When a climber falls, force is applied which pulls on the stem causing the lobes to expand even further. This outward pressure is transferred to the rock, and combined with friction in a good placement, the cam holds in place.
(There are a lot of people who have explained this a lot better than I can.)
Here's a nice diagram that Metolius provides:
Anyway, this blog started out as a post about my first time falling on trad gear. Thing is, since I started this blog, I've fallen on the gear more than once, and the excitement and luster of trad virginity has slightly faded. Only slightly though. It's turning into trad love. I fell on the yellow TCU (Metolius "three-cam unit") on a climb called "Extreme Unction" in the most beautiful canyon in the Wasatch.
I fell again on a yellow TCU in the San Rafael Swell on a climb called "Anchors from Hell".
I'm going to take the liberty of linking a photo that Adam (friend) took of Alex climbing on "Old Bushmill's" crack. I was actually able to climb this crack cleanly and didn't have to test any of the gear that I placed. (It would have held fine)This crack is in a really wide dihedral and goes from rattly fingers up to big hands in size.
I climbed it twice before I felt like I was ready to lead it, and by then, it was the end of the day. I was sure shaky, and almost slipped out of the crack in a few places. It was exhilarating to get up there and clip the anchors. I almost hated to be lowered back down to the horizontal world. If anyone ever gets a chance to climb something like this, don't let it go. Crack climbing is dangerous and painful and so so rewarding.
Climb safe out there.
Price