Sounding out bearing lubrication

Mark Fletcher delves into a UK-based research project which will attempt to demystify the measurement and analysis of bearing lubrication films

Anyone who has had an engine seize in their car or motorbike is quite abruptly, and often rather dangerously, made aware of the virtues of a decent, functioning lubrication system. Unfortunately it is only when the worst happens that the lubrication problem becomes apparent in the first place. And by this stage, it is often too late for any remedial action. Seizure of manufacturing systems and machinery is just a damaging as downtime can play havoc with schedules, costs, asset planning and, most importantly, your customers.
Without some kind of lubrication between metal surfaces industry world wide could literally grind to a halt due to its reliance on bearings. Indeed, it is for this very reason that companies are eager to extol the virtues of lubrication schedules, lubrication monitoring and lubrication materials. Many bearing companies also have advanced mathematical formula for predicting life and MTBF figures but these can tend to be heavily reliant on schedules being adhered to and the correct operating conditions being met.
Bringing a new level of proactivity in to the equation, researchers at the Universities of Sheffield and Bristol are currently engaged in development work which should make the all important measurement of bearing lubricant film thickness far more effective and much better for basing lubricant decisions on. Their research centres around the use of transducers 'pinging' a bearing and then hardware and software analysing the reflected results much like sonar. As the research evolves in to a commercial measurement system it will have a glut of applications across a great many sectors – not just manufacturing and machinery.
According to Dr. Bruce Drinkwater of Bristol University and Dr. Rob Dwyer-Joyce of Sheffield University, applications for their LubeSense hardware and the principles behind it will include instrumentation for machine development, condition monitoring and lubrication integrity checking. With further cost reductions, as the technology matures, the company also envisages the system being used as an automotive engine sensor, process and plant monitoring and as an aid to increase bearing functionality as bearings could be supplied with integral sensors.
The LubeSense hardware will provide the means to measure the lubricant film thickness in a non-invasive manner, it will produce localised high-resolution readings, provide online rapid measurements and will determine films as low as 50nm, with no upper limit. It does this by recording the response of a lubrication film to an ultrasonic pulse generated by a transducer mounted on the external shell of the bearing. The mounting requires no special procedures just a reasonably clean surface and a good coupling.
The sensor emits a specially pre-conditioned ultrasonic pulse that passes through the bearing shell and reflects back from the lubricated interface. This reflected signal is then recorded and processed to obtain the oil lubricant film thickness to within an accuracy of ±2%. Signals can be monitored continuously or triggered to capture specific lubrication events and as the method exhibits high spatial resolution it can be used to map film thickness in a lubricated component.
The researchers have created three devices in the LubeSense range. LubeSense A is designed to work with any thick fluid films greater than 100µm; LubeSense B is suitable for the measurement of typical hydrodynamic lubrication films in journal bearings and pad-type bearings between the ranges of 10 to 100µm; and LubeSense C which has been developed to measure thin elastohydrodynamic lubricant films in concentrated contacts between 0.05 and 20µm. Instrumentation and sensors are supplied as a standard package with software and signal processing tailored to each lubrication case. The device can act as a standalone unit, displaying the relevant film thickness data in real time or the data can be returned for analysis.
Tribology at Sheffield University
Bristol University

Head: So, how does it work?

Although simple in practice, the processing and data extrapolation behind the LubeSense concept is fraught with complications and is prone to the foibles of the materials, the lubricant and the lubricant film thickness . Each of the different units has to employ varying methods of measurement depending on the physical conditions encountered.
When ultrasound is incident on a boundary between two different media some of the energy is reflected and some is transmitted. The reflection and transmission behaviour of the waves at the boundary is dependent on the acoustical properties of the two media. If the lubricant later is sufficiently thick or the ultrasound 'packet ' sufficiently small then the reflections from the top and bottom bearing surface exhibit a discrete time signature (time of flight or TOF) which can be measured if the speed of sound in the lubricant is known. One of the problems is that the oil, when under pressure, becomes a glassy solid with a higher bulk modulus, this changes the conditions for measurement, especially those which rely on the speed of sound of the output and feedback signals.
With very thin layers the reflected pulses can overlap making it impossible to discern discrete reflections, typically, even the thickest lubricant films are less than 50µm so the TOF approach is not applicable. The researchers had to adopt a number of other approaches for these thinner films which measured the resonance of the lubrication film to produce results which could then be interpreted by a PC and turned into useable figures.
One of these approaches is to measure the frequency at which the film resonates. The incident ultrasonic pulse causes the film to vibrate; then resonant frequencies within the wave packet are transmitted. The thickness of the oil film can be deduced from these frequencies. If the film is very thin (sub-micron), like in a ball bearing for example, then the frequency at which it resonates is above that which can be propagated through the bearing shell. In this case, the approach is to determine the stiffness of the layer from the proportion of the sound wave reflected. This stiffness can then be used to deduce the oil film thickness.

Tom Shelley

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