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Cam Physics Explanation Please!
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ARowland wrote: If you do a free body diagram of the flake, you'll see what I mean. There's 2x at the cam, and the reacting force at the attachment of the flake.Okay, if you include the flake attachment then it can be modeled as a wedge with a mechanical advantage ~ 1/tan(wedge angle) so for 1/tan(14x2 deg) is about 2. So maybe the oft quoted 4x factor must include the pulley effect? From outdooradventureclub.com/ne… : Brand Cam Angle Outward Force multiplier Metolius 13.25º 4.36 WIld Country 13.75º 4.2 Black Diamond approx 15º 3.88 Aliens 16º 3.64 "So what does it mean? First off it means that it's true, cams do generate roughly four times the downward force as outward force." ARowland wrote: I thing a carabiners efficiency has been meansured to be in neighborhood of .7 IIRC. 1.7x isn't so far off. And hey, maybe the top piece is a DMM revolver?From Luebben's anchor book: books.google.com/books?id=I… "The pulley effect. The top protection gets loaded by the falling climber and the belayer, increasing the load by 60 to 70 percent above the force on the climber." |
Aric Datesman wrote: Except in cases where it's keyholed, in which case it's square peg/round hole and there's no outward force. Long story short, there's a whole lot of "it depends" going on with passive pro, and it's much harder to quantify than with active gear.Same applies for a cam when placed above a constriction. The cam angle changes and a different outward force is applied. What about a #1 Camelot placed as a passive device. There's a shitload of depends but I think for the sake of arguing let's keep it to the basics. Wouldn't the outward force = tangent(cam angle) * downward force ? Not taking friction into account. |
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Downforce / tangent(cam angle) = outward force. |
hikingdrew wrote: Okay, if you include the flake attachment then it can be modeled as a wedge with a mechanical advantage ~ 1/tan(wedge angle) so for 1/tan(14x2 deg) is about 2. So maybe the oft quoted 4x factor must include the pulley effect?I think what's going on is that the 4x figure is just being misenterpreted. It's not that is wrong really, but applied in the wrong way. The reason why you would want to use the 4x figure is in computing the friction neccesary to keep a cam from slipping. On a given cam lobe, or side of the cam, or even the cam as a whole, the ratio of the outward forces to the vertical forces is going to be 4, which means that the coefficient of friction must be at least .25 in order to prevent slipping. Because this is an important figure for the relative security of a cam, I can understand why it's listed for various models. |
ARowland wrote: ...the coefficient of friction must be at least .25 in order to prevent slipping. Because this is an important figure for the relative security of a cam, I can understand why it's listed for various models.Sure, but when climbing or talking about cam placement, what I'm interested in is what is the force multiplier from the fall to the flake/crack I've stuck the cam in. It has to be burly enough to take the several x times fall force.. We used to say that it should be able to hold a truck (few tons ~ tens kilonewtons) |
ARowland wrote: I thing a carabiners efficiency has been meansured to be in neighborhood of .7 IIRC. 1.7x isn't so far off. And hey, maybe the top piece is a DMM revolver?It depends on the load. I have measured carabiner efficiency values as low as 42% for a top rope fall with a light person. For a lead fall, one can expect around 1.6ish, as you said. I have tested values ranging from 1.55 - 1.7. The UIAA assumes 1.66x for the purpose of writing some of their standards. |
Aric Datesman wrote: Except in cases where it's keyholed, in which case it's square peg/round hole and there's no outward force. Long story short, there's a whole lot of "it depends" going on with passive pro, and it's much harder to quantify than with active gear.Ok, but how about for an idealized case, match fit between rock and nut, and, if you like, frictionless rock? Just for a ballpark comparison between nut and cam... |
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Wild Country's "The Cam Book" was helpful for my understanding of cam mechanics. |
Optimistic wrote: Ok, but how about for an idealized case, match fit between rock and nut, and, if you like, frictionless rock? Just for a ballpark comparison between nut and cam...If it's frictionless, then it should reduce to the wedge case where the force increases by 1/tan(wedge angle).. |
hikingdrew wrote: If it's frictionless, then it should reduce to the wedge case where the force increases by 1/tan(wedge angle)..Which doesn't really apply nowadays, given that with most nuts now have curved surfaces. Then you also have to look at the yield point of the material and point loading to see if the nut slides or lets the rock bite in, which then puts us back to the square peg/round hole scenario. As I said, it's much harder to quantify this for passive gear. |
nicelegs wrote:I just stick it in and hope for the best.I agree, let the engineers do the math. As long as you know what a good placement is your good to go |
climber57 wrote: I agree, let the engineers do the math. As long as you know what a good placement is your good to go(engineer chuckling amusedly over the implication that strength of materials is that intuitive.) Ever climb shale, limestone or tufa? :) |
Aric Datesman wrote: Ever climb shale, limestone or tufa? :)Tufa is limestone.... |
taylor januskiewiecz wrote: Tufa is limestone....So Frankenjura is full of Tufa? News to me, and climbs very different IMO. |
Aric Datesman wrote: So Frankenjura is full of Tufa? News to me, and climbs very different IMO.Are you maybe thinking of "tuff", ie welded tuff? I'm no geologist, but I've always heard the word tufa applied to a rib or column on limestone, not to a distinct rock type. Tuff, I'd thought, was the rock type, some kind of conglomerate. |
Aric Datesman wrote: So Frankenjura is full of Tufa? News to me, and climbs very different IMO.The Frankenjura is limestone from the Jurassic period, hence it´s name. Tufa (in the form of calcerous sinter) is rare around here, I´ve never seen anything substantial in the Franken but probably in the caves there is some. Tuff is consolidated volcanic ash, there is none in the Franken. |
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Welded Tuff is found in areas like Smith Rock and Big Chief. It can be pretty hard and protected with cams, but isn't generally as solid as Granite. |
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Ah, yes. Tuff, not tufa. Sorry sbout that. |
