 Forgings Help Preserve Maritime History
Three
unique forgings were instrumental in granting North Carolina's
storied Cape Hatteras Lighthouse a reprieve from the encroaching
Atlantic Ocean. The components are part of a colossal jacking
system used in a $12 million relocation project that lifted
the lighthouse from its original foundation on the Outer
Banks, moved it over a half mile and lowered it onto its
new location.
The lighthouse was built in 1870
on Hatteras Island, near Buxton, to warn mariners of menacing
Diamond Shoals. Its site was 1600 ft. from the open ocean,
a distance that the builders believed would make the lighthouse
invulnerable. In the ensuing 129 years, however, beach erosion
has advanced the sea almost to the base of the light.
The National Park Service, which
oversees the Cape Hatteras National Seashore, had two alternatives:
Move the structure to safety or let it succumb to the advancing
surf. In an era when large-vessel navigators rely on global
positioning satellites, the Cape Hatteras Lighthouse represents
maritime history worth preserving. And for craft not equipped
with GPS, it is still a potential lifesaver.
At 208 ft., the Cape Hatteras Lighthouse
is the tallest brick structure of its kind in the United
States, a fragile 4800-ton masonry spire with aging mortar
joints. Moving the lighthouse required lifting and rolling
while maintaining it absolutely plumb. Tilting could at
the least result in massive cracking. At worst, the lighthouse
could be reduced to a pile of historic rubble. Jahns Structure
Jacking Systems (JSJS) has confronted similar problems before.
The Elburn, Illinois company manufactures specialized equipment
for lifting and moving structures, and has previous experience
in lighthouse relocation: Rhode Island's much lower Block
Island light in 1993.
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Unified jacking system
JSJS builds unified jacking systems
that lift uniformly, said Bill Jahns, President. "A unified
jacking system is a master cylinder connected mechanically
to a series of displacement or slave cylinders. The displacement
cylinders, in turn, power the lifting jacks hydraulically.
The displacement cylinders supply an equal volume of hydraulic
fluid to each jack. Regardless of jack tonnage, they lift
the same distance because they all receive the same amount
of oil."
For the lighthouse move, JSJS built
the largest unified jacking machine ever produced in the
U.S. A master cylinder with a 17.5-in. bore and a 54-in.
stroke drives 60 displacement cylinders, which also stroke
54 in.
The three 1045 normalized carbon
steel open-die forgings, produced by Scot Forge, are the
cylinder, piston and end plate in the master cylinder assembly.
The 8130-lb. cylinder is the most complex; it is 62-in.
long, with a 51.5-in. deep blind cylinder bore forged in
the center. Forging was done in a 3000-ton press at Scot
Forge's Spring Grove, Illinois, headquarters plant.
"We formed the ID over a tapered
pin held by the forging manipulator," said Mark Brouwer,
of the inside sales and estimating office at Scot Forge.
"The forging billet is partially pierced, and the action
of the forging press works the billet along the pin.
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Reducing weight nearly 2 tons
"Forging the blind hole reduced
the metal requirement by 1740 lbs. If the forging had been
solid, that is the amount of metal that would have been
drilled out. The savings in both material and machining
exceeded the additional cost of forging the hole."
The cylinder forging has a major
outside diameter of 34.25 in., which tapers at a 30 degree
angle to a 28.25-in. OD. The major OD is the flange to which
eighteen 80-in. long tie rods are attached. The end plate
is bolted to the other ends of the tie rods, forming a rigid
cage in which the 60 displacement cylinders are installed.
There was no practical alternative
to forging for the master cylinder, according to Bill Jahns.
"The wall thickness of the cylinder is 5.25 in.," he noted.
"We couldn't buy drawn tube with the diameter and wall thickness
to fabricate the master cylinder. We would have needed a
forged tube body anyway, so it was to our advantage to forge
the master cylinder in one piece.
"A fabricated master cylinder would
be a three-piece weldment: a forged body, plus a flange
and a plug for the bottom. This assembly would be much more
costly than a one-piece forging, and not as strong."
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Superior performance
The master cylinder could have been a one-piece centrifugal
casting, Mark Brouwer pointed out, but the performance of
a forging is superior. "The grain flow of a forging is continuous,
which provides greater impact and directional strength than
the columnar grain structure of a centrifugal casting. When
we forge a step or a shoulder, the grain flows along the
step. This gives the part high structural integrity, and
is also easier to machine than columnar grain. Forging also
eliminates internal voids and porosity by consolidating
the ingot center."
The piston weighs 4450 lbs. and was also forged in the
3000-ton press. The as-forged dimensions are a major diameter
of 35.25 in for a length of 5.375 in., stepped down to 18.5
in. for the remainder of the 55-in. overall length. The
3150-lb. end plate is 34.25 in. in diameter, with a thickness
of 12 in. It was forged in a 1250-ton press.
Scot Forge received the order for the three components
late in November 1998, and completed the master cylinder
forging in just over five weeks. JSJS delivered the unified
jacking system to the job site the end of May 1999, and
the moving process began in June.
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One hundred lifting points
Fifty of the 60 displacement cylinders were used to lift
the lighthouse 57 in. Each cylinder was connected via a
tee fitting to two lifting jacks, for a total of one hundred
50-ton jacks. The lifting capacity is 200 tons greater than
the load.
The master cylinder was pressurized by two hydraulic pumps
of 6 and 21 gpm capacity, both driven by a 200-hp turbo-diesel
engine. The lower-capacity pump advanced the jacks slowly
to take up the slack in the crib supporting the lighthouse,
an assembly of 9000 oak 6x6s, each 4-ft. long. When the
settling of the ground, the compression of the crib and
the stretch in the hydraulic hoses were compensated for,
the movers switched to the larger pump to complete the lift.
A combination of computerized sensors and a traditional
plumb bob tracked the perpendicularity of the lighthouse.
Manual control of jacking pressures corrected for tilt.
Propelled by hydraulics
Horizontal travel was also hydraulically powered, using
five 30-ton jacks with 5-ft. strokes. The lighthouse rode
on steel rollers and a track made of seven parallel I-beams.
The push cylinders were clamped to the beams to move the
lighthouse in 5-ft. increments.
"To complete each stroke, the cylinder clamps were loosened,
the cylinders retracted and the clamps reattached," Bill
Jahns explained. "It was a push-and-grip process."
"As the lighthouse advanced, the support beams were dismantled
at the rear and reassembled at the front. The greatest distance
traveled in one day was 355 ft."
The lighthouse was moved 2900 ft. in a southwesterly direction
to a point that is the original 1600-ft. distance from the
ocean. The rate of beach erosion is variously estimated
at 10 to 29 ft. per year. "How long the relocation lasts,"
Jahns concluded, "depends on the number of severe storms.
It could be as few of 55 years, or as many as 160."
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847-587-1000 |
©2008 Scot Forge. All Rights Reserved.
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800-435-6621
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