Major League Epair Job
25 March 2008
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 was a major repair job on the stadium's movable roof involving replacement of the 10 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 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 inline fashion. But 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. The problem was in the cylindrical wheel with a wide bearing surface, where the outside of the wheel traveled farther than the inside - by about 5½ inches in the worst case.
The wheels on the replacement bogies were designed with spherical rolling surfaces to allow for minor bogie tilt, and the wheel axles are turned such that the bogie naturally follows the curved track. In addition, 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 on its location. They are powered by 60 horsepower motors via gearboxes and roller chains.
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 the consulting engineer, explaining that jacking brackets to lift against had to be designed, fabricated, and installed, as well as jacking platforms. According to Millwright's general foreman and 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 10 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.
The weight lifted ranged up to about 800 tons, so a capacity safety margin was provided by 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 horsepower, 10,000 psi electric pump. The jacks had 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 set up included a valve with a venturi feature to deliver negative pressure when needed. The hydraulic system was assembled and tested before use. Jim Ronning explains that to provide for lateral movement during lifts, the jacks rested on a 1½ inch thick steel plate, then a sheet of Teflon, and then a sheet of polished stainless steel.
For this lift, the hydraulic jacks were arranged in a quad-cluster, fitted with spherical load caps and rested on a Teflon “sandwich” to
Two gantry legs were set up on each side of the railway track with the header beam going across the tracks allow for lateral movement. All 10 of the new bogies are in place, and all work was expected to be complete before opening day 2007. Total cost is estimated at $13 to $15 million, with the variability partly due to the unknowns of winter working conditions.