Theatre Design & Technology - Spring 2021 - 22

roll off the grooves of the blocks could be
excessive, therefore causing friction. The
sheave grooves could not have proper angles to support the wire rope. What types
of sheaves are used--steel, cast, Nylatron?
What kind of bearings do the blocks
have--ball, tapered, roller, plain? Are the
bearings properly lubricated? If there are
idler pulleys, what kind of bearings do
they have? And, there are other causes,
to be sure. All of those factors involving
blocks can impact the friction in the line
set. Or, the operating rope could be too
taut causing friction as it passes through
the rope lock. The rope lock could be
worn and not adjusted properly to compensate for the wear. Are the operating
ropes and their grooves in the sheaves
sized properly for each other?

The Unicorn's Home

The Imperial opened in 1913 as part of
the Keith chain of vaudeville/movie palaces. The original rigging was a hemp system. The Imperial, like many theatres of
its vintage, spent most of the 20th century
as a cinema. In the late 1980s, however,
the community decided that it needed a
performing arts center. The Imperial was
the place that answered those needs. So
in 1993, the front of house was renovated and the original stage house was replaced with a full fly tower equipped with
50 counterweight line sets.
Getting a superior system involves the
consultant, the owner, the architect, the
manufacturer, the installer, the general
contractor and last, but not least, the user.
Bob Brannigan of Robert Brannigan and
Associates was rigging consultant. J.R.
Clancy of Syracuse, New York, manufactured the rigging equipment. And, GC
Stage of Montreal, Quebec, installed the
rigging. These were three of the key partners whose skillful efforts brought the
unicorn into existence.
On the day of my inspection, the core
of Imperial Theatre's system was in perfect working order. By my estimation,
the core of the system, which is the most
costly element to replace, will remain
serviceable for the foreseeable future
with proper maintenance and inspection.
After 27 years, most system cores have
begun to fail, so questions to consider
include: What is different about this one?
And why does it operate so smoothly?
What does it take to put in place a superior counterweight rigging system? Does

it cost more? If so, is it worth any extra
cost? The Imperial Theatre system is a
lesson for us all.

The Unicorn's Design Team

A major ingredient to getting a unicorn, is
having a good rigging consultant, though
not every theatre chooses to hire a rigging consultant at all. The manufacturers of rigging equipment are quite willing
and capable of providing the information
required and to specify their equipment.
In fact, this is how much of the industry
operated from the 1920s until the mid1980s when theatre consultants became
more common. Manufacturers still provide specifications today. And, to be sure,
it is better to have a manufacturer specify
a system than an architect who may not
be well versed in counterweight rigging
systems and may lack the industry-wide
equipment familiarity of a consultant.
Robert Brannigan Associates was a
busy theatre consultancy in the last portion of the 20th century. The system that
exists today is evidence that he was very
particular about this project. A number of
small details speak to the longevity of this
installation.
One detail in particular is the structural bracing installed for the fly galleries.
A fly gallery is supported at one end by
the proscenium wall and at the other end
by the upstage wall. If the gallery is long
enough, it may also be suspended from
the roof steel. What results is a long gallery that tends to flex toward and/or away
from the side wall. To prevent the flexing,
the gallery is braced to the wall in a small
number of places. Generally, each brace
interferes with one line set, thus eliminating that line set. In this installation, there
are three braces, which would have resulted in the loss of three line sets. But
Brannigan had a special brace designed
that allowed the arbor of the line set to
pass through it, thereby allowing for
three extra line sets.
The bracing, however, is a side issue.
The relevant detail that reduces friction
is the loft block set Brannigan selected.
When counterweight systems were first
used in North America, the practice was
to mount the loft blocks on top of a walking grid. By the time the Imperial was
rebuilt, the industry had moved away
from grid-mounted loft blocks and had
adopted blocks that are underhung from
the overhead loft beams.

22 | THEATRE DESIGN & TECHNOLOGY | SPRING 2021

The easiest and least-expensive way
to make this transition is to take a gridmounted block and invert it to hang under the loft beams. However, the longer
lift lines sag severely, so idler pulleys are
added to prevent the sagging. The problem is that idler pulleys add friction to the
line set. The extra fleeting of the lines required to maneuver them back and forth
among the unaligned grooves of the load
sheave and the idler pulleys adds friction.
The fleeting between idler pulleys and
the main sheave of the next block will be
approximately 0.5°. The friction of this
fleet angle is compounded by the fact
that the grooves on the idlers are more
" U " shaped, than " V " shaped. Often,
individual idlers are used, which compounds the friction. Sometimes multigroove idlers are used. Sometimes the
idler bearings are a simple hole inserted
onto a bolt. Sometimes idlers have ball
bearings. Depending on these details,
the idler pulleys will impart more or less
friction. In all cases, the idlers impart friction. Over time, idler pulleys, which are
one more thing that can go wrong, can
rotate less easily to the point of freezing
up completely.
However, Brannigan did not specify
loft blocks with idlers. He specified
multi-groove loft blocks. A multi-groove
block set has a first block with a sheave
with five grooves, the next block has one
with four grooves, then one with three,
and then two, and then one groove as
you move away from the arbor (in a five
lift set, for example). A benefit of a multigroove block set is that the fleeting of the
lift lines between one loft block and the

A simple drawing to show the difference between a
V groove and a U groove on a sheave. See how the
wire rope is in greater contact with the sides of the
groove in the U groove. | Image courtesy of Rick
Boychuk.



Theatre Design & Technology - Spring 2021

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