494 research outputs found

    Ultrasonic reflection from mixed liquid-solid contacts and the determination of interface stiffness

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    In thin film or boundary lubricated contacts there is a possibility of potentially damaging asperity contact occurring. Whilst there are many models of this contact mechanism, experimental verification of the proportion of solid contact is difficult to achieve. Electrical methods will only indicate that contact has occurred. Whereas, optical methods can be used to determine the proportion of contact, but only when one surface is transparent. In this work the use of ultrasonic reflection is investigated as a means to analyse these types of mixed solid-liquid contacts. A pulse of ultrasound is partially reflected at the contact between two rough surfaces. The proportion of the wave reflected can be readily used to determine the stiffness of the interface. Experimental data has been obtained from grit-blasted surfaces pressed together, both with and without liquid at the interface. The interface stiffness can be modelled by two springs in series, one of them representing the solid contact stiffness, Ksolid and the other the stiffness of the liquid fluid, Kliquid. The variation of these stiffness values with contact pressure has been investigated. At this stage it is not possible to directly determine the proportion of liquid or solid contact from the stiffness. The results however, give qualitative comparisons and information about the approach of the surfaces and hence the mean thickness of the liquid layer at the interface

    Evaluation of an ultrasonic method for measurement of oil film thickness in a hydraulic motor piston ring

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    The efficiency of a hydraulic motor depends on the lubrication performance of the piston ring. If the film is too thin then wear occurs quickly, if it is too thick then oil is lost into the cylinder and efficiency is reduced. In this paper a technique for oil film measurement based on ultrasonic reflection is investigated. This has the potential to be used non-invasively on real components. An ultrasonic pulse will reflect from a thin film interposed between two solids. The proportion of the pulse that is reflected depends on the stiffness of the intermediate layer. If the acoustic properties of the film material are known, then the stiffness can readily be used to determine the film thickness. This principle has been employed for the piston ring lubrication case. A piston/cylinder test bench has been used to evaluate the ultrasonic method. A focusing piezo-electric transducer is mounted outside the cylinder and ultrasonic pulses reflected back from the inner bore. The variation of these pulses as the piston ring passes underneath is investigated and used to determine oil film thickness. Films in the range 0.7 to 1.3 μm were measured; the thickness did not depend strongly on either ring speed or sealed pressure. Several practical aspects were investigated such as, attenuation in the cylinder material, response time, and transducer resolution. Whilst this study demonstrated that film thickness measurement is feasible, there are a number of practical considerations that require further work, principally the focusing and coupling of the ultrasonic transducer and the response time

    Viscosity measurement in thin lubricant films using shear ultrasonic reflection

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    When a shear ultrasonic wave is incident on a solid and liquid boundary, the proportion that is reflected depends on the liquid viscosity. This is the basis for some instruments for on-line measurement of bulk liquid viscosity. In machine elements, the lubricant is usually present in a thin layer between two rubbing solid surfaces. The thin film has a different response to an ultrasonic shear wave than liquid in bulk. In this work, this response is investigated with the aim of measuring viscosity in situ in a lubricating film. The proportion of the wave reflected at a thin layer depends on the layer stiffness. A shear wave is reflected by the shear stiffness of the thin layer. For a thin viscous liquid layer, the stiffness is a complex quantity dependent on the viscosity, wave frequency, and film thickness. This stiffness is incorporated into a quasi-static spring model of ultrasonic reflection. In this way, the viscosity can be determined from shear-wave reflection if the oil-film thickness is known. The approach has been experimentally evaluated on some static oil film between Perspex plates. Predictions of the spring model gave good measurement up to layer thicknesses of around 15 μm. For thicker layers, the shear stiffness reduces to such an extent that almost all the wave is reflected and the difference associated with the layer response is hard to distinguish from background noise

    Operating limits for acoustic measurement of rolling bearing oil film thickness

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    An ultrasonic pulse striking a thin layer of liquid trapped between solid bodies will be partially reflected. The proportion reflected is a function of the layer stiffness, which in turn depends on the film thickness and its bulk modulus. In this work, measurements of reflection have been used to determine the thickness of oil films in elastohydrodynamic lubricated (EHL) contacts. A very thin liquid layer behaves like a spring when struck by an ultrasonic pulse. A simple quasi-static spring model can be used to determine the proportion of the ultrasonic waves reflected. Experiments have been performed on a model EHL contact between a ball and a flat surface. A transducer is mounted above the contact such that the ultrasonic wave is focused onto the oil film. The reflected signals are captured and passed to a PC for processing. Fourier analysis gives the reflection spectrum that is then used to determine the stiffness of the liquid layer and hence its thickness. In further testing, an ultrasonic transducer has been mounted in the housing of a deep-groove ball bearing to measure the film generated at the outer raceway as each ball passes. Results from both the ball-flat and ball bearing measurements agree well with steady-state theoretical EHL predictions. The limits of the measuring technique, in terms of the measurable rolling bearing size and operating parameters, have been investigated

    Combating automative engine valve recession

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    [INTRODUCTION] Valve recession occurs when wear of the valve or seat inserts in an automotive engine has caused the valve to sink or recede into the seat insert (as shown in Figure 1). Excessive recession leads to valves not seating correctly and cylinder pressure loss. Leaking hot combustion gases can also cause valve guttering or torching, which will accelerate valve failure. Although new valve materials and production techniques are constantly being developed, these advances have been outpaced by demands for increased engine performance and wear related problems remain an issue. Dynamometer engine testing is often used to establish short-term solutions. This is time consuming and does not necessarily reveal the actual causes of wear. A long-term approach is required in order to understand fundamental wear mechanisms and the effect of varying engine operating conditions or design changes to the valve train. This information can then be used to develop tools for predicting wear and for solving problems more quickly if they do occur. In this case study, such tools were developed using a combination of component failure analysis, bench test work and wear modelling

    Wear mechanisms and transitions in railway wheel steels

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    The need to improve safety and reduce costs means that new specifications are being imposed on railway wheel wear. These mean that more durable wheel steels are required. In order to develop such materials, a greater understanding is needed of the wear mechanisms and transitions occurring in wheel steels. In this work, twin-disc wear testing has been carried out to study the wear characteristics of R8T railway wheel steel. The results have indicated that, compared with previous wheel steels, R8T offers greater wear resistance. Three wear regimes were identified; mild, severe, and catastrophic. Wear rates were seen to increase steadily initially and then to level off, before increasing rapidly as the severity of the contact conditions increased. This paper is concerned with the form of the data and the reasons for the transitions. Analysis of the contact conditions indicated that the first transition in the wear rate was caused by the change from partial slip to full slip conditions at the disc interface. Temperature calculations for the contact showed that the large increase in wear rates seen at the second wear transition may result from a thermally induced reduction in yield strength and other material properties. This improved understanding will help in progressing towards the aim of eventually attaining a wear modelling methodology reliant on material properties rather than wear constants derived from testing

    Wear effects and mechanisms of soot-contaminated automotive lubricants

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    A study has been carried out to investigate the influence of soot-contaminated automotive lubricants in the wear process of a simulated engine valve train contact. Previous research on this topic has been mainly performed from a chemical point of view in fundamental studies, with insufficient relevance to real engine conditions, i.e. load and geometry. This study investigates the conditions under which wear occurs through specimen testing. The objective of the work was to understand the wear mechanisms that occur within the contaminated contact zone, to help in future development of a predictive wear model to assist in the valve-train design process. The effects of soot in lubricants have been tested using a reciprocating test-rig specifically designed for this application, where a steel disc is held in a bath of oil and a steel ball (replicating a valve train contact) is attached to a reciprocating arm. The materials, contact geometry and loading conditions are all related to specific conditions experienced within an engine's valve train. The testing was carried out under various contact conditions, using carbon black as a soot simulant. Wear measurements were taken during the tests and wear scar morphology was studied. The results have revealed how varying lubrication conditions changes the wear rate of engine components and determines the wear mechanism that dominates for specific situations

    Measurement of interface pressure in interference fits

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    When components such as bearings or gears are pressed onto a shaft, the resulting interference induces a pressure at the interface. The size of this pressure is important as many components fail because fatigue initiates from press-fit stress concentrations. The aim of the present work was to develop ultrasound as a tool for non-destructive determination of press-fit contact pressures. An interference fit interface behaves like a spring. If the pressure is high, there are few air gaps, so it is very stiff and allows transmission of an ultrasonic wave. If the pressure is low, then interface stiffness is lower and most ultrasound is reflected. A spring model was used to determine maps of contact stiffness from interference-fit ultrasonic reflection data. A calibration procedure was then used to determine the pressure. The interface contact pressure has been determined for a number of different press- and shrink-fit cases. The results show a central region of approximately uniform pressure with edge stress at the contact sides. The magnitude of the pressure in the central region agrees well with the elastic Lamé analysis. In the more severe press-fit cases, the surfaces scuffed which led to anomalies in the reflected ultrasound. These anomalies were associated with regions of surface damage at the interface. The average contact pressure in a shrink-fit and press-fit joint were similar. However, in the shrink-fit joint more uneven contact pressure was observed with regions of poor conformity. This could be because the action of pressing on a sleeve plastically smooths out long wavelength roughness, leading to a more conforming surface

    Wear of a chute in a rice sorting machine

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    In a rice sorting machine, rice grains drop onto and slide down an anodised aluminium chute. The purpose of the chute is to separate the grains and provide a controlled distribution. At the bottom of the chute the grains are examined optically and contaminants or defective grains are removed from the stream by jets of air. The machine has the ability to sort low quality rice which contains a large element of contaminants such as husk. The husk is extremely abrasive and this, along with other factors, can lead to a reduction in the life of the chute by wear of the surface. In this work a failure analysis process was undertaken to establish the nature and causes of the chute surface wear and the mechanisms of material removal. Wear occurs initially at the location where the grains first strike the chute and at subsequent regions down the chute where bounce occurs. An experimental and analytical examination of the rice motion on impacting the chute was also carried out along with some friction testing of potential replacement chute materials. The evidence gathered during the failure analysis along with the experimental analysis was used to propose possible material/design improvements

    Wheel/Rail Contact Isolation Due to Track Contamination

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    An experimental study has been carried out to investigate the effect of sanding on the electrical isolation of a wheel/rail contact. Sand is applied to the wheel/rail interface to increase adhesion in both braking and traction. Train detection, for signalling purposes, can be by means of track circuits. Signalling block occupancy is triggered by the wheelset of the train ‘shorting out’ the track circuit. Sand in the wheel/rail interface means that contact between the wheelsets and the track may be compromised, inhibiting train identification. Static tests were performed using sections cut from wheels and rail and dynamic tests on a twin disc machine where rail and wheel steel discs are loaded together and driven under controlled conditions of rolling and slip. The electrical circuit used was a simplified simulation of the TI21 track circuit. The application of sand was carried out under a range of mild and severe test conditions. The results indicated that a transition exists in the amount of sand applied, below which there is a measurable, but not severe, change in voltage, but above which the contact conductance decreases by an order of magnitude. A model of electrical isolation has been developed assuming either full disc separation by a sand layer or partial disc contact with some sand present. Idealisations inherent in both test methods mean that they represent a severe case. Given these limitations, it is likely that the test methods, at their present stage of development, should be used as a means to qualitatively assess the relative effects on electrical isolation of different contaminants
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