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esander4
Mar 23, 2011, 12:03 AM
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Just a curiosity question, what happens to the cam angle when a cam is placed in a constricting or flaring crack? Does the cam angle change? How can a cam angle remain constant when not all cracks are parallel?
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hafilax
Mar 23, 2011, 12:15 AM
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esander4 wrote: Just a curiosity question, what happens to the cam angle when a cam is placed in a constricting or flaring crack? Does the cam angle change? How can a cam angle remain constant when not all cracks are parallel?
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esander4
Mar 23, 2011, 12:44 AM
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No, it doesn't. I already know about what a cam angle is and the physics, but that's assuming a parallel crack as previously said. My question was about a downward constricting or flaring crack. I guess I should have been more specific, of course the cam angle changes, but how does that effect the outward force? Without the parallel crack do the physics still apply or do you need another method of calculating the force?
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viciado
Mar 24, 2011, 3:26 PM
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Although the Totem document is aimed at showing how their cam is a good solution to the challenge of reliable placements in flared cracks, they do provide not only the answer to the OP, but in the process also help us see to some extent the effective range of (their) cams is in a flared crack. Their document adds numbers to what most eyeball and yank test evaluations of standard cams in flared placements will tell you.
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Rudmin
Mar 24, 2011, 3:41 PM
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This is all for regular cams (aka Mastercams):
Flares are pretty straight forward, as the flare angle increases from 0 to something around 28 degrees, the effective cam angle goes to 0 at which point the cam should roll out. Even at small effective cam angles, a tiny slip or shift of a cam lobe could quickly tip out a cam beyond its range.
Also, as the flare angle increases, the axle or lobes will experience larger than normal stress. The typical cam has something like a 4:1 ratio of expansion load to hanging load in a parallel crack. As the crack flares that ratio approaches infinity. This will stress the cam and the rock a huge amount.
One of the advantages of a flaring placement is that it is much harder to walk a cam into it.
Some people might think that flares are good because they increase the "holding power" of a cam. While they do increase the expansion forces, the support the exact same coefficient of friction as a parallel crack because the relative angle of applied forces is still the same at the wall. A cam that slips in a parallel crack will also slip in a flaring crack.
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ptlong2
Mar 25, 2011, 12:02 AM
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Good summary, Rudmin.
The actual cam angle doesn't change of course. It's constancy is the main feature of the logarithmic spiral.
What changes is the outward force. You can think of this in terms of an "effective" cam angle, equal to the actual cam angle minus half the flare angle.
The concept is a little bit muddy since the stability criterion remains the same regardless of the flare, and also because the frictional and normal forces cannot be described exactly with this simple substitution.
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