A 90-ton operable roof on the Starlight Theatre in Rockford, Ill., incorporates six triangular panels that, when open, give theater-goers a star-shaped view of the night sky.

Usually when you go to the theater, the only stars you see are the ones on stage. Not so at Rock Valley College’s Starlight Theatre in Rockford, Ill., where theater-goers sit under the stars in the sky.

The open theater has been a landmark in the community for 30 years, but weather had a way of raining on the plays. The theater’s director wanted to keep the under-the-stars experience, minus the rainouts. Using the theater’s name for inspiration, architects designed a roof of six triangular panels that open to the sky, forming a six-point star audience members can see from their seats. When it came to actually moving the panels, Minneapolis-based Uni-Systems LLC was called in. The company is no stranger to moving large structures — it designed the retractable roofs on Houston’s Minute Maid Park and Reliant Stadium — but this particular project required motion unlike any roof they had designed in the past.

Act one: Motion

When viewed from below, the roof panels form a hexagon perimeter. They rotate up, one after the other in a clockwise motion, 54° about the perimeter to form the sixpoint star shape the audience sees.

“Our goal was to maintain the architect’s and theater director’s vision of having the roof open, but make the activating mechanism invisible to the people in the audience,” says Alan Wilcox, mechanical engineer at Uni-Systems. “One of the biggest challenges was getting all the components to fit inside the shape, while still making the area where the mechanisms are enclosed easy to access and maintain. Significant geometry changes go on inside as the panels rotate 54°, so we had to make sure there was clearance for all the components.”

The solution was found in simple mechanics. A 20-in.-diameter stationary torque tube spans the inside cavity of the panels, which are 36 ft wide, 42 ft long, and weigh about 30,000 lb. Two supports, or load plates, connect the torque tube to a ring truss structure below. Four large, engineered-plastic-lined radial bearings connect the panel and torque tube and provide circular arc motion. The torque tube is completely hidden inside the panel. Only the 2-in.-wide load plates are visible holding up the cantilevered structure. Audience members can’t see any gears or motors. In fact, they can’t see anything that indicates how the roof moves.

“The resulting panel architecture integrates the mechanical and structural systems so seamlessly, the panels appear to float open,” says Frank Worms, Uni-Systems’ architect and Starlight Theatre project manager.

So, how does it move? A 50-ton keyed machine screw jack from Dayton-based Joyce/Dayton sits between the roof panel frame and torque tube in a double-clevis arrangement to power each panel. As the machine screw jack extends out, the panel slowly rises to its fully open position. The jack’s inherent design, as opposed to a ballscrew jack, doesn’t let it backdrive under the weight of the panel alone.

Spherical bearings at each end of the jack allow for any misalignment between the moving panel and torque tube. A torque-arm assembly supports and balances the weight of the jack and the off-center weight from the gearmotor and motor mount assembly. Thus, all components stay level throughout the entire motion while giving the jack freedom to float on spherical bearings.

Gearmotors from Nord Gear in Waunakee, Wis., activate each machine screw jack. They consist of a helical-bevel gearbox with a 5-hp, 480-Vac, three-phase motor and spring-set safety brake. A reversing contactor lets the gearmotors run in both directions to raise or lower the panels. When the motor is de-energized, the brake automatically sets. An added safety device, the brake holds the panel in position and further prevents the screw jack from retracting.

“The term ‘non-backdrivable’ does not take vibration into account,” says Wilcox. “Even though our system is not technically in a vibration environment, the whole structure is subject to some oscillation while in a static condition because of wind and other factors. The brake is for these situations.”

A Browning torque clutch, made by Emerson Power Transmission, Maysville, Ky., couples the gearmotor to the screw jack and serves as a torque-limiting, automatic-resetting, protection device. In normal operation, the torque clutch engages to provide a rigid coupling between the gearmotor and screw jack. If an internal jack failure prevents the jack from extending or retracting, or a sensor malfunction causes the jack to drive into the rigid safety stops, the torque clutch will disengage or decouple.

Continue on page 2