In recent years, the 4.45 MN DWM started to show similar indications of damage as had earlier been observed in the two other DWMs of similar chain-loading design.
The largest DWM is the 4.45 MN apparatus, which has a height of over 25 m and has been in use in the NIST Force Laboratory since 1965.
Inhouse NIST intercomparisons between these measurement points and the corresponding measurement points in the 1.33 MN DWM showed good agreement, which indicates that the damage has not affected the accuracy of the machine.
In addition to the in-house intercomparisons and machine history, two international intercomparisons also reinforce confidence in the integrity of the forces realized in the 4.45 MN DWM. In 1989, an intercomparison between NIST and Physikalisch-Technische Bundesanstalt (PTB), Germany demonstrated agreement to within [+ or -] 40 ppm for forces up to 900 kN .
At this point we discuss potential damage mechanisms in relation to the damage observed on the 4.45 MN DWM. Galling is a form of adhesive wear that results in macroscopic particles of material being torn (and possibly transferred) between two surfaces, constituting large and irreversible damage.
In this paper, we model the conical contact joints of the 4.45 MN DWM and perform parametric studies in order to gain some understanding of the origin of the damage observed, and of potential means of preventing similar damage from reoccurring in the future.
We now describe structural finite element modelling of the components of the 4.45 MN DWM, which was performed using the software package COMSOL Multiphysics 4.4 (2).
This is a slight approximation, as the axisymmetry of the 4.45 MN DWM is interrupted by the presence of boltholes, but is justified based on the uniform wear pattern observed on the conical seat joints.
All calibrated components of the 4.45 MN DWM are made of stainless steel.
In this study we model the behavior of the 4.45 MN DWM using each of the two material constitutive relations.
We note that the 1.33 MN DWM that showed similar damage in the early 1970s and was treated with a lubricant, namely MoS2 solid or semi-solid, has not experienced a reoccurrence of such damage up to the present.
(iii) With the benefit of hindsight and a finite-element simulation (results not shown in this manuscript), it is found that a larger contact diameter would have been beneficial by reducing the contact stresses at the pick-up joints in the upper part of the DWM.