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patto
Apr 5, 2006, 2:18 PM
Post #26 of 36
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In reply to: In reply to: Based on the WC cam angle of 13.75 we can work out the grip strength. (ignoring the spring tension which is negligable once proper load is applied) Load Expansion pressure on rock (spread over all cams) 0kN 0kN 0.1kN 0.409kN 1kN 4.09kN 10kN 40.9kN Would you mind posting the formula you're using to calculate this?
[Force]/tan(13.75)
1/tan(13.75)=4.08666
So just multiply by 4 to get the total force.
And since we have the calculator out now:
From this we can conclude that for the cam to hold the coefficient of friction between the rock and the cam must be greater than 0.25. (The rock would have to be completely smooth and glassy for this to be the case)
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phunkdified
Apr 5, 2006, 2:41 PM
Post #27 of 36
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ok patto......the basic physics that the camming devices are based on is......wait for it......FRICTION
the expansion of the cam lobes facilitates the cam being placed in a range of crack sizes, not to increase grip.
while i would agree that by forcing the cam to try and expand in an inexpandable space would increase the forces normal perpendicular to the sides of the rock........this is not the force that is important to your protection staying put, the force you are trying to overcome is the force pulling parallel to the rock edges(ie the force of your fall...if you oriented it right)....this force is over come by a force of equal or greater magnitude in the opposite direction (newtons law not mine).....that force is friction
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reg
Apr 5, 2006, 2:46 PM
Post #28 of 36
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In reply to: There is one thing that gets my head spinning in climbing. Fall Factor vs. Kilo Newtons (KN) The fall on gear is rated in FF ( fall factor ) The strength of gear is rated in KN ( Kilo Newtons )
well not really - as you know FF is a ratio of rope out ~ lenght of fall. that's it - i do not believe it was ever intended to speak to force (kn) on the gear because again as we know ropes limit force on climbers to 12kn max and each rope has a different amount of elasticity so that force is different between ropes - some more, some less but none over 12kn. so a ff2 with one rope might be lower/higher in kn then another.
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microbarn
Apr 5, 2006, 3:14 PM
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In reply to: In reality, larger surface area cams can cause lower friction due to the presence of sand. (I would be more careful with FAT cams around sand) For a similar reason, fat slick tires can be terrible in the wet.
Be careful with this. Taking this to extremes, we can see the error of blatantly applying this logic. One grain of sand under a wide cam lobe is not going to change friction at all like slick tires on a wet road. A hand full of sand under the lobe could be compared to hydroplaning, but not one grain.
In the case of one grain of sand, wide lobes deform and contact with the good rock far away from the grain of sand. Narrow lobes would have their contact area reduced more drastically in the case of one grain of sand.
Narrow lobes will cut through a hand full of sand to make better contact as you state, but wide lobes will cope with a few generated grains of sand better.
Dan
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greenketch
Apr 5, 2006, 3:26 PM
Post #30 of 36
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In reply to: In reply to: There is one thing that gets my head spinning in climbing. Fall Factor vs. Kilo Newtons (KN) The fall on gear is rated in FF ( fall factor ) The strength of gear is rated in KN ( Kilo Newtons ) well not really - as you know FF is a ratio of rope out ~ lenght of fall. that's it - i do not believe it was ever intended to speak to force (kn) on the gear because again as we know ropes limit force on climbers to 12kn max and each rope has a different amount of elasticity so that force is different between ropes - some more, some less but none over 12kn. so a ff2 with one rope might be lower/higher in kn then another.
This has all the potential to degrade into the same discussion that has happened many times here. I think this thread is about forces that pull out gear.
For those that are newer at thinking through those forces the entire discussion of FF and Kn is a little challenging. Styndall started down the road in his example. Yes, the kinetic energy increases with the size of the fall. The missing portion for some is that the length of time over which this energy is dissappaited determines the force applied at any moment. That is why there is a Fall Factor variable. A short fall develops less energy but with less rope to catch on that energy is spent in a very short time. With a larger fall factor the time of dissapation goes up faster than the energy gained from a bigger fall.
One must keep in mind that the rope itself is a major buffering element in the system. Anyone that has experienced a short stiff catch (think FF 1, 2 foot fall, caught on a sling direct to the anchor) knows that it is much harder and briefer than a big soft whipper (think FF 1, 30 footer, caught on floss).
I would like to see some data that shows cams pulling at 6Kn or other low numbers. I would suspect the placements in some fashion over the cams themselves.
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patto
Apr 5, 2006, 3:45 PM
Post #31 of 36
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In reply to: ok patto......the basic physics that the camming devices are based on is......wait for it......FRICTION the expansion of the cam lobes facilitates the cam being placed in a range of crack sizes, not to increase grip. while i would agree that by forcing the cam to try and expand in an inexpandable space would increase the forces normal perpendicular to the sides of the rock........this is not the force that is important to your protection staying put, the force you are trying to overcome is the force pulling parallel to the rock edges(ie the force of your fall...if you oriented it right)....this force is over come by a force of equal or greater magnitude in the opposite direction (newtons law not mine).....that force is friction
:lol: :lol:
As you rightly point out friction is the force keeping the cam in place, it is the force parallel to the rock. However what causes the friction? It doesn't appear magically. It is a direct consequence of the force perpendicular to the rock, and increase linearly with this perpendicular force. Thus your original statement is completely wrong.
If the cam angle was say 60 degrees instead of 13.5 then there is no chance in hell for you magical friction to stop the cam pulling, as the perpendicular force is not high enough.
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reg
Apr 5, 2006, 3:46 PM
Post #32 of 36
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[quote="greenketch"]In reply to: In reply to: There is one thing that gets my head spinning in climbing. Fall Factor vs. Kilo Newtons (KN) The fall on gear is rated in FF ( fall factor ) The strength of gear is rated in KN ( Kilo Newtons ) well not really - as you know FF is a ratio of rope out ~ lenght of fall. that's it - i do not believe it was ever intended to speak to force (kn) on the gear because again as we know ropes limit force on climbers to 12kn max and each rope has a different amount of elasticity so that force is different between ropes - some more, some less but none over 12kn. so a ff2 with one rope might be lower/higher in kn then another.
In reply to: . Anyone that has experienced a short stiff catch (think FF 1, 2 foot fall, caught on a sling direct to the anchor) .
that's a senerio where people get broke, harnesses get broke, and death.
it was drummed into me early on : never ever climb above your gear on a sling. good rule of thumb but i can not speak to the exact forces of falling 2' on a sling but it's high - i bet over 5kn.
good stuff to talk about.
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trenchdigger
Apr 5, 2006, 4:29 PM
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Oh boy, here we go again...
In reply to: (pushes his glasses up onto his nose) actually the myth that the harder a cam is pulled the harder it grips is incorrect. the whole concept of the cam is based on an optimum friction angle which is why the cam has that nice organic flared shape, it is based on natural shapes like shells that grow at a constant angle.
The harder a cam is pulled, the harder it presses outward on the rock. The harder it presses outward on the rock, the greater the force of friction. It's not a myth, it's basic physics.
In reply to: the guy that created the first camming device (Jardine?? i think) was a smart dude he figured that the optimum angle for maximum friction was somewhere between 11 and 14 degrees so he set the constant angle of the camming device to a range of these "friends" and then tested them to find out what the best angle is this is why smooth cams hold better or at least as well as grooved cams, smooth ones have more surface area in contact with the rock, therefore more friction......
In reply to: anyway if there is a discrepancy between the empirical test strength and the actual breaking strength of cams in the real world, i would bet that the variables that are inherent within climbing would be the main culprit. the tested strength of the units are based on specifically set up parameters. since gear placement is based on the perceived direction of pull in the event of a fall there are bound to be some disparity between the two numbers, also the amount of fatigue in the material or small imperfections due to wear and tear........i dunno but for my own piece of mind I'm going to rationalize the hell out of this.....either that or throw out all my gear and take up golf........ :lol: |
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styndall
Apr 5, 2006, 4:52 PM
Post #34 of 36
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In reply to: In reply to: Pretend you have two fall situations. Situation 1 has fall factor .1. Situation 2 has fall factor 1.5. Which system experienced the greatest force? You have no idea. Correct me if I am wrong. In a typical climbing situation with a dynamic rope, and using the same climber taking the whip, the FF 1.5 is ALWAYS going to create more force than a FF 1.
If you assume identical situations, you're correct. I pointed out above that things like dynamic belays and screamers can greatly reduce the forces felt by the top piece.
Note that I am saying that fall factor is a general guide to relative forces felt by pieces placed by a particular climber, you can in no way accurately judge the amount of force in the system from fall factor alone.
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brent_e
Apr 5, 2006, 5:17 PM
Post #35 of 36
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In reply to: Pretend you have two fall situations. Situation 1 has fall factor .1. Situation 2 has fall factor 1.5. Which system experienced the greatest force? You have no idea. Fall factor alone provides no information as to the forces in a system. If I hold a 24 inch sling by one end, clip a carabiner to the other end, then raise and drop the biner, my arm has just held a factor 2 fall. If I rig some industrial cable to thirty foot steel shafts driven into the side of a mountain, then attach my car so that when I drive off a cliff, the car falls 10 feet onto a hundred feet of cable, the fall factor is .1. However, in spite of the first situation having a factor 2 fall and the second having a factor .1 fall, the anchors in the second situation experience forces orders of magnitude greater than my arm does in the first situation. If you know that two situations were identical except for the fall factor, then you can make some claims about the various forces. Even in climbing situations, where the setup will generally be fairly similar, fall factor might be misleading. A screamer and a very dynamic belay might well mean the force felt by the top piece in a fall with a high fall factor would turn out less than in a fall with a lower fall factor but a static belay and no screamer. This is the point I'm trying to make.
well put, Styndall. I get you, now.
I guess my description only works when everything is held equal.
Thanks for the clarity.
Brent
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microbarn
Apr 5, 2006, 5:23 PM
Post #36 of 36
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I did some hand calculations to estimate the force required to rip a cam out of a perfect parallel placement.
By my estimates, if I round the force to the nearest 6, then I arrive at 6.
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