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 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/closing. The problem was traced back to the five pivot locations behind home plate, high above the upper deck.
Designed with seven fan-like panels, the roof consists of one stationary panel along each base side and five moveable panels.
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 City, engineers from Timken (www.timken.com) analyzed the conditions and began working toward a plan to replace the five bearings.
The moveable dome roof weighs 12,200 tons and has a surface area of 369,760 square feet. Designing new bearings for each pivot point required detailed understanding of the demands placed on each bearing. The center panel is the largest, and it produces a bearing load of 2.2 million pounds. Each of the remaining panels bears a thrust load of more than 1.5 million pounds. 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, while the roof was at rest, produced stresses from weather and caused it to shift slightly at the pivot points. Dynamic conditions, while the roof was in motion, forced engineers to consider the bearing load while in motion plus the increased load created by weather. With constant 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 pounds before set into the housing, the bearing's basic dynamic thrust capacity is 3,033,000 pounds, and its static capacity is 12,933,000 pounds. The bearing measured less than four feet in diameter -- an outside diameter is 1050 millimeters, a bore of 580 millimeters and a stack height of 258 mm.
"Each bearing design has set application limits for static and dynamic conditions," said 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.