The troublesome retractable roof of Milwaukee's Miller Park Stadium may finally be permanently repaired, as reengineering and replacement of the support bogies and overloaded bearings is being completed. The total cost will be approximately $15 million, paid for by a settlement from Mitsubishi Heavy Industries of America.

Since before its opening in 2001, problems and bearing failures have plagued the bogies which hold up and move the roof's five wedge-shaped panels.

Over the ten minutes it takes to open or close, bogies drive the roof panels left and right along an 8-inch track. The pieces nest at each side, above each other -- three move to the left of home plate (two and the top section), two move to the right. They meet at a center pivot point behind home plate, where five large bearings carry the weight on a massive pier.

Miller Park design model with roof open

This fan-shaped retractable roof dome is the only one of its kind in North America. The dome allows unimpeded flight for a high-hit baseball, and helps reduce snow loading. Inside, the stadium is approximately 200 feet high; the dome adds another 100 feet with 330 feet being its highest point.

Between 2001 and 2005, bearing failures plagued the pivot assembly and bogie assemblies. They were repeatedly addressed by Mitsubishi and others, with no long-term success.

In late 2002, the entire five-bearing center pivot assembly, where the panels converge, had to be replaced at a cost of $11 million. It had been making noise from the time the park opened; blamed on contamination and overloading. Although the bogies continued to give problems, the redesigned center pivot bearings have been trouble-free.

Three years of litigation and repeated attempts to repair problems linked to the design of the roof operating mechanisms were finally settled in January 2005. Mitsubishi Heavy Industries of America, its insurers, and the stadium authority reached settlement for $30 million.

The entire roof weighs 12,000 tons (11 million kg), but the pieces are not all the same size and weight; the center unit is the highest and largest, the next two are slightly smaller, the next two are smaller yet, and the final two are fixed.

Consequently, the new drive system employs ten 24-foot bogies: two 65-ton for each of the center three panels, and two 50-ton for each of the outer two panels.

Bogie unit with traction motor and chain drive

All of the new bogies have four wheels; three idlers and one chain-driven by a 60hp traction motor. The original Mitsubishi-designed bogies had only two wheels.

Bogies travel on a single rail, with horizontal locating rollers (3/16" clearance) for stability. On top of each, a large spindle bearing carries the roof panel weight.

Being able to close the roof is particularly important in Milwaukee, where cold weather can persist through the baseball season. Approximately three hours after closing the roof, the stadium inside can be as much as 30°F warmer.

The roof, with 12,140 tons of steel alone, is designed to withstand the additional weight of 12 feet of snow; its rounded shape should make that load impossible.

There are "Uecker Seats" behind home plate, named for Bob Uecker, which sell for $1 because the view from there is blocked by the massive five-bearing pivot assembly.

Another point of contention is the fact that the roof leaks during rainstorms, so people, players and the field still get wet. The designers say it was never designed to be watertight, but the stadium authority is looking into making it as watertight as possible. This might also help address the potential for corrosion.

The new bogies were manufactured by Steward Machine in Birmingham, Alabama [website].

Mike Duckett, stadium district Executive Director, said: "We're saying the bogies will last for the life of the facility."