Physical disassembly of the 4.45 MN (1,000,000 lbf) deadweight machine (DWM) began in FY15 in order to address material galling that developed in key structural components within the stainless steel weight stack. Repair of the weights required shutdown of the machine, disassembly of at least part of the weight stack, remachining of affected components, recalibration of all removed weights and reassembly and testing of the rebuilt machine. Estimated duration of the downtime is eight to ten months or longer.
The 4.45 MN DWM has been in use in the NIST Force Laboratory since 1965 and is frequently characterized as the largest deadweight force standard in the world. Thousands of measurements through many force calibrations have been performed over the lifespan of the machine. The machine was the centerpiece of two multi-year, multi-NMI international key comparisons that assured comparability of measurements throughout the world for large primary forces, thus, the 4.45 MN DWM is an international resource that has disseminated force units to a worldwide customer base since its inception and is a source of US national pride in the world of metrology.
The lifting frame of the machine is manufactured of various grades of steel; all force-generating, calibrated mass components of the machine consist of AISI 410 and 416 series stainless steel. The design of the weight stack is described as chain-loading or serial-loading meaning each 222.4 kN (50 klbf) weight in the stack must support all weights below it when forces are generated during machine use. The individual weights are among the largest mass objects ever calibrated.
This reconditioning process is extremely time-consuming and labor intensive requiring coordination of many differing disciplines under the project manager: NIST Plant Division, professional riggers, machine shop services, truck transportation, tribology, mass calibration and financial backing.