Raising the roof at Miller Park

Sept. 1, 2007
Just a few years into its existence, Miller Park's retractable roof suffered several problems with its drive system. The Milwaukee Brewers' baseball stadium,

Just a few years into its existence, Miller Park's retractable roof suffered several problems with its drive system. The Milwaukee Brewers' baseball stadium, built by Mitsubishi Heavy Industries in 2001, featured a retractable roof with combined design elements from two different concepts, which quickly developed problems. Three bogie drives were replaced, five electric motors failed, and spindle bearing problems plagued the operation of opening and closing the roof panels.

To combat these issues, a new drive system is being installed, which includes a more robust gear drive with dual output shafts. Hardesty & Hanover LLP (H&H), New York, decided to double the number of bogie wheels or rollers to increase the load capacity and provide a greater safety factor. Previously, the rollers only were powered on one side, so now it became necessary to deliver power to both sides of the bogie. John Gimblette, mechanical engineer at H&H, says, “By using a dual output shaft arrangement, we were able to minimize the overhung load on the two shafts. This was really one of our guidelines for sizing the drives, because the original units were short of meeting the overhung load rating.”

While the initial redesign concept incorporated a differential drive to ensure that the load was evenly distributed to both drive output shaft extensions, the added complexity of such a system combined with the desire to use off-the-shelf components led to considering other approaches. H&H contacted Rexnord Industries LLC, Milwaukee, because the gearing within the unit itself is more straightforward. Rexnord performed a finite element analysis that helped determine that the reducer and other elements within the drive system would accommodate an uneven load between the two output shafts.

The roof mechanism incorporates 10 bogies, each made of several sub-assemblies. These include an expansion assembly, two truck assemblies, and the drive machinery. One truck assembly is an idler, while the other is the driven truck. The drive machinery is mounted to the driven truck and consists of a Toshiba 60 hp, 1,800 rpm electric motor; a Magnetek 100-in. shoe-type hydraulic thruster brake on the back end of the motor; a Falk Steelflex 1080 grid coupling connecting the motor to the high speed, low torque end of a Falk A-Plus right-angle speed reducer with dual output shafts; a sprocket on each output shaft; and a Super H double-strand roller chain driving the corresponding bogie wheel.

Because of size differences among the roof panels they handle, six of the bogies use 485 AB4 Model C reducers, while the remaining four use smaller 465 AB4 Model C reducers. Although there are two different bogie sizes, the same motor size is used on all 10 units for efficiency in manufacturing and maintenance.

The drives incorporate carburized ground gearing, which provides a higher rating in a more compact package. H&H was looking for a low-profile drive that would fit easily beneath the roof panels. To balance the loads between the chain drives on the two output shafts, the drive needed to achieve this load sharing. Rexnord's finite element analysis of the entire low-speed shaft assembly, including the gear on the shaft, the output gear, the shaft itself, the sprockets, and the chain showed that there was an inherent load sharing because of the elasticity in the system, including the shaft, sprocket teeth, and chain.

Rexnord engineers also determined that the load differences from one side of the bogie to the other could be accommodated by using shrink disc-type shaft-hub locking devices instead of keyways to secure the sprockets to the shafts.

The new drives are also sized to include a substantial safety factor. The two drive sizes have safety ratings from 1.6 to 2. The reducers need to withstand a momentary overload equal to three times the full-load torque of the drive motors without any component reaching 75% of its yield strength.

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