Current Projects

Computational Fluid Dynamics (CFD) Modelling of Elastohydrodynamic Lubrication (EHL)

Researcher: Damon LeeCFD

Supervisor: Professor Daniele DiniDr Amir Kadiric

Sponsor: SKF, EPSRC (CASE)

Summary

 Rolling element bearings, gears and many other machine elements operate in the Elastohydrodynamic Lubrication (EHL) regime. In this regime, the lubricant creates a very thin protective film between the contacting elements, improving reliability as well as reducing friction. Therefore, understanding of EHL lubrication allows optimisation of these components in terms of reliability and efficiency, through predictions of EHL film thickness and EHL frictional losses.

Typical methods for predicting the EHL oil film behaviour are either empirical relationships or numerical solutions to simplified fluid flow equations (Reynolds equation), coupled with an approximation to the linear elasticity equations. These methods rely on a number of assumptions that may not always hold. This project utilises finite volume Computational Fluid Dynamics (CFD) and linear elasticity to model the EHL contact through a complete solution of Navier-Stokes equations in the fluid domain, coupled with the Navier Cauchy equations in the solid domain, as well as the heat equation in all domains. A cavitation model is also implemented. This provides for more accurate treatment of relevant physical principles and allows for inclusion of additional effects such as surface roughness, surface coatings or inlet shear heating for example. EHL film thickness and friction predictions are more accurate as the full continuum mechanics description of the system is solved, resolving all gradients. The modelling domain is larger than the immediate contact so that, for instance, the entire flow, viscosity and temperature fields can be studied at the entrance to the contact. The complete shear stress field is predicted, hence providing an accurate way of studying EHL friction.

In addition to improving the EHL modelling tools, the project will attempt to generate charts that indicate the contact conditions where the simplified EHL solutions may be used with sufficient accuracy and those where the full CFD solution may be needed. Investigation will also be made into suitable rheology models for grease as a lubricant.

Film Formation and Friction in Grease-Lubricated Contacts with Focus on Low Speed Operation

Researcher: Yuta Kanazawa

Supervisor: Dr Amir Kadiric

Sponsor: SKF

Summary

This study presents the influence of grease formulation on friction and film thickness in non-conformal rolling-sliding contacts. Custom made greases are conducted in ball-on-disc tribometers under fully-flooded conditions and rolling bearings. Friction coefficient and film thickness are measured over a range of entrainment speeds, loads and temperatures. Specimens with variable surface roughnesses are used in order to cover a wide range of lambda ratios. To understand the fundamental behaviour of grease, additive-free greases (having similar ASTM worked penetration) and their base oils are used as test lubricants. The greases are lithium complex and diurea based in the same polyalphaolefins base oils. The base oils are also tested as a comparison.

The frictional behaviour of the greases is observed with following methods;

• Film thickness measurement in a single contact with EHD rig
• Friction coefficient measurement in a single contact with Mini Traction Machine (MTM)
• Film forming measurement in a full bearing with a bearing lubrication performance tester
• Friction torque measurement in a full bearing with a modified four ball machine

Improvement in Bearing Technology Through European Research Collaboration (iBETTER)

Researcher: Dr Achilleas Vortselas, Dr Jie (Jason) Zhang, Dr Sebastian Echeverri Restrepo, Bo Peng, Rachel Januszewski

Supervisor: Dr Amir Kadiric

Sponsor: SKF, European Commision

Summary

iBETTER is a research programme under the FP7 Industry – Academia Partnerships and Pathways scheme. The project involves collaboration between four partners: SKF Engineering and Research Centre (ERC), the Netherlands, SKF Condition Monitoring Centre (CMC), UK, Lulea University of Technology (LTU), Sweden and Imperial College London (IC), UK.

The project started in September 2013. The main research objective of the programme is to integrate fundamental understanding of tribological contact with condition monitoring technologies in order to improve the performance of rolling element bearings and in particular, to provide an integrated prognostic methodology for reliable predictions of remaining useful life of a bearing operating in a machine. A successful implementation of such a methodology can substantially reduce the maintenance costs, downtime and safety risks associated with operation of mechanical systems. In addition, the strategic objective is to strengthen the collaboration and create new partnerships between the industrial and academic participants involved.

Work Description :
- Physical Modelling of Tribological Contact
- Condition Signal Acquisition and Processing
- Contact Performance Measurements
- Data Integration Models for Accurate Diagnosis and Prognosis
- Industrial Pilot Projects and Demonstration of New Technologies

Influence of Grease Composition on Friction in Elastohydrodynamic (EHD) Contacts

Researcher: Nicola De Laurentisgrease

Supervisor: Dr Amir Kadiric

Sponsor: SKF

Summary

The aim of this project is to examine the relationship between bearing grease composition and rolling-sliding friction in lubricated contacts. The friction coefficient and lubricating film thickness of a series of commercially available bearing greases and their bled oils will be measured in laboratory tribometers. Test greases will be selected to cover a wide spectrum of thickener and base oil types, and base oil viscosities. The trends in measured friction coefficients will be analysed in relation to grease composition in an attempt to establish the relative influence of individual grease components on friction.

Influence of Roughness Characteristics on Friction in Nominally Dry Sliding Contacts

Researcher: Daniel Lofgren

Supervisor: Dr Amir Kadiric

Sponsor: SKF

Summary

Influence of Steel Microstructure and Composition on the Formation and Effectiveness of Lubricant Boundary Layers

Researcher: Kostas PagkalisBearing

Supervisor:  Dr Amir Kadiric

Sponsor: Afton Chemical, SKF

Summary

In the boundary lubrication regime, lubricant films are very thin so surface roughness asperities of the contacting bodies come into contact during rubbing. The load in boundary lubrication is supported mostly by contacting asperities as the local pressure exceeds what can be supported by the lubricant. Under such conditions, the protection for the contacting components can be provided by a boundary film, which is formed through the adsorption/reaction of various chemicals (additives) present in oil with the contacting solid surfaces. The formation of the boundary layers can obviously be affected by the contact conditions, but also by the composition and microstructure of the surface materials (in engineering components, this would usually be steel). Oil additives may form films preferentially with certain alloying elements and microstructures of steel. Although boundary lubrication has received significant attention in tribology research, this tends to be focused on the actual additives whereas the complete system i.e. the interaction between the steel composition and microstructure and the oil, has received little attention.

The current study will examine the influence of different steel compositions and microstructures on the formation and effectiveness of boundary films under concentrated contact and with a selection of different additives/lubricants. Different kinds of steel microstructures and compositions should be used to understand their influence with different additives and how they affect the boundary film formation. The aim of the project is to find which alloying elements with the existing lubricants are mostly beneficial to the boundary film formation.

Premature Failures in Bearing Steels Associated with White Etching Cracks (WECs)

Researcher: Francesco Manieri

Supervisor: Dr Amir Kadiric

Sponsor: SKF

Summary

Premature failure of components is a significant problem in the energy and transport industries, particularly since energy requirements have become more ambitious and demanding. It is well known that components in gearboxes, especially at bearing location, tend to fail below the expected life. Generally, one tends to identify the problem of premature failure with a particular failure mode, i.e. white etching cracks (WECs) as they are very likely to appear in premature failures. A WEC is a crack accompanied by a microstructural change that appears white after etching. The aim of this project is to reproduce premature failures under controlled laboratory conditions, using a triple-contact rig, clarifying the relationship with WECs and identify their root causes.

Propagation of Surface Initiated Rolling Contact Fatigue Cracks in Bearing Steel

Researcher: Dr Pawel RycerzCrack

Supervisor: Dr Amir Kadiric, Professor Andrew Olver

Sponsor: SKF

Summary

Surface initiated rolling contact fatigue, leading to a surface failure known as pitting, is a life limiting failure mode in many modern machine elements, particularly rolling element bearings. Most research on rolling contact fatigue considers total life to pitting. Instead, this work studies the growth of rolling contact fatigue cracks before they develop into surface pits in an attempt to better understand crack propagation mechanisms.

A triple-contact disc machine will be used to perform pitting experiments on bearing steel samples under closely controlled contact conditions in mixed lubrication regime. Crack growth across the specimen surface will be monitored and crack propagation rates extracted. The morphology of the generated cracks will be observed by preparing sections of cracked specimens at the end of the test.

Scuffing in Non-Conformal Contacts

Researcher: Bo Peng

Supervisor: Dr Amir Kadiric

Sponsor: SKF

Summary

Surface Crack Behaviour under Moving Contact Load: An XFEM Study

Researcher: Dr Yilun Xuxfem

Supervisor: Dr Amir Kadiric

Sponsor: SKFMarie Curie

Summary

Three-dimensional extended finite element method (XFEM) calculations that simulate penny shape surface-break crack behaviour under a rolling contact are conducted. The interaction between the surface-break crack and the moving contact is investigated under various working scenarios, including coefficient of friction, relative position of the crack cut and the contact size, etc. Stress intensity factors (SIFs) are evaluated along the crack front based on the stress field, which are further applied to estimate the propagation rate of the crack under a certain scenario. Numerical results illustrate the most vulnerable position of contact with respect to the crack that leads to a fast crack propagation. Besides, this research sheds light on the dependence of SIFs values along a crack front upon the parameters that define a rolling contact scenario. Relative slips between crack surfaces are also investigated under various rolling contact scenarios in this project.

Tribology of Rolling Element Bearings

Researcher: Dr He Liang (Holly)Bearing

Supervisor: Dr Amir Kadiric

Sponsor: SKF, European Commision

Summary

This project uses a custom-made, model ball bearing rig to directly observe and measure lubricant films in the rolling track as well as EHL films in the ball-ring contacts at contact pressures and rotational speeds commensurate with those present in a real rolling bearing. Glass ring is used as the outer bearing race allowing full optical access to the EHL contact. Lubricant films in the rolling track and contact inlet are measured using fluorescence technique, while optical interferometry is utilized to measure thin EHL films in ball-ring contacts. The results are presented to illustrate the influence of multiple factors including entrainment speed and oil fill level on oil films in and around the contact.

Understanding and Prevention of False Brinelling Failure Mode in Rolling Element Bearings

Researcher: Rachel JanuszewskiBearing

Supervisor: Dr Amir Kadiric

Sponsor: SKF

Summary

False brinelling is a type of surface damage that most commonly occurs in non-conformal, nominally stationary contacts that are subjected to externally generated vibration. All machine elements that rely on non-conformal, rolling-sliding contacts in their operation can suffer from false brinelling, but it is most commonly observed in rolling element bearings, especially in stand-by equipment stored near running machines and in the transport of automotive vehicles by rail or sea.

False brinelling is a specific type of a more general contact damage mechanism of fretting, often referred to as fretting corrosion. The underlying mechanisms causing fretting and false brinelling are thought to be similar, but false brinelling in rolling bearings has an added complication that the oscillatory motion is not pure sliding but also involves rolling of rolling elements on bearing raceways.
The aim of the proposed research is firstly, to gain a better understanding of the factors that drive the onset and progression of false-brinelling damage and secondly, to provide potential preventative measures, be it through the improvements in bearing design or lubricant formulation.

Wear, Plasticity and Damage Accumulation Modelling for Rolling Contact Fatigue in Bearings

Researcher: Dr Achilleas VortselasWear

Supervisor: Dr Amir Kadiric

Sponsor: SKF

Summary

The aim of this work is to develop new physical models for selected tribological operating conditions and failure including: wear model, crack initiation and propagation model, contact stress and temperature predictions, plasticity and damage accumulation. The fundamental models developed here have a wide applicability to many machine elements including, bearings, gears, cam-followers and constant velocity joints.

Specifically, it involves the development new semi-analytical model for elastic-plastic contact along the lines of Chiu’s theory for stresses and displacements in a half space due to a cuboidal volume with uniform residual strains. This will enable the team to study damage accumulation and crack propagation in rolling-sliding contacts, using various stress, strain and energy based criteria. In conjunction with experimental observations and numerical simulations on crack growth, surrogate models of the bearing material’s degradation behaviour can be developed, to be used in stochastic simulations of bearing life.

Recent Projects

Computational Fluid Dynamics (CFD) modelling of Elasto-Hydrodynamic Lubrication (EHL)

  • Leaders:Daniele Dini, Tamer Zaki
  • Staffing: PhD Student
  • Staff: Ali Hajishafiee

Task:

To investigate the limitations of available theoretical methods for modelling fluid flow (both numerical and analytical) under Elasto-Hydrodynamic Lubrication conditions with application to bearings.  The research will include application of advanced methods for implementing fluid-solid interaction (FSI) and complex rheological models including compressibility, thermal effects and cavitation..

Date:

 2009- 2012

 

Effect of lubricant composition on crack development

 

  • Leaders: A Olver, H Spikes, A Kadiric
  • Staffing: PhD Student
  • Staff: Pawel Rycerz

Task:

The program of research covers investigation of the influence of oil base stock and additives, roughness and material hardness on initiation and propagation of cracks under rolling and rolling sliding conditions.  Appropriate methods of crack monitoring will be employed.

Date:

2010 - 2013

Lubricated cracks in Rolling Contacts

  • Leaders: Daniele Dini, Andy Olver
  • Staffing : PhD Student + Post Doctoral Researcher
  • Staff: Robbie Balcombe, Mark Fowell

Task:

FluidCrackTo investigate role and importance that entrapment and pressurisation of lubricant within surface breaking cracks may play in the propagation of such cracks under rolling and sliding conditions.  The work includes investigation of the influences of lubricant physical properties on crack development.  The experimental work includes lubricant and crack face displacement tracking .  Numerical investigations are based on a custom boundary element code developed within the project for cracked bodies.

Date:

 2007 - 2011

 

Monitoring Hydrogen Contamination in Lubricated Contacts

Researcher: Dr Arnaud Ruellan du Crehu

Supervisor: Dr Amir Kadiric

Sponsor: SKF, Marie Curie

Summary

Presence of hydrogen in bearing steels is known to promote the onset of unpredictable rolling contact fatigue related failures. In order to better predict the risk of these failures and design countermeasures, there is a need to better identify hydrogen sources in lubricated contacts.

Bearing steel itself inevitably contains a certain amount of hydrogen, but evidence suggests hydrogen can also be generated through the tribological decomposition of lubricant compounds, subsequently promoting atomic hydrogen permeation. Hydrogen contamination remains very difficult to monitor due to very low concentrations, its versatility and diffusivity.
This project aims to apply different hydrogen monitoring techniques on tribometers simulating rolling/sliding lubricated contacts of bearing steels. Focus of the study is on comparative measurements to assess the influence of various parameters such as steel and lubricant composition, water contamination, temperature and frictional energy on the evolution of hydrogen and to try to correlate hydrogen gas evolution and hydrogen absorption into the bearing steel.

 

Numerical modelling of multi-layered rough contacts

  • Leader: Ritchie Sayles
  • Staffing: PhD Student
  • Staff: Jessika Nyqvist

Task:

To develop advanced modelling capabilities for layered materials (containing more than one layer) in dry non-conformal contacts.  The research includes optimisation of layer properties in relation to contact pressures and deformations and sub-surface stress field. The three-dimensional model accounts for real rough surfaces and an arbitrary contact geometry..

Date:

2007- 2011

 

Short Term Projects


  • Film thickness and friction measurements with different surface topography
  • Rheology of selected oils
  • Fluorescence observation of lubricant behaviour on molecular level
  • Contact mechanics of rough surfaces
  • Thermo-mechanical modelling
  • Lubrication of bearing surfaces
  • Contact mechanics modelling of dry layered surfaces