Understanding the mechanism of acoustic emission generation due to surface asperity interaction in mixed lubrication conditions

When they mesh together, gear teeth are placed under very high loads and separated by thin films of lubricant often only a few microns thick. High temperatures and pressures develop within the oil film between the gear teeth. Real gears, particularly in aerospace transmissions, may have finely machined surfaces and appear smooth but on the microscopic scale of the lubricant film they are rough. The peaks of these rough surfaces (known as asperities) often come into contact with each other. These conditions can lead to failure of the lubricant film itself, wear, surface damage and gradual cracking (fatigue), none of which are desirable in an aircraft engine for example!
When a material undergoes deformation (such as occurs when rough surfaces come into loaded contact) a rapid release of energy in the form of transient elastic waves occurs, known as Acoustic Emission (AE). These waves can be recorded using sensors mounted on the material, and AE methods have been widely used in monitoring structures where they have been shown to out-perform other methods. In gear systems, asperity contact, wear, surface damage or cracking causes acoustic emissions to occur.
This project focusses on the development of an advanced computational model to predict the levels of AE generation due to asperity interaction within a lubricated contact, such as those found between gear teeth. This asperity interaction has been found by previous researchers to be a significant part of the AE signal from a pair of meshing gears. The work will form a key part of a long-term aim of developing the use of AE monitoring techniques for high speed, heavily loaded power transmission gearing.
The work will include both the development of the computational models, which are based on strain energy calculations and advanced simulations of pressures and shear stresses within the lubricant film, together with a comprehensive experimental programme.

The proposed programme of research forms part of the PI's overall research roadmap leading to the development of acoustic emission (AE) based techniques for health monitoring of power transmission gearing. This will ultimately benefit users of heavily loaded, high-speed power transmission gearing in a range of industries. Chiefly, the current research will investigate the generation of acoustic emission by asperity contact in lubricated contacts, typical of those in power transmission gearing, which is a key component of the proposed gear health monitoring system.

Users of gearing in safety-critical environments, such as the civil and military aviation industries, will derive benefit from this in a number of ways. Firstly, the eventual system will directly improve safety levels as it will provide continuous monitoring of gear health and earlier identification of problems when compared to the sole use of current techniques. It will also allow reductions in ownership and operation costs, by allowing increased maintenance and inspection intervals, together with increased aircraft availability and reduced un-planned downtime. Within the aerospace industry, some of the most arduous operating conditions for gear systems can be found in helicopters, and it is anticipated that the long-term outcomes of this research will enable the UK to take a world-lead in helicopter logistics and operations.

Other potential users of the AE techniques include the power generation (both conventional and renewable energy), marine and automotive industries. For example, a closer understanding of asperity contact levels, and the ability to monitor these levels, will be of great benefit to those seeking to reduce surface-failure issues within bearings and gears in wind turbine gearboxes. Furthermore, monitoring techniques are becoming essential as renewable energy generation capacity is installed in inaccessible, or inhospitable conditions both on and off-shore.

The research will also benefit the Acoustic Emission industry itself, by providing a key part of the scientific knowledge necessary to develop a commercially-viable monitoring system for power transmission gearing. This can introduce AE technology to new markets in which such methods were not previously available. The industry will benefit via increased software and hardware sales and consultancy activities, leading to higher employment levels and increased revenue generation.

Finally, the UK scientific and engineering base will benefit via the development of highly skilled researchers and PhD students, which this grant will facilitate.