Nachrichten 56 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 Mitteilungen der ÖTG 1 Abstract Oil film thickness represents the main tribological parameter in a lubricated tribocontact, however in-situ measurement of film thickness is still not sufficiently solved. In this work we present major advancements for the application of ultrasonic reflectance sensing technology for the condition monitoring of tribo-contacts in modeltribometers and on real components. Ultrasonic waves can travel non-invasively through solids, thus allowing a direct investigation of tribocontacts. A new class of piezoelectric devices is developed to apply this ultrasonic technology to standard tribometers and rolling element bearings. Direct measurements of film thickness are provided together with a correlation to the Hamrock- Dowson analytical solution. Keywords: ultrasonic sensors, film thickness, online measurement, bearing, condition monitoring, tribology. 2 Introduction Sensing devices play a crucial role in the monitoring of tribological contacts. In lubricated contact film thickness and viscosity are of major importance as they correlate to the frictional losses and to the efficiency of the tribosystems. Standard online sensing devices include thermocouples, accelerometers, capacitance-, impedance-, and acoustic sensors. However, none of these sensors provide a direct insight into the lubrication condition in the tribocontact. Rather, a change in the operation of the system is detected when already a (small) damage is measurable. Therefore, costs of maintenance of such components usually are very high, as they necessitate almost immediate action once failure is detected. This is especially critical for assets that are not easily accessible (e.g., offshore wind turbines gear boxes). Ultrasonic waves reflectance-based measurement techniques at high ultrasonic frequencies above 1 MHz hold a great potential for in-line and non-invasive tribological monitoring. Differently from standard acoustic emission sensors, reflectance-based instruments generate an ultrasonic wave at high frequency and detect the attenuation of this wave at the tribological contact [1]. Ultrasonic waves are elastic waves that can travel through metal, liquids, and gases and which are reflected at boundaries. Such reflections correlate to the lubricant’s physical properties in a tribocontact. This method of contact evaluation is well known since the early days of tribology. For example, ultrasonic monitoring techniques based on the reflection of shear waves were developed by Barlow and Lamb in the 1960s [2] to measure viscosity of oils in operating conditions that are typical of bearings. The reflection of longitudinal waves was used by Tattersall [3] to measure the real contact area between two components in dry contact. This concept has been then applied for detecting lubricated contact thickness by Dwyer- Joyce [1]. Taking these advantages into account, why high frequency ultrasonic reflectance sensors are not widely utilized in industry for tribological condition monitoring? Condition monitoring experts could argue the following two aspects of active tribo-acoustics sensors: i) Low reliability and long-term measurement response drift and ii) difficult in the integration with existing industrial operating protocols. This research deals with the solution to the problem i). A method for the selection of piezoelectric materials, the heart of the tribo-acoustic sensor, and the manufacturing of the sensor is outlined. Furthermore, the application of the developed sensor for long-term in-line monitoring of a tapered rolling element bearing is presented. 3 Method and Materials The main problem associated with the practical application of the ultrasonic reflectance method is that the amplitude measured experimentally is subject to an error induced by the thermal response of the piezoelectric material. This issue can be tackled by using lead metaniobate (LM) piezoelectric transducers. The response of LM is linear, and no hysteresis is present throughout tribological testing. Therefore, the use of this material will allow for a long-term stable measurement [4]. To prove this, LM transducers are applied to a rolling element bearing test rig shown in Figure 1 and operated over a large time span. The LM piezoelectric material is bonded on the external cage of a 32008 X SKF bearing used in an axially loaded DIN 51350-6 standard rig “looking” towards the movement of the rolling elements in the centre. To test the sta- Online Tribomonitoring with Advanced Ultrasonic Sensors Michele Schirru, Fabio Tatzgern, Markus Varga* * AC2T research GmbH, Viktor-Kaplan-Straße 2/ C, 2700 Wiener Neustadt, Austria TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 56 Nachrichten 57 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 bility of the ultrasonic sensor, tests are executed in a range of temperatures, loads and rotational speeds on a lithium-based grease with NLGI consistency of 2 in a long duration test of 21 days. Tests are conducted at steps in which speeds and loads are varied. The speed was in the range of 1000 to 2000 rpm, while the load varied from 5 to 15 kN. 4 Results Figure 2a shows the amplitude acquired over the passing of 9 rollers in front of the ultrasonic sensor. The amplitude reduces when the rolling element passes and increases when the sensor measures the bulk grease in between the rollers’ passes. Figure 2b shows the film thickness calculated over time and at different load and speed levels from these measured data. The calculation is based on the reflection coefficient technique [1]. The Hamrock-Dowson (H&D) equation is used for comparison with the experimental data as it is used for the calibration of other instruments for film thickness measurements in rolling element contacts that are based on capacitance and resistance measurements [5]. The average film thickness results measured using the ultrasonic sensor at the different load-rotational speed steps are in good agreement with the H&D analytical film thickness solution. A reduction of the film thickness with both, load and speed is noticed. The decrement of the thickness at higher rotational speeds is thought to be entailed by the increment of temperature (Figure 2b) throughout the experiment. This temperature increase goes conform with a reduction in viscosity and a consequent thinner film thickness. Increased loads naturally lead to higher pressures and thinner films. 5 Conclusion The choice of the correct piezoelectric material allows for long-term precise and repeatable ultrasonic reflectance measurement of film thickness e.g., in bearings. In this work we investigated a rolling element bearing with grease lubrication over hundreds consecutive hours of operation. The test was monitored by a lead metaniobate piezoelectric element targeted from the outside onto the rolling elements passing in the bearing. The calculation of the film thickness is based on the measurement of the ultrasonic reflection coefficient. It was proven, that a long-term stable measurement with the ultrasonic sensor is possible. Further, the measured film thickness was in good agreement with the analytical Hamrock-Dowson equation. The results of this study clearly show that ul- Figure 1: Application of ultrasonic film thickness sensor to bearing test rig Figure 2: a) In-line and real time acquisition of ultrasonic amplitude from rolling element contacts, (b) calculated oil film thickness in the tribocontact for different loads and speeds. TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 57 Nachrichten 58 Tribologie + Schmierungstechnik · 68. Jahrgang · 5/ 2021 trasonic reflectance sensors have the potential to offer online condition monitoring of lubricant properties for a long-term industrial use. 6 Acknowledgement The work presented was funded by the Austrian COMET program (Project InTribology, no. 872176) and carried out at the “Excellence Centre of Tribology” (AC2T research GmbH). 7 References [1] Drinkwater, B. W., Dwyer-Joyce R.S., and Cawley P. “A study of the interaction between ultrasound and a partially contacting solid - solid inter-face.” Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences 452.1955 (1996): 2613- 2628. [2] Barlow, A. J., and Lamb J. “The visco-elastic behaviour of lubricating oils under cyclic shearing stress.” Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 253.1272 (1959): 52-69. [3] Tattersall, H. G. “The ultrasonic pulse-echo technique as applied to adhesion testing.” Journal of Physics D: Applied Physics 6.7 (1973): 819. [4] Schirru, M., and Adler M. “An Ultrasonic Rheometer to Measure Gas Absorption in Ionic Liquids: Design, Calibration and Testing.” Sensors 20.12 (2020): 3544. [5] Hamrock, B.J., and Dowson D. “Isothermal elastohydrodynamic lubrication of point contacts: Part 1—Theoretical formulation.” J. of Lubrication Tech. (1976): 223-228. TuS_5_2021.qxp_TuS_5_2021 10.12.21 11: 05 Seite 58