The Ground Shifts in a New Exhibit at the Museum of Science and Industry, Chicago.

Science Storms at the Museum of Science and Industry (MSI), Chicago lets guests investigate the basic scientific principles behind nature’s most awe-inspiring events in a 26,000-square-foot permanent exhibition. One of the displays lets guests trigger an avalanche in a 20’ rotating disk to reveal the beauty of granular dynamics.

Designed by Architect Jack Pascarosa, AIA, from Evidence Design and environmental artist Ned Kahn, the controllable avalanche disk was engineered, fabricated, and installed by Production Resource Group (PRG). Christopher Wilson, Senior Project Manager of Museum Exhibits for MSI oversaw the 16,500 lb display that provided the safe and educational method of understanding the power of an avalanche.

“The 20’ diameter disk is set at a 23 degree angle to horizontal,” explains Sara Rockwell, I.E. Mechanical Engineer with PRG. “The disk spins between 0 and 8-RPMs; the speed is based on the control wheel that is connected to the disk. Guests use the wheel, similar to a steering wheel, to change the speed of the disk rotation and see how the mixture of glass beads and garnet sand inside the disk reacts to different speeds. It allows an attendee to see how a solid can act like a liquid, which is the concept of an avalanche.”

For the turning mechanism PRG used a 15-HP motor connected to a reducer. A four-foot diameter ring gear is the main component mounted to the disk. There is a spur gear attached to the gear box that turns the ring gear which then turns the disk. “We needed to build in a safety factor because we didn’t want the inertia generated by the disk to back drive our gear box in the event that someone suddenly changed the speed from 8-RPM to zero,” says Rockwell. “We needed to accommodate a full stop in less than five seconds. To do this, we inserted a friction slip hub into the spur gear, which allowed the gear box to slip if the torque we set for the hub was exceeded. When the disk slows down enough then the clutch engages again and is driven normally. The most severe torque on the system typically happens only at start up and shut down.” PRG worked with SEW-Eurodrive, Inc. for the gear motors and Mayr Corporation for the brakes.

For installation in the exhibit hall Wilson was very pleased with PRG’s solution. “PRG’s approach to the avalanche was a game changer for us,” comments Wilson. “We were considering building the avalanche disk as a single piece. The concept of building it in small pieces, bringing it into the hall and assembling it, then pouring a special urethane compound to make the final disk surface was all PRG’s idea and it saved us a lot of time, a lot of money, and allowed us to do a full test in their shop.”

Rockwell explains that the load was an important consideration, “We weren’t to exceed a maximum load of 250 Pounds per Square Foot (PSF) for floor loading. Since we were dealing with dynamic motion within the avalanche, we needed to subtract for the live load, so we really only had 150 PSF, which is not a lot when you think about it.” The avalanche display did not need to be tied into the building structure however, since it is heavy enough and stable enough to sit on the floor without moving. Geiger Engineering worked with PRG on the building impact for the avalanche.


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