173 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
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
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
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
Wear and Fatigue of Railway Track Caused by Contamination, Sanding and Surface Damage
The wheel rail contact operates in an arduous
environment. Damage to the surface of either
component is possible during manufacture, installation,
or operation. The question arises as to how tolerant is
the railway wheel or section of track to surface
indentation or damage.
In this work a twin disc simulation has been used
to relate the level of surface damage (as well as the way
it is generated) to the fatigue life of the surfaces. A
related problem is the presence of solid contamination
on the track. Sand (applied for improved adhesion) or
track ballast material can cause damage to the rail and
wheel surfaces. These mechanisms have been explored
to assess the effect on contact fatigue life and wear. The
disc specimens have been either artificially damaged
(with dents and scratches) or run with particles of sand
or ballast material. The discs were then loaded and
rotated at realistic conditions of contact pressure and
controlled slip.
For normal operation of the contact, either dry or
with water lubrication, surface dents and scratches have
little effect on fatigue life. The normal plastic flow in
the rail surface layer acts to close up dents. The failure
of the disc is then by fatigue cracking across the whole
surface with no particular preference to the dent
location. Alternatively, if the contact is lubricated with
oil then this plastic flow is greatly reduced and the dents
act as stress raisers and fatigue cracks initiate from their
trailing edge.
Sand or ballast particles are crushed as they enter
the wheel/rail contact. The fragments indent the surfaces
and rapidly roughen the contact faces. The surface
indentation is relatively minor, but the presence of
particles increases the level of traction (over the wet
case) and promotes further surface plastic flow. This
can reduce the residual fatigue life of the contact.
Further, high concentrations of sand were shown to
promote a low cycle fatigue process that caused very
high wear by the spallation of material.
The twin disc simulations have shown that, under
conditions similar to that of wheel/rail operation,
surface damage is not a primary cause of fatigue failure.
However, wear is greatly accelerated by the presence of
solid contaminants and some evidence of a low cycle
fatigue process was observed for sanded contacts
- …