This article focuses on controlling of a 13,000-ton retractable steel sodium roof of the Safeco field. The roof is built in three telescoping sections, which spans 8.9 acres, travels atop two parallel elevated runways along the length of the stadium’s north and south sides. The basic intent behind Safeco Field was to honor the open-air tradition of vintage ballparks, such as Boston’s Fenway Park and Chicago’s Wrigley Field, while protecting players and fans from the fickle Northwest weather. When needed, the roof will move out over the field at a speed of 30 feet per minute, with the leading end section traveling the longest distance, 532 feet. This giant umbrella relies on a network of 96 adjustable-voltage dc motors driving 36-inch-diameter wheels along steel rails, all load balanced with the help of “cutout” gear couplings, specially modified for quick, easy deployment.
When the Seattle Mariners square off with the San Diego Padres on July 15, the roar of the fans will welcome the Mariners in a spectacular new setting. It will be the inaugural game for the Seattle team’s new home, the 47,000-seat Safeco Field. Players and spectators alike will see some striking contrasts between the new stadium and the Kingdome, the team’s old quarters. While the Kingdome is fully enclosed and carpeted with artificial turf, Safeco is an open stadium with real grass underfoot and the Northwest sky overhead.
But whenever the weather turns foul, 13,000 tons of state-of-the-art retractable roofing will roll out over the new field in less than 20 minutes. Built in three telescoping sections, the roof, which spans 8.9 acres, travels atop two parallel elevated runways along the length of the stadium’s north and south sides. Looking like railway trestles, the north runway rises 66 feet behind the left field bleachers, while the south runway—higher to accommodate grandstands, skyboxes, and press facilities—rises 119 feet behind the first base line.
Convertible Top vs. Sunroof
The basic intent behind Safeco Field was to honor the open-air tradition of vintage ballparks, such as Boston’s Fenway Park and Chicago’s Wrigley Field, while protecting players and fans from the fickle Northwest weather. These seemingly contrary objectives ruled out previous designs such as that used in Toronto’s Sky- dome—a fully enclosed facility that provides only an overhead view of the sky through its retractable roof.
“Safeco Field’s sunroof was envisioned as a convertible auto top compared to Skydome’s sunroof,” according to Neil Skogland, president of Seattle-based Ederer Inc., which designed the drive system that powers the roof. The three separate sections of the roof design are arched steel structures that nest in their retracted position beyond the stadium’s outfield seating sections. Two 3,000- ton end sections fit under the 7,000-ton middle section when parked at one end of their 816-foot-long rails. Its lower trusses clear the baselines by more than 230 feet.
Because the stadium’s grass field requires a lot of sun and water, the roof is expected to remain in the retracted position as much as possible. When needed, the roof will move out over the field at a speed of 30 feet per minute, with the leading end section traveling the longest distance, 532 feet.
This giant umbrella relies on a network of 96 adjustable-voltage dc motors driving 36-inch-diameter wheels along steel rails, all load balanced with the help of “cutout” gear couplings, specially modified for quick, easy deployment. Separated by about 645 feet across the width of the stadium, the wheels must move along both runways at exactly the same speed, so that the ends of the roof are synchronized to within less than one foot.
Each top end roof section stands on four legs, two on either side. Each leg is centered on a 35-foot-long, eight-wheel truck system that resembles a giant Rollerblade skate, called a “corner.” The roof’s center section, rising higher to pass over the two top end sections, stands on eight legs, four on either side, again with each leg mounted over the same type of truck assembly. All three roof sections ride on a total of 16 corner assemblies. The center section’s wheels roll along the outer rail atop each trestle, while the two end sections roll on a parallel inner rail 12 feet away.
The eight wheels in each corner assembly are installed as four two-wheel trucks. In each of these eight-wheel sets, six wheels are individually powered by 10-hp motors, while the two end wheels roll freely. Of the total of 128 wheels, 96 are under power. The wheels are similar to railroad wheels, except that they are double flanged.
For each powered wheel, torque is transmitted through a right-angle reducer to a spur pinion, driving a bull gear on each wheel, explained Ederer design engineer Don McGhee. This configuration was selected to allow all the motors in each section to be load-balanced, he said.
“The type of motor we selected allows electrical engineers to balance electrical resistance to ensure that all the motors carry the same speed all the time, even during startup,” McGhee said. “Balancing involves measuring current draws and adjusting voltages to compensate for differing wire feed lengths and motor characteristics, and it requires that the drives be run under no load.” Each wheel in the roof drive system normally bears more than 205,000 lbs. of weight. This makes it unrealistic to jack up each wheel to adjust, test, and readjust motor speed.
McGhee looked for a mechanical power transmission coupling that could be disconnected easily, without disturbing power transmission drive components, while satisfying other truck design considerations. Ederer had already specified a motor, reducer housing, and C-face adapter that had side-access openings, he recalled. This meant that the coupling had to be small enough to fit inside the adapter and provide adequate space for a technician’s hands to reach in and disconnect it, yet be big enough to transmit sufficient torque. Considering the number of motors per section and the load on each motor, McGhee calculated starting torque at 788 in.-lbs. and normal operating torque at 525 in.-lbs.
After evaluating several coupling possibilities, McGhee settled on a Lovejoy/Sier-Bath Series F flanged sleeve gear coupling, supplied by Lovejoy Inc. of Downers Grove, 111. The coupling offered a high ratio of torque capacity to outside diameter in a compact design. Specified in double engagement design—with hub-to-sleeve gear meshes in both hubs—this hub coupling accommodates up to 3 degrees of angular misalignment as well as relatively high parallel offset and end float. A patented gear- tooth form enforces a broader contact area of mating gear teeth than conventionally crowned tooth couplings. According to the maker, the design reduces tooth stress and maintains torque capability if misalignment increases.
The company’s standard F I model met Ederer’s needs without requiring the installation of custom shafts, said McGhee. “The FI couplings connect each l5/s-inch motor output shaft to each l!/8~inch reducer input shaft,” he said. “Their maximum torque and rpm ratings comfortably exceed our roof-drive requirements, plus they met all of our space restrictions.” The coupling had a full diameter around its flanges of 49/u, inches and a hub sleeve diameter of slightly over 3 inches, leaving room for hand movement within the C-face adapter.
Lovejoy modified the hub and sleeve assembly of the standard FI coupling to allow quick disconnecting for balancing. The coupling was modified by machining the hub to receive an internal stop ring, and tapping the hub’s sleeve flange to house a locating screw.
When the screw is backed out, the assembled sleeve is free to slide axially to disengage its gear teeth from the hub’s gear teeth. This disconnects the motor from the reducer and allows the motor to turn freely for balancing. Connection is re-established by sliding the sleeve back to the engaged position and tightening the locating screw to hold it in place. The sleeve flanges remain bolted together and slide as a single unit, so lubrication is retained within the ring seals that line the outer ends of the sleeve.
Ederer’s design called for eight distributed programmable logic controllers riding along on the moving roof sections to help provide precise speed control. Each PLC supervises 12 motors, six each on two trucks. The PLCs are linked via fiber-optic cables to a central computer beneath the outfield stands. The fiber-optic cables ride along with the motor power cables, which are paid out and taken up by huge reels mounted on the moving truck systems.
On rainy days, the grounds crew will walk along each runway to make sure the 6-inch-high rails are clear. Control room operators power up the drive system electronic controls; programmable controllers will lift anchor pins that moor the drive trucks to the concrete runways; brakes will be released as all 96 drive motors power up to full torque and rpm together, and roof sections will begin moving. As roof sections reach their extended positions, proximity switches signal when the anchor pins are aligned with their mooring holes. Power drops off, brakes reset, anchor pins go down, and the game continues with players and fans both protected and treated to a mechanical spectacle as exciting as winning a close game. Well, almost.