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\technote{5}{Dan Ports}{2004/04/10}{drkp@mit.edu}
\tntitle{Rigging for Two-Dimensional Motion}

\section{The problem}
\label{sec:problem}

It is fairly straightforward to rig an object to move vertically
(using a simple pulley configuration) or horizontally (by means of a
traveler track). However, frequently a production calls for an object
to move both horizontally and vertically. This note presents a design
that combines these two mechanisms to solve this problem.


\section{The design}
\label{sec:design}
This design was presented by David Krajec of Cardinal Stritch College
in Milwaukee, Wisconsin \cite{tsg-2d-rigging}. It uses a modified
traveler truck that moves horizontally across a standard track and
allows an object (in his example, a chandelier) hanging from the truck
to be moved vertically.

Horizontal motion is achieved in essentially the same manner as
rigging a curtain. A single truck moves along a standard traveler
track.  The load is attached to this truck. By attaching a line to
each side of the truck, it can be moved back and forth just like a
curtain.

To rig an object to fly vertically, we would normally attach it to a
lift line that passe s through a
loft block and head block % FIXME: is this correct?
before reaching the operator, who can then raise and lower it. The
most obvious approach to the two-dimensional problem would be to use
this design, placing the pulley on the traveler truck. This can be
made to work, but it has a problem: as the truck is moved back and
forth along the track, the vertical line's length will change, causing
the load to move up and down. In some situations, this might be
tolerable (e.g. if it's being moved during a scene change, not in view
of the audience). However, generally a better solution is needed.

The solution is to anchor one end of the lift line to the far wall,
and have it pass through a pulley on the load and on each side of the
traveler truck, as in Figure~\ref{fig:rigging}. Two eye bolts with
washers are added to the truck. One pulley is attached to each of
these eye bolts using a quick link. A third pulley is attached to the
top of the load. Finally, another eye bolt is added offstage at the
end of the track opposite the operator's control line. The line is
then tied off to this eye bolt, passes through the first pulley on the
truck down to the load pulley and back up to the second truck pulley,
and finally to a block at the other end of the track and down to the
operator. The result is that the vertical line can be let in and out
in order to move the load up and down, and the horizontal line can be
used to move the load left and right \emph{without affecting the
  vertical position.}

The operator is presented with two control lines: one for moving the
load horizontally and one for moving it vertically; they act
independently. A cleat should be used to tie off the lines to prevent
the load from moving.

One issue not fully addressed by this design is that the load may
rotate when moving. This can be dealt with by attaching the load's
pulley to the load using some sort of fixed attachment that ensures
that it cannot rotate. For certain objects, such as the chandelier
previously mentioned, this sort of attachment is difficult to
accomplish. In such cases, using an appropriate braided cord may serve
to minimize undesired spin.

This design requires only slightly more space than a standard traveler
track, and thus is suitable for use in theaters with little or no fly
space (a problem we are all too familiar with, especially here).


In addition, it provides a $2:1$ mechanical advantage since there are
pulleys both at the top of the load and on the truck. This is a
side-effect of the design for vertical motion rather than an
intentional choice, but it has the convenient property that a larger
load can be controlled by a single operator. For even heavier loads,
one could imagine using a counterweight rigging system for the
vertical lift line. This would mean that the operator's ability to
lift the load would no longer be the limiting factor.



\nocite{backstage-handbook}
\bibliography{21m735-tns}

\begin{sidewaysfigure}[htbp]
  \centering
  \includegraphics[height=5.5in]{21m735-tn5-rigging}
  \caption{Rigging overview}
  \label{fig:rigging}
\end{sidewaysfigure}

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