Road Test: Strong Technobeam, page 40 - PLSN.com

PRODUCT SPOTLIGHT
PROJECTION LIGHTS & STAGING NEWS
Ron StageMaster
By RichardCadena
Do you know how you can tell a rookie
entertainment professional from
a veteran? When they walk into a
show, the rookie cranes his neck to admire
the lighting rig while the veteran cranes his
neck to see if it’s safe to walk under the rig.
As shows get larger and more complex,
rigging issues become more serious. A small
error in calculations can result in a big disaster.
And when you have multiple rigging
points with dynamic loads, things can get
really hairy. And that’s no time to compromise
on safety, especially when lives are at
stake and there are people under the rig.
When more than two hoists are being
used on a truss, or more than three hoists
are used on a structure, you can encoun-
The computer-based system runs on
a laptop while communicating wirelessly
with up to 16 or 32 load cells per channel,
which are rigged in line between the chain
hoist and the load. Load cells are available
for loads ranging from a half ton to six tons
and they are small and lightweight. They
run on four AA batteries or on 110/220VAC.
The load cells can operate up to 1,500
hours on one set of disposable batteries.
Measured end to end, the smallest load
cells are 5.5 inches and weigh 2.5 pounds
while the largest are 7.9 inches and weigh
4.5 pounds each. The wireless system has
a range of up to 450 feet and operates at
radio frequency. The receiver is about the
size of a PDA and it is housed in a light-
When more than two hoists are being used
on a truss, or more than three hoists are
used on a structure, you can encounter an
unpredictable load distribution, or a phenomenon
known as “statically indeterminate
structure.”
ter an unpredictable load distribution, or a
phenomenon known as “statically indeterminate
structure.” Even if the load appears
to be balanced and equally distributed
among multiple points, some of the hoists
may be overloaded. It can happen to inexperienced
and experienced riggers alike.
One good way to avoid this situation is
to monitor the load in real time. Ron Stage-
Master is a wireless, multi-load-cell system
that does just that. In addition to monitoring
loads in real time, it also detects overload
and underload situations. When certain
conditions are sensed, it can set off an
alarm, stop the hoists, or both, depending
on how the system is configured.
weight ABS plastic. It connects directly to
the laptop with a cable.
The software includes a load map that
can be overlaid onto an AutoCAD drawing,
or any other drawing converted to bitmap,
to provide immediate identification of each
rigging point. Names can be assigned to a
group of points and individual points can be
monitored or summaries of several points
or the entire structure can be accessed. For
larger systems, multiple systems can be
used to monitor the entire rig.
Some of the parameters that the system
can monitor include overload, danger,
zero load, tare weights, load sums
and maximum load. In addition, the resolution
for the entire system can be preset.
The data can be downloaded from the
data log and it can store weeks worth of
continuous measurements including the
operator’s name, time, weights, danger
alerts, overload situations, and the cumulative
weight for each group. The software
can be configured to display units of measure
in metric tons, short tons, kilograms,
pounds, newtons, deca-newtons or kilonewtons.
With any rigging system, it’s always a
good idea to design with a healthy safety
factor. In this case, the load cells have a
built-in safety factor of five or 10. The resolution
of the load cells varies from two
pounds to 10 pounds, depending on the
load rating of the cell.
The system deals with the out of doors
rather well. The load cells can operate in
temperatures ranging from -15ºF to 175ºF
and they are IP rated 65 (NEMA 4). And since
there are no messy cables to deal with, the
load cells can be installed quickly and easily.
The show must always go on, but
there’s no reason it can’t do so safely. The
Ron StageMaster system is your safety insurance
policy for any rigging situation. It
is an easy system to deploy and use, and it
provides an extra layer of safety that can’t
be matched by a rigging expert alone.
Putting the Brakes on Aerial Risks
continued from page 59
If you take a second motor side brake and put it
right next to the first one, you have now added
protection in case the first brake fails.” But, he
cautions, “there can be several gear meshes and
couplings between the cable drum and these
brakes.
If any of these fail, the motor shaft will still be
locked in place, but the load will fall. In this case,
you had better be very confident that the gearbox
and intermediate couplings will not fail.
“Now, take the second brake and move it
onto the end of the drum (or sprocket) shaft so
that it is at the opposite end of the mechanical
drive train from the motor brake. This is a load
side brake. Now, if one brake fails, the other will
still hold the load. Also, if the gearbox or any
other coupling or shaft between the motor and
the drum fails, the load side brake can still hold
the load.”
An analogy might be that of a car with an
automatic transmission parked on a steep hill
without a parking brake engaged. There is a
brake inside the automatic transmission that
keeps the drive shaft from rotating. The drive
shaft, in turn, prevents the differential gears
from moving, which keeps the rear axles from
moving, which then keeps the wheels from
moving. If any one of these elements lets go, the
car is going to roll down the hill. Contrast this
with the parking brake: it clamps directly onto
the wheel and keeps the car from rolling even if
the drive shaft falls completely off of the car.
Simply matching the same style of brake on
the load side might not be enough, however.
“There is no gear ratio between the new brake
and the cable drum, so the brake needs to be
larger than the one on the motor shaft.” The
paper also notes that “as hoists become larger,
brake units large enough to be applied as load
side brakes become very expensive,” and that
for a large enough load “it can be more costeffective
to use two complete sets of motors,
gearboxes and motor side brakes than it is to
use a single load side brake.” Even so, the admonition
bears repeating: “Even the most carefully
designed and manufactured systems are subject
to factors such as material flaws, misuse, or
lack of inspection and maintenance.” And the
simple warning, repeated as a mantra, works as
an antidote to complacency:
“It is the nature of machinery to fail.”
Me So Stupid
continued from page 60
So as long as the loads are equal in
value, then the line current is not twice
the single-phase current, but 1.73 times
the single phase current.
In this case, we have 7.2 amps in
phase A and 7.2 amps in phase B. So
the resulting line current is 7.2 × 1.73
amps = 12.5 amps.
If we added another automated light
across phases A and C, then all three
legs will have 12.5 amps going through
them. We can confirm this by using the
formula for three phase power, which is
W = V × I × PF × 1.73. In this case, with
all three lights hooked up the wattage
is 4500, so we have 4500 = 208 × I × PF
(we’ll assume it’s 1) × 1.73, or I = 4500 ÷
(208 x 1.73) = 12.5 amps three-phase.
It turns out that our friend, the 30-year
electrician who uses the formula I = (W ×
2) ÷ V, is close, but technically not correct.
However, every good electrician always
builds in a de-rating factor, usually about
20%. If we take that magic number, 1.73,
and give it a 20% overhead, we end up with
– you guessed it – 2, or something very close
to it (1.73 × 1.2 = 2.076).
Now I can sleep at night knowing that
the classic formula for a three-phase load
does indeed produce the correct answer.
But I might never have confirmed it had I not
had a satisfactory philosophy of ignorance
and welcomed the doubt and discussion
that drives both science and art.
When the author is not waving to complete
strangers he can be reached at
rcadena@plsn.com.
34
PLSN MARCH 2008