking||Quicker on big pitches. Less tiring.||Intricate system required. Slow on short pitch since putting on gear requires long time. Starting off causes some people problems.|
|Sit-stand||Simple. Quick on short pitches. Easy to rest.||Slow on big pitches. Overhangs difficult.|
|Gibbs type ropewalker||Strong. Grips on muddy and icy rope.||Needs to be dissembled to put on rope. Bits are separate and can be lost.|
|Jumar type spring cam jammer||All one piece. Easy to put on/take off rope.||Can slip on muddy, etc. rope. Some types subject to spring failure.|
It was with some interest, therefore, that amongst a pile of mangled krabs Dave Brooks was showing us in his lecture at the 1980 BCRA conference in Nottingham I saw a novel rope walker developed at the University of Warsaw, which had run off a batch of about 500 of them. This particular one was slightly bent in the testing procedure Dave had put it through, but other than saying that it was 'surprisingly strong' no comment was passed on it. I found that rather puzzling since this highly innovative device was a single piece. There were no strings to snag, bog chains to break, wires to bend, holes to clog up with mud or parachute pins whose balls could drop off. Fired by enthusiasm and with mental images of me shooting up Flat Iron, overtaking the sit-standers and pouring scorn on the rope walkers who were pouring scorn on the sit-standers, I borrowed this thing, took it to pieces (the essence of scientific technique is to take something apart to understand how it works and then why it won't when you've reassembled it), sketched it and took the drawing back to Oxford.
On getting back to Oxford I showed the blue print (actually a grey print since they only let me use pencils in here) to an engineering friend of mine, to ask his advice on constructional details. When he'd got back onto his chair (I can only assume he had swooned at the excellence of my T.D. and that he was holding his sides and shaking merely to improve his breathing) I showed him which way up it should be and which of the grubby doodlings he was supposed to be considering. We then got down to a discussion of the how, what and whys.
Since the device has effectively only got one side (see figure 1 for overall view) I was worried about its bursting strength. I therefore wanted to use a thickish (3/32") piece of stainless for the sheath. Being a practical chap, however, Phil, pointed out to me that I had to bend it and that I would be better off using a thick piece of Dural. The advantage of Dural is that it is less dense than stainless steel and, since the bending moment is proportional to thickness squared, doubling the thickness would increase the bursting strength whilst reducing the weight of the sheath. Since the flexural modulus of Dural is greater than 50% of that of stainless I could make a stronger, lighter sheath by using thick Dural.
This I did and then made the cam, again of Dural (it's light, cheap and there's plenty of chunks of it knocking round our workshop) and bolted the whole lot together with a 3/8" stainless steel bolt, hammering over the threads and then covering the lot in Araldite to fix it and smooth it over at the same time.
With my nice, new, shiny ascenders in my paw I trotted off to test the things out on one of the trees surrounding Alum Pot. (I chicken out of a 220' freehanging pitch on untried ascenders. It's an allergy I've called safety.) The system I used them in was a standard rope walking system - one Warsaw walker on my ankle, one on my knee and a Lewis ascender on my shoulder with a Howie belt.
They ran up surprisingly smoothly - I didn't need any weight on the rope to start off. Then the acid test - how easily would they accidentally come off the rope? I sat down 15' above the ground with all my weight on my top (Lewis) ascender (i.e. no weight at all on the Warsaw walkers) and then I got Martin to vigorously shake the rope. I wasn't sure whether he thought he was skipping or whipping but the rope flew around for about a minute and both Warsaw walkers stayed on the rope - success!
a) pull the krab hole up, lie the upper part of the rope along the cam and thence hook it inside the sheath (figures 2, 3).
b) pull the krab hole down, lie the lower part of the rope along the underside of the cam and then hook the rope inside the lower lip of the sheath (figure 4, 5).
To take the device off the rope the process must be exactly reversed.
In Spain I found these gizmos to be very good indeed. They did wear a bit so that I had to perform the usual tricks that rope walkers do to start off on a pitch - maybe a more advanced bearing and more attention to the top toothed part of the cam would improve this. At no time did these things come off the rope, except when I took them off - all in all they performed very well. Some people may wish to make a hole or two in the sheath to fix elastics for rope walking - that's easy, but you should remember that if you fix both ends of the elastic to the cam you must fix them both to the same side of the sheath otherwise you need to undo the elastic when removing the walker from the rope - defeating the object. Finally a few points in conclusion, some obvious and some maybe not.
|Pro:||All one piece - no bits to lose.
Can be put onto rope whilst still attached to you - no dropping them down pitches.
Work like rope walkers - i.e. on muddy and icy ropes.
|Con:||Cannot be used for self lining - I haven't checked the bursting
strength but I guess self lining might be pushing it.
Cannot be used on shoulder - you can't get them off the rope if your weight is on the rope below the device (I didn't have any problems on Flat Iron though with the weight of 100m of rope below) - even though they never come off in use I reckon a Petzl or Gibbs on the shoulder makes me feel safer (a Petzl made more sense since the whole point is to eliminate Gibbs).
(Ed's note: as a successful rope walking caver, I'd like to comment on this point. 1) I can't think how you'd mount a Petzl on your shoulder so that it would run up the rope easily and hold you close in to the rope. 2) Neither the Petzl nor the Gibbs are as efficient a device for the shoulder as an ascender containing rollers, e.g. the Lewis (Caves and Caving 13, p. 26). It's philosophically pleasing to consider a system with three one piece ascenders but I think that the loss in efficiency due to the Petzl on the shoulder would outweigh any of the advantages.)