Journal bearings are found in a staggering variety of systems, from motorcycles to cruise ships. As such, understanding their performance could have a substantial positive impact in many industries. With increased understanding comes the ability to optimise systems, leading to improved efficiencies and reduced emissions.
One key parameter in hydrodynamic journal bearings is the thickness of the lubricant layer at the shaft-bearing interface. Due to the journal bearing's enclosed geometry and the minute thickness of the oil film, typically in the order of 1's to 10's microns, conventional techniques often struggle to provide effective measurements. A promising alternative is the ultrasonic method. Ultrasound has previously been shown to accurately measure thin oil films and furthering this research will allow a greater understanding of the technology's potential.
The aim of this project is to develop new ultrasonic techniques for journal bearing lubricant film thickness measurement under a wide range of operating conditions, including variable load, speed, temperature, alignment and oil type.
To achieve this, two bespoke journal bearing platforms have been designed. The first is a static loading test platform instrumented with ultrasonic transducers embedded within the shaft. Testing under normal operating conditions demonstrated that the combined application of multiple ultrasonic techniques enables accurate circumferential film measurements when compared against a numerical solution. The test platform and ultrasonic method were demonstrated to be highly repeatable, enabling a robust comparison between different lubricant types.
How oil films behave under more severe conditions has also been investigated. Testing under shaft misaligned and offset-loading conditions demonstrated the ability of ultrasound to measure misalignment angle. Results compared against numerical simulations demonstrated the dramatic effect tiny changes in edge film thickness can have on peak pressure. Run-down testing, in which rotation speed was steadily reduced, allowed the system to clearly transition through lubrication regimes. The transition point between the hydrodynamic and mixed regime was found to be viscosity-dependent. Starvation testing showed that by observing changes in signal response at the thick film side of the bearing, the onset of low lubricant supply could be identified earlier than via conventional methods.
The second system is a dynamic loading test platform, with both bearing and shaft mounted ultrasonic transducers. This platform can apply a wide variety of complex loading patterns via a hydraulic power-pack controlled by an electro-proportional programmable valve. The effects of rotation speed and load pattern on oil film rupture and recovery have been studied, with results in good agreement with numerical predictions.
