The Petzl Tibloc is very similar to an old ascender design patented by Kent Kammerrer in 1985 (US Patent 4,667,772). The design uses a U-shaped shell with a diagonal slot. A carabiner fits into the slot, and when the climber's weight is applied, the carabiner slides downward with respect to the shell, wedging the rope and providing enough friction to support the climber's weight. The inside of the Tibloc has numerous down-sloping conical teeth in a (3.2.2.3.2)(1S1)^7 pattern to ensure that the shell does not slide downward when weight is applied.
The instructions that come with the Tibloc state that the carabiner must have a round cross section with a diameter between 10 and 12 mm. inclusive; however, Bruce Smith of On Rope 1 has demonstrated (but does not recommend) a ropewalker system using 3.5 mm trapezoidal rapide links.
It is
essential that the Tibloc be set firmly so that most of the teeth
are engaged, since if only a few are set, the Tibloc will slide
down the rope, tearing the sheath in the process. Petzl emphasizes
this in their instructions, and recommends the techniques shown
in the figure to the left. Misuse will damage the rope - note
the shredded rope in the figure to the right. Bruce found that
filing the top three teeth off helps reduce the likelihood of
this occurring, but of course this voids any warrantee. I find
the risk of rope damage to be so severe that I avoid using my
Tibloc except for demonstration purposes. I'm not the only one
- the Tibloc is banned from the NSS Convention vertical contests
for the same reason.
The Tibloc has attracted a lot of attention in caving circles,
probably because most cavers are looking for a lighter climbing
system. I'm not as impressed as they are - Salewa introduced a
much smaller and lighter (albeit semi-mechanical) ascender
decades ago. The Tibloc might be a be a nice lightweight device
for emergency use, but an RBS
knot doesn't require the Tibloc. Prusik
knots are lighter still and don't require the carabiner.