What are Rappel devices (descenders) and what kinds are there?
These devices are friction brakes which are designed for descending ropes. Many belay devices can be used as descenders, but there are descenders that are not practical for belaying, since it is too difficult to feed rope through them.
Sometimes just called "eight", this device is most commonly used as a descender, but may also be used as a belay device in the absence of more appropriate equipment.
It is an aluminium (or occasionally steel) "8" shaped device, but comes in several varieties. Its main advantage is efficient heat dissipation. A square eight, used in rescue applications, is better for rappelling than the traditional 8. Because of the "ears" or "wings" on the rescue 8, there is less chance of forming a girth hitch whilst rappelling very quickly.
A figure eight descender
Figure eights allow fast but controlled descent on a rope. They are easy to set up and are effective in dissipating the heat caused by friction but have a tendency to put a twist in the rope. Holding the brake hand off to the side twists the rope, whereas holding the brake hand straight down, parallel to the body, allows a controlled descent without twisting the rope. Because of the many bends it puts into the rope, an 8 descender can wear a rope quicker than a tube style belay/rappel device. Many sport climbers also avoid them because of the extra bulk an 8 puts on the rack. However, many ice climbers prefer to use the 8, because it is much easier to thread with stiff or frozen rope.
A rescue eight is a variation of a figure eight, with "ears" or "wings" which prevent the rope from "locking up" or creating a girth hitch, thus stranding the rappeller on the rope. Rescue eights are frequently made of steel, rather than aluminium.
This consists of a 'U' shaped frame, attached to the belayer's harness, into which snap multiple bars that pivot from the other side of the frame. The rope is woven through as many of the bars as are required to provide sufficient friction. This arrangement allows for variations in rope diameter and condition, as well as controlled rate of descent.
on Jan 14, 2014