 Field of Dreams
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.
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| Blacksmith measures a finished rolled
ring for accuracy. |
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 moto
rs 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.
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| 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.
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| 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.
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| Spindles in one of Scot Forge's quench
tanks for heat treat. |
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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.
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