EHD phenomena in grease lubricated contacts

Nagata, Yuichiro (2011) EHD phenomena in grease lubricated contacts. Doctoral thesis (DPhil), University of Sussex.

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This thesis examines the rheological behaviour and film formation of greases and oils under
elastohydrodynamic (EHD) conditions. It approaches the lubrication of non-conforming
contacts from tribological as well as from dielectric point of view.

The experimental work was carried out on a point contact formed between a ball and the
flat of a disc, which was either transparent, for optical interferometry study or steel in order
to evaluate the conditions in real-life machine components. In the second case electrical
capacitance method has been employed to study film formation. The experimental
equipment has also been adapted to study the starvation behaviour of grease lubricants in
point EHD contacts under vibrations.

Dielectric properties of lubricants have been studied in static conditions and correlated to
their performance in EHD conditions. The dielectric constant of greases indicates in general
higher value than corresponding base oils and the dielectric relaxation time of greases is
proved to be shorter. It has been found that in EHD conditions higher-polar greases show
higher shear stress, while the dielectric constant of the lubricating film decreases with
increasing contact pressure.

The effect of three parameters upon grease EHD films was evaluated: operating speed, load
and vibration of the contact. The results showed that greases possess an intrinsic time to
starvation related to the operating speed but independent of the exerted load. The results
also showed that a high speed was not necessarily associated with the high likelihood of
occurrence of starvation. The vibration tests revealed that the lateral motion perpendicular
to the rotating direction helped maintain the inlet region flooded and, under oscillations,
starvation of the contact seems to never occur. This would suggest that machine elements
such as rolling element bearings could operate under the fully flooded conditions as
vibrations are almost inevitable in any machinery.

Finally, the cavitation phenomenon was also investigated and found that the rheological
model of greases could be predicted from the observation of the cavity pattern.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Engineering and Informatics > Engineering and Design
Subjects: T Technology > TJ Mechanical engineering and machinery
Depositing User: Library Cataloguing
Date Deposited: 17 Feb 2011 15:24
Last Modified: 13 Aug 2015 14:31

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