Theatre Design & Technology - Summer 1999 - 53

tech EXPO
1999

SHOP BUILT
FRICTION DRIVE
- - - - Dan Brinker
and
David Stellhorn
University of
Illinois

NEAR SECTION
SHAFT COLLAR

At the University of Illinois we have put together a few stock self-propelled friction dlives in configurations for both palette wagons and turntables. These units are quiet and powerful, fitting into spaces less
than 9 inches high and can move castered loads in excess of 2000 pounds with ease. The mechanical
components sit in a frame bolted to the offstage end of the wagon's strucrure where two 6-inch dianleter
rubber wheels support the load and drive the wagon across the stage floor. At the heal1 of the drive is a 3phase AC 3 hp (yes, fractional horsepower) motor coupled \\~th a parallel shaft speed reducer. This powers
the dtive wheels through a 13!is" axle-drive shaft using standard #40 chain and sprockets. This dtive shaft
spans tile width of tile wagon and (ides on standard pillow blocks bolted to tile drive frame. (See figure 1.)
Our first drive unit was developed for a University of Illinois production of A Tale ofTwo Cities. The
scenic design called for a moving palette wagon 5 feet wide by 14 feet long, but standing no more than 9
inches in total height above the deck. Discussion with the director established that the wagon had to
move at human walking speed. We designed the mechanical drive to move the wagon a bit faster than the
director asked since we were using a variable speed controller on the motor which would allow some
flexibility. Our target speed was 25 inches per second-which is a pretty fast walk-but we expected to
slow it to the desired speed. I decided that the drive wheels would be 6 inches in diameter as this is a
standard size and would fit easily under the restricted height of a 9-inch tall wagon. Knowing these have a
circumference of 18.85 inches, we calculated needing 1.5 revolutions per second at the drive shaft. Using
standard physics formulae, I selected a 3 hp electric motor and matching speed reducer to supply the
required torque and speed. The speed reducer, with a 19.1:1 ratio offers 215 inch-pounds of torque at
90 rpm or 1.5 rev/sec. Coupling the output shaft of the gear reducer to the drive shaft with a matched set
of #40 chain sprockets and a length of chain meant we could modify the top speed further by adjusting
the ratio of teeth on the sprockets on the drive shaft and speed reducer. (See figure 2.)
For the drive wheels I ultimately chose Colson Hi-Tech PERFORMA wheels which have a high-density
plastic hub with a molded rubber tire. My search for these was difficult, and I learned that industrial
"Drive Wheels" are exceptionally expensive and built for extreme conditions. I needed something that
was strong and inexpensive, so I nlrned to caster manufacnlrers. I found several caster wheels which I
thought would work, but all had very
large bores to accommodate the bearings used in casters. I had planned to
use a JA" drive shaft keyed to the drive
wheels, but after getting several very
high quotes on making adapters that
would key a JA" shaft to a 13!is" inch
bore wheel, I decided to change the drive
shaft to the larger diameter. This increased
the cost of both the shaft and pillow blocks
that support it, but had the advantage
of being relatively stiff and free from
the danger of bending through misuse-making it well worth the extra
cost.
The platform for this wagon was
constructed of square steel rube and plywood, with straight casters mounted on
the underside. The drive unit, built into a
OF MOTOR FRAME REMOVED
2" x 3" steel rube frame bolted to the
underside of the wagon at the offstage
end. Standard Browning Pillow blocks
SECTION VIEW

DRIVEWHEEL:

COLSON HI-TECH PERFORMA 6X2 3/16"
DRIVESHAFT:

PILLOW BLOCK:

1-3/16" COLD ROLLED W/
MATCHED #40 CHAIN
SPROCKETS (17 TOOTH)

GRAINGER 5X685 1-3/16 BORE

Figure 2

Figure
TZ>&T

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Table of Contents for the Digital Edition of Theatre Design & Technology - Summer 1999