I acquired my Controlled Rate Descender new-in-bag on eBay in January 2006.
The Controlled Rate Descender consists of three bollards, mounted between two side plates that pivot on a fourth attachment post. An adjustment screw assembly helps vary the friction in the device.
Each side plate is a roughly pentagonal piece of black anodized 1/8-in (3.3 mm) aluminum. The plates are 132 mm. high with a curved top. The top half of each plate is 60 mm. wide for the top 66 mm, then taper to join a 22 mm. diameter circular arc at the bottom. The side plates are attached to a 16 mm. diameter steel attachment post at the center of the bottom circular arc by round-head hex-socket cap screws and washers. The front plate is tightly bolted to the post, while the rear plate is loosely bolted. A spring acts to close the rear plate. The top and bottom rope bollards are bolted to the front plate with two small 3.5(?)-mm round-head hex-socket cap screws to prevent rotation. The center bollard is similarly bolted to the rear plate.
The bollards are made of unfinished aluminum. Each rope bollard has a groove that fits into a corresponding notch in the opposite plate. The bollards are 16 mm. wide, measured from between the insides of the side plates. The top three bollard diameters are 26.5 mm. in the center, widening to 28 mm. about 1 mm. from the side plate, then constant. The three bollards are mounted 106.5 mm, 65 mm. and 44.5 mm. from the center of the pivot point. When the rope is inserted, this asymmetrical arrangement ensures that the rope is pinched between the center and bottom bollards only.
The rear plate has a small aluminum block bolted to the upper left inside of the plate. A 10 mm. threaded rod (with about 6 right-hand threads per centimeter) passes through this plate and presses against the upper bollard. A plastic wing-nut attaches to the outside of the rod. The sides of the cross piece have red and green painted dots on them.
The front piece is stamped "P\LOADED," "B\ No 20." and "SWL 2 MEN." A sticker on the front plate has "CRD" and a rigging illustration. A sticker on the rear plate has "A411."
According to Dave Allport, Fallright licensed the Troll Allp design for their Controlled Rate Descender. The Controlled Rate Descender uses a right-hand threaded rod while the one on the Troll Allp has a left-hand thread. Most right-handed users seem to prefer the left-hand thread on the Troll Allp since turning the wing nut away (clockwise) adds braking. Although I am right-handed, I usually I rappel left-handed (unless cave conditions favor right-handed rappelling), so I prefer the right-hand thread on the Controlled Rate Descender. Actually, getting used to either arrangement is a minor matter, but randomly switching between the two can create momentary confusion, particularly in the dark.
By analogy with the Troll Allp, the Controlled Rate Descender appears to be designed for 7/16-in (11 mm) to 1/2-in (13 mm) rope. It performs much like the Troll Allp as well. Part of the friction is obtained by squeezing the rope between the center and bottom bollard. As one turns the rod clockwise, it forces the rear plate in the direction that separates the center and lower bollard, reducing the rope squeezing action. The friction decreases accordingly. This process only continues until there is no more squeezing. After that, there is very little change in friction. This is rather obvious, because there is no large change in the snubbing angle once the bollards are separated. The practical effect is that if one starts a rappel with the rod screwed in and finds out that the friction is insufficient, it will take many turns before the friction is significantly increased. One should always start with the rod unscrewed as far as practicable for the conditions.
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©2007, Gary D. Storrick