Measurement of interface pressure in interference fits

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

An ultrasonic approach for contact stress mapping in machine joints and concentrated contacts

The measurement of pressure at a contact in a machine part is important because contact stresses frequently lead to failure by seizure, wear or fatigue. While the interface might appear smooth on a macroscale, it consists of regions of asperity contact and air gaps on a microscale. The reflection of an ultrasonic pulse at such a rough contact can be used to give information about the contact conditions. The more conformal the contact, the smaller is the proportion of an incident wave amplitude that will be reflected. In this paper, this phenomenon has been used to produce maps of contact pressure at machine element interfaces. An ultrasonic pulse is generated and reflected at the interface, to be received by the same piezoelectric transducer. The transducer is scanned across the interface and a map of reflected ultrasound (a c-scan) is recorded. The proportion of the wave reflected can be used to determine the stiffness of the interface. Stiffness correlates qualitatively with contact pressure, but unfortunately there is no unique relationship. In this work, two approaches have been used to obtain contact pressure: firstly by using an independent calibration experiment, and secondly by using experimental observations that stiffness and pressure are linearly related. The approach has been used in three example cases: a series of press fitted joints, a wheel/rail contact and a bolted joint

Measuring wheel/rail contact stresses using ultrasound

The investigation of contact area and pressure distribution in a wheel/rail contact is essential information required in fatigue and wear calculations to determine design life, regrinding requirements, and maintenance schedules. The aim of this work was to use ultrasound to non-destructively determine wheel/rail contact pressures. Three different contacts were investigated those resulting from; un-used, sand damaged, and worn wheel/rail specimens. A wheel/rail interface behaves like a spring. If the pressure is high the interface is very stiff, with few air gaps, and allows the transmission of an ultrasonic sound wave. If the pressure is low, interfacial stiffness is low and almost all the ultrasound is reflected. A spring model was used to determine maps of contact stiffness from wheel/rail ultrasonic reflection data. Pressure was then determined using a calibration experiment. Separate calibrations were performed for each of the three sets of wheel/rail specimens investigated. Measured contact pressure distributions are compared to those determined using analytical and computer bases numerical techniques

Experimental characterization of wheel-rail contact patch evolution

The contact area and pressure distribution in a wheel/rail contact is essential information required in any fatigue or wear calculations to determine design life, re-grinding, and maintenance schedules. As wheel or rail wear or surface damage takes place the contact patch size and shape will change. This leads to a redistribution of the contact stresses. The aim of this work was to use ultrasound to nondestructively quantify the stress distribution in new, worn, and damaged wheel-rail contacts. The response of a wheel/rail interface to an ultrasonic wave can be modeled as a spring. If the contact pressure is high the interface is very stiff, with few air gaps, and allows the transmission of an ultrasonic sound wave. If the pressure is low, interfacial stiffness is lower and almost all the ultrasound is reflected. A quasistatic spring model was used to determine maps of contact stiffness from wheel/rail ultrasonic reflection data. Pressure was then determined using a parallel calibration experiment. Three different contacts were investigated; those resulting from unused, worn, and sand damaged wheel and rail specimens. Measured contact pressure distributions are compared to those determined using elastic analytical and numerical elastic-plastic solutions. Unused as-machined contact surfaces had similar contact areas to predicted elastic Hertzian solutions. However, within the contact patch, the numerical models better reproduced the stress distribution, as they incorporated real surface roughness effects. The worn surfaces were smoother and more conformal, resulting in a larger contact patch and lower contact stress. Sand damaged surfaces were extremely rough and resulted in highly fragmented contact regions and high local contact stress. Copyright © 2006 by ASME

Ultrasonic characterisation of wheel hub/axle interference fit pressures

Railway wheels are secured onto the axle by means of an interference fit. The wheel is press fitted onto a pre-lubricated axle, and the resulting interference fit induces a contact pressure at the interface. Occasionally railway wheels fail by fatigue, with the initiation point for the failure frequently traced to the interference fit. The aim of this work is to use ultrasonic reflection to non-destructively determine contact conditions in the interference fit. The rough surface contact at the interference fit interface behaves like a spring. If the contact pressure is high the interface is conformal with few air gaps, the stiffness is then high and the transmission of an ultrasonic wave is permitted. However, when pressure is low more air gaps exist, interfacial stiffness is then reduced and more of the ultrasound is reflected. Normalised contact pressure was determined from this stiffness. Maps of the interface have been produced which show the contact pressure to peak at the edges of the fit, and to experience a continuous variation about a mean value elsewhere

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

SVtL: System Verification through Logic: tool support for verifying sliced hierarchical statecharts

SVtL is the core of a slicing-based verification environment for UML statechart models. We present an overview of the SVtL software architecture. Special attention is paid to the slicing approach. Slicing reduces the complexity of the verification approach, based on removing pieces of the model that are not of interest during verification. In [18] a slicing algorithm has been proposed for statecharts, but it was not able to handle orthogonal regions efficiently. We optimize this algorithm by removing false dependencies, relying on the broadcasting mechanism between different parts of the statechart model

Characterising pressure and bruising in apple fruit

A large percentage of apples are wasted each year due to damage such as bruising. The apple journey from orchard to supermarket is very complex and apples are subjected to a variety of static and dynamic loads that could result in this damage occurring. The aim of this work was to use a novel ultrasonic technique to study apple contact areas and stresses under static loading that may occur, for example, in bulk storage bins used during harvesting. These results were used to identify load thresholds above which unacceptable damage occurs. They were also used to validate output from a finite element model, which will ultimately be developed into a packaging design tool to help reduce the likelihood of apple damage occurring

Detecting Damage on Wind Turbine Bearings Using Acoustic Emissions and Gaussian Process Latent Variable Models

This paper presents a study into the use of Gaussian Process Latent Variable Models (GP-LVM) and Probabilistic Principal Component Analysis (PPCA) for detection of defects on wind turbine bearings using Acoustic Emission (AE) data. The results presented have been taken from an experimental rig with a seeded defect, to attempt to replicate an AE burst generated from a developing crack. Some of the results for both models are presented and compared, and it is shown that the GP-LVM, which is a nonlinear extension of PPCA, outperforms it in distinguishing AE bursts generated from a defect over those generated by other mechanisms