The amount of weight any production can hang in a venue has long dictated the size of many productions. As touring productions get larger and use more rigging points, it’s become imperative that the weights on each point become precise to avoid overloading a hoist or the beam from which it is suspended. Over times, rigging weights have been measured from produce style looking scales to electronic devices with miles of thin wires. Today’s load cells are a strain gauge transducer that works like a digital scale. The business of figuring out rigging weights has gone wireless with the addition of BroadWeigh to the market. But let’s look at what led our industry to this achievement.
Some History
We’ve all helped someone move a couch into a house. At some point, you reach the stairs and you must choose who gets the front and who should lift the lower back. But which end weighs more? Thirty years ago, I asked Dean Hart from Stage Rigging this question. His answer was, “I think I know, but let’s prove it.” He had an analog dynamometer in his workbox. Back then, the riggers would hang this device between the hoists and the shackle that held the spansets to weigh every point. It looked like a giant produce scale. We took a stick of truss over to a forklift. We hung one end from a blade with a three-foot spanset and the other with a six-foot spanset (alternating the dyno between sides). We found that the lower end weighed considerably more that the raised end. The weight had shifted to the guy at the low end of the stairs with the couch. If you involve another person, it gets even more complex. (This is, then, the realm of static indeterminacy).
The importance of load monitoring became a real issue when lighting designers started moving giant pods of lights during performances. Pink Floyd bought a set in 1987 for their live show. In the 1990s, video tiles started getting added to live event productions. The first ones were bulky and incredibly heavy. Two-ton hoists were often used. One motor might have been two inches higher than the adjacent ones on either side, but be lifting 500 more pounds. This was dangerous, especially for the venue’s rigging beams that needed the weight to be distributed evenly. Load cells became necessary to balance this weight. With trusses moving and tilting at shows, it became more of an issue. It is complex to determine the precise weight of any point when angled, and then equally difficult to hang the truss in that exact same position a second time. All those estimated weights the lighting company provided the production are no longer valid.
The late 80’s saw the advent of actual load cells that got their current through 5- or 7-pin XLR lines while reading back the weights on a computer or device designed by the manufacturer. These were cumbersome, involved many extra skinny cables and were basically hated by any lighting tech or rigger who handled them, as they turned into spaghetti during load out.
The Future
Mantracourt Electronics Ltd. is a U.K.-based company that specializes in designing devices that gauge the strain on different things, ranging from pressure sensors to torque transducers to temperature sensors. They expanded their operations to include devices that utilize wireless telemetry to broadcast data through transmitters that can send a signal more than a half mile (2,800 feet). They developed a product line called BroadWeigh, which deals with wireless load cells that could well become an industry standard in trade shows and live events.
The speed and ease in which this system is set up is now no harder than attaching a standard shackle at work. BroadWeigh teamed up with Crosby, a popular shackle manufacturer. The result is two separately sized shackles that emulate the popular 5/8-inch and ¾-inch shackles we predominantly use in lifting. The bell (or bow) of the shackle is just like a typical Crosby shackle, but the screw pin is replaced with a pin that has strain gauges inside it and a transmitter on one end to send a wireless signal back to a base station. The pin is held in place by a threaded nut and an R clip (cotter pin).
The transmitter is permanently fastened to the shackle pin and includes a bracket that keeps the shackle pin from rotating. It gets its power from two easily changeable alkaline AA batteries. The transmitters can be told by the base station or handheld remote to go into low power mode, and that low power mode is deactivated as soon as the units are connected to another device and given a signal. The user can also adjust each load cell to time each transmission of weight data; as in send new data to the monitor station at one second. This simply saves battery life. The batteries themselves will last 2,000 hours of normal use, so turning them off all the time is no major concern.
The shackles come in two models, depending on weight limits. The 5/8-inch model, called the BW-S325, can hold up to 3.25 metric tonnes. The larger BW—S475 has a limit of 4.75 tonnes. Two-ton hoists require the larger load cell, obviously. The load cells are calibrated and tested before shipping. The user can re-zero the cell from a PC at any time. They are fully weatherized with an IP67 rating and can work in freezing (-20° C/(-4° F) and hot (up to 50° C/122° F) temperatures.
Each shackle has its own identifier, the data tag, which allows the receivers to know where the data has come from. The transmitters have a red LED on the side, which tells the user they are active and working. The user can power up the load cells from a handheld device, which in turn displays the data tag of the shackle on an LCD screen. One can read the weight then as well.
The BroadWeigh system comes with software called the BW—LOG100 that the user can monitor from any PC laptop. The PC connects to the base station through a USB port, transmitting the data to the software, which can monitor up to 100 load cells at once. This software is optional, and one could just use the handheld device to individually check weights. But the software is free, simple and easy to learn. There are safety functions such as being able to set warning signs when a hoist has been overloaded or even approaching the dedicated weight limit the user defines for each hoist. BroadWeigh can display the mass load in pounds, kilograms, tonnes (metric ton), U.S. ton or kN (kilo newton; a measure of force). The system can recall times during the show that it took measurements and other nifty functions a rigger may wish to watch over when moving loads during a live performance.
The base station can be set to any of 15 different wireless frequencies to ensure a good uninterrupted signal. The signal can go a long way, but the company also offers an active repeater, which doubles the range. This is useful for festival sites with multiple stages or a venue where the monitoring computer is in a separate room (think about lifting retractable LED walls and permanent installed scoreboards, etc.). BroadWeigh offers a durable road case with foam inserts and locking latches to protect them as a separate purchase.
Once configured, the user should make a chart to ensure each BroadWeigh shackle is properly utilized in the same location every show, allowing the user to reference the rigging point with the correct measurement. A useful thing with the system is that the software comes with a layout view, where the user could draw a simulated rigging plot, then have the current weight appear next to the hoist on the plot. One can easily locate the load cell in question immediately. The map also will show you the entire load weight of all the cells combined.
For more information, please visit the BroadWeigh website at www.broadweigh.com.
