World’s Largest Deadweight Machine Completes 16-Month Overhaul
Ian Wright posted on June 06, 2016 |
NIST’s three story, million-pound machine calibrates load cells for aerospace applications.
Rigging crew reassembling the million-pound weight stack in March 2016. (Image courtesy of NIST PML.)
Rigging crew reassembling the million-pound weight stack in March 2016. (Image courtesy of NIST PML.)
The National Institute of Standards and Technology (NIST) has brought the world’s largest deadweight machine back online, after more than a year of maintenance. This marks the first time the machine has ever been recalibrated since its construction in 1965.

That might sound like a long time to go without any recalibration, but it makes sense when you realize that the machine’s total height is roughly ten stories. This includes the laboratory/control room and the large hydraulic ram above it which lifts the three-story weight stack.

Schematic of the NIST deadweight machine, with the hydraulic ram at the top, the weight stack at the bottom and the laboratory/control room in between. (Image courtesy of NIST PML.)
Schematic of the NIST deadweight machine, with the hydraulic ram at the top, the weight stack at the bottom and the laboratory/control room in between. (Image courtesy of NIST PML.)
The machine is designed to generate forces of up to 4.45 million newtons (equivalent to one million pounds-force) for the calibration of load cells. Quality professionals then use the calibrated cells to measure large forces, such as rocket or jet engine thrust or the deflection of airplane wings.

NIST engineers generate calibration forces using the machine like a conventional mechanical scale—hanging stainless steel weights from the load cell under calibration. The weight stack itself consists of a calibrated lift frame and 19 stainless steel discs with an average mass of 22,696 kg (approximately 50,000 lbs).

 

Calibrating the World’s Largest Deadweight Machine

“Taking apart a machine for the first time in 50 years was a major undertaking, one that we didn’t take lightly,” said Carl Williams, deputy director of NIST’s Physical Measurement Laboratory (PML). “Observing its dismantling and repair was a great honor–a once-in-a-lifetime opportunity.”

The 16-month refurbishment included a complete overhaul of half the stainless disks comprising the weight stack. NIST engineers suspected that galling had begun to occur inside several key joints in the assembly, causing pieces to bind up or stick.

Damage from galling inside one of the stainless steel disks. (Image courtesy of NIST.)
Damage from galling inside one of the stainless steel disks. (Image courtesy of NIST.)
“The system is literally worth millions of dollars, which meant that any mistakes during any phase of the operation could have unknown and severe consequences for NIST and our capacity to disseminate large forces to our customer base,” said project leader Rick Seifarth. “Translation: We could have ruined the machine.”

The initial disassembly involved removing (or in stubborn cases, destroying) the bolts connecting the individual steel disks. The disks were then inspected and those with galling damage sent to an off-site machine shop. The shop repaired the damage by shaving off as little as a millimeter of steel from the affected joints.

The resulting lost mass was replaced via pre-calibrated stainless steel weights that were inserted into pre-existing cavities designed for such adjustments and then sealed. The subsequent recalibration of the individual weights was one of the largest parts of the refurbishment project.

Sam Ho (left) and Kevin Chestnutwood working with large masses in December 2015 as part of the recalibration process. (Image courtesy of Jennifer Lauren Lee/NIST PML.)
Sam Ho (left) and Kevin Chestnutwood working with large masses in December 2015 as part of the recalibration process. (Image courtesy of Jennifer Lauren Lee/NIST PML.)
The refurbishment crew also coated the surfaces with a molybdenum disulfide lubricant, which was not available when the machine was originally built in 1965. With their work finished, the crew is confident that the system will last at least another 50 years before needing recalibration.

That’s good news not only for NIST, but for many aerospace manufacturers as well. “Many of our large force calibration customers have had to put their own calibrations for their customers on hold until this repair was completed,” said Seifarth. “And that’s because their measurements are directly traceable back to the international unit of force through our calibrations. So without us in place, they were stuck.”

For more information or to watch a video featurette on NIST’s Million-Pound Makeover, click here.

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