When Milwaukee's Miller Park baseball stadium opened for the 2001 season, fans welcomed the new facility and the ability to enjoy the game, come rain or come shine, thanks to a new retractable roof. But a year and a half after the stadium opened, the roof began to make loud noises when opening and closing. The problem was traced back to the five pivot locations behind home plate, high above the upper deck, and a fundamental design problem.
Miller Park has a one-of-a-kind roof design that brings America's favorite pastime indoors in 10 minutes. Designed with seven fan-like panels, the roof consists of one stationary panel along each base side and five moveable panels. The mobile pieces are supported at the pivot locations behind home plate and the other end is supported on a track behind the outfield. When the roof is opened, three panels rest on the third base side and two on the first base side.
Although noise was the noticeable symptom, the problem lay within the misalignment of the pivot bushings at each pivot location. Working with Hardesty & Hanover (www.hardesty-hanover.com), moveable-bridge engineers from New York, engineers from Timken (www.timken.com) analyzed the conditions and began working toward a plan to replace the five bearings.
Taking into account the size of the roof, engineers reassessed the requirements for the job. The moveable dome roof weighs 12,200 tons and has a surface area of 369,760 ft2. Designing new bearings for each pivot point required detailed understanding of the demands placed on each pivot. The center panel is the largest, and it produces a bearing load of 2.2 million lbs. Each of the remaining panels bears a thrust load of more than 1.5 million lbs. Beyond bearing load, engineers had to take into account movement at slow speeds, minimal rotation, and marginal lubrication conditions.
Analysis also revealed that static conditions when the roof was at rest produced stresses from weather and caused it to shift slightly at the pivot points. Dynamic conditions when the roof was moving forced engineers to consider the dynamic bearing load plus the increased load created by weather. With such continuous motion at the pivot points, the bearing was susceptible to constant misalignment.
Extensive research, testing, and redesign resulted in a customized spherical roller thrust bearing, offering low rotational friction and misalignment capabilities. Weighing 1,971 lbs. before set into the housing, the bearing's basic dynamic thrust capacity is 3,033,000 lbs, and its static capacity is 12,933,000 lbs. The bearing has an outside diameter of 1,050 mm (41.3 inches), a bore of 580 mm (22.8 inches), and a stack height of 258 mm (10.2 inches).
"Each bearing design has set application limits for static and dynamic conditions," says Cam Hyde of Timken. "We designed around those limits and made sure we didn't exceed what was good engineering design practice for us." Knowing the bearing was susceptible to constant misalignment, engineers included a special envelope dimension, a case hardened inner ring and air removal features. To minimize wear and reduce friction, an engineered surface coating was applied to the rollers.
With a deadline of opening day 2003 set for the six replacement bearings, five for installation and one as a spare, the bearings were installed in the housings and then shipped to Milwaukee. The last of the bearings arrived at Miller Park three weeks before the March 31 opening game. A team of ironworkers and rigging experts were on site making final preparations for the panels to accept the new bearings. Workers also designed a special jacking system of steel beams to raise the pivot locations when replacing each bearing.
"The bearings were in place in time for opening day—and that's what counts,"
said Hans Landin of Timken. "We were involved at every stage of the bearing
installation and have high expectations that the only sounds fans hear when the
roof opens will be baseball-related."
Pivot Man: Five original pivots for the
retractable stadium roof were the source of loud noise during movement.