Part 4. Counterweight Rigging
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112 Counterweight Rigging Introduction The invention of counterweight rigging was the next logical step in the progression of flying equipment for the stage. It began to appear in the first quarter of the twentieth century. The early systems employed a rack, or arbor, in which to stack metal weights. This arbor was attached to the hemp lift lines, and a single hand line was attached to the arbor. The shrinking and stretching of the hemp lift lines because of changes in humidity still posed a problem. This was solved by using a wire rope for the lift lines (lead lines). Wire guides gave the arbors some vertical stability. Eventually, T-bar–guide rails appeared and have become the most common form of guide systems. For efficiency, the counterweight system is a great improvement over hemp rigging. The onstage load can be counterweighted much faster than a hemp set can be bagged. The Part 4 4.01 113 single hand line, wire-rope lift lines, and lock rail reduce work and thus save time. Single-Purchase Counterweight System A single-purchase counterweight system is used when there is clear wall space on one side of the stage from grid height to the stage floor (fig. 4.1). This system typically consists of 1. Head block for lift line and hand line 2. Loft blocks (mule blocks as needed) 3. Wire-rope lift lines 4. Batten 4.02 Fig. 4.1. Single-purchase counterweight set 114 5. Hand line (Purchase line) 6. Counterweight arbor 7. Lock rail 8. Tension block 9. T-bar–guide rails 10. Loading bridge Counterweights are used in a 1:1 ratio, that is, 1 lb. of counterweight is needed for each pound of load weight. Double-Purchase Counterweight System A double-purchase system is used when some obstruction prevents full travel of the arbor from grid to stage floor. Note the compound rigging of both hand line and lift line in figure 4.2. The bat4 .03 Fig. 4.2. Double-purchase counterweight set 115 ten travels 2´ for every 1´ of travel for the arbor. Consequently, 2 lb. of counterweight are required for every 1 lb. load. The arbors must be sized larger than a single-purchase system in order to have the same lifting capacity. Larger arbors often make loading and unloading more difficult and thus more dangerous . Quite often two loading bridges are required to safely load the longer arbors. A typical double-purchase counterweight system consists of 1. Head block for lift line and hand line 2. Loft blocks (mule blocks as needed) 3. Wire-rope lift lines 4. Batten 5. Hand line 6. Counterweight arbor 7. Lock rail 8. Tension block 9. T-bar–guide rails 10. Loading bridge 11. Arbor blocks 12. Hand-line tie-off 13. Hand-line and lift-line tie-off Miscellaneous Hardware Knowing the rigging system includes knowing the capacity of every part of a system. A line set is only as strong as the weakest piece of hardware in it, and this includes the miscellaneous hardware: wire-rope clips, swage fittings, shackles, pear rings, chain, and other items used for rigging purposes but not made by manufacturers of theatrical-rigging hardware. When using this type of hardware for rigging applications, use only hardware with a manufacturer’s name and load, or application, rating on it and, if possible, a Product Identification Code (PIC.) When a manufacturer places its name on a product, it is willing to stand behind the product. It guarantees that the product will do the job for which it is designed. The companies that manufacturer this type of hardware have engineering departments that are willing to answer your questions about the use and application of their products. Shackles, pear rings, chain, and other load-bearing products generally have a rated working load limit. This is a percentage of the ultimate breaking strength, well below the yield point of the material from which it is made (see section 1.06.C). Many 4.04 116 manufacturers put the design factors in their catalogs, simplifying the task of selecting hardware with adequate design factors. The PIC tells the year that the product was manufactured, the plant where it was manufactured, and the heat number of the metal used for that particular item. The PIC, along with the manufacturer ’s name, assures the user of complete traceability in case of component failure and of the manufacturer’s liability for the quality of its product. By purchasing...