Theatre Design & Technology - Winter 2005 - 22

FIGURE 4. PINCH ROLLER 1, WITH BENT SHAFT

frame (fig. 5). Once again we assembled the rollers on the
hose, verified that only 400 psi was coming from the pump
and eased the lever open on the valve. The roller cage tilted
slightly toward the pressurized side of the hose but that was it.
It moved not a whit nor a jot. The pressure gauge on the power
unit indicated 400 psi, but a little pop-up indicator on the inline filter showed that the pump was producing no flow. The
pump had reached the set pressure and ceased pumping, but
the rollers were not moving. Matt and I scratched our heads
and tried to figure out the problem. If 400 psi were available at
one side of the rollers and the rollers were still not moving,
then they were obviously screwed down too tight. We loosened
the rollers slightly and tried again. This time the rollers moved
about six inches and then stopped again. Now the pump was
showing flow at 400 psi, but the rollers were just not moving.
It was clear that fluid was leaking past the rollers, because the
hose on both sides of the rollers was as hard as concrete.
We tinkered, in our ignorance, with the settings on the
power unit. First we increased the pressure to 600 psi (scary!),
then backed off to 300 psi. We fiddled with the pressure relief
valve, backing it out all the way, then screwing it all the way in.
Nothing made a difference. We were skunked. I put a call into
Parker Hannifin. The engineer I originally contacted had moved
on to another job, so I had to go through the whole process of explaining the project, overcoming initial skepticism and incredulity to get to the troubleshooting part. The pinch roller system
being outside the realm of their experience, the engineers at
Parker were sympathetic to our plight, but had no suggestions as
to what might be wrong, other than maybe it was not such a
good idea in the first place. After considerable haranguing and
cajoling, Parker agreed to send out an engineer.
In the meantime, Matt and I designed yet another version
of the rollers. We reasoned that if the rollers pinched too tight
to allow the unit to move, and loosening them up just let the
fluid go by, then perhaps a pair of double rollers might allow
us to distribute the pinch pressure over a greater area and still
maintain a good seal (fig. 6). This worked no better than the
others. I was now beginning to panic. I had $6,100 of USITT's

FIGURE 5. PINCH ROLLER 2, WITH THREE SAME SIZE ROLLERS

money and all I had to show for it was a fifty foot long, highpressure sausage lying on the trap room floor.
We tried another version of the rollers. This time we used
just two rollers, one on each side of the hose. This format
made tensioning more difficult, since four bolts had to be adjusted instead of two (fig. 7). This is the design that was on the
hose when we had our first modicum of success.
The engineer from Parker took a look at the system and
could find nothing wrong. He noted that the pump would compensate for any leakage past the rollers to keep the pressured
side at system pressure. Anything getting past the rollers
should just return to the reservoir. On a whim, he disconnected the remote compensation control and the pressure
dropped to 200 psi on the gauge, which is the minimum pressure available from the power unit. At that pressure, there is
still a little flow, since no hydraulic system is completely free
from internal leakage. Lo and behold! The rollers began moving, albeit slowly, along the hose. He reconnected the compensation and the rollers leapt ahead about a foot and then
stopped again. We tried it over and over. With compensation
the rollers would not move except for an initial leap; without
compensation the rollers would crawl slowly along the hose.
Not only did they crawl, they did so with the force one
might expect at 200 psi. The formula for determining the force
available at an actuator is:
F = PA
Where F = force in pounds
P = pressure in pounds per square inch
A = area
With a theoretical area of 1.76 in2, at 200 psi the rollers were
developing over 353 pounds of force! (It turns out we were off in
our calculations of area by a factor of 2, but that discovery would
come later. We were just very pleased to see the thing running.)
Presumably, if the rollers could move at lower pressure
but not at a relatively high pressure, then the problem must be
pressure-related. To reduce the variables by one step, I turned
to our fall-back, domestic water supply system. I lay the hose
TD & T

W I N T E R

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Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 2005