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Mindless testiculation, but given the way it's bent I'd say the close cams were on the bottom. I remember being told many years ago that it was best to put the wider cams on the bottom to resist rotation.
It also looks like a fairly flared placement, which allowed the cam to rotate in line with the pull and collapse that lobe. It probably would have been better to slot it in a little further left, already oriented to the load that got put on it. The placement may not have allowed this.
Very sensibly the climber had a just in case piece straight off the ledge. Rule 1: Never have a single piece of gear between you and hitting the deck unless you can live with the consequences (i.e. the first piece and second pieces should be boulderable).
oval pockets (rather than parallel letter-box slots) are terrible for cams. The side loading doesn't add to the camming force (and bends lobes). The side load also adds friction to the camming action which renders it inneffectual.
In sever flares the cam will rip out with little or no resistance
In moderate flares the forces on the cam lobes are actually greater than a parallel placement. Thus the cam lobes will compress/rotate until the bead/break or until the contact angle become 0 and then they'll rip.
I'd argue that latter happened here.
In such situations a cam that performs in flares would be preferable. Totems are excellent and Aliens are useful to a lesser extend.
I'm personally a big fan of placing cams at 45 degrees backwards on traverses, assuming they hold and once you stop swinging thats where they forces on the lobes are balanced. But, sometimes you've just got to work with whatever placement you have!!! Good video. Thanks for the post.
The extent of my mechanical knowledge ends at discovering that the pointy bit of the hammer is not in fact used for smaller nails. So, could anyone explain how the horizontal force adds friction, reducing the effectiveness of the camming action?
I think mikllaw is talking about horizontal movement related to this diagram (and the way the cam was setup).
If you look at the diagram (great sketch of the problem by the way Mikllaw), the vertical bars are the bits of the cam that move and "cam" into the rock. Because only the corners of the cam arms are touching the angled rock rather than sitting square to the rock, they move in the direction of the arrows when they are weighted.
The cams are only designed to move vertically in this diagram, so the fact that the arrows aren't perfectly vertical means they've got some horizontal movement. This means they'll try to move sideways. The sideways movement will create friction, not camming.
Don't know if that clears anything up or makes it more confusing, but that's the best I can think of...
On 22/10/2013 daave wrote:
>The extent of my mechanical knowledge ends at discovering that the pointy
>bit of the hammer is not in fact used for smaller nails. So, could anyone
>explain how the horizontal force adds friction, reducing the effectiveness
>of the camming action?
This was one occasion were MikeL didn't describe things clearly. That said it isn't an easy thing to explain...
A simple and non technical explanation is to look at the diagram and realise that the outer cam lobes are on a slope. A significant proportion of the "grip" of the out cam lobes is used just trying to avoid sliding down that slope. Thus there is less "grip" available to rest the outward pull of the climbers fall.
When the forces are too great for the lobes' grip the outside lobes slide down the slope a bit and bends the lobes as the cam gets ripped out.