The end of grease life is the point at which the grease can no longer build up a sufficiently thick film to fully separate the contacting surfaces in a rolling bearing. Empirically, it has been shown that grease life can be determined via measurement of grease consistency (given by its yield stress) and/or base oil content. Via modeling, the end of grease life can be estimated by calculating the point in time where the film thickness is equal to the interfacial roughness. Such a model does not currently exist. The film thickness in a grease-lubricated bearing is essentially the compressed combined thickness of the lubricant layers entering the tribological (ball-ring) contacts. These layers are reduced in thickness by side flow induced by the high pressures in the contacts and are fed by grease bleed and replenishment. Grease bleed is the slow release of base oil from the grease into the contacts, providing a feed mechanism for the lubricant film. Replenishment is related to the base oil viscosity and the remaining grease volume, which is related to the yield stress of the grease. Recent works have shown that mechanical degradation of lithium and polyurea greases (the most widely used grease types) leads to a decrease in the yield stress and an increase in grease bleed.
The goal of this project is to develop a model for the prediction of the remaining useful life of grease via the grease yield stress and oil content, including the effects of temperature changes.
•             You will study the rheological behavior of some model greases.
•             You will study the thermodynamics of grease degradation.
•             You will study how grease behavior changes in a rolling bearing by “mechanical load” on the grease matrix (the matrix deteriorates due to large shear and pressure).
•             You will study the oil separation behavior in a ball bearing.
•             You will study the change in micro-structure of grease taken from the bearing tests using Atomic Force Microscopy (AFM) or other techniques.
•             You will develop criteria for using oil content to measure end of grease life.
•             You will develop criteria for using yield stress to measure end of grease life.
•             You will generate enough data for the validation of the remaining grease life concept and for the applicabili

•             You hold an MSc degree in Physics or Mechanical Engineering or a closely related discipline.

•             You have good knowledge/demonstrated awareness of modelling using Python, C++ and/or MatLab.

•             You have good experimental skills.

•             You have a strong analytical mind and an eye for detail, as well as a critical attitude towards assessment of experimental results.

•             You like to present your results to colleagues, the industrial partner and at international conferences.

•             You are passionate about learning new concepts and doing original research.

•            You are fluent in English and express yourself easily both in speech and in writing.

·         We provide excellent mentorship and a stimulating research environment with excellent facilities.

·         You are offered a professional and personal development program within Graduate School Twente.

·         A starting salary of  € 2.395 gross per month in the first year and a salary of € 3.061 in the fourth year gross per month.

·         A holiday allowance of 8% of the gross annual salary and a year-end bonus of 8.3%.

·         A solid pension scheme.

·         Minimum of 29 holidays per year in case of fulltime employment.

The SKF University Technical Centre for Grease Lubrication of the University of Twente has an opening for a PhD candidate. This project operates at the interface of fluid dynamics, rheology, complex fluids, thermodynamics and tribology. The expected outcome of this 4-year project will be a model that can predict the remaining grease life based on the condition of the grease for bearings operating at medium and low temperatures (typically temperatures lower than 90oC). This requires a thorough understanding of the grease rheological and thermodynamic properties, as well as degradation mechanisms in rolling bearing applications, among others. The project is executed in close collaboration with SKF, a leading global bearing manufacturer.

Lubricating grease is a visco-elastic semi-solid, consisting of a thickener matrix (usually a soap, 5-15%), base oil (mineral or synthetic) and additives (anti-corrosion, anti-wear, 1-5%).  At small strains, lubricating grease is nearly solid. This quality makes grease easy to use: it does not easily leak out of a bearing and acts as a seal, preventing contamination from entering the bearing. Only creep flow may occur. This is ascribed to the thickener matrix that forms a network filled with oil. At higher shear rates, the viscous properties dominate. At very high shear rates, grease viscosity approaches that of its base oil.