Theatre Design & Technology - Oct 1970 - 17

IPANELI

TECHNICAL
DEVELOPMENTS

FLUID POWER SYSTEMS
IN THE THEATRE
William Cruse
Theatre Consultant, New York
e are all aware of the developments in
electric motors, rotary motors, and rotary motion, and of the necessity for changing rotary motion into linear motion adaptable to theatrical problems.

The specific pump unit I will discuss is
for a specific installation. It has 150horsepower, is approximately four feet by
eight feet, and has a double-ended motor
drive. The pump unit is located in a mechanical equipment room with the building's
noisy apparatus: air conditioning, heating,
etc.

The system I will describe provides a
breakthrough in fluid power and is a sensi'
ble, commonsense approach to the problems
of motion in the theatre.
The system is a low-cost, automatic,
total-stage machinery system. It encompasses elevator actuation, stage wagons,
and horizontal motion on the stage floor.
And vertically, 1'h-inch pipes can rise on
6-inch centers to the stage gridiron.
The system has the three basic components that most fluid power systems do.
The first of them is the pump or power unit,
depending on- the kind of pump or power
situation involved. Then we have the actuator itself, the component key to our particular system in wagon and baton drive
components. The pump sends fluid at the
required pressure down two main lines to
an actuator, which is either a cable cylinder,
or a rod-type piston.
The cylinder on all of our installations
on wagon or baton drives is particularly suitable to theatrical applications. The system's
unique adaptability provides a streamlined
linear drive mechanism that allows the theatre architect maximum flexibil ity. He need not
chitect maximum flexibility. That he need not
be concerned about the problem of doublepurchase counterweights, counterweights
themselves, the catwalks necessary to load
the counter weights, or the myriad problems that arise with conventional, manual,
or electric systems.
Perhaps the most important element of
the system is the simple low-voltage control system that almost exactly resembles
the SCR dimmer system. A low-voltage control activates a valve, which, in turn, emits
fluid to either end of the actuator.

£!l!!!O. THEATER DESIGN AND TECHNOLOGY

A typical pump unit.
The key thing that the control energy
source allows us is a system parameter
consistent with theatrical operation. We are
able to size the power unit in terms of the
horsepower required to lift or drive mechanisms on the stage. This may be five or ten
thousand pounds, depending on the facility
and the performance requirements.

The cable cylinder is a very simple device, developed by the Air Force for use on
bombay doors. It is a double-ended actuator
that has broad advantages. There is a piston, which is a simple piece of steel with
a set of rings on each end. A sheave within
the piston houses a cable fitting, and from
that cable fitting a nylon-coated cable runs
from each side of the piston through seals
on each end and around the sheave to a
clew which is located exactly opposite from
the piston. So as we emit forward or to one
side, we achieve a linear motion within the
length of the actuator. This is the unique
featu re of cable cyl inders as compared to
rod cylinders, which require an extension of
their own lengths on actuation.
The drive is remote controlled and electrically actuated. The valves come in sixmount stacks, which simplifies the plumbing considerably. The key feature is that a
12-volt signal is sent from the control console (just as in a SCR dimmer system) to
the valve, telling the valve in which direction
and at what speed the motion is to take
place. This valve is proportional as as you
move the joystick forward or backward, you
achieve both direction and proportionate control of acceleration or deceleration of the
scenic unit.
Now let us examine a typical college installation. The cable cylinder is adjacent to
the stage wall. A small control rail runs up
and down stage in conventional counterweight style. Lines come from loft blocks
across the head block. The conventional

An exploded view of the basic cable cylinder actuator.

OCTOBER,1970

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