ARLINGTON, Va.—How do you move a wounded, 2,080-pound patriot?
Very carefully. Recently, under the watchful eyes of curators, conservators,
surveyors, and engineers, a team of riggers deftly lifted the fragile
Liberty Bell off of the supports on which it has been resting for a
quarter century and confirmed the Bell can be safely moved into its
new home this fall.
Recognizable across the globe with its famous crack, the 250-year-old
bell is remarkably frail for its size. The metal is far from pure.
Rather, it is a mixture of various metals, voids and contaminants,
intermingled "like a piece of fruitcake," said Karie Diethorn, Supervisory
Museum Curator at Independence National Historical Park in Philadelphia.
The park is operated by the National Park Service.
Of particular concern is a hairline fracture that extends from the
main crack to the rear of the Bell—if the hairline fracture splits,
so does the icon.
"In terms of assessing its fragility, we really don't know, so our
plan is to subject it to the least amount of stress possible," said
Diethorn.
The Liberty Bell has traveled several times, even across the country,
before settling in the 1976 Bicentennial facility where it now rests.
The latest move will bring the Bell closer to Independence Hall, its
original home.
Although the Bell will move only 200 yards into the new museum, curators
are taking every precaution to assure its safety. Steve Arms, president
of MicroStrain, Inc. tracked movements of the metal along the crack
using tiny, wireless sensors he developed as part of the NSF Small
Business Innovation Research program. The devices are extremely sensitive,
able to detect motion as small as 1/100th the width of a
human hair.
The researchers used custom attachments to place two metal sensors,
originally developed for the semiconductor industry, on the metal around
the Bell's main crack. One sensor monitored stresses that could widen
the crack, another monitored stresses from shearing motions. Stresses
along the main crack will warn of dangers that could spread to the
hairline fracture. The team also hung a third sensor inside the Bell
to monitor potentially jarring rocking motions.
The Bell surface is extremely sensitive to both scratching and chemical
damage, so the team could not use glues or directly touch metal to
metal. Ultimately, Andrew Lins, chief conservator of the Philadelphia
Museum of Art, worked with MicroStrain's Steven Mundell to carefully
clamp the devices to the Bell, inserting a special paper where the
sensors would otherwise touch, and potentially mar, the surface.
Fortunately, Arms's sensors detected no motions greater than several
millionths of a meter, tiny movements that do not seem to stress the
Bell.
"We can use the data that we collected to create upper and lower
limits for vibration, and sound an alarm during the move to warn the
riggers if the limits are exceeded," said Arms. "That will allow us
to move the bell to the new location and basically mimic the conditions
that we got here today, which we feel are safe." he added.
Because of the nature of the move, all of the technology had to be
small and wireless. Despite these restrictions, the devices are capable
of continuously streaming enormous amounts of data to a laptop that
Arms monitored at the site. However, the proprietary software is web
compatible, so all of the monitoring could theoretically have been
done from anywhere in the world.
MicroStrain's work with the Liberty Bell, and all of the stress sensors
and equipment, were provided pro bono. "I just thought this would be
a fun project to take on, and a challenging one," said Arms, "we wanted
to try to protect a national treasure."
The Liberty Bell was forged in 1752 at Whitechapel Bell Foundry in
England — the same foundry that forged Big Ben (the 13-ton and,
ironically, cracked bell within the Great Clock of Westminster) and
the bells of Washington National Cathedral.
According to historians, the Pennsylvania Assembly probably ordered
the Bell in 1751 to commemorate the 50th Anniversary of
Pennsylvania's Charter of Privileges, religious and political freedoms
that the state had enjoyed since its founding. The Assembly asked for
the biblical inscription: Proclaim Liberty throughout all the Land
Unto all the Inhabitants thereof - Leviticus 25:10
The Bell was to hang in the clock tower of the Pennsylvania State
House. The State House was later renamed Independence Hall, and the
Bell — once known as the State House bell —was renamed "Liberty
Bell" by abolitionists who adopted it as their symbol in the 1800s.
The Bell cracked soon after its arrival in Philadelphia and was recast
(from the original metal) by local craftsmen John Pass and John Stow
in 1753. Even that casting had problems, and the Bell that now rests
in the display hall is the third casting.
Over the next century of continual use, a crack had begun to form
that had to be filed down to prevent a jarring noise when the Bell
was struck (the filing marks are still apparent today). In February,
1846, the Bell was repaired and rung in commemoration of George Washington's
birthday. The repair is visible today as a wide jagged crack spanned
in two places by rivets. While it once rang the pitch of E-flat, the
Bell has not pealed since 1846.
Liberty Bell Videos
The following videos require the RealPlayer Plug-in.
Click on the real player icon to get your copy of the RealPlayer
Plug-in.
|
|
Segment 1
Attaching Sensors
Segment 2
Riggers Prepare for Lifting
Segment 3
Lifting the Liberty Bell
Segment 4
Comments from Steve Arms, President, MicroStrain, Inc.
Segment 5
Comments from Karie Diethorn, Supervisory Museum Curator at Independence National Historical Park in
Philadelphia
| |
Public Relations, MicroStrain Inc: Lynne McMinn, (802) 326-4215,
lmmcminn@together.net Public Affairs Officer, Independence National
Historical Park: Phil Sheridan, (215) 597-0060; phil_sheridan@nps.gov
Official National Park Service Web site on the Liberty Bell: http://www.nps.gov/inde/liberty-bell.html
MicroStrain Web site: http://www.microstrain.com/index.cfm
Big Ben at Whitechapel Foundry Web site: http://www.whitechapelbellfoundry.co.uk/bigben.htm
NSF is an independent federal agency that supports fundamental research
and education across all fields of science and engineering, with an
annual budget of nearly $5 billion. NSF funds reach all 50 states through
grants to nearly 2,000 universities and institutions. Each year, NSF
receives about 30,000 competitive requests for funding, and makes about
10,000 new funding awards. NSF also awards over $200 million in professional
and service contracts yearly.
Receive official NSF news electronically through the e-mail delivery
system, NSFnews. To subscribe, send an e-mail message
to join-nsfnews@lists.nsf.gov.
In the body of the message, type "subscribe nsfnews" and then type
your name. (Ex.: "subscribe nsfnews John Smith")
Useful Web Sites:
NSF Home Page: http://www.nsf.gov
News Highlights: http://www.nsf.gov/home/news.html
Newsroom: http://www.nsf.gov/od/lpa/news/media/start.htm
Science Statistics: http://www.nsf.gov/sbe/srs/stats.htm
Awards Searches: http://www.fastlane.nsf.gov/a6/A6Start.htm
|
|
Liberty
Bell Videos
The Liberty Bell in its current pavilion located on Market Street in
Philadelphia, Pennsylvania. Independence Hall is visible in the background
on the left.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 421KB)
A view of the famous crack as viewed from within the Liberty Bell.
The crack was originally a hairline, and by the 1840s it was a threat
to the bell. In an attempt to repair the crack, part of it was drilled
out (creating the widened, visible crack familiar to visitors) and
rivets were inserted to hold the two sides of the fissure together.
Unfortunately, the "repair" also deadened the sound, and the Bell can
no longer ring. Also visible within the Liberty Bell is the spider,
a structural brace that helps the Bell support its own weight-much
like a flying buttress helps distribute weight away from a cathedral
roof. Without the spider, the fractured Bell might collapse on itself.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 209KB)
Andrew Lins (sitting), chief conservator of the Philadelphia Museum
of Art and metals conservation consultant to the National Park Service
on preserving the Liberty Bell, worked with MicroStrain's Steven Mundell
to carefully clamp sensor devices to the icon.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 517KB)
A close-up view of the MicroStrain NANO-DVRT wireless sensors clamped
to the Liberty Bell (the top and bottom sensors are the same design,
but oriented perpendicular to each other). With even tiny motions of
the crack-on the scale of millionths of a meter-the metal rod shifts
its placement. The movement is detected by electrical coils, and that
information is transmitted to a wireless receiver. The wires visible
in this image are connecting the sensors to a wireless transmitter
inside the Bell.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 684KB)
A close-up view of the spider, a structural brace that helps the Bell
support its own weight. Also visible is the wireless transmitter that
conveys the signals from the sensors on the crack to the computer that
is recording the stresses (black box on top in blue tape) and a MicroStrain
G-link sensor that detects rocking motion (black box near lip of Bell).
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 403KB)
Riggers from the George Young Co. carefully position tackle that will
be used to hoist the Liberty Bell.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 483KB)
The Liberty Bell hangs after being hoisted out of its supports. Visible
are the straps that hold the Bell, with protective padding separating
the strap fabric and the Bell metal.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 480KB)
MicroStrain president Steve Arms monitors the incoming stream of sensor
data as the Bell is lifted.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 384KB)
Animation
Liberty Sensor in Action
Animation depicting the motion of the DVRT sensor core within the NANO-DVRT
sensor. As the core moves further into the electrical coils within
the block at the base of the diagram, the coils change the manner in
which they conduct current. The current change is proportional to the
motion, and the device can be sensitive to motions of less than one
millionth of a meter.
Photo Credit: Stephen Pendo, MicroStrain, Inc.
Steve Arms conducts initial measurements on the Liberty Bell.
Photo Credit: MicroStrain, Inc.
Select image for larger version
(Size: 42KB)
A wooden model of the Liberty Bell crack which the researchers at MicroStrain
used to design their custom sensor mounts.
Photo Credit: MicroStrain, Inc.
Select image for larger version
(Size: 49KB)
Larger versions (Total
Size: 3,672KB) of all images from this document
Note
About Images
|