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by Chris Harmston (firstname.lastname@example.org)
NdE: since this correspondence, Chris has done more experiments and Greg Luebben has written a more detailed paper on this topic which should be of general interest to any ice climber, since it challenges some "truths". Most of the contents of the paper is published in Climbing no 172 (Nov 97), pp106-115, which I encourage everyone to read. Apart from the angle issue described here, two other important findings are the weakness of ice hooks (ie BD spectre), and the failure of tie-offs (the screw bends, the tie-off slips and then is cut at the hanger). For a copy of the full paper, send Chris email.
The screws (BD style threads) actually are stronger when angled in the direction of the fall. The reason for this is that the threads do in fact have a high pullout strength. What I have seen is that the ice around the surface of a screw placed as you describe is the limiting factor. For a screw placed perpendicular to the surface or slightly angled back the top surface layer of ice breaks at fairly low loads (500 to 2000 lbs) and then you have the lever effect going on which is weaker. When the screw is angled down there is much less force on the top surface layer and it helps support the shaft so the screw does not flex and bend. However, placements are always limited by the ice quality. In "good" ice I think all screws will hold regardless of the angle (within limits of course). In marginal ice the situation is not so clear. In my mind, the best placement is always perpendicular to the ice if possible. Obviously, if the ice is thin and you know you have to tie off then it is deffinitely better to angle the screw up so the tie off stays next to the ice. But, in tests I have done on 22 cm screws tied off the shaft flexes and about 2000 lbs the sling slips to the end of the hanger anyway and pullout occurs. Note, the reason I did not correct your post in rec.climbing is because I am in the process of doing further investigations on this exact issue. I am looking into the effects of impact (i.e. I am going to take some whippers without sharp things attached of course) on different placements and actually measuring the forces with a protable load cell. My only problem is that there is no ice here anymore. I suspect I will have to go to Colorado in January to achomplish my testing. I have about 50 screws to play with and see what I can do. I plan on investigating the angle issue in both good ice and marginal ice (if I can find it!) with screws tied off at different lengths. My theory is that only the first 3 inches hold the load unless placed downward so the shear strength of the threads can help. I say this because the testing I have done in the lab on 10 cm screws shows that these are just as strong (and more consistent) than the 22 cm screws. Obviously this is a controlled condition with only one ice temperature and no air pockets so this may not be true. Anyway, if you have any good ideas on how I might better bash gear let me know.
Note, all the above comments are based on a limited amout of data in controlled conditions and may in fact be statisticaly wrong. This is the reason I am doing more testing.
In our batch testing (fairly consistent with a fair bit of data) we always place the screws perpendicular to the ice. The screws either flex and pullout, snap off in the middle of the tube, or bends the hanger off the head of the screw. The loads at which this happens range between 2400 lbs and 6500 lbs. This huge variation is due to many variables which I have not been able to narrow down (unless we freeze them in place). If we were to place a 3 sigma rating on the "ice" strength it would be near or below zero. Obviously this does not tell us anything since they do in fact hold more than 10 kN (2248 lbs) and this is why we rate the hanger strength 3 sigma and the minimum strength in ice is 10 kN.
Chris Harmston (email@example.com) Black Diamond Equipment Ltd. 2084 East 3900 South, SLC, UT 84124 phone: 801-278-5552 DISCLAIMER: Unless otherwise indicated, this correspondence is personal opinion and NOT an official statement of Black Diamond Equipment Ltd.
At the Ice Festival in North Conway, we did an interesting experiment. One of the guides placed a 17cm BD screw at a substantial negative angle (more than recommended by BD). Eight climbers tied into it with a static rope (aprox. 1500 lbs.), then pulled as hard as we could. The end of the screw flexed about 1/8" and that was it - no evidence of loosening after ten minutes of yanking on it. The ice was good, but certainly not the best I''ve seen. It was also pretty cold (single digits) and the surface ice was brittle, but there was no cracking around the screw. Overall, I was pretty impressed. Granted, we didn''t simulate a true shock load, but this test definitely gives me more confidence that a screw in decent ice wil actually hold a fall and that the negative angle is a "positive" idea.Contributed by Brian Nystrom (firstname.lastname@example.org) on February 26, 1998.
I have been placing icescrews at a negative angle for about two years now. From personal experience during falling and thanking the ice screw for holding, I have found that in most cases the negative angle works considerably better. During an out and (and unfortunately) downward fall the upward forces (Newtons Law) applied at the apex of the screw are minimised, in turn minimising cracking of the ice, bending of the shaft and head, and therefore maximising the retentive properties of the thread. Obviously there is good and bad ice which will cause screws to ''pop'' no matter where they''re positioned, but I personally believe that it only makes sense to have the screw positioned perpendicular to the pulling load, not perpendicular to the cliff face because you can bend an ice screw but lets see you stretch it! I''d be very interested to see the results of experiments on this hot debate one way or the other.Contributed by Tony Eldridge (email@example.com) on March 22, 1998.
Hi, I have been conducting real-life testing of the differences in negative vs positive angled ice screws in New Zealand over the last season and hopefully will continue into next season. So far the results of the differences are not conclusive and more tests are required but if anyone would like the preliminary results and conclusions or have any useful comments feel free to contact me.Contributed by Alexis Shead (firstname.lastname@example.org) on August 22, 2001.
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