Repairs on 12,000-Ton Movable Roof
Product Announcement from ENERPAC
January 19, 2007 - The day after the Milwaukee Brewers finished their final game of the season at Miller Park in September 2006, a different team went into action. This time the coaches were engineers, and the first-string players were millwrights, ironworkers, operating engineers, and laborers.
The task for which they had been preparing was a major repair job on the stadium's movable roof: replacement of the ten bogies (powered carriages) on which the five movable sections of the roof are carried. The 12,000-ton roof is designed in a fan-shape, with each of five movable sections pivoted at its home-plate end and riding on two bogies at its wide (outfield) end 600 feet away.
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. Therefore, 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 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. Doing it 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 4-6 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.
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 Teflon, and then a sheet of polished stainless steel.
The bottom line
All ten of the new bogies are in place, and all work is expected to be complete before opening day 2007 rolls around. Total cost is estimated at $13-15 million, with the variability partly due to the unknowns of winter working conditions.
Miller Park hosts a lot of loyal fans, and a 2005 Sports Illustrated fan survey rated it as the best ballpark based on value per dollar spent. Stadium officials are proud that not one of the more than 550 events held during its six-year history has been cancelled because of weather, so they are understandably pleased at the prospect of a much more reliably operating roof.