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Forgings support retractable roof-moving system for Seattle Mariners' new ballpark

As Seattle Mariners fans look out over a bright green natural-grass playing field in the city's new outdoor ballpark next July, some may be inclined to ask, "Is this heaven?" Indeed, the completion of the park will be the fulfillment of a dream for many--from the team's owners and designers, contractors, and even the fans. Most expect attendance levels to increase over the characterless indoor Kingdome's. The park's open-air design evokes the nostalgia of more traditional ballparks, and has views of the Seattle skyline. Nonetheless, players and fans won't be left out in the rain--thanks to a moving roof that extends over the stadium when showers threaten, then retracts when it clears.

Blacksmith measures a finished ring

A computer-controlled, steel wheels-and-rails roof-moving mechanism opens and closes the roof with the push of a button. "It's like putting the top down on a car, and as easy to use as a garage-door opener," says Neil Skogland, president of Ederer Incorporated, the Seattle-based crane company responsible for making the roof extend and retract. Although the roof is intended to cover, not seal, the stadium, the design must stand up to tough conditions. The high-strength wheel axles, gear pinions, and connecting pins that anchor the roof trusses to the wheeled trucks must stand up to 70-mph winds, earthquakes with lateral-ground accelerations of up to one-third the force of gravity, and six-ft snow drifts. That's why Ederer selected open-die forging for these critical components.

Other critical components include 96 10-hp dc motors from Baldor, dc drives and PLCs from Allen Bradley Co., and gearboxes from Sumitomo Machinery Corp.

Sound forged centers

According to Ederer Project Manager Steve Hertel, the sheer weight of the roof, combined with just moderate winds, makes for high loads. But worst-case scenario, the design load exceeds 300,000 lb per wheel. "That's why cold-rolled steel, used in typical wheel-and-rail systems, didn't meet our strength requirements," Hertel explains. "Castings and fabrications, that are subject to internal defects, don't provide the sound-part centers that result from open-die forging. The process really has an edge over other metalworking processes in terms of directional, structural, and impact strength."

Finding a manufacturer to deliver in the required lead time at a competitive cost was a challenge because of the sheer volume of large parts, 432 in all. "One of the larger connecting pins alone weighs 5,000 lb.," notes Hertel. After comparing quotes from forge shops around the world, Ederer awarded the contract to Scot Forge (Spring Grove, IL) based on its ability to meet the demand and its cost-effective approach. "We have a long-standing relationship with Scot Forge and know them to be reliable," Hertel adds.

In addition to meeting demand, Scot Forge provided unique solutions for minimizing costs. The pinions that drive the wheel bull gears are integral to the gearbox output shaft. The parts are typically produced by machining a larger-diameter forging to create the step for the pinion. "At a count of 96, this approach would have been wasteful in terms of material usage. And machining costs would have been high. By working the step into the product during the forging process instead, we cut costs significantly," says Hertel.

One of a kind

Only one other retractable roof exists in the U.S., the Arizona Diamondbacks' Bank One Ballpark in Phoenix. The Diamondbacks' roof uses a cable-drawn system, not designed for the more volatile weather conditions of Seattle. The Seattle Mariners' roof weighs 11,000 tons, with the roof-moving equipment bringing the total weight to 13,000 tons. Yet while most people would consider the roof heavy, Hertel points out that each roof section catches the wind like a sail. The tricky part for Ederer was engineering a system with high wheel loads and large lateral loads due to the wind and seismic forces that the roof must withstand.

The Roof

The roof must stand up to 70-mph winds, earthquakes with lateral-ground accelerations of up to one-third the force of gravity, and six-ft snow drifts. Ederer's roof-moving concept won a design competition sponsored by the ballpark's Seattle-based architects NBBJ, and structural engineers Skilling Ward Magnusson Barkshire, to generate solutions. Expertise gained as principle vendor to NASA for rocket and Space Shuttle handling cranes helped Ederer win the bid.

To make the three roof sections glide over the ballpark, 16 wheeled assemblies support the roof sections. Eight wheeled assemblies roll along each of two 816-ft rails, mounted to the stadium's north and south sides. Each wheeled assembly, or truck, uses eight wheels and weighs 130,000 lb. One wheel alone weighs a ton. Sensors ensure that each side moves at the same speed, and a computer controls the 96 10-hp motors that power the trucks. The roof is over 600 feet wide, and moves at approximately six inches per second-about the speed of a leisurely stroll. "It takes about 20 minutes to completely open or close the roof," notes Skogland.

Ederer specified AISI 4340 steel, heat treated to 311/352 BHN. The grade's alloying elements (nickel, chromium, and molybdenum) deliver the desired mechanical properties throughout the entire cross section. "Through-hardening, especially important in the gears, allows them to transmit high torque and to have long life," Hertel states.

Wheeled trucks

The roof uses open-die forged parts including: 96 bull gears; 96 step shafts linking the gears to the wheels; and 240 wheel-axle and connecting pins that anchor the roof trusses to the wheeled trucks.

On schedule

Scot Forge Account Specialist Jason Artner runs down a list of parts the firm has delivered: 96 forged rings to be cut into bull gears, 96 forged pinion shafts linking the gears to the gearboxes, and 240 forged bars to be used as wheel axles and as pins connecting the truck assemblies to the roof trusses. The 5.75-inch-thick rings used for the gears have a 36.75-inch OD and a 9-inch ID. The 51.75-inch-long pinion shafts have 3.03-inch diameters, with a 6.75-inch step in diameter.


The bars, or pins, range in length from 31.625 to 69.5 inches, and the diameters range from 9 to 18 inches. Artner explains, "The wheeled assemblies connect to the roof trusses using a series of horizontal sills secured by the forged pins. The pins connect the lowermost sills up to intermediate sills and top sills. Each sill, and the pins that connect them, get progressively larger with altitude. Sixteen upper connecting pins each have an 18-inch diameter and are 69.5 inches long. Weighing 2.5 tons each, they carry over 2,500,000 lb. vertical load. Forged pins also make up the axles for the 128 wheels on the eight trucks. Considering the enormous loads riding on the pins, it's important that they have sound forged centers."

With the ballpark scheduled for completion in July 1999, the three roof sections should be completed by the end of the year. "Scot Forge is helping us to meet that deadline," Hertel asserts. Ederer delivered all components for the first section of the roof in May and the second section in August. As of this writing, the rest of the components are finish machined, assembled and stored, and ready for delivery.

The Mariners' new stadium is truly a "field of dreams," allowing fans to enjoy a game under clear skies. But for all who participated in the roof's construction, the realization of the dream occurs when they are comfortably watching the game from the stands on a rainy day, with the roof rolled securely in place overhead.