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

PROJECTION LIGHTS & STAGING NEWS
TECHNOPOLIS
Putting the Brakes on Aerial Risks
Today’s high-flying productions are hoisting a heavier array of equipment and scenery,
raising the need for strategically redundant braking systems.
By FrankHammel
Motorized rigging eliminates the
need to figure out how to safely
adjust rigged loads with counterweights
in a conventional fly system. You
just push a button, and up it goes.
And so far, for the most part, it’s stayed
there. Catastrophic hoist brake failures have
been exceedingly rare. With the scarcity of
major hoist failures, we want to avoid breeding
a new kind of danger: complacency.
The Danger of Complacency Tech
As lighting and stage designers push for
spectacular shows beyond anything tried
before, however, the combined weight of
scenery, video and lighting is moving hoisted
loads from a few hundred pounds to
multiple tons of potentially lethal gear. The
consequences of any failure can be dire,
and so the mechanical designs of hoisting
systems frequently incorporate redundant
brakes in order to keep the performers and
audiences safe.
“It is the nature of machinery to fail,”
notes Pete Svitavsky, a mechanical engineer
with J.R. Clancy, a Syracuse, NY-based
supplier of rigging equipment. “Even the
most carefully designed and manufactured
systems are subject to factors such as material
flaws, misuse, or lack of inspection and
maintenance. A design for any machine
that lifts or holds loads over the heads
of people requires consideration of what
happens when any one given component
in the system fails. Wherever possible, we
want to make sure that if a component fails,
some other component or feature prevents
the load from being released.”
Beyond hoist braking action, there are
also issues raised by load imbalances posed
by multiple-winch rigging setups. Any time
there are more than two hoists working
together, there is an opportunity for load
distribution imbalances, according to Ron
Crane Scales, maker of the Ron StageMaster
system of devices used to alert riggers
to imbalanced loads.
“Overload situations might happen to
the most experienced riggers, even if each
hanging point’s load is calculated and the
calculation falls within the limitations of the
nominal hoist and truss capacity,” the company
says. “The actual load distribution on
the truss is still unpredictable and cannot
be calculated, or even estimated” from a visual
assessment alone.
Another issue is the effect on the structural
integrity of the truss, or even the
whole building, if tons of equipment were
to be caught in a sudden, jerking stop as
opposed to a slow, gradual braking action.
As secondary brakes become more widely
specified in the U.S. market, J. R. Clancy
recognized the need to clarify some of the
subtleties of how brakes are described and
integrated into the machinery. For this reason
they published a white paper titled, appropriately
enough, “Brakes for Theatrical
Hoists.”
The first issue that the paper addresses
is the possibility that people who are writing
and reading the specs for hoists and
braking systems will have a different idea of
what the same words mean. The paper provides
a list of terms and definitions based
on those used by the American Society
of Mechanical Engineers, recent drafts of
ESTA’s proposed ANSI E1.6 standard, “Powered
Hoisting Systems for Places of Public
Assembly.” and J. R. Clancy’s internal standards.
Knowing the difference between a
“holding” or “parking” brake and a “stopping
brake” is important, for example, not
just because they are radically differewnt
in what they do and how they work, but
because the friction lining for a stopping
brake is likely to wear out far sooner than
that for a holding or parking brake, and will
need to be maintained accordingly.
Motor Side vs. Load Side
Tech
Another important distinction is made
between “motor side brakes” and “load side
brakes.” “Consider a simple drum hoist comprised
of a frame, a drum, a gear reducer, and
an electric motor with an integral brake. The
brake is on the high speed side of the gearbox,
so it is a motor side brake,” Svitavsky says.
“Now, take that same drum hoist and add an
electric brake mounted to the drum shaft.
The new brake is a load side brake because it
is on the same shaft as the drum.” The reason
for making the distinction is this: If a brake or
a motor is not attached directly to the drum
of a hoist, the failure of any component between
them and the drum could cause the
load to be released.
In his discussion of load side brakes versus
motor side brakes, Svitavsky points out
that this effect needs to be considered when
building redundant brakes into theatrical
rigging systems. “Many gear motors feature
a motor side brake attached directly to the
shaft of the rotor. continued on page 34
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2008 MARCH PLSN 59