The effect of vibrations on the behaviour of lubricated elastohydrodynamic contacts

Zhang, Xingnan (2017) The effect of vibrations on the behaviour of lubricated elastohydrodynamic contacts. Doctoral thesis (PhD), University of Sussex.

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It is known that rolling element bearings are the second most extensively used machine components in various mechanisms and machines. Consequently, the demand of pursuing higher bearing performance, increased load carrying capacity and prolonged operation life is crucial. The lubrication regime in which rolling element bearings operate is called Elastohydrodynamic Lubrication (EHD). The EHD lubrication regime in steady-state conditions, where working parameters such as load, geometry or speed of the surfaces are constant, is nowadays well understood. Nevertheless, in practice EHD contacts never work in completely steady-state regimes. For example, the contacts of rolling element bearings operating in a machine, which runs at constant speed, has no shock loading from the end effector still experience variable loading when the rolling elements enter and exit the loaded zone and variable speed at the start up and shut down of the machine. Factors related to the working cycle of the machine or mechanism cannot be avoided or simply the start or stop of those will always introduce unsteadiness into the EHD contacts. Vibrations in EHD contacts cause variation of load, and implicitly pressure and contact size, fluctuations of film thickness, which in their turn cause localized fluctuations of the pressure. The vibrational behavior is directly related to noise generation, which significantly influences bearing performance.
The current research is focused on the effect of forced harmonic vibrations on oil and grease lubricated EHD contacts. Experiments were carried out using a test rig that simulates real – life EHD contacts by a steel ball on glass disc contact. The method used for measuring the lubricant film thickness was optical interferometry, adapted for the specifics of this research. Lubricant film thickness was measured in systematically – designed tests where a wide range of lubricants and working parameters was employed. The trends of EHD film behaviour under variable loading were evaluated and maps of the likelihood of film thickness perturbation occurrence were drawn for the first time. An analytical model of the lubricant film thickness variation during rapid variation of load was also devised and successfully compared to the experimental findings. The systematic study of the effect of vibrations upon EHD film recovery in limited lubricant supply conditions was also carried out and published. An experimental study on the effect of vibrations was successfully extended into grease-lubricated contacts.
The scientific information gained will be a useful tool in the hands of both, lubricants and machine elements producers, with the aim of extending the life expectancy of those elements.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Engineering and Informatics > Engineering and Design
Subjects: T Technology > TJ Mechanical engineering and machinery > TJ1040 Machinery exclusive of prime movers
Depositing User: Library Cataloguing
Date Deposited: 19 Dec 2017 11:13
Last Modified: 19 Dec 2017 11:13

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