Page 51 - Rappelling

(working load limit) rating, which is a strength rat- There are two basic styles of ⁄4-inch Rawl Drive
1
ing given by the manufacturer of the maximum rec- bolts. The buttonhead design has a mushroom-like
ommended load, which for quick links is typically head and is pounded into the hole with the hanger
one-fifth of the MBS. Because of the outsourcing to preattached. The threaded Rawl Drive has threads
cheap manufacturers, and quality control issues with with a nut on the end to hold the hanger in place, a
Asian manufacturers, it is wise to never trust your weaker configuration, since the threads can weaken
life to a single, non-CE-certified quick link, or any the shear strength of the shaft if the hanger is at
cheap hardware store doodad for that matter. the level of the threads. But more significantly, the
threaded design has a serious flaw: Pulling straight
Artificial Anchors out on the bolt hanger will only be as strong as the
holding power of the nut on the threads, a dangerous
problem if the nut is at the very end of the threads.
Bolts
The shear strength on a brand-new ⁄4-inch
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The most common fixed anchor is a two-bolt Rawl Drive bolt is roughly 2,000 pounds, but
anchor. Some knowledge of the history, character- the problem with contraction bolts is not shear
istics, and specifications of bolts will improve your strength but pullout strength, which varies drasti-
ability to assess the reliability of bolt anchors. cally depending on the quality and hardness of the
In the 1960s and 1970s, bolts were placed by rock. In very soft sandstone, the pullout strength of
hand drilling—an arduous process where a drill bit a ⁄4-inch contraction bolt is extremely low, render-
1
was inserted into a drill holder, then a hammer was ing the bolt unsafe.
used to pound on the holder to painstakingly drill The buttonhead Rawl Drive bolts were also
into the rock. Once the hole was deep enough, a sold in ⁄16-inch diameters, these being far more
5
bolt, with a hanger attached, was hammered into reliable as long as they were placed in good, hard,
the hole. The most common bolt during that era fine-grained granite. The ⁄16-inch buttonhead, for
5
was the ubiquitous ⁄4-inch contraction bolt, called example, has a shear and pullout strength in excess
1
the Rawl Drive, manufactured by the Rawl Com- of 4,000 pounds and for many years was the bolt of
pany and designed for the construction industry for choice for first ascensionists who were hand drilling
anchoring in masonry or concrete. A contraction bolts. The ⁄16-inch buttonhead Rawl Drive was dis-
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bolt has a split shaft that is wider than the diameter continued, but the ⁄8-inch buttonhead is still on the
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of the hole. When pounded into the hole, the two market today, with a shear strength of 7,000 pounds
bowed shaft pieces are forced to straighten slightly, and a pullout strength of over 4,000 pounds in the
contracting under tension in the hole. This works best granite.
fine for hard granite, but in soft rock, like sandstone, Probably the most disconcerting problem associ-
the split shaft doesn’t really contract all that much, ated with bolts from the ⁄4-inch era is not the bolts
1
and there is little tension to keep it in the hole, themselves but the hangers. During that time, hang-
resulting in very weak pullout strength (i.e., pulling ers made for rock climbing were manufactured pri-
straight out on the bolt). marily by the SMC company. Thankfully, the hangers
Another problem with ⁄4-inch bolts is that they are easily identified, as the “SMC” brand is stamped
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came in various lengths, some as short as ⁄4 inch on them. There were two series of hangers, one
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long, and once placed in the rock, there was no way good, and one very bad. The bad hangers were nick-
for future climbers to determine the length of the named the SMC “death hanger,” since some of them
bolt merely by inspection. failed under body weight after only a few seasons of

40 RAPPELLING

Rappelling_i-174_3pp_CS55le.indd 40 7/24/13 10:15 AM