213 research outputs found
Viscosity measurement in thin lubricant films using shear ultrasonic reflection
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
Combating automative engine valve recession
[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
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
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
Wear of a chute in a rice sorting machine
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
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
Wheel/Rail Contact Isolation Due to Track Contamination
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
The mechanisms of pedestrian slip on flooring contaminated with solid particles
Statistics by the UK Health and Safety Executive (HSE) suggest that slips, trips and falls account for up to one in three major workplace accidents. The vast majority of these accidents are the result of contaminant (fluid or solid) within the shoe-floor contact. Though the lubrication mechanisms for liquid contaminants within the contact are well understood, the same cannot be said for particulate contaminants. This paper considers the key parameters controlling friction in a shoe-floor contact contaminated with various particles of different diameters and shape factors and floors with different roughness values (Rz). Experiments were conducted using a Stanley Pendulum Tester, which is the floor friction tester recommended by the HSE. Results suggest that the adhesive friction is significantly affected by particulate contaminants, while the hysteretic component is not. Three lubrication mechanisms identified as sliding, shearing and rolling have been observed depending on floor roughness, particle size and shape factor and have been plotted in a simple map to predict behaviour
Ultrasound for the non-invasive measurement of IC engine piston skirt lubricant films
Measurements of piston skirt film thickness in-recoprocare are relatively scarce and generally require significant modification to a test engine. However, the trend of engine original equipment manufacturers to downsize capacities whilst maintaining power outputs, increases the demands placed on the piston components. As a result, the relative pressure loading on the piston skirt is increasing and requires that lubrication be optimised to maintain low friction. This paper outlines experimental skirt film thickness measurements obtained from a fired single cylinder engine using reflected ultrasound. The profile of the film structure is observed and an insight into some of the motions of the piston can be determined. This study shows that quantitative film thickness measurements can be made using ultrasound and which are comparable with other established techniques
Integrating Dynamics and Wear Modelling to Predict Railway Wheel Profile Evolution
The aim of the work described was to predict wheel
profile evolution by integrating multi-body dynamics
simulations of a wheelset with a wear model.
The wear modelling approach is based on a wear
index commonly used in rail wear predictions. This
assumes wear is proportional to Tγ, where T is tractive
force and γ is slip at the wheel/rail interface. Twin disc
testing of rail and wheel materials was carried out to
generate wear coefficients for use in the model.
The modelling code is interfaced with
ADAMS/Rail, which produces multi-body dynamics
simulations of a railway wheelset and contact conditions
at the wheel/rail interface. Simplified theory of rolling
contact is used to discretise the contact patches
produced by ADAMS/Rail and calculate traction and
slip within each.
The wear model combines the simplified theory of
rolling contact, ADAMS/Rail output and the wear
coefficients to predict the wear and hence the change of
wheel profile for given track layouts
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