Theatre Design & Technology - Summer 1986 - 7

stop the fall.
The single-failure-proof principles
can be best understood by describing
certain specific projects. These principles can be applied to any system, be
it manual winch-operated scenery pipe,
a movable acoustical canopy, a light
bridge, or a stage lift. Whether or not
the application of these principles is
necessary is an engineering decision
that depends on the risk factors involved and on the damage a potential
failure could cause.
More than twenty years ago a high
school in New Jersey experienced a seri0us accident. The light pipe fell on the
students sitting below and many were
seriously injured. The failure was
caused by raising the light pipe (see Fig.
1) against the loft blocks by a manually
operated worm gear winch. In all probability, the cable was tightened to such
an extent that the head block (actually
a loft block had been used for this purpose) shaft sheared. The cable pulled
the sheave out of the head block housing, the light batten dropped and the resulting impact caused a chain reaction
of failures, resulting in the light batten
falling on the students.

small initial drop would have relieved
the overload.
Another fairly simple example is a
gear rack-type stage lift. Fig. 3 is a simplified underside view of a stage lift
which climbs up and down on the gear
racks. Lifts like this are commonly
used in Europe and in a number of
countries elsewhere. The Ii fting system
consists of a centrally located motor or
motor / reducer unit, connected by
shafting to right-angle pinion drive gear
reducers which in turn are connected to
drive pinions through cross-slatting.
All machinery is mounted to the underside of the lift platform. Drive pinions
engage the stationary gear racks and are
driven by the motor through shafting
and gearing to raise or lower the Ii ft
platform by climbing the racks.
The system, as shown, is not singlefailure-proof. Failure of a pinion drive
gear reducer would cause one end of the
lift platform to fall.
In contrast, the system shown on
Fig. 4 employs a single-failure-proof
design. In this case, each pinion is driven by a separate self-locking pinion
drive gear reducer. Should one of these
fail, the additional load would be trans-

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FIGURE 2
There is no doubt that the head
block shaft failed because of overload.
This in itself, however, did not have to
cause the accident. A spacer of adequate size and strength placed between
the head block side plates below the
cable (Fig. 2) could have prevented the
cable from dropping and would have
made the head block single- failureproof. The drop of the light pipe would
have amounted only to a few inchesnot nearly enough to cause a sequence
of impact failures. Furthermore, this

ferred to the adjacent pinion drive gear
reducer and the lift platform would be
held level by three remaining pinions.
The drive would jam or make plenty of
noise until repaired, but a potentially
disastrous accident would be prevented.
The failure of one gear rack or pinion
would be likewise single-failure-proof
for both systems and would not cause
the lift platform to fall.
The above description again is a
simplified one. Using four self-locking
pinion drive gear reducers does not

alone make the system safe. All other
components must be designed accordingly. The gear racks and pinions, for
example, must have sufficient strength
for sudden load transfer and the platform framing must be capable of supporting the loads when carried by diagonally opposed gear racks. This, however, does not double the size of the
mechanical components because the
factors of safety used for normal operating (including fatigue and other
considerations) can be reduced for a
short emergency period. A thorough
engineering evaluation is necessary.
As a further example, a singlefailure-proof design for this lift can be
achieved by other means. The lift
shown in Fig. 5 has the same drive system as described for the lift in Fig. 3.
The difference lies in the guides-they
are placed vertically far apart from
each other at both ends of the lift platform. In this case, the lift platform
would be held level by the force couple
developed in the guide shoes if a pinion
drive gear reducer fails. All components, of course, have to be designed to
have adequate strength. This system
requires additional depth in the lift pit
to accommodate the guide brackets and
thus may be more costly than other
solutions.
The theatre stage has historically
been a rather dangerous place, full of
rigging that is sometimes improvised on
the spot for a particular performance or
sometimes during stage construction
just to make the equipment fit. Auditoriums, however, have been better
protected by fire curtains separating the
stage from the audience and by other
code requirements.
Probably one of the most dangerous
practices in the recent design of the performing arts facilities is the application
of loose stage equipment standards to
the design of movable auditorium elements for changing the acoustical characteristics of the auditoriums and for
creating mUlti-purpose spaces. Much
of this work is done by stage consultants with the help of the manufacturers' sales personnel and not by engineers. Thus, as the last description
of a single failure proof design, an
acoustical canopy can serve us here as
a good example.
For purposes of simplicity, the
counterweights, which could be used to
balance the known part of the weight,
are omitted for this description. Claims

USITT / Summer 1986/Theatre Design & Technology

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