Motorized rigging eliminates the need to figure out how to safely adjust rigged loads with counterweights in a conventional fly system. You just push a button, and up it goes. And so far, for the most part, it’s stayed there. Catastrophic hoist brake failures have been exceedingly rare. With the scarcity of major hoist failures, we want to avoid breeding a new kind of danger: complacency.
The Danger of Complacency
As lighting and stage designers push for spectacular shows beyond anything tried before, however, the combined weight of scenery, video and lighting is moving hoisted loads from a few hundred pounds to multiple tons of potentially lethal gear. The consequences of any failure can be dire, and so the mechanical designs of hoisting systems frequently incorporate redundant brakes in order to keep the performers and audiences safe.
“It is the nature of machinery to fail,” notes Pete Svitavsky, a mechanical engineer with J.R. Clancy, a Syracuse, NY-based supplier of rigging equipment. “Even the most carefully designed and manufactured systems are subject to factors such as material flaws, misuse, or lack of inspection and maintenance. A design for any machine that lifts or holds loads over the heads of people requires consideration of what happens when any one given component in the system fails. Wherever possible, we want to make sure that if a component fails, some other component or feature prevents the load from being released.”
Beyond hoist braking action, there are also issues raised by load imbalances posed by multiple-winch rigging setups. Any time there are more than two hoists working together, there is an opportunity for load distribution imbalances, according to Ron Crane Scales, maker of the Ron StageMaster system of devices used to alert riggers to imbalanced loads.
“Overload situations might happen to the most experienced riggers, even if each hanging point’s load is calculated and the calculation falls within the limitations of the nominal hoist and truss capacity,” the company says. “The actual load distribution on the truss is still unpredictable and cannot be calculated, or even estimated” from a visual assessment alone.
Another issue is the effect on the structural integrity of the truss, or even the whole building, if tons of equipment were to be caught in a sudden, jerking stop as opposed to a slow, gradual braking action. As secondary brakes become more widely specified in the U.S. market, J. R. Clancy recognized the need to clarify some of the subtleties of how brakes are described and integrated into the machinery. For this reason they published a white paper titled, appropriately enough, “Brakes for Theatrical Hoists.”
The first issue that the paper addresses is the possibility that people who are writing and reading the specs for hoists and braking systems will have a different idea of what the same words mean. The paper provides a list of terms and definitions based on those used by the American Society of Mechanical Engineers, recent drafts of ESTA’s proposed ANSI E1.6 standard, “Powered Hoisting Systems for Places of Public Assembly.” and J. R. Clancy’s internal standards. Knowing the difference between a “holding” or “parking” brake and a “stopping brake” is important, for example, not just because they are radically differewnt in what they do and how they work, but because the friction lining for a stopping brake is likely to wear out far sooner than that for a holding or parking brake, and will need to be maintained accordingly.
Motor Side vs. Load Side
Another important distinction is made between “motor side brakes” and “load side brakes.” “Consider a simple drum hoist comprised of a frame, a drum, a gear reducer, and an electric motor with an integral brake. The brake is on the high speed side of the gearbox, so it is a motor side brake,” Svitavsky says. “Now, take that same drum hoist and add an electric brake mounted to the drum shaft. The new brake is a load side brake because it is on the same shaft as the drum.” The reason for making the distinction is this: If a brake or a motor is not attached directly to the drum of a hoist, the failure of any component between them and the drum could cause the load to be released.
In his discussion of load side brakes versus motor side brakes, Svitavsky points out that this effect needs to be considered when building redundant brakes into theatrical rigging systems. “Many gear motors feature a motor side brake attached directly to the shaft of the rotor. If you take a second motor side brake and put it right next to the first one, you have now added protection in case the first brake fails.” But, he cautions, “there can be several gear meshes and couplings between the cable drum and these brakes.
If any of these fail, the motor shaft will still be locked in place, but the load will fall. In this case, you had better be very confident that the gearbox and intermediate couplings will not fail.
“Now, take the second brake and move it onto the end of the drum (or sprocket) shaft so that it is at the opposite end of the mechanical drive train from the motor brake. This is a load side brake. Now, if one brake fails, the other will still hold the load. Also, if the gearbox or any other coupling or shaft between the motor and the drum fails, the load side brake can still hold the load.”
An analogy might be that of a car with an automatic transmission parked on a steep hill without a parking brake engaged. There is a brake inside the automatic transmission that keeps the drive shaft from rotating. The drive shaft, in turn, prevents the differential gears from moving, which keeps the rear axles from moving, which then keeps the wheels from moving. If any one of these elements lets go, the car is going to roll down the hill. Contrast this with the parking brake: it clamps directly onto the wheel and keeps the car from rolling even if the drive shaft falls completely off of the car.
Simply matching the same style of brake on the load side might not be enough, however. “There is no gear ratio between the new brake and the cable drum, so the brake needs to be larger than the one on the motor shaft.” The paper also notes that “as hoists become larger, brake units large enough to be applied as load side brakes become very expensive,” and that for a large enough load “it can be more cost-effective to use two complete sets of motors, gearboxes and motor side brakes than it is to use a single load side brake.” Even so, the admonition bears repeating: “Even the most carefully designed and manufactured systems are subject to factors such as material flaws, misuse, or lack of inspection and maintenance.” And the simple warning, repeated as a mantra, works as an antidote to complacency:
“It is the nature of machinery to fail.”
