Research
From March 2005 to September 2008 I took on a PhD at Brunel University in Tribology. My main research area was developing a method for simulating the response of complex multilayer coatings to contact problems commonly experienced by gearing systems. This had a particular focus on component life cycle prediction through the application of shakedown principles and low cycle fatigue theory. Below is a brief summary of my work and findings and links to a few papers that have been published.
Contact Mechanics & Automotive Transmissions
Contact mechanics, as its name suggests, is the study of the mechanics of surfaces in contact. It is simply the term given to the science of defining the response of materials that are transmitting loads through interfacial contact. This type of contact exists everywhere from the soles of your feet when you stand or walk to complex mechanical systems we rely on everyday.
During my PhD, my main area of focus was on automotive transmission components. As gears in a transmission mesh, the various teeth around the circumference engage and disengage with their mating gear, transmitting the torque from one shaft to the other. These gear teeth, like the one simulated above experience a mixture of rolling and sliding contact during this meshing process.
[Walton & Goodwin, 1998]
As a pair of teeth begin to engage, sliding contact dominates giving rise to high friction forces and stress concentrations near the surface of the contact. As the teeth continue to engage, they reach a point of pure rolling, this occurs at the pitch point of the gear pair. Past the pitch point, sliding contact occurs once more. This variation of mixed rolling and sliding contact is complex, varies in magnitude and causes movement of the stress field through an individual tooth flank. Over time, these gear flanks will begin to wear creating noise & vibration, material debris, increased contact friction and ultimately reduced efficiency. In severe cases it could eventually lead to failure and removal of the gear teeth.
The past decade has seen a significant drive to improve engine efficiency and power output whilst continuing to reduce production of harmful emissions. Liturature suggests around 15% of an engines brake power output is used to overcome mechanical friction forces and over 5% is lost in the transmission alone. In an attempt to reduce these losses, transmission components are being designed smaller, lighter and with improved surface finishes, however this trend is putting more and more strain on engine and transmission components.
Surface Engineering & Coatings
One solution that has emerged over the past couple of decades is that of surface engineering and coatings. By carefully engineering the surfaces of components in contact through surface modification and the addition of advanced surface coatings, components can be made smaller and lighter with improved surface strength and durability.
Although coating development is well established, the selection of coatings for a particular application is complex. Coating development is often a hit and miss process with a great deal of trial and error. This results in significant expense in terms of personnel and expertise as well as materials, time and testing equipment.
A multi-layer coating development tool
My research focused on the development of a computational model that allows the quantitative comparison and rapid modification of complex multi-layer coating systems. Using advanced Finite Element Analysis techniques, complemented with a number of physical tests, this novel tool simulates the response of multi-layer coatings to mixed rolling-sliding contact under a variety of loading conditions. Its versatility also allows the simple modification of the coatings mechanical properties, layer thicknesses, coating composition and overall model contact geometry.
My research focused on investigating a comparable wear effects of three advanced surface coatings; Titanium Nitride, Diamond-like Carbon and Tungsten-carbide carbon. Each of these coatings were simulated and tested and showed significant resistance to rolling-sliding induced wear when compared to an uncoated substrate.
Current Research
The recent growth in the use of wind turbines as a viable source of clean energy has been phenomenal. Wind turbine gearboxes are being produced in their thousands and are being installed all over the world. As the technology is still relatively immature a number of issues are occurring with gearbox components some of which result in failure. Wind turbines have a very high cost of repair through inaccessibility (particularly for offshore), high part cost and addition cost through lost revenue during power generation downtime. Through my research with automotive transmissions, I have been investigating the use of coating technology to improve the reliability of wind turbine gearbox components.
As one example, during high wind periods turbines are halted to avoid overload damage. The rolling element bearings can therefore be subjected to periods of zero movement. This can result in poor lubrication conditions for some of the elements. When movement is then reintroduced again, significant bearing damage can occur. Advanced surface coatings have the potential to offer base line lubrication protecting these bearing surfaces before the primarily lubricant is able to take over. This lubrication protection may significantly reduce bearing damage and improve overall bearing life which could greatly reduce costly bearing failures and maintenance bills.
My current research is focussing on investigating the use of coatings in bearings, quantifying the benefits and investigating the potential failure mechanisms.
Published Material:
- Farley, J. et al., ‘The Use of CAE Techniques and Analysis for Improved Reliability of Wind Turbine Gearboxes’, China Wind Energy conference, 2009.
- Farley, J et al.. ‘Innovative Technology for Wind Turbine Gearboxes’, China Wind Energy Conference 2009.
- Farley, J., Wrobel, L., ‘Performance evaluation of multilayer thin film coatings under mixed rolling-sliding contact conditions’, Wear, doi:10.1016/j.wear.2009.08.001 , 2009
- Farley, J. & Brown, J., ‘Investigation into roller bearing misalignment damage through advanced non-linear contact mechanics’, NAFEMS World Congress 2009. [Download paper]
- Farley, J., Mao K. & Wrobel L., ‘Development of a Computational Method of Low Cycle Fatigue Prediction for Multi-Layer Surfaces under Rolling/Sliding Contact Conditions’, SAE World Congress, 2007.
- Farley, J., Mao K. & Wrobel L., ‘Multi-layer Contact Mechanics and its application in the Automotive Industry’, 50th Annual Tribology Conference, 2006. [ Download paper]
- Farley, J., ‘Multi-layer Contact Mechanics and its application in the Automotive Industry’, Mission of Tribology, Vol 14, 2005. [ Download paper]
[ SAE Paper]