<P> Double purchase counterweight systems are sometimes used where the vertical travel of the counterweight arbor would be inadequate due to limited fly space or stage - level wing space . In systems of this type, the operating and lift lines are double - purchased so that the batten will travel twice the distance of the arbor . In other words, for every foot of arbor travel, the batten travels two feet . This often results in the arbors remaining well above the stage deck, leaving the otherwise occupied wing space usable for cast and crew . </P> <P> In a conventional counterweight system the operating and lift lines terminate at the top of the arbor after passing over the head block . In a double - purchase system, however, after passing over the head block the operating and lift lines pass through another block, which is mounted to the top of the arbor, before rising back up and terminating below the head block . In addition, the opposite end of a double - purchase operating line terminates at the fly gallery, off - stage wall, or stage deck, instead of the underside of the arbor, after passing through a block mounted at the underside of the arbor . The additional blocks result in the arbor moving at half the rate of the lift and operating lines . </P> <P> In order to compensate for the reduced arbor travel, the loaded arbors must weigh twice as much as the batten load, resulting in arbors that are twice as tall . The additional mass on the arbor increases inertia, and the additional blocks increase friction, resulting in linesets that are more difficult to operate . In addition, double - purchase linesets are more expensive to install and maintain . For those reasons, double - purchase line sets are generally avoided, or limited to a few sets within a counterweight system, unless space issues preclude the use of a single - purchase system . The use of an arbor pit is an alternative approach to dealing with limited space for arbor travel . </P> <P> Electrical hoists (also referred to as winches) can facilitate coordination with cues, move extremely heavy line - sets, and significantly limit the required population of the fly crew . Despite those potential benefits, most hoists can fly line sets at only a fraction of the speed that an experienced flyman can achieve manually . </P>

Difference between single purchase double purchase fly system