Theatre Design & Technology - Fall 1978 - 22

Air Supply System. Two one inch air lines move the air supply to a
series of manifolds. Each individual air line is fitted with a needle
valve to adjust the pressure.

structing a steel frame turntable thirty feet in diameter. It was
to float on floor-mounted air casters in order to prevent the
usual problems of weight, noise, and friction.

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Theatre Design & Technology

First, the frame was constructed from one-inch square
steel tubing (.065" thickness) welded into a grid of squares on
sixteen-inch centers. The table was divided in halves to facilitate handling and these halves were bolted together at installation. A slip or pivot bearing was mounted in the center, and
all struts were cut from this center. Owing to the rigidity of the
frame there was no need to reinforce the perimeter. Thus, the
frame was light in weight, and measurements indicated that it
was within one eighth of an inch of being round.
Next, two bearing races were cut from half-inch cdx plywood and screwed to the bottom of the frame. Since air
casters read the slightest irregularity in the surfaces they
encounter, the races were covered with quarter-inch Masonite and all seams were filled with auto body putty. Later, when
the frame was flipped over, several of these seams showed
cracks which were then covered with Scotch filament tape,
refilled with body putty and sanded smooth. All bearings and
races were sprayed with silicone lubricant at installation.
The fi nished turntable was seated on a system of two-byfour wood frames, toe-nailed to the stage floor. These provided additional support to the steel frame and prevented the
table from shifting when not in motion. Spaced around and
beneath the location of the outer race were sixteen twelveinch diameter S.T.D. heavy duty Rolaire casters mounted on
one-inch thick plywood pads which placed the casters at the
correct height for inflation. Another four casters were similarly mounted under the inner race, thus providing plenty of lift
and balance for the structure when inflated. One-inch air lines
ran from each caster to a series of needle valves mounted on
manifolds. Since we did not have enough air lines, some
casters had to be "twofered" together. Although this can be
done on a limited basis, the air supply system should have
one needle valve per bearing in order that the pressure delivered to each bearing can be separately adjusted. Finally, the
manifolds were fed by two one-inch air supply lines. We
found that one one-inch line delivering about 100 Ibs. P.S.1. of
air could not supply the volume needed, but two were more
than enough to lift the turntable.
The scenery was mounted and a rim drive system using an
automobile wheel and tire was engaged. We found the system to work quite efficiently. It was reasonably free from noise
(the sound of air hissing was much less than the rumble of
straight casters), and turned with only a small amount of
friction. With an unbalanced load of scenery and five actors
on board, the table could have been manually turned by one
stagehand. Since the table was truly round, traction was no
problem for the tire, but to insure against any slippage, the
quarter-inch plywood facing was treated with sand-imbedded
paint.
Some problems, however, were encountered. Due to the
heavy and unbalanced scenery load, several of the seals on
the bearing races again developed cracks. This problem was
once again remedied with auto body fillei·.
If another system of this sort were to be built, I would
suggest that this problem might be avoided by welding the
steel grid from 1'12" square steel tube on two-foot centers.
This would increase the rigidity of the structure, and thereby
prevent the races from cracking before mounting the turntable in place. Secondly, the races themselves could have
been supported by adding a track of steel tube to the grid
design. This backing would provide continuous support, further insuring rigidity, particularly if a heavy load were later
mounted on the turntable.
USITT /Fall, 1978

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