I acquired this descender from Al Haarr in 1976.
The rappelevator begins as a toroidal steel tie-down ring for an Atlas rocket. The proper ring has an ID of 85.4 mm. and a minor diameter of 22.9 mm. A 17.5 mm. slot is cut though the ring at a nominal 45° angle, and a 13 mm. hole is drilled 45° away from the slot and inset 4 mm. towards the inside of the ring. The result is an 85.2 mm. diameter, 22.9 mm. thick descender that weighs 955 g.
The rappelevator was invented by Joe Voigt and manufactured in the late 1950's.
The rappelevator works on the same principal as the carabiner wrap. The minor diameter of the rappelevator is much larger than that of a carabiner, so the rope is not strained as much. Many people warn that the rope must wrapped around the rappelevator so that the wraps cross the slot at a 90° angle rather than lying parallel to it. This is geometrically impossible for a planar slot and helical coils, so the comment should be ignored. In any case, there is essentially no tendency for the coils to migrate towards the slot.
The rappel is quite smooth if the proper number of wraps are used. The proper number depends on the type and state of the rope and the cavers weight. I typically use four or five wraps on clean 11 mm. nylon ropes. It is possible to add or remove wraps while on rappel, but it requires lifting the trailing end of the rope over the head while maintaining a firm grip with the braking hand. The maneuver is probably too risky to employ in practice.
The rappelevator introduces some spin on free drops, but not nearly as much as the Patten's hook or the Top.
The rappelevator has one capability in whichn it surpasses almost any other rappel device that I am familiar with: it passes knots with ease. In general, one can pass a knot in the main line, be it a simple overhand or a barrel knot joining two ropes, by simply slowing down, braking with the balance hand while moving the braking hand to below the knot, then resuming the rappel. The knot will pass through the device with only a slight initial bump.