u-stem camalot failure

I got lucky last weekend and thought I should share the details in case there are any old timers still using original u-stem camalots.

I hand placed a #2 in a perfectly sized vertical slot at about a 30 degree angle from plumb, on an aid line, stood on it, moved past it, then took about a 3 foot fall on to it. The cam ripped right out. I fell about 17', landed on a ledge and in a tree below my belayer. The rope just started to come tight as I slammed my head into the tree. The helmet, the 0 tcu, my partner and my guardian angel probably saved me. I walked out with only minor injuries.

I have seen some u-stem cams ripped apart by hydraulic tools, but I was not aware of any real world failures, they were state of the art when I started climbing.

There are a few design flaws that are now quite obvious;
Each cam lobe has a hole in it rather than being solid, this allows the inner lobe to move so much that the cam twists out. The stiffness of the plastic sheathing and the thickness of the lobes also.

I think if the cam had been placed closer to plumb it would not have placed that much strain on the center lobes.

Happy climbing

hole in lobe

bent

defect allowing excessive play
gouges

Glad you are ok! Thanks for posting this!

Play back and forth in the lobes actually makes them less likely to deform, and essentially all cam lobes have holes in them.

That said, glad you're okay, and hopefully your lead head is still in a good place next time you go out.

Out of 12 of the old u-stems on my rack, 9 developed cracks in the axle housing.
Ancient gear-
Decommission and Defenestrate.

brian benedon wrote:Each cam lobe has a hole in it rather than being solid, this allows the inner lobe to move so much that the cam twists out. The stiffness of the plastic sheathing and the thickness of the lobes also.

Can someone explain this to me?

Every cam I own has holes in it (each lobe). At the minimum, that's how it mounts on the axle.

The stiffness of the plastic sheathing and the thickness of the lobes also WHAT?

My smaller Metolius cams have solid lobes - no (weight-saving) holes, except for where the axle goes through.

brian benedon wrote:I hand placed a #2 in a perfectly sized vertical slot at about a 30 degree angle from plumb, on an aid line, stood on it, moved past it, then took about a 3 foot fall on to it.

When you took the 3 foot fall, was this a direct fall onto the cam?

Edit to add: A direct fall seems unlikely given you have been at this awhile and that your injuries were minor. The described failure is simply that surprising to me without greater apparent forces. And so this could just be about some good calibrating info regarding a failure at a relatively modest angle of mis-alignment with that sized cam/type. Moves my hexes up a little higher on the rack. ;-)

You said you stood on it before you fell. Could you have twisted it sideways before the fall? I have a .75 U stem. Thanks for the post, I'll place it cautiously until I can replace it.

It is interesting that the cam didn't simply "walk" into alignment with the fall force. When some talk about cams being "omni-directional" in that they might walk into alignment and hold ... well, the OP's experience is one counter example.

Bill Czajkowski wrote: Can someone explain this to me? Every cam I own has holes in it (each lobe). At the minimum, that's how it mounts on the axle.

I don't necessarily disagree with Jake's explanation. Even so, I think the failure could have started something like this ...

First, as everyone here probably knows, the ideal for cam functioning is for 100% of the fall force to translate to cam lobe rotation about the axle which the lobe spiral and crack walls prevent. Instead, the fall force is applied (and multiplied) to one directly into the crack walls which proportionately increases holding ability.

If instead, the fall force is not aligned with the stem, some percentage of that fall force initially is in a direction that could bend the cam lobe to its' side. And with the tolerance / allowance / slop that Brian is pointing out, the misalignment before something gives only gets worse ... perhaps allowing significant amounts of the fall force to not be converted to holding ability. Instead, the cam lobes can smear sideways and so slide out or the lobes can bend with same result.

If you don't have four lobes on perpendicular rock, it's suspect regardless of the cam brand.

Cams are a lot more finicky than some people assume.

Politically Correct Ball wrote:If you don't have four lobes on perpendicular rock, it's suspect regardless of the cam brand. Cams are a lot more finicky than some people assume.

Perpendicular is typically the relationship between two adjacent walls in a rectangular room ... a cam in that corner would be super sketchy.

All double axle cams must have an open space in then to allow for the second axle. It's not a flaw in the design.

I agree that those old BD cams are garbage.

Bill Lawry wrote: Perpendicular is typically the relationship between two adjacent walls in a rectangular room ... a cam in that corner would be super sketchy.

Well OBVIOUSLY (although not obvious to some?) I mean perpendicular to the lobe.

rocknice2 wrote:I agree that those old BD cams are garbage.

Based on...

Mainly based on the number of them that have cracked and exploded. This is actually really old news - no one should be using these anymore. I bailed off them as junk after about three days of climbing on them when they first came out. Had a well-placed one completely disintegrate in a fall and got stopped by the pink tricam underneath it. Done.