/ Cams, tri-cams and flakes

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fire_munki on 14 Oct 2016
Flakes and cams don't mix that's a given, but what about flakes and tri-cams?

Do tri-cams put as much expansive force if you set one behind and fell off? I'd presume not as there is only one 'cam' in play.
GHawksworth on 14 Oct 2016
In reply to fire_munki:

The force exerted outwards relates to the camming angle, not the number of 'cams' or lobes. So with a tri-cam, you'll still exert roughly the same force to the flake as a normal cam but just on one point.

I wouldn't do it.
CurlyStevo - on 14 Oct 2016
In reply to fire_munki:

I doubt its as simple as that. Also I've read the camming angle on a tricam varies in a parallel depending how open it is.

I would imagine their is a position on the tricam that the forces are pretty equivalent but that's just a guess!
2
SenzuBean - on 14 Oct 2016
In reply to GHawksworth:

http://www.rockclimbing.com/forum/Climbing_Disciplines_C6/Trad_Climbing_F20/forces_tricams_generate_...

I did a quick calculation yesterday and got a outward force of F/tan(theta) for a logarithmic spiral cam and F/(mu+tan(theta)) for a tricam assuming that the stinger is acting as a pivot. F is the pull force, theta is the camming angle and mu is the coefficient of friction.

mu seems to be around .3 to .38 for aluminum on granite. Assuming .3 the outward force from a tricam would be around 1.7 to 2 times the applied force whereas for a 14 degree cam it's around 4 times the applied force so by my calculations a tricam puts about have as much outward force but strongly depends on the exact friction.


If that guys maths is correct, then a tri-cam exerts about half of the amount of shear force that a camming device does.
GHawksworth on 14 Oct 2016
In reply to fire_munki:

Fair enough! it still exerts force outwards so I still wouldn't. But yeh that answered that! I'd still prefer an offset nut if it fits.
springfall2008 - on 14 Oct 2016
In reply to GHawksworth:

> The force exerted outwards relates to the camming angle, not the number of 'cams' or lobes. So with a tri-cam, you'll still exert roughly the same force to the flake as a normal cam but just on one point.

> I wouldn't do it.

It depends, if it's the only gear you have then I think you would? There's some pretty big secure flakes and some small dodgy ones....
1
duchessofmalfi - on 14 Oct 2016
In reply to springfall2008:

"mu seems to be around .3 to .38 for aluminum on granite. Assuming .3 the outward force from a tricam would be around 1.7 to 2 times the applied force whereas for a 14 degree cam it's around 4 times the applied force so by my calculations a tricam puts about have as much outward force but strongly depends on the exact friction."

This is incorrect - the force calculation for the cam is missing a factor of 2 (because there are opposite cams sharing the load) - the sideways force exerted by a cam is ~2x the load. Exception is the totem which exerts a higher for because the wires are pulling on the opposite of the lobe rather than the centre axle and exert a additional torque.



1
bpmclimb on 15 Oct 2016
In reply to fire_munki:

I don't know how much difference there is between cams and tricams (if any), but I reckon a more significant variable is where exactly the placement is. A deep placement behind a thick part of the flake is far less likely to lever it off than a shallow one behind a thin, projecting flake edge.

Although there's a case for considering the type of gear used, to my mind more important questions are: Does the flake look fragile? Do I really need that particular runner? Are alternatives available?
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CurlyStevo - on 17 Oct 2016
In reply to duchessofmalfi:

Depends what the guy meant, did he mean the sum of the magnitude of the forces away from the axel or the force on each wall of the crack?

Certainly a cam is more able to multiply the force outwards than a tri cam, which I think has a greater friction component in there. However tricams seem quite complex in exactly how they multiply force to me and I don't know much about their camming angles etc and I can't find and articles on line to help.

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