rob Posted October 20, 2008 Posted October 20, 2008 OK, maybe these questions have been asked before, but whatever. First of all, how many of you have ever actually fallen on a screw? Whenever I place one, I always think "yeah right." Which leads me to my next question. I understand the mechanics of a good cam in good rock. But, what keeps a screw in, anyway? Is it the threads? Or is it held in by the same compression mechanics as a picket? Or is it both? One more question, while I'm here. What's the story with the angle of a screw? Some people tell me to place it perpendicular, some say to angle it down, some say to angle it up? WTF. Quote
el jefe Posted October 20, 2008 Posted October 20, 2008 wishful thinking is what keeps the screw there. also, i've found that sprinkling a little pixie dust on it never hurts. and no, i've never fallen on one, either. Quote
G-spotter Posted October 20, 2008 Posted October 20, 2008 the threads keep it in. the picket mechanism does not, and in fact can lead to ice breakage and screw failure. that's why you should put it in perpendicular to slightly down. falling on ice is more likely to result in injury from crampon points catching ice on way down/getting stuck into leg/whatever or you hitting a mushroom/feature and breaking something, than screw failing. i've never fallen on one myself but i've caught two. Quote
Gary_Yngve Posted October 20, 2008 Posted October 20, 2008 The holding power of the screw comes from the threads. The failure of a screw, assuming good ice, generally comes from a stress cone breaking off around the surface of the screw and the screw getting levered/deformed. The most important thing to realize is that a screw is best when in all the way. Tying off (when there's an inch or more sticking out) is better than clipping at the end for dealing with a screw that bottoms out, but nowhere near as good as using a shorter screw to begin with. Some said to angle the screw up to get a picket-like effect. Others said angle down to make the stress cone friendlier. I think it's probably better to put it in at the angle near perpendicular that is the least awkward/tiring to place. Quote
mike_m Posted October 20, 2008 Posted October 20, 2008 Never fallen on one, but held 2 falls onto screws. The best testing data came out several years ago, that Chris Harmston did while at BD. He was pulling screws to failure, using solid water ice at cold temps (-10???), a load cell, and I believe a high-speed camera; I've got his white paper at home. He found what Gary related above, screws fail when loaded and break out the stress cone, then the tube buckles where it is no longer supported by the surrounding ice. He found the optimal angle, in those conditions, to be 5-10 degrees down, and rely on the threads for holding power, with the screws failing at much higher loads that when placed at the old CW, 10-15 up. Interestingly, I think I recall his finding that 17cm screws were failing at nearly the same load levels as the longer 22's--both lengths have the same amount of threads; I'll have to double-check the white paper when I get home. Quote
catbirdseat Posted October 20, 2008 Posted October 20, 2008 I fell once on a screw and it held. It was a 40 foot fall after rope stretch. I think I was about 15 feet above the screw when I fell. Weekend_Climberz caught me and injured his chin when he was pulled into the ice. I was uninjured. I was very, very lucky, however. WC went up to remove the screw. He said the ice was a little damaged around it but the screw itself was undamaged. I remember having placed the screw into very good ice, perpendicular to the surface. Quote
tomtom Posted October 20, 2008 Posted October 20, 2008 Testing and more testing. [video:youtube]Nfjxrzw_jZs Quote
rob Posted October 20, 2008 Author Posted October 20, 2008 great links! A final word of caution relative to equalizing two screws. Ice generally fails horizontally. Placing two screws horizontally also increases the force on the screws due to the triangle force multiplier (American Triangle). Place the screws vertically with one screw above the other. This will give the best possible chance for both screws to hold. Interesting. Does that mean when equalizing two v-threads, they should also be placed vertically? Quote
kevino Posted October 20, 2008 Posted October 20, 2008 On the subject of this, what are people favorite screamers? I know yates makes them, so does petzl etc. Quote
Gary_Yngve Posted October 20, 2008 Posted October 20, 2008 v-threads break from either failure of the cord or failure of the ice. in the latter case, a whole chunk of ice (say a 2-foot-wide block) will break away (unless the ice is just really really bad). you want to place each v-thread in good ice. you want the v-threads equalized in such a way that the angles are small. you want the v-threads placed so they are independent of each other (the failure of one won't affect the strength of the other) Quote
mike_m Posted October 20, 2008 Posted October 20, 2008 Yates shorties, but more brand-loyalty than anything else; they are what I first had, and stuck with it. Quote
alps Posted October 20, 2008 Posted October 20, 2008 A few years ago, the German Alpine Club published a test conducted in real waterfall ice. I'll try to summarize the results as I remember them: ice screws in solid, compact ice seemed to hold about as well as bolts (in fact, they broke quite a few bolt hangers in the process - they used them to anchor their test setup). Also, making screws longer than medium lengths doesn't seem to improve their strength in solid ice very much. In not-so-great or bad ice (lots of air in the ice), the results are somewhat unpredictable. They also tested snargs and v-threads. Snargs seem pretty crappy in terms of what they hold, for v-threads in good ice it was usually the cord which broke. In case anyone can read German and wants to read the article, here is a link: DAV test. And personally, I haven't fallen on an ice screw (and I don't really want to). Quote
alps Posted October 20, 2008 Posted October 20, 2008 BTW, for the optimum angle they got similar results as the study mentioned above. Quote
chriss Posted October 21, 2008 Posted October 21, 2008 http://www.jjgeng.com/html/body_ice_screw.html A FEA done on both screw and ice. Interesting for those who care for this type of thing. Quote
montypiton Posted October 22, 2008 Posted October 22, 2008 threads do help a screw stay in the ice, but a greater factor in the screw's holding capacity is refreezing -- once the tube stops turning, assuming the ice is below freezing temp, the tube freezes in place, and both the threaded exterior surface and the smooth interior of the screw present bonding surfaces... this has led, on occasion, to some terrifying experiences on warm days / sloppy ice... Quote
cbcbd Posted October 22, 2008 Posted October 22, 2008 Haven't fallen on a screw and don't plan on doing it, but I do know that the strength comes from the threads and that a 22cm screw has the same holding strength as a 13cm because they both have the same number of threads. You use longer screws just to get to "better" ice deeper in the surface. But longer screws do hold your head at peace more than shorter screws. Quote
G-spotter Posted October 22, 2008 Posted October 22, 2008 threads do help a screw stay in the ice, but a greater factor in the screw's holding capacity is refreezing -- once the tube stops turning, assuming the ice is below freezing temp, the tube freezes in place, and both the threaded exterior surface and the smooth interior of the screw present bonding surfaces... this has led, on occasion, to some terrifying experiences on warm days / sloppy ice... that's just not true. in fact, it's almost complete bullshit. although, on warm days you get heat conducted into the ice by the screw and a surface water film/less thread contact with ice. Quote
Ade Posted October 22, 2008 Posted October 22, 2008 I took a reasonably significant fall onto a screw last season. Can't say it's something I care to repeat. Get very skinny bouncy ropes would be my advice. Ade Quote
rbw1966 Posted October 22, 2008 Posted October 22, 2008 I've taken a significant fall on a screw that also had a screamer on it (yates). I had placed it perpendicular to the ice (less than optimcal ice) and it held. The screamer fully deployed. I do not recommend doing this yourself. Quote
chriss Posted October 23, 2008 Posted October 23, 2008 Haven't fallen on a screw and don't plan on doing it, but I do know that the strength comes from the threads and that a 22cm screw has the same holding strength as a 13cm because they both have the same number of threads. You use longer screws just to get to "better" ice deeper in the surface. But longer screws do hold your head at peace more than shorter screws. Actually BD's 13cm screw has more threads than the 22. So it should hold more. But, every test has shown this to be false. It's all about the quality of the ice. Quote
alps Posted October 23, 2008 Posted October 23, 2008 Actually BD's 13cm screw has more threads than the 22. So it should hold more. But, every test has shown this to be false. It's all about the quality of the ice. On the shorter ice screws, the threads extend all the way to the hanger. So the outermost threads don't have much to hold on to... Quote
cbcbd Posted October 23, 2008 Posted October 23, 2008 (edited) Haven't fallen on a screw and don't plan on doing it, but I do know that the strength comes from the threads and that a 22cm screw has the same holding strength as a 13cm because they both have the same number of threads. You use longer screws just to get to "better" ice deeper in the surface. But longer screws do hold your head at peace more than shorter screws. Actually BD's 13cm screw has more threads than the 22. So it should hold more. But, every test has shown this to be false. It's all about the quality of the ice. Assuming that the quality of the ice is the same (ie. good) throughout, the threads "should" hold the same. Craig Luebben's How to Ice Climb book has a good chapter on screws and tests. Even in controlled conditions there were huge variances in results. eg. in 17 tests with 10cm stubbies (which definitively have less threads then the rest) they held between 300-4100lbs - ice is variable and a gamble - you just gotta try your best with getting good gear in and just not fall. I took a pic of my older 22 BD express, older 16 BD turbo, and new gen 13 BD Turbo. I am not sure if they changed the new 22cm screws, but the threads on all 3 were the same: Edited October 23, 2008 by cbcbd Quote
cbcbd Posted October 23, 2008 Posted October 23, 2008 I didn't say that they did. I was comparing screws of different lengths with the same number of threads - meaning 13cm and up Quote
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