The crane orbits the sculpture overhead at a 40" radius, raises and lowers it, and also supplies power to it through the 2 hang cables. To accomplish this mission, it must have a few unusual features.
The close-up at right illustrates some of those features... Obviously, 2 cables are required to maintain heading, thus it has a dual hoist. And for the sculpture to remain level, both hoists must play out cable equally. A single layer helical wrap strategy works well in this respect. Guide forks riding on trolleys (white, which are synced to hoist rotation), guide the cable onto the spool at the desired lateral position, insuring that the cable wraps on neatly as a single layer. Also, to initially level the Undanate, spools are "unclampable" for adjustment.
The lateral-moving trolleys serve an additional purpose - keeping track of the sculpture's vertical position and automatically limiting it. Microswitches disengaged by the trolleys prevent the disaster scenarios of lifting into the hoist and snapping cables, or lowering into altitudes with obstacles.
Power is transmitted to each cable via a slip ring (brass ring) / brush (copper-colored "U") system, and this must be electrically insulated from the structure.
As with all overhead applications, safety (for the artwork as well as any humans below) is crucial. Redundancy was employed where possible, as was conservative design and lots of over-stress testing.
The base (right) is attached to the ceiling via an adjustable leveling mount and carries the central axis (1" high strength steel tube) on ball bearings. Slip rings allow power to pass through this continuous-rotation joint to the hoist(s) and sculpture(s) (the crane is designed for 2 hoists).
A 218:1 Pittman gearmotor slowly and smoothly turns the central axis via a V-belt.
The control system is very simple: variable DC voltages for all motors originate from a wired control box down below, and are set manually with knobs and toggles.
Where the 2 cables meet the sculpture is a tricky area: these junctions must conduct electricity reliably, be easily removable, withstand repetitive bending, and, if possible, be visually unobtrusive.
In a 2 cable system like this, some left/right repetitive tipping of the sculpture (aka roll oscillation) is inevitable due to the sculpture's internal movements, wind, etc. Repetitive bending can eventually fail a cable, thus the concern. A physical test was set up to see just how much repetitive end-bending under load the cable could withstand. A molded rubber cable exit was created (right) which restricts bend radius. This system tested very well and is used in the final installation.
