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sixleggedinsect
Jun 18, 2009, 2:40 AM
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quoted from the "its not the design" thread..
adatesman wrote: Axle centering was tested using software developed by John Field specifically for finding cams that have axles where they shouldn't be.
im intrigued by that program, and i have a question for the experts- im not going to pull test (or whip on) my aliens, but i would like to minimize my risk just a little bit. if i run my cams through that program, what is an acceptable range of cam angles, and when should consider a cam unsafe or unusable?
what is the range of values seen from the companies who are never (rarely?) accused of poor manufacturing practice? if i cam-angle-tested 20 metolius cams, and 20 bd cams, would they be 14 (or whatever) deg +/- 2 degrees? or less? or right on every time?
im assuming there is some play in the real-world manufacturing processes, and some further uncertainty would be introduced by my own photo taking and curve-fitting. i know one way to control for this is to do lots of angle checks on all my other cams, but im curious if there's an answer nonetheless.
(This post was edited by sixleggedinsect on Jun 18, 2009, 2:43 AM)
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blondgecko
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Jun 18, 2009, 4:15 AM
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Aric posted a link to Vaino Kodas' site discussing the physics of cams. The pertinent bit is that, in order to not slip out, the tan of the camming angle must be smaller than the coefficient of friction.
The non-intuitive bit of cam physics is this: the critical angle is independent of the force applied. Up until the point where something deforms or breaks (whether that be the rock or the cam) a cam that holds under body weight should hold under any weight - it's highly unlikely that it will be the aluminium-rock interface that gives first.
Below, I've plotted the relationship between the coefficient of friction and cam angle. The blue boxes cover the approximate range of aluminium-rock friction coefficients found in the wild. Purple is the static friction coefficient between aluminium and steel (ie. Aric's rig), and green is the sliding friction coefficient between the same.
Short version: in situations where the friction is good, cam angles as high as 20-25 degrees may hold. I would hazard that this actually describes a fairly wide range of climbing conditions. In other situations (very hard, smooth, parallel rock with an intrinsically low friction coefficient) even perfectly-made cams are already pushing right up against the critical angle.
We have at least three climbing areas here in Australia that fit this description: Frog Buttress, some parts of the Glasshouse Mountains, and (especially) Mount Arapiles. I know of accidents in all three places that involved cams pulling out - in two cases, under body weight.
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sixleggedinsect
Jun 19, 2009, 3:18 AM
Post #3 of 5
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i realize now that the right place for this question is in the mis-centered axle thread (mods, feel free to shift things), and that some of my original questions are answered there. apologies for the blind posting.
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sixleggedinsect
Jun 19, 2009, 4:10 AM
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blondgecko wrote: Below, I've plotted the relationship between the coefficient of friction and cam angle. The blue boxes cover the approximate range of aluminium-rock friction coefficients found in the wild. Purple is the static friction coefficient between aluminium and steel (ie. Aric's rig), and green is the sliding friction coefficient between the same.
forgive me a dense quesiton- the friction coefficients for Al/Steel are based on smooth surfaces, right?.. and woudlnt really apply to a knurled/textured steel plate test fixture, where im assuming there is a larger-scale mechanical resistance to the lobes slipping out after the initial pull deforms the aluminum slightly.
am i imagining this?
i remember some folks being surprised about the relative friction coefficients between the metals and the rocks and whatnot.
the granite/Al friction coefficient.. is that also a perfect surface theoretical thing? i find it hard to believe a cam in a smooth steel jig would hold better than a cam in a rock jig, but this may be because im used to smooth metal, and ive never placed a cam between two sheets of polished (ie: smooth) granite (and i doubt too many folks will).
basically, i guess im saying those theoretical friction coefficients dont matter much in this situation, and that in the end the test jig is solely to check out whether the cam is structurally sound, not whether it will slip.
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blondgecko
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Jun 19, 2009, 4:17 AM
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sixleggedinsect wrote: blondgecko wrote: Below, I've plotted the relationship between the coefficient of friction and cam angle. The blue boxes cover the approximate range of aluminium-rock friction coefficients found in the wild. Purple is the static friction coefficient between aluminium and steel (ie. Aric's rig), and green is the sliding friction coefficient between the same. forgive me a dense quesiton- the friction coefficients for Al/Steel are based on smooth surfaces, right?.. and woudlnt really apply to a knurled/textured steel plate test fixture, where im assuming there is a larger-scale mechanical resistance to the lobes slipping out after the initial pull deforms the aluminum slightly. am i imagining this? i remember some folks being surprised about the relative friction coefficients between the metals and the rocks and whatnot. the granite/Al friction coefficient.. is that also a perfect surface theoretical thing? i find it hard to believe a cam in a smooth steel jig would hold better than a cam in a rock jig, but this may be because im used to smooth metal, and ive never placed a cam between two sheets of polished (ie: smooth) granite (and i doubt too many folks will). basically, i guess im saying those theoretical friction coefficients dont matter much in this situation, and that in the end the test jig is solely to check out whether the cam is structurally sound, not whether it will slip.
I think this pretty much nails it - which makes the fact that the Aliens slip out anyway all the more scary, IMHO.
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