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Waters Corporation

Waters is an international company which designs, manufactures and sells analytical equipment to a wide range of industries, including pharmaceutical, biochemical and quality assurance. Waters had a leadscrew assembly operating in a vacuum environment, however the working parts occasionally seized under the extreme conditions of long term endurance test. Service and inspection revealed the presence of metal particles, which were presumed to have been transferred from the leadscrew. Waters wished to establish the surface hardness of the leadscrew component and required TWI to perform nanohardness tests on it and the corresponding bore face of a ring bearing. TWI was also asked to investigate the depth of a feature which is sometimes present in the crest of the leadscrew thread caused by issues in the rolling process.

For indentation of the thread region of the leadscrew, a specialist dual axis goniometer jig was sourced and adapted to allow the tapered screw thread to be mounted parallel to the nanoindenter tip. Five sets of five horizontal indents, 30 µm apart,  were made at 250µm intervals from the screw thread root, along the thread flank, to the thread crest. The goniometer was adjusted between indents to ensure contact between the screw and indenter tip was perpendicular.

To investigate the crest feature, a region rich in these features was removed from the leadscrew and then sectioned longitudinally, mounted in resin and polished. Several of the largest crest features were imaged using scanning electron microscopy (SEM), and several locations around two of the features were also examined with energy-dispersive X-ray spectroscopy (EDX) to determine the composition of the material inside this region. SEM analysis on the cross-sections showed that there were large voids, from which fine cracks extended through the bulk material.

The EDX compositional analysis taken from the pores and notches were characteristic of stainless steel, however, the investigation also evidenced the presence of calcium, which would not otherwise be expected. This suggested that the feature was an artefact of the thread forming process, a known consequence of roll forming threads. Roll forming results in high levels of plastic deformation at both the thread crest and the thread root. Whilst no features were visible at the thread root, the nature of deformation at this point suggested it was the most likely to be caused by the presence of the contaminating material observed.

This knowledge has given Waters a better understanding of their materials which has allowed them to make informed choices about the components used in future work. This should increase not only the lifetime of the component but also the system as a whole, and increase production as the system should stop seizing during service.

For more information, please email contactus@twi.co.uk

White light interferometry of crest feature. Image is at 20x magnification.
White light interferometry of crest feature. Image is at 20x magnification.
Specialised dual axis nanoindenter jig with screw.
Specialised dual axis nanoindenter jig with screw.
Nanoindentation of screw thread
Nanoindentation of screw thread
Backscatter SEM images of screw thread crest feature.
Backscatter SEM images of screw thread crest feature.
Avatar Chris Graham Project Leader – Tribology and Coatings Engineering

Prior to working with TWI, Chris studied his BSc at Teesside University, before completing a part-time graduate internship with TWI. Chris then joined TWI full time in 2016 as a Project Leader in the Tribology and Coatings Engineering team.