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outward force on cams when loaded |
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14-Jun-2015 12:07:00 PM
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I was out climbing in the youyangs the other day on 'things that go bump in the night' at nightfall pinnacles which tackles a short and very sharp loose and hollow exfoliation flake. In placing cams behind the flake I was wondering the relationship between downward forces and outward forces on the cam lobes, so I could gauge the likelihood that IF I fell would the cam pop the flake off.
After a little googling I found this, I'm no mathematician but the simple idea that the outward force is 2 x the downward force seems easy to understand.
http://www.vainokodas.com/climbing/cams.html
In answer I would say that most likely I would have popped the flake off!
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14-Jun-2015 1:48:16 PM
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Basic rule is don't use camming devices in expanding placements, use passive gear like nuts. Try and get the first nut to be really good and holding the initial expansion, and try and set further nuts with some space in reserve. Hexs can work well as they give some range without the force of a cam. If you need to use a cam try and set it as deep in solid rock as you can ...
Be careful if you have to weight any placement as it can sometimes expand the flake that little bit more, just enough to allow lower gear to fall out, which is actually one of the many humorous joys of hard aid
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14-Jun-2015 9:35:24 PM
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Cams in that sort of flake is a really really bad idea. Huge outward force which is why they work so good.
If you noticed a large amount of rock debris on the ground it is the remains of a 2m tall by 10cm thick chunk of flake I pulled off from there on abseil with the lightest of prods - which had become dangerously loose I suspect from from someone i know previously placing a cam in there and falling on it. Could have really ruined someones day. It was pretty scary watching that thing fall and explode.
What remains is a lot more solid than before and probably ok with hexes and nuts but I still wouldn't be using cams on it.
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15-Jun-2015 8:28:46 AM
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Yep I rapped the route prior and tried to pull the flakes after spotting the flake that had already popped. Its a fair bit easier than 19 but you can see the original line. Just thought it was interesting to quantify the forces.
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15-Jun-2015 9:12:32 AM
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Nuts placed in their normal fashion (aka as a wedge) also put significant outward force on the constriction. In many cases this many be GREATER than that of a cam. So the logic that nuts are better in such circumstances does not hold.
Also the outward force of cams is 4x not 2x. Or more precisely it is given by 1/tan(¥) where ¥ is the cam angle.
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15-Jun-2015 11:21:38 AM
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Yes nuts can also put significant outward force but only at the upper extreme of their range, however if set shallowly i.e. thick nut with constriction down to wire width then the load is minimal as the direction of force is almost straight down. At upper limit, widest point it becomes more like a 'T' with larger forces . . Mostly nuts will be set in the lower range with lower resultant forces . Obviously it is better to use a suitable nut.
Also the expansion range of a nut is fairly minimal compared to a cam, so a nut will simply slip out whilst a cam will continue to expand and blow the flake. Dual axle cams are even worse for this as they expand further . .
No the outward force is not 4x as the load is shared between both sides, so the resultant force is the same as in the single cam analysis @2x ( actually slightly less), see the referenced doc . . .
These general rules re nuts as opposed to cams on expando flakes have been pretty well sorted out by aid climbers. The logic is solid, even if the placements aren't . . .
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15-Jun-2015 5:14:39 PM
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The force is 2x for each side of the crack, but in a flake one side of the crack is the cliff and therefore immovable and all of the force is transferred to moving the expando side. So patto is still right with 4x force outward on a flake. Have I got this right?
Nuts vs Cams? I'm thinking if there is not much friction between the nut with a wedge shape and the rock allowing it to slide easily then the outward force could be very large. But if there is a lot of friction reducing any sliding movement then most of the force could be downward.? Need someone to go out and do a real world test, bounce up and down and see which will break the rock first.
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15-Jun-2015 7:26:15 PM
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Did you just volunteer??
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15-Jun-2015 9:29:02 PM
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On 15/06/2015 harold wrote:
>The force is 2x for each side of the crack, but in a flake one side of
>the crack is the cliff and therefore immovable and all of the force is
>transferred to moving the expando side. So patto is still right with 4x
>force outward on a flake. Have I got this right?
No. You ain't got it right.
If it didn't apply force to both sides it would fall out.
If both sides are 'immoveable' where does the force go? Does it just drift off into space? Maybe it goes into Peter Croft's forearms in anticipation of his next hand jam.
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15-Jun-2015 10:59:32 PM
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Harold, as mentioned earlier due to the minimal expansion range of nuts compared to the massive amount available in cams a nut is safer because it will fall out before breaking the flake that a cam might break . . .
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16-Jun-2015 7:50:50 AM
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2x ?
"If b = 13 degrees, then:
Fx = T/2tan(14-13) = 28.64T"
ie depends on the angles.
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16-Jun-2015 8:23:59 AM
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On 15/06/2015 Macciza wrote:
>No the outward force is not 4x as the load is shared between both sides,
>so the resultant force is the same as in the single cam analysis @2x (
>actually slightly less), see the referenced doc . . .
Fair call. I was wrong.
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16-Jun-2015 11:16:33 AM
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Black circle is for a BD cam in a parallel placement (14.5 deg - 0 deg). If you were to place a C4 in a crack that was flared by 10 deg (if it even held!) you'd be at the far left of this graph (~6x).
A similar analysis would be valid for nuts, but it's the angle of the constriction that really matters in that case.
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16-Jun-2015 11:18:57 AM
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>These general rules re nuts as opposed to cams on expando flakes have been pretty well sorted out by aid climbers. The logic is solid, even if the placements aren't . . .
as done by the illogicle frinj element could this be why aid climbin like the placements is so illogicle ?
how many dfallen aid climbers did it take till they got sorted?
read something a while back about harding and his batso products, seems like aiding has been like it foreva.
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16-Jun-2015 12:19:48 PM
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On 16/06/2015 OodlesDownHere wrote:
>2x ?
>"If b = 13 degrees, then:
>Fx = T/2tan(14-13) = 28.64T"
>
>ie depends on the angles.
Yep, and do you realise what the example you are giving actually represents??
That is the resultant force of a 13deg cam in a placement with both sides flaring 14 deg from vertical for a total 28deg flare - effective camming angle of 1deg - not a standard parallel placement which gives the ~2x result .....
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