The application of ultrasonic NDT techniques in tribology

The use of ultrasonic reflection is emerging as a technique for studying tribological contacts. Ultrasonic waves can be transmitted non-destructively through machine components and their behaviour at an interface describes the characteristics of that contact. This paper is a review of the current state of understanding of the mechanisms of ultrasonic reflection at interfaces, and how this has been used to investigate the processes of dry rough surface contact and lubricated contact. The review extends to cover how ultrasound has been used to study the tribological function of certain engineering machine elements

Monitoring of lubricant film failure in a ball bearing using ultrasound

A lubricant-film monitoring system for a conventional deep groove ball bearing (type 6016, shaft diameter 80 mm, ball diameter 12.7 mm) is described. A high-firequency (50 MHz) ultrasonic transducer is mounted on the static outer raceway of the bearing. The transducer is focused on the ball-raceway interface and used to measure the reflection coefficient of the lubricant in the "contact" ellipse between bearing components. The reflection coefficient characterizes the lubricant film and can be used to calculate its thickness. An accurate triggering system enables multiple reflection measurements to be made as each lubricated contact moves past the measurement location. Experiments are described in which bearings were deliberately caused to fail by the addition of acetone, water and sand to the lubricant. The ultrasonic reflection coefficient was monitored as a function of time as the failure occurred. Also monitored were the more standard parameters, temperature and vibration. The results indicate that the ultrasonic measurements are able to detect the failures before seizure. It is also observed that, when us,ed in parallel, these monitoring techniques offer the potential to diagnose the failure mechanism and hence improve predictions of remaining life

Oil film measurement in polytetrafluoroethylene-faced thrust pad bearings for hydrogenerator applications

There is a growing trend in the replacement of the babbit facing in thrust pad bearings with a composite polytetrafluoroethylene (PTFE) surface layer. The PTFE-faced bearings have been shown to allow a greater specific pressure, reduce thermal crowning, and, in some cases, negate the need for an oil-lift (jacking) system. These designs of bearing require new methods for the measurement of oil film thickness both to assist in their development and for plant condition monitoring. In this work, an ultrasonic method of oil film measurement is evaluated for this purpose. An ultrasonic transducer is mounted on the back face of the thrust pad. Pulses are generated and transmitted through the pad material, bonding interlayer, and PTFE surface layer. The proportion of the wave that reflects back from the oil film layer is determined. This is then related to the oil film thickness using a series of calibration experiments and a spring stiffness model. In practice, the reflected signal is difficult to distinguish, in the time domain, from other internal reflections from the pad. Signals are compared with reflections when no oil film is present and processing is carried out in the frequency domain. Experiments have been performed on a full size PTFE-faced thrust pad destined for a hydroelectric power station turbine. The instrumented pad was installed in a test facility and subjected to a range of loading conditions both with and without oil lift. Although there were some problems with the robustness of the experimental procedure, oil films were successfully measured and used to study the effect of the oil-lift system on film formation. © IMechE 2006

Non-Destructive Techniques for the Condition and Structural Health Monitoring of Wind Turbines: A Literature Review of the Last 20 Years

A complete surveillance strategy for wind turbines requires both the condition monitoring (CM) of their mechanical components and the structural health monitoring (SHM) of their load-bearing structural elements (foundations, tower, and blades). Therefore, it spans both the civil and mechanical engineering fields. Several traditional and advanced non-destructive techniques (NDTs) have been proposed for both areas of application throughout the last years. These include visual inspection (VI), acoustic emissions (AEs), ultrasonic testing (UT), infrared thermography (IRT), radiographic testing (RT), electromagnetic testing (ET), oil monitoring, and many other methods. These NDTs can be performed by human personnel, robots, or unmanned aerial vehicles (UAVs); they can also be applied both for isolated wind turbines or systematically for whole onshore or offshore wind farms. These non-destructive approaches have been extensively reviewed here; more than 300 scientific articles, technical reports, and other documents are included in this review, encompassing all the main aspects of these survey strategies. Particular attention was dedicated to the latest developments in the last two decades (2000–2021). Highly influential research works, which received major attention from the scientific community, are highlighted and commented upon. Furthermore, for each strategy, a selection of relevant applications is reported by way of example, including newer and less developed strategies as well

The Real-Time Characterisation of Dry Machine Element Contacts Using Ultrasonic Reflectometry

For two components to be in contact, they must be physically touching. If two solids are touching, the contact is by definition inaccessible. How do engineers develop an understanding about a contact if the interface is inaccessible? Load, geometry and material properties govern the contact pressures of touching components. As they move against one another, the result is often wear and this inherently changes the contact behaviour. By understanding how interfaces interact in terms of contact pressure, contact area and wear, components can be optimised to reduce cost and maximise efficiency. Ultrasonic reflectometry has been widely used in medical imaging and non-destructive testing. It is a non-invasive technique that has the ability to probe deep into solid structures and extract information regarding the material and the interface. Two methods have been developed to measure wear in real-time based on ultrasonic time-of-flight and the resonant frequency model. The ultrasonic technique has also been applied to learn more about the contacting parts of machine elements. By analogising the interface asperities as springs, ultrasonic reflectometry can be used to calculate the interfacial stiffness and from this, contact pressure of a tribosystem. Previous work has been limited to laboratory based static measurements. This work builds on this technology to characterise dynamic machine elements with the hopes of developing industrial condition monitoring tools. This thesis is aimed to be a guide for those who wish to use ultrasound as a tool to measure and monitor a dry dynamic tribosystem. More specifically, this work highlights a series of recommendations and pitfalls for ultrasonic measurements of contact pressure and wear in industrial applications

An ultrasonic method for measuring fluid penetration rate into threaded contacts

Various methods have been employed to study the efficacy of multipurpose penetrating oils but these techniques do not investigate the rate which these oils penetrate surfaces. This paper outlines a novel, non-invasive ultrasonic method that provides a direct means of mapping fluid penetration in threaded systems. An apparatus with piezoelectric elements was developed to pulse ultrasonic waves into a nut specimen where the waves reflected from the threaded interface. The reflected signal amplitude shifted as fluid penetrated the thread, allowing the fluid to be mapped to provide a measure of ingress rate. The results for three fluid samples are presented. Measurements suggest the fluid tracks helically down the unloaded side of the thread and radially into the loaded thread surfaces

The use of ultrasound for detecting particles suspended in lubricant and hydraulic fluids

Imperial Users onl

Rayleigh Wave Interactions with Tribological Contacts

The use of ultrasonic reflectometry is a proven method of condition monitoring machine systems. The working principles are simple. A burst of ultrasonic signal is sent to the interface of interest where reflection of the signal takes place. The manner in which reflection takes place depends on the properties of the interface. As such, the reflected signal carries information regarding the interface that can be extracted with proper techniques. However, the use of ultrasonic reflectometry in condition monitoring is not without its limitations. Conventional ultrasound techniques make use of ultrasonic bulk waves that travel through the body of a given material. Problems arise when the medium through which the wave travels is attenuative. This prevents any passage of ultrasonic signals as most of the energy will be absorbed by the material. In addition, most components have complex designs, requiring that the signal pass through multiple interfaces before reaching the interface of interest. Reflections occur at these intermediate interfaces, reducing the overall energy content of the signal. In order to overcome these issues, the use of Rayleigh wave as an alternative is researched in the work carried out here. Instead of having to travel through the bulk of the material, Rayleigh waves function by propagating along the free surface of the said material, thereby circumventing the existing issues with the use of conventional bulk waves. The research here was carried out to seek an understanding of how Rayleigh waves interact with a contact interface. This was performed on three separate fronts. First, a novel analytical model was developed by modelling the contact interface as a series of springs. It is discovered that the stiffness of the springs are directly proportional to the reflection coefficient of the Rayleigh wave incident upon the interface. The implication of this finding is that rough interfaces will have a lesser reflection coefficient (due to decreased stiffness), with a perfectly smooth interface giving the maximum reflection coefficient obtainable from a particular interface. This was then followed by studies performed using both finite element simulations as well as experimental work. Data from all three studies (analytical model, finite element simulations and experimental work) were compared against each other and it was shown that a good agreement exists between all three methods. Exploratory work on lip seals were performed in order to research the potential of using Rayleigh wave as a condition monitoring tool. By measuring the delay in the time of arrival of a Rayleigh wave pulse reflected from the sealing zone, it is possible to measure the extent of misalignment that is present in a lip seal. Axial misalignments of the lip between 6mm to 8mm were successfully measured. Additional work in measuring the degradation of a lubricating film via evaporation was qualitative in nature, with the amplitude of the reflected pulse slowly decreasing as the layer of fluid at the sealing zone diminishes via evaporation

Development of a Novel Bearing Concept for Improved Wind Turbine Gearbox Reliability

The wind industry has experienced rapid growth in recent years as a result of ever increasing concerns over fossil fuel power generation and the associated impact this has upon climate change. Wind turbine technology has however displayed relatively poor reliability from the outset, largely due to the highly variable nature of the wind, which subsequently places system components under extremely harsh loading conditions. Such reliability issues are becoming increasingly problematic for the wind turbine operator, as wind farms – and indeed the turbines themselves – continue to be up-scaled thus increasing the complexity and cost of maintenance, particularly in off-shore environments. Gearbox failures have been found to be responsible for a large proportion of wind turbine downtime, and this is very often linked to the failure of bearings at certain locations within the gearbox. One such critical location is the epicyclic stage, where planetary support bearings are often found to exhibit damage to a localised portion of their inner raceways, corresponding to the location of the applied load. A concept has been proposed to extend the life of such bearings by periodically rotating the normally static inner raceway so as to avoid the build of damage to one localised region. The concept has been termed the MultiLife(TM) mechanism and a key aim of this thesis was to establish proof-of-concept for this system. Initial analytical work was performed, through which it was identified that a five-fold enhancement to bearing life would be theoretically achievable. The concept was subsequently validated experimentally through testing on a bespoke test platform. In addition to this, the ultrasound technique has been explored as a means to provide early warning of bearing failure and thus support the operation of the MultiLife system. This technique has previously been proven as a method to measure the thickness of the oil films that form within bearing contacts. It was identified for this study that oil films would be too small to measure due to the low viscosity lubricants and high bearing loads utilised to accelerate bearing failures. Nonetheless, a general reduction in the amount of ultrasonic energy reflected from the rolling contact was observed during bearing failure, which was linked to a breakdown of the lubricating oil layer due to degradation of the rolling surface. In this case the ultrasound technique did not provide any advanced warning of bearing failure over more classical condition monitoring techniques; however, it was identified that the use of higher resolution ultrasound sensor arrays would enhance the capabilities considerably. Currently, very few monitoring techniques are applied to wind turbine gearbox bearings, and as a further part of this work a condition monitoring system has been installed within an operational 600kW wind turbine. A high speed shaft bearing has been instrumented with a variety of sensors, including ultrasound instrumentation, to assess the potential of such a system in providing early warning of impending gearbox failures

Direct Load Monitoring in Rolling Element Bearing by Using Ultrasonic Time of Flight

Rolling element bearings find widespread use in numerous machines and they are one of key components in involved systems. Bearing failures can cause catastrophic events if they are not detected in time and result in increasing downtime and maintenance cost. The need for longer endurance life with less cost drives research on bearing condition monitoring. Abstract Load monitoring provides significant information for bearing design and residual service life prediction as load applied by each rolling element on a bearing raceway controls friction and wear. It is possible to infer bearing load from load cells or strain gauges on the shaft or bearing housing. However this is not always simply and uniquely related to the real load transmitted by rolling elements directly to the raceway. Firstly, the load sharing between rolling elements in the raceway is statically indeterminate. And secondly, in a machine with non-steady loading the load path is complex and highly transient being subject to dynamic behavior of the transmission. This project develops a non-invasive, safe and portable technique to measure the load that transmitted directly by a rolling element to the raceway by using ultrasound. Abstract The technique works by monitoring the time-of-flight (ToF) of ultrasound that travels in a raceway and reflects back from the contact face. A piezoelectric sensor was permanently bonded onto the external surface of the stationary raceway in a rolling element bearing. The ToF of an ultrasonic pulse from the sensor to the raceway-rolling element contact was measured which depends on the wave speed and the thickness of the raceway. Abstract The speed of an ultrasonic wave in a component changes with the state of the stress; known as the acoustoelastic effect. The thickness of the element varies when deflection occurs as the contacting surfaces are subjected to load. Therefore, the ultrasonic ToF in a raceway is load dependent. In practical measurements, it was found that the phase of the wave reflected from rolling contacts varied with contact conditions. The phase was determined by the contact stiffness and in simple peak to peak measurement, this appeared as a change in the ToF. For typical rolling contacts, the ToF changes caused by deflection and acoustoelastic effect are of the order of nanoseconds, while the apparent time shift from the phase change effect is in the same order. Abstract Despite the phase change having effect on reflected signals, it does not affect the envelope of these signals. In this work the Hilbert transform was used to calculate the envelope of the reflected pulses and thus this contact dependent phase shift was eliminated. Time difference between the envelope of reflected pulses in unloaded and loaded state was a result of load effect alone. Abstract Ultrasonic measurements have been carried out on a model line contact formed between a steel plate and a cylindrical bearing steel roller, and line contacts in a cylindrical roller bearing which was used for the planet gear of a wind turbine epicyclic gearbox, as well as on elliptical contacts in a radially loaded ball bearing (deep groove). The ToF changes under different contact loads were recorded and used to determine the deflection of the raceway. This was then related to load using a simple elastic contact model. Measured load from the ultrasonic reflection was compared with the applied load upon the contact and good agreement has been achieved. The ultrasonic ToF technique shows promise as an effective method for load monitoring in real bearing application