The 22-foot long original equipment bogies, two at each of the far corners of each fan-shaped roof section, were fitted with pairs of double-flanged wheels to ride on an eight-inch-wide circular track approximately 138 feet above ground level. Three-phase power for the bogie drive motors is fed out along each roof section from the home plate pivot end, eliminating any need for sliding contacts.
The two double-flanged wheels on each bogie were arranged to ride the single track in an in-line fashion. Additional safety is provided by guide rollers that follow single tracks mounted on both sidewalls of the main track bed.
The original equipment bogies proved inadequate for their massive burden, and the day after the Brewers’ September 2006 home finale, the roof had to be left in a partially open position when a bogie guide roller shattered. Additionally, a drawback of the old bogies was that their wheel faces were cylindrical. “The problem was in the cylindrical wheel with a relatively wide bearing surface where the outside of the wheel wanted to travel farther than the inside, about 5.5 inches in the worst case. It was this anomaly that may have caused the snapping sound as the bogies rolled along the rail,” explains Jim Ronning, a consulting engineer responsible for specifying the jacking arrangements.
“The wheels on the new bogies have spherical rolling surfaces to allow for minor bogie tilt, and the wheel axles are turned such that the bogie naturally follows the curved track,” says Ronning. “Additionally, the new bogie design employs four wheels arranged in two pairs, instead of the previous two-wheel design.” The new bogies are 24 feet long and each weighs either 49 or 66 tons, depending upon its location. They are powered by 60-hp motors via gear boxes and massive roller chains.
Raising the Roof
In principle, the replacement of each bogie was straightforward: lift the roof, remove the old bogie, then position the new bogie and lower the roof. Executing the replacement was a little more complicated. “There was extensive work to prepare the roof panels for jacking,” says engineer Ronning, explaining that jacking brackets to lift against had to be designed, fabricated, and installed, as well as jacking platforms.
According to Dale Anderson, Millwright General Foreman/Project Manager for Price Erecting, contractor for much of the work, lateral movement during the lifting process also had to be taken into account. Working 600 feet from the pivot ends of the roof panels, thermal expansion and contraction were significant, and wind effects could not be ignored.
The stadium roof sections were jacked in ten separate lifts, one for each bogie replacement. Each time, the roof was lifted 4 to 6 inches, the old bogie driven out under its own power, a new bogie rolled in, and the roof lowered back into place on a spindle bearing. A 500-ton crane moved bogies to and from ground level.
Two hydraulic jacks are lifting an add-on jacking bracket. Another two jacks are on the other side of the tracks(Photo courtesy of Price Erecting Co., Milwaukee, Wisconsin).
Hydraulics Provide Lifting Muscle
The weight lifted ranged up to about 800 tons, so a capacity safety margin was provided by using four Enerpac 300-ton, 10,000 psi, 12-inch stroke cylinders for each lift. The cylinders were connected to a common manifold fed by an Enerpac 12.5 hp, 10,000 psi electric pump.
The jacks were equipped with lock rings to guarantee load holding, and a locking valve was used in the pump-to-manifold feed line. The locking valve incorporates a check valve with a manually controlled pilot operator.
The 300-ton jacks were a single-acting load-return type. To provide positive pull-down, the Enerpac pump setup included a valve with a venturi feature to deliver negative pressure when needed. The hydraulic system was assembled and tested before being put into service.
Jim Ronning explains that to provide for lateral movement during lifts, the jacks rested on a 1.5-inch-thick steel plate, then a sheet of coated plastic, and then a sheet of polished stainless steel.
The cylinders were connected to a common manifold fed by a 9.3 kW, 700 bar electric pump.
The Bottom Line
Work was completed successfully before opening day in 2007 on this $13-15 million project.
Miller Park hosts many loyal fans of the Milwaukee Brewers as well as rival Chicago Cubs fans who enjoy the relatively quick trip to the Milwaukee “away” games. A recent Sports Illustrated fan survey rated it as the best ballpark based on value per dollar spent. Stadium officials are very pleased with their much more reliable roof.
Grainger offers a wide variety of Enerpac products, from the most basic lifting set for small jobs to custom configured systems for large scale lifts similar to that featured in this article.
Article courtesy of Enerpac